Comparison of accumulation rates and stable isotope ratios

Since 1979/80, glaciological studies have been carried out at Ekströmisen, Antarctica, including accumulation-stake measurements, snow-pit and shallow-firn-core studies. Snowstratigraphy, chemical properties and stable-isotope ratios (d18O) were investigated. This study focuses on three cores taken between 1982 and 1998. The 1998 core was dated using dielectric profiling, d18O profiles and stake measurements. Accumulation rates showhigh interannual and spatial variability due to the extreme wind influence. No significant trend was found for the last 50 years; during the first half of the 20th century, accumulation decreased. The high spatial and interannual variability, however, means that trends must be interpreted with care. In spite of the highly irregular accumulation distribution, stable-isotope ratios show little spatial variability. The mean annual d18O values of cores B04 and FB0198 agree fairly well for the time period 1955-82 covered by both cores. d18O values have increased during most of the 20th century; since the late 1980s a decrease is observed. This change is not related to air temperature, since mean annual air temperatures at Neumayer show no significant trend over the last two decades.

Stable isotopes and calcium carbonate at DSDP Holes 85-574 and 85-574A

Detailed stable isotopic and calcium carbonate records (with a sampling resolution of 3000 yr.) from the middle Miocene section of hydraulic piston corer (HPC) Hole 574A provide a sequence that records the major shift in the oxygen isotopic composition of the world's oceans that occurred at about 14 Ma. The data suggest that this transition was rapid and spans about 30,000 yr. of sediment deposition. In intervals before and after the shift, the mean d18O values are characterized by a constant mean with a high degree of variability. The degree of variability in both the d18O and d13C records is comparable to that observed for the Pliocene and earliest Pleistocene and does not show a significant change before or after the major shift in the d18O record. Whereas the oxygen isotopic record is characterized by relatively stable mean values before and after the middle Miocene event, the d13C record shows a number of significant offsets in the mean value separated by intervals of high-frequency variations. Time and frequency domain analysis of all records from Hole 574A indicate that the frequency components shown to be related to orbital changes in the Pleistocene record are also present in the middle Miocene. The high variability observed in the Site 574 isotopic records places important constraints on models describing the role of formation of the Antarctic ice sheet during the middle Miocene climatic transitions. Thus, HPC Hole 574A provides a valuable sequence for detailed study of climatic variability during an important time in the Earth's history, although we cannot provide a definitive explanation of the major oxygen isotopic event of the middle Miocene.

Stable oxygen and carbon isotope ratios of foraminifera and test results after different sample cleaning procedures from ODP Leg 207 sediments

We report oxygen and carbon stable isotope analyses of foraminifers, primarily planktonic, sampled at low resolution in the Cretaceous and Paleogene sections from Sites 1257, 1258, and 1260. Data from two samples from Site 1259 are also reported. The very low resolution of the data only allows us to detect climate-driven isotopic events on the timescale of more than 500 k.y. A several million-year-long interval of overall increase in planktonic 18O is seen in the Cenomanian at Site 1260. Before and after this interval, foraminifers from Cenomanian and Turonian black shales have d18O values in the range -4.2 per mil to -5.0 per mil, suggestive of upper ocean temperatures higher than modern tropical values. The d18O values of upper ocean dwelling Paleogene planktonics exhibit a long-term increase from the early Eocene to the middle Eocene. During shipboard and postcruise processing, it proved difficult to extract well-preserved foraminifer tests from black shales by conventional techniques. Here, we report results of a test of procedures for cleaning foraminifers in Cretaceous organic-rich mudstone sediments using various combinations of soaking in bleach, Calgon/hydrogen peroxide, or Cascade, accompanied by drying, repeat soaking, or sonication. A procedure that used 100% bleach, no detergent, and no sonication yielded the largest number of clean, whole individual foraminifers with the shortest preparation time. We found no significant difference in d18O or d13C values among sets of multiple samples of the planktonic foraminifer Whiteinella baltica extracted following each cleaning procedure.

Age determination and stable isotope record of sediment cores at 9°S in the Indian Ocean

A continuous 10-m-long section consisting of roughly two thirds Ethmodiscus rex (a diatom) and one third mixed planktonic foraminifera was identified in a core from 3800 m depth at 9°S on the Indian Ocean's 90°E Ridge. Radiocarbon dates place the onset of deposition of this layer at >30,000 years B.P. and its termination at close to 11,000 years B.P. However, precise dating of the foraminifera from the Ethmodiscus layer itself proved to be impossible owing to the presence of secondary calcite presumably precipitated from the pore waters. During the Holocene, high calcium carbonate content ooze free of diatoms was deposited at this locale. As the site currently lies beneath the pathway taken by upper ocean waters entering the Indian Ocean from the Pacific (via the Indonesian Straits), it appears that during glacial time, thermocline waters moving along this same path provided the silica and other nutrients required by these diatoms.

Stable oxygen isotopes of Antarctic snow pits

Stable oxygen analyses and snow accumulation rates from snow pits sampled in the McMurdo Dry Valleys have been used to reconstruct variations in summer temperature and moisture availability over the last four decades. The temperature data show a common interannual variability, with strong regional warmings occurring especially in 1984/85, 1995/96 and 1990/91 and profound coolings during 1977/78, 1983/84, 1988/89, 1993/94, and 1996/97. Annual snow accumulation shows a larger variance between sites, but the early 1970s, 1984, 1997, and to a lesser degree 1990/91 are characterized overall by wetter conditions, while the early and late 1980s show low snow accumulation values. Comparison of the reconstructed and measured summer temperatures with the Southern Oscillation Index (SOI) and the Antarctic Oscillation (AAO) yield statistically significant correlations, which improve when phase-relationships are considered. A distinct change in the phase relationship of the correlation is observed, with the SOI-AAO leading over the temperature records by one year before, and lagging by one year after 1988. These results suggest that over the last two decades summer temperatures are influenced by opposing El Niño Southern Oscillation and AAO forcings and support previous studies that identified a change in the Tropical-Antarctic teleconnection between the 1980s and 1990s.

Planktonic foraminifer assemblages and stable oxygen and carbon isotope record of sediment core MD07-3076Q

We report high-resolution planktonic foraminifer census counts and stable oxygen and carbon isotope measurements of the planktonic foraminifera G. bulloides and N. pachyderma s. from sediment core MD07-3076Q for the last deglaciation, the last glacial maximum and Marine Isotope Stage 3. These data provide insights into the marine cycling of carbon and frontal dynamics in the sub-Antarctic Atlantic during the last 68 ka.

Age model and stable isotope record of eastern equatorial Pacific sediment cores

Six deep sea cores from the eastern equatorial Pacific (EEP) were analyzed for planktonic foraminifera and stable isotopes in order to reconstruct sea surface temperatures (SST) for the last 40 ka. South of the Equatorial Front the abundance of Globorotalia inflata increased, and SST decreased by >5°C (core ODP846B), creating a stronger SST meridional gradient and advection of the Peru Current than present for the ~16-35 ka interval. A sharper SST meridional gradient forced stronger Choco jet events and a moisture increase in western Colombia, which supplied, through the San Juan River and the south-flowing equatorial and the Peru-Chile countercurrents, abundant hemipelagic quartz over the northern Peru basin (core TR163-31B). The Choco jet, and its associated mesoscale convective cells, provoked an increase in snow precipitation over the Central Cordillera of Colombia and the advance of the Murillo glacier. In synchrony with the intensified Choco jet events, the "dry island" effect over the Eastern Cordillera of Colombia intensified, and the level of Fuquene Lake dropped.

Stable foraminifer isotope composition and age of ODP Sites 184-1147 and 184-1148, South China Sea

Sites 1147 (18°50.11'N, 116°33.28'E; water depth = 3246 m) and 1148 (18°50.17'N, 116°33.94'E; water depth = 3294 m) are located on the lowermost continental slope off southern China near the continent/ocean crust boundary of the South China Sea Basin. Site 1147 is located upslope ~0.45 nmi west of Site 1148. Three advanced piston corer holes at Site 1147 and two extended core barrel holes at Site 1148 were cored and combined into a composite (spliced) stratigraphic section, which provided a relatively continuous profile for the lower Oligocene to Holocene (Wang, Prell, Blum, et al., 2000, doi:10.2973/odp.proc.ir.184.2000; Jian, et al., 2001, doi:10.1007/BF02907088) for studying stratigraphy and paleoceanography. A total of 1047 planktonic foraminifers stable isotope measurements were performed on 975 samples covering the upper 409.58 meters composite depth (mcd) at ~42-cm intervals (Tables T1, T2), and a total of 1864 benthic foraminifers measurements were performed on 1650 samples in the upper 837.11 mcd at ~51-cm intervals (Tables T3, T4). We significantly improved the time resolution of the benthic stable isotope record in the upper 476.68 mcd by reducing the average sample spacing to ~29 cm. This translates into an average sampling resolution of ~16 k.y. for the Miocene sequence and ~8 k.y. for the Pliocene-Holocene interval, assuming a change in sedimentation rates from ~1.8 to ~3.5 cm/k.y., as suggested by shipboard stratigraphy. These data sets provide the basis for upcoming studies to establish an oxygen isotope stratigraphy and examine the Neogene evolution of deep and surface water signatures (temperature, salinity, and nutrients) in the South China Sea.

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.