fractionation of sulfur isotopes during bacterial sulfate reduction in deep ocean sediments at elevated temperatures abstract a numerical model of sulfate reduction and isotopic fractionation has been applied to pore fluid so 4 2 and 34 s data from four sites drilled during ocean drilling program odp leg 168 in the cascadia basin at 48 n where basement temperatures reach up to 62 c there is a source of sulfate both at the top and the bottom of the sediment column due to the presence of basement fluid flow which promotes bacterial sulfate reduction below the sulfate minimum zone at elevated temperatures pore fluid 34 s data show the highest values 135 yet found in the marine environment the bacterial sulfur isotopic fractionation factor is severely underestimated if the pore fluids of anoxic marine sediments are assumed to be closed systems and rayleigh fractionation plots yield erroneous values for by as much as 15 in diffusive and advective pore fluid regimes model results are consistent with 1 077 0 007 with no temperature effect over the range 1 8 to 62 c and no effect of sulfate reduction rate over the range 2 to 10 pmol cm 3 d 1 the reason for this large isotopic fractionation is unknown but one difference with previous studies is the very low sulfate reduction rates recorded about two orders of magnitude lower than literature values that are in the range of mol cm 3 d 1 to tens of nmol cm 3 d 1 in general the greatest 34 s depletions are associated with the lowest sulfate reduction rates and vice versa and it is possible that such extreme fractionation is a characteristic of open systems with low sulfate reduction rates

the seawater sr ca ratio in the past 50 myr from bulk carbonate sediments corrected for diagenesis abstract the strontium calcium sr ca ratio in paleoseawater is important for evaluating the global fluxes of continental weathering products and carbonate deposition in the oceans existing reconstructions of the paleoseawater sr ca ratio using different methods give conflicting results we use a modeling approach to estimate paleo sr ca for the past 50 million years that uses pore fluid chemistry and bulk sediment measurements of carbonate ooze and chalk from the ontong java plateau as input the effects of diagenesis in modifying the sr ca ratio recorded in bulk carbonate sediments are estimated using a reactive transport model and those effects are accounted for in determining the initial sr ca ratio of the biogenic carbonate sediments the sr ca of the paleo oceans is calculated based on an effective partition coefficient k sr bio 0 198 inferred from sr ca in modern sediments the model simulates deposition of sediments on the seafloor reactions between pore fluid and solid dissolution and precipitation of calcite and diffusion within the pore fluid measured pore fluid sr ca and sulfate concentrations provide input for the sr model this approach applied to four ocean drilling program odp sites 803 805 806 807 that have long cenozoic records of carbonate deposition indicates that the sr ca ratio of seawater has increased slowly over the past 50 million years from a value of about 8 mmol mol to the modern value of 9 4 with fluctuations of about 1 over 10 myr periods this result is consistent with previous estimates based on foraminifera and coral aragonite but differs from recent estimates obtained with other approaches the near constant seawater sr ca over the past 50 million years is consistent with likely weathering sources to the oceans the fluctuations suggest that there were modest shifts in the aragonite calcite ratio of marine carbonate

reevaluating the carbon sink due to sedimentary carbonate formation in modern marine sediments abstract previous attempts to quantify the amount of sedimentary carbonate precipitation in modern marine sediments have been derived from the flux of calcium into the sediments due to diffusion under assumed steady state the application of fick s first law and then extrapolation of these site specific rates to the global ocean sediment column this approach is limited however as much of the ocean floor has not been sampled we take a machine learning approach to update and refine the estimate of the amount of sedimentary carbonate precipitation as well as define whether sedimentary carbonate precipitation is driven by organoclastic microbial sulfate reduction or anaerobic methane oxidation we identify areas where there is sedimentary carbonate formation using machine learning based upon oceanic physical and chemical properties including bathymetry temperature water depth distance from shore and tracers of primary production and data from the global odp iodp database our results suggest that the total amount of sedimentary carbonate formation is much lower than previous estimates at 1 35 0 5 10 11 mol c yr we suggest that this rate is a lower estimate and discuss why machine learning approaches may always produce lower bound estimates of global processes our calculations suggest that the formation of sedimentary carbonate today is mainly driven by anaerobic methane oxidation 77 with the remainder attributed to organoclastic sulfate reduction we use our machine learning results to speculate the impact that sedimentary carbonate precipitation may have had on the carbon isotope composition of the surface dissolved inorganic carbon reservoir over earth history

two opposing effects of sulfate reduction on carbonate precipitation in normal marine hypersaline and alkaline environments comment sulfate reduction sr has been linked to carbonate mineral precipitation and has been invoked as a possible mechanism for microbialite formation lyons et al 1984 1 kempe 1990 2 although there is evidence to support the contribution of sr in carbonate mineral precipitation from both

landscape modification in response to repeated onset of hyperarid paleoclimate states since 14 ma atacama desert chile the landscape of the hyperarid atacama desert in northern chile records extremely slow change on earth s surface disputed ages for the onset of hyperaridity range from the late paleogene through the pleistocene a long term paleoclimate record is recorded in a nonmarine basin whose fill is primarily alluvial strata for this setting the primary proxies for climate state are the mineralogical and chemical composition of soil which varies across a precipitation gradient and the landforms and deposits of alluvial fans during the most recent 15 million years five climate related landscape stages are resolved for the pampa del tamarugal sedimentary basin with each successively younger stage inset lower in the local topography than its predecessor the earliest landscape stage is expressed as a set of alluvial strata inherited from a time of arid or semi arid climate ca 14 12 ma the younger four landscape stages generated a composite long lasting exposure surface predominantly hyperarid conditions have persisted since ca 12 1 ma during which four intervals of arid to semi arid climate occurred each wet interval was short lived a million years or less whereas some of the hyperarid periods were lengthy 1 5 m y the hyperarid intervals were roughly 11 5 5 ma 4 5 4 ma 3 6 2 6 ma 2 2 1 ma and repeated intervals during the last 1 m y the onset of hyperaridity ca 12 ma likely reflects the growth of the andes mountains above a climate threshold in contrast sea surface temperature variability likely has controlled atacama paleoclimate changes since the late miocene

marine pore water sulfate profiles indicate in situ methane flux from underlying gas hydrate marine pore water sulfate profiles measured in piston cores are used to estimate methane flux toward the sea floor and to detect anomalous methane gradients within sediments overlying a major gas hydrate deposit at the carolina rise and blake ridge u s atlantic continental margin here sulfate gradients are linear implying that sulfate depletion is driven by methane flux from below rather than by the flux of sedimentary organic matter from above thus these linear sulfate gradients can be used to quantify and assess in situ methane flux which is a function of the methane inventory below

effect of paleoseawater composition on hydrothermal exchange in midocean ridges variations in the mg ca sr and so4 concentrations of paleoseawater can affect the chemical exchange between seawater and oceanic basalt in hydrothermal systems at midocean ridges mor we present a model for evaluating the nature and magnitude of these previously unappreciated effects using available estimates of paleoseawater composition over phanerozoic time as inputs and 87sr 86sr of ophiolite epidosites and epidote quartz veins as constraints the results suggest that modern hydrothermal fluids are not typical due to low ca and sr relative to mg and so4 in modern seawater at other times during the last 500 million years particularly during the cretaceous and ordovician hydrothermal fluids had more seawater derived sr and ca a prediction that is supported by sr isotope data the predicted 87sr 86sr of vent fluids varies cyclically in concert with ocean chemistry with some values much higher than the modern value of 0 7037 the seawater chemistry effects can be expressed in terms of the transfer efficiency of basaltic ca and sr to seawater in hydrothermal systems which varies by a factor of 1 6 over the phanerozoic with minima when seawater mg and so4 are low this effect provides a modest negative feedback on seawater composition and 87sr 86sr changes for the mid cretaceous the low 87sr 86sr of seawater requires either exceptionally large amounts of low temperature exchange with oceanic crust or that the weathering flux of continentally derived sr was especially small the model also has implications for mor hydrothermal systems in the precambrian when low seawater so4 could help explain low seawater 87sr 86sr

a late quaternary paleoenvironmental record in sand dunes of the northern atacama desert chile this paper reports a previously unidentified paleoenvironmental record found in sand dunes of the atacama desert chile long term aeolian deflation by prevailing onshore winds has resulted in the deposition of sand on the irregular surface of a miocene aged anhydrite outcrop two deposits 25 km apart along the prevailing wind trajectory were hand excavated then analyzed for vertical and temporal changes in physical and chemical composition radiocarbon ages of organic matter embedded within the deposits show that rapid accumulation of sediment began at the last glacial maximum and slowed considerably after the pacific ocean attained its present post glacial level over this time period grain sizes are seen to increase while accumulation rates simultaneously decrease suggesting greater wind speeds and or a change or decrease in sediment supply changes in 34 s values of sulfate in the sediment beginning 10 ka indicate an increase in marine sources similarly 2 h values from palmitic acid show a steady increase at 10 ka likely resulting from aridification of the region during the holocene due to the extreme aridity in the region these sand dunes retain a well preserved chemical record that reflects changes in elevation and coastal proximity after the last glacial maximum

the relationship between organoclastic sulfate reduction and carbonate precipitation dissolution in marine sediments abstract pore fluid sulfate calcium and magnesium concentrations in ocean drilling sites 981 984 925 929 and 803 807 are modeled to estimate the rates of sulfate reduction and carbonate precipitation dissolution these sites represent scenarios where the sulfate reduction rate is high moderate and low results show that the rate of carbonate precipitation decreases with decreasing sulfate reduction rate when sulfate reduction rate is sufficiently low carbonate minerals dissolve sulfate reduction produces h and bicarbonate which compete in driving carbonate minerals to dissolve or precipitate the results suggest that the carbonate reactions are mainly driven by the produced h when sulfate reduction rate is low and by the produced bicarbonate when the sulfate reduction rate is high

molluscs as evidence for a late pleistocene and early holocene humid period in the southern coastal desert of peru 14 5 s the southern peruvian coastal desert around palpa southern peru 14 5 s is currently characterized by hyper arid conditions however the presence of two species of molluscs scutalus pupoides and desert loess deposits indicates the past development of semi desert and grassland ecosystems caused by a displacement of the eastern desert margin due to hydrological changes radiocarbon dating shows that the transition to a semi arid climate in the southern peruvian coastal desert took place during the greenland interstadial 1 13 5 cal ka bp at the beginning of the holocene the mollusc fauna vanished due to increasing humidity and the development of grasslands dust particles were fixed by the grasses as indicated by abundant poaceae phytoliths and desert loess was formed the humid period we observe here is out of phase with the palaeoenvironmental records from the titicaca region which indicates dry conditions at that time this paper offers a new idea for this contradiction an orbitally driven meridional shift of the bolivian high might have altered the moisture supply across the andes

presence of oxygen and aerobic communities from sea floor to basement in deep sea sediments the depth of oxygen penetration and microbial activity in marine sediments varies by region sediment cores from the south pacific gyre host oxygen and aerobic microbial communities to at least 75 metres below the sea floor

stratigraphic evolution of the late jurassic hanifa formation along the tuwaiq escarpment saudi arabia evidence for a carbonate ramp system abstract a sequence stratigraphic framework of the late jurassic oxfordian hanifa formation at its exposure in central arabia is presented for the first time this study offers the first high resolution stratigraphic framework of the hanifa along the tuwaiq escarpment by measuring 15 sections 770 m total thickness over an oblique to dip distance of 260 km and collecting 295 samples for petrographic analysis on the basis of these data the hanifa formation can be subdivided into eight facies 1 tabular cross bedded quartz peloidal skeletal grainstone 2 cross bedded skeletal peloidal grainstone 3 bioturbated foraminiferal wackestone mud dominated packstone 4 oncolitic rudstone 5 stromatoporoid coral biostrome bioherm 6 peloidal composite grain grain dominated packstone grainstone 7 bioturbated spiculitic wackestone mud dominated packstone and 8 thinly bedded argillaceous mudstone wackestone the vertical and lateral distributions of these facies along the exposure define their sequence setting using the principals of sequence stratigraphy by recognizing erosional surfaces facies offset and changes in facies proportions five third order sequences with an average duration of 1 1 myr are interpreted for the hanifa formation the correlation of the sequences across the study area shows that only four sequences are preserved in the north where shallow water deposits are well developed facies trends within these sequences are further illustrated in depositional models representing the highstand systems tracts hst and the transgressive systems tracts tst of the hanifa formation these proposed models represent depositional settings of a carbonate ramp with normal open marine conditions the hst depositional model is characterized by a high energy shoreline and depicts the presence of an offshore structurally controlled skeletal peloidal shoal body described here for the first time at the hanifa exposure in the hozwa area this work provides a predictive framework and outcrop analog for applications in hydrocarbon exploration and development furthermore a basinal setting predicted to the south of the study area is a potential site for unconventional plays

