Douglas F. Williams

Surface circulation of the Indian Ocean during the last glacial maximum, approximately 18,000 yr B.P.

A seasonal reconstruction of the Indian Ocean during the last glacial maximum (∼18,000 yr B.P.) reveals that its surface circulation and sea surface temperature patterns were significantly different from the modern Indian Ocean. This reconstruction is based on the planktonic foraminiferal biogeography and estimated sea surface temperatures in 42 Indian Ocean samples. Compared to modern conditions, the polar front was 5° to 10° latitude further north during the last glacial maximum; the Subtropical Convergence was 2° to 5° latitude further north. The West Australian Current was more intense as part of the West Wind Drift was deflected northward along the coast of Australia. The Agulhas Current was cooler and weaker during the summer and more saline and subtropical during the winter. In general, the low latitudes underwent little temperature change. The western Arabian Sea was warmer which implies less upwelling and a weaker Southwest Monsoon. On the average, the Indian Ocean was 1.9°C cooler in February and 1.7°C cooler in August during the last glacial maximum.

Chronology of the pleistocene oxygen isotope record: 0–1.88 m.y. B.P

Abstract Detailed oxygen isotope records from various ocean basins and representing the last 1.88 m.y. are correlated using nannofossil biostratigraphy and paleomagnetic stratigraphy. These correlations establish the global nature of oxygen isotope stages 23 through 63 in the early Pleistocene to latest Pliocene. A composite isotope record for the last 1.88 m.y. reveals that the mid-Pleistocene change in climate regime was a complicated response that lasted from approximately 0.9 to 0.6 Ma and not a simple shift from one climate mode to another. The proposed chronology for the extended isotope stages provides a chronostratigraphic framework for detailed studies of paleoceanographic processes in the early Pleistocene and paves the way for application of oxygen isotope stratigraphy to early Pleistocene deep-sea and continental margin drilled sections.

Dynamics of the Laurentide ice sheet during the last deglaciation: evidence from the Gulf of Mexico

Oxygen isotopic analyses of planktonic foraminifera from an anoxic basin (Orca Basin) in the northwestern Gulf of Mexico record a negative isotopic anomaly of 3.6‰ from 16,500 to 11,600 years before present, indicative of a discharge phase of glacial meltwater from the ablating Laurentide ice sheet in northern North America. The isotopic anomaly represents two major meltwater discharges of magnitudes 2.0‰ and 2.6‰ and each discharge shows evidence of several smaller isotopic pulses of magnitudes ranging from 0.7 to 1.4‰. The intermittent nature of theseisotopic shifts implies that the southern margin of the Laurentide ice sheet experienced several retreats and advances before finally decaying to a northern position when most of the subsequent meltwater flowed to the east 11,600 years ago. The oxygen isotope record of the Gulf of Mexico is in good agreement with the reconstructed history of the Laurentide ice sheetbased on continental evidence.

The age, origin, and volcanological significance of the Y-5 ash layer in the Mediterranean

Abstract The Y-5 ash is the most widespread layer in deep-sea sediments from the eastern Mediterranean. This ash layer was previously correlated with the Citara-Serrara tuff on Ischia Island and dated at approximately 25,000 yr B.P. New data on the glass chemistry of the Y-5 ash and pyroclastic deposits from the Neopolitan volcanic province suggest that the layer is correlative with the large-volume Campanian ignimbrite and not with the deposit from Ischia Island. The volume of the Y-5 ash is approximately 65 km 3 which is comparable in magnitude to the volume of the Campanian ignimbrite. An interpolated age of approximately 38,000 yr B.P. is estimated based on sedimentation rates derived from δ 18 O stratigraphy. There is a discrepancy between this estimate and previously reported radiocarbon ages which range from 24,000 to 35,000 yr B.P. We propose that the “Campanian tuff ash layer” should be adopted as the full stratigraphic name for the Y-5 ash. The deep-sea ash layer is divisible into two units in proximal localities, probably correlating with two major phases of the eruption: plinian and ignimbrite.

Growth history and ecology of the Atlantic surf clam, Spisula solidissima (Dillwyn), as revealed by stable isotopes and annual shell increments☆

Abstract The shells of the Atlantic surf clam, Spisula solidissima (Dillwyn), contain a record of both life history and environmental changes. These shell records were investigated using oxygen and carbon stable isotopic analyses (δ18O, δ13C) and shell growth increment analyses. δ18O variations across annual shell increments reflect the yearly cycle of sea-water temperatures off the New Jersey coast, further documenting the proposed annual periodicity of the major shell increments. The 11-yr shell record analyzed here confirms that shell growth is most rapid in spring-early summer, slow in late summer-fall, and extremely slow or non-existent in winter. Shell growth appears to occur in isotopic equilibrium with sea water and measured δ18O values are used to refine the aragonite-water temperature scale. Variations in the timing of annual growth increment formation are noted as well as ontogenetic effects upon the range of isotopic values recorded in shell carbonate. Both the δ18O and δ13C profiles are influenced by changes in the sea-water temperature regime over the 11-yr period studied (1965–1976) and record these effects in the shell. The combination of stable isotope and growth increment analyses provides a powerful tool for interpreting the shell records of both modern and fossil molluscs.

