Stable isotope ratios in shells of marine organisms

Oxygen and carbon isotope analyses have been carried out on calcareous skeletons of important recent groups of organisms. Annual temperature ranges and distinct developmental stages can be reconstructed from single shells with the aid of the micro-sampling technique made possible by modern mass-spectrometers. This is in contrast to the results of earlier studies which used bulk sampIes. The skeletons analysed are from Bermuda, the Philippines, the Persian Gulf and the continental margin off Peru. In these environments, seasonal salinity ranges and thus annual variations in the isotopic composition of the water are small. In addition, environmental parameters are weIl documented in these areas. The recognition of seasonal isotopic variations is dependant on the type of calcification. Shells built up by carbonate deposition at the margin, such as molluscs, are suitable for isotopic studies. Analysis is more difficult where chambers are added at the margin of the shell but where older chambers are simultaneously covered by a thin veneer of carbonate e. g. in rotaliid foraminifera. Organisms such as calcareous algae or echinoderms that thicken existing calcareous parts as weIl as growing in length and breadth are the most difficult to analyse. All organisms analysed show temperature related oxygen-isotope fractionation. The most recent groups fractionate oxygen isotopes in accordance with established d18O temperature relationships (Tab. 18, Fig. 42). These groups are deep-sea foraminifera, planktonic foraminifera, serpulids, brachiopods, bryozoa, almost all molluscs, sea urchins, and fish (otoliths). A second group of organisms including the calcareous algae Padina, Acetabularia, and Penicillus, as weIl as barnacles, cause enrichment of the heavy isotope 18O. Finally, the calcareous algae Amphiroa, Cymopolia and Halimeda, the larger foraminifera, corals, starfish, and holothurians cause enrichment of the lighter isotope 16O. Organisms causing non-equilibrium fractionation also record seasonal temperature variations within their skeletons which are reflected in stable-oxygen-isotope patterns. With the exception of the green algae Halimeda and Penicillus, all organisms analysed show lower d13C values than calculated equilibrium values (Tab. 18, Fig. 42). Especially enriched with the lighter isotope 12C are animals such as hermatypic corals and larger foraminifera which exist in symbiosis with other organisms, but also ahermatypic corals, starfish, and holothurians. With increasing age of the organisms, seven different d13C trends were observed within the skeletons. 1) No d13C variations are observed in deep-sea foraminifera presumably due to relatively stable environmental conditions. 2) Lower d13C values occur in miliolid larger foraminifera and are possibly related to increased growth with increasing age of the foraminifera. 3) Higher values are found in planktonic foraminifera and rotaliid larger foraminifera and can be explained by a slowing down of growth with increasing age. 4) A sudden change to lower d13C values at a distinct shell size occurs in molluscs and is possibly caused by the first reproductive event. 5) A low-high-Iow cycle in calcareous algae is possibly caused by variations in the stage of calcification or growth. 6) A positive correlation between d18O and d13C values is found in some hermatypic corals, all ahermatypic corals, in the septa of Nautilus and in the otoliths of fish. In hermatypic corals from tropical areas, this correlation is the result of the inverse relationship between temperature and light caused by summer cloud cover; in other groups it is inferred to be due to metabolic processes. 7) A negative correlation between d18O and d13C values found in hermatypic corals from the subtropics is explained by the sympathetic relationship between temperature and light in these latitudes. These trends show that the carbon isotope fractionation is controlled by the biology of the respective carbonate producing organisms. Thus, the carbon isotope distribution can provide information on the symbiont-host relationship, on metabolic processes and calcification and growth stages during ontogenesis of calcareous marine organisms.

Gas composition, concentration and stable isotope ratios (Table 2, 3, 4)

The deployment of CCS (carbon capture and storage) at industrial scale implies the development of effective monitoring tools. Noble gases are tracers usually proposed to track CO2. This methodology, combined with the geochemistry of carbon isotopes, has been tested on available analogues. At first, gases from natural analogues were sampled in the Colorado Plateau and in the French carbogaseous provinces, in both well-confined and leaking-sites. Second, we performed a 2-years tracing experience on an underground natural gas storage, sampling gas each month during injection and withdrawal periods. In natural analogues, the geochemical fingerprints are dependent on the containment criterion and on the geological context, giving tools to detect a leakage of deep-CO2 toward surface. This study also provides information on the origin of CO2, as well as residence time of fluids within the crust and clues on the physico-chemical processes occurring during the geological story. The study on the industrial analogue demonstrates the feasibility of using noble gases as tracers of CO2. Withdrawn gases follow geochemical trends coherent with mixing processes between injected gas end-members. Physico-chemical processes revealed by the tracing occur at transient state. These two complementary studies proved the interest of geochemical monitoring to survey the CO2 behaviour, and gave information on its use.

Stable isotope ratios of planktonic foraminifera from the West Equatorial Pacific

Differential dissolution affects the isotopic composition of different species of planktonic foraminifera in different ways. In the two species studied here in cores from Ontong Java Plateau, the less resistant species, Globigerinoides sacculifer, is more readily affected at a shallower depth than the more resistant species, Pulleniatina obliquiloculata (2.9 versus 3.4 km), but shows a smaller and less predictable response to partial dissolution (0.2 to 0.3 per mil versus 0.6 to 0.7 per mil). Comparison of isotopic values from the last glacial period with those from the late Holocene indicates that the apparent dissolution effect is considerably reduced during the last glacial, presumably due to reduced dissolution intensity during glacial time. A change in the level of the lysocline of about 400 m is suggested. In the published isotope records from Pacific cores V28-238 and V28-239, the dissolution-generated difference in delta18O (noted previously by Shackleton and Opdyke [1976]) is seen to describe a mid-Brunhes dissolution maximum, between 300 and 500 thousand years ago. This mid-Brunhes dissolution excursion is well known from the Pacific and the Indian Ocean.

Neodymium isotope ratios of fish debris from late Cretaceous black shales

Neodymium isotopes of fish debris from two sites on Demerara Rise, spanning ~4.5 m.y. of deposition from the early Cenomanian to just before ocean anoxic event 2 (OAE2) (Cenomanian-Turonian transition), suggest a circulation-controlled nutrient trap in intermediate waters of the western tropical North Atlantic that could explain continuous deposition of organic-rich black shales for as many as ~15 m.y. (Cenomanian-early Santonian). Unusually low Nd isotopic data (epsilon-Nd(t) ~-11 to ~-16) on Demerara Rise during the Cenomanian are confirmed, but the shallower site generally exhibits higher and more variable values. A scenario in which southwest-flowing Tethyan and/or North Atlantic waters overrode warm, saline Demerara bottom water explains the isotopic differences between sites and could create a dynamic nutrient trap controlled by circulation patterns in the absence of topographic barriers. Nutrient trapping, in turn, would explain the ~15 m.y. deposition of black shales through positive feedbacks between low oxygen and nutrient-rich bottom waters, efficient phosphate recycling, transport of nutrients to the surface, high productivity, and organic carbon export to the seafloor. This nutrient trap and the correlation seen previously between high Nd and organic carbon isotopic values during OAE2 on Demerara Rise suggest that physical oceanographic changes could be components of OAE2, one of the largest perturbations to the global carbon cycle in the past 150 m.y.

Stable oxygen and carbon isotope ratios of foraminifera from Oligocene to Miocene sediments of DSDP Hole 39-354 from Ceara Rise, West Altlantic (Table 1)

The oxygen- and carbon-isotope compositions of planktic and benthic foraminifera and calcareous nannofossils from Middle Oligocene-Early Miocene Equatorial Atlantic sediments (DSDP Site 354) indicate two important paleoceanographic changes, in the Late Oligocene (foraminiferal Zone P.21) and in the Early Miocene (foraminiferal Zone N.5). The first change, reflected by a delta18O increase of 1.45? in Globigerina venezuelana, affected only intermediate pelagic and not surface, deep or bottom waters. The second change affected surface and intermediate waters, whereas deep and bottom waters showed only minor fluctuations. In the case of the former the isotope effect of the moderate ice accumulation on the Antarctic continent is amplified in the Equatorial Atlantic by changes in the circulation pattern. The latter paleoceanographic change, reflected by a significant increase in 18O in both planktic and benthic forms (about 1.0? and 0.5?, respectively), may have been caused by ice volume increase and temperature decrease. Both oxygen- and carbon-isotope compositions indicate a marked depth-habitat stratification for planktic foraminifera and calcareous nannofossils. Three different dwelling groups are recognized: shallow Globigerinoides, Globoquadrina dehiscens, Globorotalia mayeri and nannofossils; intermediate Globigerina venezuelana; and deep Catapsydrax dissimilis. The comparison of foraminifera and calcareous nannofossils suggests that the isotopic compositions of nannofossils are generally controlled by the same parameters which control the isotopic composition of shallow-dwelling foraminifera, but the former are more enriched in 18O.

Table 1. Pressure, temperature and isotope ratios between H2O and CO2 for all experiments

Traditionally, the application of stable isotopes in Carbon Capture and Storage (CCS) projects has focused on d13C values of CO2 to trace the migration of injected CO2 in the subsurface. More recently the use of d18O values of both CO2 and reservoir fluids has been proposed as a method for quantifying in situ CO2 reservoir saturations due to O isotope exchange between CO2 and H2O and subsequent changes in d18OH2O values in the presence of high concentrations of CO2. To verify that O isotope exchange between CO2 and H2O reaches equilibrium within days, and that d18OH2O values indeed change predictably due to the presence of CO2, a laboratory study was conducted during which the isotope composition of H2O, CO2, and dissolved inorganic C (DIC) was determined at representative reservoir conditions (50°C and up to 19 MPa) and varying CO2 pressures. Conditions typical for the Pembina Cardium CO2 Monitoring Pilot in Alberta (Canada) were chosen for the experiments. Results obtained showed that d18O values of CO2 were on average 36.4±2.2 per mil (1 sigma, n=15) higher than those of water at all pressures up to and including reservoir pressure (19 MPa), in excellent agreement with the theoretically predicted isotope enrichment factor of 35.5 per mil for the experimental temperatures of 50°C. By using 18O enriched water for the experiments it was demonstrated that changes in the d18O values of water were predictably related to the fraction of O in the system sourced from CO2 in excellent agreement with theoretical predictions. Since the fraction of O sourced from CO2 is related to the total volumetric saturation of CO2 and water as a fraction of the total volume of the system, it is concluded that changes in d18O values of reservoir fluids can be used to calculate reservoir saturations of CO2 in CCS settings given that the d18O values of CO2 and water are sufficiently distinct.

Stable isotope ratios and geochemical composition of authigenic carbonates from ODP Sites 186-1150 and 186-1151

Several carbonaceous layers or fragments were recovered from sediments of Sites 1150 and 1151 on the deep-sea terrace of the Japan Trench during Leg 186. The X-ray diffraction analysis (XRD) data indicate that these are predominantly dolomitic. In this study, carbon and oxygen isotopes of these carbonates recovered at Sites 1150 and 1151 are presented. The oxygen isotope ratios of the dolomites analyzed range from +0.4 per mil to +4.1 per mil vs. Peedee formation belemnite (PDB) and those of calcites from +0.6 per mil to +2.8 per mil PDB. The isotopic composition of carbon varies from -7.0 per mil to +12.3 per mil PDB in dolomite and from -13.4 per mil to -24.1 per mil PDB in calcite. The wide range of carbon isotopic compositions indicates that the carbonate samples were formed by the decomposition of organic matter through reactions such as oxidation, sulfate reduction, and methane formation during diagenesis.

Stable isotope ratios of foraminifera from various DSDP/ODP sites

The equator to high southern latitude sea surface and vertical temperature gradients are reconstructed from oxygen isotope values of planktonic and benthic foraminifers for the following five time intervals: late Paleocene, early Eocene, early middle Eocene, late Eocene, and early Oligocene. Paleotemperatures are calculated using standard oxygen isotope/temperature equations with adjustments to account for (1) variations in sea water delta18O related to changes in global ice volume over time and (2) latitudinal gradients in surface water delta18O. These reconstructions indicate that sea-surface temperatures (SST) of the Southern Oceans in the early Eocene were as high as 15°C, whereas temperatures during the late Paleocene and early middle Eocene reached maximum levels of 10°-12°C. By the late Eocene and early Oligocene high latitude SST had declined to 6 and 4°C, respectively. For most of the early Paleogene, low latitude sub-tropical temperatures remained constant and well within the range of Holocene temperatures (24°-25°C) but by the late Eocene and early Oligocene declined to values in the range of 18° to 22°C. The late Paleogene apparent decline in tropical temperatures, however, might be artificial because of dissolution of near-surface foraminifera tests which biased sediment assemblages toward deeper-dwelling foraminifera. Moreover, according to recent plate reconstructions, it appears that the majority of sites upon which the late Eocene and early Oligocene tropical temperatures were previously established were located either in or near regions likely to have been influenced by upwelling. Global deepwater temperature on average paralleled southern ocean SST for most of the Paleogene. We speculate based on the overall timing and character of marine sea surface temperature variation during the Paleogene that some combination of both higher levels of greenhouse gases and increased heat transport was responsible for the exceptional high-latitude warmth of the early Eocene.

