Testing Southern Ocean Control of Atmospheric CO2 using Diatom-based Paleo-proxies

University Of Massachusetts, Dartmouth, North Dartmouth MA

Investigators

Abstract

This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports a project to investigate the role of the Southern Ocean in regulating atmospheric carbon dioxide (CO2) during glacial-interglacial cycles. The polar Southern Ocean is the primary region where the deep-sea exchanges carbon dioxide (CO2) with the atmosphere due to shoaling of very-cold seawater to the near-surface layer. Consequently, of the numerous theories that have been advanced to explain oceanic control of atmospheric CO2, most of those that remain viable point to a dominant role for this region. Increasing consumption of upwelled CO2 and nutrients (e.g. increasing export production) or decreasing rates of supply to the surface (e.g. increasing stratification or sea-ice cover) may thus be responsible for reduced atmospheric CO2 during glacial maxima. Since at present only a small fraction of available macronutrients (nitrates and phosphates) are consumed in Southern Ocean surface waters, either scenario would produce a substantial increase in relative nutrient utilization, thus making it a master variable. The history of Southern Ocean surface nutrient utilization will be reconstructed for the last 60,000 years at three Ocean Drilling Program (ODP) sites that span the polar front. These remarkable Atlantic sector sites have continuous, high sediment accumulation rates (>15 centimeters per thousand years (cm/kyr)) for the entire period to be studied. They are also unique for the Southern Ocean in being within the modern zone of high biogenic opal content as well as having sufficient foraminiferal abundance for oxygen isotope chronostratigraphy. Centennial resolution will be achieved over the past 60,000 years to determine phasing with changes in climate and atmospheric CO2 to within 500 years with emphasis on millennial-scale events between 20,000 and 60,000 years ago as well as the last glacial termination. Since sea-ice has been implicated as a driving mechanism, detailed comparisons will also be made for existing sea-ice proxy data from the ODP sites as well as the Vostok ice core climate and CO2 record. Nitrogen isotopic ratio will be the primary proxy for nutrient utilization, which has been verified in detail through studies of the modern Southern Ocean. To avoid artifacts associated with organic matter diagenesis, we will conduct analyses primarily on diatom-frustule bound organic matter. The three ODP sites have relatively high biogenic opal content over the time period of interest. Though not completely unambiguous, diatom carbon isotopic data obtained at no extra cost will be useful for constraining possible scenarios particularly since it will be obtained synoptically with the other tracer data. It will reflect primarily changes in surface CO2 and/or diatom growth rate. In collaboration with Dr. Christina De La Rocha (Cambridge U.), silicon isotopes will be measured on a subset of the samples analyzed for nitrogen and carbon isotopic ratio. To achieve the best possible chronology for nutrient utilization for the last 40,000 years, we will explore radiocarbon dating of diatom frustule-bound organic matter using preparation methodology recently developed by John Hayes' group at the Woods Hole Oceanographic Institution (WHOI) Accelerator Mass Spectrometry (AMS) facility. Subsets of samples analyzed for nitrogen, carbon, and silicon isotopic ratio will be sent to WHOI for sample preparation and AMS radiocarbon analysis. This approach not only permits dating of critical intervals where there is insufficient foram material for radiocarbon dating but also ensures that the dates obtained pertain directly to our nutrient utilization record. Verification of this new method will be made at each site by dating diatom frustules and forams from the same sediment depths where the latter are sufficiently abundant. The following major hypotheses will be tested: #1 Southern Ocean surface relative nutrient utilization is the primary driver of past changes in atmospheric CO2 and hence global climate change. #2 Seasonal sea ice cover is an important regulator of Southern Ocean relative nutrient utilization and CO2 providing a positive feedback between Antarctic and global climate. #3 Glacial reduction of the silicate to nitrate utilization ratio resulted in elevated silicate in Subantarctic surface waters, northward movement of opal-rich sediments, and changes in the nutrient composition of intermediate waters formed in polar frontal region.