three dimensional topography correction applied to magnetotelluric data from sikkim himalayas abstract magnetotelluric mt method is one of the powerful tools to investigate the deep crustal image of mountainous regions such as himalayas topographic variations due to irregular surface terrain distort the resistivity curves and hence may not give accurate interpretation of magnetotelluric data the two dimensional 2 d topographic effects in transverse magnetic tm mode is only galvanic whereas inductive in transverse electric te mode thus tm mode responses is much more important than te mode responses in 2 d in three dimensional 3 d the topography effect is both galvanic and inductive in each element of impedance tensor and hence the interpretation is complicated in the present work we investigate the effects of three dimensional 3 d topography for a hill model this paper presents the impedance tensor correction algorithm to reduce the topographic effects in mt data the distortion caused by surface topography effectively decreases by using homogeneous background resistivity in impedance correction method in this study we analyze the response of ramp distance from topographic edges conductive and resistive dykes the new correction method is applied to the real data from sikkim himalayas which brought out the true nature of the basement in this region

groundwater electromagnetic imaging in complex geological and topographical regions a case study of a tectonic boundary in the french alps very low frequency vlf audiomagnetotelluric amt and water geochemistry surveys were performed on the sur fretes ridge in the french alps to evaluate the groundwater circulation system within the ridge at this site temporal variations of the electric field have been observed in association with water level variations of neighboring artificial lakes the sur fretes ridge is 1 km wide and trends east west water samples were collected at 52 points distributed throughout the area vlf soundings were carried out along three parallel east west profiles and 7 amt soundings were carried out along an east west profile on the top of the ridge this site is characterized by a rugged topography of the ridge where geological and topographic trends are almost perpendicular making the structure fully three dimensional we constructed a 2 d resistivity model of the ridge from 2 d and 3 d analyses of the vlf and amt data associating geology and topography models when combined with the water chemistry data a m

phosphorus cycling in sediments of the bohai and yellow seas using a box sampler surface and core sediment samples were collected from the bohai and yellow seas in 1998 1999 the sediments were sequentially extracted to determine six phosphorus fractions and non sequentially extracted for total p tp and inorganic p ip where the difference between tp and ip was assumed to be organic p op it was found that more than 70 of the total p is inorganic p the concentrations of phosphorus were high near the river mouth reflecting the influence from the huanghe and changjiang sediment loads the phosphorus levels in the bohai sea are higher than those in the yellow sea which is related to the sediment sources and distribution of chlorophyll a in these areas concentrations of tp in the bohai and yellow seas are comparable to the other worldwide coastal areas among the six forms of phosphorus phosphorus distribution ranges were 0 8 2 for loosely sorbed p 3 10 for iron bound inorganic p 4 19 for leachable organic p 3 15 for authigenic apatite 35 66 for detrital apatite and 14 36 for refractory organic p potentially bioavailable phosphorus accounts for a small amount of tp while phosphorus load associated with eroded soil may be larger than the anthropogenic load the regeneration of dissolved p from sediments is a slow process which could play a role in maintaining low phosphate levels in the seas most of the phosphorus was regenerated in a water column and almost all of the remaining phosphorus was buried in the sediments after its accumulation the high values of p burial efficiencies in the study areas were related to low benthic phosphate flux which is due to the high percent of detrital apatite that is not bioavailable and the high sediment accumulation rate

phosphorus cycling in marine sediments from the continental margin off namibia abstract in this study we investigate benthic phosphorus cycling in recent continental margin sediments at three sites off the namibian coastal upwelling area examination of the sediments reveals that organic and biogenic phosphorus are the major p containing phases preserved high corg porg ratios just at the sediment surface suggest that the preferential regeneration of phosphorus relative to that of organic carbon has either already occurred on the suspension load or that the organic matter deposited at these sites is already rather refractory release of phosphate in the course of benthic microbial organic matter degradation cannot be identified as the dominating process within the observed internal benthic phosphorus cycle dissolved phosphate and iron in the pore water are closely coupled showing high concentrations below the oxygenated surface layer of the sediments and low concentrations at the sediment water interface the abundant presence of fe iii bound phosphorus in the sediments document the co precipitation of both constituents as p containing iron oxyhydr oxides however highly dissolved phosphate concentrations in pore waters cannot be explained neither by simple mass balance calculations nor by the application of an established computer model under the assumption of steady state conditions phosphate release rates are too high as to be balanced with a solid phase reservoir this discrepancy points to an apparent lack of solid phase phosphorus at sediment depth were suboxic conditions prevail we assume that the known active fast and episodic particle mixing by burrowing macrobenthic organisms could repeatedly provide the microbially catalyzed processes of iron reduction with authigenic iron oxyhydro oxides from the oxic surface sediments accordingly a multiple internal cycling of phosphate and iron would result before both elements are buried below the iron reduction zone

solid phase phosphorus pools in highly organic carbonate sediments of northeastern florida bay currently few studies have investigated sediment phosphorus p pools or identified the chemical processes important in the p cycle of fine grained carbonate sediments particularly in coastal estuaries with high organic matter to determine the role of fine grain calcium carbonate and high organic matter on sedimentary p we investigated the solid phase p pools in seagrass sediments of north eastern ne florida bay at the bay mangrove ecotone sediments were fractionated by sequential extractions into seven chemically defined groups exchangeable inorganic and organic p reducible inorganic and organic p fe bound acid extractable inorganic and organic p ca bound and residual organic p calcium bound p accounted for approximately 56 of total p and 96 of inorganic p our total calcium bound p was in the range 34 151 gpg 1 reported for coarse grained low organic sediments while the organic p associated with this fraction was slightly 10 higher than those reported for other carbonate systems the second dominant p fraction was residual organic p 30 71 gpg 1 accounting for 42 of tp this high residual pool suggests the importance of fringing mangrove and seagrass detritus in long term p storage in contrast to temperate estuaries the iron bound p fraction in ne florida bay sediments was low 70 gg 1 at the surface and undetectable 1 gg 1 below 10 cm based on our findings we hypothesize that carbonate chemisorption reactions along with potential reactive organic surfaces and the sequestering of p into recalcitrant organic pools maintain low exchangeable and pore water p concentrations across the ne florida bay estuary and account for the reported autotrophic p limitation 2001 academic press

cycling of phosphorus maintains the production of microphytobenthic communities in carbonate sediments of a coral reef the phosphorus p cycle that maintains the production of microphytobenthic communities in carbonate sediments was investigated in the fringing shiraho coral reef system located off the east coast of ishigaki island japan production rates of the diatom dominated microphytobenthic communities and the p flux at the sediment water interface were determined by in situ benthic enclosure experiments the daily net production 6 9 24 mmol c m 22

a side scan sonar and high resolution chirp sub bottom profile study of the natural and anthropogenic sedimentary record of lower lough erne northwestern ireland a geophysical survey side scan sonar and high resolution sub bottom profiling combined with established chrono and bio stratigraphic markers are used to reconstruct the evolutionary history of lower lough erne northwestern ireland since the end of the last glacial discrete acoustic units define four prominent sedimentation cycles one late glacial and three holocene within the seismic stratigraphy basin development is predominantly controlled by climate until the mid holocene when the effects of cultural activity are related to pronounced signatures in the seismic record an enhanced reflector marking the termination of the early holocene sedimentation cycle is associated with the elm decline and forest clearance during the neolithic between approximately 5800 cal bp and 5300 cal bp the associated increase in sediment input by infilling minor topographic lows and basins has a smoothing effect on the morphologic expression of the original basin onlapping and truncated reflectors in the penultimate acoustic unit define further modification of lake bed relief by removal of sediment from the system during engineered lowering of the lake levels that began in the 1880s acoustic anomalies identified on the side scan sonar data are linked with cultural activity and are identified as having the potential to facilitate the recent establishment and migration of invasive zebra mussels through the erne system c 2005 elsevier ltd all rights reserved

accumulation and biological mixing of peru margin sediments abstract rates of sediment accumulation and biological particle mixing in eight cores from the peru shelf have been estimated by application of uranium series disequilibrium techniques and radiocarbon dating activities of 210pb 226ra 234th and 238u have been determined in closely spaced intervals in each sediment core biological particle mixing coefficients were determined via a two box advection diffusion steady state mixing model based on the distribution profiles of excess 210pb and in some cases excess 234th activities the sedimentation rates estimated were in the order of a few millimeters yr except for cores enriched in phosphatic components which displayed rates a few orders of magnitude slower bioturbation was significant in most cores studied with the exception of one core collected from within the middle of the oxygen minimum zone estimated biological mixing coefficients were in the order of 100 101 cm2 yr 1 for the surface mixed layer and 10 1 100 cm2 yr 1 for the layer below sediment slumping appears to have affected the 210pb distribution of two or three of our cores

evolution of the academician ridge accommodation zone in the central part of the baikal rift from high resolution reflection seismic profiling and geological field investigations new high resolution seismic reflection data from the central part of lake baikal provide new insight into the structure and stratigraphy of academician ridge a large intra rift accommodation zone separating the central and north baikal basins four seismic packages are distinguished above the basement a thin top of basement unit seismic stratigraphic unit x seismic stratigraphic unit a and seismic stratigraphic unit b units a and b were cored on selected key locations the four packages are correlated with a series of deposits exposed on the nearby western shores the ularyar sequence oligocene the tagay sequence lower to middle miocene the sasa sequence upper miocene to lower pliocene the kharantsy sequence upper pliocene and the nyurga sequence lower pleistocene based on stratal relationships sedimentary geometries distribution patterns and principal morphostructural elements both onshore and offshore we propose a new palaeogeographic evolution model for the area in this model progressive tectonic subsidence of the baikal basins and successive pulses of uplift of various segments of the rift margins lead to a formation of the ridge as a structural and morphological feature separating the central and north baikal basins during the middle to late miocene b gradual flooding of the main parts of the ridge and establishment of a lacustrine connection between the two rift basins during the late miocene and c total submergence of the top parts of the crest of the ridge during the latest pleistocene this new model helps to better constrain numerous phases in the structural evolution of the baikal rift in which the academician ridge as an accommodation zone plays a crucial role

vivianite is a key sink for phosphorus in sediments of the landsort deep an intermittently anoxic deep basin in the baltic sea phosphorus p is an essential nutrient for marine organisms its burial in hypoxic and anoxic marine basins is still incompletely understood recent studies suggest that p can be sequestered in sediments of such basins as reduced iron fe p but the exact phase and the underlying mechanisms that lead to its formation are unknown in this study we investigated sediments from the deepest basin in the baltic sea the landsort deep site m0063 that were retrieved during the integrated ocean drilling project iodp baltic sea paleoenvironment expedition 347 the record comprises the whole brackish marine littorina sea stage including past intervals of extensive hypoxia in the baltic sea that occurred during the holocene thermal maximum htmhi and the medieval climate anomaly mca1hi and mca2hi various redox proxies e g the presence of laminations and high mo contents suggest almost permanent bottom water hypoxia during the littorina sea stage in the landsort deep the bottom waters were likely even seasonally anoxic or sulfidic during the mca1hi and mca2hi and permanently sulfidic during the htmhi with the use of micro analysis of sieved minerals sem eds xrd and synchrotron based xas we show that mn and mg rich vivianite crystals are present at various depths in the littorina sea sediments we also have indications for vivianite in the mca1hi mca2hi and htmhi deposits the formation of vivianite thus likely explains the high fe bound p fraction throughout the whole littorina sea stage shuttling of fe and mn from the shelves into the basin and high inputs of p in settling organic matter are likely key drivers for vivianite formation our study shows that vivianite can likely form in near surface sediments under a broad range of bottom water redox conditions varying from hypoxic and anoxic to sulfidic

first study of fossil diatom flora from middle miocene lower pliocene lacustrine sediments in barguzinvalley baikal rift zone abstract a subsequent formation of oligotrophic tectonic paleo lakes bodon 1 and bodon 2 is reconstructed through study of fossil diatom flora in sediments of the upper stratum of the tankhoy formation in the south of barguzin valley the first lake is characterized by the development of middle late miocene planktonic species aulacoseira sp 1 and 2 diatom zones 1 and 2 then of the species pseudoaulacoseira moisseeviae diatom zone 3 the second lake is identified due to early pliocene species a islandica a anadensis and eunotia polyglyphoides diatom zone 4 the diatom flora of zone 3 of paleo lake bodon 1 is correlated to similar zone in paleo lakes in vitim plateau it was assumed that these lakes were formed simultaneously with the paleo lake mukhal on the vitim plateau ca 13 12 million years ago and the paleo lake yuzhnoe ca 10 9 million years ago and later

understanding lotem data from mountainous terrain long offset transient electromagnetic lotem data from the vesuvius volcano in italy show that the em response of the topography is a potential cause of data distortions a modeling study was carried out to simulate the effect of mountainous terrain on vertical magnetic field time derivatives using a 3 d finite difference code the objectives were to assess the importance of topographic effects and to help identify them in existing field data the total effect of topography on the lotem response can be considered as a combination of four distortions of the corresponding responses for a flat terrain first the receiver is at some height above the flat surface second the mountain acts as a conductive body displacing air third large loop receivers are nonhorizontal and sense a combination of horizontal and vertical magnetic fields finally the electromagnetic coupling between the mountain and deeper lying structure modifies the structure response each of the effects can be identified in field data

meso cenozoic building of the northern central asian orogenic belt thermotectonic history of the tuva region abstract the tuvinian and west sayan mountain ranges form part of the central asian orogenic belt caob more specifically they align along the altai sayan hangay zone its precambrian paleozoic basement has been subjected to mesozoic and cenozoic tectonic reactivation two north south transects across the mountain belts and intervening basins of tuva were sampled for apatite fission track aft thermochronology in order to elucidate the thermal history of the tuvinian basement in relation to the mesozoic and cenozoic reactivation of the caob most aft ages are late cretaceous and range between 55 and 115 ma mean lengths of confined fission tracks are relatively long with most values between 13 and 14 m thermal history modeling shows a rapid late jurassic cretaceous cooling for the sampled tuvinian crystalline rocks related to exhumation of the paleozoic basement this exhumation is possibly related to the building and subsequent orogenic collapse of the mongol okhotsk orogen that formed between the siberian and north china mongolian sino korean or amurian continental blocks during the late mesozoic far field effects of this orogeny and its collapse might have affected the baikal altai and sayan units of the central asian orogenic belt including the tuvinian basement also at the mesozoic southern eurasian margin growth of the asian continent continued and several collision accretion events asserted distal tectonic influence into the caob after a paleogene period of stability thermal history models for some samples hint at a renewed period of basement cooling during the neogene in support of this neogene event a single sample from the main west sayan fault zone contains an apatite population with 2 ma reset aft ages this is interpreted in the framework of ongoing building of the modern central asian orogens and associated fault movements and exhumation of the basement presumably related with the ongoing india eurasia convergence

fluorine mobility during early diagenesis of carbonate sediment an indicator of mineral transformations abstract the abundant occurrence of calcium carbonate minerals in marine sediment their well documented dissolution or precipitation during diagenesis and their high f content suggest that carbonate mineral diagenesis may be an important influence on f behavior in marine sediment to test this hypothesis we have examined the geochemistry of f in shallow carbonate mud bank sediment of florida bay usa the f content of biogenic calcium carbonate in florida bay varies with mineralogy positively correlates with the mg content of calcite and occurs in similar abundance to sr high mg calcite f sr 2 4 mol mol aragonite 0 53 0 79 mol mol low mg calcite 0 22 mol mol models of porewater distributions direct solute flux measurements and reaction rate estimates over the upper 0 16 cm of sediment from bob allen key bank predict net fluxes out of the sediment of 170 mol f m2 d 230 mol sr m2 d and 5 6 mmol ca m2 d the net solute flux ratio of f sr 0 71 mol mol is consistent with dissolution of aragonite or high mg calcite but f ca and sr ca ratios 28 mmol mol and 40 mmol mol respectively are 3 10 that of biogenic carbonate sources selective dissolution of a high f and sr content phase or more likely based on dissolution experiments concurrent dissolution and reprecipitation of phases with different f and sr contents account for this discrepancy the loss of f sr and ca to fluorapatite precipitation as predicted from a stoichiometric model of phosphate release can be added to the transport reaction model predicted net fluxes a resulting more complete mass balance model incorporating both carbonate mineral reactions and fluorapatite formation yields total release estimates of f sr and ca of 770 mol 332 mol and 53 mmol m2 d respectively calcium carbonate minerals apparently undergo transformation whereby 34 mmol high mg calcite m2 d and 19 mmol aragonite m2 d dissolve and 46 mmol low mg calcite m2 d precipitates ignoring other cations besides ca if no fluorapatite formation occurs the required fluxes are that 23 mmol aragonite m2 d dissolves and 3 4 mmol high mg calcite m2 d and 14 mmol low mg calcite m2 d precipitate net loss of caco3 from the deposit by dissolution is 6 7 mmol m2 d 3 of the accumulation flux the magnitude of these fluxes could cause significant mineralogical and chemical changes on rapid time scales in nearshore carbonate sediment fluorine is probably mobile in other carbonate deposits undergoing diagenetic alteration on short and longer time scales and is a powerful additional constraint on the rates and nature of carbonate mineral diagenesis

global seawater redox trends during the late devonian mass extinction detected using u isotopes of marine limestones abstract the late devonian extinction ranks as one of the big five phanerozoic extinctions affecting up to 80 of marine species and is characterized by two major extinction events that are separated by 238 u across a 7 my interval of well dated upper devonian marine limestones from the devil s gate formation in nevada usa the measured 238 u curve shows no co variation with local sediment derived redox trends water depth dependent facies changes redox sensitive metals toc or diagnostic elemental ratios from these relationships we infer that the 238 u curve represents global seawater redox conditions two negative 238 u excursions indicating more reducing seawater are observed with durations of 3 8 my late frasnian and 1 1 my early famennian respectively steady state modeling of the observed 0 2 to 0 3 shifts in 238 u points to a 5 15 increase in the total area of anoxic seafloor during these excursions the late frasnian negative excursion is broadly coincident with the first extinction event late rhenana zone or lower kellwasser event whereas the early famennian negative excursion lower middle triangularis zones occurs after the most intense frasnian famennian boundary f f extinction event upper kellwasser event compilations of local sediment redox conditions from upper devonian marine deposits with conodont zone level age control indicates that the extinction events were coincident with widespread anoxic deposits accumulating in subtropical epeiric sea and some open ocean settings supporting previous interpretations that widespread marine anoxia had an important influence on the late devonian extinctions the temporal relationships between global ocean redox trends represented by the 238 u curve and the newly compiled subtropical marine redox sediment trends indicates that local epeiric seawaters carried a similar u isotopic composition as the open ocean for the majority of the studied interval except for a brief interval

crustal structure of central lake baikal insights into intracontinental rifting this is the published version copyright 1976 american geophysical union all rights reserved

dissolved inorganic phosphorus export to the coastal zone results from a spatially explicit global model 1 here we describe test and apply a spatially explicit global model of river borne dissolved inorganic phosphorus dip export called news dip among the innovations in news dip are increased spatial resolution 0 5 0 5 explicit treatment of sewage fertilizer manure and weathering p sources and inclusion of reservoir retention and consumptive water use terms the news dip model performed better than pre existing global models in predicting dip yield for both calibration and validation basins r2 0 72 and 0 56 respectively news dip predicts that of the 34 tg of p yr 1 loaded on watersheds by human activity globally approximately 3 reaches river mouths as dip anthropogenic sources account for 65 0 71 tg yr 1 of the dip exported to the coastal zone with the remainder 0 38 tg yr 1 attributable to natural weathering processes dip yields range over 5 orders of magnitude from less than 0 01 to 1153 kg p km 2 yr 1 with highest predicted dip yields clustering in east asia europe and indonesia human sewage is the largest anthropogenic source of dip to the coastal zone on all continents and to all ocean basins news dip also suggests that despite regional variability at the global scale non point sources of dip such as inorganic p fertilizer and manure are much less important in determining coastal export of dip than point sources and natural weathering processes

quaternary sedimentation and subsidence history of lake baikal siberia based on seismic stratigraphy and coring the long continuous high latitude stratigraphic record of lake baikal was deposited in three broad sedimentary environ ments defined by high resolution seismic reflection and coring meth ods 1 turbidite depositional systems by far the most widespread characterizing most of the margins and floors of the main basins of the lake 2 large deltas of major drainages and 3 tectonically or topographically isolated ridges and banks holocene sedimentation rates based on radiocarbon ages vary by more than an order of mag nitude among these environments from less than about 0 03 mm yr on ridges and banks to more than about 0 3 mm yr on basin floors ex trapolating these rates with a correction for compaction yields ten tative estimates of about 25 and 11 ma for the inception of rifting in the central and north basins respectively and less than 6 ma for the 200 m sediment depth on academician ridge the selenga delta has the distinctive form of a classic prograding gilbert type delta but its history appears to represent a complex com bination of tectonism and sedimentation the central part of the delta is underlain by prograding shallow water sequences now several hun dred meters below the lake surface these deposits and much of the delta slope are mantled by fine grained deep water hemipelagic de posits whose base is estimated to be about 650 000 years old modern coarse grained sediment bypasses the delta slope through fault con trolled canyons that feed large subaqueous fans at the ends of the south and central basins these relations along with abundant other evidence of recent faulting and the great depths of the central and south basins suggest that these two rift basins have experienced a period of unusually rapid subsidence over the last 650 000 years dur ing at least part of which sedimentation has failed to keep pace

evidence of iron deficiency in seagrasses growing above carbonate sediments a comparison of available data on iron concentration in seagrass tissues shows iron concentrations in seagrass leaves growing on carbonate sediments to be below critical levels for angiosperms 100 g fe g dw 1 iron concentrations of leaves in caribbean seagrasses declined by 5 10 fold from terrigenous to carbonate sediments inner gulf of mexico to the caribbean coast off the yucatan peninsula this observation provided evidence ofpossible iron deficiency in seagrasses which was confirmed by an experimental demonstration that adding iron to the sediments stimulates growth of the caribbean seagrass thalassia testudinum and increases the chlorophyll a concentration of both t testudinum and syringodium filiforme growing above coralline carbonate sediments we conclude that seagrasses growing above carbonate sediments are likely to experience iron deficiency

the marine phosphorus cycle phosphorus introduced to the ocean must ultimately be removed if a steady state is to be maintained over geologic time aeolian input is insignificant and there is evidently no hydrothermal contribution rivers dominate p input to the ocean the fluvial p flux is poorly quantified due to the interactions of two factors 1 man s influence on river nutrient fluxes and 2 inherent complexities in understanding the biological and inorganic associations of p in estuarine and coastal waters our best estimate of the natural or pre man p input flux is about 10 x 10 sup 9 moles p cm sup 2 ocean surface year of the identified flux of phosphorus to the sediments about 90 percent is in the form of biogenic debris or its regeneration products the burial of phosphatic fish debris is negligible p removal by direct reaction of phosphate in seawater with sea floor basalts during mid ocean ridge hydrothermal circulation is insignificant p removal by processes such as sorption on clay surfaces or formation of authigenic fe and al phosphate phases is presently unquantified and remains a serious uncertainty in the phosphorus cycle the identified sources and sinks approximately balance yielding a residence time for p in the oceans of about 10 sup 5 yrs

phosphate regeneration from sediments of the peru continental margin by dissolution of fish debris generally oxidative regeneration of phosphate from anoxic sediments is by microbially mediated sulfate reduction processes stoichiometric modelling of such reactions takes into consideration varying proportions of decomposable organically bound p to account for the ratios among nutrients in depth concentration profiles of near surface sediments new results of interstitial water composition from sediments underlying the water masses influenced by coastal upwelling of the eastern boundary current system off peru indicate that dissolution of phosphatic fish debris represents a mechanism for remineralization of phosphate comparable to or larger in magnitude than that by oxidative regeneration of organically bound p dissolved interstitial phosphate from fish debris is revealed by an excess amount of phosphate over that predicted from a simple stoichiometric oxidative regeneration model and by anomalously high dissolved interstitial fluoride concentrations phosphate flux estimates based on diffusion from the sediment suggest that this mechanism may generate up to 10 of the nutrient pool in the waters of the peru undercurrent partitioning of p among the two sources reveals further that fish debris phosphate is about four times more important than organically bound p in nutrient generation from sediments of the peru continental margin not only does this mechanism of regeneration affect the nutrient cycling but may also control widespread phosphorite formation in this area

