Stable isotope record of a Bermuda coral

Records of skeletal delta18O in monthly and Sr/Ca ratios in half-yearly resolution were obtained from a Bermuda coral (Diploria labyrinthiformis) for the time period 1520-1603 (+/-15 yr) AD within the Little Ice Age. Annual and decadal averages of both sea-surface temperature proxies indicate temperature variabilities of 0.5°C (standard deviation) and 0.3°C, respectively. Both numbers are close to recent instrumental observations. Approximately 30% of the interannual time series variance of delta18O is concentrated in broad bands centered at periods of ~30, 16, and 7.8 yr, the last two reflecting the influence of the North Atlantic Oscillation. Although this large-scale climate signal is present in the record, there is no correlation with other contemporaneous northern hemisphere proxy data, resulting from spatial differences in climate variability.

Quaternary carbonate and stable isotope record of ODP Holes 133-817A and 133-818B

The Quaternary history of metastable CaCO3 input and preservation within Antarctic Intermediate Water (AAIW) was examined by studying sediments from ODP Holes 818B (745 mbsl) and 817A (1015 mbsl) drilled in the Townsville Trough on the southern slope of the Queensland Plateau. These sites lie within the core of modern AAIW, and near the aragonite saturation depth (~1000 m). Thus, they are well positioned to monitor chemical changes that may have occurred within this watermass during the past 1.6 m.y. The percent of fine aragonite content, percent of fine magnesian calcite content, and percent of whole pteropods (>355 µm) were used to separate the fine aragonite input signal from the CaCO3 preservation signal. Stable d18O and d13C isotopic ratios were determined for the planktonic foraminifer Globigerinoides sacculifer and, in Hole 818B, for the benthic foraminifer Cibicidoides spp. to establish the oxygen isotope stratigraphy and to study the relationship between intermediate and shallow water d13C of Sum CO2 and the relationship between benthic foraminiferal d13C and CaCO3 preservation within intermediate waters of the Townsville Trough. Data were converted from depth to age using oxygen isotope stratigraphy, nannostratigraphy, and foraminiferal biostratigraphy. Several long hiatuses and the absence of magnetostratigraphy did not permit time series analysis. The principal results of the CaCO3 preservation study include the following (1) a general increase in CaCO3 preservation between 0.9 and 1.6 Ma; (2) a CaCO3 dissolution maximum near 0.9 Ma, primarily expressed in the Hole 818B fine aragonite record; (3) an abrupt and permanent increase of fine aragonite content between 0.86 and 0.875 Ma in both Holes 818B and 817A probably reflecting a dramatic increase of fine carbonate sediment production on the Queensland Plateau; (4) an improvement in CaCO3 preservation near 0.87 Ma, which accompanied the increase of sediment input, indicated by the first appearance of whole pteropods in the deeper Hole 817A and a "spike" in the percent whole pteropods in Hole 818B; (5) a period of strong CaCO3 dissolution during the mid-Brunhes Chron from 0.36 to 0.41 Ma; and (6) a complex CaCO3 preservation pattern between 0.36 Ma and the present characterized by a general increase in CaCO3 preservation through time with good preservation during interglacial stages and poor preservation during glacial stages. The long-term aragonite preservation histories for Holes 818B and 817A appear to be similar in general shape, although different in detail, to CaCO3 preservation records from the deep Indian and central equatorial Pacific oceans as well as from intermediate water sites in the Bahamas and the Maldives. All of these areas have experienced CaCO3 dissolution at about 0.9 Ma and during the mid-Brunhes Chron. However, the late Quaternary (0 to 0.36 Ma) glacial to interglacial preservation pattern in Holes 818B and 817A is out of phase with CaCO3 preservation records for sediments deposited in Pacific deep and bottom waters. The sharp increase in bank production and export from the Queensland Plateau and the coincident improvement of CaCO3 preservation between 0.86 and 0.875 Ma may have been synchronous with the initiation of the Great Barrier Reef and roughly coincides with an increase in carbonate accumulation on the Bahama banks, in the western North Atlantic Ocean, and on Mururoa atoll, in the central South Pacific Ocean. The development of these reef systems during the middle Quaternary may be related to the transition in the frequency and amplitude of global sea level change from 41 k.y. low amplitude cycles prior to 0.9 Ma to 100 k.y. high amplitude cycles after 0.73 Ma. Carbon isotopic analyses show that benthic foraminiferal d13C values (Cibicidoides spp.) have been heavier than planktonic foraminiferal d13C values (G. sacculifer) throughout most of the last 0.54 m.y., which may indicate that 13C-enriched intermediate water (AAIW) occupied the Townsville Trough during much of the late Quaternary. Furthermore, both planktonic and benthic foraminiferal d13C values are often observed to be heaviest during interglacial to glacial transitions, and lightest during glacial to interglacial transitions. We suggest that this pattern is the result of changes in the preformed d13C of Sum CO2 of AAIW and may reflect changes in nutrient utilization by primary producers in Antarctic surface waters, changes in the d13C of upwelled Circumpolar Deep Water, or changes in the extent and/or temperature of equilibration between surface water and atmospheric CO2 within the Antarctic Polar Frontal Zone (the source area for AAIW). Finally, the poor correlation between percent of whole pteropods (aragonite preservation) and d13C of Cibicidoides spp. may be the result of a decoupling of d13C from CO2 due to the numerous and complex variables that combine to produce the preformed d13C of AAIW.

Stable isotope organic chemistry of ODP Leg 105 sites

Stable carbon and nitrogen isotopic compositions as well as organic carbon and total nitrogen contents are reported for Site 645 in Baffin Bay and Sites 646 and 647 in the Labrador Sea. Both low-resolution analyses (1 sample/section) and high-resolution results (up to 7 samples/section) are presented. These records indicate that large-scale changes in productivity since the middle Miocene have occurred in Baffin Bay. Such changes are not evident in samples from the Labrador Sea. Isotopic records of all the sites are influenced strongly by rapidly changing influxes that combine terrigenous debris with planktonic production. In parts of the cores, relationships to other phenomena, such as ice stages, are present. However, these correlations are not common and indicate that these events were masked by the dynamic changes in sources of organic matter occurring in this complex system.

Barium concentrations und stable isotope ratios of late Paleocene deep-sea sediments

Deep-sea sediment Ba* (Ba/Al2O3(sample) * 15% - Ba(aluminosilicate) records show increasing values synchronous with the evolution of the late Paleocene global d13C maximum, reflecting an increase in marine surface primary production and biogenic barite formation at this time. At two oligotrophic locations, Deep Sea Drilling Project (DSDP) Sites 384 and 527 in the North and South Atlantic, respectively, Ba* increases from 160-360 ppm in the early Paleocene to 1100-3000 ppm during the d13C maximum. At equatorial DSDP Site 577, positioned within or near the high-productivity zone, Ba* increases from ~15,500 ppm in the early Paleocene to ~25,400 ppm in conjunction with late Paleocene maximum d13C values. Linear fitted correlation plots of sediment Ba* content versus surface water d13C in all three regions support barite originating in the euphotic zone. The early to late Paleocene relative increase in Ba* illustrates how burial rates of Corg (relative to Al2O3) accelerated by a factor of ~1.8 and ~6.0 in the eutrophic and oligotrophic areas, respectively. A tentative estimate, weighing our result for the entire ocean, suggests that accumulation rates of organic carbon increased by a factor of 2 during the late Paleocene d13C maximum.

Stable isotope geochemistry of pore waters and marine sediments from the New Jersey shelf: Methane formation and fluid origin

Interstitial water and sediment samples of the Integrated Ocean Drilling Program (IODP) expedition 313 "New Jersey Shallow Shelf" were analyzed for chemical composition and stable isotope ratios. A total of 222 water samples were collected from the cores by Rhizon samplers and squeezing of fresh core material. Water was analyzed for its stable oxygen and hydrogen isotope geochemistry (d2H and d18O) at sites M0027A and M0029A, and the carbon isotope composition of the dissolved inorganic carbon (d13CDIC) (all sites). In addition, organic material (Corg) and inorganic carbonates from sediments were analyzed for their carbon ratios (d13Corg and d13Ccarb), and in case of the carbonates also for oxygen (d18Ocarb). Carbon isotopes were also analyzed in samples containing enough methane gas (d13Cmeth). Pore fluids from site M0027A were analyzed for the sulfur isotope composition of dissolved sulfate (d34S). The combination of isotope analyses of all phases (interstitial water, sediment, and gas) with pore water chemistry is expected to enable a better understanding of processes in the sediment and will help to identify the origin of fluids under the New Jersey shelf.

