A top-down and bottom-up comparison of paleoproductivity proxies: calcareous nannofossil Sr/Ca ratios and benthic foraminiferal accumulation rates

Waite, A.J., Gibbs, S.J., Diester-Haass, L., Rickaby, R.E.M. and Billups, K. (2008) A top-down and bottom-up comparison of paleoproductivity proxies: calcareous nannofossil Sr/Ca ratios and benthic foraminiferal accumulation rates. Geochemistry, Geophysics, Geosystems, 9, Q06005. (doi:10.1029/2007GC001812).

Abstract

We investigate whether a relationship exists between organic matter production at the sea surface, recorded by nannofossil carbonate Sr/Ca, and its consumption on the seafloor, measured by benthic foraminiferal accumulation rates (BFAR). A mid-Pliocene (3.67-3.90 Ma) section of Ocean Drilling Program Site 926 in the northwestern tropical Atlantic was sampled, where previous work established that the calcareous nannoplankton assemblages vary with insolation linked changes in surface water productivity. Our results reveal that coarse fraction nannofossil Sr/Ca varies with changes in assemblage composition and may be predominantly controlled by the geochemistry of the warm water oligotrophic genus Discoaster. BFAR also have a positive relationship with Sr/Ca and Discoaster abundances, implying times of relatively low surface water nutrients coincide with enhanced BFAR. This result is opposite of what one would expect given the assumption of a direct relationship between primary and export production. We speculate that the BFAR are stimulated by enhanced organic carbon export associated with ballasting by nannofossil assemblages dominated volumetrically by large, robust taxa such as Discoaster species. These results highlight the complexity of interpretations of bulk nannofossil Sr/Ca ratios, as well as BFAR data, with respect to paleoproductivity.

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