Was the deep Atlantic dominated by southern source waters during the LGM? A conservative view based on the oxygen isotopic ratio of benthic foraminifera

University Of Connecticut, Storrs CT

Investigators

Abstract

The cause of glacial-interglacial atmospheric CO2 cycles remains one of the most important unresolved questions in the field of paleoclimatology. It has long been known that Earth’s ice sheets expand when atmospheric CO2 is low and shrink when CO2 rises. The underlying climate mechanisms that regulate CO2 levels however, remain unclear. One possibility is that changes in ocean circulation influence air-sea gas exchange. It has been hypothesized that expansion of deep waters from the Southern Ocean limited the release of carbon during the last ice age (~20,000 years ago). Reconstructing ocean circulation in the past is therefore essential to understanding what regulates atmospheric CO2 over long time periods. In this study, we will use a new method to reconstruct the ocean circulation using oxygen isotope analyses of microscopic fossils from marine sediment cores. Students from all levels, high school to undergraduate to graduate, will participate in this research. Earth’s climate during the LGM (Last Glacial Maximum ~20,000 years ago) was characterized by lower atmospheric CO2 levels, cooler surface temperatures, and lower global sea level. The Atlantic Ocean circulation was also likely marked by shoaling of deep waters that emanate from the North Atlantic and incursion of abyssal waters from the Southern Ocean. Our understanding of the circulation is based mainly on carbon isotope analyses of microfossils that suggest southern source waters replaced northern source waters in the North Atlantic. The image of southern source waters dominating the deep Atlantic has become a defining feature of the LGM, as familiar as well-known changes in pCO2, surface temperature, and sea level. More recently, however, results from neodymium isotopes imply the deep North Atlantic was influenced primarily by northern source waters, challenging the canonical view of the LGM circulation. The goal of the proposed work is to determine which of these two scenarios is most likely correct using oxygen isotopes as a circulation tracer. The oxygen isotopic composition of microfossils is conservative because it varies mainly as a function of temperature. Once the oxygen isotope ratio is set at the sea surface, it changes only through ocean circulation and mixing. The carbon isotope composition of microfossils, on the other hand, is non-conservative because it is also influenced by biological processes that compromise its utility as a circulation tracer. The proposed work will benefit the paleoclimate community by creating the first section for the LGM Atlantic based on a conservative water mass tracer. The oxygen isotope results will also facilitate validation of other circulation proxies and lead to improved model simulations of the ocean circulation and its role in the carbon cycle. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.