Opening the carbon isotope "vital effect" black box, 2, Quantitative model for interpreting foraminiferal carbon isotope data

Interpretation of carbon isotope records from late Quaternary planktonic foraminifers are confounded due to the presence of a significant physiological component in the carbon isotopic signal. A quantitative carbon isotope (QC) model is presented which relates the carbon isotopic composition of a foraminiferal shell to the physiological processes of respiration and symbiont photosynthesis and to the δ13C value of seawater ΣCO2. The QC model is calibrated with physiological and stable isotopic data from laboratory experiments with living planktonic foraminifers. Model simulations of chamber and shell δ13C values with the symbiont-bearing foraminifers, Orbulina universa and Globigerinoides sacculifer, suggest (1) variations in symbiont density and photosynthetic rate (light level or habitat depth) are the primary physiological parameters controlling intraspecific carbon isotopic variability in these species, (2) respiration has little effect on the δ13C of O. universa, and (3) each chamber in a multichambered foraminiferal test will have a distinct δ13C value depending on its position in the test whorl. Size:δ13C value relationships reported for G. sacculifer from fossil assemblages can be explained as a function of increasing symbiont density during ontogenetic development.

Seasonal stable isotopic variations in living planktonic foraminifera from bermuda plankton tows

Abstract Seasonal variations in surface water temperature and the 18O/16O ratios of calcite tests of living planktonic foraminifera collected in the upper 10 mare are coincident in the Sargasso Sea off Bermuda. The δ 18O of seasonally restricted Globorotalia truncatulinoides and Globigerinoides ruber (pink and white) may be used to determine the seasonal temperature contrast of the oceans in the past. A preliminary model based on yearly absolute abundance and isotopic composition of these two species is able to approximate the composite δ 18O of fossil specimens of these species. Carbon and oxygen isotopic compositions of most species are size-dependent, with the largest differences occuring in δ 13C as a function of size. The calcite tests of all living species were depleted in δ 13C relative to the δ 13C of total dissolved CO2 at 0 and 50 m, with no seasonal pattern in δ 13C being discernible in any species. In almost all cases, fossil specimens from box core sediments are significantly enriched in δ 13C and slightly enriched in δ 18O relative to their counterparts in the living fauna because the sediment populations are generally composed of larger-sized specimens compares to the plankton. of larger-size specimens compared to the plankton.

Periodic Freshwater Flooding and Stagnation of the Eastern Mediterranean Sea During the Late Quaternary

Major negative oxygen isotopic anomalies in planktonic foraminifera are associated with deep-sea anoxic mud layers (sapropels) deposited 9000 and 80,000 years ago in the eastern Mediterranean. The isotopic depletion in surface-dwelling foraminifera is significantly greater than in mesopelagic foraminifera. This difference in isotopic response suggests that surface-water salinities were drastically reduced during times of sapropel formation, possibly as the result of meltwater runoff from the Fennoscandian ice sheet into the eastern Mediterranean by way of the Black and Aegean seas.

Seasonal temperature-salinity changes and thermocline development in the mid-Atlantic Bight as recorded by the isotopic composition of bivalves

Stable isotope records across annual growth increments in specimens of the surf clam Spisula solidissima from the mid-Atlantic Bight shelf from 10 m and 45 m depths reflect the changes in temperature and nutrient concentrations on the shelf over the year. The δ 18 O and δ 13 C records from clams at the two depths record well-mixed conditions in the water column during the winter months and the development of a thermocline during the summer. Spring high productivity and a transient salinity excursion in surface waters are also recorded. Reconstructing the paleoceanography of late Cenozoic temperate continental shelves may be possible using stable isotope records from fossil Spisula solidissima and other bivalves.

Anoxic events in the Mediterranean Sea in relation to the evolution of late Neogene climates

Stable isotopic and micropaleontological studies were made of selected sapropels (organic-rich sediments) deposited in the Mediterranean Sea during the last 5.0 m.y. to determine the processes responsible for their formation. Distinct isotopic and faunal changes occur across sapropels of late Pleistocene, early Pleistocene and latest Pliocene age, while smaller isotopic changes and more stable faunal assemblages are associated with the early and mid-late Pliocene sapropels. The large δ 18O depletions and euryhaline fauna associated with latest Pliocene—Pleistocene sapropels supports a density stratification model with a low salinity surface layer. In contrast, early Pliocene and mid-late Pliocene sapropels appear to have been formed as the result of sluggish circulation and low oxygen contents in bottom waters of the eastern Mediterranean due to the stable, warm climatic conditions of that time period.