Development and Application of AMS Carbon-14 Chronologies from Organic Carbon in Antarctic Marine Sediments

This is a project to design, test and apply a new method for high resolution dating of organic carbon in highly-siliceous marine sediment cores lacking carbonate microfossils. In the Antarctic Ocean, especially south of the Polar Front Zone and in the Pacific sector, the principle obstacles to reconstructing detailed paleohistories for the last climatic cycle is an inability to date sediments by traditional isotope stratigraphic correlations of benthic or planktonic forams and by AMS-14C measurements of several thousand hand-picked forams (N. pachyderma) . This gap has prevented the testing of several important theories of global climate change:

plunger photo (1) Did abrupt changes comparable to those in the North Atlantic and Greenland ice cores occur in the Southern Hemisphere (frontal migration, ice sheet discharge, Dansgaard-Oeschger-Heinrich-Bond cycles)

(2) Were these changes synchronous within the Southern Hemisphere and in phase or out of phase with those in the Northern Hemipshere on Milankovitch vs. sub-Milankovitch time scales. Are there leads and lags of Southern Hemisphere climate change vs. Northern Hemisphere climate change on millenial time scales?

(3) Do changes in surface-deep water exchange rates, nutrient uptake efficiencies, sea-ice coverage and stratification in the Antarctic lead or lag the observed changes in atmospheric CO₂ in the ice cores? Are there detectable phase relationships that might provide a test of the High Latitude Nutrient and Iron Hypotheses of carbon export? In order to identify the timing of these important events in the Southern Ocean it is necessary to have an independent chronological method which is routine, amenable to continuous high resolution down-core application, and not dependent on the rare presence of carbonate microfossils.

Previous attempts to 14C date organic matter in opal-rich sediments in the Antarctic has been limited by an as-yet unexplained incorporation of modern carbon, raising the 14C content of "dead" samples to over 2% modern. We propose to eliminate this source of contamination and develop a routine methodology to AMS 14C date Antarctic sediments. We will test methods based on both bulk and diatom-intrinsic carbon. The established protocol will then be tested in several cores which contain enough planktonic foraminifera (N. pachyderma sin) for a paired comparison via AMS analysis. To the extent possible, we will utilize volcanic ash layers from known dated volcanoes to establish surface ocean reservoir corrections.

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