Stable and strontium isotope ratios and age determination of ODP Site 130-803

Stratigraphic information from strontium, oxygen, and carbon isotopic ratios has been integrated with diatom and planktonic foraminifer datums to refine the Oligocene to early Miocene chemostratigraphy of Site 803. The Sr isotope results are based on analyses of mixed species of planktonic foraminifer and bulk carbonate samples. 87Sr/86Sr ratios of bulk carbonate samples are, in most cases, less radiogenic than contemporaneous seawater. Estimated sediment ages based on planktonic foraminifer 87Sr/86Sr ratios, using the Sr-isotope-age relation determined by Hess and others in 1989, are in moderately good agreement with the biostratigraphic ages. Chronological resolution is significantly enhanced with the correlation of oxygen and carbon isotope records to those of the standard Oligocene section tied to the Geomagnetic Polarity Time Scale at Site 522. Ages revised by this method and other published ages of planktonic foraminifer datums are used to revise the Oligocene stratigraphy of Site 77 to correlate the stable isotope records of Sites 77 and 803. Comparison of the Cibicidoides stable isotope records of Sites 77 and 574 with paleodepths below 2500 m in the central equatorial Pacific, and Site 803 at about 2000-m paleodepth in the Ontong Java Plateau reveals inversions in the vertical d18O gradient at several times during the Oligocene and in the early Miocene. The shallower water site had significantly-higher d18O values than the deeper water sites after the earliest Oligocene 18O enrichment and before 34.5 Ma, in the late Oligocene from 27.5 to at least 25 Ma, and in the early Miocene from 22.5 to 20.5 Ma. It is not possible to ascertain if the d18O inversion persisted during the Oligocene/Miocene transition because the deeper sites have hiatuses spanning this interval. We interpret this pattern to reflect that waters at about 2000 m depth were cold and may have formed from mixing with colder waters originating in northern or southern high-latitude regions. The deeper water appear to have been warmer and may have been a mixture with warm saline waters from mid- or low-latitude regions. No apparent vertical d13C gradient is present during the Oligocene, suggesting that the age difference of these water masses was small.

Stable isotope ratios of benthic foraminifera in mid-Pleitsocene sediments of ODP Site 108-664 in the equatorial Atlantic

Five delta13C records from the deep ocean, extending back to 1.3 Ma, were examined in order to constrain changes in mean ocean carbon isotope composition and thermohaline circulation over the 41- to 100-ka climate transition. These data show that significant perturbations in mean ocean carbon chemistry were associated with the mid-Pleistocene climate transition. Notable features of the last 1.3 Myr are (1) a pronounced ~0.3? decrease in mean ocean delta13C between 0.9 and 1.0 Myr, followed by a return to pre-1.0 Ma values by 400 ka B.P., which we propose was due to the onetime addition of isotopically depleted terrestrial carbon to the ocean, possibly associated with an increase in global aridity (and decrease in the size of the biosphere) across the 41- to 100-ka transition; (2) no change in the Atlantic-Pacific (A-P) delta13C gradient over the last 1.3 Myr, suggesting no change in mean ocean nutrient content accompanied the addition of light carbon; and (3) stronger vertical nutrient fractionation in the North Atlantic in the middle Pleistocene between sites 607 and 552, suggesting weaker North Atlantic Deep Water formation at this time relative to the early and late Pleistocene. We also find evidence for a more pronounced deep recirculation gyre in the western North Atlantic basin in the early Brunhes, as evidenced by "aging" of deep northern basin water (site 607) relative to deep water in the equatorial Atlantic (site 664).

Stable carbon isotope ratios of Cibicidoides wuellerstorfi of MIS 2 time slices

The interval of time represented by marine isotope stages 11 and 12 (~360-470 ka) contains what may be the most extreme glacial and interglacial climate conditions of the Late Pleistocene. It has been suggested that sea level rose by ~160 m at the termination of glacial stage 12. This is 30% greater than the sea level rise that followed the most recent glacial maximum. There have been few detailed studies of the unique conditions that existed during the stage 11-12 time period because of the lack of high-quality core material. This problem has been addressed by the collection of high deposition rate cores from sediment drifts in the western North Atlantic during Ocean Drilling Project Leg 172. Benthic foraminiferal d13C data from cores collected between ~4600 and 1800 m were used to reconstruct bathymetric gradients in deep and intermediate water properties for selected time slices during this glacial-interglacial cycle. During glacial stage 12, the deep western North Atlantic was filled by a water mass that was more nutrient-enriched than modern Antarctic Bottom Water. Above 2000 m, a more nutrient-depleted water mass existed during this glacial stage. Such an intermediate water mass has been described for more recent glacial periods and presumably forms in a more proximate region of the North Atlantic. Interglacial stage 11 water mass properties closely resemble those of the present-day western North Atlantic. A nutrient-depleted water mass (d13C of 0.75-1.0 per mil), similar to modern North Atlantic Deep Water existed between 3500 and 2000 m. This was underlain by a water mass with lower d13C values (<0.75 per mil) that probably was derived from a southern source. Using Leg 172 data, along with previously published results from the Atlantic and Pacific oceans, we estimate a mean global d13C change of 0.95 per mil from stage 12 to stage 11. This is twice the whole ocean ?13C change reported for the transition from the last glacial maximum to the Holocene.