Paleoproductivity Reconstructions for the Paleogene Southern Ocean: A Direct Comparison of Geochemical and Micropaleontological Proxies

摘要

The reliability of paleoproductivity proxies must be determined before assessing the role of the oceanic carbon (C) cycle in affecting past climate changes. We compare paleoproductivity records of newly generated micropaleontological data (benthic foraminiferal accumulation rates, BFAR) to those of existing geochemical data (reactive phosphorus [reactive P] mass accumulation rates [MAR] and biological barium [bio-Ba] MAR) for the same Paleogene-aged sediments. Sediments are from the Atlantic (Maud Rise, Ocean Drilling Program Sites 689/690) and the Indian (Kerguelen plateau, Ocean Drilling Program Site 738) sectors of the Southern Ocean. Reactive P MAR, but not bio-Ba MAR, correlates to varying degree with BFAR for all three sites investigated. Export productivity, delivery of organic C to the seafloor, and organic C burial calculated here using bio-Ba MAR, BFAR, and reactive P MAR, respectively, for these sites during the Early Paleogene span 2 orders of magnitude (~0.01 to 1 g C·cm~(-2)·kyr~(-1)). Differences in magnitude of reconstructed organic C fluxes are expected because different proxies record different aspects of the biological pump, and these aspects did not behave proportionally similar for all periods. Proxies studied here indicate that transfer efficiency, the fraction of exported organic matter from 100 m that reaches the deep ocean, was low for the Early Paleogene Southern Ocean, similar to today. Despite this, absolute organic carbon burial was similar or higher than today because export productivity was similar or higher. Elevated temperatures may have increased both biological production and respiration in the Early Paleogene Southern Ocean.