acoustic stratigraphy of bear lake utah idaho late quaternary sedimentation patterns in a simple half graben a277 kmnetworkofhigh resolutionseismic reflectionprofiles supplementedwithasidescan sonarmosaicofthelakefloor was collected in bear lake utah idaho in order to explore the sedimentary framework of the lake s paleoclimate record the acoustic stratigraphy is tied to a 120 m deep continuously cored drill hole in the lake based on the age model for the drill core the oldest continuously mapped acoustic reflector in the data set has an age of about 100 ka although older sediments were locally imaged the acoustic stratigraphy of the sediments below the lake indicates that the basin developed primarily as a simple half graben with a steep normal fault margin on the east and a flexural margin on the west as expected for a basin controlled by a listric master fault seismic reflections steepen and diverge toward the fault bounding eastward thickening sediment wedges secondary normal faults west of the master fault were imaged beneath the lake and many of these faults show progressively increasing offset with depth and age several faults cut the youngest sediments in the lake as well as the modern lake floor the relative simplicity of the sedimentary sequence is interrupted in the northwestern part of the basin by a unit that is interpreted as a large 410 km paleodelta of the bear river the delta overlies a horizon with an age of about 97 ka outcrops at the lake floor and is onlapped by much of the uppermost sequence of lake sediments a feature interpreted as a wave cut bench occurs in many places on the western side of the lake the base of this bench occurs at a depth 22 24 m similar to that 20 25 m of the distal surface of the paleodelta pinch outs of sedimentary units are common in relatively shallow water on the gentle western margin of the basin and little holocene sediment has accumulated in water depths of less than 30 m on the steep eastern margin of the basin sediments commonly onlap the hanging wall of the east bear lake fault however no major erosional or depositional features suggestive of shoreline processes were observed on acoustic profiles in water deeper than about 20 25 m d 2005 elsevier b v all rights reserved

magnetotelluric images of the crust and mantle in the southwestern taurides turkey inversion of the magnetotelluric data across the southwestern taurides reveals two subzones of crust with varying thicknesses conductive lower crust b75 v m overlain by resistive n350 v m upper crust with four resistive cores n2000 v m separated by three relatively conductive vertical zones the first and second vertical zones coincide with surface faults interpreted in anatolia suchasfethiyeburdurfaultzone thethirdoneisthemostconductiveandliesin continuitywiththestrabofaultzone in the mediterranean sea a hypocentral cross section of earthquakes along the profile shows more dense seismic activity in the second resistive core where the conductive crust is not present beneath it the depth of the crust upper mantle boundary varies between 30 and 50 km and has an undulating character the resistivity of the upper mantle reaches 500 1000 v m d 2004 elsevier b v all rights reserved

total alkalinity flux in coral reefs estimated from eddy covariance and sediment pore water profiles ocean acidification decreases the ph of seawater and the seawater saturation state with respect to caco3 minerals in the event of ocean acidification mg calcite is considered to be the first mineral to dissolve dissolution of mg calcite is more prevalent at depth in the sediment than at the sediment interface because of production of co2 resulting from microbial decomposition of organic matter rates of caco3 dissolution can be estimated from total alkalinity at fluxes calculated from concentration gradients and diffusion coefficients we estimated at flux in a sandy area of the shiraho coral reef under natural hydrodynamic conditions using eddy covariance and sedimentary at profiles the calculated nighttime at flux at the sediment water interface was 0 4 2 6 mmol m 2 h 1 analysis of the sedimentary profile at a depth of 0 20 mm indicated that respiration by organisms consumed oxygen and produced co2 during night and that photosynthesis enhanced o2 concentrations during the day however dissolved oxygen was depleted at all times in sediments deeper than 20 mm the pore water aragonite saturation state a was constant at 3 0 which is equivalent to a value of 1 0 for the saturation state with respect to foraminifera fora as determined in a previous study both organic reactions e g respiration and inorganic mg calcite dissolution occur in the sediment leading to a constant fora value in the sediment these data confirm the metastable equilibrium of pore water with respect to mg calcite from foraminifera which is the most soluble phase in the sediment

a comparison of the geological and geoelectrical structures in the eastern marmara region nw turkey abstract northwestern anatolia contains different tectonic units which are presently bounded by the two branches of the north anatolian fault the pontide zone in the north consists of the istanbul zonguldak unit to the west and the ballidag kure unit a remnant of paleo tethyan ocean in the east the sakarya zone in the south is juxtaposed against the pontide zone due to the closure of paleo tethys the armutlu ovacik zone represents a tectonic mixture of both zones in the north and south neotectonic units such as the izmit sapanca corridor and the northern sakarya low which developed south of the istanbul zone and north of the sakarya zone respectively represent an extensional tectonic regime the relationship and continuations of these tectonic entities in the region were examined by a 90 km long nw se trending magnetotelluric profile electrical resistivity is quite low 10 ohm m beneath the northern sakarya low and the sakarya zone at a depth of 26 km compared to the northern areas this low resistivity zone may provide an indication of thinning in the total crustal thickness and a viscoelastic lower crust furthermore in the northern part of the profile the five layered conductive sequence obtained by magnetotelluric modelling may correspond to fragments of the istanbul zone and the ballidag kure unit underlying it

rates of carbonate dissolution in permeable sediments estimated from pore water profiles the role of sea grasses in this study we estimate sediment carbonate dissolution rates for sandy sea grass sediments on the bahamas bank using an inverse pore water advection diffusion reaction model constrained by field observations this model accounts for sea grass o2 input to these sediments and also parameterizes pore water advection through these permeable sediments as a nonlocal exchange process the resulting rates of carbonate dissolution are positively correlated with sea grass density and are comparable with previous rate estimates for florida bay sediments in contrast the advective uptake of o2 by these sediments decreased with increasing sea grass density this suggests that the competing interplay between bottom water flow near seabed pressure gradients and the presence of a sea grass canopy is important in controlling this type of sediment oxygen uptake when the carbonate dissolution rates estimated here are examined in the context of carbonate budgets for shallow water carbonate platforms systems they suggest that carbonate dissolution may be a significant loss term in these budgets sea grass mediated carbonate dissolution may also exert a negative feedback on rising atmospheric co2 although the magnitude of this effect remains to be quantified

stratigraphic and structural evolution of the selenga delta accommodation zone lake baikal rift siberia seismic reflection profiles from the lake baikal rift reveal extensive details about the sediment thickness structural geometry and history of extensional deformation and syn rift sedimentation in this classic continental rift the selenga river is the largest single source of terrigenous input into lake baikal and its large delta sits astride the major accommodation zone between the central and south basins of the lake incorporating one of the world s largest lacustrine deltas this depositional system is a classic example of the influence of rift basin structural segmentation on a major continental drainage more than 3700 km of deep basin scale multi channel seismic reflection mcs data were acquired during the 1989 russian and the 1992 russian american field programs the seismic data image most of the sedimentary section including pre rift basement in several localities the mcs data reveal that the broad bathymetric saddle between these two major half graben basins is underlain by a complex of severely deformed basement blocks and is not simply a consequence of long term deltaic deposition maximum sediment thickness is estimated to be more than 9 km in some areas around the selenga delta detailed stratigraphic analyses of the selenga area mcs data suggest that modes of deposition have shifted markedly during the history of the delta the present mode of gravity and mass flow sedimentation that dominates the northern and southern parts of the modern delta as well as the pronounced bathymetric relief in the area are relatively recent developments in the history of the lake baikal rift several episodes of major delta progradation each extending far across the modern rift can be documented in the mcs data the stratigraphic framework defined by these prograding deltaic sequences can be used to constrain the structural as well as depositional evolution of this part of the baikal rift an age model has been established for this stratigraphy by tying the delta sequences to the site of the baikal drilling project 1993 drill hole although the drill hole is only 100 m deep and the base of the cores is only 670 ka in age ages were extrapolated to deeper stratigraphic intervals using the reflection seismic radiocarbon method of cohen et al 1993 the deep prograding delta sequences now observed in the mcs data probably formed in response to major fluctuations in sediment supply rather than in response to shifts in lake level this stratigraphic framework and age model suggest that the deep delta packages developed at intervals of approximately 400 ka and may have formed as a consequence of climate changes affiliated with the northern hemisphere glaciations the stratigraphic analysis also suggests that the selenga basin and syncline developed as a distinct depocentre only during the past 2 3 ma

three dimensional topography corrections of magnetotelluric data summary topographic effects due to irregular surface terrain may prevent accurate interpretation of magnetotelluric mt data three dimensional 3 d topographic effects have been investigated for a trapezoidal hill model using an edge finite element method the 3 d topography generates significant mt anomalies and has both galvanic and inductive effects in any polarization this paper presents two different correction algorithms which are applied to the impedance tensor and to both electric and magnetic fields respectively to reduce topographic effects on mt data the correction procedures using a homogeneous background resistivity derived from a simple averaging method effectively decrease distortions caused by surface topography and improve the quality of subsurface interpretation nonlinear least squares inversion of topography corrected data successfully recovers most of structures including a conductive or resistive dyke

sources deposition rates and decomposition of organic carbon in recent sediment of sachem head harbor long island sound the vertical distributions of excess 210pb and fall out 239 240pu imply a uniform sedimentation rate of 1 4 1 6 cm year 1 from 0 to 105 110 cm this sediment accumulation rate is compatible with sulfate reduction rate data from this location below 70 cm only aged refractory carbon is present cr 1 8 c with an age of approximately 2400 years this phase is present in a number of locations across long island sound planktonic carbon cp is present above the 60 67 cm horizon a value of 1 0 for ap 14c activity at 32 37 cm was taken ap 1 285 was used for contemporary plankton this was obtained by correcting the measured ap of a plankton tow sample for admixed refractory carbon these values were then used to calculate cr cp and cf fossil carbon at 32 37 cm and 6 12 cm the only values compatible with the known sulfate reduction rate data are cr equal to pre 60 67 cm levels 1 6 1 8 c cf being 0 3 c at both depths and cp decreasing with depth from 0 3 to 0 4 c at 6 12 cm to close to zero at 32 37 cm

the diagenesis of phosphorus in a nearshore marine sediment abstract chemical analyses have been made for total inorganic phosphorus total organic phosphorus and organic carbon in plankton and in anoxic sediments from long island sound u s a when combined with laboratory experiments the measurements of other studies on the same sediments and mathematical modelling the results indicate that 1 1 a large proportion of dissolved phosphate in the upper 10 cm of sediment zone of bioturbation is provided by the release of adsorbed phosphate during the reduction of ferric oxyhydroxides 2 2 below the zone of bioturbation phosphate is liberated to solution solely via organic matter decomposition and the release is stoichiometrically coupled to bacterial sulfate reduction and ammonia formation 3 3 a steady state diagenetic model can be used to predict the profile of organic phosphorus from that of dissolved phosphate at depths below the zone of bioturbation 4 4 the flux of dissolved phosphate out of the sediment must arise largely from phosphate liberation occurring very close 5 5 there is a large preferential loss of phosphorus relative to carbon from organic matter at the time of or prior to burial whereas after burial phosphorus is not lost preferentially at this location

controls on gravel termination in seven distributary channels of the selenga river delta baikal rift basin russia the selenga river delta lake baikal russia is 600 km2 in size and contains multiple distributary channels that receive varying amounts of water and sediment discharge the delta is positioned along the deep water 1600 m margin of lake baikal a half graben styled rift basin qualifying it as a modern analogue of a shelf edge delta system this study provides a detailed field survey of channel bed sediment composition channel geometry and water discharge the data and analyses presented here indicate that the selenga delta exhibits downstream sediment fining over tens of kilometers ranging from predominantly gravel coarse pebble and sand near its apex to silt and sand at the delta lake interface we developed an analytical framework to evaluate the downstream elimination of gravel within the multiple distributary channels the findings include the following 1 the selenga river delta consists of at least eight orders of distributary channels 2 with increasing channel order downstream channel cross sectional area width depth ratio water discharge boundary shear stress and sediment flux systematically decrease 3 the downstream elimination of gravel in distributary channels is caused by declining boundary shear stress as a result of water discharge partitioning among the bifurcating channels 4 over longer time scales gravel is contained on the delta topset due to frequent and discrete seismic events that produce subsidence and accommodation so that coarse sediment cannot be transported to the axis of the baikal rift basin the distribution of sediment grain size in deltaic channels as related to hydrodynamics and sediment transport plays a critical role in influencing stratigraphy because the sustained tectonism leads to high preservation potential of the delta topset sedimentary deposits therefore the selenga river delta provides an opportunity to explore the interactions between modern deltaic sedimentation processes and tectonics that affect the production of basin stratigraphy

spatial distribution of sedimentary p pools in a mediterranean coastal lagoon albufera d es grau minorca island spain the method for the sequential extraction of p proposed by jensen et al mcglathery k j marino r howarth r w 1998 limnol oceanogr 43 799 810 was used to study the spatial distribution of sedimentary p in superficial sediments of a mesohaline coastal lagoon located in a watershed formed by carboniferous sandstone in the western mediterranean minorca island spain dissolved inorganic phosphate ip dissolved organic phosphate op fe ca al and f were analyzed in the extractions to assess adequacy of this method to clay sediments the elemental composition of the solid phase al fe k ti si mg and ca was also analyzed to relate concentrations of p pools to the mineral composition samples from marine carbonated sediments rock and some materials of biological origin tubes of polychaete ficopomatus enigmaticus bivalve shell debris were analyzed for comparative purposes the sequential procedure allowed to extract almost all sedimentary p from carbonate sediments and biogenic debris but only 70 of total phosphorus tp from clay sediments and rock main ip pools in the lagoon were fe bound p 16 6 of tp caco3 bound p mainly from ficopomatus tubes 12 0 of tp and detrital carbonate fluorapatite 7 8 of tp the most abundant p pool was refractory p 20 3 of tp which appeared associated to the fe k al silicate fraction and to humic p this indicated that clay humic organic p complexes were the main p reservoir in the lagoon spatial distribution of p pools reflected differential sedimentation of allochthonous materials authigenic precipitation of fe oxides and fe bound p as well as the differential distribution of organisms such as ficopomatus

atypical heat flow near gas hydrate irregularities and cold seeps in the baikal rift zone abstract in this paper we address the irregular behaviour and geometry of the gas hydrate stability zone hsz inferred from reflection seismic data in relation to heat flow measurements the study area lies in the hanging wall of the posolsky fault in the southern baikal basin sbb side scan sonar imagery already revealed an undulating antithetic active fault structure and several isolated active vent structures remarkably these fluid discharge structures occur only where the base of the hydrate stability zone bhsz as inferred from seismic reflection profiles is fluctuating and discontinuous independent of lake floor morphology the correlation between the interpreted bhsz and heat flow data across the malenki seep is reasonable on a seismic profile south of the fluid escape features the bhsz is expressed as an oscillatory but continuous reflection and shows poor correlation with heat flow measurements in nearly all cases measured heat flow exceeds inferred heat flow additionally the local inferred minima are anomalously low compared to the expected background values in the sbb these observations suggest that the present day hydrate accumulation and its meta stability are more complicated than originally suspected the limited area of these anomalies their amplitudes and their occurrence in the immediate vicinity of faults and fluid escape features suggest that fluid convection cells disturb local gas hydrate stability conditions

pore waters of the central pacific ocean nutrient results interstitial waters from the manop siliceous and calcareous ooze sites in the central pacific ocean were recovered by shipboard squeezing and centrifuging techniques and an in situ harpoon sampler the pore waters were analyzed for nutrients carbonate system species mn 2 ca 2 total alkaline earth metals and dissolved oxygen siliceous ooze site only in calcareous sediments the co 2 alkalinity po 4 3 and ca 2 results obtained from shipboard techniques are significantly different than those recovered in situ the other species measured as well as all of the data from the siliceous ooze site reveal no significant dependence on the method used to sample the pore water the relative changes in o 2 no 3 mn 2 alkalinity and co 2 indicate that respiration using o 2 no 3 and mno 2 as electron acceptors is the dominant reaction affecting the pore water species we develop a model which simulates the fate of pore water oxygen and nitrate and particulate organic carbon in sediments this model includes the effects of pore water diffusion oxygen and nitrate respiration sediment accumulation sediment mixing and changes in sediment porosity we observe that factor of 2 variations in the rates of sediment mixing and accumulation have only a minor effect on the pore water profiles the major factor controlling the pore water profiles is the flux of organic carbon across the sediment water interface we conclude that the large intra site variability observed in the interstitial water nitrate profiles is probably caused by small scale variations in the flux of organic material

evolution of the offshore western gulf of corinth the young gulf of corinth rift in central greece is an ideal place to study processes occurring during the initiation and early stages of continental extension at the east end of the 100 km long e w rift holocene extension on n dipping faults equates to geodetic extension rates at the western end however estimated extension rates on n dipping onshore faults account for only 20 40 of the geodetically measured rates 10 15 mm yr we use high quality multichannel seismic reflection and swath bathymetry data to investigate the tectonics of the western gulf and quantify the contribution of offshore fault activity toward total rift extension five major offshore faults generate the variable basement topography and a ca 0 4 ma unconformity separates stratigraphy into two main packages basin fill geometry indicates that during the early stages of subsidence of the offshore western gulf s dipping faults were the dominant border fault structure this is still the case at the western end of the western gulf however elsewhere n dipping faults on the south margin are now relatively more dominant and cause stratigraphy to tilt south the temporally and spatially varying rift structure contrasts with the simple consistently n dipping border fault half graben geometry of the east gulf of corinth combined west gulf offshore and onshore holocene fault extension rates total 5 14 5 mm yr within the range calculated geodetically and render enhanced slip on a low angle detachment surface beneath the western gulf unnecessary