hydrogen isotope ratios of palmitic acid in lacustrine sediments record late quaternary climate variations the rich paleoclimate information preserved in lacustrine sedimentary organic matter can be difficult to extract because of the mixed terrestrial and aquatic inputs herein we demonstrate that compound specific hydrogen isotope analysis of palmitic acid pa a ubiquitous compound in lacustrine sediments captures the d signals of lake water samples collected across a diverse range of 33 north american lakes show a strong correlation between water and dpa values at crooked pond massachusetts the dpa changes in a 14 k y sediment record parallel temperature trends inferred from fossil pollen downcore changes reveal differences between climatic trends in new england and in greenland that are consistent with important regional differences in climate controls

significant contribution of authigenic carbonate to marine carbon burial carbon is removed from the earth s surface through the formation and burial of carbon bearing rocks and minerals an analysis of pore water profiles collected from marine sediments around the globe suggests that the precipitation of authigenic calcium carbonate accounts for around 10 of the carbonate that accumulates in marine sediments globally carbon is removed from the earth s surface through the formation and burial of carbon bearing rocks and minerals1 2 the formation of calcium carbonate and its burial in marine sediments accounts for around 80 of the total carbon removed from the earth s surface however the fraction of calcium carbonate that precipitates in the oceans versus that which precipitates authigenically in marine sediments is unclear here we compile measurements of the calcium concentration of pore fluids collected at 672 seafloor sites around the globe to calculate the global flux of calcium within marine sediments we use these data combined with alkalinity measurements of pore fluids to quantify authigenic calcium carbonate precipitation we estimate that the net calcium flux into marine sediments that can be ascribed to authigenic carbonate precipitation amounts to around 1 1012 mol yr 1 as such we estimate that authigenic carbonate precipitation accounts for at least 10 of global carbonate accumulation we show that much of the precipitation occurs along the eastern margins of ocean basins where organic matter delivery to the sea floor is likely to be high we suggest that authigenic calcium carbonate precipitation represents a non negligible component of the global carbon cycle

fluid seepage velocities through marine sediments constrained by a global compilation of interstitial water so42 mg2 and ca2 profiles the depth dependence of the so42 mg2 and ca2 contents of interstitial waters extracted from sediments in 140 deep sea drilling project ocean drilling program integrated ocean drilling program drill holes on oceanic crust has been fit using a model of transport advection and diffusion and reaction these drill holes come from a range of crustal ages sediment types sediment thicknesses and sediment accumulation rates the best fitting specific discharge through the sediment at these locations is estimated to be generally 500 m myr 1 0 05 cm yr 1 where sediments are thicker than 100 m although an order of magnitude faster seepage is estimated for some locations with sediment tens of meters thick assuming that the drill holes are globally representative then seepage of fluids through marine sediments at specific discharges of a few hundred m myr 1 is estimated to be only a few percent of the total ridge flank hydrothermal fluid flux this is consistent with the previous suggestion that hydrothermal fluid exchange between the ocean and the crustal aquifer primarily occurs through basement outcrops chemical fluxes of so42 mg2 and ca2 across the sediment water interface globally excluding continental margins are estimated to be less than 10 of the riverine input to the ocean

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

vertical variations in bassanite distribution patterns in near surface sediments southern egypt abstract nile valley sediments of the el adaima area in southern egypt contain bassanite caso 4 0 5h 2 o associated with gypsum caso 4 2h 2 o in the upper 1 m of the deposits the presence of bassanite at this site formed by dehydration of gypsum is marked by variations in mode of occurrence with depth in the lowest bassanite bearing interval bassanite occurs mainly along the contact between gypsum crystals in pores which is related to the protection of bassanite from rehydration in those parts of the gypsum aggregates at a higher level bassanite occurs along the sides of lenticular gypsum crystals in the sediment matrix near the top of the studied sequence bassanite mainly appears as inclusions in gypsum crystals which represents a texture that developed by repeated dehydration and rehydration elsewhere in the upper part of the deposits bassanite occurs as aggregates of small crystals in pores without associated gypsum which was entirely dehydrated to bassanite in these parts these vertical variations in bassanite distribution patterns record differences in the diagenetic history of the deposits between depth intervals

