HR: 16:20h
AN: PP42D-02 INVITED     [PDF]
TI: Organic Carbon Cyclicity in the Kimmeridge Clay (Dorset, UK)
AU: * Tyson, R V
EM: r.v.tyson@ncl.ac.uk
AF: School of Civil Engineering and Geosciences, University of Newcastle, Newcastle, NE17RU United Kingdom
AU: Weedon, G
EM: graham.weedon@luton.ac.uk
AF: Centre for Environmental Change, University of Luton, Luton, LU13JU United Kingdom
AB: The type section of the Late Jurassic Kimmeridge Clay Formation (KCF) of Dorset (UK) has recently been completely cored and studied during the NERC RGGE project. The middle 250m of the formation is characterized by conspicuous metre-scale Milankovitch cyclicity, expressed primarily by marine total organic carbon (MTOC) content, and thought to record 38ka obliquity and 19ka precession cycles. The cyclicity is clearest where mean compacted sedimentation rates are $<$80m/Ma, the low dilution amplifying the effect of productivity and redox cycles on MTOC content. Overall the dominant first MTOC mode varies symmetrically from 0.5% at the top and base of the KCF, to 4-5% in the central eudoxus-wheatleyensis interval, reflecting the long term relative sea level trend and its effects on clastic dilution and bottom water oxygenation. Superimposed third order cycles also influence the cyclicity. Algorithms relating sedimentation rate, carbon delivery flux, bottom water oxygen, burial efficiency and TOC in modern marine sediments yield low to moderate mean paleoproductivity estimates of 40-150 gC/m2/a. Given low dilution and good preservation high absolute paleoproductivity is not required to explain typical MTOC values, but MTOC variation is influenced by the interaction of input, preservation and dilution (IPD) factors in a complex multivariate fashion. In this distal facies phytoclast and palynomorph concentration data (No. per mg rock) are primarily controlled by inorganic sediment dilution, not organic matter supply, which forces positive correlations between genetically unrelated particles; the concentration of refractory phytoclasts may thus provide an intra-cycle proxy for relative mineral dilution. Using this approach, comparison of different cycles suggests that the MTOC is also influenced by dilution at this scale, but changes in all the IPD variiables are probably important. Excess organic-walled plankton concentrations are sometimes correlated with heavier d13Com values, suggestive of short-term productivity spikes (calculated palaeoproductivities of up to 400 gC/m2/a), while other d13Com anomalies are apparently associated with higher contents of isotopically heavy sulphurised organic matter.
DE: 4267 Paleoceanography
DE: 4802 Anoxic environments
DE: 4825 Geochemistry
DE: 9609 Mesozoic
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting