chemical and physical denudation in the amazon river basin we present major and trace element data on the suspended and dissolved phases of the amazon river and its main tributaries the sr isotopic composition of the dissolved load is also reported special attention is paid to the abundances of ree and to their fractionation between the dissolved and suspended phase the rivers of the amazon basin are among the richest in dissolved ree and are similar to the rivers of the congo system however a greater range of fractionation between lree and hree is reported here at a global scale the rivers have intermediate patterns between those of the congo system and those of high ph rivers such as the indus and mississippi rivers only few elements rb u ba k na sr and ca are mobilized by silicate weathering these elements are strongly depleted in the suspended phase with respect to upper continental crust in the dissolved load these elements are controlled by atmospheric inputs and the weathering of the main lithologies we propose a model based on mass budget equations that allow the proportions derived from the different sources to be calculated as a consequence silicate carbonate and evaporite weathering rates can be estimated as well as the consumption of co2 by weathering of each of these lithologies physical weathering rates can be estimated by two complementary approaches on the one hand the multi year average of suspended sediments yields can be used to estimate physical denudation on the other hand we have developed a steady state model of erosion that allows us to calculate physical erosion rates on the basis of the dissolved load of rivers a mean crustal composition is assumed in this model for the rock sources of the drainage basins comparison of the rates predicted by the model to the observed rates shows good agreement for the lowland rivers but a strong discrepancy for the rivers derived from the andes andean rivers solimoes madeira and amazon have observed sediment yields much greater than those predicted according to the steady state model of chemical and physical weathering two interpretations can account for this inconsistency the first is that these rivers are not in steady state and hence that the soils are being destroyed the second requires that the local continental crust is different from the average continental crust of taylor and mclennan and contains a large proportion of sedimentary rocks using the measured sediment yields and assuming a steady state we can estimate the amount of sediment recycling for each drainage basin for the amazon at santarem we find that at least 25 of the mass of the upper continental crust of the amazon drainage basin is constituted of recycled material

dissolution of biogenic carbonates effects of skeletal structure abstract dissolution of biogenic shallow water carbonates exposed on deep sea moorings indicates that skeletal structure is important for the rate of disintegration of biogenic carbonates besides mineralogy and grain size of particles the aragonites and high mg calcites used represent a wide spectrum of mineralogies and types of skeletal framework the particles were deployed at different water depths on a mooring in the drake passage for 52 days weight loss curves for the various types of particles show the relative importance of the different structural factors for the disintegration of these biogenic carbonates organic coatings intraskeletal pore spaces and sizes and shapes of individual crystallites in the skeletons may be more important than carbonate mineralogy and particle size in cases the presence of internal sediments cement aggregates and natural contaminations and of diatoms incorporated during growth into carbonate skeletons strongly influence the disintegration of the skeletal materials the first step of particle disintegration is the selective removal of impurities this step is analogous to a cleaning of specimens it is succeeded by initial dissolution by strong dissolution and finally by disintegration

controls of eustasy and diagenesis on the 238u 235u of carbonates and evolution of the seawater 234u 238u during the last 1 4 myr using a leaching protocol designed for the study of u isotopes in recent carbonates we measured the u isotope composition both 238 u 235 u and 234 u 238 u of modern and ancient corals n 6 a limestone and a dolostone as well as 43 shallow water carbonate sediments from the odp leg 166 site 1009 drill core on the slope of the bahamas platform although bulk corals record the seawater 238 u value within 0 02 differences of up to 0 30 in the 238 u of individual leachates suggest a control of the coral structure and a more positive 238 u 235 u ratio in the centers of calcification the drill core 238 u data shows that the 238 u 235 u ratio of shallow water carbonates is controlled mainly by 1 variations in sea level through the mixing of different amounts of platform derived sediments with 238 u 0 50 0 60 heavier than seawater and pelagic sediments with seawater like 238 u values 2 authigenic u enrichment via pore watercirculation and u reduction both on the platform and down to 5 m below the surface mbsf after deposition of the sediment and to a lesser extent by 3 early diagenetic processes i e carbonate dissolution and or recrystallization during sediment burial the global effect of these processes leaves the 238 u values of shallow water carbonates offset relative to that of seawater by carbonates sw 0 24 0 06 95 ci including all samples this shift can be used in seawater paleoredox reconstructions based on carbonates deposited on shallow water platform shelf and slope environments i e most of the carbonate sedimentary record prior to the mesozoic to account for the average effect of carbonate diagenesis assuming that the 238 u 235 u ratio of carbonate platform sediments directly records the seawater 238 u 235 u ratio would underestimate the extent of ocean seafloor anoxia by at least a factor 10 the rapid fluctuations in 238 u values due to sea level changes i is a factor that should be considered before interpreting 238 u variations as reflecting changes in oceanic paleoredox conditions and ii reinforces the need for statistically meaningful data sets the 234 u data suggest that the 234 u 238 u ratio of the seawater has remained within 20 of the modern seawater value during the last 1 1 4 myr furthermore we find that small scale 1 15 variations in seawater 234 u mirror sea level changes during the penultimate glacial interglacial period 140 to 200 ka thus confirming the record of lower 234 u sw during periods of low sea level stand and expanding it to at least the last two glacial interglacial events i e 0 23 ma such fluctuations in 234 u initial values should be taken into account when screening carbonate sediments u th ages on the basis of the initial 234 u 238 u ratios of the samples

calcite dissolution in sediments of the ceara rise in situ measurements of porewater o2 ph and co2 aq in situ electrode measurements of porewater oxygen and ph from four sites on the ceara rise in the western tropical atlantic unequivocally demonstrate that calcite dissolution driven by metabolic co2 produced within the sediments is a significant part of the diagenesis of sedimentary calcite dissolving at least 20 of the calcite rain to the seafloor the first ever in situ absorbance based measurements of porewater co2 aq support the dissolution estimates based on models of the observed ph distributions and are incompatible with scenarios excluding metabolic co2 driven dissolution our dissolution estimates are consistent with those estimated by an independent study of porewater calcium and alkalinity profiles martin and sayles 1996 at this location the consistency of these disparate porewater measurements and our ability to interpret them with simple models of respiration and dissolution implies that no special mechanism is required to describe this process the calcite dissolution rate constant in the sediments of the deepest station in this study is significantly higher than determined by in situ studies at other locations values approach those determined by early laboratory measurements the dissolution rate constant here must be at least two orders of magnitude higher than for calcite in sediments on the ontong java plateau based on in situ electrode measurements there hales and emerson 1996 the calcite solubility consistent with these data is not compatible with that consistent with the ontong java data the reason for this difference is currently unresolved but may be due to the high order of the empirical dissolution rate law assumed in this study

progressive dissolution of fine carbonate particles in pelagic sediments abstract large systematic variations occur in the proportion of coccolith sized carbonate particles relative to other types of carbonate particles within the

quantification of foraminifer and coccolith carbonate in south atlantic surface sediments by means of carbonate grain size distributions this study presents a differentiated carbonate budget for marine surface sediments from the mid atlantic ridge of the south atlantic with results based on carbonate grain size composition upon separation into sand silt and clay sub fractions the silt grain size dis tribution was measured using a sedigraph 5100 we found regionally characteristic grain size distributions with an overall minimum at 8 mm equivalent spherical diameter esd sem observations reveal that the coarse particles 8 mm esd are attributed to planktic for aminifers and their fragments and the fine particles 8 mm esd to coccoliths on the basis of this division the regional variation of the contribution of foraminifers and coccoliths to the carbonate budget of the sediments are calculated foraminifer carbonate dominates the sed iments in mesotropic regions whereas coccoliths contribute most car bonate in oligotrophic regions the grain size of the coccolith share is constant over water depth indicating a lower susceptibility for car bonate dissolution compared to foraminifers finally the characteristic grain size distribution in fine silt 8 mm esd is set into context with the coccolith assemblage counted and biometrically measured using a sem the coccoliths present in the silt fraction are predominantly large species length 4 mm smaller species length 4 mm belong to the clay fraction 2 mm esd the average length of most frequent coccolith species is connected to prominent peaks in grain size distri butions esd with a shape factor the area below gaussian distri butions fitted to these peaks is suggested as a way to quantitatively estimate the carbonate contribution of single coccolith species more precisely compared to conventional volume estimates the quantitative division of carbonate into the fraction produced by coccoliths and that secreted by foraminifers enables a more precise estimate for source sink relations of consumed and released co 2 in the carbon cycle the allocation of coccolith length and grain size esd suggests size windows for the separation or accumulation of distinct coccolith species in investigations that depend on non to slightly mixed signals e g isotopic studies

late quaternary variations in calcium carbonate preservation of deep sea sediments in the northern cape basin results from a multiproxy approach in this study we test various parameters in deep sea sediments bulk sediment parameters and changes in microfossil abundances and preservation character which are generally accepted as indicators of calcium carbonate dissolution we investigate sediment material from station geob 1710 3 in the northern cape basin eastern south atlantic 280 km away from the namibian coast well outside today s coastal upwelling as northern benguela upwelling cells were displaced westward and periodically preceded the core location during the past 245 kyr volbers et al submitted geob 1710 3 sediments reflect these changes in upwelling productivity results of the most commonly used calcium carbonate dissolution proxies do not only monitor dissolution within these calcareous sediments but also reflect changes in upwelling intensity accordingly these conventional proxy parameters misrepresent to some extent the extent of calcium carbonate dissolution these results were verified by an independent dissolution proxy the globigerina bulloides dissolution index bdxp volbers and henrich submitted the bdxp is based on scanning electronic microscope ultrastructural investigation of planktonic foraminiferal tests and indicates persistent good carbonate preservation throughout the past 245 kyr with the exception of one pronounced dissolution event at early oxygen isotopic stage ois 6 the early ois 6 is characterized by calcium carbonate contents sand contents and planktonic foraminiferal concentrations all at their lowest levels for the last 245 kyr at the same time the ratio of radiolarian to planktonic foraminiferal abundances and the ratio of benthic to planktonic foraminiferal tests are strongly increased as are the rain ratio the fragmentation index and the bdxp the sedimentary calcite lysocline rose above the core position and geob 1710 3 sediments were heavily altered as attested to by the unusual accumulation of pellets aggregates sponge spicules radiolaria benthic foraminifera and planktonic foraminiferal assemblages solely the early ois 6 dissolution event altered the coarse fraction intensely and is therefore reflected by all conventional calcium carbonate preservation proxies and the bdxp we attribute the more than 1000 m rise of the sedimentary calcite lysocline to the combination of two processes a a prominent change in the deep water mass distribution within the south atlantic and b intense degradation of organic material within the sediment preserved as maximum total organic carbon content creating microenvironments favorable for calcium carbonate

sequence stratigraphy of the palaeozoic welsh basin the timing and extent of unconformities in the welsh basin are investigated using rock preservation curves derived from outcrop stratigraphic logs four basin wide unconformities occur focussed in late precambrian late tremadoc pusgillian early ashgill and mid devonian times these bound three megasequences equating with newly defined lithostratigraphic units the dyfed gwynedd and powys supergroups less extensive unconformities bound 18 component sequences the majority of the sequence boundaries reflect a component of tectonic or volcanotectonic activity rather than a pure eustatic sea level change the megasequence boundaries are attributed to late precambrian to early cambrian onset of rifting to form a passive margin tremadoc onset of subduction with intra arc then back arc extension late caradoc end to subduction and late early devonian collisional deformation the megasequences and controlling events can be tentatively matched with other basins on the avalonian margin more generally this study shows that sequence analysis is feasible in onshore basins lacking well and seismic data and that the global eustatic interpretation of sequence stratigraphy is only partially applicable to active margin basins

deepwater source variations during the last climatic cycle and their impact on the global deepwater circulation the degree of similarity of the 13c records of the planktonic foraminiferal species n pachyderma and of the benthic foraminiferal genus cibicides in the high latitude basins of the world ocean is used as an indicator of the presence of deepwater sources during the last climatic cycle whereas continuous formation of deep water is recognized in the southern ocean the norwegian sea stopped acting as a sink for surface water during isotope stage 4 and the remainder of the last glaciation however deep water formed in the north atlantic south of the norwegian sea during the last climatic cycle as early as isotope substage 5d and this area was also the only active northern source during stages 4 2 a detailed reconstruction of the geographic distribution of 13c in benthic foraminifera in the atlantic ocean during the last glacial maximum shows that the most important deepwater mass originated from the southern ocean whereas the glacial north atlantic deep water cannot be traced south of 40 n at shallower depth an oxygenated 13c rich intermediate water mass extended from 45 n to 15 s in the pacific ocean a ventilation higher than the modern one was also found in open ocean in the depth range 700 2600 m and is best explained by stronger formation of intermediate water in high northern latitudes

uptake of elements from seawater by ferromanganese crusts solid phase associations and seawater speciation marine fe mn oxyhydroxide crusts form by precipitation of dissolved components from seawater three hydrogenetic crust samples one phosphatized and two hydrothermal mn oxide samples were subjected to a sequential leaching procedure in order to determine the host phases of 40 elements those host phase associations are discussed with respect to element speciation in seawater the partitioning of elements between the two major phases mn oxide and fe oxyhydroxide can in a first order approximation be explained by a simple sorption model related to the inorganic speciation of the elements in seawater as has been proposed in earlier models free and weakly complexed cations such as alkali and alkaline earth metals mn co ni zn tl i and partly y are sorbed preferentially on the negatively charged surface of the mno2 in hydrogenetic crusts the driving force is a strong coulombic interaction all neutral or negatively charged chloro cd hg tl carbonate cu y pb and u and hydroxide be sc ti fe zr nb in sn sb te hf ta bi th and tl iii complexes and oxyanions v cr as se mo and w bind to the slightly positively charged surface of the amorphous feooh phase while coulombic interaction can explain the sorption of the negatively charged species the binding of neutral species is based on specific chemical interaction organic complexation of elements in deep ocean water seems to be at most of minor importance surface oxidation can explain some strong metal associations e g of co and tl with the mno2 and te with the feooh sorption reactions initially driven by coulombic forces are often followed by the formation of specific bonds between the adsorbate and the atoms of the oxide surface differences in the associations of some metals between the non phosphatized and phosphatized hydrogenetic crusts and between the hydrogenetic and the hydrothermal samples reflect the different physico chemical environments of formation and speciations in oxic seawater vs less oxic fluids especially for the redox sensitive metals such as mo and v these environmental related differences indicate that the methodology of chemical speciation used here in combination with spectroscopic methods may allow for the detection of changes in paleoceanographic conditions recorded during the several tens of millions of years of crust growth

global ocean redox variations across the smithian spathian boundary linked to concurrent climatic and biotic changes abstract the smithian spathian boundary ssb was an interval characterized by a major global carbon cycle perturbation climatic cooling from a middle late smithian boundary hyperthermal condition and a major setback in the recovery of marine necto pelagic faunas from the end permian mass extinction although the ssb has been linked to changes in oceanic redox conditions key aspects of this redox variation e g duration extent and triggering mechanisms and its relationship to coeval climatic and biotic changes remain unresolved here we report a high resolution middle smithian to middle spathian u isotope 238u record based on marine carbonates of the zuodeng south china and jesmond british columbia sections to investigate the timing and global extent of ocean redox variation across the ssb our 238u record reveals values similar to or slightly heavier than modern seawater 0 39 during the middle smithian a rapid negative shift to highly negative values during the early late smithian a positive shift at the ssb and a rapid shift back to more negative values in the early middle spathian a simple u isotope mass balance modeling suggests that the global area of anoxic seafloor expanded strongly during the late smithian and the early middle spathian covering 11 of total seafloor area but that it contracted sharply during the ssb 2 the redox pattern documented by our 238u record shows a good first order correspondence to tropical sea surface temperature sst data for the smithian spathian in particular peak anoxia coincided with the middle late smithian boundary hyperthermal event and diminished anoxia with a pronounced decline in ssts at the ssb the temporal correlation between anoxia and low biodiversity levels of many marine clades e g conodonts and ammonoids during the late smithian indicates that oceanic anoxia may have played a role in the ssb biocrisis

uranium isotope fractionation in saanich inlet a modern analog study of a paleoredox tracer abstract uranium isotope fractionation was studied in the sediment and water of saanich inlet a seasonally anoxic fjord on the east coast of vancouver island the concentration of dissolved u is the same above and below the redoxcline at 120 m depth with an average 238u value of 0 45 0 06 2 s e which is indistinguishable from seawater values reported in other studies this finding is consistent with water renewal times for the inlet that are roughly seasonal in frequency thus hiding the impact of the u losses to the anoxic sediment of the inlet complete digests of the anoxic sediment yielded higher 238u values than seawater 0 21 0 11 2 s e by contrast complete digests of the sediment trap samples yielded lower 238u values than seawater 0 52 0 10 2 s e and marine plankton from the inlet yielded the lowest 238u value of 1 24 0 17 2 cross plots of 238u and 234u vs th u reveal strong correlations indicative of two component mixing when the data from traps and anoxic inlet sediment are combined one mixing end member is fractionated uranium of seawater origin with a 238u value of 0 17 0 14 2 the other is detrital uranium with a 238u value of 0 83 0 12 2 the detrital end member is lower than the range of 238u values reported in the literature for granitoid igneous rocks 0 44 to 0 17 2 telus et al 2012 suggesting that continental weathering fractionates uranium isotopes with preferential release of 238u development and application of u isotopes as a paleoredox proxy has its basis in the nuclear volume fractionation the data from saanich inlet meets this expectation with a positive fractionation factor aq vi sed iv of 0 62 0 17 2 calculated as the difference in 238u between authigenic u in anoxic sediments and sediment traps 0 17 and u dissolved in seawater 0 45 however it is widely believed that u vi reduction in the marine environment occurs on the surfaces of particles and that the negative isotope effect associated with u vi sorption to plankton seawater plankton 0 79 0 17 2 opens up the possibility that the particulate pool of seawater derived u vi is fractionated from the dissolved pool accordingly the reduction of u vi in the marine environment might involve two steps 1 u vi sorption to particles with a negative fractionation and 2 reduction to u iv on particles with a positive fractionation environmental factors that are not yet well understood may influence the relative reaction rates for the two steps thus affecting the magnitude and sign of the overall fractionation this has the potential to explain conflicting results reported for u vi reduction experiments in the literature laboratory and field where both positive and negative fractionations have been observed and in some instances no fractionation at all variation in the fractionation factor has the potential to complicate application of u isotopes as a paleoredox proxy but this study and a study of the black sea romaniello 2012 give the same results within their respective uncertainties suggesting that 0 62 0 17 2 is a robust assessment of the apparent u isotope fractionation factor associated with reductive deposition of seawater u vi in both anoxic and euxinic marine sediments of the present day

thallium isotope evidence for a permanent increase in marine organic carbon export in the early eocene article the first high resolution thallium tl isotope records in two ferromanganese crusts fe mn crusts cd29 and d11 from the pacific ocean are presented the crusts record pronounced but systematic changes in 205 tl 203 tl that are unlikely to reflect diagenetic overprinting or changes in isotope fractionation between seawater and fe mn crusts it appears more likely that the fe mn crusts track the tl isotope composition of seawater over time the present day oceanic residence time of tl is estimated to be about 20 000 yr such that the isotopic composition should reflect ocean wide events new and published os isotope data are used to construct age models for these crusts that are consistent with each other and significantly different from previous age models application of these age models reveals that the tl isotope composition of seawater changed systematically between 55 ma and 45 ma using a simple box model it is shown that the present day tl isotope composition of seawater depends almost exclusively on the ratio between the two principal output fluxes of marine tl these fluxes are the rate of removal of tl from seawater via scavenging by authigenic fe mn oxyhydroxide precipitation and the uptake rate of tl during low temperature alteration of oceanic crust it is highly unlikely that the latter has changed greatly therefore assuming that the marine tl budget has also not changed significantly during the cenozoic the low 205 tl

latest precambrian to early cambrian basin evolution fortune bay newfoundland fault bounded basin to platform uppermost precambrian and lowest cambrian sediments of the rencontre chapel island and random formations in the fortune bay area western avalon zone consist of conglomerates sandstones and shales deposited in fluvial marginal marine and open marine settings deposition began in a small fault bounded basin alternation between nonmarine and marine deposition resulted from pulses in fault activity uplift and coarse terrestrial clastics separated by quiescent periods of basinwide thermal subsidence transgressive fine grained marine units overstepping earlier basin margins the tectonic regime may have been extensional rift or transtensional strike slip pull apart a strike slip origin is easiest to defend and is consistent with current tectonic models for the avalon zone involving prolonged extension or transtension but little spreading the fortune bay basin persisted until the beginning of the cambrian period at which time eustatic sea level rise led to widespread transgression and d

fast reconnaissance of carbonate dissolution based on the size distribution of calcareous ooze on walvis ridge se atlantic ocean we present a new index of carbonate fragmentation based on the size distribution of bulk sediments in core md962094 from walvis ridge se atlantic ocean the carbonate fragmentation index is constructed by taking a ratio of the two coarsest fractions in the grain size distributions of the bulk calcareous ooze the coarsest two fractions 25 90 small mu greekm and 90 small mu greekm of the bulk sediments consist primarily of complete shells and fragments of adult foraminifera shells and juvenile foraminifera shells and fragments respectively the ratio of the proportions of the two fractions is interpreted as a measure of fragmentation of the foraminifera shells caused by carbonate dissolution downcore changes in our carbonate fragmentation index compare very well with those in the coarse carbonate fragmentation index in sediments from a nearby core on walvis ridge the latter commonly used fragmentation index is defined as a ratio of foraminifera fragments over whole foraminifera in the 150 small mu greekm fraction as seen with a light microscope fragmentation is relatively high during glacial stages and relatively low during interglacial stages during the last 300 kyr caused by the combined effect of wind driven upwelling of corrosive water and increased production of organic matter decreasing the preservation potential of carbonates both during and after deposition the carbonate fragmentation index we present here provides a precise and fast method to establish a downcore fragmentation record it can be applied to bulk sediments that are carbonate rich caco3 68 and to all other deep marine sediments of which the grain size distribution of the carbonate free fraction is available

late quaternary changes of western equatorial atlantic surface circulation and amazon lowland climate recorded in ceara rise deep sea sediments today the western tropical atlantic is the most important passage for cross equatorial transfer of heat in the form of warm surface water flowing from the south into the north atlantic circulation changes north of south america may thus have influenced the global thermohaline circulation system and high northern latitude climate here we reconstruct late quaternary variations of western equatorial atlantic surface circulation and amazon lowland climate obtained from a multiproxy sediment record from ceara rise variations in the illite smectite ratio suggest drier climatic conditions in the amazon basin during glacials relative to interglacials the 230thex normalized fluxes and the 13c 12c record of organic carbon indicate that sea level fluctuations shelf topography and changes of the surface circulation pattern controlled variations and amplitude of terrigenous sediment supply to the ceara rise we attribute variations in thermocline depth reconstructed from vertical planktic foraminiferal oxygen isotope gradients and abundances of the phytoplankton species florisphaera profunda to changes in southeast trade wind intensity strong trade winds during ice volume maxima are associated with a deep western tropical atlantic thermocline strengthening of the north brazil current retroflection and more vigorous eastward flow of surface waters

correlation of late precambrian and early palaeozoic sequences by eustatic sea level changes and the selection of the precambrian cambrian boundary abstract the author argues for a pragmatic approach to the problem of defining the precambrian cambrian boundary a boundary at first occurrence of shelly fauna as recommended by the precambrian cambrian working group will create severe difficulties in worldwide correlation by placing the boundary stratigraphically somewhat higher i e the fallotaspis zone with a more abundant fauna there is a much better basis for biostratigraphic correlation eustatic sea level changes may be another important tool for late precambrian early palaeozoic correlation and more detailed facies interpretation of such sequences should be carried out in order to recognise these eustatic sea level changes

the influence of organic matter diagenesis on caco3 dissolution at the deep sea floor abstract in situ benthic flux chamber measurements were performed at seafloor locations in the atlantic and pacific oceans where the bottomwaters were supersaturated approximately saturated and undersaturated with respect to calcite there was no evidence of significant dissolution at or above the saturation horizon below the saturation horizon however significant benthic fluxes of titration alkalinity and calcium out of the sediments were observed indicating appreciable dissolution where observed the rate of dissolution is consistent with a rate expression of reaction order 4 5 and a rate constant in the range of 0 05 to 0 5 per day a numerical simulation was constructed to represent organic matter and caco3 diagenesis at the seafloor at the supersaturated and near saturation sites the model over predicts the influence of metabolically produced co2 on the extent of caco3 dissolution it is possible that these results can be reconciled by a better understanding and parameterization of organic matter mineralization and porewater transport processes in surface sediments the primary factors to be considered include 1 the oxidation state of the carbon being oxidized 2 the vertical distribution of oxidation rates including the role of doc in sediment remineralization 3 the influence of nondiffusive transport of near surface porewaters and 4 the influence of episodic depositional events on organic and inorganic carbon cycling

a cenozoic seawater redox record derived from 238u 235u in ferromanganese crusts oceanic oxygen levels are projected to drop in certain areas due to warming climate but the net effect to the overall ocean redox state is difficult to predict here we measured the stable uranium isotope composition 238u 235u in globally representative hydrogenous ferromanganese crusts in order to reconstruct the redox evolution of the global ocean throughout the cenozoic samples averaging 3 myr intervals have analytically indistinguishable 238u 235u throughout the cenozoic combined with a u isotope mass balance model we suggest that the overall ocean redox state has remained remarkably stable on million year time scales throughout the cenozoic despite large surface temperature fluctuations during this time this suggests that stabilizing feedbacks for example nutrient limitation in low oxygen zones may have prevented dramatic large scale shifts in oxygen levels in the ocean however the fe mn crust record will be unlikely to reflect rapid perturbations in ocean redox state to investigate these events sediment archives with faster accumulation rates and redox proxies with faster response time must be explored

correlation of co concentration and growth rate a method for age determination of ferromanganese crusts abstract based on the analysis and comparison of metal content and growth rate determination by the 10be method an empirical law has been developed to calculate the growth rate evaluating after 520 chemical analyses we have shown that the flux of co into ferromanganese seamount crusts has been constant over the entire growth period of all crusts investigated the amount of co supplied per unit of time and seabottom area is nearly constant in the oceanic water column the layered texture of ferromanganese crusts reflects the change of phases and hydrogenetically precipitated amounts as well as their different growth rates the most important indicator revealing the growth rate is the co enrichment of each crust layer the higher the growth rate the lower the co concentration therefore it is possible to recalculate the age and growth rate of each ferromanganese crust layer by g mm ma 1 1 28 co 0 24 based on the intense phosphorite impregnation of the lower part of the ferromanganese crust which has grown within the time period 18 12 ma so called older crust two additional effects have to be taken into account a the intense phosphorite impregnation causing dilution and b the chemical dissolution of the ferromanganese crusts caused by the phosphate rich pore water both reduce the co concentration and the relating co contents co x and co x have thus to be compensated a co x co x m mn co x mn co b b co x co x 1 0 05 p 1 where co x double corrected co concentration of the layer x of the older crust generation co x corrected co concentration of the layer x of the older crust generation concerning the phosphate dilution co x m measured co concentration of the layer x of the older crust generation mn co x mn co ratio measured of the layer x of the older crust generation mn co b mn co ratio measured of the boundary layer between younger and older crust generation and p difference between the apatite fraction of the older crust of the layer x and the average value of the younger crust generation this simple cobalt calculation model makes it possible to recalculate the growth rate and age of each simple layer of ferromanganese crusts

changes in the vertical structure of the north atlantic ocean between glacial and modern times the distribution of planktic foraminiferal 18o over the north atlantic ocean has been interpreted by duplessy et al 1991 1992 in terms of the distribution of surface water salinity during the last glacial maximum lgm and the younger dryas yd we present here the implications of this surface salinity distribution for changes in deep convection during these periods temperature salinity density diagrams of the water column have been constructed using the plancktic and benthic foraminifera 18o values together with sea surface temperature estimates obtained by using micropaleontological transfer functions the precision of these reconstructions is limited by the remaining uncertainties in the two basic assumptions upon which the methods are based the sea surface temperature reconstruction by climap are valid within the statistical uncertainties the water salinity 18o relationship is well constrained for surface and intermediate ocean waters a significant difference between the modern and the reconstructed glacial water structure is observed during the lgm the modern relatively warm and salty norwegian sea surface water and the north atlantic deep water nadw were replaced by low salinity surface water in the northern atlantic ocean and the norwegian sea and cold deep water 0 1 c however surface water salinity in the central north atlantic ocean 52 54 n 25 40 w was sufficiently high to permit deep water convection as a source of north atlantic intermediate and deep waters open ocean convection at the northern limit of the subtropical surface waters around 40 n may have contributed to the ventilation of the intermediate waters the waters deeper than 2000 m were the result of mixing between poorly oxygenated southern waters and well ventilated north atlantic deep waters a similar reconstruction for the cold yd period indicates that the modern type of circulation with warm and ventilated nadw was largely in operation and that the hydrologic pattern was different from that prevailing during the lgm

dissolution of calcite in deep sea sediments ph and o2 microelectrode results abstract we present microelectrode profiles of oxygen ph and porosity measured at in situ depth across the sediment water interface from stations above and below the calcite saturation horizon in the equatorial atlantic resistivity electrode data indicate that the diffusion coefficient for solutes is attenuated by nearly a factor of two within the top centimeter of the sediment lattice oxygen and porosity data are used to estimate that the organic carbon respiration rate of the sediment is 10 15 mol cm 2 yr 1 in these locations comparison of the ph data with numerical model results reveals that sedimentary calcite dissolves in response to respiration in the top few centimeters of sediment even above the saturation horizon we calculate that the effective in situ dissolution rate constant for calcite required to explain these data is the neighborhood of 10 100 d 1 although more data are needed to confirm this result it appears to be at least an order of magnitude less than the rate observed in laboratory stirred reactor experiments

late quaternary productivity changes in the western equatorial atlantic evidence from 230th normalized carbonate and organic carbon accumulation rates paleoproductivity changes in the western equatorial atlantic have been estimated from carbonate and marine organic carbon accumulation for the last 180 000 yr from a ceara rise sediment core geob 1523 1 accumulation rates were calculated by normalizing to excess 230th activity paleoproductivity from carbonate accumulation was estimated by a correlation between carbonate and organic carbon fluxes derived from sediment traps deployed in oligotrophic waters results indicate minor productivity changes varying around 30 gc m 2 yr 1 for organic carbon derived estimations and around 45 gc m 2 yr 1 for carbonate derived estimations suggesting that the study area remained a low productivity region throughout the time period examined maxima in western atlantic paleoproductivity occurred during the warm substages of glacials and interglacials in contrast to the eastern tropical atlantic where maxima are recorded in the cold substages this contrast might be caused by a deepening of the thermocline and nutricline in the west connected with a synchronous shallowing in the east the carbonate preservation record was examined for the last 380 000 yr based on variations in percent planktonic foraminiferal fragments and percent coarse fraction only minor carbonate dissolution above 3300 m water depth is observed except for three main dissolution events at isotope stages 4 8 4 and 10 considerable carbonate loss during these intervals is attributed to a decreased production of north atlantic deep water which is associated with vertical expansion of southern source deep water

the final miocene carbonate crash in the atlantic assessing carbonate accumulation preservation and production abstract in many palaeoceanographic studies low carbonate contents in neogene deep sea sediments are interpreted as dissolution events related to prominent shifts in intermediate and deep water circulation in the middle to late miocene such carbonate crash events were a widespread phenomenon throughout the world s ocean basins however low carbonate contents may also derive from terrigenous dilution and carbonate production changes in surface waters as shown for late miocene records of the south atlantic diester haass et al 2004 westerhold et al 2005 in order to contribute to solve this question we investigated the youngest of the miocene crash events 10 5 ma to 9 0 ma at a prominent key location of global deep water circulation i e the equatorial atlantic ceara rise depth transect odp sites 926 927 and 928 by analysing different carbonate preservation proxies like carbonate content silt grain size parameters sand content fragmentation index and mass accumulation rates we were able to quantify dissolution dilution and production and to assess previous conflicting interpretations critical evaluation of the different dissolution proxies and comparison to former studies revealed that silt preservation proxies do not entirely reproduce the depth dependant dissolution in the miocene sediments as indicated by foraminifer fragmentation in the investigated late miocene time slice three minima of carbonate accumulation were detected the predominant control on these events changed from dissolution of carbonate to lowered production the last event at 9 6 ma is accompanied by an increased preservation but reduced carbonate accumulation during this time the first prominent pulses of northern component water ncw occurred sourced most probably from the labrador sea this may be understood as a first short term precursor of long time persistent and much stronger ncw production that is characteristic for younger periods and marks the turnover to the modern type of basin to basin fractionation between the atlantic and pacific furthermore preservation trends of caribbean and ceara rise records were in phase in contrast to former hypotheses that suggested antithetical preservation during carbonate crash events the similarity of the preservation records in the studied ocean basins in conjunction with the inconsistency in timing of the cc events and proposed ncw estimates indicates that changes in the carbon cycle other than dissolution controlled the cc events prior to 9 9 ma

on the duration of the paleocene eocene thermal maximum petm 1 the paleocene eocene thermal maximum petm is one of the best known examples of a transient climate perturbation associated with a brief but intense interval of global warming and a massive perturbation of the global carbon cycle from injection of isotopically light carbon into the ocean atmosphere system one key to quantifying the mass of carbon released identifying the source s and understanding the ultimate fate of this carbon is to develop high resolution age models two independent strategies have been employed cycle stratigraphy and analysis of extraterrestrial helium heet both of which were first tested on ocean drilling program odp site 690 these two methods are in agreement for the onset of the petm and initial recovery or the clay layer main body but seem to differ in the final recovery phase of the event above the clay layer where the carbonate contents rise and carbon isotope values return toward background values here we present a state of the art age model for the petm derived from a new orbital chronology developed with cycle stratigraphic records from sites drilled during odp leg 208 walvis ridge southeastern atlantic integrated with published records from site 690 weddell sea southern ocean odp leg 113 during leg 208 five paleocene eocene p e boundary sections sites 1262 to 1267 were recovered in multiple holes over a depth transect of more than 2200 m at the walvis ridge yielding the first stratigraphically complete p e deep sea sequence with moderate to relatively high sedimentation rates 1 to 3 cm ka where a is years a detailed chronology was developed with nondestructive x ray fluorescence xrf core scanning records on the scale of precession cycles with a total duration of the petm now estimated to be 170 ka the revised cycle stratigraphic record confirms original estimates for the duration of the onset and initial recovery but suggests a new duration for the final recovery that is intermediate to the previous estimates by cycle stratigraphy and heet

proterozoic ocean redox and biogeochemical stasis the partial pressure of oxygen in earth s atmosphere has increased dramatically through time and this increase is thought to have occurred in two rapid steps at both ends of the proterozoic eon 2 5 0 543 ga however the trajectory and mechanisms of earth s oxygenation are still poorly constrained and little is known regarding attendant changes in ocean ventilation and seafloor redox we have a particularly poor understanding of ocean chemistry during the mid proterozoic 1 8 0 8 ga given the coupling between redox sensitive trace element cycles and planktonic productivity various models for mid proterozoic ocean chemistry imply different effects on the biogeochemical cycling of major and trace nutrients with potential ecological constraints on emerging eukaryotic life here we exploit the differing redox behavior of molybdenum and chromium to provide constraints on seafloor redox evolution by coupling a large database of sedimentary metal enrichments to a mass balance model that includes spatially variant metal burial rates we find that the metal enrichment record implies a proterozoic deep ocean characterized by pervasive anoxia relative to the phanerozoic at least 30 40 of modern seafloor area but a relatively small extent of euxinic anoxic and sulfidic seafloor less than 1 10 of modern seafloor area our model suggests that the oceanic mo reservoir is extremely sensitive to perturbations in the extent of sulfidic seafloor and that the record of mo and chromium enrichments through time is consistent with the possibility of a mo n colimited marine biosphere during many periods of earth s history

new age for ferromanganese crust 109d c and implications for isotopic records of lead neodymium hafnium and thallium in the pliocene indian ocean 1 this study presents a high resolution record of osmium and thallium isotopes in a ferro manganese fe mn crust from the indian ocean antipode 109d c these results when combined with additional new os isotope data from odp hole 756b in the southeast indian ocean define a new best estimate for the age at the base of this crust of 6 5 ma which is significantly different from a previous estimate of 15 ma based on co flux modeling the tl isotope record obtained for the indian ocean resembles that for the pacific ocean with a small but well defined increase occurring over the last 5 myr this contrasts with two records from the atlantic ocean which do not have resolvable variations ocean basin scale tl isotope variation may be inconsistent with the inferred modern marine residence time for tl of 20 kyr but could be explained by an increase in ocean crust production rates in the pacific and indian oceans since 10 ma the improved age model for 109d c reveals that the hf isotope composition of indian ocean bottom waters has remained homogenous over the last 6 myr thus this isotope system does not bear any evidence that the influence of north atlantic deep water in the formation of indian ocean bottom waters has changed during that time however because of the lack of knowledge about hf isotopes as a tracer of ocean circulation we cannot conclude that export of nadw decreased over the last 6 myr

carbonate preservation patterns at the ceara rise evidence for the pliocene super conveyor enhanced atlantic overturning during the pliocene was first proposed almost 10 yrs ago evidence for this pliocene super conveyor scenario has been collected using a number of proxies e g benthic 13c nd isotopic composition of manganese crusts the present study contributes to the existing evidences by using carbonate dissolution and current vigour history of early pliocene sediments from the ceara rise odp sites 927 and 929 in order to reveal carbonate dissolution history a number of commonly used and newly established proxies were applied i e sand and carbonate contents foraminifer fragmentation index bulloides dissolution index and carbonate silt grain size distributions terrigenous silt grain size distributions were used to unravel variations in relative current strength and sediment input to the two sites overall good carbonate preservation at the shallow site 927 3314 m water depth shows that this level was bathed in north atlantic deep water throughout the early pliocene the contrastingly poor carbonate preservation record of the deeper site 929 4358 m water depth at present exposed to antarctic bottom water is frequently interrupted by phases of good carbonate preservation these results indicate that the depth of the calcite lysocline was mainly tied to present level 4200 m water depth and sometimes even dropped to water depths greater than 4360 m due to even more enhanced circulation surprisingly the expansion of nadw is not clearly reflected by an increase in current speed as shown by continuously fine terrigenous grain size

sequential leaching of marine ferromanganese precipitates genetic implications abstract sequential leaching experiments were carried out on twenty one hydrogenetic crust samples from different locations in the central pacific and the results are compared with four crust and nodule samples of different genetic origin fe vernadite mno2 is the most important minor and trace metal bearing mineral phase and shows high concentrations of co ni cd ti ba zn and cu which are present in seawater mainly as hydrated and labile complexed cations elements forming carbonate and hydroxide complexes and oxyanions in seawater like pb mo v and minor proportions of cu and zn are bound in the fe00h fraction the ca carbonate phase and the residual fraction which consists of aluminosilicates and crystalline oxides show only minor heavy metal associations e g cu and zn only a small part of the high ti concentrations in hydrogenetic crusts is of detrital origin ti mainly forms a hydrogenetic phase probably consisting of ti02 2h20 intergrown with the amorphous fe00h phase analyses of different sample types and crust layers show that aging and diagenetic effects can change the phase associations of some elements the most notable change was observed for pb which during phosphatization of crusts is transferred from the mn fe oxide fraction to the apatite fraction a colloid chemical model for the hydrogenetic precipitation of ferromanganese crusts on seamounts is proposed in the first stage mn2 rich water from the oxygen minimum zone is mixed with oxygen rich deep water and oxidized mn iv and other metals like fe ti al and si form oxide and hydroxide colloids these form mixed colloidal phases and scavenge trace metals by sorptive processes which are dominated by coulombic and chemical interactions between colloid surfaces and dissolved metal species in stage two the colloidal phases precipitate on the substrate rocks of the seamounts as ferromanganese oxide encrustations incorporating the sorbed heavy metals into the mineral phases

a comparison of benthic nutrient fluxes from deep sea sediments off namibia and argentina abstract benthic fluxes of nitrate phosphate silicate and oxygen estimated c org input to the sea floor and c org content of surface sediments at various stations in the argentine basin and the cape basin south atlantic were used to characterize the benthic mineralization pattern in these areas diffusive benthic fluxes across the sediment water interface reveal very intense subsurface biodegradation of organic matter and dissolution of biogenic silica across the namibian continental margin diffusive benthic fluxes generally decrease with increasing water depth but a considerable site to site variation exists along the 1300 m isobath mineralization patterns indicate that the input flux of organic matter seems to be significantly controlled by vertical transport through the water column a nearly reverse pattern can be observed in the western argentine basin intense nutrient release is observed even at locations on the deeper slope for this whole area a dependence of nutrient release and water depth is not observed an exception is the rio de la plata region where stations between 1800 and 3500 m water depth reveal the highest nitrate and phosphate fluxes decreasing towards the shelf and into the deeper basin silicate fluxes off the rio de la plata increase continuously with water depth below 4000 m water depth this is probably due to the input of antarctic diatoms by the antarctic bottom water mineralization processes on the continental margin of the argentine basin do not correlate with the estimated organic matter input from surface waters in accordance with previous sedimentological studies we conclude that intense sediment mass flows from the shelf are the main source for the input of reactive organic matter to the deeper parts of the continental margin

a new geochemical classification of sedimentary environments because of the relative constancy of ph in most subaqueous sediments and the general lack of measurability of eh these parameters are not practically useful for classifying sedimentary environments in their place a new classification is offered which derived from studies of modern sediments is based on the presence or absence of dissolved oxygen and dissolved sulfide in the sediments at the time of authigenic mineral formation sedimentary environments are first of all divided into oxic and anoxic depending upon the presence or absence of measurable dissolved oxygen anoxic environments in turn are divided into sulfidic and non sulfidic depending upon the presence of measurable dissolved sulfide anoxic nonsulfidic environments are further divided into postoxic or resulting from oxygen removal without sulfate reduction weakly reducing and methanic or resulting from complete sulfate reduction with consequent methane formation strongly reducing the environments are shown to succeed one another during early diagenesis in the order oxic post oxic sulfidic methanic iron and manganese minerals characteristic and stable in each environment are listed and discussed so that they may be used to distinguish the environments when studying the ancient geological record

present water mass calcium carbonate corrosiveness in the eastern south atlantic inferred from ultrastructural breakdown of globigerina bulloides in surface sediments abstract the atlantic is regarded as a huge carbonate depocenter due to an on average deep calcite lysocline however calculations and models that attribute the calcite lysocline to the critical undersaturation depth hydrographic or chemical lysocline and not to the depth at which significant calcium carbonate dissolution is observed sedimentary calcite lysocline strongly overestimate the preservation potential of calcareous deep sea sediments significant calcium carbonate dissolution is expected to begin firstly below 5000 m in the deep guinea and angola basin and below 4400 m in the cape basin our study that is based on different calcium carbonate dissolution stages of the planktic foraminifera globigerina bulloides clearly shows that it starts between 400 and 1600 m shallower depending on the different hydrographic settings of the south atlantic ocean in particular coastal areas are severely affected by increased supply of organic matter and the resultant production of metabolic co2 which seems to create microenvironments favorable for dissolution of calcite well above the hydrographic lysocline

suborbital timescale variability of north atlantic deep water during the past 200 000 years we generated 200 kyr long proxy records of surface and deepwater variability from a subpolar north atlantic core v29 202 enabling us to assess the linkage between surface and deepwater changes on suborbital timescales in particular we used a benthic 13c record to evaluate the deep water response to dansgaard oeschger temperature oscillations and to heinrich events times of massive iceberg delivery to the north atlantic we found that the reduction of north atlantic deep water nadw production was generally associated with cold or dropping sea surface temperatures ssts as indicated by planktonic foraminiferal assemblages the nadw contribution to the site did not drop appreciably during heinrich events h4 through h2 probably because these events followed intervals of prolonged surface cooling already characterized by low rates of nadw production by contrast nadw reduction appears to have been synchronous with h5 sst rise associated with both dansgaard oeschger oscillations and heinrich events was usually accompanied by increasing nadw strength in a few cases the nadw recovery appeared to lag the sst rise however the apparent delay is most likely an artefact of the sedimentary record low concentrations of benthic foraminifera as a result of low benthic foraminiferal abundances during stage 6 the stage 6 benthic foraminiferal 13c record is of lower resolution than the younger part of the record the stage 6 proxy records for surface hydrography nevertheless reveal millennial scale oscillations similar to those seen in stage 3 the available 13c data suggest that nadw weakened in association with the cold portions of stage 6 sst oscillations we also sought to confirm a recent study which concluded that there was little nadw variability during the peak of the last interglaciation marine oxygen isotope substage 5e eemian isotope stage 5 was marked by a trend of increasing benthic 13c in v29 202 rising 13c through isotope stage 5 is also seen in tropical surface water records and at some deep atlantic sites and may reflect the common derivation of these water masses variations of 0 5 superimposed on this rising 13c trend within substage 5e in v29 202 are so far not evident in tropical feed waters and may therefore indicate that nadw production was weaker during the late than mid eemian an electronic supplement of this material may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type get and the name of the file to get it finally type quit to leave the system paper 95pa02089 suborbital timescale variability of north atlantic deep water during the past 200 000 years by d w oppo and s j lehman diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order

uranium removal in oceanic sediments and the oceanic u balance pore water uranium data from the nw atlantic ocean cariaco trench black sea and san clemente basin are used to calculate fluxes across the sediment water interface all samples were collected in situ and show depletions in pore water u relative to the overlying water concentration the uptake rate of u derived from pore water gradients reflects the redox environment with lowest fluxes 10 g cm 2 ky in oxic su ic sediments of the nw atlantic dominated by no 3 reduction and higher fluxes 25 70 g cm 2 ky in the anoxic sulfate reducing sediments of the cariaco trench black sea and san clemente basin the anoxic fluxes compare well with previous estimates based on solid phase data calculations indicate that the areally integrated removal of u into su ic hemipelagic sediments is of greater magnitude than that into anoxic sulfate reducing sediments these fluxes are discussed in the context of the oceanic u balance the removal of u in all oceanic sediments is estimated to account for about 65 of the riverine input and this sink plus removal of u in altered oceanic crust balances the most probable value for riverine input

global enhancement of ocean anoxia during oceanic anoxic event 2 a quantitative approach using u isotopes during the mesozoic greenhouse world the oceans underwent several oceanic anoxic events oaes characterized by intervals during which organic rich black shales were deposited indicating strong oxygen depletion in the marine realm the cenomanian turonian oae2 ca 93 ma represents one of the most prominent events of the cretaceous with significant perturbations of the global carbon cycle although oae2 likely reached a global scale the spatial extent of seawater anoxia during this oae is poorly constrained here we demonstrate that variations in the 238u 235u isotope ratio 238u a newly developed paleoredox proxy can be used to quantify the extent of marine anoxia for black shales from the mid cretaceous oae2 we find a systematic shift toward lighter 238u and lower u concentrations as compared to modern equivalent organic rich sediments from the black sea this shift translates to a global increase of oceanic anoxia during oae2 by at least a factor of three as compared to the present day or to periods before and after oae2 the constant offset in u concentrations and isotope compositions of black shales throughout oae2 compared to modern black sea sediments indicates an enhancement of oceanic anoxic conditions already prior to the onset of oae2

environmental processes of the ice age land oceans glaciers epilog knowledge of the state of the earth at the last glacial maximum lgm an interval around 21 000 years ago is an important benchmark for understanding the sensitivity of global environmental systems to change much progress in understanding climates of the lgm has occurred in the 20 years since the end of the climap project of the 1970s climate long range investigation mapping and prediction here we review this progress based on presentations and discussion at a rst open science meeting of the epilog project environmental processes of the ice age land oceans glaciers held in delmenhorst germany may 1999 we outline key controversies and document protocols for epilog contributions so that a new synthesis of the lgm earth can emerge as an open project of the world s community of scientists 2001 elsevier science ltd all rights reserved

the isotopic composition of authigenic chromium in anoxic marine sediments a case study from the cariaco basin chromium cr isotopes are an emerging proxy for tracking redox processes at the earth s surface however there has been limited exploration of the cr isotope record of modern and recent marine sediments the basic inorganic chemistry of cr suggests that anoxic marine basins should factor prominently in the global cr cycle and that sediments deposited within anoxic basins may offer a valuable cr isotope archive throughout earth s history here we present 53 cr data from sediments of the cariaco basin venezuela a type environment for large perennially anoxic basins with a relatively strong hydrological connection to the global oceans we document a marked positive shift in bulk 53 cr values following the termination of the last glacial maximum followed by relative stasis based on a suite of independent redox proxies this transition marks a switch from oxic to persistently anoxic and sulfidic euxinic depositional conditions within the basin we find good agreement between two independent approaches toward estimating the 53 cr composition of authigenic cr in euxinic cariaco basin sediments and that these estimates are very similar to the 53 cr composition of modern open atlantic ocean seawater these data together with considerations of reaction kinetics and mass balance within the cariaco basin are consistent with the hypothesis that anoxic marine settings can serve as a chemical archive of first order trends in seawater 53 cr composition additionally the cariaco basin data suggest that there has been secular stability in the average 53 cr value of atlantic seawater over the last 15 kyr

60 myr records of major elements and pb nd isotopes from hydrogenous ferromanganese crusts reconstruction of seawater paleochemistry we compare the time series of major element geochemical and pb and nd isotopic composition obtained for seven hydrogenous ferromanganese crusts from the atlantic indian and pacific oceans which cover the last 60 myr average crust growth rates and age depth relationships were determined directly for the last about 10 myr using be 10 be 9 profiles in the absence of other information these were extrapolated to the base of the crusts assuming constant growth rates and constant initial be 10 be 9 ratios due to the lack of additional information co contents have also been used previously to estimate growth rates in co rich pacific and atlantic seamount crusts puteanus and halbach 1988 a comparison of be 10 be 9 and co based dating of three co rich crusts supports the validity of this approach and confirms the earlier chronologies derived from extrapolated be 10 be 9 based growth rates back to 60 ma our data show that the flux of co into co poor crusts has been considerably lower the relationship between growth rate and co content for the co poor crusts developed from these data is in good agreement with a previous study of a wider range of marine deposits manheim 1986 the results suggest that the co content provides detailed information on the growth history of ferromanganese crusts particularly prior to 10 12 ma where the be 10 based method is not applicable the distributions of pb and nd isotopes in the deep oceans over the last 60 myr are expected to be controlled by two main factors a variations of oceanic mixing patterns and flow paths of water masses with distinct isotopic signatures related to major paleogeographic changes and b variability of supply rates or provenance of detrital material delivered to the ocean linked to climate change glaciations or major tectonic uplift the major element profiles of crusts in this study show neither systematic features which are common to crusts with similar isotope records nor do they generally show coherent relationships to the isotope records within a single crust consequently any interpretation of time series of major element concentrations of a single crust in terms of paleoceanograghic variations must be considered with caution this is because local processes appear to have dominated over more basin wide paleoceanographic effects in this study co is the only element which shows a relationship to pb and nd isotopes in pacific crusts a possible link to changes of pacific deep water properties associated with an enhanced northward advection of antarctic bottom water from about 14 ma is consistent with the pb but not with the nd isotopic results the self consistent profiles of the pb and nd isotopes suggest that postdepositional diagenetic processes in hydrogenous crusts including phosphatization events have been insignificant for particle reactive elements such as pb be and nd isotope time series of pb and nd show no systematic relationships with major element contents of the crusts which supports their use as tracers of paleo seawater isotopic composition

orbital forcing of bottom current enhanced sedimentation on feni drift ne atlantic during the mid pleistocene feni ridge is a plastered sediment drift deposited along the northwestern flank of rockall trough ne atlantic under the influence of deep geostrophic currents formed by intermittent overflows of arctic intermediate water from the norwegian sea across the iceland scotland ridge nsow lower deep water ldw a derivative of antarctic bottom water from the south atlantic mixed with old north atlantic deep water nadw also bathes the lower flanks of the drift below 2500 m deep sea drilling project site 610 was drilled in a sediment wave field on the surface of feni drift 2417 m though the sediments recovered proved to be typically pelagic preserving a high resolution 5 cm kyr 1 quaternary record of orbitally modulated ice rafting and biogenic productivity cycles but showing little evidence of current affected deposition from particle size and compositional analyses of the biogenic and lithogenic 10 m silt which we then compared with records of the percentage and flux of ice rafted detrital ird sand foraminifera and coccoliths for the same interval cross spectral comparisons were then made between these records and variations in a model of northern hemisphere ice volume ice sheet growth and decay in response to orbitally forced insolation changes at 65 n variations in the relative proportions and fluxes of ird sand foraminifera and coccoliths all covary closely with orbitally forced ice volume fluctuations but with differing amplitude coherency and phase relationships at each milankovitch frequency however there is a significant component of variance in pelagic and ice rafted sedimentation which is controlled by bottom currents and this shows excellent phase and coherency agreement with the ice volume model at all orbital frequencies the relative enrichment coarseness and flux of the contourite silt fraction of feni drift sediments all increase during glacial or cold stages of the mid pleistocene relative to warm stages suggesting that currents were stronger during these periods possible explanations for such changes in current activity include 1 an increased flux of nsow via the wyville thompson ridge into the rockall trough during glacials but not due to increased production of nsow 2 ldw presently lying below 2500 m rises during glacials by at least 50 60 m thereby flooding over the surface of feni ridge at site 610 3 the path of a major recirculation gyre in the lower nadw ldw system presently situated marginally south of site 610 encroaches upon the southern tip of feni ridge during glacials and 4 the rockall trough itself is a site of deep water formation during glacials analogous to labrador sea deep water formation today

seawater 234 u 238 u during the last 800 thousand years abstract constraining the history of seawater 234u 238u is important because this ratio is used to assess the validity of u th ages and because it provides information about the past rate of physical weathering on the continents this study makes use of u rich slope sediments from the bahamas in an attempt to reconstruct seawater 234u 238u for the last 800 kyr for the last 360 kyr u th dating of these sediments provides ages and initial 234u 238u values sixty seven samples largely from marine highstands have initial 234u 238u which scatter somewhat about the modern seawater value 1 145 but neither this scatter nor the average value increases with age of sample these data contrast with published coral data and suggest that seawater 234u 238u has remained within 15 of the modern value for the last 360 kyr this confirms the rejection of coral u th ages where the initial 234u 238u is significantly different from modern seawater data from older highstands dated with 18o stratigraphy or by the presence of the brunhes matuyama b m reversal at 780 kyr allow seawater 234u 238u to be assessed prior to the range of the 230th chronometer unfortunately diagenetic scatter in the data between the b m reversal and 360 kyr is rather large probably relating to low u concentrations for these samples but there is no indication of a trend in seawater 234u 238u with age high u samples from close to the b m reversal show less diagenetic scatter and an initial 234u 238u that averages 1 102 this lower value can be explained by lower seawater 234u 238u at the time of the b m reversal or by progressive loss of 234u from the sediment by recoil a simple box model is presented to illustrate the response of seawater 234u 238u to variations in riverine input such as might be caused by changes in continental weathering comparison of the bahamas 234u 238u data with model results indicates that riverine 234u 238u has not varied by more than 65 for any 100 kyr period during the last 360 kyr it also indicates that the ratio of physical to chemical weathering on the continents has not been higher than at present for any extended period during the last 800 kyr

nitrogen isotope evidence for water mass denitrification during the early toarcian jurassic oceanic anoxic event bulk sedimentary nitrogen isotope 15ntot data have been generated from lower jurassic black carbon rich shales in the british isles and northern italy deposited during the early toarcian oceanic anoxic event a pronounced positive 15ntot excursion through the exaratum subzone of the falciferum zone defined by characteristic ammonites in the british isles broadly correlates with a relative maximum in weight percent total organic carbon and in some sections with a negative 13corg excursion upwelling of a deoxygenated water mass that had undergone partial denitrification is the likely explanation for relative enrichment of 15ntot and parallels may be drawn with quaternary sediments of the arabian sea gulf of california and northwest mexican margin the development of early toarcian suboxic water masses and consequent partial denitrification is attributed to increases in organic productivity approximately coincident phenomena include the following a relative climatic optimum realignment of major oceanic current systems and a possible release of methane gas hydrates from continental margin sediments early in the history of the oceanic anoxic event

carbonate dissolution revealed by silt grain size distribution comparison of holocene and last glacial maximum sediments from the pelagic south atlantic the current issue of global warming and the role of the ocean in global exchange of co2 increases the interest in solid budgets of marine carbonate production and dissolution the present study utilizes grain size composition of pelagic sediments in order to trace spatial and temporal variability of carbonate sedimentation in the south atlantic for the holocene and last glacial maximum lgm 19 23 cal kyr bp a decrease in grain size e g sand content mean grain size of coarse carbonate silt indicates increased carbonate dissolution as a result of increased fragmentation of calcareous microfossils the spatial grain size pattern suggests a threshold water depth below which a gradual grain size decrease becomes increasingly rapid this water depth is considered as the sedimentary lysocline for the holocene time slice a constant gradual decrease of foraminifer carbonate of about 5 10 per 1000 m water depth above the lysocline gives evidence for supra lysoclinal dissolution the water depth of the lysocline for the holocene is tied to the interface of north atlantic deep water and antarctic bottom water aabw ca 4100 m submarine ridges which restrict intrusion of aabw into the angola basin cause an asymmetry in carbonate preservation across the mid atlantic ridge the lysocline was reconstructed at ca 3100 m for the lgm these data suggest that the ca 1000 m rise of the lysocline eradicated the holocene east west asymmetry

inverse correlation between the molybdenum and uranium isotope compositions of upper devonian black shales caused by changes in local depositional conditions rather than global ocean redox variations abstract coupled mo u isotope data from the upper devonian kettle point formation ontario canada provide a cautionary tale regarding interpretation of global ocean redox conditions using data from euxinic black shales in the gore of chatham core the kettle point black shales have high mo concentrations 48 473 g g and consistently high mo u ratios 3 times the mo u ratio of modern seawater suggesting a euxinic depositional environment these shales yield an inverse correlation r 0 89 p

concentration oxidation state and particulate flux of uranium in the black sea abstract in contrast to its unreactive behavior in the open ocean uranium is removed from seawater to sediments of anoxic marine basins four biogeochemical processes have been proposed to account for this removal active uptake by organisms whose remains are preserved by the anoxic conditions of the sediments complexation of u vi by particulate organic matter chemical reduction of soluble u vi to insoluble u iv which is scavenged to the sediments by settling particles and precipitation of uranium within the sediments themselves the black sea is the largest anoxic marine basin in the world today waters below 150 200 m are anoxic and h2s levels build up to about 400 mol 1 1 in the bottom water maximum depth 2200 m concentrations of total u and the oxidation state of dissolved u were measured throughout the water column while particulate u fluxes were determined from time series sediment trap samples collected in the deep anoxic zone bottom waters were depleted in u by 40 relative to the initial u content of the water two independent methods gave residence times of dissolved u in the deep water column of about 103 years uranium occurred throughout the water column as soluble chemically labile reducible u vi rather than the thermodynamically favored u iv uranium is neither reduced to u iv by the high levels of h2s nor is it rapidly scavenged from the water column as would be expected for strongly hydrolyzed tetravalent actinides like u iv fluxes of particulate authigenic u measured with sediment traps are at least two orders of magnitude less than the rate of authigenic u burial in black sea sediments therefore the first three processes listed above contribute negligibly to the deposition of u in the sediments of anoxic marine basins such as the black sea the dominant removal process must involve precipitation of u in the sediments rather than scavenging from the water column

carbon isotope excursions in precambrian cambrian boundary beds morocco the precambrian cambrian boundary is one of the most significant points on the geological timescale because it marks the change in the fossil record from soft bodied organisms to skeletonized faunas the cambrian explosion or radiation event1 saw the evolution of many new life forms especially various molluscs and archaeocyathids from the base of the cambrian tommotian stage others including the trilobites and bivalves evolved a little later in the succeeding atdabanian stage2 3 a massive increase in the biomass of the photic zone through phytoplankton blooms has also been postulated for this time4 particularly in the late riphean early vendian and in the early cambrian5 there has been much discussion concerning where precisely to place the boundary6 7 and here are several contenders in the world for the type section8 i present here a carbon isotope profile from the precambrian cambrian boundary beds in morocco where two distinctive positive 13c excursions are revealed these are interpreted as records of increased organic productivity as part of the precambrian cambrian explosion of life 13c excursions could be useful points for correlation between boundary sections

two major cenozoic episodes of phosphogenesis recorded in equatorial pacific seamount deposits seamount phosphorites have been recognized since the 1950s but this is the first study to provide an in depth exploration of the origin and history of these widespread deposits representative samples from equatorial pacific cretaceous seamounts were analyzed for chemical mineralogical and stable isotope compositions the phosphorites occur in a wide variety of forms but most commonly carbonate fluorapatite cfa replaced middle eocene and older carbonate sediment in a deep water environment 1000 m element ratios distinguish seamount phosphorites from continental margin plateau and insular phosphorites uranium and thorium contents are low and total rare earth element ree contents are generally high ree ratios and shale normalized patterns demonstrate that the rees and host cfa were derived from seawater strontium isotopic compositions compared with inferred cenozoic seawater curves define two major episodes of cenozoic phosphatization late eocene early oligocene 39 34 ma and late oligocene early miocene 27 21 ma three minor events are also indicated the major episodes occurred at times of climate transition the first from a nonglacial to glacial earth and the second from a predominantly glacial to warm earth the paleoceanographic conditions that existed at those times initiated and sustained development of phosphorite by accumulation of dissolved phosphorus in the deep sea during relatively stable climatic conditions when oceanic circulation was sluggish fluctuations in climate sealevel and upwelling that accompanied the climate transitions may have driven cycles of enrichment and depletion of the deep sea phosphorus reservoir as temperature gradients in the oceans increased antarctic glaciation expanded and oceanic circulation and upwelling intensified expansion and intensification of the oxygen minimum zone may have increased the capacity for midwater storage of phosphorus supplied by dynamic upwelling around seamounts however the bottom waters never became anoxic during the phosphogenic episodes fluctuations in the ccd and lysocline co2 fluxes and changes in bottom water circulation and temperatures may have bathed the seamount carbonates in more corrosive waters which coupled with increased supplies of dissolved phosphorus promoted replacement processes the late eocene early oligocene phosphogenic episode recorded in seamount deposits is not matched by large phosphorite deposits in the geologic record whereas the late oligocene early miocene episode and middle miocene event are matched by large deposits distributed globally the seamount phosphorites are exposed at the surface of the seamounts and have been for most of the neogene and oligocene the phosphorites do not show signs of etching that would indicate substantial undersaturation of seawater phosphate with respect to cfa mass balance calculations indicate that about 5 4 19 1012 g of p2o5 are locked up in equatorial pacific seamount phosphorites that amount is equivalent to about 2 7 years of the present annual input from rivers

introduction to u series geochemistry during the last century the earth sciences underwent two major revolutions in understanding the first was the recognition of the great antiquity of the earth and the second was the development of plate tectonic theory these leaps in knowledge moved geology from its largely descriptive origins and established the modern quantitative earth sciences for any science and particularly for the earth sciences time scales are of central importance until recently however the study of time scales in the earth sciences was largely restricted to the unraveling of the ancient history of our planet for several decades earth scientists have used a variety of isotope chronometers to unravel the long term evolution of the planet a fuller understanding of the physical and chemical processes driving this evolution often remained elusive because such processes occur on time scales 1 105 years which are simply not resolvable by most conventional chronometers the u series isotopes however do provide tools with sufficient time resolution to study these earth processes during the last decade the earth sciences have become increasingly focused on fundamental processes and u series geochemistry has witnessed a renaissance with widespread application in disciplines as diverse as modern oceanography and igneous petrology the uranium and thorium decay series contain radioactive isotopes of many elements in particular u th pa ra and rn the varied geochemical properties of these elements cause nuclides within the chain to be fractionated in different geological environments while the varied half lives of the nuclides allows investigation of processes occurring on time scales from days to 105 years u series measurements have therefore revolutionized the earth sciences by offering some of the only quantitative constraints on time scales applicable to the physical processes that take place on the earth the application of u series geochemistry to the earth sciences was thoroughly summarized in 1982

oxygen minimum zones omzs in the modern ocean in the modern ocean oxygen minimum zones omzs are potential traces of a primitive ocean in which archean bacteria lived and reduced chemical anomalies occurred but omzs are also keys to understanding the present unbalanced nitrogen cycle and the oceans role on atmospheric greenhouse control omzs are the main areas of nitrogen loss as n2 n2o to the atmosphere through denitrification and anammox and could even indirectly mitigate the oceanic biological sequestration of co2 it was recently hypothesized that omzs are going to spread in the coming decades as a consequence of global climate change despite an important omz role for the origin of marine life and for the biogeochemical cycles of carbon and nitrogen there are some key questions on the structure of omzs at a global scale there is no agreement concerning the threshold in oxygen that defines an omz and the extent of an omz is often evaluated by denitrification criteria which at the same time are o2 dependent our work deals with the identification of each omz the evaluation of its extent volume and vertical structure the determination of its seasonality or permanence and the comparison between omzs and denitrification zones at a global scale the co existence in the omz of oxic in its boundaries and suboxic even anoxic in its core conditions involves rather complex biogeochemical processes such as strong remineralization of the organic matter removal of nitrate and release of nitrite the quantitative omz analysis is focused on taking into account the whole water volume under the influence of an omz and adapted to the study of the specific low oxygen biogeochemical processes a characterization of the entire structure for the main and most intense omzs o2 10 mu m maximum zone the larger omz relative to the extent of denitrification zone suggests that the unbalanced nitrogen cycle on the global scale could be more intense than previously recognized and that evaluation of the omz from denitrification could underestimate their extent

seawater oxygenation during the paleocene eocene thermal maximum uncertainty over the trajectory of seawater oxygenation in the coming decades is of particular concern in the light of geological episodes of abrupt global warming that were frequently accompanied by lowered seawater oxygen concentrations here we present an assessment of global seawater oxygenation from an interval of one of these warming episodes the paleocene eocene thermal maximum petm 55 9 m y ago our results obtained from integrated ocean drilling program expedition 302 site m0004 in the arctic ocean are based on molybdenum isotope determinations and molybdenum rhenium and uranium abundances these data indicate a small global expansion of low oxygen marine environments in the upper part of the petm interval compared with the present day more extensive seawater deoxygenation may have occurred for as long as 100 k y associated with a high rate of global warming and carbon oxidation at the start of the petm our data also reveal molybdenum isotope compositions in arctic ocean deposits that are outside the range currently documented in marine environments these exceptional compositions could reflect either the influence of hydrothermal inputs or equilibrium isotope fractionations associated with molybdenum sulfide speciation

variability of silt grain size and planktonic foraminiferal preservation in plio pleistocene sediments from the western equatorial atlantic and caribbean abstract records of mean sortable silt and planktonic foraminiferal preservation from the ceara rise western equatorial atlantic and from the caribbean are presented to analyze the pliocene 3 5 2 2 ma to pleistocene 1 6 0 3 ma evolution of near bottom current strength and the carbonate corrosiveness of deep water during the mid pleistocene climate transition 1 ma a drastic decrease in glacial bottom current strength and an increase in carbonate corrosiveness is registered demonstrating a substantial decrease in the glacial contribution of the lower north atlantic deep water lnadw to the atlantic ocean also an increased sensitivity to eccentricity orbital forcing is registered after the mpt by contrast carbonate preservation increases considerably in the deep caribbean in response to a strong and persistent stable contribution of upper north atlantic deep water unadw we found evidence for the strongest and most stable circulation within the lnadw cell during the northern hemisphere cooling period between 3 2 and 2 75 ma this is in agreement with the superconveyor model which postulates that the highest nadw production took place prior to 2 7 ma a considerable decrease in bottom current strength and planktonic foraminiferal preservation is observed synchronous with the first occurrence of large scale continental ice sheets in the northern hemisphere this documents the final termination of the superconveyor at ca 2 75 ma however our data do not support a superconveyor in the interval between 3 5 and 3 2 ma when high amplitude fluctuations in bottom current flow and preservation in planktonic foraminifera are observed because of the great sensitivity of nadw production to changes in surface water salinity we assume that the high amplitude fluctuations of lnadw circulation prior to 3 2 ma are linked to changes in the atlantic salinity budget after 2 75 ma they are primarily controlled by ice sheet forcing in contrast to the stepwise deterioration of planktonic foraminiferal preservation in the western deep atlantic a trend toward better preservation from the pliocene to pleistocene is observed in the deep caribbean this indicates a long term increase in the contribution of unadw to the atlantic ocean

rapid expansion of oceanic anoxia immediately before the end permian mass extinction tive of an expansion of oceanic anoxia and higher th u ratios from 0 06 to 0 42 indicative of drawdown of u concentrations in seawater using a mass balance model we estimate that this isotopic shift represents a sixfold increase in the flux of u to anoxic facies implying a corresponding increase in the extent of oceanic anoxia the intensification of oceanic anoxia coincided with or slightly preceded the eh and persisted for an interval of at least 40 000 to 50 000 y following the eh these findings challenge previous hypotheses of an extended period of whole ocean anoxia prior to the end permian extinction

nitrogen cycling during the cretaceous cenomanian turonian oceanic anoxic event ii low 15n values ranging from 1 2 to 3 9 and atomic carbon nitrogen c n ratios of 25 50 are characteristic of black shales deposited during the cenomanian turonian boundary interval and oceanic anoxic event ii oae ii an observed antithetic relationship between c n ratios and 15n could suggest a predominance of terrestrially derived organic matter or a diagenetic control on 15n variability shifting bulk 15n values lower however hydrogen indices hi generally 450 and a positive correlation of hi with c n mitigates against a significant terrestrial organic matter fraction high c n values are likely the result preferential degradation of labile n rich compounds during early diagenesis and loss of n as ammonium from sediments through time a hypothetical model that considers the degradation of a 15n enriched labile protein fraction yields only small 1 2 negative shifts in 15n however 15n depletion during diagenesis is contrary to normal isotope kinetics which should result in 15n enrichment of bulk organic matter therefore we conclude that the bulk 15n values in this study reflect primary changes in the nitrogen cycle the 15n data support the hypothesis of expanded nitrogen fixation driven by upwelling of nutrient nitrogen poor phosphorus replete waters during oae ii and from the mid cenomanian to santonian at demerara rise the low 15n values which are significantly lower than bulk 15n values in modern regions where nitrogen fixation is known to be important probably result from a more significant fraction of dissolved inorganic nitrogen being produced by nitrogen fixation during the peak of oae ii a marked shift to lower 15n values is observed this shift possibly reflects greater utilization of 15n depleted ammonium during a chemocline upward excursion cue dominance of low 15n values from other periods of more widespread marine anoxia is likely the result of similar processes

evolution of nd and pb isotopes in central pacific seawater from ferromanganese crusts abstract hydrogenetic ferromanganese crusts incorporate elements from ambient seawater during their growth on seamounts by analysing nd pb and be isotope profiles within crusts it is possible to reconstruct seawater tracer histories depth profiles of 10be 9be ratios in three pacific ferromanganese crusts have been used to obtain growth rates which are between 1 4 and 3 8 mm ma nd and pb isotopes provide intact records of isotopic variations in pacific seawater over the last 20 ma or more there were only small changes in pb isotope composition in the last 20 ma this indicates a constant pb composition for the erosional sources and suggests further that erosional nd inputs may have been uniform too end values vary considerably with time and most probably reflect changes in ocean circulation the end values of the crusts not only vary as a function of age but also as a function of water depth from 25 to 0 ma crust va13 2 from 4 8 km water depth has a similar pattern of end variation to the two shallower crusts from 1 8 and 2 3 km but about 1 0 to 1 5 units more negative this suggests that end stratification in pacific seawater as demonstrated for the present day has been maintained for at least 20 ma each crust shows a decrease in end from 3 5 ma to the present which is interpreted in terms of an increase in the nadw component present in the pacific from 10 to 3 5 ma ago the crusts show an increase in end this suggests a decreasing role for a deep water source with end less than circum pacific sources in this regard the panamanian gateway restriction from 10 ma with final closure at 3 5 ma may have played an important role in reducing access of atlantic derived nd to the pacific

