HR: 0830h
AN: PP11A-0219    [PDF]
TI: Carbon and Nitrogen Stable Isotope Systematics in Three Mid-Pleistocene Sapropels from ODP Site 974 in the Tyrrhenian Basin
AU: Bernasconi, S
EM: stefano@erdw.ethz.ch
AF: Geologisches Institut, ETH-Zentrum, Zuerich, 8092 Switzerland
AU: * Meyers, P A
EM: pameyers@umich.edu
AF: Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1063 United States
AU: Arnaboldi, M
EM: marna@umich.edu
AF: Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1063 United States
AB: The modern Mediterranean Sea is oligotrophic, yet its sediment record contains layers of organic-carbon-rich sapropels at 21 ky (precessional) spacing that imply periods of elevated productivity in the past that were equivalent to the high rates of modern upwelling systems. This paradox is resolved by lines of evidence suggesting that the mode of primary productivity changed from one dominated by algae to one dominated by photosynthetic bacteria during times of sapropel deposition. We have made a high-resolution comparison of the organic carbon and nitrogen isotopic compositions of three sapropels and their background sediments in a 3-m sequence that covers 1001 to 946 ka. Del 13C values systematically increase from -25 to -22 per mil and del 15N values systematically decrease from 5 to -1 per mil as organic carbon mass accumulation rates increase. The increase in carbon isotope values mirrors the increase in marine productivity indicated by the increased mass accumulation rates. The decrease in nitrogen isotope values implies major contributions of nitrogen-fixing cyanobacteria to the total productivity. The precessional minima with which sapropels coincide were times of wetter climate, which stratified the surface Mediterranean Sea and increased delivery of soil-derived phosphorus. Cyanobacteria function best when an amplified oxygen minimum zone extends into the photic zone, which also enhances recycling of phosphorus from detrital organic matter and thereby sustains nutrient availability. Our high-resolution study reveals several relatively rapid excursions into and out of the high-productivity mode, which indicates that sapropel deposition was a surface-driven phenomenon that did not require basin-wide stagnation.
DE: 4267 Paleoceanography
DE: 4802 Anoxic environments
DE: 4805 Biogeochemical cycles (1615)
DE: 4845 Nutrients and nutrient cycling
DE: 4870 Stable isotopes
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting