Reconstructions of deep ocean climate variability from foraminiferal geochemistry.

摘要

Reconstructing changes in deep ocean chemistry is important to elucidating controls on Quaternary climate changes because the deep ocean plays an important role in transporting heat, redistributing nutrients, and regulating atmospheric carbon dioxide. Variations in the chemical composition of the calcite shells of benthic foraminifera recovered from deep sea sediments are one of the most valuable records of past deep ocean chemistry. A rapid and precise analytical method (ICP-MS) is used to acquire a suite of trace metal data (Cd, Ba, Mn, Sr, and Mg) from benthic foraminifera to evaluate changes in glacial ocean circulation, temperature, and chemistry.; Paleochemical data from eastern tropical Atlantic core V30-49 (3.1km) reveal large changes in nutrient concentrations of deep Atlantic waters over the past 250 kyr. Comparison of regional Cd/Ca records defines dramatic water mass changes in the tropical Atlantic and implies changes in deep water Ba and δ13C composition. Tight coupling of Ba/Ca and δ 13C records suggests that compositional changes in deep waters from the tropical Atlantic may result from productivity changes.; A correlation between benthic foraminiferal shell Mg/Ca and bottom water temperature (BWT) suggests that fossil Mg/Ca may be useful in assessing Quaternary BWT changes. Benthic Mg/Ca changes in eastern tropical Atlantic core M16772 (3.9km) imply BWT shifts of 2–4°C over the last 330 kyr; these estimates are consistent with previous Atlantic BWT estimates and circulation changes inferred from nutrient proxy records, demonstrating the potential of benthic Mg-paleothermometry. Mg/Ca data from eastern tropical Pacific core TR163-31P (3.2km) imply slightly smaller glacial-interglacial BWT oscillations and show clear millennial-scale oscillations correlated with oxygen isotope peaks.; Benthic and planktonic foraminiferal Sr/Ca records show globally coherent oscillations (5 ± 1%) over the past 250 kyr, with higher Sr/Ca recorded during glacial intervals. Major Sr/Ca oscillations appear too large to be explained entirely by paleoenvironmental changes and are most readily explained by changes in mean ocean Sr/Ca. If foraminifera records reliably record higher glacial sea water Sr/Ca, coral Sr paleothermometry would underestimate SST during glacial episodes.