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42908-AC2
Top Down and Bottom Up: Testing the Fidelity of Two Paleoproductivity Proxies in the Context of the Late Miocene to Early Pliocene 'Biogenic Bloom'

Katharina Billups, University of Delaware

In this study, we explore the relationship between two paleoproductivity proxies reflecting differing aspects of primary productivity. These are organic matter production at the sea surface (as recorded by nannofossil Sr/Ca) and its consumption on the seafloor (as recorded by benthic foraminiferal accumulation rates, BFAR). We hypothesize that paleoproductivity variations will introduce evidence for a relatively simple relationship between these two proxy records when measured in the same samples. To test this idea we have re-sampled a mid-Pliocene (3.67 Ma to 3.90 Ma) section of Ocean Drilling Program Site 926 in the northwestern tropical Atlantic. Previous work at this site has established that mid-Pliocene calcareous nannoplankton (nannofossil) assemblages vary on orbital timescales suggesting that insolation linked changes in surface water productivity occurred during the study interval. Our results, however, reveal that while a statistical relationship between the surface and bottom dwellers is apparent during the majority of the record, the nature of the association is a complex one. Size fractioned (8-20 microns) nannofossil Sr/Ca ratios vary positively with the abundance of particular warm water oligotrophic genus (Discoaster). BFARs show a positive relationship with Sr/Ca ratios and Discoaster abundances, which would imply that times of relatively low surface water nutrients also reflect times of enhanced BFAR. Mg/Ca ratios provide evidence that D. pentaradiatus dominate the carbonate fine fraction. These results suggest that phytoplankton assemblages and community structure impact export production. We speculate that the robust and dissolution resistant D. pentaradiatus may be stimulating benthic foraminiferal production along the seafloor.

This research project was the focus of A. Waite's master thesis at the University of Delaware. She presented her work at the 2005 Fall Meeting of the AGU (Waite et al., 2005), and has submitted a manuscript for consideration for publication in the journal GEOCHEMISTRY, GEOPHYSICS, GEOSYSTEMS. Amanda is now enrolled in the Ph.D. program at the University of Miami, in the Rosenstiel School of Marine and Atmospheric Science. The research also contributed to graduate student D. Weirauch's Master Thesis as he was able to learn geochemical techniques and interpretation of paleoproductivity proxies. The research also involved two female undergraduate students, J. Wenke and L. Montone. J. Wenke is an adult reentry finishing a Bachelors in Science. She is now employed part time by an environmental consulting firm in Delaware, and she will be a full time employee after graduating form the University of Delaware. L. Montone is now applying to graduate schools to obtain an advanced degree in Micropaleontology/Paleoceanography. During the grant period, I was promoted to Associate Professor with tenure at the University of Delaware.

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