thallium isotope variations in seawater and hydrogenetic diagenetic and hydrothermal ferromanganese deposits results are presented for the first in depth investigation of tl isotope variations in marine materials the tl isotopic measurements were conducted by multiple collector inductively coupled plasma mass spectrometry for a comprehensive suite of hydrogenetic ferromanganese crusts diagenetic fe mn nodules hydrothermal manganese deposits and seawater samples the natural variability of tl isotope compositions in these samples exceeds the analytical reproducibility 0 05 by more than a factor of 40 hydrogenetic fe mn crusts have 205tl of 10 to 14 whereas seawater is characterized by values as low as 8 205tl represents the deviation of the 205tl 203tl ratio of a sample from the nist srm 997 tl isotope standard in parts per 104 this 2 difference in isotope composition is thought to result from the isotope fractionation that accompanies the adsorption of tl onto ferromanganese particles an equilibrium fractionation factor of 1 0021 is calculated for this process ferromanganese nodules and hydrothermal manganese deposits have variable tl isotope compositions that range between the values obtained for seawater and hydrogenetic fe mn crusts the variability in 205tl in diagenetic nodules appears to be caused by the adsorption of tl from pore fluids which act as a closed system reservoir with a tl isotope composition that is inferred to be similar to seawater nodules with 205tl values similar to seawater are found if the scavenging of tl is nearly quantitative hydrothermal manganese deposits display a positive correlation between 205tl and mn fe this trend is thought to be due to the derivation of tl from distinct hydrothermal sources deposits with low mn fe ratios and low 205tl are produced by the adsorption of tl from fluids that are sampled close to hydrothermal sources such fluids have low mn fe ratios and relatively high temperatures such that only minor isotope fractionation occurs during adsorption hydrothermal manganese deposits with high mn fe and high 205tl are generated by scavenging of tl from colder more distal hydrothermal fluids under such conditions adsorption is associated with significant isotope fractionation and this produces deposits with higher 205tl values coupled with high mn fe

vanadium in foraminiferal calcite as a tracer for changes in the areal extent of reducing sediments we have used the vanadium concentration in cleaned foraminiferal calcite as a tracer of seawater v changes in the past since the benthic flux of vanadium is sensitive to the redox po tential of sediments changes in the vanadium concentration of seawater should be a reflection of changes that control the redox state of sediments v ca in g sacculifer from an eastern equatorial atlantic core eno66 17ggc is 21 6 2 8 nmol v mol ca this value does not change over 35 kyr indicating that there was no measurable change in seawater vanadium levels over this period potential artifacts from partial dissolution are not significant based on low constant values for foraminiferal fragmentation 6 7 in the top 50 cm of the core a minor correction to account for vanadium associated with mn carbonate overgrowths estimated from two caribbean cores where this mixed phase dominates the deeper v ca values has been applied changes in the areal extent of anoxic and suboxic sediments are thus constrained by this constant value and the standard deviation of the measurement 12 based on a mass balance for vanadium where suboxic sedi ments are a source and anoxic sediments are a sink to the ocean suboxic sediments are predicted to have changed by no more than 0 5 1 5 times the current value assuming no changein the areal extent of anoxic sediments this correspondsto 1 3 3 5 of total ocean sediments given a con stant area of suboxic sediments the areal extentof anoxic sediments didonot increase by more than fivefold or 1 5 of the ocean floor over th e past 35 kyr the significant reductions in deep water oxygen levels and consequent changes in sedimen t redox conditions required by polar nutrient de pletion scenarios are not reflected in the foraminiferal vanadium data over the past 35 kyr this suggests that models which call on less or no changes in deep water oxygen are more likely alternatives

uranium isotopes fingerprint biotic reduction knowledge of paleo redox conditions in the earth s history provides a window into events that shaped the evolution of life on our planet the role of microbial activity in paleo redox processes remains unexplored due to the inability to discriminate biotic from abiotic redox transformations in the rock record the ability to deconvolute these two processes would provide a means to identify environmental niches in which microbial activity was prevalent at a specific time in paleo history and to correlate specific biogeochemical events with the corresponding microbial metabolism here we demonstrate that the isotopic signature associated with microbial reduction of hexavalent uranium u i e the accumulation of the heavy isotope in the u iv phase is readily distinguishable from that generated by abiotic uranium reduction in laboratory experiments thus isotope signatures preserved in the geologic record through the reductive precipitation of uranium may provide the sought after tool to probe for biotic processes because uranium is a common element in the earth s crust and a wide variety of metabolic groups of microorganisms catalyze the biological reduction of u vi this tool is applicable to a multiplicity of geological epochs and terrestrial environments the findings of this study indicate that biological activity contributed to the formation of many authigenic u deposits including sandstone u deposits of various ages as well as modern cretaceous and archean black shales additionally engineered bioremediation activities also exhibit a biotic signature suggesting that although multiple pathways may be involved in the reduction direct enzymatic reduction contributes substantially to the immobilization of uranium

sea surface temperatures in the equatorial and south atlantic ocean during the last glacial maximum 23 19 ka 1 we used planktic foraminiferal assemblages in 70 sediment cores from the tropical and subtropical south atlantic ocean 10 n 37 s to estimate annual mean sea surface temperatures ssts and seasonality for the last glacial maximum with a modified version of the imbrie kipp transfer function method iktf that takes into account the abundance of rare but temperature sensitive species in contrast to climap project members 1981 the reconstructed ssts indicate cooler glacial ssts in the entire tropical subtropical south atlantic with strongest cooling in the upwelling region off namibia 7 10 c and smallest cooling 1 2 c in the western subtropical gyre in the western atlantic our data support recent temperature estimates from other proxies in the upwelling regions in the eastern atlantic our data conflict with sst reconstructions from alkenones which may be due to an environmental preference of the alkenone producing algae or to an underestimation of foraminiferal ssts due to anomalous high abundances of n pachyderma sinistral

diagenetic effects on uranium isotope fractionation in carbonate sediments from the bahamas abstract uranium isotope variations 238u recorded in sedimentary carbonate rocks are a promising new proxy for the extent of oceanic anoxia through geological time however the effects of diagenetic alteration on the u isotopic composition in carbonate sediments which are crucial to understand the accurate reconstruction of marine 238u are currently poorly constrained here we examine the effects of the aragonite to calcite transition in the pleistocene key largo limestone of south florida and assess the effects of vadose meteoric phreatic meteoric and phreatic marine diagenesis on u isotope fractionation in carbonate sediments from the bahamas transect including the well studied clino unda and odp site 1006 drill cores our results suggest that early diagenetic processes in bahamas carbonate sediments fractionate u isotopes by an average of 0 27 0 14 1 sd heavier than contemporaneous seawater downcore variations of 238u in slope and basin sediments display little if any correlation with u concentration and common geochemical indicators of diagenesis 13c 18o mn sr mg ca sr ca enrichments of redox sensitive elements or rare earth elements anomalies we propose two possible mechanisms to interpret the positive change in the 238u during carbonate diagenesis authigenic enrichment of isotopically positive u iv in carbonates and preferential incorporation of isotopically positive aqueous u vi species into carbonates these processes likely operate during early syndepositional diagenesis on the banktop further diagenesis during deeper burial is limited by the low solubility of u iv under reducing pore water conditions the early diagenetic behavior of u isotopes in bahamas carbonate sediments is likely broadly representative of carbonate diagenesis in the geological past we suggest that the mean diagenetic offset determined in this study be applied when reconstructing seawater 238u from ancient carbonates furthermore early diagenesis induces significant statistical variability in sediment 238u values pointing to the need for large high resolution data sets in order to average out stochastic variations in individual bulk sediment samples

north atlantic thermohaline circulation during the past 20 000 years linked to high latitude surface temperature during a surface cooling event 10 000 to 12 000 years ago higher cd ca and lower 13c 12c ratios are observed in benthic foraminifera shells from rapidly accumulating western north atlantic sediments data from sediment cores show that marked nutrient depletion of intermediate waters occurs in association with reduced glacial north atlantic deep water flux it is proposed that cold high latitude sea surface temperatures enhance intermediate water formation at the expense of deep water formation

sediment transport time measured with u series isotopes results from odp north atlantic drift site 984 abstract high precision uranium isotope measurements of marine clastic sediments are used to measure the transport and storage time of sediment from source to site of deposition the approach is demonstrated on fine grained late pleistocene deep sea sediments from ocean drilling program site 984a on the bjorn drift in the north atlantic the sediments are siliciclastic with up to 30 carbonate and dated by 18o of benthic foraminifera nd and sr isotopes indicate that provenance has oscillated between a proximal source during the last three interglacial periods volcanic rocks from iceland and a distal continental source during glacial periods an unexpected finding is that the 234u 238u ratios of the silicate portion of the sediment isolated by leaching with hydrochloric acid are significantly less than the secular equilibrium value and show large and systematic variations that are correlated with glacial cycles and sediment provenance the 234u depletions are inferred to be due to recoil loss of 234th and are used to calculate comminution ages of the sediment the time elapsed between the generation of the small 50 m sediment grains in the source areas by comminution of bedrock and the time of deposition on the seafloor transport times the difference between comminution ages and depositional ages vary from less than 10 ky to about 300 to 400 ky for the site 984a sediments long transport times may reflect prior storage in soils on continental shelves or elsewhere on the seafloor transport time may also be a measure of bottom current strength during the most recent interglacial periods the detritus from distal continental sources is diluted with sediment from iceland that is rapidly transported to the site of deposition the comminution age approach could be used to date quaternary non marine sediments soils and atmospheric dust and may be enhanced by concomitant measurement of 226ra 230th 230th 234u and cosmogenic nuclides

the geochemistry of redox sensitive trace metals in sediments abstract we analyzed the redox sensitive elements v mo u re and cd in surface sediments from the northwest african margin the u s northwest margin and the arabian sea to determine their response under a range of redox conditions where oxygen penetrates 1 cm or less into the sediments mo and v diffuse to the overlying water as mn is reduced and remobilized authigenic enrichments of u re and cd are evident under these redox conditions with the onset of sulfate reduction all of the metals accumulate authigenically with re being by far the most enriched general trends in authigenic metal accumulation are described by calculating authigenic fluxes for the 3 main redox regimes oxic reducing where oxygen penetrates 1 cm and anoxic conditions using a simple diagenesis model and global estimates of organic carbon rain rate and bottom water oxygen concentrations we calculate the area of sediments below 1000 m water depth in which oxygen penetration is 1 cm to be 4 of the ocean floor we conclude that sediments where oxygen penetrates 1 cm release mn v and mo to seawater at rates of 140 260 60 150 and 5 10 of their respective riverine fluxes using the authigenic metal concentrations and accumulation rates from this work and other literature these sediments are sinks for re cd and u with burial fluxes of 70 140 30 80 and 20 40 respectively of their dissolved riverine inputs we modeled the sensitivity of the response of seawater re cd and v concentrations to changes in the area of reducing sediments where oxygen penetrates 1 cm our analysis suggests a negligible change in seawater re concentration whereas seawater concentrations of cd and v could have decreased and increased respectively by 5 10 over 20 kyr if the area of reducing sediments increased by a factor of 2 and by 10 20 if the area increased by a factor of 3 the concentration variations for a factor of 2 increase in the area of reducing sediments are at about the level of uncertainty of cd ca and v ca ratios observed in foraminifera shells over the last 40 kyr this implies that the area of reducing sediments in the ocean deeper than 1000 m 4 has not been greater than twice the present value in the recent past

cenozoic marine geochemistry of thallium deduced from isotopic studies of ferromanganese crusts and pelagic sediments cenozoic records of tl isotope compositions recorded by ferromanganese fe mn crusts have been obtained such records are of interest because recent growth surfaces of fe mn crusts display a nearly constant tl isotope fractionation relative to seawater the time series data are complemented by results for bulk samples and leachates of various marine sediments oxic pelagic sediments and anoxic marine deposits can be distinguished by their tl isotope compositions both pelagic clays and biogenic oozes are typically characterized by 205tl greater than 2 5 whereas anoxic sediments have 205tl of less than 1 5 205tl is the deviation of the 205tl 203tl isotope ratio of a sample from nist srm 997 tl in parts per 104 leaching experiments indicate that the high 205tl values of oxic sediments probably reflect authigenic fe mn oxyhydroxides time resolved tl isotope compositions were obtained from six fe mn crusts from the atlantic indian and pacific oceans and a number of observations indicate that these records were not biased by diagenetic alteration over the last 25 myr the data do not show isotopic variations that significantly exceed the range of tl isotope compositions observed for surface layers of fe mn crusts distributed globally 205tl 12 8 1 2 this indicates that variations in deep ocean temperature were not recorded by tl isotopes the results most likely reflect a constant tl isotope composition for seawater the growth layers of three fe mn crusts that are older than 25 ma show a systematic increase of 205tl with decreasing age from about 6 at 60 50 ma to about 12 at 25 ma these trends are thought to be due to variations in the tl isotope composition of seawater which requires that the oceans of the early cenozoic either had smaller output fluxes or received larger input fluxes of tl with low 205tl larger inputs of isotopically light tl may have been supplied by benthic fluxes from reducing sediments rivers and or volcanic emanations alternatively the tl isotope trends may reflect the increasing importance of tl fluxes to altered ocean crust through time

