Top down and bottom up: a comparison of nannofossil strontium/calcium and benthic foraminiferal accumulation rates as paleoproductivity indicators
Date
2005
Authors
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Publisher
University of Delaware
Abstract
This study compares productivity records from two different proxies, nannofossil Sr/Ca as an indicator of surface ocean primary productivity and benthic foraminiferal accumulation rates (BFAR) as an indicator of consumption at the seafloor, using late Miocene-early Pliocene sediments from Ocean Drilling Program (ODP) cores at Atlantic Ocean Sites 982 and 925. The research provides the unique opportunity to relate multiple aspects, both physical and geochemical in nature, from a single set of synchronous samples in order to reconstruct paleoproductivity. Despite a number of uncertainties uniquely related to each proxy, the productivity reconstructions share numerous similarities. Records from both sites show a marked increase in productivity in the long term trend during the study interval, including increasing values of BFAR [Diester-Haas et al., in press] and bulk (0.8 to 20.0 μm) nannofossil Sr/Ca at ~5 Ma at Site 982, and an increase in measured BFAR [Diester-Haas et al., in press] and fine fraction (0.8 to 8.0 μm) Sr/Ca that are synchronous at ~6 Ma at Site 925. Additionally, the records share a number of similarities in small scale maxima and minima. This suggests that a link is preserved in the geologic record between surface ocean productivity, export production and the reflection of them on the seafloor. Furthermore, the results of this study support evidence for an ocean wide increase in productivity during the late Miocene-early Pliocene study interval. ☐ Species effects on the Sr/Ca record are examined here using a high resolution nannofossil species abundance record published by Gibbs et al. [2004] for ODP Site 926. Results of this analysis show cyclic insolation-like fluctuations in measurements of the coarse fraction (8.0 to 20.0 μm) nannofossil Sr/Ca only. These patterns are consistent with orbital forcing and parallel fluctuations in Discoaster and Florisphaera profunda abundances. This suggests that coarse fraction Sr/Ca may be dominated by a record of discoaster productivity, as total Discoaster abundances are at their minima in association with high magnetic susceptibility and low insolation. Furthermore, this suggests that Discoaster abundances (and potentially calcification and/or growth) may be controlled by environmental conditions (nutrient levels and/or temperature) varying on orbital time scales. This may be associated with, at least in part, orbitally driven terrigenous supply (including clays, organic matter and nutrients) to the Ceara Rise from the Amazon River or insolation guided changes in the upper ocean hydrography (photic zone and/or thermocline depth). ☐ Overall, this study documents remarkably good agreement between the BFAR and nannofossil Sr/Ca proxies for productivity, as well as good support for an increase in Atlantic Ocean productivity during the late Miocene-early Pliocene interval. The presence of some inconsistencies between the proxy records (e.g. differing trends between fine and coarse fraction Sr/Ca at Site 925 and Site 926 as well as excursions in Site 982 nannofossil Sr/Ca) highlight some of the difficulties faced when interpreting such reconstructions. From here, it might be helpful to utilize improved size fractionation methods and complete down-core assemblage analysis to better constrain species effects on the Sr/Ca records. Also, the construction of synchronous coccolith alkenone records could contribute to the geochemical data presented here by revealing any temperature effects that may be present in the nannofossil Sr/Ca data. Both the extension of the Site 926 record through the Miocene- Pliocene interval and heightened resolution at Site 925 may also enhance interpretations. Ultimately, this research emphasizes the complex nature of paleoproductivity reconstructions and their interpretation and provides a strong argument for the coupling of numerous paleoproductivity proxies in paleoceanographic studies.