distribution and origin of protodolomite from the late miocene pliocene red clay formation chinese loess plateau the pliocene epoch is considered the most recent analog of modern warming because co2 levels were similar to the present to explore the carbonate minerals formed in the warmer pliocene epoch we studied two continuous sections of the red clay formation on the chinese loess plateau clp by x ray diffraction xrd fourier transform infrared spectroscopy ftir scanning electron microscopy sem and stable isotope mass spectrometry the red clay formation on the clp exhibits diagnostic ftir absorption features of calcite and protodolomite this allowed quantification of the two carbonate minerals by the ftir method using the ftir method we estimate the average concentration of protodolomite in bajiazui is 3 6 whereas the duanjiapo section is 6 0 protodolomite occurrence is more consistent and the concentration is higher from similar to 6 5 4 2 ma b p and decreases markedly from 4 2 2 6 ma b p red clay protodolomite is depleted in both delta c 13 pdb and delta o 18 pdb ranging from 4 1 to 10 4 and from 6 7 to 11 6 respectively and has a slightly higher delta o 18 value than the calcites sem observations show that red clay protodolomite is composed of euhedral rhombic crystals that range from 1 20 mu m in diameter grow into the soil voids and coexist with authigenic calcite and palygorskite these observations imply that the protodolomite grew in situ and is authigenic from pedogenesis dolomitization in the red clay sequence appears to be the result of overcoming kinetic barriers we propose that in the red clay a warm climate with seasonal dry conditions leads to the formation of calcrete from soil pore waters thereby enriching the pore solutions with respect to mg2 and significantly increasing the mg ca ratio bringing about the formation of protodolomite

withdrawn petrographic and origin of authigenic carbonate minerals and the associated fluid evolution in the lacustrine carbonate siliciclastic mudrocks a case from the low cretaceous of the erennaoer sag erlian basin ne china

sediment transport mechanisms on soil mantled hillslopes landscape evolution is modeled widely using a simple creep law for complex processes of sediment transport here field data show how a new transport model combined with an exponential soil production law better captures spatial variations of soil thick ness on hillslopes we combine parameterizations of simple and depth dependent creep with overland flow to predict soil thickness and suggest how soil distribution evolves in response to climatic and tectonic forcing we present an empirical expression for the response time of the system to external forcing that shows strong dependence on relief and is independent of soil production rate we suggest that this parameterization may be used to quantify upland carbon storage and removal and predict impacts of defor estation or rapid climatic changes

occurrence and formation of dolomite in organic rich continental margin sediments dolomite is presently forming throughout a large part of the oceans in continental margin marine sediments accumulating at rates less than 500 m m y and having organic carbon contents greater than 0 5 wt at higher sedimentation rates the dolomite content of the sediments is greatly diluted at lower organic carbon content the porewater chemistry precludes dolomite formation the reactants for dolomite formation have the following sources magnesium supplied from overlying seawater calcium supplied by the dissolution of calcium carbonate and carbon supplied from carbonate dissolution and organic carbon oxidation the rate of dolomite formation appears to be limited by the rate of calcium carbonate dissolution this rate can be quantitatively calculated from the magnesium diffusional and advective fluxes conversely in ancient sediments the depth and timing of dolomite formation can be calculated from the amount of dolomite and the sedimentation rate in dolomite rich sediments these calculations demonstrate that most of the dolomite formed within a few tens of meters of the sea floor within the zone of microbial sulfate reduction such dolomites have ne ative values of dgr13c dolomite in dolomite poor sediments may form at greater burial depth below the zone of sulfate reduction resulting in positive values of dgr13c the distribution coefficient of strontium in dolomite has been determined to have an average value of 0 06 using this value predicted strontium contents for submarine dolomites range from 150 ppm in dolomite rich sections to 1 290 ppm in dolomite poor sections

role of bacterial mats in oxygen deficient marine basins and coastal upwelling regimes preliminary report bacterial mats play an important role in the sedimentation history of many marine biogenic regimes observations of the living mats suggest that in shallow waters they are dominated by photosynthetic cyanobacteria while in deeper waters the mats are dominated by nonphotosynthetic sulfur oxidizing bacteria fossilized remains of analogous mat structures from the miocene monterey formation are commonly associated with oil producing and phosphate rich zones preliminary observations suggest that bacterial mats may be a source of the organic matter that in time is thermally altered to hydrocarbons

carbon isotopic composition and lattice bound carbonate of peru chile margin phosphorites new light stable carbonate carbon isotope and lattice bound co2 data from quaternary peru chile margin phosphatic nodules crusts and pelletal grains and from associated dolomicritic concretions are presented which provide constraints on the timing and mechanisms of growth of these phases in organic carbon rich sediments comparison of 13c values from carbonate fluorapatite cfa nodules and pelletal grains 4 8 to 0 0 and 2 9 to 1 0 pdb respectively with pore water total dissolved 13c values from these sediments suggests early authigenic cfa precipitation from pore waters within a few centimeters of the sediment water interface in association with suboxic to perhaps anoxic microbial degradation of organic matter in contrast the dolomicritic cores of nodules recovered from about 12 s display both strongly negative to positive 13c values 10 8 to 6 1 characteristic of formation deeper in the sediments in association with methanogenic and perhaps sulfate reduction microbial processes the amount of structural carbonate in cfa suggests that carbonate substitution generally increases as 13c in cfa decreases a probable consequence of increasing carbonate and accompanying charge balancing substitutions in the cfa lattice in response to increasing pore water carbonate ion concentrations with depth below the sediment water interface in one buried upward growing nodule decreasing cfa 13c and increasing structural co2 also correspond to decreasing cfa growth rates these data suggest that in addition to other constraints such as pore water phosphorus and fluoride availability the lower limit of cfa precipitation in suboxic to anoxic sediments may be controlled by lattice poisoning due to excessive dissolved carbonate ion concentrations in organic rich peru chile margin sediments this depth threshold appears to be at approximately 5 10 cm below the sediment water interface where maximum cfa co2 contents of about 6 wt occur in less organic rich settings greater depths of precipitation of cfa may be anticipated below this relatively shallow depth of cfa precipitation on the peru shelf high pore water alkalinity and associated elevated total dissolved carbon and carbonate ion concentration apparently favor the precipitation of authigenic carbonates

a sediment transport model for incision of gullies on steep topography 1 we have conducted surveys of gullies that developed in a small steep watershed in the idaho batholith after a severe wildfire followed by intense precipitation we measured gully length and cross sections to estimate the volumes of sediment loss due to gully formation these volume estimates are assumed to provide an estimate of sediment transport capacity at each survey cross section from the single gully forming thunderstorm sediment transport models commonly relate transport capacity to overland flow shear stress which is related to runoff rate slope and drainage area we have estimated the runoff rate and duration associated with the gully forming event and used the sediment volume measurements to calibrate a general physically based sediment transport equation in this steep high shear stress environment we find that a shear stress exponent of 3 corresponding to drainage area and slope exponents of m 2 1 and n 2 25 match our data this shear stress exponent of 3 is approximately 2 times higher than those for bed load transport in alluvial rivers but is in the range of shear stress exponents derived from flume experiments on steep slopes and with total load equations the concavity index of the gully profiles obtained theoretically from the area and slope exponents of the sediment transport equation c m 1 n agrees well with the observed profile concavity of the gullies our results although preliminary because of the uncertainty associated with the sediment volume estimates suggest that for steep hillslopes such as those in our study area a greater nonlinearity in the sediment transport function exists than that assumed in some existing hillslope erosion models which calculate sediment transport capacity using the bed load equations developed for rivers

how well can hillslope evolution models explain topography simulating soil transport and production with high resolution topographic data the morphology of hillslopes is a direct refl ection of tectonic forcing and climatic and biologic processes that drive soil production mobilization and transport soil transport on hillslopes affects river incision by providing tools for channel abrasion and controls the distribution of sediment that infl uences aquatic habitat although numerous hillslope transport relationships have been proposed over the past 60 years a comprehensive analysis of model predictions for a real landscape has not been performed here we use high resolution topographic data obtained via airborne laser swath mapping alsm to simulate the long term evolution of oregon coast range hillslopes and test three published transport models and a new model that accounts for nonlinear depth and slopedependent transport analysis of one dimensional steady state solutions for these four models suggests that plots of gradient curvature may be diagnostic for distinguishing model predictions to evaluate two dimensional model predictions for our fi eld site we assumed local steady state erosion for a 72 000 m 2 sequence of hillslopes and valleys after calibrating each of the four models we imposed constant base level lowering for cells within the valley network simulated 500 000 yr of soil production and transport and determined which transport model best preserved morphologic patterns that describe the current landscape form models for which fl ux varies proportionally with hillslope gradient generated broadly convex hilltops inconsistent with the sharp crested steep sided slopes of our study site whereas the two nonlinear slope dependent models produced convex planar slopes consistent with current hillslope form our proposed nonlinear slope and depth dependent model accounts for how soil thickness controls the magnitude of biogenic disturbances that drive transport this model best preserved the current landscape form particularly the narrow sharply convex hilltops characteristic of the oregon coast range according to our formulation which provides an explicit linkage for relating the distribution of biota to hillslope processes the degree of hilltop convexity varies nonlinearly with the ratio of erosion rate to maximum soil production rate highlighting the profound infl uence of soil depth on hillslope evolution

testing the model of late archean terrane accretion in southern west greenland a comparison of the timing of geological events across the qarliit nunaat fault buksefjorden region abstract the prevailing tectonic model for the archean gneiss complex of southern west greenland holds that terranes with different metamorphic histories and protolith ages were juxtaposed between 2 80 and 2 72 ga this model was tested by using detailed mapping and u pb geochronology to compare the timing of geological events across the qarliit nunaat fault the boundary between terranes in the buksefjorden region the fault was interpreted as a terrane boundary mostly because it is a narrow zone of mylonitic rocks that separates granulite or retrograde amphibolite facies gneisses of the tasiusarsuaq terrane tt from prograde amphibolite facies gneisses of the akulleq terrane at and it separates tonalitic gneisses of different age 3 87 3 60 and 2 80 ga in the at versus 2 92 2 86 ga in the tt dating of similar homogeneous gneisses in each terrane yielded ages of 2843 3 ma in the tt tonalite and 2824 2 ma in the at granodiorite ikkattoq gneiss granulite facies metamorphism in the tt is thought to have coincided with intrusion of a 2805 2 ma orthopyroxene pegmatite and zircon and titanite from nearby amphibolitic gneiss yielded ages of 2779 2 and 2660 10 ma respectively metamorphic minerals in the at are younger zircon and titanite from amphibolitic gneiss are 2717 3 and 2510 10 ma respectively and monazite from pelitic schist is 2732 5 and 2708 4 ma a concordant pegmatite layer that underwent a substantial amount of shearing along the qarliit nunaat fault was dated at 2725 2 other pegmatites that intruded relatively late in the structural history crystallized at 2710 2 in the tt and 2541 2 ma in the at these results support the prevailing tectonic model by showing that i similar homogeneous gneisses in each terrane are different age ii 2805 ma granulite facies metamorphism in the tt predated 2740 2700 ma amphibolite facies metamorphism in the at and iii shearing along the terrane boundary occurred after 2725 ma

local topography and erosion rate control regolith thickness along a ridgeline in the sierra nevada california the ridgelines of mountain ranges are a source of geomorphic information unadulterated by the arrival of sediment from upslope studies along ridgecrests therefore can help identify and isolate the controls on important regolith properties such as thickness and texture a 1 5 km section of ridgeline in the sierra nevada ca with a tenfold decrease in erosion rate inferred from ridgetop convexity provided an opportunity to conduct a high resolution survey of regolith properties and investigate their controls we found that regolith along the most quickly eroding section of the ridge was the rockiest and had the lowest clay concentrations furthermore a general increase in regolith thickness with a slowing of erosion rate was accompanied by an increase in biomass changes in vegetation community broader ridgeline profiles and an apparent increase in total available moisture the greatest source of variation in regolith thickness at the 10 100 m scale however was the local topography along the ridgeline with the deepest regolith in the saddles and the thinnest on the knobs because regolith in the saddles had higher surface soil moisture than the knobs we conclude that the hydrological conditions primarily driven by local topography i e rapid vs slow drainage and water storage potential provide the fundamental controls on regolith thickness through feedbacks incorporating physical weathering by the biota and chemical weathering moreover because the ridgeline saddles are the uppermost extensions of first order valleys we propose that the fluvial network affects regolith properties in the furthest reaches of the watershed

the root of branching river networks models and field measurements together show that the branching patterns of fine scale river networks are the result of coupled instabilities in the erosional processes that drive valley incision the mechanisms that create the familiar patterns seen in branching river networks are poorly understood here the authors use a simple numerical model together with field measurements to establish that the branching patterns of fine scale river networks are the result of coupled instabilities in the erosion processes involved in valley widening and channel incision this means that river networks are not a consequence of random topology but of erosional mechanics and the branching patterns can provide information about the geological and environmental conditions during their formation branching river networks are one of the most widespread and recognizable features of earth s landscapes and have also been discovered elsewhere in the solar system1 2 but the mechanisms that create these patterns and control their spatial scales are poorly understood theories based on probability3 4 5 or optimality3 6 7 8 have proven useful9 but do not explain how river networks develop over time through erosion and sediment transport here we show that branching at the uppermost reaches of river networks is rooted in two coupled instabilities first valleys widen at the expense of their smaller neighbours and second side slopes of the widening valleys become susceptible to channel incision each instability occurs at a critical ratio of the characteristic timescales for soil transport and channel incision measurements from two field sites demonstrate that our theory correctly predicts the size of the smallest valleys with tributaries we also show that the dominant control on the scale of landscape dissection in these sites is the strength of channel incision which correlates with aridity and rock weakness rather than the strength of soil transport these results imply that the fine scale structure of branching river networks is an organized signature of erosional mechanics not a consequence of random topology

the geochemistry of carbonate diagenesis the past present and future stable carbon and oxygen isotopes 18o and 13c values and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years as valuable as these insights have been many problems regarding the interpretation of geochemical signals within mature rocks remain for example while the 18o values of carbonate rocks are dependent both upon the temperature and the 18o value of the fluid and additional information including trace element composition aids in interpreting such signals direct evidence of either the temperature or the composition of the fluids is required such information can be obtained by analysing the 18o value of any fluid inclusions or by measuring the temperature using a method such as the clumped isotope technique such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well known tendency for 18o values of carbonate rocks to decrease with increasing age unlike the 18o 13c values of carbonates are considered to be less influenced by diagenesis and more a reflection of primary changes in the global carbon cycle through time however many studies have not sufficiently emphasized the effects of diagenesis and other post depositional influences on the eventual carbon isotopic composition of the rock with the classic paradigm that the present is the key to the past being frequently ignored finally many additional proxies are poised to contribute to the interpretation of carbonate diagenesis although the study of carbonate diagenesis is at an exciting point with an explosion of new proxies and methods care should be taken to understand both old and new proxies before applying them to the ancient record

uplift histories from river profiles 1 longitudinal river profiles where elevation of a river bed is plotted as a function of distance along the river bed contain information about uplift rate when a region adjacent to a reference level e g sea level is uplifted a rapid change in gradient occurs near the river mouth the erosional process causes this change in gradient to migrate upstream thus a river profile is effectively a tape recording of the uplift rate history provided that the erosional process can be adequately parameterized here we use a non linear equation to relate the shape of a river profile z x to uplift rate history u t if erosion is assumed to be dominated by knickpoint retreat an inverse model can be formulated and used to calculate uplift rate histories our model builds upon standard stream profile analysis which focuses on the relationship between profile slope and drainage area we have applied this analytical approach to river profiles from the bie dome angola calculated uplift rate histories agree with independent geologic estimates

advection of pore fluids through sediments in the equatorial east pacific measurements of the ratio of helium 4 to helium 3 and of calcium ion in the pore waters of sediments at two locations in the eastern equatorial pacific indicate that solution advection is occurring through the sediments both the helium ratio and the calcium ion profile yield velocity values for advective flow of about 20 centimeters per year mass balance constraints are also consistent with the interpretation presented flow appears to be occurring thorugh relatively thick sediments on the order of 300 meters

geologic constraints on bedrock river incision using the stream power law denudation rate in unextended terranes is limited by the rate of bedrock channel incision often modeled as work rate on the channel bed by water and sediment or stream power the latter can be generalized as ka m s n where k represents the channel bed s resistance to lowenng whose variation with lithology is unknown a is drainage area a surrogate for discharge s is local slope and m and n are exponents whose values are debated we address these uncertainties by simulating the lowering of ancient river profiles using the finite difference method we vary m n and k to match the evolved profile as closely as possible to the corresponding modern river profile over a time period constrained by the age of the mapped paleoprofiles we find at least two end member incision laws ka 0 3 0 5 s for australian rivers with stable base levels and k f a 0 1 0 2 s n for rivers in kauai subject to abrupt base level change the long term lowering rate on the latter expression is a function of the frequency and magnitude of knickpoint erosion characterized by k f incision patterns from japan and california could follow either expression if they follow the first expression with m 0 4 k varies from 10 7 10 6 m 0 2 yr for granite and metamorphic rocks to 10 5 10 4 m 0 2 yr for volcaniclastic rocks and 10 4 10 2 m 0 2 yr for mudstones this potentially large variation in k with lithology could drive strong variability in the rate of long term landscape change including denudation rate and sediment yield

origin of dolomite in the arab d reservoir from the ghawar field saudi arabia evidence from petrographic and geochemical constraints a significant proportion of oil production from the kim meridgian arab d strata in the ghawar field saudi arabia originates from dolomitized rocks stratigraphic petrographic and geochemical data suggest that at least four episodes of dolomitization affected these sediments the lower portion of the arab d zone 3 is only partially dolomitized with the dolomite frequently being associated with firm grounds we propose that these dolomites were formed on an outer ramp setting with a maximum water depth of 50 m during a period of nondeposition with the dolomitization process being promoted by the oxidation of organic material and the diffusion of mg 21 from the overlying seawater the dolomites in zone 2b are geochemically dis tinct compared to those in zone 3 in that they have relatively positive oxygen isotope compositions 2 1t o22 compared to 26 5 the relatively positive oxygen isotope composition and the geochemical sim ilarity of zone 2b to the dolomites in zone 1 which are intimately associated with the overlying evaporites has led us to conclude that the zone 2 dolomites probably formed by the reflux of hypersaline fluids through the sediments these hypersaline fluids bypassed zone 2a by moving through the grain dominated sediments early cemen tation and dolomite formation made these units more susceptible to later fracturing that affected the entire arab d formation this frac turing allowed higher temperature fluids to leach the dolostones there by removing any remaining calcite and partially resetting the oxygen isotope composition of some of the dolomites as a result of this later dolomitization event rocks that were only partially dolomitized were leached creating units with extremely high permeability and porosity super k intervals dolomites in the lower zone 3 were recrystallized during burial by the normal geothermal gradient leading to the pre sent negative oxygen isotope values zone 1 dolomites are petrograph ically distinct from zone 2 dolomite in that they are mimetic and fabric preserving although they are geochemically similar this mimetic style of dolomitization occurs immediately adjacent to the overlying anhy drite and is interpreted to have formed very shortly after deposition from hypersaline brines recovery and their presence is inferred from a combination of production data and the diameter of the bore hole as measured using the caliper log as a result of the importance of dolomite in the formation of the super k intervals within the arab d an understanding of the paragenesis of these zones is important not only in predicting super k formation within the arab d but also as an analogue for understanding other dolomitized res ervoirs with high permeability and porosity previous characterization work of dolostones from the arab d in the ghawar field cantrell et al 2004 has identified three types of dolomite with distinctive petrographic stratigraphic and geochemical characteristics fabric preserving non fabric preserving and baroque dolomite fabric pre serving dolomite is very finely crystalline in which petrographic details of the original limestone fabric are usually well preserved fabric preserving dolomite contains low concentrations of fe average 247 ppm fe and relatively positive oxygen isotope values average

the illusion of diffusion field evidence for depth dependent sediment transport soil covered upland landscapes are common in much of the habitable world and our understanding of their evolution as a function of different climatic tectonic and geologic regimes is important across a wide range of disciplines erosion laws direct quantitative study of the processes shaping earth s surface and form the basis of landscape evolution modeling but are based on limited field data here we use in situ produced cosmogenic 1 0 be and 2 6 al concentrations from granitic saprolite to quantify an exponential decline in soil production with increasing soil thickness for a new field site in point reyes california results are similar to soil production functions from two different previously studied field sites and are used with extensive measurements of soil thickness to quantify depth integrated sediment transport flux plots of calculated sediment fluxes against the product of soil depth and hillslope gradient provide the first field based evidence that soil transport is a nonlinear depth dependent function data from all sites suggest that the widely used linear diffusion equation is only appropriate for shallow gradient convex up regions while the depth dependent transport law is more broadly applicable quantifying both the mobile soil thickness and landscape morphology is therefore critical to understanding how landscapes evolve

an empirical stream power formulation for knickpoint retreat in appalachian plateau fluviokarst summary hydrologically abandoned caves on tributaries of the upper cumberland river tennessee usa record a wave of river incision that advanced up the drainage basin in the late pliocene and early pleistocene geomorphic and geologic evidence suggests that incision occurred as a migrating knickpoint generated by sudden base level lowering the passage of a knickpoint up the cumberland river tributaries was modeled as a perturbation to steady state incision according to the stream power law e kq m s n and tested using dated incision events recorded in cave sediments knickpoint migration rates generated by this model were 0 1 0 18 m yr over the entire stream network and 4 0 m yr for the mainstem cumberland river channel the ratio m n 0 79 was consistent with previously published parameters however the values of m 1 91 and n 2 39 were much higher than those reported in previous field studies these results suggest the stream power model may be used to model knickpoint migration in the study area provided values for the constants m and n are larger this may be due to the influence of fluviokarst where surface drainage is interrupted due to diversion into the underlying karst aquifer field measurements of channel and basin geometry in fluviokarstic tributaries to the upper cumberland river show 1 a stronger variance between channel slope and discharge 2 a nonlinear relationship between discharge and drainage area and 3 stream width to be nearly invariant as opposed to non karst watersheds because the stream power model relies heavily on the substitution of discharge for drainage area the behavior of channel incision and knickpoint migration in fluviokarst may differ substantially from that of non karst channels

variable methane fluxes in shallow marine systems over geologic time abstract carbon cycling is an important but poorly understood process on passive continental margins in this study we use the ionic and stable isotopic composition of interstitial waters and the petrology mineralogy and stable isotopic composition of authigenic carbonates collected from ocean drilling program odp leg 174a sites 1071 and 1072 to constrain the origin of the carbonates and the evolution of methane on the outer new jersey shelf the pore fluids of the new jersey continental shelf are characterized by 1 a fresh brackish water plume and 2 organic matter degradation reactions which proceed through sulfate reduction however only minor methanogenesis occurs the oxygen isotopic composition of the pore fluids supports a meteoric origin of the low salinity fluids authigenic carbonates are found in nodules thin 1 cm layers and carbonate cemented pavements siderite is the most common authigenic carbonate followed by dolomite and calcite the oxygen isotopic composition of the authigenic carbonates i e 1 3 6 5 peedee belemnite pdb indicates an origin in marine pore fluids the carbon isotopic composition of dolomite cements range from 16 4 to 8 8 pdb consistent with formation within the zone of sulfate reduction siderite 13c values show a greater range 17 67 16 4 but are largely positive mean 2 8 and are interpreted to have formed throughout the zone of methanogenesis in contrast calcite 13c values are highly negative as low as 41 7 and must have formed from waters with a large component of dissolved inorganic carbon derived from methane oxidation pore water data show that despite complete sulfate reduction methanogenesis appears not to be an important process presently occurring in the upper 400 m of the outer new jersey shelf in contrast the carbon isotopic composition of the siderites and calcites document an active methanogenic zone during their formation the methane may have been either oxidized or vented from shelf sediments perhaps during sea level fluctuations if this unaccounted and variable methane flux is an areally important process during neogene sea level fluctuations then it likely plays an important role in long term carbon cycling on passive continental margins

channel changes in badlands stripping of the vegetation and soil from a 13 hectare site in virginia underlain by coastal plain sediments created a rapidly evolving badland topography two types of channels developed 1 sand bed alluvial channels were graded to transport the bed material load supplied from slope erosion with available runoff but they also generally eroded their beds slowly and 2 steeper bedrock floored channels incised rapidly in bedrock channels the erosion rate was proportional to the 4 9ths power of drainage area and the 2 3rds power of gradient these exponents are consistent with a model in which the erosion rate is proportional to the bed shear during high flows due to rapid mass wasting and reduced runoff the alluvial channels became as much as 50 steeper during the winter than the summer with an attendant yearly cycle of winter aggradation and summer entrenchment the gradients their seasonal variability and their downstream hydraulic geometry were consistent with the predictions of total load transport formulas for sand beds and high loads the hydraulic geometry of alluvial channels in the virginia badlands were similar to that on the morrison formation in the western united states

landscape response to tectonic forcing digital elevation model analysis of stream profiles in the mendocino triple junction region northern california the topographic evolution of orogens is fundamentally dictated by rates and patterns of bedrock channel incision quantitative field assessments of process based laws are needed to accurately describe landscape uplift and denudation in response to tectonics and climate we evaluate and calibrate the shear stress or similar unit stream power bedrock incision model by studying stream profiles in a tectonically active mountain range previous work on emergent marine terraces in the mendocino triple junction region of northern california provides spatial and temporal control on rock uplift rates digital elevation models and field data are used to quantify differences in landscape morphology associated with along strike northwest to southeast changes in tectonic and climatic conditions analysis of longitudinal profiles supports the hypothesis that the study area channels are in equilibrium with current uplift and climatic conditions consistent with theoretical calculations of system response time based on the shear stress model within uncertainty the profile concavity of the trunk streams is constant throughout the study area 0 43 as predicted by the model channel steepness correlates with uplift rate these data help constrain the two key unknown model parameters the coefficient of erosion k and the exponent associated with channel gradient n this analysis shows that k cannot be treated as a constant throughout the study area despite generally homogeneous substrate properties for a reasonable range of slope exponent values n best fit values of k are positively correlated with uplift rate this correlation has important implications for landscape evolution models and likely reflects dynamic adjustment of k to tectonic changes due to variations in orographic precipitation and perhaps channel width sediment load and frequency of debris flows the apparent variation in k makes a unique value of n impossible to constrain with present data