Stable isotope and trace element geochemistry of carbonate sediments at DSDP Holes 87-577 and 6-47.2

Detailed analyses of well-preserved carbonate samples from across the Cretaceous/Tertiary boundary in Hole 577 have revealed a significant decline in the d13C values of calcareous nannoplankton from the Maestrichtian to the Danian Age accompanied by a substantial reduction in carbonate accumulation rates. Benthic foraminifers, however, do not exhibit a shift in carbon composition similar to that recorded by the calcareous nannoplankton, but actually increase slightly over the same time interval. These results are similar to the earlier findings at two North Pacific Deep Sea Drilling Project locations, Sites 47.2 and 465, and are considered to represent a dramatic decrease in oceanic phytoplankton production associated with the catastrophic Cretaceous/Tertiary boundary extinctions. In addition, the change in carbon composition of calcareous nannoplankton across the Cretaceous/Tertiary boundary at Hole 577 is accompanied by only minor changes in the oxygen isotope trends of both calcareous nannoplankton and benthic foraminifers, suggesting that temperature variations in the North Pacific from the late Maestrichtian to the early Danian Age were insignificant.

Stable isotope record of the Okhotsk Sea

Stable isotope data on benthic foraminifera from more than 30 cores on the northern Emperor Seamounts and in the Okhotsk Sea are synthesized in paleohydrographic profiles spanning the depth range 1000-4000 m. Holocene (core-top) benthic foraminiferal d18O and d13C data are calibrated to modern hydrographic properties through measurements of d13C of SumCO2 and d18O of seawater. Cibicidoides stable isotope ratios are close to the d13C and equilibrium d18O of seawater, whereas Uvigerina d18O and d13C are variably offset from Cibicidoides. Glacial maximum d13C of Cibicidoides displays a different vertical profile than that of the Holocene. When results are adjusted by +0.32 per mil to account for the secular change in d13C during the last glacial maximum, the data coincide with the modern seawater and foraminiferal curves deeper than ~2 km. However, at shallower depths d13C gradually increases by as much as 1 per mil above the modern value. Furthermore, above 2 km the benthic d18O decreases by ~0.5 per mil. These results are consistent with a benthic front at ~2 km in the North Pacific (see Herguera et al., 1992), but they differ from interpretations based on trace metal data which indicate a source of nutrient-depleted deep water during glaciation. The isotopic data suggest that during glaciation there was a better ventilated watermass at intermediate depths in the far northwestern Pacific, it was relatively fresher than deep waters there, and deep waters were as nutrient-rich as today.

Stable isotope ratios of foraminifera and sea surface temperature estimation of sediment core W2804A-14

Carbon isotopically based estimates of CO2 levels have been generated from a record of the photosynthetic fractionation of 13C (epsilon p) in a central equatorial Pacific sediment core that spans the last ~255 ka. Contents of 13C in phytoplanktonic biomass were determined by analysis of C37 alkadienones. These compounds are exclusive products of Prymnesiophyte algae which at present grow most abundantly at depths of 70-90 m in the central equatorial Pacific. A record of the isotopic compostion of dissolved CO2 was constructed from isotopic analyses of the planktonic foraminifera Neogloboquadrina dutertrei, which calcifies at 70-90 m in the same region. Values of epsilon p, derived by comparison of the organic and inorganic delta values, were transformed to yield concentrations of dissolved CO2 (c e) based on a new, site-specific calibration of the relationship between epsilon p and c e. The calibration was based on reassessment of existing epsilon p versus c e data, which support a physiologically based model in which epsilon p is inversely related to c e. Values of PCO2, the partial pressure of CO2 that would be in equilibrium with the estimated concentrations of dissolved CO2, were calculated using Henry's law and the temperature determined from the alkenone-unsaturation index UK 37. Uncertainties in these values arise mainly from uncertainties about the appropriateness (particularly over time) of the site-specific relationship between epsilon p and 1/c e. These are discussed in detail and it is concluded that the observed record of epsilon p most probably reflects significant variations in Delta pCO2, the ocean-atmosphere disequilibrium, which appears to have ranged from ~110 µatm during glacial intervals (ocean > atmosphere) to ~60 µatm during interglacials. Fluxes of CO2 to the atmosphere would thus have been significantly larger during glacial intervals. If this were characteristic of large areas of the equatorial Pacific, then greater glacial sinks for the equatorially evaded CO2 must have existed elsewhere. Statistical analysis of air-sea pCO2 differences and other parameters revealed significant (p < 0.01) inverse correlations of Delta pCO2 with sea surface temperature and with the mass accumulation rate of opal. The former suggests response to the strength of upwelling, the latter may indicate either drawdown of CO2 by siliceous phytoplankton or variation of [CO2]/[Si(OH)4] ratios in upwelling waters.