HR: 1340h
AN: PP33B-0940    [Abstracts]
TI: Multi-Site Evidence for Marine Nitrogen Fixation in Mid-Cretaceous Black Shales
AU: * Yum, J
EM: paulyum@umich.edu
AF: Department of Geological Sciences, The University of Michigan, 426 East University Avenue, Ann Arbor, MI 48109-1063 United States
AU: Meyers, P A
EM: pameyers@umich.edu
AF: Department of Geological Sciences, The University of Michigan, 426 East University Avenue, Ann Arbor, MI 48109-1063 United States
AU: Bernasconi, S
EM: stefano@erdw.ethz.ch
AF: Geologische Institut, ETHZ, Sonneggstrasse 5, Zuerich, CH-8092 Switzerland
AB: High concentrations of organic carbon in Cretaceous black shales imply levels of sustained export production of organic matter that are unknown in the modern ocean where marine productivity is usually limited by availability of dissolved nitrate. However, if a mid-water anoxic zone expands upward into the photic zone, then nitrogen-fixing cyanobacteria can flourish. These organisms produce organic matter having an isotopic composition close to atmospheric nitrogen (0 per mil). We have compared the carbon and nitrogen isotopic and total organic carbon compositions of Albian to Santonian black shale sequences from the Demerara Rise in the equatorial Atlantic, the Kerguelan Plateau in the southern Indian Ocean, the Hatteras Rise in the western North Atlantic Ocean, the Angola Basin in the eastern South Atlantic Ocean, and the Cape Verde Rise in the eastern North Atlantic Ocean . Nitrogen isotope compositions that become lighter as organic carbon concentrations increase indicate that organic matter production was enhanced by a consortium of primary producers that included nitrogen-fixers. Expansion of an intensified oxygen minimum zone into the photic zone probably permitted coexistence of algae and of cyanobacteria, the latter functioning best under low-oxygen conditions and not being limited by nitrate availability. Improved preservation of the exported organic matter in an intensified near-surface oxygen minimum zone is implied by C/N ratios that increase to 40 as organic carbon concentrations increase. Periods of wetter climate evidently created periods of increased surface stratification of Cretaceous oceans that led to enhanced cyanobacterial primary productivity, magnified organic matter export, and deposition of the organic-carbon-rich black shales. Our multi-site comparison suggests that climate-related gradients in the degree of surface stratification led to associated gradients in export production of organic matter.
DE: 3344 Paleoclimatology
DE: 4267 Paleoceanography
DE: 1040 Isotopic composition/chemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1055 Organic geochemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting