HR: 0800h
AN: PP51D-0640    [Abstracts]
TI: Sediment Composition and Cyclicity in the Mid-Cretaceous at Demerara Rise (ODP Leg 207): Productivity or Anoxia Driven?
AU: Nederbragt, A J
EM: a.nederbragt@ucl.ac.uk
AF: Dept. Earth Sciences, University College London, Gower Street, London, WC1E 6BT United Kingdom
AU: * Thurow, J
EM: j.thurow@ucl.ac.uk
AF: Dept. Earth Sciences, University College London, Gower Street, London, WC1E 6BT United Kingdom
AU: Pearce, R
EM: rp1@noc.soton.ac.uk
AF: School of Ocean and Earth Sciences, University of Southampton, National Oceanography Centre, Southampton, SO14 3ZH United Kingdom
AU: Kemp, A E
EM: aesk@noc.soton.ac.uk
AF: School of Ocean and Earth Sciences, University of Southampton, National Oceanography Centre, Southampton, SO14 3ZH United Kingdom
AB: Characteristic patterns of orbital cycles are present in pre-Neogene sediments, but the climate forcing that produces the observed lithologic variation are still poorly understand. We present an interpretation of cycle patterns in mid-Cretaceous organic rich sediments from Demerara Rise. The sediments shows decimeter to meter scale alternations between carbonate- and organic-rich sediment. The color variable b* (yellow) is used as an indicator for cabonate content, in combination with thin section analysis of representative lithologies. There are two main types of carbonate rich intervals, pelagic carbonate beds and planktonic foraminiferal packstone layers. Pelagic carbonate rich intervals are the dominant carbonate lithology at the deep water sites (ODP Sites 1257/1258). Planktonic foraminiferal packstones are the dominant carbonate lithology in the lower part of the sequence at Sites 1259-1261 but decrease in frequency higher in the sequence (continued subsidence); they persist longer at the shallowest sites (Sites 1259/1261) than at the intermediate waterdepth site (Site 1260). The packstones are attributed to either current or wave induced winnowing in relatively shallow water. There is a positive correlation between cycle thickness and carbonate content in a pelagic interval, indicating that carbonate production was highest during high-amplitude precession cycles. Intervals with planktonic foraminiferal packstones show a variable relation between b* and cycle thickness, which is attributed to variable interplay between carbonate rain rates and subsequent removal due to winnowing.We conclude that the packstone layers are time equivalent with deposition of pelagic carbonates in deeper water. Strong eccentricity/precession bundles are inferred to represent an atmospheric and/or oceanic circulation signal, which controlled upwelling intensity and surface water productivity. Winnowing within carbonate rich levels implies that general circulation, and thus any upwelling, was the most vigorous during deposition of carbonate rich levels, which ultimately implies that organic rich sediments are primarily related to enhanced anoxia.
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
DE: 4910 Astronomical forcing
DE: 4944 Micropaleontology (0459, 3030)
DE: 4946 Milankovitch theory
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005