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Integrated Quantitative Palynology and Chemostratigraphy of Upper Devonian Black Shales: Implications for Anoxia-Related Controls on Green Algal Distributions

Sarah R. de la Rue
University of Idaho, Department of Geological Sciences Moscow, ID 83844, USA; [email protected]

High quantities of total organic carbon (>4 wt %) in black shales are thought to have resulted from increased primary production and subsequent oxygen-depleted bottom waters, thus increasing carbon sequestration. This inference is based on large yields of the green alga Tasmanites and elemental ratios interpreted as being larger than Redfield C:N:P ratios required for modern photosynthesis. Previous black shale studies, however, have not accounted for species-specific metabolic reactions of organic-walled phytoplankton to varying quantities and types of major elements and trace metals.

I will test two hypotheses on Upper Devonian rocks from high- and low-latitudes: 1) oxygen-deficient bottom-waters and ensuing changes in geochemical regimes were required for a turnover in surface-water phytoplankton ecosystems, and 2) anoxia resulted in optimal growth conditions for green algae, while previously-dominant organic-walled phytoplankton species experienced heavy metal toxicity, thus explaining their rarity or absence during times of anoxia.

Testing of the first hypothesis utilizes downcore same-sample changes in lithology, phytoplankton assemblages (morphology, species composition, and concentrations), chemostratigraphic profiles, and elemental suites. Recovered trace-elements and biological proxies will be compared to requirements for cell growth of modern correlative phytoplankton, allowing testing of my second hypothesis concerning enhanced or inhibited cell growth in reaction to changing sea-water chemistry.

The significance of this project is that it will show an association between anoxia-related shifts in oceanographic elemental suites and the biolimiting controls on cell growth of Paleozoic phytoplankton. This association will provide a biologically-oriented explanation tying Tasmanites abundance with economically-important black shale deposits.

AAPG Search and Discovery Article #90083 © 2008 AAPG Foundation Grants in Aid