erosional equilibrium and disequilibrium in the sierra nevada inferred from cosmogenic 26al and 10be in alluvial sediment we used cosmogenic 26 al and 10 be in stream sediment to measure landscape scale erosion rates for topographically diverse catchments at seven sierra nevada sites at three sites erosion rates and hillslope gradients are strongly correlated increasing with proximity to fault scarps and river canyons which appear to have accelerated local base level lowering rates and thus increased catchment erosion rates by up to 15 fold at four other sites far from fault scarps and river canyons erosion rates are much more uniform and less sensitive to average hillslope gradient our measurements show that contrasts in landscape erosion rates cannot be inferred from hillslope gradients alone because landscapes can evolve toward a state of erosional equilibrium in which steep and gentle slopes erode at similar rates

statistical description of slope dependent soil transport and the diffusion like coefficient 1 for hillslopes undergoing diffusive soil transport it is often assumed that the soil flux is proportional to the local land surface gradient where the coefficient of proportionality is like a diffusion coefficient inasmuch as transport involves quasi random soil particle motions related to biomechanical mixing and similar dilational processes a slope dependent relation arises from a balance between particle fluxes that tend to loft a soil and gravitational settling of particles into available pore space a specialized form of the fokker planck equation adapted to such particle motions clarifies how the particle flux involves advective and diffusive parts this in turn contributes to a kinematic description of the diffusion like coefficient ingredients of this coefficient include an active soil thickness a characteristic particle size the porosity in excess of a consolidated porosity and the rate of particle activation as a function of depth these last two ingredients vertical porosity structure and activation rate in effect characterize the magnitude and frequency of settling particle motions related to biological activity and thereby set the rate constant of the transport process the significance of land surface slope is that it is a measure of the downslope component of slope normal lofting that is balanced by settling because the diffusion like coefficient contains the soil thickness the analysis suggests that the soil flux is proportional to the depth slope product the analysis is consistent with published profiles of soil creep displacement and with published estimates of soil flux obtained by downslope integration of soil production rates for hillslopes in california and australia

topographic evolution and morphology of surfaces evolving in response to coupled fluvial and hillslope sediment transport 1 this paper quantifies how the ratio of sediment transport on hillslopes to sediment transport in channels influences surface and channel network morphologies and the dynamics of topographic evolution this problem is investigated by development and investigation of a simple deterministic model incorporating mass balance of sediment and runoff coupled with a law combining dispersive and concentrative sediment transport processes our analysis includes the identification of a new nondimensional parameter de that is a function of rainfall system size rock type and hydraulic regime and that is a measure of the relative importance of fluvial and hillslope sediment transport we show that de has an important influence on the surface morphology e g total exposed surface area and interface width which reflects surface roughness and relief channel network form e g channel sinuosity channel spacing and timescale of surface evolution surface and channel network morphologies are also strongly influenced by the overall surface slope relative to the magnitude of initial topographic roughness topographic evolution occurs in distinct phases of relief growth and decay the transition between which is controlled by a saturation phenomenon related to the growth of spatial correlations the scaling behavior of simulated topography with respect to both time and space is obtained and is shown to be independent of de roughness exponents are found to be independent of de but dependent on the magnitude of initial roughness interface width is shown to grow and decay as a logarithm of time

channel network extraction from high resolution topography using wavelets 1 the availability of high resolution topography from lidar offers new opportunities for objectively extracting the channels directly from a dem using local topographic information instead of inferring them indirectly based on global criteria such as area or area slope threshold relationships here we introduce the use of wavelet filtering to delineate threshold curvatures for defining valleys and threshold slope direction change for defining probable channeled portions of the valleys this approach exploits the topographic signatures uniquely found in high resolution topography and reveals the fuzzy topographic transition in which local weakly convergent areas lie at the transition between hillslopes and valleys

sealevel changes control diagenetic dolomite formation in hemipelagic sediments of the peru margin abstract the first experimentally determined temperature dependent oxygen 18 fractionation factor between dolomite and water at low temperatures vasconcelos c mckenzie j a warthmann r bernasconi s 2005 calibration of the 18o paleo thermometer with dolomite formed in microbial cultures and natural environments geology 33 317 320 allows now the precise calculation of temperatures during early diagenetic dolomite precipitation we use 18o values of early diagenetic dolomite beds sampled during odp legs 112 and 201 on the peru continental margin sites 1227 1228 and 1229 meister p mckenzie j a vasconcelos c bernasconi s frank m gutjahr m schrag d p 2007 dolomite formation in the dynamic deep biosphere results from the peru margin opd leg 201 sedimentology 54 1007 1032 to calculate paleo porewater temperatures at the time of dolomite precipitation we assumed unaltered seawater 18o values in the porewater which is supported by 18o values of the modern porewater presented in this study the dolomite layers in the pleistocene part of the sedimentary columns showed oxygen isotope temperatures up to 5 c lower than today since sites 1228 and 1229 are located at 150 and 250 m below sealevel respectively their paleo porewater temperatures would be influenced by considerably colder surface water during glacial sealevel lowstands thus pleistocene dolomite layers in the peru continental margin probably formed during glacial times this finding is consistent with a model for dolomite precipitation in the peru margin recently discussed by meister et al meister p mckenzie j a vasconcelos c bernasconi s frank m gutjahr m schrag d p 2007 dolomite formation in the dynamic deep biosphere results from the peru margin opd leg 201 sedimentology 54 1007 1032 where dolomite forms episodically at the sulphate methane interface it was shown that the sulphate methane interface migrates upwards and downwards within the sedimentary column but dolomite layers may only form when the sulphate methane interface stays at a fixed depth for a sufficient amount of time we hypothesize that the sulphate methane interface persists within toc rich interglacial sediments while this zone is buried by toc poor sedimentation during glacial times thus the presented oxygen isotope data provide additional information on the timing of early diagenetic dolomite formation and a possible link between episodicity in dolomite formation and sealevel variations a similar link between early diagenesis and oceanography may also explain spacing of dolomite layers in a milankovitch type pattern observed in the geological record such as in the miocene monterey formation

tectonic and climatic controls on knickpoint retreat rates and landscape response times 1 the rate at which transient knickpoints propagate through a landscape fundamentally controls the rate of geomorphic response to tectonic and climatic perturbation here we present knickpoint retreat rates upstream of active faults for 19 bedrock catchments in turkey and 11 bedrock catchments in italy where we have very good constraints on both the magnitude and timing of the tectonic perturbation and where climate histories are well documented we show that the knickpoints have average retreat rates of between 0 2 and 2 mm yr for catchments with drainage areas between 6 and 65 km2 and we test whether differences in rock mass strength and catchment size are sufficient to explain this range in retreat rates our analysis suggests that even accounting for these two variables knickpoint propagation velocities differ markedly and we show that channels crossing faults with higher throw rates have knickpoints that are retreating faster the dependence of knickpoint retreat velocity on throw rate is at least as important as catchment drainage area these results indicate counterintuitively that landscapes forced by large amplitude tectonic perturbations will have shorter response times than those perturbed by smaller amplitude changes the link between the knickpoint propagation velocity and throw rate is largely but not completely explained by channel narrowing in areas of high uplift rate channel steepening upstream of the active faults may explain all of the residual dependency of knickpoint retreat rate on fault throw rate but only if the slope exponent n in the standard stream power model is greater than 1 3 however we cannot rule out a role for sediment supply in driving enhanced knickpoint retreat rate in addition to the well documented channel narrowing effect finally we find that mean knickpoints retreat rates in turkey are only half of those in italy for catchments of equivalent size crossing faults with similar throw rates this difference in fluvial response time is accounted for by long term differences in the ratio of precipitation to infiltration in the two areas over the last 1 my

mg and ca isotope signatures of authigenic dolomite in siliceous deep sea sediments abstract authigenic carbonates in marine sediments frequently have carbon isotope ratios that reflect local organic carbon processing rather than the c 13 of the global dic dissolved inorganic carbon reservoir but their contributions to ancient sedimentary sections are difficult to assess in this study of authigenic dolomite from the miocene age monterey formation of offshore california mg and ca isotopes are shown to vary with stratigraphic depth as a result of early diagenetic processes the dolomite is a pre compaction authigenic phase that occurs as beds and nodules with c 13 ranging from 16 to 9 light c 13 values were likely acquired from the sedimentary zone of microbial sulfate reduction while heavy c 13 values were acquired from the zone of methanogenesis mg and ca isotopes are roughly anti correlated with intervals of negative c 13 associated with low mg 26 and higher ca 44 40 values the variability is observed over a wide range of length scales from 10 2 meters within individual authigenic beds nodules to 10 2 meters over the entire stratigraphic column and can be understood as the consequence of dolomite precipitation in pore fluids where mg supply is limited by diffusive transport the relationship of mg 26 and ca 44 40 to the more common stable isotope measurements of c 13 and o 18 represents a new diagenetically robust geochemical fingerprint for identifying synsedimentary authigenic carbonates in the geological record

role of sulfide oxidation in dolomitization sediment and pore water geochemistry of a modern hypersaline lagoon system dolomite precipitation has been reported in sediments of marginal marine hypersaline lagoons near rio de janeiro brazil and has been related to microbial sulfate reduction our field study of pore water and sediment geochemistry in two lagoons reveals only modest degrees of so4 reduction and even h2s oxidation significant differences in dolomite contents salinity and carbon sulfur cycling occur in the two lagoons pore waters from the most dolomite rich sediments exhibit significantly elevated so4 cl ratios relative to seawater moreover the o isotope composition of so4 in all lagoonal waters has evolved significantly from seawater values this suggests active sulfur recycling via h2s oxidation which provides acid that decreases pore water saturation state for high mg calcite and aragonite thermodynamically favoring the precipitation of dolomite the open hydrogeochemical nature of these lagoons maintains so4 concentrations below gypsum saturation allowing dolomite to form from fluids with normal marine mg ca

drainage density and relative relief in humid steep mountains with frequent slope failure previous studies in japanese mountains have shown that drainage density d correlates negatively with relief r and the erosion rate e whereas elsewhere both r and e correlate positively with d to investigate the inconsistency this paper compares two types of d r relations for eight mountain river basins in central japan r is computed from a digital elevation model for 1109 morphometric samples of area 0 5 km 0 5 km drainage networks in these cells were first constructed by map criteria applied previously in japan deeply notched v shaped contours with an angle 53 the resulting d correlates negatively with r confirming preceding studies when drainage lines along shallower hollows were added however the calculated d essentially constant these relations arise from active landsliding in high relief terrains which has eroded steep channel banks into gentle ones the decline of channel banks with increasing r is accelerated in terrains underlain by soft rocks because of rapid erosion the constant d for all the drainage lines indicates a uniform frequency or spacing of ridges and hollows on hillslopes in rugged humid mountains because the d r and d e relations for japan reflect a uniquely japanese physiographic setting characterized by frequent landsliding they differ from those relations for other regions where channelization by gullying predominates 1997 by john wiley sons ltd

pore fluid and diagenetic evolution of carbonate cements in lacustrine carbonate siliciclastic rocks a case from the lower cretaceous of the erennaoer sag erlian basin ne china lower cretaceous lacustrine carbonate siliciclastic rocks including siltstones and mudstones form a unique tight oil reservoir in the erennaoer sag in the erlian basin of northeastern china that is characterized by various carbonate cements and volcanic material based on integrated analysis of the petrology mineralogy and carbon and oxygen isotope geochemistry the origin compositions and evolution of carbonate cements and pore fluids are determined five types of carbonate cements are identified from oldest to youngest penecontemporaneous micritic crystalline ferroan dolomite 1 eogenetic micro crystalline ferroan dolomite 2 sparry micro crystalline ferroan calcite 1 mesogenetic micro crystalline non ferroan dolomite 3 and calcite 2 the evolution of the pore fluids also controlled the compositions and distribution of the carbonate cements of the lacustrine mudstone during different periods this may be advantageous to predict the distribution of high quality reservoirs the geochemistry of the paleo pore fluids in these mixed lithological reservoirs are more complicated thus the origin and distribution of pore fluids and their influences on carbonate cementation fluid rock interaction and reservoir quality are complicated and certainly worth studying for hydrocarbon exploration in addition the evolution of the pore fluids of the arshan formation have experienced different salinity fluid environments under alkaline to weak acid conditions

constraining dolomitization by mg isotopes a case study from partially dolomitized limestones of the middle cambrian xuzhuang formation north china the dolomite problem refers to the rare dolomite formation in modern oceans that is in sharp contrast to the widespread ancient dolostone in rock record as well as failure of laboratory inorganic dolomite precipitation at near earth surface temperature novel mg isotope systematics provides a promising tool in resolving the dolomite problem here we develop a protocol to place constraints on the dolomitization process by using mg isotopes in this study we measured mg isotopic compositions 26mg of two batches of partially dolomitized limestone samples from the middle cambrian xuzhuang formation in north china 26mg varies between 0 55 and 3 18 and shows a negative linear correlation with 1 mg suggesting that 26mg can be described by a binary mixing between the calcite and dolomite components mg isotopic composition of the dolomite component 26mgdol for the lower sample set that is collected from a 4 m stratigraphic interval containing three high frequency ribbon rock packstone cycles is 1 6 while 26mgdol for the upper sample set from a thick sequence of ribbon rock is significantly higher 0 3 however neither mineralogical and elemental compositions carbon and oxygen isotopes nor crystal morphologies of dolomite provides diagnostic criteria to differentiate these two batches of samples 26mgdol of the xuzhuang limestone is simulated by the advective flow af and the diffusion advection reaction dar models the af model assumes that mg is transported by advective fluid flows while the dar model simulates a contemporaneous seawater dolomitization process in which mg is delivered by diffusion the af modeling result indicates that 26mg of the dolomitization fluid is 0 4 and 1 7 for the lower and upper sample sets respectively these values are significantly higher than modern and cenozoic seawater mg isotopic composition suggesting that the dolomitization fluid is not contemporaneous seawater the af model also predicts spatially heterogeneous 26mgdol with progressive enrichment in 26mg along the fluid flow pathway in the dar model both dolomite content and 26mgdol of the lower sample set can be simulated by using seawater mg isotopic composition of 0 75 thus contemporaneous seawater dolomitization may explain 26mgdol of the xuzhuang limestone furthermore the dar model demonstrates spatially homogeneous 26mgdol to differentiate the af and dar models samples from multiple sections are required nevertheless this study implies that mg isotope might be a useful tool in the study of dolomitization

quantification of the transient response to base level fall in a small mountain catchment sierra nevada southern spain 1 we integrated optically stimulated luminescence dating and 10be cosmogenic nuclide measurements to quantify short to medium term 102 104 years catchment dynamics and response to active tectonics in the 27 km2 rio torrente catchment sierra nevada southern spain rapid base level fall has triggered knickpoint migration up both trunk and tributary channels resulting in two distinct geomorphic zones 1 a steep lower catchment with concordant rates of hillslope erosion and channel incision over both short 100 years 5 8 mm yr 1 and medium 12 ka 5 1 mm yr 1 timescales and 2 a low relief soil mantled upland topography with uniformly low bedrock and hillslope erosion rates 0 05 0 02 mm yr 1 the uniformity of erosion in this upland surface indicates that catchment topography was previously in steady state rapid river incision below the channel knickpoints has resulted in the development of steep landslide dominated hillslopes that are essentially tracking the incising channel the magnitude of base level fall required to generate these steep hillslopes is 100 m at least 50 m of this base level fall occurred during the past 21 ka these steep hillslopes are eroding back into the low angled upland surface at a much slower rate than the channel knickpoint consequently the trunk channel knickpoint has already reached the catchment headwaters while hillslopes continue to adjust to the new base level indicating that the channel profile will regain equilibrium form long before hillslopes thus hillslopes are the limiting factor for the duration of landscape transience in this small mountain catchment

fluvial incision and tectonic uplift across the himalayas of central nepal the pattern of fluvial incision across the himalayas of central nepal is estimated from the distribution of holocene and pleistocene terraces and from the geometry of modern channels along major rivers draining across the range the terraces provide good constraints on incision rates across the himalayan frontal folds sub himalaya or siwaliks hills where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces farther north and upstream in the lesser himalaya prominent fill terraces were deposited probably during the late pleistocene and were subsequently incised the amount of bedrock incision beneath the fill deposits is generally small suggesting a slow rate of fluvial incision in the lesser himalaya the terrace record is lost in the high range where the rivers are cutting steep gorges to complement the terrace study fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate this approach allows quantification of the effect of variations in channel slope width and discharge on the incision rate of a river the determination of incision rates requires an additional lithological calibration the two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared in the sub himalaya river incision is rapid with values up to 10 15 mm yr it does not exceed a few millimeters per year in the lesser himalaya and rises abruptly at the front of the high range to reach values of 4 8 mm yr within a 50 km wide zone that coincides with the position of the highest himalayan peaks sediment yield derived from the measurement of suspended load in himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range the observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat ramp flat geometry of the main himalayan thrust fault the morphology of the range reflects a dynamic equilibrium between present day tectonics and surface processes the sharp relief together with the high uplift rates in the higher himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the tibetan plateau driven by late cenozoic climate change or late miocene reactivation of the main central thrust

experimental evidence for hillslope control of landscape scale landscape evolution theory suggests that climate sets the scale of landscape dissection by modulating the competition between diffusive processes that sculpt convex hillslopes and advective processes that carve concave valleys however the link between the relative dominance of hillslope and valley transport processes and landscape scale is difficult to demonstrate in natural landscapes due to the episodic nature of erosion here we report results from laboratory experiments combining diffusive and advective processes in an eroding landscape we demonstrate that rainsplash driven disturbances in our experiments are a robust proxy for hillslope transport such that increasing hillslope transport efficiency decreases drainage density our experimental results demonstrate how the coupling of climate driven hillslope and valley forming processes such as bioturbation and runoff dictates the scale of eroding landscapes

an organodiagenetic model for marinoan age cap carbonates abstract the nuccaleena formation cap carbonate is the global stratotype for the marinoan glaciation and ediacaran gssp a designation that emphasises the importance of cap carbonates to our current understanding of global neoproterozoic biogeochemistry sedimentology and stratigraphy however to date there is no agreed depositional model for cap carbonates and there remains minimal detailed paragenetic data for developing process based depositional models here an early diagenetic organodiagenetic dolomite cementation model for marinoan cap carbonate formation is hypothesised and explored it is demonstrated how this process based model can explain the three main atypical sedimentary structures that are the basis for correlation of marinoan age cap carbonates giant tepee like structures sheet veins and tubestones these features are interpreted to be products of fluid overpressure deformation induced by organodiagenetic expansive cementation giant tepee like structures the hallmark of marinoan cap carbonates conform to a simple structural analysis that is consistent with the predicted stress field induced by expansive cementation driven fluid overpressure the undeformed to highly deformed gradient of deformation features in cap carbonates is consistent with the range of rheologies in a shallowly buried cementing carbonate modification of matrix textures and bedding surfaces also conforms to the range of expected textures within this model overall it is hypothesised that the aggradational condensed section architecture of cap carbonates was the primary control over the generation of these atypical sedimentary features the available paragenetic analysis suggests that the geochemical data particularly the c isotopic data can be reinterpreted as supportive of the organodiagenetic model and that the neoproterozoic ocean was perhaps similar to that of today the proposed model indicates that the current criteria for the placement of the ediacaran gssp are non unique and potentially non isochronous perhaps the most important aspect of this model is that it is testable and is a call for focused research on the much overlooked paragenesis of cap carbonates

upper cretaceous campanian phosphorites in jordan implications for the formation of a south tethyan phosphorite giant a record of sedimentary authigenic and biological processes are preserved within the upper cretaceous campanian alhisa phosphorite formation ap in central and northern jordan the ap formed near the eastern extremity of the south tethyan phosphorite province stpp a carbonate dominated upper cretaceous to eocene phosphorite giant that extends from colombia north africa to the middle east multidisciplinary research of the ap and associated cherts chalks and oyster buildups indicate that phosphatic strata formed on a highly productive storm dominated east west trending epeiric platform along the south tethyan margin the onset of phosphogenesis and the accumulation of economic phosphorite coincided with a rise in relative sea level that onlapped peritidal carbonates of the ajlun group pristine phosphates are associated with well developed micrite concretionary horizons and contain abundant non keeled spiral planktic foraminifera and a low diversity benthic assemblage of buliminacean foraminifera suggesting that pristine phosphates are a condensed facies and phosphogenesis was stimulated by the effects of a highly productive surface ocean and the suboxic diagenesis of sedimentary organic matter the bulk sediment composition and absence of fe bearing authigenic phases such as glauconite pyrite including pyrite molds siderite and goethite within pristine phosphates suggests that deposition and authigenesis occurred under conditions of detrital starvation and that iron pumping played a minimal role in phosphogenesis authigenic precipitation of phosphate occurred in a broad array of sedimentary environments herein termed a phosphorite nursery that spanned the entire platform this is a non uniformitarian phenomenon reflecting precipitation of sedimentary apatite across a wide depositional spectrum in a variety of depositional settings wherever the conditions were suitable for phosphogenesis sedimentologic data indicate that pristine phosphates were concentrated into phosphatic grainstones through storm wave winnowing and storm generated shelf parallel geostrophic currents economic phosphorites formed through the amalgamation of storm induced event beds stratigraphic packaging of phosphatic strata indicates that temporal variations in storm frequency were a prerequisite for the formation of economic phosphorite syndepositional phosphogenesis reworking and amalgamation to form phosphorites contrasts sharply with the principles of baturin cycling a transgressive systems tract coupled with high surface productivity created detritally starved settings favourable for phosphogenesis storm reworking of pristine

dissolved sulfide catalyzed precipitation of disordered dolomite implications for the formation mechanism of sedimentary dolomite abstract dolomite is a common mineral in the rock record however the rarity of modern dolomite and the notorious difficulty in synthesizing dolomite abiotically under normal earth surface conditions result in the long standing dolomite problem in sedimentary geology some modern dolomites are associated with sediments where microbial sulfate reduction is active however the role of sulfate reducing bacteria in dolomite formation is still under debate in this study we tested the effect of dissolved sulfide on the precipitation of ca mg carbonates which has been never explored before although dissolved sulfide is one of the major products of microbial sulfate reduction our results demonstrated that dissolved sulfide with a concentration of as low as several millimoles can enhance the mg2 incorporation into the calcitic structure and promote the crystallization of high magnesian calcite and disordered dolomite we also conducted seeded precipitation in experimental solutions containing dissolved sulfide which showed that calcite seeds can inhibit the precipitation of aragonite and monohydrocalcite caco3 h2o and induce more mg2 incorporation we propose that accumulated dissolved sulfide in pore waters in organic rich sediments may trigger the precipitation of disordered dolomite which can be considered as a precursor of some sedimentary dolomite our adsorption experiments revealed a strong adsorption of dissolved sulfide onto calcite faces we suggest that adsorbed dissolved sulfide can lower the energy barrier to the dehydration of mg2 water complexes on the growing carbonate surfaces this study sheds new light on understanding the role of sulfate reducing bacteria in dolomite formation and the formation mechanism of sedimentary dolomite

hillslope response to tectonic forcing in threshold landscapes hillslopes are thought to poorly record tectonic signals in threshold landscapes numerous previous studies of steep landscapes suggest that large changes in long term erosion rate lead to little change in mean hillslope angle measured at coarse resolution new lidar derived topography data enables a finer examination of threshold hillslopes here we quantify hillslope response to tectonic forcing in a threshold landscape to do so we use an extensive cosmogenic beryllium 10 10 be based dataset of catchment averaged erosion rates combined with a 500 km 2 lidar derived 1 m digital elevation model to exploit a gradient of tectonic forcing and topographic relief in the san gabriel mountains california we also calibrate a new method of quantifying rock exposure from lidar derived slope measurements using high resolution panoramic photographs two distinct trends in hillslope behavior emerge below catchment mean slopes of 30 modal slopes increase with mean slopes slope distribution skewness decreases with increasing mean slope and bedrock exposure is limited above mean slopes of 30 our rock exposure index increases strongly with mean slope and the prevalence of angle of repose debris wedges keeps modal slopes near 37 resulting in a positive relationship between slope distribution skewness and mean slope we find that both mean slopes and rock exposure increase with erosion rate up to 1 mm a in contrast to previous work based on coarser topographic data we also find that as erosion rates increase the extent of the fluvial network decreases while colluvial channels extend downstream keeping the total drainage density similar across the range our results reveal important textural details lost in 10 or 30 m resolution digital elevation models of steep landscapes and highlight the need for process based studies of threshold hillslopes and colluvial channels

mineralogy early marine diagenesis and the chemistry of shallow water carbonate sediments abstract shallow water carbonate sediments constitute the bulk of sedimentary carbonates in the geologic record and are widely used archives of earth s chemical and climatic history one of the main limitations in interpreting the geochemistry of ancient carbonate sediments is the potential for post depositional diagenetic alteration in this study we use paired measurements of calcium 44ca 40ca or 44ca and magnesium 26mg 24mg or 26mg isotope ratios in sedimentary carbonates and associated pore fluids as a tool to understand the mineralogical and diagenetic history of neogene shallow water carbonate sediments from the bahamas and southwest australia we find that the ca and mg isotopic composition of bulk carbonate sediments at these sites exhibits systematic stratigraphic variability that is related to both mineralogy and early marine diagenesis the observed variability in bulk sediment ca isotopes is best explained by changes in the extent and style of early marine diagenesis from one where the composition of the diagenetic carbonate mineral is determined by the chemistry of the fluid fluid buffered to one where the composition of the diagenetic carbonate mineral is determined by the chemistry of the precursor sediment sediment buffered our results indicate that this process together with variations in carbonate mineralogy aragonite calcite and dolomite plays a fundamental and underappreciated role in determining the regional and global stratigraphic expressions of geochemical tracers 13c 18o major minor and trace elements in shallow water carbonate sediments in the geologic record our results also provide evidence that a large shallow water carbonate sink that is enriched in 44ca can explain the mismatch between the 44 40ca value of rivers and deep sea carbonate sediments and call into question the hypothesis that the 44 40ca value of seawater depends on the mineralogy of primary carbonate precipitations e g aragonite seas and calcite seas finally our results for sedimentary dolomites suggest that paired measurements of ca and mg isotopes may provide a unique geochemical fingerprint of mass transfer during dolomitization to better understand the paleo environmental information preserved in these enigmatic but widespread carbonate minerals

early dolomitization and recrystallization of carbonate in an evaporite basin the middle devonian ratner laminite in southern saskatchewan canada this study provides an example of early dolomitization of basinal limestone associated with a giant evaporite deposit the middle devonian ratner carbonate is extensively to completely replaced by microcrystalline to finely crystalline dolomite in the southern and central part of southern saskatchewan in the elk point basin in the northern part however individual cored intervals of the ratner laminite are either completely dolomitized or contain no microcrystalline to finely crystalline dolomite stratigraphic petrographical and sr isotopic data suggest that the ratner dolomite has formed penecontemporaneously from dense evaporitic seawater that percolated downward the distribution of the ratner dolomite is highly localized in the northern part of the study area suggesting that the downward flow of mg 2 rich fluids was restricted by interbedded anhydrite but where the fluids penetrated the underlying carbonate deposits were dolomitized the depletion in 18 o isotope varying from 4 5 to 6 6 pdb and the coarser and inhomogeneous textures of the ratner dolomite are considered to be the result of recrystallization which is interpreted to occur at elevated temperatures related to a thermal anomaly during the late devonian and carboniferous burial

constraining pathways of microbial mediation for carbonate concretions of the miocene monterey formation using carbonate associated sulfate abstract carbonate concretions can form as a result of organic matter degradation within sediments however the ability to determine specific processes and timing relationships to particular concretions has remained elusive previously employed proxies e g carbon and oxygen isotopes cannot uniquely distinguish among diagenetic alkalinity sources generated by microbial oxidation of organic matter using oxygen nitrate metal oxides and sulfate as electron acceptors in addition to degradation by thermal decarboxylation here we employ concentrations of carbonate associated sulfate cas and 34scas along with more traditional approaches to determine the specific nature of concretion authigenesis within the miocene monterey formation integrated geochemical analyses reveal that at least three specific organo diagenetic reaction pathways can be tied to concretion formation and that these reactions are largely sample site specific one calcitic concretion from the phosphatic shale member at naples beach yields 34scas values near miocene seawater sulfate 22 vcdt abundant cas ca 1000 ppm depleted 13ccarb 11 vpdb and very low concentrations of fe ca 700 ppm and mn ca 15 ppm characteristics most consistent with shallow formation in association with organic matter degradation by nitrate iron oxides and or minor sulfate reduction cemented concretionary layers of the phosphatic shale member at shell beach display elevated 34scas up to 37 cas concentrations of 600 ppm mildly depleted 13ccarb 6 moderate amounts of mn ca 250 ppm and relatively low fe ca 1700 ppm indicative of formation in sediments dominated by sulfate reduction finally concretions within a siliceous host at montana de oro and naples beach show minimal cas concentrations positive 13c values and the highest concentrations of fe ca 11 300 ppm and mn ca 440 ppm consistent with formation in sediments experiencing methanogenesis in a highly reducing environment this study highlights the promise in combining cas analysis with more traditional techniques to differentiate among diagenetic reactions as preserved in the geologic record and shows potential for unraveling subsurface biospheric processes in ancient samples with a high degree of specificity

basins and bedrock spatial variation in 10be erosion rates and increasing relief in the southern rocky mountains usa we used measurements of cosmogenic 10 be in alluvium to estimate erosion rates on a 10 3 10 4 yr time scale for small 0 01 47 km 2 unglaciated basins in northern colorado southern wyoming and adjacent western nebraska western united states basins formed in proterozoic cores of laramide ranges are eroding more slowly 23 7 mm k y 1 n 19 than adjacent basins draining weakly lithified cenozoic sedimentary rocks 75 36 mm k y 1 n 20 erosion rates show a relationship to rock resistance and for granitic rocks to basin slope but not to mean annual precipitation we estimated longer term 10 5 yr time scale erosion rates for the granitic core of the front range by measuring the concentration of 10 be and 26 al produced mainly by muon interactions at depths 1 7 10 m below the surface concentrations imply erosion rates of 9 31 mm k y 1 similar to shorter term erosion rates inferred from alluvial sediment the spatial distribution of erosion rates and stratigraphic evidence imply that relief in the southern rocky mountains increased in the late cenozoic modern relief probably dates from post middle miocene time

a generalized power law approximation for fluvial incision of bedrock channels 1 sediment flux is known to influence bedrock incision rates in mountain rivers although the widely used stream power incision model lacks any explicit representation of sediment flux the model appears to work in a variety of real settings we address this apparent contradiction using numerical experiments to explore the morphology of fluvial landscapes evolved with four different incision models three of which include the influence of sediment flux on incision rate the numerical landscapes have different spatial patterns of uplift and are at steady state we analyze these landscape using the common stream power approach which views incision rates to be primarily a function of the local channel gradient s and the upstream drainage area a we find that incision rates i for these landscapes are well described by an empirical power law equation i k am sn this equation is functionally equivalent to the widely used stream power model with the important distinction that the parameters k m and n are entirely empirical these parameters take on constant values within a single landscape but can otherwise be quite different between landscapes mainly due to differences in the pattern of rock uplift within the drainage in particular the parameters m and n decrease as the rate of rock uplift becomes more focused in the upland part of a mountain belt the parameter m is particularly important in that it describes the sensitivity of a tectonically active mountain belt to changes in precipitation or tectonic accretion it also defines how incision rates will change as the discharge becomes flashier

the paragenetic history of a marinoan cap carbonate abstract carbonates that cap late neoproterozoic glacial diamictites contain large atypical tepee like structures that have been interpreted as evidence for seafloor dolomite cementation the nuccaleena formation in the adelaide fold thrust belt is the stratotype marinoan age cap dolostone at parachilna gorge the nuccaleena formation contains paired cogenetic tepees and growth faults that demonstrate these structures grew well below the sediment water interface these tepees and their associated sheet veins were the result of pore fluid overpressure driven by expansive early diagenetic dolomite cementation fluid overpressure resulted in two forces 1 macroscale lateral compression that overcame the lithostatic weight to buckle sediments upward forming the tepee structure and 2 macro to microscale extension that overcame the lithologic tensile strength to form sheet veins the physical processes of fluid overpressure within the parachilna gorge cap carbonate are in many ways analogous to the fluid overpressure dynamics in mud volcanoes and were sufficiently high to generate veins extending 17 m below the cap carbonate overpressure driven dewatering removed pore fluid clay and organic matter from the dolomicrospar matrix into dewatering structures and fabrics the latter are hierarchically arranged from small linear clay fabrics to clay lined dendritic arrays to clay choked curvilinear veins to large sheet veins packets of red clay laminae were deposited when pore fluids vented to the sea floor the early diagenetic chemical evolution of the parachilna gorge cap carbonate was equally dynamic in which an initial reducing trend was supplanted by an oxidation trend brought about by tepee sheet vein deformation once established the sheet vein network also reinvigorated diffusional processes that likely enhanced early diagenetic cementation of the cap dolostone the detailed paragenetic history also suggests that calcite not dolomite was the initial carbonate precipitated from the post glacial adelaidean sea the nuccaleena formation exhibits the dolomicrospar isotopic signatures and tepee like structures that distinguish marinoan age cap carbonates at parachilna gorge these typical features are a product of early diagenesis suggesting that other marinoan age cap carbonates may also be early diagenetic in origin

controls on the channel width of rivers implications for modeling fluvial incision of bedrock on the basis of the manning equation and basic mass conser vation principles we derive an expression for scaling the steady state width w of river channels as a function of discharge q channel slope s roughness n and width to depth ratio a w 5 a a1 2 2 3 3 8 q 3 8 s 23 16 n 3 8 we propose that channel width to depth ratio in addition to roughness is a function of the material in which the channel is developed and that where a river is con fined to a given material width to depth ratio and roughness can be assumed constant given these simplifications the expression emulates traditional width discharge relationships for rivers incis ing bedrock with uniformly concave fluvial long profiles more sig nificantly this relationship describes river width trends in terrain with spatially nonuniform rock uplift rates where conventional discharge based width scaling laws are inadequate we suggest that much of observed channel width variability in river channels confined by bedrock is a simple consequence of the tendency for water to flow faster in steeper reaches and therefore occupy smaller channel cross sections we demonstrate that using conventional scaling relationships for channel width can result in underestima tion of stream power variability in channels incising bedrock and that our model improves estimates of spatial patterns of bedrock incision rates

a coupled fluid inclusion and stable isotope record of paleofluids in the monterey formation california determining the history of fossil hydrologic systems that is recorded in vein minerals can be complicated by heterogeneity in the oxygen and carbon isotope compositions at millimeter and smaller scales several samples of diagenetic dolomite from two location in the miocene monterey formation california have 18o and 13c values that vary over a few millimeters by 28 and 6 5 respectively relative to the pee dee belemnite isotope standard these variations reflect episodic flow and mixing of waters from different sources with distinct diagenetic histories on the other hand isotopic conpositions of dolomite in samples with gradual shifts in isotopic compositions and correlated 18o and 13c values may have been controlled by mass diffusion and thermal conduction between episodic flow events primary fluid inclusion assemblages from samples collected at one location yielded homogenization temperatures of 87 8 3 6 c 2 these temperatures were combined with measured 18o values very nearly collocated in each dolomite sample to show that 18owater values ranged between 0 8 and 2 5 relative to standard mean ocean water the samples from this monterey formation dolomite thus indicate their precipitation from water more enriched in 18o than late miocene and younger seawater and must reflect modification of the 18o values of the involved waters by mineral dehydration reactions prior to dolomite precipitation samples from the other location contain no primary fluid inclusion assemblages and few abrupt shifts in isotopic compositions perhaps reflecting recrystallization small scale variations in isotopic compositions of vein minerals could be interpreted as the result of a changing regional hydrologic pattern but we think such an interpretation for the monterey formation samples would be incorrect instead we infer that these variations provide important information about localized effects on the ancient hydrologic system and that these effects bear on the origins of the paleofluids responsible for the mineral precipitation

channel width response to differential uplift 1 the role of channel width and slope adjustments to differential uplift in rivers within actively deforming terrains remains contentious here high resolution topographic surveying of formerly antecedent outwash channels demonstrates marked changes in channel width as a primary response to differential uplift for five late quaternary alluvial paleochannels crossing small folds along the active ostler fault zone of southern new zealand nearly continuous measurements of paleochannel width and concomitant incision reveal abrupt narrowing of widths toward minimum values at channel positions coincident with the initial uplift when the magnitude of differential uplift is sufficiently small narrowing alone permits these channels to remain antecedent in the context of a unit stream power model for fluvial erosion observed limits on the magnitude of channel narrowing suggest that above some threshold amount of differential uplift continued incision requires concomitant changes in channel gradient thus when crossing small growing folds alluvial rivers simply narrow their channels whereas larger folds that demand greater incision prompt an initial narrowing followed by channel steepening

the relationship between drainage density erosion rate and hilltop curvature implications for sediment transport processes drainage density is a fundamental landscape metric describing the extent of the fluvial network we compare the relationship between drainage density dd and erosion rate e using the channel hillslope integrated landscape development child numerical model we find that varying the channel slope exponent n in detachment limited fluvial incision models controls the relationship between dd and e with n 1 resulting in increasing dd with e if all other parameters are held constant this result is consistent when modeling both linear and nonlinear hillslope sediment flux we also test the relationship between dd and e in five soil mantled landscapes throughout the usa feather river ca san gabriel mountains ca boulder creek co guadalupe mountains nm and bitterroot national forest id for two of these field sites we compare dd to cosmogenic radionuclide crn derived erosion rates and for each site we use mean hilltop curvature as a proxy for erosion rate where crn derived erosion rates are not available we find that there is a significant positive relationship between dd e and hilltop curvature across every site with the exception of the san gabriel mountains ca this relationship is consistent with an n exponent greater than 1 suggesting that at higher erosion rates the transition between advective and diffusive processes occurs at smaller contributing areas in soil mantled landscapes

anaerobic methane oxidation and the formation of dolomite abstract we examine the link between organic matter degradation anaerobic methane oxidation amo and sulfate depletion and explore how these processes potentially influence dolomitization we determined rates and depths of amo and dolomite formation for a variety of organic rich sites along the west african margin using data from ocean drilling program odp leg 175 rates of amo are calculated from the diffusive fluxes of ch4 and so4 and rates of dolomite formation are calculated from the diffusive flux of mg we find that the rates of dolomite formation are relatively constant regardless of the depth at which it is forming indicating that the diffusive fluxes of mg and ca are not limiting based upon the calculated log iap values log ksp values for dolomite were found to narrowly range between 16 1 and 16 4 dolomite formation is controlled in part by competition between amo and methanogenesis which controls the speciation of dissolved co2 amo increases the concentration of co32 through sulfate reduction favoring dolomite formation while methanogenesis increases the pco2 of the pore waters inhibiting dolomite formation by regulating the pco2 and alkalinity methanogenesis and amo can regulate the formation of dolomite in organic rich marine sediments in addition to providing a mechanistic link between amo and dolomite formation our findings provide a method by which the stability constant of dolomite can be calculated in modern sediments and allow prediction of regions and depth domains in which dolomite may be forming