sediment flux to lake baikal siberia russia modeling approach preliminary results of modeling of annual variability in suspended particle matter spm in lake baikal are presented together with outline of major present day sedimentary environments and variation in sedimentation patterns in drilled sections spanning several million years from the lake the model predicts the spm concentration sedimentation resuspension mixing and mineralization in the lake on a monthly basis using parameters describing the lake geometry and the climate in the catchment results of the modeling are generally in agreement with empirical data and indicate that the model accounts well for the relatively large volume and variable conditions occurring in lake baikal the model also seems to have large potential in predicting variability of sediment flux and mass balance in the lake in the past

the importance of topographic corrections on magnetotelluric response data from rugged regions of anatolia topographic irregularities cause some distortions of magnetotelluric mt fields in the vicinity of a topographic feature the tm mode distortion increases with the height and inclination of the slope it is well known that tm mode topographic effects are much greater than te mode distortions the authors have made a study of mt anomalies in tm mode due to two dimensional topography in order to reduce these effects the distortion tensor stripping technique was used after corrections the resulting data can be interpreted as if they were obtained over a flat surface and depend only on the subsurface structure however this technique sometimes causes some geometrical distortions of the real subsurface structure one of the aims is to overcome this failure the authors have modified the correction coefficients by considering the actual one dimensional geology model studies showed that this approach is especially useful in removing the terrain effects on complex 2d subsurface structures the other purpose of this study is to emphasize the importance of a proper terrain correction for data from sites having mountainous topography over complex geology e g strike slip faults suture zones and rift valleys some examples of mt data sets collected from the north anatolian fault zone and from the thrust regions ofmore the western taurides will be presented less

relation between sedimentation rate and burial of organic phosphorus and organic carbon in marine sediments organic carbon and organic phosphorus concentrations in marine sediment cores with known sedimentation rates were determined and collected from the literature with the exception of organic rich sediments from upwelling areas organic phosphorus concentrations in marine sediments do not correlate linearly with organic carbon concentrations the c p ratios of organic matter buried in marine sediments vary systematically with sedimentation rate over the range studied 0 0001 to 2 5 cm y low c p ratios 200 atomic occur in sediments with sedimentation rates both less than 0 002 cm y and greater than 1 cm y higher c p ratios up to 600 are found at intermediate sedimentation rates a simple steady state model is developed for the early diagenesis of organic carbon and organic phosphorus in marine sediments the processes included are sediment burial pore water diffusion and bacterial decomposition of organic matter by oxic respiration and sulfate reduction preferential regeneration of phosphorus relative to carbon is assumed to take place during oxic respiration the modeling shows that both organic matter preservation and the degree of elemental fractionation between phosphorus and carbon depend strongly on the overall sedimentation rate very high sedimentation rates coastal deltaic environment result in a good preservation of deposited organic matter with an initial c p ratio close to the redfield value at very slow sedimentation rates pelagic environment the oxidation of the sedimentary organic matter is nearly complete and produces a low c p residual organic material this residual organic matter may represent either refractory detrital organic compounds enriched in phosphorus or in situ synthesized low c p bacterial remains the high c p ratios at intermediate sedimentation rates slope and shelf environment reflect the preferential regeneration of phosphorus during the incomplete decomposition of the metabolizable organic matter

gas hydrate of lake baikal discovery and varieties abstract this paper summarizes the results of recent gas hydrate studies in lake baikal the only fresh water lake in the world containing gas hydrates in its sedimentary infill we provide a historical overview of the different investigations and discoveries and highlight some recent breakthroughs in our understanding of the baikal hydrate system so far 21 sites of gas hydrate occurrence have been discovered gas hydrates are of structures i and ii which are of thermogenic microbial and mixed origin at the 15 sites gas hydrates were found in mud volcanoes and the rest six near gas discharges additionally depending on type of discharge and gas hydrate structure they were visually different investigations using mir submersibles allowed finding of gas hydrates at the bottom surface of lake baikal at the three sites

morphotectonic and paleoseismological studies of late holocene deformation along the primorsky fault baikal rift abstract the baikal rift is the largest cenozoic continental rift in asia and has developed at the boundary between the siberian craton and a collage of microplates composing the amurian lithospheric plate gps data show that the amurian plate moves southeastward at a rate of 3 4 mm yr with respect to stable eurasia but the slip rates along the main faults that control the opening of basins within the baikal rift system are still poorly quantified the recent graben system of the olkhon region of the central baikal rift is a key example for studying of synrift faulting we discovered a previously unknown paleoseismic structure in the course of our morphometric study of the primorsky fault scarp one of the youngest grabens within the baikal rift detailed topographic mapping of the deformation area and trenching across the fault scarp revealed two paleoseismic events responsible for scarp formation dating of organic remnants found in the trench walls allowed us to estimate a late holocene age for the paleoearthquakes and bracket the vertical slip rates along the primorsky fault to between 0 5 0 1 mm yr min and 0 9 0 2 mm yr max over the last 2 5 kyr the geomorphological and paleoseismological data in combination with the analysis of recent seismicity and horizontal geodetic rates suggest that the primorsky fault has accumulated elastic stress with the potential for a m 6 6 earthquake large earthquakes of this magnitude have occurred at least twice on the investigated fault segment during the last 1 4 kyr

the formation of proval bay as an episode in the development of the baikal rift basin a case study abstract this paper provides a cumulative review of important specific features in the formation and development of proval bay lake baikal as a large seismic dislocation element this bay appeared during one of the largest historical earthquakes in siberia m lh 7 5 on january 12 1862 as a result more than 230 km 2 of the shore was submerged the paper considers the formation of proval bay in the context of analysis conducted on general morphological characteristics of the baikal rift and in terms of the occurrence of the main elements in the mechanism of its neotectonic development it is precisely these seismotectonic phenomena associated with the subsidence of large tectonic blocks which primarily cause the growth of lake baikal basin in spite of the fact that the northwestern side of the rift has a more pronounced morphological structure whose general elements are high and steep monolithic tectonic escarpments major lithospheric extension and its associated extension of the rift thinning and rearrangement of blocks in the upper lithosphere slab occur on the more gently sloping eastern side

a new model for atmospheric oxygen over phanerozoic time a mathematical model has been constructed that enables calculation of the level of atmospheric o2 over the past 570 my from rates of burial and weathering of organic carbon c and pyrite sulfur s burial rates as a function of time are calculated from an assumed constant worldwide clastic sedimentation rate and the relative abundance and c and s contents of the three rock types marine sandstones and shales coal basin sediments and other non marine clastics red beds arkoses by our model values of o2 versus time using a constant total sedimentation rate agree with those for variable sedimentation derived from present day rock abundances and estimates of erosional losses since deposition this agreement is the result of our reliance on the idea that any increase in total worldwide sediment burial with consequently faster burial of c and s and greater o2 production must be accompanied by a corresponding increase in erosion and increased exposure of c and s on the continents to o2 consumption via weathering it is the redistribution of sediment between the three different rock types and not total sedimentation rate that is important in o2 control to add stability to the system negative feedback against excessive o2 fluctuation was provided in the modeling by the geologically reasonable assignment of higher weathering rates to younger rocks resulting in rapid recycling of c and s we did not use direct o2 negative feedback on either weathering of c and s or burial of c because weathering rates are assumed to be limited by uplift and erosion and the burial rate of c limited by the rate of sediment deposition the latter assumption is the result of modern sediment studies which show that marine organic matter burial occurs mainly in oxygenated shallow water and is limited by the rate of supply of nutrients to the oceans by rivers results of the modeling indicate that atmospheric o2 probably has varied appreciably over phanerozoic time during the late carboniferous and permian periods o2 was higher than previously because of the rise of vascular land plants and the widespread burial of organic matter in vast coal swamps a large decrease in o2 during the late permian was due probably to the drying up of the coal swamps and deposition of a large proportion of total sediment in c and s free continental red beds sensitivity study shows that major parameters affecting results are the mean c concentration in coal basins and the relative sizes of the reservoirs of young rapidly recycled versus old rocks less sensitivity was found for changes over time in total land area undergoing weathering and the use of direct o2 negative feedback on marine carbon burial good agreement for rates of c burial calculated via our model and via independent models which are based on the use of stable carbon isotopes indicates that the dominant factor that has brought about changes in atmospheric o2 level and the isotopic composition of dissolved inorganic carbon in seawater over phanerozoic time is sedimentation and not weathering or higher temperature phenomena such as basalt seawater reaction

the role of organic acid exudates in liberating phosphorus from seagrass vegetated carbonate sediments sediment bound phosphorus p is a potential nutrient source for p limited seagrasses inhabiting carbonate sediments we explored the role of organic acid oa exudation by seagrasses in liberating mineral p from carbonate sediments organic acids can act to increase available p by dissolving carbonate sediment competing with p for binding sites and complexing dissolution end products and also by fueling microbial processes that change pore water ph we used dialysis tubing placed around individual roots in situ to quantify dissolved species immediately adjacent to roots root zone and compared these to bulk pore water concentrations in vegetated and nonvegetated sediments total oa concentrations were highest in the root zone 29 8 6 1 8 mmol l21 compared to bulk measures of 15 5 6 1 9 and 7 5 6 0 6 mmol l21 in vegetated and nonvegetated sediments respectively phosphate concentrations were also highest in the root zone and were linearly related to oa concentrations r2 5 0 63 organic acid concentrations increased along a seagrass productivity gradient and ratios of oa concentrations to productivity showed a significant response to a gradient in p limitation of seagrasses organic acid concentrations found in and around roots compared to those found in bulk sediment measures indicate that seagrasses are a significant source of oa sampling at small spatial scales mm immediately adjacent to the roots is critical because bulk sediment pore water measures did not capture the observed fluctuations caused by the rapid reaction and consumption of oa in the sediment root zone processes can liberate considerable quantities of p and oa exudates likely contribute significantly to the success of t testudinum in p limited environments

forms and availability of sediment phosphorus in carbonate sand of bermuda seagrass beds primary production by seagrasscs in tropical and subtropical carbonate sediments often appears to be phosphorus p limited and several studies have sought to identify the possible sources of p for long term growth here we quantify concentrations of particulate and dissolved p and fluoride f in carbonate sediments sediment water p exchange and leaf tissue p concentrations in three seagrass beds in bermuda we also present data on the sequential extraction of p and f from the sediments at each site total sediment p tp in the upper 20 cm ranged from 650 to 1 250 mmol p m 2 and was some 500 fold larger than the pool of p dissolved in pore waters loosely adsorbed inorganic p comprised 2 of tpsed while a surface bound inorganic p pool extracted by dithionite buffer constituted 15 20 most of the remaining p and nearly all of the ca and f present in the sediment was recovered when the remaing sediment was dissolved in an acetic acid buffer solution this pool includes calcium fluoroapatite cfa which is considered the primary diagenetic sink for p in carbonate sands dissolved inorganic p and fwere both elevated in the rhizosphere pore waters from the three seagrass beds in combination with our analysis of sediments by sequential extraction this result indicates that the carbonate matrix of the sediment is being dissolved in the rhizosphere with a resulting release of p to the pore water and hence to the seagrass roots we suggest that this is important in the p economy of these subtropical seagrass thalussia testudinum beds and note that the carbonate bound p pool is large enough to sustain seagrass p requirements for decades nonetheless primary production in such seagrass systems can remain p limited if the rate of p release from sediment dissolution is too slow to support maximum seagrass growth rates analysis of seagrass leaf tissues revealed that p availability was highest at the site with the highest anthropogenic influences this site was also characterized by the highest sediment p release indicating that high p availability may also be partly due to higher release of p as sediments are dissolved phosphorus was released from the sediment mainly as dissolved organic p causing a net efflux of dissolved p at all sites in april and at two sites in august

phosphorus release with carbonate dissolution coupled to sulfide oxidation in florida bay seagrass sediments we hypothesized that caco3 dissolution coupled to sulfide oxidation is an important mechanism by which solid phase inorganic p ip becomes available to seagrass in tropical carbonate sediments to examine this supposition we measured field sulfate reduction rates and simulated the acidity 10 50 mmol h2so4 cm23 sediment generated by subsequent sulfide oxidation from high western and low eastern total p tp sediments in florida bay dissolution experiments were conducted using sediment slurries at field ph pore water ph 6 5 7 5 while caco3 dissolution maximum 1 3 of sediment dry weight was evidenced at all sites by leaching of ca2 into slurries at three of the four sites po 3 4 was primarily recovered in the sediment exchangeable pools and at a lower p ca ratio than observed in source sediments in contrast no po 3 4 was recovered from an eastern bay site with the lowest tp and finest grained sediments suggesting that po 3 4 was either tightly adsorbed or incorporated into the carbonate matrix post acidification the potential for tight po 3 4 resorption by sediment from the low tp site was also supported by the rapid rate of isotopic exchange of 33po 3 4 into the sediment matrix these adsorptive and incorporation processes may explain the low po 3 4 in pore waters in the eastern vs western regions of florida bay even upon dissolution of carbonates carbonate dissolution coupled to sulfide oxidation could potentially provide 1 8 and 23 5 mmol ip m22 d21 at eastern and western bay sites respectively meeting 5 of eastern and 29 of western bay seagrass thalassia testudinum p requirements

impact of sea grass density on carbonate dissolution in bahamian sediments carbonate dissolution has been widely observed in shallow water tropical sediments however sediment budgets have generally not been closed with respect to the amount of acid required to produce the observed carbonate dissolution recently it has been suggested that enhanced oxygen transport into sediments through the roots and rhizomes of sea grasses might play a role in resolving this mass balance problem we conducted studies of sea grass carbonate sediment interactions around lee stocking island exuma islands bahamas to further examine this problem our studies showed that alkalinity total dissolved inorganic carbon sco2 and ca 21 increased with depth in the pore waters while ph and calculated carbonate ion concentration decreased with depth these observations are consistent with the occurrence of carbonate dissolution in these sediments the magnitude of pore water alkalinity sco2 and ca 21 changes was also related to sea grass density with the largest gradients seen in the sediments of dense sea grass beds calculations suggested that less than 50 of the o2 needed to drive aerobic respiration and ultimately carbonate dissolution via co2 production could be supplied by transport processes such as diffusion bioturbation and physical pore water advection furthermore the o 2 needed to balance the carbonate dissolution budget could be provided by the transport of 15 of the photosynthetically derived o2 to the sediments through sea grass roots and rhizomes without enhancing the removal of carbonate dissolution end products thus sea grasses play an important role in controlling the rates of carbonate dissolution in shallow water tropical marine sediments

catastrophic events in the quaternary outflow history of lake baikal abstract lake baikal is earth s deepest lake and an iconic site of scientific study this vast basin holds sedimentary archives of environmental change dating back to the miocene and its array of palaeoshorelines and surrounding relief record the past 1 3 ma of lake levels and outflows here we present an extensive review of previous work alongside a new set of observations concerning the quaternary development of lake baikal with special focus on lake level fluctuations and the formation and evolution of the lake s three known outlets the sequence of shoreline terraces indicates that lake levels were both higher and lower in the past lake baikal stood 200 m higher during the last interglacial i e marine isotope stage mis 5e and dropped to 40 m below present day during the last glacial maximum mis 2 the relative lake level variations reflect climate factors and gradual or sudden coseismic tectonic impacts on the elevation of the lake s outlet thresholds three successive outlets are known i the palaeo goloustnaya manzurka associated with the manzurka alluvium ii the palaeo irkut and iii the currently active angara river outlet we propose that the manzurka alluvium is the product of catastrophic events in lake baikal the sudden possibly coseismic collapse of the 15 3 km goloustnaya fault block into lake baikal triggered a mega tsunami that thrust overwash deposits across neighbouring drainage divides above lake baikal and the valleys of the goloustnaya manzurka river system the age of the manzurka alluvium remains poorly constrained but the mega tsunami is potentially traceable to an unconformity in drill core sediments at 0 8 1 0 ma although older late pliocene and younger 125 ka ages have also been proposed the irkut outlet existed between mis 6 and mis 5 when lake level was 200 m higher than present 640 650 masl and a large bay extended into the tunka rift at baikal s south west tip lake baikal retreated from the tunka rift when lake level fell by up to 100 m in early mis 5e we propose that the lake level fall is connected to a partial collapse of primorsky ridge at listvenichny bay which caused baikal to overspill into the angara river thereby forming a new outlet the release of a 4000 km3 megaflood down the angara river valley caused large scale modification and reworking of valley fills mis 5e at the end of mis 2 further collapse of primorsky ridge lowered the outlet threshold an additional 50 60 m and prompted a second megaflood down the angara river valley which left a widespread unconformity where horizontal bedded sands dated at 11 8 13 4 ka overlie cryoturbated deposits of the earlier megaflood the central role of catastrophic processes at lake baikal suggests that rather than being rare events coseismic landsliding and mega tsunami may be more frequent than hitherto recognised

insights into the effects of oblique extension on continental rift interaction from 3d analogue and numerical models abstract continental rifts often develop from linkage of distinct rift segments under varying degrees of extension obliquity these rift segments arise from rift initiation at non aligned crustal heterogeneities and need to interact to develop a full scale rift system here we test the effects of 1 oblique extension and 2 initial heterogeneity seed offset on continental rift interaction with the use of an improved analogue model set up x ray computer tomography ct techniques are used to analyse the 3d models through time and the results are compared with additional numerical models and natural examples the experimental results reveal that increasing extension obliquity strongly changes rift segment structures from wide rifts in orthogonal settings to narrower rifts with oblique internal structures under oblique extension conditions to narrow strike slip dominated systems towards the strike slip domain we also find that both decreasing seed offset and increasing extension obliquity promote hard linkage of rift segments through the formation of continuous rift boundary faults at the surface initial soft linkage through the formation of relay ramps is more likely when seed offset increases or extension is more orthogonal rather than linking at depth the rift boundary faults curve around each other at depth and merge towards the surface to form a continuous trough orthogonal extension promotes the formation of intra rift horsts which may provide hydrocarbon traps in nature

aeolian deposits of rift zones the evolution of contemporary aeolian processes in continental rift zones is one of the poorly explored issues in quaternary geology associated with rift zones is the formation of aeolian zircon rutile ilmenite sands which are rich in various mineral resources and primarily in titanium zirconium uranium and in other rare earth elements using the baikal rift system as an example the characteristics of the occurrence at the rift zones hollows of contemporaneous unfixed aeolian sands are examined analysis of the morphological elements of aeolian deposits their substance composition and development patterns is based on long term field work on the study of aeolian sands in the depressions of the baikal rift system as well as on graphical representations of riftogenic structures on remotely sensed material and primarily on space acquired images

accommodating large scale intracontinental extension and compression in a single stress field a key example from the baikal rift system the baikal rift system in southern siberia is one of the main intracontinental extensional features on earth the rift system represents the northwestern boundary of the amuria plate and in that respect can be considered as an evolving plate boundary the baikal rift system has been widely studied both in terms of geology and geophysics and many models have been proposed for its formation and evolution however the age of the initiation of deformation and the mechanism driving this deformation are still largely debated while major extension has occurred since the late miocene pliocene the onset of extension seems older than the india asia collision implying that several driving mechanisms may have acted together or in relay through time in this work we review the available data and models for deformation in an area encompassing the baikal rift system the sayan ranges to the west and the transbaikal to the east using a synthesis of this data and our own field and mapping observations we show that the baikal rift system along with transpressional deformation in the sayan ranges and transtension in the transbaikal area can be explained through major left lateral strike slip systems the deformation is strongly controlled by inherited crustal and lithospheric structures and is distributed over a wide area within the western amuria plate that consequently cannot be considered as a rigid block such distributed deformation is likely to have a strong effect on the structure of the future continental margin if extension evolves towards the formation of oceanic crust

active hydrate destabilization in lake baikal siberia abstract in this paper we present new seismic and heat flow data that show the base of the hydrate stability zone bhsz in lake baikal to be locally characterized by abnormal variations in depth with distinct regions of deeper than normal and regions of shallower than normal bhsz these variations are related to strong lateral variations in heat flow and occur in close association with important rift basin faults areas of shallow bhsz are also characterized by the presence of several methane seeps and mud volcanoes at the lake floor we infer that the seeps are the surface expression of escape pathways for overpressured fluids generated by the dissociation of pre existing hydrates in response to a thermal pulse caused by an upward flow of hydrothermal fluids towards the bhsz it thus seems that present day hydrate dissociation in lake baikal is modulated by the tectonic activity in the rift rather than by climatically controlled changes in lake level or water temperature

the detailed record of climatic events during the past 75 000yrs bp from the lake baikal drill core bdp 93 2 abstract by correlating the pronounced interstadial type diatom peak with mis 3 the high resolution and well dated record bdp 93 2 from the selenga delta area of lake baikal complements the classical long paleoclimatic records from academician ridge which lack the pronounced mis 3 warm signal this correlation helps to resolve the age model controversies related to this interstadial interval the content and c n ratios of total organic carbon as well as its stable isotope composition and diatom abundance reveal high frequency oscillatory variations reflecting the more unstable conditions of mis 3 interstadial in the lake baikal region as compared with the holocene interglacial an important part of the regional climatic instability during the past 75 000 yrs bp revealed for the first time in the baikal record is the recurrence of the abrupt erosional events leading to the deposition of the detrital carbonate material at the hemipelagic bdp 93 site there were at least five such events at mis 5 4 transition in the middle of mis 4 and three during mis 3 stratigraphic and ams radiocarbon age models allow the baikal erosional carbonate events to be confidently correlated to the intervals of heinrich layers in the north atlantic this correlation suggests a surprisingly strong climatic teleconnection of the remote mid continent baikal location with the dansgaard oeschger events and the bond cycles in the north atlantic region

early diagenesis of organic matter in peru continental margin sediments phosphorite precipitation abstract pore water chemistry total dissolved co 2 nh 4 no 3 no 2 po 4 si oh 4 ca mg fe mn so 4 h 2 s and f and titration alkalinity solid phase chemistry c org p org c tot n tot f si opal and s ii and sediment characteristics porosity dry bulk density and formation factors were determined on a centimeter scale spacing in the upper 20 40 cm of sediments under intense upwelling areas on the peru continental shelf these data demonstrate that carbonate fluorapatite cfa is precipitating from pore waters in the upper few centimeters of a gelatinous mud with high organic carbon content up to 20 c org very high porosity 0 96 ml cm 3 and very low dry bulk density 3 dissolved phosphate concentrations at the sediment water interface range from 20 to 100 m orders of magnitude higher than bottom water concentrations and much higher than predicted from regeneration of organic matter the mechanism of this interfacial phosphate release is unclear but is apparently uncoupled from carbon and nitrogen metabolism and thus may be linked either to dissolution of fish debris or to the presence of a microbial mat in surficial sediments fluoride is incorporated into cfa by diffusion from the overlying seawater and carbonate ions are provided from pore water alkalinity magnesium concentrations in this reaction zone are not significantly different from those of seawater suggesting that magnesium depletion is not a necessary prerequisite for cfa precipitation the environment of precipitation is interface linked rather than driven by organic diagenesis of phosphorus deeper in the sediment most of the cores display a wide range of diagenetic characteristics below the immediate interfacial region but almost all show the precipitation signature near the interface this interface linked early diagenetic porewater environment for the precipitation of cfa explains many of the geochemical characteristics of phosphorites and provides a testable model to compare the modern phosphogenic analog with ancient phosphorite deposits two of the cores display very high solid phase phosphorus and fluoride contents reflecting the presence of apparently modern pelletal apatites

sedimentation processes and new age constraints on rifting stages in lake baikal results of deep water drilling with this paper we present a first attempt to combine the direct results on lithology composition and age dating in the boreholes bdp 93 bdp 96 and bdp 97 with geological and seismic data from the areas where those sections were drilled the sedimentary environments represented by the bdp boreholes are markedly different and possess characteristic lithological features the results of the deep drilling provide the essential means for testing numerous age models used in geological reconstructions of the lake baikal rifting dynamics neither the basin wide unconformity interpreted from seismic data nor the interpreted change from shallow water to deep water facies at the boundary of the seismic stratigraphic complexes were found in the bdp 96 boreholes on academician ridge also lithology does not support the proposed reconstructions of intense lake level fluctuations and transgressions during the pliocene at academician ridge the continuous deep water hemipelagic sedimentation at academician ridge has existed for the past 5 ma the beginning of an intense rifting phase of the neobaikalian sub stage and related drastic changes in sedimentation processes were interpreted on seismic sections as the basin wide unconformity b10 different age estimates for this boundary ranged from late pliocene 3 5 ma to plio pleistocene boundary as shown by bdp 96 borehole b10 is associated with a lithological change from diatomaceous ooze to dense silty clay and not with an erosional contact the new age for this boundary in bdp 96 is approximately 2 5 ma this new age constraint suggests that the upper sedimentary strata of northern baikal 1 5 1 7 km thick have formed during the past 2 5 ma with average sedimentation rates of 60 70 cm ka the bdp 93 boreholes at buguldeika suggest that uplift in primorsky range took place prior to 1 07 1 31 ma a date which exceeds the age of previous geological models