two opposing effects of sulfate reduction on carbonate precipitation in normal marine hypersaline and alkaline environments reply abstractsulfate reduction has been suggested as a mechanism to induce precipitation of calcium and magnesium carbonates in marine sediments and microbial mats through most of earth s his tory however sulfate reduction also causes a drop in ph that favors dissolution rather than precipitation of carbonates model results obtained in this study show that in modern seawa ter modern hypersaline water and assumed precambrian alkaline seawater sulfate reduction initially lowers the saturation of carbonates due to a rapid decrease in ph with continu ing sulfate reduction the ph stabilizes between 6 5 and 7 and carbonate saturation slowly increases as a result of increasing dissolved inorganic carbon concentration however sulfate reduction in surface microbial mats is not suf cient to cause such an increase in saturation with increasing salinity sulfate reduction becomes even less ef cient to induce carbonate pre cipitation in an alkaline precambrian ocean where large amounts of carbonate were formed induction through sulfate reduction was entirely ineffective other metabolic pathways or abi otic factors must be responsible for inducing carbonate formation in microbial mats through earth s history

dolomite formation in the dynamic deep biosphere results from the peru margin early diagenetic dolomite beds were sampled during the ocean drilling programme odp leg 201 at four reoccupied odp leg 112 sites on the peru continental margin sites 1227 684 1228 680 1229 681 and 1230 685 and analysed for petrography mineralogy 13c 18o and 87sr 86sr values the results are compared with the chemistry and 13c and 87sr 86sr values of the associated porewater petrographic relationships indicate that dolomite forms as a primary precipitate in porous diatom ooze and siliciclastic sediment and is not replacing the small amounts of precursor carbonate dolomite precipitation often pre dates the formation of framboidal pyrite most dolomite layers show 87sr 86sr ratios similar to the composition of quaternary seawater and do not indicate a contribution from the hypersaline brine which is present at a greater burial depth also the 13c values of the dolomite are not in equilibrium with the 13c values of the dissolved inorganic carbon in the associated modern porewater both petrography and 87sr 86sr ratios suggest a shallow depth of dolomite formation in the uppermost sediment 30 m below the seafloor a significant depletion in the dissolved mg and ca in the porewater constrains the present site of dolomite precipitation which co occurs with a sharp increase in alkalinity and microbial cell concentration at the sulphate methane interface it has been hypothesized that microbial hot spots such as the sulphate methane interface may act as focused sites of dolomite precipitation varying 13c values from 15 to 15 for the dolomite are consistent with precipitation at a dynamic sulphate methane interface where 13c of the dissolved inorganic carbon would likewise be variable a dynamic deep biosphere with upward and downward migration of the sulphate methane interface can be simulated using a simple numerical diffusion model for sulphate concentration in a sedimentary sequence with variable input of organic matter thus the study of dolomite layers in ancient organic carbon rich sedimentary sequences can provide a useful window into the palaeo dynamics of the deep biosphere

constraining magnesium cycling in marine sediments using magnesium isotopes abstract magnesium concentrations in deep sea sediment pore fluids typically decrease down core due to net precipitation of dolomite or clay minerals in the sediments or underlying crust to better characterize and differentiate these processes we have measured magnesium isotopes in pore fluids and sediment samples from ocean drilling program sites 1082 1086 1012 984 1219 and 925 that span a range of oceanographic settings at all sites magnesium concentrations decrease with depth at sites where diagenetic reactions are dominated by the respiration of organic carbon pore fluid 26mg values increase with depth by as much as 2 because carbonates preferentially incorporate 24mg low 26mg the increase in pore fluid 26mg values at these sites is consistent with the removal of magnesium in mg carbonate dolomite in contrast at sites where the respiration of organic carbon is not important and or weatherable minerals are abundant pore fluid 26mg values decrease with depth by up to 2 the decline in pore fluid 26mg at these sites is consistent with a magnesium sink that is isotopically enriched relative to the pore fluid the identity of this enriched magnesium sink is likely clay minerals using a simple 1d diffusion advection reaction model of pore fluid magnesium we estimate rates of net magnesium uptake removal and associated net magnesium isotope fractionation factors for sources and sinks at all sites independent estimates of magnesium isotope fractionation during dolomite precipitation from measured 26mg values of dolomite samples from sites 1082 and 1012 are very similar to modeled net fractionation factors at these sites suggesting that local exchange of magnesium between sediment and pore fluid at these sites can be neglected our results indicate that the magnesium incorporated in dolomite is 2 0 2 7 depleted in 26mg relative to the precipitating fluid assuming local exchange of magnesium is minor at the rest of the studied sites our results suggest that magnesium incorporated into clay minerals is enriched in 26mg by 0 to 1 25 relative to the precipitating fluid this work demonstrates the utility of magnesium isotopes as a tracer for magnesium sources sinks in low temperature aqueous systems

