ice stream surface texture sticky spots waves and breathers the coupled flow of ice till and water this paper addresses the coupling of flows of ice till and water and the issue of whether such coupling provides mechanisms for meso scale kilometres to tens of kilometres variability in ice sheet flow and texture the question of whether effective pressures at the ice bed interface are statically or hydraulically controlled is examined in this paper the answer is scale dependent and has a significant effect on the relationship between ice surface and basal topography the consequences of these considerations on till flow coupled ice till flow and coupled ice till and water flow are examined an analysis of till flow kinematics and shock formation is carried out the linear stability of coupled long wavelength ice till flow is analysed and regions in parameter space where this flow is unstable with rather small rate constants are found upstream moving ice surface waves are predicted the linear stablity of coupled ice till water flow is examined where water flow is modelled using a basal flow system with effective pressure dependent properties again regions in parameter space where the system is linearly unstable are found this time with relatively rapid rate constants the water pressure exhibits breather modes these analyses assume that there is a substantial basal traction a problem with models of ice streams wholly restrained at the side is identified they seem to predict erosion rates which are unfeasibly large there appears to be sufficient variability in the ice till water system to potentially explain texture in ice stream surfaces variations in ice stream thickness of tens of metres not directly relatable to topography and waves moving upstream or downstream most importantly the ice stream bed system is shown to exhibit meso scale variability simply by coupling ice flow according to the shallow ice approximation till flow according to the hydrostatic thin till approximation and water flow according to an effective pressure dependent hydraulics

controlling factors of ca isotope fractionation in scleractinian corals evaluated by temperature ph and light controlled culture experiments abstract in this study the 44ca 40ca ratios of porites australiensis grown under three different culture experiments composed of temperature ph and light controlled culture experiments are measured the temperature dependent isotope fractionation of 0 02 c deduced from this study is similar to inorganic aragonite but the degree of isotope fractionation is about 0 4 offset in corals these observations agree with earlier results on different coral species suggesting ca isotope fractionation during ca transmembrane transport in corals while in cultured corals a significant temperature dependence of 44ca is observed the relationships between calcium isotope fractionation and ph as well as light intensity are negligible therefore variation of 44ca in porites corals is mainly controlled by temperature a combination of 44ca and sr ca of corals in temperature controlled experiments cannot be explained by rayleigh type fractionation directly from a fluid which is seawater like in terms of 44ca and sr ca through coral specific biomineralization processes overall mean 44ca of scleractinian corals including previous studies are different from biogenic aragonites secreted by sclerosponges and pteropods but are comparable with those of bivalves as well as calcitic coccolithophores and foraminifers these findings are important for better understanding biomineralization in corals and in order to constrain the ca isotopic composition of oceanic ca sinks in response to climate changes and associated with shifts of calcite and aragonite seas

acid hot springs discharged from the rehai hydrothermal system of the tengchong volcanic area china formed via magmatic fluid absorption or geothermal steam heating across mainland china tengchong is geothermally unique because both magmatic hydrothermal systems and late cenozoic volcanic activities have been observed there rehai located in the tengchong volcanic area is the only hydrothermal system in mainland china discharging acid springs with very low ph values two types of acid geothermal waters were identified one is sulfate waters with low chloride concentrations and ph levels as low as around 2 the other has slightly higher chloride concentrations and is marked with diversified hydrochemical types both types of acid springs at rehai are immature waters and far from being equilibrated with a thermodynamically stable mineral assemblage comprising albite k feldspar muscovite and clinochlore characteristic for felsic rock hosted reservoirs in contrast to those neutral chloride springs at rehai that are formed via boiling conductive cooling or mixing with cold groundwaters of a parent geothermal liquid pgl the sulfate rich acid waters discharged from the diretiyan area the dagunguo area and the shizitou spring of rehai are essentially locally perched groundwaters heated by h2s rich steam separated from deep geothermal fluid the acidity of these waters originates from the oxidation of h2s in near surface environment in contrast the acid waters with higher chloride concentrations are the results of mixing between steam heated waters and neutral chloride waters the geochemical geneses of the rehai geothermal springs inferred based on stable hydrogen and oxygen isotopes are generally identical unlike active volcanic areas across the world there is a lack of acid waters at rehai formed through partial neutralization of magmatic fluid possibly owing to the comparatively deep seated magma chamber there no less than 7 km deep

a simple biomineralization model to explain li mg and sr incorporation into aragonitic foraminifera and corals abstract in contrast to li ca and mg ca li mg is strongly anticorrelated with temperature in aragonites precipitated by the benthic foraminifer hoeglundina elegans and a wide range of scleractinian coral taxa we propose a simple conceptual model of biomineralization that explains this pattern and is consistent with available abiotic aragonite partition coefficients under this model the organism actively modifies seawater within its calcification pool by raising its ca2 using a pump that strongly discriminates against both li and mg2 rayleigh fractionation during calcification effectively reverses this process removing ca2 while leaving most li and mg2 behind in the calcifying fluid the net effect of these two processes is that li mg in the calcifying fluid remains very close to the seawater value and temperature dependent abiotic partition coefficients are expressed in the biogenic aragonite li mg ratio we further show that coral sr ca is consistent with this model if the ca2 pump barely discriminates against sr2 in h elegans the covariation of sr ca and mg ca requires either that the pump more strongly discriminates against sr2 or that cation incorporation is affected by aragonite precipitation rate via the mechanism of surface entrapment in either case li mg is minimally affected by such vital effects which plague other elemental ratio paleotemperature proxies

calcite and aragonite precipitation from seawater solutions of various salinities precipitation rates and overgrowth compositions abstract the reaction rate and composition of calcite and aragonite overgrowths precipitated from seawater solutions of various salinities i e s 5 15 25 35 44 were determined at 25 c and 10 2 5 atm co2 partial pressure using a constant disequilibrium seeded technique the rate data were fitted to an empirical rate law of the form log r n c or a 1 log k where n is the empirical reaction order and k is the rate constant calcite precipitation rates in seawater solutions do not vary appreciably as a result of salinity variations over the range investigated while those for aragonite show an increase in going from the higher i e s 35 44 to the lower i e s 5 15 25 salinity range this study also confirms previously published findings that above a given saturation state i e c 2 6 aragonite precipitates more rapidly than calcite at 25 c the incorporation of sr2 in aragonite and mg2 in calcite overgrowths are independent of the precipitation rate the partition coefficient of sr2 in aragonite is approximately equal to unity and is unaffected by salinity variations between 5 and 44 however the mg2 partition coefficient in calcite increases with decreasing salinity of the parent seawater solutions possibly as a result of variations in the sulfate content of the solutions and solids the experimental results were discussed in the context of a number of geological environments

temperature dependence of δ 7 li δ 44 ca and li ca during growth of calcium carbonate abstract lithium and calcium isotope ratios together with the li ca element ratio have been measured in calcites precipitated inorganically at temperatures ranging from 5 to 30 c and on seasonal growth bands from a coral these measurements indicate the potential uses of these three environmental proxies carbonate li isotopes are lighter than the solution from which they grow probably due to equilibrium fractionation this fractionation is not significantly influenced by temperature and because li is not involved in any known biological process this suggests that carbonates will provide a recorder for the past li isotope composition of natural waters and hence for past weathering conditions as has been observed previously for inorganic aragonite the ca isotopes in inorganic calcite are lighter than the growth solution and are only weakly dependent on temperature this confirms that the temperature dependence observed in some species of foraminifera geochem geophys geosyst 1 2001 reflects additional isotope fractionation due to biological processes during mineralization ca isotopes may therefore provide a powerful tool with which to investigate the processes of biomineralization finally li ca ratios in inorganic and biogenic carbonates increase as temperatures decrease with a sensitivity of 4 per c this temperature dependence of li ca may prove useful to assess past temperatures in cold environments such as the deep ocean

ca and mg isotope constraints on the origin of earth s deepest δ13c excursion abstract understanding the extreme carbon isotope excursions found in carbonate rocks of the ediacaran period 635 541 ma where 13 c of marine carbonates 13 c carb reach their minimum 12 for earth history is one of the most vexing problems in precambrian geology known colloquially as the shuram excursion the event has been interpreted by many as a product of a profoundly different ediacaran carbon cycle more recently diagenetic processes have been invoked with the very negative 13 c values of ediacaran carbonates explained via meteoric alteration late stage burial diagenesis or growth of authigenic carbonates in the sediment column thus challenging models which rely upon a dramatically changing redox state of the ediacaran oceans here we present 257 44 40 ca and 131 26 mg measurements along with mg mn and sr data from carbonates of the ediacaran aged wonoka formation fm of south australia to bring new isotope systems to bear on understanding the shuram excursion data from four measured sections spanning the basin reveal stratigraphically coherent trends with variability of 1 5 in 26 mg and 1 2 in 44 40 ca this ca isotope variability dwarfs the 0 2 0 3 change seen coeval with the permian triassic mass extinction the largest recorded in the rock record and is on par with putative changes in the 44 40 ca value of seawater seen over the phanerozoic eon changes in both isotopic systems are too large to explain with changes in the isotopic composition of ca and mg in global seawater given modern budgets and residence times and thus must be products of alternative processes relationships between 44 40 ca and sr and 26 mg and mg are consistent with mineralogical control e g aragonite vs calcite limestone vs dolostone on calcium and magnesium isotope variability the most pristine samples in the wonoka dataset preserving sr concentrations in the 1000s of ppm range and 44 40 ca values inherited from an originally aragonitic polymorph have 13 c carb of 8 to 7 thereby providing strong geochemical evidence that extremely negative 13 c carb values are primary products of the ediacaran surface environment

calcium isotopic fractionation in microbially mediated gypsum precipitates abstract gypsum caso4 2h2o precipitation experiments were carried out at low ph in the presence of the sulfur oxidizing bacterium acidithiobacillus thiooxidans the observed ca isotopic fractionation expressed as 44 40cas f 44 40casolid 44 40cafluid at the end of each experimental time period 50 to 60 days was 1 41 to 1 09 in the biotic experiments 1 09 in the killed control and 1 01 to 0 88 in the abiotic controls as there were no strong differences in the solution chemistry and the rate at which gypsum precipitated in the biotic and abiotic controls we deduce a biological ca isotope effect on the order of 0 3 the isotope effect correlates with a difference in crystal aspect ratios between the biotic experiments 8 05 3 99 and abiotic controls 31 9 8 40 we hypothesize that soluble and or insoluble organic compounds selectively inhibit crystal growth at specific crystal faces and that the growth inhibition affects the fractionation factor associated with gypsum precipitation the experimental results help explain ca isotopic variability in gypsum sampled from a sulfidic cave system in which gypsum crystals exhibiting a diversity of morphologies microcrystalline to cm scale needles have a broad range of 44 40ca values 1 2 0 4 relative to the limestone wall 44 40ca 1 3 in light of the laboratory experiments the variation in ca isotope values in the caves can be interpreted as a consequence of gypsum precipitation in the presence of microbial organic matter and subsequent isotopic re equilibration with the ca source

ca isotope study of ordovician dolomite limestone and anhydrite in the williston basin implications for subsurface dolomitization and local ca cycling abstract a difference of 0 61 is reported between dolomite 1 66 and its precursor limestone 1 05 in the yeoman formation red river equivalent of the williston basin southeastern saskatchewan the significance of the large difference found and the preference for light isotope enrichment in the dolomite is evaluated with assistance from 87 sr 86 sr ratios and dolomitization models previously proposed for the studied rocks in particular i explore the possibility that 44 ca values in dolomite reflect the 44 ca values of the dolomitizing fluids without any correction for mineral fluid fractionation this hypothesis is based on recent studies showing negligible isotopic fractionation between carbonates and ca 2 bearing waters at very low precipitation rates fantle and depaolo 2007 jacobson and holmden 2008 if correct calcium isotopes hold promise as a tool for discriminating among numerous hydrological models of dolomite formation as a case in point the 44 ca value of the yeoman dolomite 1 66 is too low to reflect ca derived from the following sources 1 the original limestone 1 05 2 overlying beds of anhydrite 1 28 or 3 evaporated seawater 0 25 paleozoic carbonates ranging as low as 1 7 appear to be the lightest source of ca in the basin succession suggesting that the mg containing dolomitizing fluid was a connate water with a previous history of water rock interactions with paleozoic carbonates possibly involving earlier episodes of dolomitization and light ca release into migrating basinal fluids the relatively high seawater 44 ca values inferred from measurements of yeoman limestone and lake alma anhydrite suggest that seawater in the williston basin 450 ma was 0 22 0 46 higher in 44 ca than the contemporaneous ocean based on globally distributed brachiopods farkas et al 2007 this finding suggests that 44 ca values preserved in the deposits of epeiric seas may bias the 44 ca reconstruction of the ocean secular record if local ca cycling effects are not taken into account equations describing the isotope balance of ca in an epeiric sea indicate that seawater 44 ca values may be higher lower or equal to the 44 ca value of the ocean depending on the sizes of local scale ca deposition and weathering fluxes in relation to seawater ca exchange fluxes

evolution of the oceanic calcium cycle during the late mesozoic evidence from δ44 40ca of marine skeletal carbonates abstract the ca isotope compositions of 37 late mesozoic skeletal carbonates belemnites and brachiopods from the tethyan realm were analyzed by thermal tims and plasma mc icp ms ionization mass spectrometry a poor correlation between 44 40 ca and 18 o values of belemnites suggests only a weak temperature dependency for the ca isotope composition of belemnites likely less than 0 02 c the 44 40 ca record of belemnites was therefore used to reconstruct the ca isotope composition of paleo seawater 44 40 ca sw based on an experimentally determined fractionation factor between seawater ca and belemnite calcite cc sw of 0 9986 the inferred 44 40 ca sw record with an average stratigraphic resolution of 1 ma shows systematic temporal variation of 0 5 with the middle late jurassic 154 ma minimum of 1 4 and a subsequent general increase to the early cretaceous 124 ma maximum of 1 9 the global nature of the 44 40 ca sw record is supported by identical ca isotope compositions of coeval kimmeridgian belemnites collected from two distinct paleogeographic regions the southern new zealand and northern germany margin of the tethys ocean the observed late mesozoic 44 40 ca sw record was simulated using a simple ca isotope mass balance model and the results indicate that the variation in 44 40 ca sw record can be explained by changes in oceanic input fluxes of ca that were independent of the carbonate ion fluxes such as the hydrothermal ca flux or the release of ca to the oceans via dolomitization of marine carbonates

oxygen isotope fractionation in marine aragonite of coralline sponges oxygen isotope values of the extant caribbean coralline sponge ceratoporella nicholsoni are compared with published temperatures and 18o of water calculated from salinities the measured values from aragonitic sponge skeletons have a mean offset of 1 0 0 1 from calculated calcite equilibrium values aragonite calcite 1 0010 this is in good agreement with published values from synthetic aragonite they further agree with published near equilibrium oxygen isotope values of temperate and cold water molluscs and foraminifera extrapolated to the temperature range of the coralline sponges these results and the mode of skeleton formation of ceratoporella nicholsoni suggest that these sponges precipitate aragonite close to isotopic equilibrium the temperature dependence of oxygen isotopic fractionation between the aragonite of ceratoporella nicholsoni and water is only roughly constrained by the available data due to the narrow temperature range of the caribbean reef sites however as the data suggest oxygen isotopic equilibrium we can calculate a well constrained temperature equation combining temperate and cold water equilibrium values from molluscs and foraminifera with our sponge data full size image 1 k and full size image 1 k

the giant pacific oyster crassostrea gigas as a modern analog for fossil ostreoids isotopic ca o c and elemental mg ca sr ca mn ca proxies 1 modern analogs are an essential part of palaeoclimate studies because they provide the basis for the understanding of geochemical signatures of fossils ostreoids are common in many sedimentary sequences and because of their fast growth high temporal resolution sampling of past seasonal variability is possible here two shell structures of modern giant pacific oysters crassostrea gigas the chalky substance and foliate layers have been sampled for trace element distributions mg sr mn and stable isotope variability c o ca oxygen isotopes exhibit a clear seasonal signature mean carbon isotope values of different oysters agree within 0 1 but ontogenic variability is complicated by shell growth patterns and potential small vital effects the calcium isotope ratios are found to be constant throughout ontogeny within analytical precision at a value of 44 40ca 0 68 0 16 2 sd srm 915a which is consistent with other bivalve species calcium isotope ratios in oyster shell material might thus be a possible proxy for palaeo seawater calcium isotope ratios element ca ratios are significantly higher in the chalky substance than in the foliate layers and especially high sr ca and mn ca ratios are observed for the first growth season of the oysters mg ca ratios in the chalky substance show a negative correlation with 18o values compatible with a temperature dependence whereas this correlation is absent in the foliate layers seasonal changes of sr ca are controlled by metabolic processes whereas for mn ca an additional environmental control is evident

calcium isotopes in the marine environment and the oceanic calcium cycle abstract a systematic investigation of 44 ca 40 ca ratios in the oceanic ca cycle indicates that the average ca isotopic composition of input and output are quite different suggesting that the present ocean is not in a steady state our data show that sea water has uniform 44 ca 40 ca as expected from the long residence time of ca in the oceans it also exhibits the heaviest isotopic composition among all samples measured consistent with recent results by skulan et al 1997 and must be maintained at this value by strong biogenic isotope fractionation during the precipitation of caco 3 our measurements show that the riverine input is variable the most radiogenic river waters we have measured lowest 44 ca 40 ca are from ganges tributaries which also have high 87 sr 86 sr although there does not appear to be a systematic correlation between ca and sr isotopic compositions based on available data there is also significant isotopic variability among samples of caco 3 precipitated from the present ocean with coccolith oozes showing the most highly fractionated isotopic values relative to seawater among foraminifera ca isotopes exhibit species dependent fractionation effects but also appear to display systematic temperature dependent fractionation within inidividual species

44ca 42ca and 143nd 144nd isotope variations in cretaceous eocene tethyan francolites and their bearing on phosphogenesis in the southern tethys measurements of 4 4 ca 4 2 ca and 1 4 3 nd 1 4 4 nd isotope ratios in carbonate fluorapatite cfa through the aptian eocene section of the negev israel together with quantified rates of p and ca accumulation and bulk sedimentation permit an examination of the relationships of these five factors to tethyan phosphogenesis the data provide an aptian eocene ca 70 ma record of the ca isotope composition of 35 cfa samples 24 of which were also analyzed for nd isotopes representing 11 time stratigraphic phosphate horizons within the sequence the 4 4 ca values are significantly lower in the aptian albian samples 4 4 ca 0 11 to 0 06 than in the campanian eocene samples 4 4 ca 0 22 to 0 42 whereas the e n d values increase from continental crust like in the aptian albian e n d t 10 9 to more radiogenic pacific like e n d t 6 6 to 6 1 in the campanian both peaks of 4 4 ca and e n d t in the campanian coincide with the peak of tethyan phosphogenesis in the negev which is marked by a sharp increase of p accumulation rates from 200 mol cm 2 k y 1 in pre campanian time to 1700 mol cm 2 k y 1 in the campanian and a decrease in rates of ca accumulation and bulk sedimentation the coincident increases of 4 4 ca and e n d t values and p accumulation rates in the negev area during the campanian suggest that they are related in this time interval and were induced by the global late cretaceous sea level rise and increasing circumequatorial tethyan flow

