boron isotopes and b ca in benthic foraminifera proxies for the deep ocean carbonate system abstract accurate records of the state of the ocean carbonate system are critical for understanding past changes in pco 2 ocean acidification and climate the chemical principles underlying the proxy of oceanic ph provided by the boron isotope ratio of foraminiferal carbonate are relatively well understood but the proxy s reliability has been questioned we present 76 new multi collector inductively coupled plasma mass spectrometry mc icpms 11 b measurements on a range of benthic foraminifera from 23 late holocene samples from the atlantic that reaffirm the utility of the 11 b ph proxy our boron isotope measurements on 10 benthic foraminifera tests typically yield a precision of 0 25 at 2 s d equivalent to 0 03 ph units 11 b values of epifaunal species are within analytical uncertainty of those predicted from a simple model assuming sole incorporation of b oh 4 from seawater and no vital effects using the independently determined fractionation factor of 1 0272 between 11 b 10 b of aqueous boron species infaunal foraminifera are consistent with this model but record the combined effects of lower pore water 11 b and ph no influence of partial dissolution or shell size on 11 b is observed we have also measured the b ca ratios of the same samples for individual cibicidoides species b ca shows a good correlation with co 3 2 but the b ca of different co occurring species morphotypes varies considerably these effects are not seen in 11 b which may therefore provide a more robust proxy of the ocean carbonate system whilst in theory 11 b and b ca can be combined to provide a quantitative reconstruction of alkalinity and dissolved inorganic carbonate dic in practice this is precluded by propagated uncertainties 11 b data give significant constraints on foraminifera calcification mechanisms and seem most simply explained by incorporation of b oh 4 into a hco 3 pool which is then completely incorporated in foraminiferal caco 3 our demonstration of the predictable variation of 11 b with ph across a wide range of species and locations provides confidence in the application of mc icpms measurements of foraminiferal 11 b to reconstruct past changes in the ocean carbonate system

empirical relationship between ph and the boron isotopic composition of globigerinoides sacculifer implications for the boron isotope paleo ph proxy culture experiments were carried out with the planktonic foraminifera globigerinoides sacculifer at three different ph values 7 6 0 05 8 20 0 05 8 60 0 05 in order to establish the ph dependence of the boron isotopic fractionation between the calcitic tests of g sacculifer and seawater additional objectives were to compare the ph dependences of boron isotopic composition of g sacculifer with that of orbulina universa and inorganic calcite in order to evaluate the robustness of the boron isotope paleo ph proxy the relationship between the boron isotopic composition of g sacculifer and the culture water ph shows heavier boron isotopic values at higher ph this empirical relationship is similar but slightly offset by 3 per mil from that of o universa indicating the presence of biogenic influence vital effect on the boron isotopic composition of foraminifera however comparison of the boron isotopic composition of o universa and g sacculifer with that of inorganic calcite grown over a common ph range shows that inorganic processes are the dominant control on the boron isotopic composition of foraminifera and the vital effects play a small but not insignificant role the results imply that in spite of a small biogenic influence the variation in boron isotopic composition of foraminiferal shells with ph at least for o universa and g sacculifer is consistent with that for inorganic calcite demonstrating the potential of this isotopic signature in foraminifera as a reliable paleo ph proxy

reevaluation of the oxygen isotopic composition of planktonic foraminifera experimental results and revised paleotemperature equations cultured planktonic foraminifera orbulina universa symbiotic and globigerina bulloides nonsymbiotic are used to reexamine temperature 18o relationships at 15 25 c relationships for both species can be described by linear equations equations for o universa grown under low light ll and high light hl share a slope of 4 80 0 21 c 1 with a hl ll offset of 0 33 due to symbiont photosynthetic activity the effect of co32 on o universa is 0 002 mol 1 kg 1 and is insensitive to temperature for g bulloides ontogenetic effects produce size related trends in temperature 18o whereby larger shells are enriched in 18o relative to smaller specimens the o universa temperature 18o relationships are more accurate than previously published equations for describing plankton tow data our equations do not explain planktonic core top data with the same precision but provide a good fit to benthic cibicidoides data below 10 c temperature 18o relationships for g bulloides provide good agreement with field data for this species from the northeast pacific