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

automatic geomorphic feature extraction from lidar in flat and engineered landscapes 1 high resolution topographic data derived from light detection and ranging lidar technology enables detailed geomorphic observations to be made on spatially extensive areas in a way that was previously not possible availability of this data provides new opportunities to study the spatial organization of landscapes and channel network features increase the accuracy of environmental transport models and inform decisions for targeting conservation practices however with the opportunity of increased resolution topographic data come formidable challenges in terms of automatic geomorphic feature extraction analysis and interpretation low relief landscapes are particularly challenging because topographic gradients are low and in many places both the landscape and the channel network have been heavily modified by humans this is especially true for agricultural landscapes which dominate the midwestern united states the goal of this work is to address several issues related to feature extraction in flat lands by using geonet a recently developed method based on nonlinear multiscale filtering and geodesic optimization for automatic extraction of geomorphic features channel heads and channel networks from high resolution topographic data here we test the ability of geonet to extract channel networks in flat and human impacted landscapes using 3 m lidar data for the le sueur river basin a 2880 km 2 subbasin of the minnesota river basin we propose a curvature analysis to differentiate between channels and manmade structures that are not part of the river network such as roads and bridges we document that laplacian curvature more effectively distinguishes channels in flat human impacted landscapes compared with geometric curvature in addition we develop a method for performing automated channel morphometric analysis including extraction of cross sections detection of bank locations and identification of geomorphic bankfull water surface elevation using the slope plotted along each channel floodplain cross section we demonstrate the ability to identify and measure the height of river banks and bluffs finally we present an example that demonstrates how extracting such features automatically is important for modeling channel evolution water and sediment transport and channel floodplain sediment exchange

diagenesis of a limestone reservoir lower cretaceous abu dhabi united arab emirates comparison between the anticline crest and flanks abstract petrographic stable isotope and fluid inclusion analyses were conducted on a lower cretaceous limestone reservoir onshore abu dhabi united arab emirates in order to compare the diagenetic processes and products in the oil zone i e crest versus water zone i e flanks of a giant oilfield anticline the near seafloor shallow and intermediate burial

solutions of the stream power equation and application to the evolution of river longitudinal profiles 1 erosion by bedrock river channels is commonly modeled with the stream power equation we present a two part approach to solving this nonlinear equation analytically and explore the implications for evolving river profiles first a method for non dimensionalizing the stream power equation transforms river profiles in steady state with respect to uniform uplift into a straight line in dimensionless distance elevation space second a method that tracks the upstream migration of slope patches which are mathematical entities that carry information about downstream river states provides a basis for constructing analytical solutions slope patch analysis explains why the transient morphology of dimensionless river profiles differs fundamentally if the exponent on channel slope n is less than or greater than one and why only concave up migrating knickpoints persist when n 1 at migrating knickpoints slope patches and the information they carry are lost a phenomenon that fundamentally limits the potential for reconstructing tectonic histories from bedrock river profiles stationary knickpoints which can arise from spatially varying uplift rates differ from migrating knickpoints in that slope patches and the information they carry are not lost counterparts to migrating knickpoints called stretch zones are created when closely spaced slope patches spread to form smooth curves in distance elevation space these theoretical results are illustrated with examples from the california king range and the central apennines

diffusion and mass balance of mg during early dolomite formation monterey formation abstract diffusion from overlying seawater is capable of supplying enough mg to account for all of the early diagenetic dolomite observed in the monterey formation 0 5 20 vol the amount formed depends on the mg flux magnitude into the sediment which is proportional to the rate of dolomite precipitation and inversely proportional to the sediment accumulation rate the amount of dolomite formed is calculated for accumulation rates of 50 and 600 m ma using reaction rate constants obtained from the mg concentration profile of dsdp sites 479 533 and 147 and proposed profiles that assume significant dolomite formation in the uppermost tens of meters of sediment advective pore water flow from compaction decreases the mg flux and becomes increasingly significant as the accumulation rate increases and the reaction rate decreases advection alone during compaction supplies sufficient mg to account for a realistic maximum of 2 vol dolostone within a compacted section an approximate evaluation of the rayleigh number suggests that the low permeability of siliceous ooze prevents convective fluid flow at shallow burial depths the mg mass balance is calculated from dolomite abundance in monterey formation sections santa maria basin area california pore water and solid particle velocities are calculated as a function of sediment depth using a porosity depth relation for average siliceous sediment based on a composite of dsdp data the bulk sediment diffusion coefficient increases with sediment depth at a rate proportional to the geothermal gradient at shallow burial depths seawater is probably the only significant source of mg dolomite volcanic ash and clay minerals are the only significant sinks clay mineral organic matter and silica diagenesis may provide additional mg at deeper burial depths the more or less regular spacing of dolomitic layers may be controlled by the residence time of the dolomite forming layer within the uppermost sediment where the mg flux is a maximum

physical and chemical growth conditions of ordovician organogenic deep water dolomite concretions implications for the δ18o of early palaeozoic sea water the estimated depth of formation of authigenic dolomite concretions in the middle ordovician cloridorme formation quebec ranges from 350 mbsf 25 17 porosity in the lower ordovician levis formation outward decreasing 13cvpdb values 10 2 0 8 suggest precipitation in the methane generation zone with an increasing contribution of light carbonate derived by advection from thermocatalytic reactions at depth anomalously low 18ovpdb values centre to margin variations of 0 4 to 7 5 give reasonable temperatures for the concretion centres only if the 18o of ordovician sea water was negative 6 and the bottom water was warm 15 c the 3 5 lower values for the concretion margins compared with the centres can be explained if in addition volcanic ash alteration organic matter decomposition and or advection of 18o depleted water lowered the 18o of the pore water further by 2 0 4 0 during the first 25 200 m of burial reasonable growth temperatures for the margins of 17 20 c are compatible with a lowering of the isotopic ratios by 1 to 1 3 as a temperature effect the systematic concentric isotope zonation of the concretions suggests that the well ordered near stoichiometric dolomite is a primary feature and not the result of recrystallization diagenetic dolomite beds of the cloridorme formation appear to have formed by coalescence of concretions as shown by randomly sampled traverses that indicate formation at different subsurface depths growth of the cloridorme dolomites was probably limited by calcium availability at least 50 of which was derived from connate water and the remainder by diffusion from sea water dolomite precipitation was favoured over calcite by very high sedimentation rates the abundance of marine organic matter in the host sediment and a correspondingly thin sulphate reduction zone deep seated concretion growth in the levis formation required either internal sources for the participating ions carbonate dissolution event or porewater advection along faults

repeated occurrences of methanogenic zones diagenetic dolomite formation and linked silicate alteration in southern bering sea sediments bowers ridge iodp exp 323 site u1341 abstract diagenetic precipitates such as dolomite and the chemistry of residual deeply buried porewater often represent the only traces of past biogeochemical activity in marine sediments a 600 m thick sedimentary section recently drilled at integrated ocean drilling program iodp site u1341 on bowers ridge southern bering sea provides insight into such a 4 3 ma old paleo diagenetic archive hard lithified calcite dolomite layers and laminae of disseminated carbonate were recovered in diatom rich sediments over a depth range of 400 m carbon isotope values of the diagenetic carbonates between 16 6 and 14 4 vpdb and strontium isotope ratios of dolomites close to past seawater values suggest carbonate precipitation induced by the production of dissolved inorganic carbon dic during elevated rates of organic carbon mineralization primarily via sulfate reduction at shallow sediment depth below the paleo seafloor diagenetic carbonates at 280 440 m below seafloor were likely also produced by the intermittent onset of sulfate reduction coupled to the anaerobic oxidation of methane aom at sulfate methane transition zones smtz these microbially mediated processes do not occur in the sediment at this site at present but were likely connected to the presence of a methanogenic zone at 2 58 2 51 ma a minimum in sulfate concentrations in modern porewaters and low sedimentary ba al ratios resulting from former sulfate depletion are reminiscent of the presence of this large methanogenic zone the minimum in sulfate concentrations is reflected in a minimum in magnesium concentrations less radiogenic strontium and isotopically light calcium in the porewater it is proposed that magnesium was removed from the porewater during carbonate precipitation and volcanic ash alteration which occurred in the former methanogenic zone and also released strontium with a less radiogenic isotope ratio and isotopically light calcium into the porewater the isotopic composition of porewater calcium was also influenced by ammonium calcium exchange on clay minerals and carbonate recrystallization our study elucidates the response of porewater element concentrations and isotopic profiles interlinked with the formation of diagenetic carbonates to changes in the deposition of organic carbon in sediments of deeper water sites 2000 m water depth over prolonged timescales it shows that variations in biogeochemical processes in response to changes in oceanographic conditions and a dynamic subseafloor biogeochemical zonation have to also be taken into account at these deep water sites for a global assessment of organic carbon burial fluxes and remineralization

original sediment composition of the lower cretaceous lacustrine tight oil mudstone and influences on diagenesis and organic matter content the erennaoer sag in erlian basin ne china abstract the original sediment composition is of great value to providing an understanding of depositional environment and hydrocarbon generation potential but it is easily altered by diagenesis and difficult to recognize in this study we examine carbonate rich tight oil mudstones of the lower cretaceous arshan formation in the erennaoer sag of the erlian basin as an example based on analyses of mineralogy stable carbon and oxygen isotopes and trace rare earth elements the original composition of the arshan mudstones has been reconstructed results show that the original arshan mudstone composition was mainly siliciclastic detritus tuffaceous and clay matrix which has a major impact on diagenesis and organic matter content the erupted volcanic ashes were beneficial to the introduction of nutrients an increase in biomass and an associated increase in to the preservation of organic matter which has been suggested by an improvement of toc of arshan tuffaceous mudstones with average contents of 2 82 the volcanic ashes in the studied mudstones had also enhanced methanogensis during the shallow burial additionally the early alteration products of authigenic minerals include analcime or zeolite most of which is entirely altered to analcime and chlorite the formation of major eogenetic carbonate cements include early dolomite and calcite which were related to volcanic materials and methanogenesis at shallow depths

decoding temporal and spatial patterns of fault uplift using transient river long profiles we present detailed observations of rivers crossing active normal faults in the central apennines italy where excellent constraints exist on the temporal and spatial history of fault movement we demonstrate that rivers with drainage areas 10 km2 and crossing faults that have undergone an increase in throw rate within the last 1 my have significant long profile convexities in contrast channels that cross faults that have had a constant slip rate for 3 my have concave up profiles and have similar concavities and steepness indices to rivers that do not cross any active fault structures this trend is consistent across the central apennines and cannot be explained by appeal to lithology or regional base level change the data challenge the belief that active faulting must always be reflected in river profiles instead the long profile convexities are best explained as a transient response of the river system to a change in tectonic uplift rate moreover for these rivers we demonstrate that the height of the profile convexity as measured from the fault scales with the magnitude of the uplift rate increase on the fault and we establish that this relationship holds for throw rate variation along strike for the same fault segment as well as between faults these findings are shown to be consistent with predictions of channel response to changing uplift rate rates using a detachment limited fluvial erosion model and they illustrate that analysis of the magnitude of profile convexities has considerable predictive potential for extracting tectonic information we also demonstrate that the migration rate of the profile convexities varies from 1 5 10 mm y and is a function of the slip rate increase as well as the drainage area this is consistent with n 1 for the slope exponent in a classical detachment limited stream power erosion law but could potentially be explained by incorporating an erosion threshold or an explicit role for sediment in enhancing erosion rates finally we show that for rivers in extensional settings where the response times to tectonic perturbation are long in this case 1 my attempts to extract tectonic uplift rates from normalised steepness indices are likely to be flawed because topographic steady state has not yet been achieved

objective extraction of channel heads from high resolution topographic data fluvial landscapes are dissected by channels and at their upstream termini are channel heads accurate reconstruction of the fluvial domain is fundamental to understanding runoff generation storm hydrology sediment transport biogeochemical cycling and landscape evolution many methods have been proposed for predicting channel head locations using topographic data yet none have been tested against a robust field data set of mapped channel heads across multiple landscapes in this study four methods of channel head prediction were tested against field data from four sites with high resolution dems slope area scaling relationships two techniques based on landscape tangential curvature and a new method presented here which identifies the change from channel to hillslope topography along a profile using a transformed longitudinal coordinate system our method requires only two user defined parameters determined via independent statistical analysis slope area plots are traditionally used to identify the fluvial hillslope transition but we observe no clear relationship between this transition and field mapped channel heads of the four methods assessed one of the tangential curvature methods and our new method most accurately reproduce the measured channel heads in all four field sites feather river ca mid bailey run oh indian creek oh piedmont va with mean errors of 11 7 5 and 24 m and 34 3 12 and 58 m respectively negative values indicate channel heads located upslope of those mapped in the field importantly these two independent methods produce mutually consistent estimates providing two tests of channel head locations based on independent topographic signatures

magnesium in the marine sedimentary environment mgnh4 ion exchange abstract the release of exchangeable mg in marine sediments from displacement by ammonium ions was estimated by way of experimentally determining the parameters that govern this ion exchange equilibrium on solid geochemical phases smectite humic acid illite and opal we showed that a both the conditional selectivity constant as well as the solid concentration are important parameters in determining the relative contribution of ammonium exchangeable mg from smectite organic matter illite and opal and b that except in the cases where opal or organic matter concentrations are very high the clays are the dominant carrier phases for labile mg which is exchangeable by ammonium a model based on the sum of the contributions from the major geochemical phases present in the sediment reliably predicts the amount of mg released by exchange with ammonium in marine sediments