large climate driven changes of oceanic oxygen concentrations during the last deglaciation during the last glacial termination the solubility of gases in the ocean decreased as ocean temperatures rose however marine sediments have not unanimously recorded ocean deoxygenation throughout this time some records show increasing oxygenation since the last glacial maximum particularly in the deep sea while many document abrupt oxygenation changes often associated with apparent changes in the formation rate of north atlantic deep water here we present a global compilation of marine sediment proxy records that reveals remarkable coherency between regional oxygenation changes throughout deglaciation the upper ocean generally became less oxygenated but this general trend included pauses and even reversals reflecting changes in nutrient supply respiration rates and ventilation the most pronounced deoxygenation episode in the upper ocean occurred midway through the deglaciation associated with a reinvigoration of north atlantic deep water formation at this time the upper indo pacific ocean was less oxygenated than today meanwhile the bulk of the deep ocean became more oxygenated over the deglaciation reflecting a transfer of respired carbon to the atmosphere the observed divergence from a simple solubility control emphasizes the degree to which oxygen consumption patterns can be altered by changes in ocean circulation and marine ecosystems

geocarbsulf a combined model for phanerozoic atmospheric o2 and co2 abstract a model for the combined long term cycles of carbon and sulfur has been constructed which combines all the factors modifying weathering and degassing of the geocarb iii model berner r a kothavala z 2001 geocarb iii a revised model of atmospheric co2 over phanerozoic time am j sci 301 182 204 for co2 with rapid recycling and oxygen dependent carbon and sulfur isotope fractionation of an isotope mass balance model for o2 berner r a 2001 modeling atmospheric o2 over phanerozoic time geochim cosmochim acta 65 685 694 new isotopic data for both carbon and sulfur are used and new feedbacks are created by combining the models sensitivity analysis is done by determining 1 the effect on weathering rates of using rapid recycling rapid recycling treats carbon and sulfur weathering in terms of young rapidly weathering rocks and older more slowly weathering rocks 2 the effect on o2 of using different initial starting conditions 3 the effect on o2 of using different data for carbon isotope fractionation during photosynthesis and alternative values of oceanic 13c for the past 200 million years 4 the effect on sulfur isotope fractionation and on o2 of varying the size of o2 feedback during sedimentary pyrite formation 5 the effect on o2 of varying the dependence of organic matter and pyrite weathering on tectonic uplift plus erosion and the degree of exposure of coastal lands by sea level change 6 the effect on co2 of adding the variability of volcanic rock weathering over time berner r a 2006 inclusion of the weathering of volcanic rocks in the geocarbsulf model am j sci 306 in press results show a similar trend of atmospheric co2 over the phanerozoic to the results of geocarb iii but with some differences during the early paleozoic and for variable volcanic rock weathering lower co2 values during the mesozoic atmospheric oxygen shows a major broad late paleozoic peak with a maximum value of about 30 o2 in the permian a secondary less broad peak centered near the silurian devonian boundary variation between 15 and 20 o2 during the cambrian and ordovician a very sharp drop from 30 to 15 o2 at the permo triassic boundary and a more or less continuous rise in o2 from the late triassic to the present

a molybdenum isotope record of eocene thermal maximum 2 implications for global ocean redox during the early eocene during the early eocene a series of short term global warming events hyperthermals occurred in response to the rapid release of carbon into the oceans and atmosphere in order to investigate the response of ocean redox to global warming we have determined the molybdenum isotope compositions 98 95 mo of samples spanning one such hyperthermal eocene thermal maximum 2 etm 2 54 1ma from integrated ocean drilling program expedition 302 site m0004a in the arctic ocean the highest 98 95 mo in our sample set 2 00 0 11 corresponds to the development of local euxinia at site m0004a during the peak of etm 2 which we interpret as recording the global seawater 98 95 mo at that time the etm 2 seawater 98 95 mo is indistinguishable from a recent estimate of seawater 98 95 mo from an earlier hyperthermal paleocene eocene thermal maximum petm 55 9ma 98 95 mo 2 08 0 11 it is argued that the similarity in seawater 98 95 mo during etm 2 and the petm was caused by the development of transient euxinia in the arctic ocean during each hyperthermal that allowed sediments accumulating in this basin to capture the long term 98 95 mo of early eocene seawater our new data therefore place a minimum constraint on the magnitude of transient global seafloor deoxygenation during early eocene hyperthermals

uranium isotope systematics of ferromanganese crusts in the pacific ocean implications for the marine 238u 235u isotope system abstract variations of 238u 235u ratio 238u in sedimentary rocks have been proposed as a possible proxy for the paleo oceanic redox conditions although the marine 238u system is not fully understood here we investigate the spatial variation of 238u in modern ferromanganese fe mn crusts by analyzing u isotopes in the surface 0 3 mm depth layer of 19 fe mn crusts collected from 6 seamounts in the pacific ocean 238u values in the surface layers show little variation and range from 0 59 to 0 69 the uniformity of 238u values is consistent with the long residence time of u in modern seawater although the 238u values are lighter than that of present day seawater by 0 24 the light 238u values are consistent with the isotope offset observed in previously reported adsorption experiment of u to mn oxide these results indicate that removal of u from seawater to mn oxide is responsible for the second largest u isotope fractionation in the modern marine system and could contribute to isotopically heavy u to seawater depth profiles of u isotopes 234u and 238u in two fe mn crusts mr12 03 d06 r01 and mc10 cb07 b dated by os isotope stratigraphy were investigated to reconstruct the evolution of the oceanic redox state during the cenozoic the 238u depth profiles show very limited ranges 0 57 to 0 67 for mr12 03 d06 r01 and 0 56 to 0 69 for mc10 cb07 b and have values that are similar to those of the surface layers of fe mn crusts the absence of any resolvable variation in the 238u depth profiles may suggest that the relative amounts of oxic and reducing u sinks have not varied significantly over the past 45 myr however the 234u depth profiles of the same samples show evidence for the possible redistribution of 234u after deposition therefore the depth profile of 238u in fe mn crusts may have been also overprinted by later chemical exchange with pore water or seawater and may not reflect the paleo oceanic environmental changes to assess the potential role of u removal by mn oxides on seawater 238u we calculated seawater 238u with different u sink fluxes into mn oxides using a simple mass balance model the results of these calculations suggest that seawater 238u could have varied significantly throughout earth s history due to changes in the accumulation rate of mn oxides

sedimentary facies of glacial interglacial cycles in the norwegian sea during the last 350 ka sediment fluxes were highest in the norwegian sea during late glacial early deglacial periods i e at oxygen isotope transition 43 below transition 65 at various levels within stage 6 and below stage 9 dark diamictons deposited at these times reflect intense iceberg rafting in surface waters fed by surges along the front of the marine based parts of the continental ice sheets in the southeastern sector of the norwegian sea the high organic carbon content 0 5 1 3 in these layers reflects input from erosion of terrigenious matter rich sediments outcropping on the shelves partial oxidation of organic matter and decreased deep water renewal may explain the strong carbonate dissolution observed during these periods interglacial environments were strongly variable throughout the last 350 ka circulation patterns of stage 5e best resemble modern conditions while stage 7 and 9 sediments record a much weaker norwegian current

modeling of subsurface calcite dissolution including the respiration and reoxidation processes of marine sediments in the region of equatorial upwelling off gabon abstract mineralization of organic matter and the subsequent dissolution of calcite were simulated for surface sediments of the upper continental slope off gabon by using microsensors to measure o2 ph pco2 and ca2 in situ pore water concentration profiles of no3 nh4 fe2 and mn2 and so42 ex situ as well as sulfate reduction rates derived from incubation experiments the transport and reaction model cotrem was used to simulate the degradation of organic matter by o2 no3 fe oh 3 and so42 reoxidation reactions involving fe2 and mn2 and precipitation of fes model application revealed an overall rate of organic matter mineralization amounting to 50 mol c cm 2 yr 1 of which 77 were due to o2 17 to no3 and 3 to fe oh 3 and 3 to so42 the best fit for the ph profile was achieved by adapting three different dissolution rate constants of calcite ranging between 0 01 and 0 5 d 1 and accounting for different calcite phases in the sediment a reaction order of 4 5 was assumed in the kinetic rate law a caco3 flux to the sediment was estimated to occur at a rate of 42 g m 2 yr 1 in the area of equatorial upwelling the model predicts a redissolution flux of calcite amounting to 36 g m 2 yr 1 thus indicating that 90 of the calcite flux to the sediment is redissolved

sortable silt and fine sediment size composition slicing parameters for palaeocurrent speed and palaeoceanography fine sediment size 63 ixm is best measured by a sedimentation technique which records the whole size distribution repeated size measurement with intermediate steps of removal of components by dissolution allows inference of the size distribution of the removed component as well as the residue in this way the size of the biogenic and lithogenic noncarbonate fractions can be determined observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 ixm the dynamics of sediment erosion deposition and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10 1xm grain size and noncohesive above that size thus silt coarser than 10 ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed silt that is f mer than 10 ixm behaves in the same way as clay 2 xm useful parameters of the distribution are the 10 63 ixm mean size and the percentage 10 63 ixm in the fine fraction we cannot use size distributions to distinguish the nature of the currents therefore to infer water mass advection speeds i e the mean kinetic energy of the flow ku regions of high eddy kinetic energy ke must be avoided at the present such abyssal regions lie under the high surface k e of major current systems gulf stream kuroshio agulhas antarctic circmpolar current and brazil falldand currents in the argenthe basin this is probably a satisfactory guide for the pleistocene with regard to the carbonate subfraction of the size spectrum size modes due to both cocco iths and foramlnlferal fragments can be recognized and analyzed with the boundary between them again at about 10 lm the flux of less than 10 lm carbonate at pelagic sites above the lysocline is another candidate for a productivity indicator

using 234u 238u to assess diffusion rates of isotope tracers in ferromanganese crusts abstract ferromanganese crusts record seawater isotope history for several elements useful for paleoceanography interpreting crust isotope profiles however requires an assessment of the rate of diffusion of each element within the crust we address this issue using u and th isotope profiles coupled with concentration measurements of some trace elements whose isotopes are often measured in crusts a depth profile of 230thxs 234u for a north atlantic crust alv539 yields a growth rate of 3 05 mm myr consistent with an existing 10be 9be record but the growth rate implied by a 234u 238u profile in the same crust is 6 times faster due to diffusion of u in the crust the difference between the observed 234u 238u profile and that expected from 230thxs 234u enables the effective diffusivity of u to be calculated as 1 2 10 6 cm2 year recalculated literature data indicate that other crusts have diffusivities which range down to 4 7 10 8 cm2 yr this variation is to be expected given the range of porosity observed for ferromanganese crusts concentration measurements of various elements in the surface layer of the crust provide relative distribution coefficients for each element between seawater and crust these distribution coefficients enable the effective diffusivity of each element to be calculated by comparison with the diffusivity of u such diffusivities are assessed for crust alv539 and give a first indication of the expected preservation of isotope signals in ferromanganese crusts li os and sr have diffusivities of 2 10 4 3 10 5 and 2 10 5 cm2 year respectively and diffusion is expected to be sufficiently fast that crusts will not accurately preserve past seawater compositions th nd pb and be have diffusivities less than 10 9 cm2 year and are highly immobile in crusts a reassuring result for th and be crust chronologies and for nd and pb isotope tracing of past water masses using crusts hf has a diffusivity of 3 10 8 in alv539 fast enough to perturb but not destroy its isotope history for studies of hf in this and other crusts diffusion should therefore be considered measuring u isotope profiles provides a potential way of assessing such diffusion for hf or for other elements with relatively low distribution coefficients into ferromanganese crusts

carbonate dissolution and abyssal hydrography in the atlantic ocean abstract surface sediment samples from two regions in the eastern atlantic angola basin and cape verde basin and two regions in the western atlantic brazil basin and vema channel were examined to evaluate the relationship among carbonate dissolution abyssal hydrography and carbonate chemistry of the water column at each of these locations the position of the lysocline was determined carbonate saturation 1 was calculated and major deep water mass boundaries were identified as previously observed the correlation among these variables is strikingly different between the eastern and western atlantic in the vema channel and the brazil basin there is a first order correlation among the position of the lysocline the benthic thermocline aabw nadw boundary and the transition from saturation to undersaturation with respect to carbonate this relationship breaks down in the eastern atlantic where there is no noticeable benthic thermocline and the lysocline occurs significantly deeper than the saturation undersaturation transition