seasonal changes in species composition numbers mass size and isotopic composition of planktonic foraminifera settling into the deep sargasso sea abstract planktonic foraminifera recovered from six successive sediment trap samples collected over fourteen months at a depth of 3200 m near bermuda show varied responses to seasonal changes in the near surface hydrography these include changes in species composition and within species changes in number of individuals collected per unit time average size and weight of individuals and in oxygen and carbon isotopic composition a total of seventeen species were identified and eleven were sufficiently abundant for isotopic analysis in at least some of the samples the accumulations on the sea floor of some of these species can be expected to contain a strong seasonal component in their oxygen isotopic composition which may be utilized in paleoclimatic and paleo oceanographic reconstructions best suited to this purpose are those species which form their tests in isotopic equilibrium with surface water and whose abundance is confined to a portion of the year only the best examples are pulleniatina o bliguiloculata globigerinoides conglobatus and neogloboquadrina dutertrei globigerinoides ruber is a good estimator of the average surface water temperature because it occurs in rather even abundance throughout the year hastigerina pelagica though very abundant in surface water is practically eliminated even before arrival on the sea floor by partial resorption and structural weakening of the tests during gametogenesis orbulina universa globorotalia truncaltulinoides and globorotalia hirsuta show morphological and istopic evidence for contibnued calcification in deeper water after initial test formation near ther surface globigerina bulloides represents the most striking example of oxygen isotopic disequilibrium with sea water

alkenone and boron based pliocene pco2 records abstract the pliocene period is the most recent time when the earth was globally significantly 3 c warmer than today however the existing pco2 data for the pliocene are sparse and there is little agreement between the various techniques used to reconstruct palaeo pco2 this disagreement coupled with the general low temporal resolution of the published records does not allow a robust assessment of the role of declining pco2 in the intensification of the northern hemisphere glaciation inhg and a direct comparison to other proxy records are lacking for the first time we use a combination of foraminiferal 11b and organic biomarker alkenone derived carbon isotopes proxies to determine the concentration of atmospheric co2 over the past 5 ma both proxy records show that during the warm pliocene pco2 was between 330 and 400 ppm i e similar to today the decrease to values similar to pre industrial times 275 285 ppm occurred between 3 2 ma and 2 8 ma coincident with the inhg and affirming the link between global climate the cryosphere and pco2

effects of seawater carbonate ion concentration and temperature on shell u mg and sr in cultured planktonic foraminifera we investigate the sensitivity of u ca mg ca and sr ca to changes in seawater co3 and temperature in calcite produced by the two planktonic foraminifera species orbulina universa and globige rina bulloides in laboratory culture experiments our results demonstrate that at constant temperature u ca in o universa decreases by 25 7 per 100 mol co3 k g 1 as seawater co3 increases from 110 to 470 mol kg 1 results from g bulloides suggest a similar relationship but u ca is consistently offset by 40 at the same environmental co3 in o universa u ca is insensitive to temperature between 15 c and 25 c applying the o universa relationship to three u ca records from a related species globigerinoides sacculifer we estimate that caribbean and tropical atlantic co3 was 110 70 mol kg 1 and 80 40 mol kg 1 higher respectively during the last glacial period relative to the holocene this result is consistent with estimates of the glacial interglacial change in surface water co3 based on both modeling and on boron isotope ph estimates in settings where the addition of u by diagenetic processes is not a factor down core records of foraminiferal u ca have potential to provide information about changes in the ocean s carbonate concentration below ambient ph ph 8 2 mg ca decreased by 7 5 o universa t o 16 6 g bulloides per 0 1 unit increase in ph above ambient ph the change in mg ca was not significant for either species this result suggests that mg ca based paleotemperature estimates for the quaternary during which surface ocean ph has been at or above modern levels have not been biased by variations in surface water ph sr ca increased linearly by 1 6 0 4 per 0 1 unit increase in ph shell mg ca increased exponentially with temperature in o universa where mg ca 0 85 exp 0 096 t whereas the change in sr ca with temperature was within the reproducibility of replicate measurements copyright 2004 elsevier ltd

ground truthing the boron isotope paleo ph proxy in planktonic foraminifera shells partial dissolution and shell size effects 1 sediment samples from the ontong java plateau in the pacific and the 90 east ridge in the indian ocean were used to investigate whether shell size and early diagenesis affect 11b of the symbiont bearing planktonic foraminifer globigerinoides sacculifer in pristine shells from both study locations we found a systematic increase of 11b and mg ca with shell size shells in the sieve size class 515 865 m revealed 11b values 2 1 to 2 3 higher than shells in the 250 380 m class this pattern is most likely due to differences in symbiont photosynthetic activity and its integrated effect on the ph of the foraminiferal microenvironment we therefore suggest smaller individuals must live at approximately 50 100 m water depth where ambient light levels are lower using the empirical calibration curve for 11b in g sacculifer only shells larger than 425 m reflect surface seawater ph partial dissolution of shells derived from deeper sediment cores was determined by shell weight analyses and investigation of the shell surface microstructure by scanning electron microscopy the 11b in partially dissolved shells is up to 2 lower relative to pristine shells of the same size class in agreement with a relatively higher weight loss in smaller shells samples from the ontong java plateau show a more pronounced dissolution effect than larger shells on the basis of the primary size effect and potential postdepositional dissolution effects we recommend the use of shells that are visually pristine and in the case of g sacculifer larger than 500 m for paleoreconstructions

the universal ratio of boron to chlorinity for the north pacific and north atlantic oceans abstract we report seawater boron concentration mg kg 1 and chlorinity values measured in seawater samples n 139 collected at various depths in the north pacific and north atlantic oceans and the east japan sea located in the western temperate north pacific our results indicate that variations in seawater boron concentration are strongly coupled to variations in chlorinity and salinity yielding a mean boron to chlorinity ratio of 0 2414 0 0009 mg kg 1 1 boron to salinity ratio 0 1336 0 0005 mg kg 1 1 this ratio was surprisingly universal throughout the water column in the three marine basins and across widely different ocean surface regimes but differs from the generally accepted ratio of 0 232 0 005 mg kg 1 1 determined by uppstrom 1974 which was based on only 20 measurements at four sites in the tropical pacific ocean in converting total alkalinity to carbonate alkalinity and vice versa for thermodynamic calculations the difference between these two ratios leads to a difference of 5 mol kg 1 in estimates for ocean surface waters where the contribution of borate to total alkalinity is typically greatest we suggest the use of the new boron to chlorinity ratio for predicting seawater boron concentrations using chlorinity or salinity data

ecological and temperature controls on mg ca ratios of globigerina bulloides from the southwest pacific ocean we present mg ca data for globigerina bulloides from 10 core top sites in the southwest pacific ocean analyzed by laser ablation inductively coupled plasma mass spectrometry la icpms mg ca values in g bulloides correlate with observed ocean temperatures 7 degrees c 19 degrees c and when combined with previously published data an integrated mg ca temperature calibration for 7 degrees c 31 degrees c is derived where mg ca mmol mol 0 955 x e 0 068 x t r 2 0 95 significant variability of mg ca values 20 30 was found for the four visible chambers of g bulloides with the final chamber consistently recording the lowest mg ca and is interpreted in part to reflect changes in the depth habitat with ontogeny incipient and variable dissolution of the thin and fragile final chamber and outermost layer concomitantly added to all chambers caused by different cleaning techniques prior to solution based icpms analyses may explain the minor differences in previously published mg ca temperature calibrations for this species if the lower mg ca of the final chamber reflects changes in depth habitat then la icpms of the penultimate or older chambers will most sensitively record past changes in near surface ocean temperatures mean size normalized g bulloides test weights correlate negatively with ocean temperature t 31 8 x e 30 5xwtn r 2 0 90 suggesting that in the southwest pacific ocean temperature is a prominent control on shell weight in addition to carbonate ion levels

a new boron isotope ph calibration for orbulina universa with implications for understanding and accounting for vital effects boron isotope ratios as measured in planktic foraminifera can be a useful tracer of past ocean ph and hence help to discern the concentration of co2 in the ancient atmosphere however different species of planktic foraminifera demonstrate different patterns of boron isotope variation with ambient seawater ph therefore when applying the proxy to questions in the geological past species specific calibrations are preferable beyond the evolutionary history of a calibrated species we must rely on our understanding of the causes of the observed vital effects in the modern ocean and the applicability of that understanding to extinct species here we present a new open ocean calibration of the planktic foraminifera orbulina universa measured via multi collector inductively coupled mass spectrometry mc icpms unlike other symbiont bearing foraminifera o universa record a 11b 11b and hence ph that is lower than its surrounding seawater but with a ph sensitivity roughly equal to that of aqueous borate ion we discuss the significance of this for application of the boron isotope proxy in deep time with recommendations for best practice and future research directions

origin and evolution of a migmatite the development of a stromatic migmatite exposed east and southeast of arvika western sweden is described in four stages beginning with the country rock and following evolution through three areas characterized by low medium and high amounts of leucosomes areas l m and h respectively

opening the carbon isotope vital effect black box 1 seasonal temperatures in the euphotic zone recent laboratory experiments with orbulina universa show that the carbon and oxygen isotope signals from this symbiont bearing planktonic foraminifera record distinct environmental parameters irradiance and temperature since light levels are correlated with depth in the marine environment the carbon isotopic composition of o universa can be utilized to differentiate individuals from different depth habitats by combining 18o and 13c data from individuals in an assemblage estimates of seasonal temperature ranges are possible for the upper and lower euphotic zone of the ocean reconstruction of seasonal temperatures using individual holocene o universa from western equatorial pacific ocean boxcore erdc 83 yields ranges of 23 5 to 29 9 c and 20 1 to 24 5 c for the shallow and deep euphotic zone respectively these values are in excellent agreement with recent temperature measurements from this region analyses of individual late glacial 12 000 13 000 years b p o universa from this site suggest the seasonal upper euphotic zone temperature range was approximately 2 5 c lower than it is today a function of cooler summers

a core top assessment of proxies for the ocean carbonate system in surface dwelling foraminifers we have assessed the reliability of several foraminifer hosted proxies of the ocean carbonate system 11b b ca and u ca using holocene samples from the atlantic and pacific oceans we examined chemical variability over a range of test sizes for two surface dwelling foraminifers globigerinoides sacculifer and globigerinoides ruber measurements of 11b in g ruber show no significant relationship with test size in either atlantic or pacific sites and appear to provide a robust proxy of surface seawater ph likewise there is no significant variability in the 11b of our atlantic core top g sacculifer but we find that 11b increases with increasing test size for g sacculifer in the pacific these systematic differences in 11b are inferred to be a consequence of isotopically light gametogenic calcite in g sacculifer and its preferential preservation during postdepositional dissolution the trace element ratio proxies of ocean carbonate equilibria u ca and b ca show systematic increases in both g ruber and g sacculifer with increasing test size possibly as a result of changing growth rates this behavior complicates their use in paleoceanographic reconstructions in keeping with several previous studies we find that mg ca ratios increase with increasing size fraction in our well preserved atlantic g sacculifer but not in g ruber in contrast to previous interpretations we suggest that these observations reflect a proportionally larger influence of compositionally distinct gametogenic calcite in small individuals compared to larger ones as with 11b this influences g sacculifer but not g ruber which has negligible gametogenic calcite

b ca in planktonic foraminifera as a proxy for surface seawater ph boron isotope systematics indicate that boron incorporation into foraminiferal caco3 is determined by the partition coefficient k d frac left rm b rm ca right rm caco3 left rm b left rm oh right 4 rm hco 3 right rm seawater and b oh 4 hco3 seawater providing in principle a method to estimate seawater ph and pco2 we have measured b ca ratios in globigerina bulloides and globorotalia inflata for a series of core tops from the north atlantic and the southern ocean and in globigerinoides ruber white from ocean drilling program odp site 668b on the sierra leone rise in the eastern equatorial atlantic b ca ratios in these species of planktonic foraminifera seem unaffected by dissolution on the seafloor k d shows a strong species specific dependence on calcification temperature which can be corrected for using the mg ca temperature proxy a preliminary study of g inflata from southern ocean sediment core chat 16k suggests that temperature corrected b ca was 30 higher during the last glacial correspondingly ph was 0 15 units higher and aqueous pco2 was 95 atm lower at this site at the last glacial maximum the covariation between reconstructed pco2 and the atmospheric pco2 from the vostok ice core demonstrates the feasibility of using b ca in planktonic foraminifera for reconstructing past variations in ph and pco2

crystallographic control on the boron isotope paleo ph proxy when using the boron isotopic composition 11b of marine carbonates as a seawater ph proxy it is assumed that only the tetrahedral borate ion is incorporated into the growing carbonate crystals and that no boron isotope fractionation occurs during uptake however the 11b of the calcium carbonate from most modern foraminifera shells or corals skeletons is not the same as the 11b of seawater borate which depends on ph an observation commonly attributed to vital effects in this study we combined previously published high field 11b mas nmr and new 11b measurements on the same synthetic calcite and aragonite samples precipitated inorganically under controlled environments to avoid vital effects our results indicate that the main controlling factors of 11b are the solution ph and the mineralogy of the precipitated carbonate mineral whereas the aqueous boron concentration of the solution caco3 precipitation rate and the presence or absence of growth seeds all appear to have negligible influence in aragonite the nmr data show that boron coordination is tetrahedral bo4 in addition its 11b is equal to that of aqueous borate thus confirming the paleo ph hypothesis in contrast both trigonal bo3 and tetrahedral bo4 are present in calcite and its 11b values are higher than that of aqueous borate and are less sensitive to solution ph variations compared to 11b in aragonite these observations are interpreted in calcite as a reflection of the incorporation of decreasing amounts of boric acid with increasing ph moreover the fraction of bo3 measured by nmr in calcite is higher than that inferred from 11b which indicates a coordination change from bo4 to bo3 upon boron incorporation in the solid overall this study shows that although the observed differences in 11b between inorganic and biological aragonite are compatible with a ph increase at calcification sites the b speciation and isotope composition of biological calcites call for a more complex mechanism of boron incorporation

a radiolarian based modern analogue dataset for palaeoenvironmental reconstructions in the southwest pacific abstract radiolarian species relative abundances have been estimated for a suite of 88 surface sediment samples obtained from a broad area of the southwest pacific southern ocean bounded by 10 s to 65 s in latitude and 145 e to 170 w in longitude this allowed the documentation of a long environmental gradient 28 8 to 1 4 c for mean annual sea surface temperature sst and covered the most important surface oceanographic features in the area including the subtropical stf subantarctic saf and antarctic polar apf fronts the most abundant species in the dataset show strong relationships to surface hydrography examples are antarctissa spp 5 abundance isoline is a good indicator for the average position of the saf colonials 5 isoline approximates the average location of the tasman front between new zealand and australia dictyocoryne profunda and tetrapyle octacantha essentially absent in subantarctic antarctic waters and documenting warm water connections to higher latitudes being present in the tasmanian retroflection region and or on the campbell plateau the factor analysis model with a calibration error of 1 3 c was based on 93 taxa of the 243 taxa recognised in this study and 7 factors assemblages which explained 87 4 of the total information contained in the dataset the first five assemblages are readily interpreted in terms of water masses oceanographic fronts and regions subtropical water tasman front factor 1 circumpolar surface water factor 2 equatorial pacific factor 3 subantarctic water campbell plateau factor 4 and as a subfactor for subtropical water factor 5 a canonical correspondence analysis identified sea surface temperature as the main environmental explanatory variable for samples with sst 7 c 28 c due to the elevated nitrate concentrations in high southern latitude surface waters nitrate concentration is the main explanatory variable when all samples are considered in a companion paper this modern analogue dataset has been applied to relative abundance data from sediment core y9 in order to reconstruct the sst changes over the last 160 ka at the easternmost boundary of the campbell plateau east of new zealand

the effects of secular calcium and magnesium concentration changes on the thermodynamics of seawater acid base chemistry implications for eocene and cretaceous ocean carbon chemistry and buffering reconstructed changes in seawater calcium and magnesium concentration ca2 mg2 predictably affect the ocean s acid base and carbon chemistry yet inaccurate formulations of chemical equilibrium constants are currently in use to account for these changes here we develop an efficient implementation of the miami ionic interactionmodel to predict all chemical equilibrium constants required for carbon chemistry calculations under variable ca2 and mg2 we investigate the impact of ca2 and mg2 on the relationships among the ocean s ph co2 dissolved inorganic carbon dic saturation state of caco3 and buffer capacity increasing ca2 and or mg2 enhances ion pairing which increases seawater buffering by increasing the concentration ratio of total to free uncomplexed carbonate ion an increase in ca2 however also causes a decline in carbonate ion to maintain a given thereby overwhelming the ion pairing effect and decreasing seawater buffering given the reconstructions of eocene ca2 and mg2 ca2 20mm mg2 30mm eocene seawater would have required essentially the same dic as today to simultaneously explain a similar to modern and the estimated eocene atmospheric co2 of 1000 ppm during the cretaceous at 4 times modern ca2 ocean buffering would have been at a minimum overall during times of high seawater ca2 caco3 saturation ph and atmospheric co2 were more susceptible to perturbations of the global carbon cycle for example given both eocene and cretaceous seawater ca2 and mg2 a doubling of atmospheric co2 would require less carbon addition to the ocean atmosphere system than under modern seawater composition moreover increasing seawater buffering since the cretaceous may have been a driver of evolution by raising energetic demands of biologically controlled calcification and co2 concentration mechanisms that aid photosynthesis