a diagenetic origin for isotopic variability of sediments deposited on the margin of great bahama bank insights from clumped isotopes abstract the clumped isotope temperature proxy has been used to investigate the diagenetic history of carbonate sediments in two cores recovered during odp leg 166 on the margin of great bahama bank while periplatform sediments constitute a tempting archive of paleo ocean chemistry as they are unlikely to be subducted their primary limitation is a well documented susceptibility to post depositional diagenetic reworking the crystallization temperatures reconstructed using the clumped isotope proxy as well as the mineralogy and 13c and 18o values have been used to determine the relative effects of sediment mixing and sediment recrystallization results show that as sediments undergo diagenetic alteration at the seafloor their initially warm clumped isotope composition is overprinted at cooler benthic temperatures this process appears to occur in an environment with sufficient fluid exchange to overprint carbon isotopes an observation confirmed in a separate study by analyses of calcium a similarly rock buffered element this early reactive exchange between carbonates and fluids is likely driven by the conversion of metastable aragonite to calcite this partially occurs within the flushed zone where porewater compositions remain compositionally similar to seawater throughout the upper 40 meters of the sediment column sediments dominated by open system isotopic compositions correspond to a period of minimal sediment accumulation between 2 and 3 ma more deeply buried miocene sediments of the more platform proximal site 1003 show evidence of subsequent recrystallization incorporating the warmer geothermal t 47 values and more as well as modified water 18o values likely driven by co evolving porewater and carbonate oxygen isotopes reconstructed water 18o values of these deeper sediments at site 1003 are considerably more positive than the measured modern values suggesting that porewater 18o values were more positive during the miocene sediments deposited at the platform distal site 1006 between the early and middle miocene did not show evidence for this deeper recrystallization differences in diagenetic behavior between the two sites cannot be solely accounted for by differences in sediment accumulation rate to illustrate this time integrated models were constructed which simulated the burial of identically reactive material through the depositional history of each site the sediments deposited at site 1006 appear to stabilize after an initial phase of neomorphism whereas the more platform proximal site 1003 continues to recrystallize during deeper burial apparently in the presence of a parent fluid with a more positive 18o value than observed today we conclude that despite consisting of the same end member sediment sources and being spatially separated by less than 30 km the difference in clumped and oxygen isotopic composition between sites 1003 and 1006 can be predominantly attributed to differences in the rate and duration of recrystallization during burial

conceptual model for early diagenetic chert and dolomite amuri limestone group north eastern south island new zealand the late cretaceous to early eocene dominantly micritic amuri limestone group alg was deposited in an approximately nw trending trough in eastern marlborough new zealand the alg comprises the mead hill formation the teredo lower and middle limestone formations and the upper and lower marl formations chert and dolomite are concentrated in the mead hill formation which contains five of six recognized diagenetic zones zone i at the base of the alg consists almost entirely of chert zone ii consists solely of chert and dolomite zone iii comprises chert and limestone zone iv is composed of chert plus dolomite zone v is a chertified mudstone and the minor amounts of chert found in the middle limestone formation comprise zone vi with the exception of zones iv and v chert decreases stratigraphically upwards and away from the basin centre all the dolomites are composed of 1 mm diameter rhombohedra in discontinuous beds and lenses generally ca rich and non to slightly ferroan the dolomite contains approximately 500 900 ppm mn and 200 400 ppm sr 13c values average 1 2 pdb with 18o ratios of about 4 pdb mass balance calculations indicate that the mg2 for dolomitization was derived from sea water sr fe and mn concentrations are interpreted as indicating dolomite formation in the marine environment with no influence from meteoric waters the intimate association with pyrite implies dolomite formation in association with sulphate reduction in the upper sediment column 18o data show that the bulk of the dolomite formed at temperatures below 50 c all chert samples contain in excess of 90 wt sio2 about 1 wt al2o3 and 1 wt from losses on ignition generally all other major elements total less than 2 wt oxide 18o values range from 26 8 to 29 0 smow chert chemistry is consistent with the replacement of host carbonate and expulsion of carbonate bound components from the site of chertification and the effective dilution by sio2 of non carbonate bound insoluble residues 18o data indicate that chert formed in fluids of similar composition and temperature as the dolomite the abundance of disseminated pyrite in cherts implies an association with sulphate reduction silica for chertification is thought to have initially come from dissolution of siliceous organisms however there is insufficient biogenic silica available to form the volumes of chert observed it is suggested that the bulk of the silica came from sio2 rich pore waters generated by clay mineral reactions in the thick underlying mudstones the alg compacted down through these pore waters chert and dolomite nucleation are considered to have been penecontemporaneous dolomitization was initially probably the faster process continuing as long as sulphate reduction prevailed and there was an adequate supply of mg2 the nucleation of chert although initially slower probably due to a relatively lower initial sio2 supply continued after cessation of dolomitization to the extent of completely chertifying the dolomite intercrystalline matrix the amount of chertification decreased progressively as sio2 supplies diminished both stratigraphically upwards and away from the basin centre

geology of the point arguello discovery chevron as operator for its partners phillips champlin and impkemix discovered the point arguello oil field in 1981 the discovery well the chevron et al p 0316 1 was drilled in federal waters 8 5 mi 13 7 km south of point arguello california delineation drilling has confirmed the discovery of a giant oil field with estimated recoverable reserves in excess of 300 million bbl of oil the oil field is located within a small depocenter at the southern edge of the offshore santa maria basin this local depocenter may contain over 15 000 ft 4 600 m of neogene rocks the point arguello accumulation is trapped in a large north northwest trending anticlinal complex the primary reservoir is the middle and upper miocene monterey formation composed of fractured cherts porcellanites siliceous mudstones and dolostones calculated fracture permeabilities range up to 3 darcys crestal wells have indicated productivities after acid of approximately 6 000 bopd

beyond threshold hillslopes channel adjustment to base level fall in tectonically active mountain ranges numerous empirical and model based studies argue that in general hillslopes and river channels increase their gradients to accommodate high rates of base level fall to date however few data sets show the dynamic range of both these relationships needed to test theoretical models of hillslope evolution and river incision here we utilize concentrations of 10 be in quartz extracted from river sand on the eastern margin of the tibetan plateau to explore relationships among short term 10 2 10 5 a erosion rate hillslope gradient and channel steepness our data illustrate nonlinear behavior and a threshold in the relationship between erosion rate and mean hillslope gradient confi rming the generalization that hillslopes around the world are limited by slope stability and cease to provide a metric for erosion at high rates 0 2 mm a the relationship between channel steepness index and erosion rate is also nonlinear but channels continue to steepen beyond the point where threshold hillslopes emerge up to at least 0 6 mm a demonstrating that channel steepness is a more reliable topographic metric than mean hillslope gradient for erosion rate and that channels ultimately drive landscape adjustment to increasing rates of base level fall in tectonically active settings

the composition and diagenesis of interstitial solutions ii fluxes and diagenesis at the water sediment interface in the high latitude north and south atlantic studies of the major element composition of in situ sampled pore waters are reported for the north atlantic and southern ocean between africa and antarctica the pattern of diagenetic modification of pore water composition is similar throughout the entire atlantic enrichment of na ca2 and alkalinity and depletion of mg2 and k are nearly universal only siliceous oozes consistently provide very limited evidence of cation diagenesis the changes observed and the calculated fluxes across the seawater sediment interface are much the same as those reported previously for other areas of the atlantic and caribbean fluxes of the major cations across the interface continue to be indicated as a major factor in the geochemical cycling of these elements particularly na mg2 and k diagenetic modelling indicates that aerobic oxidation of organic matter and consequent dissolution of caco3 is a dominant reaction throughout the north atlantic the data indicate that o2 oxidation to at least 30 cm is prevalent at nearly all stations dissolution of caco3 in response to the introduction of metabolic co2 can lead to significant post depositional modification of the sediments the modelling also indicates a 1 1 stoichiometric relationship between na release and mg2 uptake by sedimentary components although dissolved silica concentrations in biogenic siliceous sediments are among the highest yet reported 700 m calculations demonstrate that solubility control cannot be through equilibria with the mineral sepiolite further the influence of cation silicate surface phases generally upon solubility is contraindicated by si oh 4 mg2 h relationships evaluation of the influence of fluid advection on pore water profiles indicates that at the stations studied in the north atlantic it is small a more general consideration of the potential contribution of fluid advection to shaping interstitial water profiles demonstrates that advection can be a dominant factor in such circumstances serious misinterpretation of the nature of diagenetic reactions may result from a lack of knowledge and consideration of fluid advection

river incision into bedrock mechanics and relative efficacy of plucking abrasion and cavitation improved formulation of bedrock erosion laws requires knowledge of the actual processes operative at the bed we present qualitative field evidence from a wide range of settings that the relative efficacy of the various processes of fluvial erosion e g plucking abrasion cavitation solution is a strong function of substrate lithology and that joint spacing fractures and bedding planes exert the most direct control the relative importance of the various processes and the nature of the interplay between them are inferred from detailed observations of the morphology of erosional forms on channel bed and banks and their spatial distributions we find that plucking dominates wherever rocks are well jointed on a submeter scale hydraulic wedging of small clasts into cracks bashing and abrasion by bedload and chemical and physical weathering all contribute to the loosening and removal of joint blocks in more massive rocks abrasion by suspended sand appears to be rate limiting in the systems studied here concentration of erosion on downstream sides of obstacles and tight coupling between fluid flow patterns and fine scale morphology of erosion forms testify to the importance of abrasion by suspended load rather than bedload particles mechanical analyses indicate that erosion by suspended load abrasion is considerably more nonlinear in shear stress than erosion by plucking in addition a new analysis indicates that cavitation is more likely to occur in natural systems than previously argued cavitation must be considered a viable process in many actively incising bedrock channels and may contribute to the fluting and potholing of massive unjointed rocks that is otherwise attributed to suspended load abrasion direct field evidence of cavitation erosion is however lacking in terms of the well known shear stress or stream power erosion law erosion by plucking is consistent with a slope exponent n of 2 3 to 1 whereas erosion by suspended load abrasion is more consistent with a slope exponent of 5 3 given that substrate lithology appears to dictate the dominant erosion process this finding has important implications for long term landscape evolution and the models used to study it

coupled organic and inorganic carbon cycling in the deep subseafloor sediment of the northeastern bering sea slope iodp exp 323 abstract we studied microbially mediated diagenetic processes driven by carbon mineralization in subseafloor sediment of the northeastern bering sea slope to a depth of 745 meters below seafloor mbsf sites u1343 u1344 and u1345 were drilled during integrated ocean drilling program iodp expedition 323 at water depths of 1008 to 3172 m they are situated in the high productivity green belt region with organic carbon burial rates typical of the high productivity upwelling domains on western continental margins the three sites show strong geochemical similarities the downward sequence of microbially mediated processes in the sediment encompasses 1 organoclastic sulfate reduction 2 anaerobic oxidation of methane aom coupled to sulfate reduction and 3 methanogenesis the sediment contains two distinct zones of diagenetic carbonate formation located at the sulfate methane transition zone smtz and between 300 and 400 mbsf the smtz at the three sites is located between 6 and 9 mbsf the upward methane fluxes into the smtz are similar to fluxes in smtzs underlying high productivity surface waters off chile and namibia our bering sea results show that intense organic carbon mineralization drives high ammonium and dissolved inorganic carbon dic production rates 4 2 mmol m 3 y 1 in the uppermost 10 mbsf and strongly imprints on the stable carbon isotope composition of dic driving it to a minimum value of 27 vpdb at the smtz pore water calcium and magnesium profiles demonstrate formation of diagenetic mg rich calcite in the smtz below the smtz methanogenesis results in 13c enrichment of pore water dic with a maximum value of 11 9 the imprint of methanogenesis on the dic carbon isotope composition is evident down to a depth of 150 mbsf below this depth slow or absent microbially mediated carbon mineralization leaves dic isotope composition unaffected ongoing carbonate formation between 300 and 400 mbsf strongly influences pore water dic and magnesium concentration profiles the linked succession of organic carbon mineralization and carbonate dissolution and precipitation patterns that we observe in the bering sea slope sediment may be representative of passive continental margin settings in high productivity areas of the world s ocean

the effects of bioturbation on soil processes and sediment transport abstract plants and animals exploit the soil for food and shelter and in the process affect it in many different ways for example uprooted trees may break up bedrock transport soil downslope increase the heterogeneity of soil respiration rates and inhibit soil horizonation in this contribution we review previously published papers that provide insights into the process of bioturbation we focus particularly on studies that allow us to place bioturbation within a quantitative framework that links the form of hillslopes with the processes of sediment transport and soil production using geometrical relationships and data from others work we derive simple sediment flux equations for tree throw and root growth and decay

early diagenetic dolomite concretions in the late cretaceous herring formation eastern marlborough new zealand abstract dolomite concretions occur in the upper part of the late cretaceous herring formation of eastern marlborough new zealand herring formation sediments are centimetre bedded poorly indurated clay mineral rich siltstones or very fine sandstones and were deposited in a nw se trending trough differential compaction of surrounding sediments around concretions is ubiquitous concretionary dolomite consists of intergrown anhedral to subhedral crystals up to 150 m across pyrite is common insoluble residue contents indicate sediment porosities of 70 or more at the time of concretion formation which is consistent with an origin in the upper few metres of the sediment column generally concretions have near stoichiometric dolomite compositions although a few are composed of calcite dolomite fe compositions average about 4 mol feco3 with mn contents in the range 623 9543 ppm all concretions contain less mn than fe the average sr content ranges from 175 to 710 ppm concretions can be separated into a light carbon group 13 c pdb 18 o pdb in the basin centre and a heavy carbon group 13 c 9 pdb 18 o 4 pdb which are found at basin margins ca2 and mg2 were derived initially from pore waters and ultimately from diffusion of overlying seawater palaeotemperatures calculated from 18o analyses indicated concretions formed at less than 60 c burial depths derived from palaeotemperatures are higher than expected for concretions formed in the upper sediment column however sr concentrations are interpreted as indicating that the bulk of dolomite formed by the dolomitization of early diagenetic calcite this process along with other competing early diagenetic reactions caused depletion in 18o in formation waters the combination of sr and 18o data are interpreted as indicating wholly marine formation without the influence of meteoric fluids the presence of pyrite and 13c data indicate concretions formed in association with so42 reduction in the upper sediment column the distribution and abundance of organic matter metabolized by so42 reducing bacteria was therefore probably important in the distribution and formation of dolomite concretions dolomitization continued as concretions were buried into the zone of bacterial fermentation in addition 13c analyses indicate that concretions formed in the basin centre contain an organic carbon component derived from upward diffusing methane whereas basin margin concretions do not concretions ceased forming when they were buried to depths below which ca2 and mg2 could diffuse and when supplies of so42 and or organic material became insufficient to support levels of so42 reducing bacteria continued burial resulted in compaction of sediments around concretions some basin margin concretions were later exhumed and phosphatised