large and rapid climate variability during the messinian salinity crisis evidence from deuterium concentrations of individual biomarkers during the messinian 6 m y ago massive sea level fall and widespread deposition of evaporites occurred in the mediterranean sea when it became isolated from the world oceans here we present the first hydrogen isotope data from individual sedimentary biomarkers n alkanes and isoprenoids that tracked climatically driven hydrographic changes in response to extreme evaporation during the messinian salinity crisis the stable hydrogen and carbon isotope compositions of these biomarkers show a range of 160 in d values and 14 in 13 c values and roughly covary this indicates that the source waters of the biomarkers were therefore in some cases extremely enriched in deuterium having average d as great as 66 vsmow vienna standard mean ocean water such values are only known from desert climates today because the offset between the d values of n alkanes and isoprenoids preserved in the miocene sedimentary rocks is similar to the offset found in modern biological samples we conclude that diagenesis did not significantly affect the primary deuterium concentrations

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

timing of the last glacial maximum from observed sea level minima during the last glacial maximum ice sheets covered large areas in northern latitudes and global temperatures were significantly lower than today but few direct estimates exist of the volume of the ice sheets or the timing and rates of change during their advance and retreat here we analyze four distinct sediment facies in the shallow tectonically stable bonaparte gulf australia each of which is characteristic of a distinct range in sea level to estimate the maximum volume of land based ice during the last glaciation and the timing of the initial melting phase we use faunal assemblages and preservation status of the sediments to distinguish open marine shallow marine marginal marine and brackish conditions and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio hydroisostatic modelling our results indicate that from at least 22 000 to 19 000 calendar years before present land based ice volume was at its maximum exceeding today s grounded ice sheets by 52 5 x 10 exp 6 cu km a rapid decrease in ice volume by about 10 percent within a few hundred years terminated the last glacial maximum at 19 000 250 years

stylolites porosity depositional texture and silicates in chalk facies sediments ontong java plateau gorm and tyra fields north sea comparison of chalk on the ontong java plateau and chalk in the central north sea indicates that whereas pressure dissolution is controlled by effective burial stress pore filling cementation is controlled by temperature effective burial stress is caused by the weight of all overlying water and sediments as counteracted by the pressure in the pore fluid so the regional overpressure in the central north sea is one reason why the two localities have different relationships between temperature and effective burial stress in the chalk of the ontong java plateau the onset of calcite silicate pressure dissolution around 490 m below sea floor bsf corresponds to an interval of waning porosity decline and even the occurrence of proper stylolites from 830 m bsf is accompanied by only minor porosity reduction because opal is present the pore water is relatively rich in si which through the formation of ca silica complexes causes an apparent super saturation of ca and retards cementation the onset of massive pore filling cementation at 1100 m bsf may be controlled by the temperature dependent transition from opal ct to quartz in the stylolite bearing chalk of two wells in the gorm and tyra fields the nannofossil matrix shows recrystallization but only minor pore filling cement whereas microfossils are cemented cementation in gorm and tyra is thus partial and has apparently not been retarded by opal controlled pore water a possible explanation is that due to the relatively high temperature silica has equilibrated to quartz before the onset of pressure dissolution and thus in this case dissolution and precipitation of calcite have no lag this temperature versus effective burial stress induced difference in diagenetic history is of particular relevance when exploring for hydrocarbons in normally pressured chalk while most experience has been accumulated in the over pressured chalk of the central north sea

rates of carbonate cementation associated with sulphate reduction in dsdp odp sediments implications for the formation of concretions dsdp odp porewater profiles in organic carbon bearing 5 org c sediments commonly show decreases in ca2 concentrations and increases in alkalinity over depths where sulphate is being removed by microbial reduction these ca2 depletion profiles represent the combined effect of diffusion advection and reaction addition by ion exchange and removal by precipitation mainly as caco3 and or dolomite a diagenetic model has been used to estimate the rate constant k for ca2 removal by precipitation during sulphate depletion over depths of 15 150 m assuming first order kinetics the rate constants for ca2 removal range from 10 14 to 10 11 s 1 in 19 dsdp odp sediments which span a range of bottom water temperatures 0 10 c lithologies calcareous to clastic and sedimentation rates 0 001 0 4 cm year 1 values of k correlate with sedimentation rate such that log k 1 16 log 10 3 indicating that faster rates of ca2 removal occur at higher sedimentation rates where there are also higher degrees of saturation with respect to caco3 and dolomite depth integrated masses of ca2 removed 100 mol cm 2 during sulphate depletion over these depth ranges are equivalent to a dispersed phase of approximately 1 5 wt caco3 or 3 wt dolomite in a compacted sediment the complete occlusion of sediment porosity observed in concretions with isotopic signatures suggesting carbonate sourced from sulphate reduction therefore requires more time a depositional hiatus more rapid sulphate reduction possibly by anaerobic methane oxidation and or the continued transport of isotopically light carbonate to the concretion site after sulphate reduction has ceased