scalings for large turbulent flow structures in gravel bed rivers 1 the relations between frequency and duration of large scale turbulent flow structures in gravel bed rivers and the distributions of the time intervals between turbulent events are investigated using flow visualisation and velocity measurements the time of occurrence and the duration and amplitude of large bursting motions were estimated independently from a five minute underwater video and from high resolution velocity time series sampled at 20 hz during 38 minutes at three heights above the bed results from both experiments show that the frequency duration relations of the large scale turbulent flow structures follow a power function with exponents ranging between 1 4 and 1 95 values that are typical of bursting systems e g solar flares the frequency distributions of time intervals between two successive ejections display an exponential tail these findings provide models for simulating sequences of large scale turbulent events in a gravel bed river an important step in linking flow turbulence and sediment transport

atmospheric carbon dioxide concentration across the mid pleistocene transition the dominant period of pleistocene glacial cycles changed during the mid pleistocene from 40 000 years to 100 000 years for as yet unknown reasons here we present a 2 1 million year record of sea surface partial pressure of co2 pco2 based on boron isotopes in planktic foraminifer shells which suggests that the atmospheric partial pressure of co2 pco2 was relatively stable before the mid pleistocene climate transition glacial pco2 was approximately 31 microatmospheres higher before the transition more than 1 million years ago but interglacial pco2 was similar to that of late pleistocene interglacial cycles 450 000 years ago these estimates are consistent with a close linkage between atmospheric co2 concentration and global climate but the lack of a gradual decrease in interglacial pco2 does not support the suggestion that a long term drawdown of atmospheric co2 was the main cause of the climate transition

environmental controls on b ca in calcite tests of the tropical planktic foraminifer species globigerinoides ruber and globigerinoides sacculifer abstract the ratio of boron to calcium b ca in the calcite tests of planktic foraminifers may serve as a proxy for past seawater chemistry but controls on b incorporation are not yet certain here we present the results of laboratory culture experiments with live specimens of globigerinoides ruber pink and globigerinoides sacculifer which provide new insight into b incorporation controls we find that in g sacculifer b ca increases with increasing ph lower hco3 higher co32 and b oh 4 but decreases with increasing total dissolved inorganic carbon dic higher hco3 and co32 constant b oh 4 this suggests competition between aqueous boron and carbon species for inclusion into the calcite lattice similar to previous experiments with the subtropical temperate orbulina universa b ca increases with salinity but not with temperature we evaluate possible carbonate system control parameters and compare our tropical culture calibrations with new and published core top data

comparison of isotopic composition of planktonic foraminifera in plankton tows sediment traps and sediments planktonic foraminifera from plankton tows sediment traps and sediments in the central north atlantic were studied in order to understand what determines their oxygen and carbon isotope composition a clear separation of species and genera on a 18o vs 13c plot for all samples suggests that their isotopic composition is controlled to a certain degree by biological factors within a species population the globorotaliids show a positive linear correlation between 18o and 13c while the shallow dwelling spinose species mostly globigerinoides species do not show a definite trend the latter species when collected in plankton tows often show slight negative deviations from isotopic equilibrium with respect to oxygen all species deviate from carbon isotope equilibrium by 1 5 to 6 these deviations from equilibrium are probably caused by incorporation of isotopically light metabolic co2 into the skeleton which is enhanced by the activity of symbiotic algae during their ontogeny the average weight per individual of most species increases which indicates that calcification continues to a depth of about 100 m this additional skeleton roughly 50 by weight is isotopically heavier because temperatures are lower and photosynthesis of symbiotic algae stops below the photic zone therefore the skeleton of foraminifera collected in sediment traps below 400 m has an overall oxygen isotope composition that seems to be in equilibrium for caco3 deposited in the upper 100 m