Self-Sedimentation of Fossil Phytoplankton Blooms, Laminated Hemipelagic Sediments and the Oceanic Carbon Cycle

Kurt A. Grimm and Carina B. Lange

The flux of phytoplankton-derived organic carbon from the surface ocean to the deep sea and underlying sediments is a non-uniform process that significantly impacts biogeochemical cycles, atmospheric pCO₂/O₂ and organic carbon enrichment in marine sediments. Some marine phytoplankton actively drive the sedimentation process by the formation of sticky transparent gels which facilitate aggregation, rapid sinking and efficient export flux. Here we present fossil evidence of unfragmented, low-diversity phytoplankton assemblages preserved as sedimentary laminations and as preserved aggregates that are attributable to a similar phytoplankton-driven sedimentary mechanism, here termed "self-sedimentation".

Heterogeneities in the texture and/or composition of sediment supply are necessary for the production of laminated hemipelagic sediments; the absence of hydraulic and biological reworking permits preservation of these sedimentary laminae. Distinctly-laminated core intervals are characterized by large compositional contrasts between adjacent laminae; many such high-bimodality couplets are attributable to self-sedimentation of phytoplankton blooms. Self-sedimentation propels the formation of some conspicuous hemipelagic sedimentary laminations and results in efficient carbon and opal flux to the sediments. These records suggest that phytoplankton-mediated changes in the efficiency of the biological carbon pump may govern many accumulations of organic-rich hydrocarbon source rock as well as many abrupt changes in atmospheric pCO₂ and climate.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California