rates of anaerobic oxidation of methane and authigenic carbonate mineralization in methane rich deep sea sediments inferred from models and geochemical profiles abstract pore water chemical data obtained from a 10 5 m long giant gravity core collected in methane rich sediments from 647 m water depth in the northern gulf of mexico n 28 04 00 w 89 43 15 defines sub bottom gradients in unprecedented detail this core penetrated the sulfate methane interface smi at 300 cm below the seafloor cmbsf at the smi dissolved inorganic carbon dic concentrations reach a maximum 13 5 mm and pore water 13 c dic 63 2 pdb and 13 c methane 89 5 pdb values are most negative below the smi pore water sulfate is nearly depleted methane concentrations rise sharply with simultaneous occurrence of a bubble textured sediment and fine grained methane derived authigenic carbonate nodules and cements are common the sharp peaks in dic concentration and isotope values centered at the smi indicate that dic is being produced by anaerobic oxidation of methane aom within a narrow zone centered at the smi the detailed sulfate and dic concentration profiles and dic 13 c values have enabled geochemical models to be constructed that explore the rate of dic formation by aom and its effect on pore water dic 13 c values model results closely match measured dic concentration and 13 c isotope profiles and indicate that microbiological conversion of methane carbon to dic is rapid in geologic terms and that aom is occurring at the present position of the smi isotope values for authigenic carbonate found immediately below the present day smi 13 c 60 2 0 7 pdb at 440 cmbsf are consistent with derivation of the carbonate carbon from methane via aom at the former location of a smi these observations and model results suggest that aom is occurring at rates that would generate the observed profiles and begin the precipitation of methane derived carbonate occur on time scales of centuries model results also show that the time needed to produce the resulting authigenic cements is an order of magnitude greater than that for aom to produce the observed dic profiles the metabolic rates for dic production by aom inferred from modeling the geochemical profiles compare favorably with available rate data obtained from laboratory microbial incubations and radiolabeled tracer experiments

late neogene lacustrine record and palaeogeography in the quillagua llamara basin central andean fore arc northern chile abstract the cenozoic quillagua llamara basin northern chile central andes is an asymmetrical intramassif fore arc basin with a relatively wide northern sector separated from a narrower southward extension by a basement threshold the northern sector was characterised by a noticeable oligocene late neogene alluvial fan and lacustrine dominated deposition which resulted in sequences up to 900 m thick whereas the southern sector was often a bypass zone with thinner fluvial and lacustrine sediment accumulation the basin infill includes two third order alluvial lacustrine unconformity bounded units which include other higher frequency 4th to 5th order sequences the evolution of the late miocene pliocene lacustrine episodes in the quillagua llamara basin was not only controlled by the regional variations from arid to hyperarid palaeoclimate conditions due to the combined influence of the pacific high pressure cell the rain shadow effect exerted by the rising andes and the northward flowing cold oceanic currents but also by a the extensional tectonics and evolution and uplift of the fore arc region which defined the location and size of the depocentres b the resulting basement palaeorelief which affected sediment thickness and facies distribution during the late basin infill episodes and c the tectonic modifications of watersheds water divides and drainage networks in the precordillera which caused considerable changes of water income in the lacustrine systems understanding of this regional tectonosedimentary evolution is a necessary first step before analysing of the low to high order lacustrine sequence changes in the region lacustrine water supply was very sensitive to tectonics even gentle tectonic tilting and uplifting in critical water divide zones could result in changes in water balance in the lacustrine basins and trigger variations in the depositional record the very conspicuous lacustrine regime changes recorded in the quillagua llamara basin infill cannot be considered in themselves conclusive proof of an exclusive climatic forcing since they took place close to either major regional drainage changes or to gentle but noticeable tectonic reactivation in the fore arc region