ca isotopes in carbonate sediment and pore fluid from odp site 807a the ca2 aq calcite equilibrium fractionation factor and calcite recrystallization rates in pleistocene sediments abstract the calcium isotopic compositions 44ca of 30 high purity nannofossil ooze and chalk and 7 pore fluid samples from odp site 807a ontong java plateau are used in conjunction with numerical models to determine the equilibrium calcium isotope fractionation factor s f between calcite and dissolved ca2 and the rates of post depositional recrystallization in deep sea carbonate ooze the value of s f at equilibrium in the marine sedimentary section is 1 0000 0 0001 which is significantly different from the value 0 9987 0 0002 found in laboratory experiments of calcite precipitation and in the formation of biogenic calcite in the surface ocean we hypothesize that this fractionation factor is relevant to calcite precipitation in any system at equilibrium and that this equilibrium fractionation factor has implications for the mechanisms responsible for ca isotope fractionation during calcite precipitation we describe a steady state model that offers a unified framework for explaining ca isotope fractionation across the observed precipitation rate range of 14 orders of magnitude the model attributes ca isotope fractionation to the relative balance between the attachment and detachment fluxes at the calcite crystal surface this model represents our hypothesis for the mechanism responsible for isotope fractionation during calcite precipitation the ca isotope data provide evidence that the bulk rate of calcite recrystallization in freshly deposited carbonate ooze is 30 40 myr and decreases with age to about 2 myr in 2 3 million year old sediment the recrystallization rates determined from ca isotopes for pleistocene sediments are higher than those previously inferred from pore fluid sr concentration and are consistent with rates derived for late pleistocene siliciclastic sediments using uranium isotopes combining our results for the equilibrium fractionation factor and recrystallization rates we evaluate the effect of diagenesis on the ca isotopic composition of marine carbonates at site 807a since calcite precipitation rates in the sedimentary column are many orders of magnitude slower than laboratory experiments and the pore fluids are only slightly oversaturated with respect to calcite the isotopic composition of diagenetic calcite is likely to reflect equilibrium precipitation accordingly diagenesis produces a maximum shift in 44ca of 0 15 for site 807a sediments but will have a larger impact where sedimentation rates are low seawater circulates through the sediment pile or there are prolonged depositional hiatuses

sr ca ratios and oxygen isotopes from sclerosponges temperature history of the caribbean mixed layer and thermocline during the little ice age we investigate aragonitic skeletons of the caribbean sclerosponge ceratoporella nicholsoni from jamaica 20 m below sea level mbsl and pedro bank 125 mbsl we use d18o and sr ca ratios as temperature proxies to reconstruct the caribbean mixed layer and thermocline temperature history since 1400 a d with a decadal time resolution our age models are based on u th dating and locating of the radiocarbon bomb spike the modern temperature difference between the two sites is used to tentatively calibrate the c nicholsoni sr ca thermometer the resulting calibration points to a temperature sensitivity of sr ca in c nicholsoni aragonite of about 0 1 mmol mol k our sr ca records reveal a pronounced warming from the early 19th to the late 20th century both at 20 and 125 mbsl two temperature minima in the shallow water record during the late 17th and early 19th century correspond to the maunder and dalton sunspot minima respectively another major cooling occurred in the late 16th century and is not correlatable with a sunspot minimum the temperature contrast between the two sites decreased from the 14th century to a minimum in the late 17th century and subsequently increased to modern values in the early 19th century this is interpreted as a long term deepening and subsequent shoaling of the caribbean thermocline the major trends of the sr ca records are reproduced in both specimens but hardly reflected in the d18o records

the δ44ca temperature calibration on fossil and cultured globigerinoides sacculifer new tool for reconstruction of past sea surface temperatures we report direct 44ca temperature calibration on cultured and fossil calcite foraminifera showing that ca isotopes are potentially a new proxy for past sea surface temperatures sst samples have been analyzed using a 43ca 48ca double spike and thermal ionization mass spectrometry tims in order to avoid species dependent isotope fractionation we focused our investigations on a single foraminifera species globigerinoides sacculifer which is known to inhabit shallow euphotic waters in tropical and subtropical oceans ca isotope measurements were performed on cultured g sacculifer that grew in seawater kept at temperatures of 19 5 26 5 and 29 5 c a 44ca change of 0 24 0 02 per 1 c is defined by the weighted linear regression through reproduced 44ca data of the three temperatures 95 confidence level application of this new method to fossil g sacculifer of an equatorial east atlantic sediment core geob1112 5 46 7 s 10 45 0 w 3125 m indicates that the 44ca difference between marine isotope stage 1 mis 1 and mis 2 is 0 71 0 24 according to the current 44ca temperature calibration this value corresponds to a temperature difference between mis 1 and mis 2 of 3 0 1 0 c

carbon isotope records from extant caribbean and south pacific sponges evolution of δ13c in surface water dic stable isotope records of demosponges from the caribbean and coral sea are described for the purpose of studying the influence of fossil fuel co2 on the carbon isotopic composition of dissolved inorganic carbon dic in surface water the slow growing sponges precipitate calcium carbonate in isotopic equilibrium with ambient sea water and are used to detect changes in 13cdic from pre industrial times early 19th century to the present we observed similar shapes and ranges in 13c curves measured on caribbean specimens collected from water depths of 25 84 and 91 m as well as a specimen collected in shallow waters off new caledonia the records reveal a highly significant correlation with atmospheric 13cco2 13cdic values for caribbean and coral sea surface waters were calculated using the 13c sponge records while 13c of atmospheric co2 decreased by about 1 4 from the early 19th century to 1990 13cdic of caribbean and coral sea surface waters decreased by 0 9 0 2 and 0 7 0 3 respectively no isotopic equilibrium between surface water dic and atmospheric co2 was observed either during the pre industrial steady state or during the last 100 years the lower amount of depletion in the surface water 13cdic with respect to the atmospheric anthropogenic signal is explained by the dilution of the surface waters by biologically altered subsurface water dic the lower 13c decrease in the coral sea points to a stronger influence of the subsurface water source compared to the caribbean

effects of ocean acidification on the marine calcium isotope record at the paleocene eocene thermal maximum highlights 44ca was measured in bulk carbonate and barite at two sites over the petm diagenetic effects on 44ca are observed associated with ocean acidification multiple sites and proxy archives necessary to reconstruct the past accurately abstract carbonates are used extensively to reconstruct paleoclimate and paleoceanographic conditions over geologic time scales however these archives are susceptible to diagenetic alteration via dissolution recrystallization and secondary precipitation particularly during ocean acidification events when intense dissolution can occur despite the possible effects of diagenesis on proxy fidelity the impacts of diagenesis on the calcium isotopic composition 44ca of carbonates are unclear to shed light on this issue bulk carbonate 44ca was measured at high resolution in two pacific deep sea sediment cores odp sites 1212 and 1221 with considerably different dissolution histories over the paleocene eocene thermal maximum petm 55 ma 55 ma the 44ca of marine barite was also measured at the deeper site 1221 which experienced severe carbonate dissolution during the petm large variations 0 8 0 8 in bulk carbonate 44ca occur in the deeper of the two sites at depths corresponding to the peak carbon isotope excursion which correlate with a large drop in carbonate weight percent such an effect is not observed in either the 1221 barite record or the bulk carbonate record at the shallower site 1212 which is also less affected by dissolution we contend that ocean chemical changes associated with abrupt and massive carbon release into the ocean atmosphere system and subsequent ocean acidification at the petm affected the bulk carbonate 44ca record via diagenesis in the sedimentary column such effects are considerable and need to be taken into account when interpreting ca isotope data and potentially other geochemical proxies over extreme climatic events that drive sediment dissolution

calcium isotope fractionation in calcite and aragonite abstract calcium isotope fractionation was measured on skeletal aragonite and calcite from different marine biota and on inorganic calcite precipitation temperatures ranged from 0 to 28 c calcium isotope fractionation shows a temperature dependence in accordance with previous observations 1000 ln cc 1 4 0 021 t c for calcite and 1000 ln ar 1 9 0 017 t c for aragonite within uncertainty the temperature slopes are identical for the two polymorphs however at all temperatures calcium isotopes are more fractionated in aragonite than in calcite the offset in 44 40 ca is about 0 6 the underlying mechanism for this offset may be related to the different coordination numbers and bond strengths of the calcium ions in calcite and aragonite crystals or to different ca reaction behavior at the solid liquid interface recently the observed temperature dependence of the ca isotope fractionation was explained quantitatively by the temperature control on precipitation rates of calcium carbonates in an experimental setting lemarchand et al 2004 we show that this mechanism can in principle also be applied to caco 3 precipitation in natural environments in normal marine settings following this model ca isotope fractionation in marine ca carbonates is primarily controlled by precipitation rates on the other hand the larger ca isotope fractionation of aragonite compared to calcite can not be explained by different precipitation rates the rate control model of ca isotope fractionation predicts a strong dependence of the ca isotopic composition of carbonates on ambient co 3 2 concentration while this model is in general accordance with our observations in marine carbonates cultured specimens of the planktic foraminifer orbulina universa show no dependence of ca isotope fractionation on the ambient co 3 2 concentration the latter observation implies that the carbonate chemistry in the calcifying vesicles of the foraminifer is independent from the ambient carbonate ion concentration of the surrounding water

reconstruction of caribbean sea surface temperature and salinity fluctuations in response to the pliocene closure of the central american gateway and radiative forcing using δ44 40ca δ18o and mg ca ratios the 18o values of planktonic foraminifera increased in the caribbean by about 0 5 relative to the equatorial east pacific values between 4 6 and 4 2 ma as a consequence of the closure of the central american gateway cag this increase in 18o can be interpreted either as an increase in caribbean sea surface mixed layer salinity sss or as a decrease in sea surface temperatures sst this problem represents an ideal situation to apply the recently developed paleotemperature proxy 44 40ca together with mg ca and 18o on the planktic foraminifer globigerinoides sacculifer from odp site 999 although differences in absolute temperature calibration of 44 40ca and mg ca exist the general pattern is similar indicating a sst decrease of about 2 3 c between 4 4 and 4 3 ma followed by an increase in the same order of magnitude between 4 3 and 4 0 ma correcting the 18o record for this temperature change and assuming that changes in global ice volume are negligible the salinity induced planktonic 18o signal decreased by about 0 4 between 4 4 and 4 3 ma and increased by about 0 9 between 4 3 and 4 0 ma in the caribbean the observed temperature and salinity trends are interpreted to reflect the restricted exchange of surface water between the caribbean and the pacific in response to the shoaling of the panamanian seaway possibly accompanied by a southward shift of the intertropical convergence zone itcz between 4 4 and 4 3 ma differences in mg ca and 44 40ca derived temperatures can be reconciled by corrections for secular variations of the marine mg casw and 44 40ca a salinity effect on the mg ca ratio and a constant temperature offset of 2 5 c between both sst proxy calibrations

biological control of calcium isotopic abundances in the global calcium cycle measurements of 44ca 40ca expressed as 44ca were made on igneous rocks and on shell and bone material from modern organisms to investigate the magnitude and origins of calcium isotopic fractionation in nature the results document a span of 4 in 44ca measured with the double spike technique to a precision of 0 15 volcanic rocks including basalt and rhyolite show little variability and cluster near 44ca 0 0 2 systematic analysis of biological samples indicates that biological processing of calcium discriminates against heavy isotopes and that biological fractionation is the primary generator of calcium isotopic fractionation in nature preliminary data suggest that calcium becomes isotopically lighter as it moves through food chains calcium carbonate shells of marine microorganisms and deep sea carbonate ooze have 44ca about 1 0 lower than seawater this fractionation causes seawater to be enriched in heavy calcium 44ca 0 9 relative to igneous rocks marine organisms consequently are isotopically heavier than their terrestrial counterparts at similar trophic level the calcium isotopic composition of living and fossil organisms may record information on diet and environment