vegetation modulated landscape evolution effects of vegetation on landscape processes drainage density and topography 1 topography acts as a template for numerous landscape processes that include hydrologic ecologic and biologic phenomena these processes not only interact with each other but also contribute to shaping the landscape as they influence geomorphic processes we have investigated the effects of vegetation on thresholds for channel initiation and landform evolution using both analytical and numerical approaches vegetation is assumed to form a uniform ground cover runoff erosion is modeled based on a power function of excess shear stress in which shear stress efficiency is inversely proportional to vegetation cover this approach is validated using data plant effect on slope stability is represented by additional cohesion provided by plant roots vegetation cover is assumed to reduce sediment transport rates due to physical creep processes rainsplash dry ravel and expansion and contraction of sediments according to a negative exponential relationship vegetation grows as a function of both available cover and unoccupied space by plants and is killed by geomorphic disturbances runoff erosion and landsliding and wildfires analytical results suggest that in an equilibrium basin with a fixed vegetation cover plants may cause a transition in the dominant erosion process at the channel head a runoff erosion dominated landscape under none or poor vegetation cover may become landslide dominated under a denser vegetation cover the sign of the predicted relationship between drainage density and vegetation cover depends on the relative influence of vegetation on different erosion phenomena with model parameter values representative of the oregon coast range ocr numerical experiments conducted using the channel hillslope integrated landscape development child model confirm the findings based on the analytical theory a highly dissected fluvial landscape emerges when surface is assumed bare when vegetation cover is modeled landscape relief increases resulting in hollow erosion dominated by landsliding interestingly our simulations underscore the importance of vegetation disturbances by geomorphic events and wildfires on the landscape structure simulated landscapes resemble real world catchments in the ocr when such disturbances are considered

responses of soil mantled hillslopes to transient channel incision rates 1 channel incision drives hillslope morphology in humid soil mantled landscapes when channel incision rates change numerous hillslope soil properties e g erosion rates soil thickness and soil production adjust in response to this change here we investigate the timescales of adjustment of hillslope soil properties when channel incision rates change in time an idealized soil mantled hillslope linear sediment flux law and soil density equal to bedrock density is investigated and the transient evolution of this hillslope is determined analytically this analysis reveals two dominant adjustment timescales the longer of these two timescales determines the rate at which the entire hillslope adjusts to a change in channel incision rates and is proportional to 4 2 2d where is the length of the hillslope and d is the sediment diffusivity numerical simulations are then used to examine responses of hillslopes that involve nonlinear behavior e g hillslopes that experience nonlinear sediment transport or have soil thicknesses that respond to the soil production function using these numerical models we show that the ratio between the soil density s and the density of the soil parent material can alter the long term response of the hillslope such that the characteristic timescale becomes 4 2 2d s in addition we show that the adjustments of the soil erosion rate the soil production rate and the soil thickness have different characteristic response timescales hillslopes that experience sediment flux that is proportional to the depth slope product respond on longer timescales than hillslopes that experience a linear sediment flux law when channel incision rates increase we illustrate how the spatial pattern of hillslope response to changes in channel incision rates can be used to constrain either channel incision histories or hillslope response timescales if the hillslope response timescale is known the pattern of hillslope disturbance can be used to constrain the celerity of a incision wave as it moves upstream through a channel if the channel incision history is known the hillslope response timescale may be evaluated on the basis of the spatial pattern of hillslope disturbance

the origin of dolomites in tertiary sediments from the margin of great bahama bank abstract based on an integrated geochemical and petrographic investigation of dolomites from two cores drilled on great bahama bank we have determined three different environments of formation for the dolomites that are common throughout the pliocene and miocene parts of these cores the first environment of dolomitization occurs in association with development of nondepositional surfaces dolomite typically forms below each of these surfaces the concentration and extent of which is governed by the length of the period of nondeposition these dolomites are recognized by their association with the nondepositional surfaces characteristic positive 18o values indicative of formation from cold bottom waters and 18o and sr profiles with depth that suggest formation in the presence of diffusive temperature and sr gradients the second environment of dolomitization occurs in pore fluids where the cation and anion profiles are governed by diffusive processes the dolomite forming here is termed background dolomite this is a microsucrosic dolomite and forms both by recrystallization of the existing sediment and precipitation directly into void space dolomitization by this mechanism uses a local source of mg2 and consequently the dolomite never constitutes more than between 5 and 10 of the sediment this type of dolomite is characterized by extremely high sr concentrations which reflect concentrations of sr2 in the pore fluids from which it formed the high concentrations of sr2 in the pore fluids arise through the continued recrystallization of metastable aragonite and high mg calcite to dolomite and low magnesium calcite driven by oxidation of organic material by sulfate not only does sulfate reduction provide an additional thermodynamic drive for recrystallization but because the absolute concentration of sr2 in the pore fluids is governed by the solubility of celestite srso4 removal of sulfate allows the sr2 ca2 ratio of the interstitial fluid to become much higher than normally encountered the final type of environment of dolomitization is associated with coarse grained reefal sediments the pervasive nature of the dolomitization and the relatively normal sr concentrations suggest the circulation of normal marine water in a relatively open system

controls on the spacing of first order valleys 1 many landscapes are composed of ridges and valleys that are uniformly spaced even where valley locations are not controlled by bedrock structure models of long term landscape evolution have reproduced this phenomenon yet the process by which uniformly spaced valleys develop is not well understood and there is no quantitative framework for predicting valley spacing here we use a numerical landscape evolution model to investigate the development of uniform valley spacing we find that evenly spaced valleys arise from a competition between adjacent drainage basins for drainage area a proxy for water flux and that the spacing becomes more uniform as the landscape approaches a topographic equilibrium valley spacing is most sensitive to the relative rates of advective erosion processes such as stream incision and diffusion like mass transport such as soil creep and less sensitive to the magnitude of a threshold that limits the spatial extent of stream incision analysis of a large number of numerical solutions reveals that valley spacing scales with a ratio of characteristic diffusion and advection timescales that is analogous to a peclet number we use this result to derive expressions for equilibrium valley spacing and drainage basin relief as a function of the rates of advective and diffusive processes and the spatial extent of the landscape the observed scaling relationships also provide insight into the cause of transitions from rill like drainage networks to branching networks the spatial scale of first order drainage basins the contributing area at which hillslopes transition into valleys and the narrow range of width to length ratios of first order basins

fluvial and slope wash erosion of soil mantled landscapes detachment or transport limited many numerical landform evolution models assume that soil erosion by flowing water is either purely detachment limited i e erosion rate is related to the shear stress power or velocity of the flow or purely transport limited i e erosion deposition rate is related to the divergence of shear stress power or velocity this paper reviews available data on the relative importance of detachment limited versus transport limited erosion by flowing water on soil mantled hillslopes and low order valleys field measurements indicate that fluvial and slope wash modification of soil mantled landscapes is best represented by a combination of transport limited and detachment limited conditions with the relative importance of each approximately equal to the ratio of sand and rock fragments to silt and clay in the eroding soil available data also indicate that detachment entrainment thresholds are highly variable in space and time in many landscapes with local threshold values dependent on vegetation cover rock fragment armoring surface roughness soil texture and cohesion this heterogeneity is significant for determining the form of the fluvial slope wash erosion or transport law because spatial and or temporal variations in detachment entrainment thresholds can effectively increase the nonlinearity of the relationship between sediment transport and stream power results from landform evolution modeling also suggest that aside from the presence of distributary channel networks and autogenic cut and fill cycles in non steady state transport limited landscapes it is difficult to infer the relative importance of transport limited versus detachment limited conditions using topography alone copyright 2011 john wiley sons ltd

paleoceanographic implications of miocene deep sea hiatuses miocene paleoceanographic evolution exhibits major changes resulting from the opening and closing of passages the subsequent changes in oceanic circulation and development of major antarctic glaciation the consequences and timing of these events can be observed in variations in the distribution of deep sea hiatuses sedimentation patterns and biogeographic distribution of planktic organisms the opening of the drake passage in the latest oligocene to early miocene 25 20 ma resulted in the establishment of the deep circumpolar current which led to thermal isolation of antarctica and increased global cooling this development was associated with a major turnover in planktic organisms resulting in the evolution of neogene assemblages and the eventual extinction of paleogene assemblages the erosive patterns of two widespread hiatuses ph 23 0 22 5 ma and nh 1 20 18 ma indicate that a deep circumequatorial circulation existed at this time characterized by a broad band of carbonate ooze deposition siliceous sedimentation was restricted to the north atlantic and a narrow band around antarctica a major reorganization in deep sea sedimentation and hiatus distribution patterns occurred near the early middle miocene boundary apparently resulting from changes in oceanic circulation beginning at this time deep sea erosion occurred throughout the caribbean hiatus nh 2 16 15 ma suggesting disruption of the deep circumequatorial circulation and northward deflection of deep currents and or intensification of the gulf stream sediment distribution patterns changed dramatically with the sudden appearance of siliceous ooze deposition in the marginal and east equatorial north pacific by 16 0 to 15 5 ma coincident with the decline of siliceous sedimentation in the north atlantic this silica switch may have been caused by the introduction of norwegian overflow water into the north atlantic acting as a barrier to outcropping of silica rich antarctic bottom water the main aspects of the present oceanic circulation system and sediment distribution pattern were established by 13 5 to 12 5 ma hiatus nh 3 coincident with the establishment of a major east antarctic ice cap antarctic glaciation resulted in a broadening belt of siliceous ooze deposition around antarctica increased siliceous sedimentation in the marginal and east equatorial north pacific and indian oceans and further northward restriction of siliceous sediments in the north atlantic periodic cool climatic events were accompanied by lower eustatic sea levels and widespread deep sea erosion at 12 to 11 ma nh 4 10 to 9 ma nh 5 7 5 to 6 2 ma nh 6 and 5 2 to 4 7 ma nh 7

recrystallization of dolomite evidence from the monterey formation miocene california dolomites from the upper calcareous siliceous member of the miocene monterey formation exposed west of santa barbara california were analysed for geochemical isotopic and crystallographic variation the data clearly document the progressive recrystallization of dolomite during burial diagenesis in marine pore fluids recrystallization is recognized by the following compositional and crystallographic variations dolomites have decreasing 18o and 13c compositions decreasing sr contents and increasing mg contents with increasing burial depths and temperatures from east to west in the study area 18o values vary from 5 3 in the east to 5 5 pdb in the west and are interpreted to reflect the greater extent and higher temperature of dolomite recrystallization in the west 13c values correlate with 18o and decrease from 13 6 in the east to 8 7 pdb in the west sr concentrations correlate positively with 18o values and decrease from a mean of 750 ppm in the east to a mean of 250 ppm in the west mol mgco3 values inversely correlate with 18o values and increase from a minimum of 41 0 in the east to a maximum of 51 4 in the west rietveld refinements of powder x ray diffraction data indicate that the more recrystallized dolomites have more contracted unit cells and increased cation ordering the fraction of the ca sites in the dolomites that are occupied by ca atoms increases slightly with the approach to stoichiometry the fraction of the mg sites occupied by mg atoms strongly correlates with mol mgco3 even in early diagenetic non stoichiometric dolomites there is little substitution of mg in ca sites during recrystallization the amount of mg substituting for ca in ca sites decreases even further most of the disorder in the least recrystallized non stoichiometric dolomites is related to substitution of excess ca on mg sites

using hilltop curvature to derive the spatial distribution of erosion rates 1 erosion rates dictate the morphology of landscapes and therefore quantifying them is a critical part of many geomorphic studies methods to directly measure erosion rates are expensive and time consuming whereas topographic analysis facilitates prediction of erosion rates rapidly and over large spatial extents if hillslope sediment flux is nonlinearly dependent on slope then the curvature of hilltops will be linearly proportional to erosion rates in this contribution we develop new techniques to extract hilltop networks and sample their adjacent hillslopes in order to test the utility of hilltop curvature for estimating erosion rates using high resolution 1 m digital elevation data published and new cosmogenic radionuclide analyses in the feather river basin california suggest that erosion rates vary by over an order of magnitude 10 to 250 mm kyr 1 hilltop curvature increases with erosion rates allowing calibration of the hillslope sediment transport coefficient which controls the relationship between gradient and sediment flux having constraints on sediment transport efficiency allows estimation of erosion rates throughout the landscape by mapping the spatial distribution of hilltop curvature additionally we show that hilltop curvature continues to increase with rising erosion rates after gradient limited hillslopes have emerged hence hilltop curvature can potentially reflect higher erosion rates than can be predicted by hillslope gradient providing soil production on hilltops can keep pace with erosion finally hilltop curvature can be used to estimate erosion rates in landscapes undergoing a transient adjustment to changing boundary conditions if the response timescale of hillslopes is short relative to channels