A core top assessment of proxies for the ocean carbonate system in surface‐dwelling foraminifers

Paleoceanography - Tập 22 Số 3 - 2007
Yunyan Ni1,2, Gavin L. Foster1, T. R. Bailey3, Tim Elliott1, Daniela N. Schmidt4, Paul N. Pearson5, Brian A. Haley1,6, Christopher D. Coath1
1Bristol Isotope Group, Department of Earth Sciences; University of Bristol; Bristol UK
2Now at Research Institute of Petroleum Exploration and Development, Beijing, China.
3National Museums and Galleries of Wales, Cardiff, UK
4Department of Earth Sciences, University of Bristol, Bristol, UK
5School of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff, UK
6Now at IFM-GEOMAR, Kiel, Germany.

Tóm tắt

We have assessed the reliability of several foraminifer‐hosted proxies of the ocean carbonate system (δ11B, B/Ca, and U/Ca) using Holocene samples from the Atlantic and Pacific oceans. We examined chemical variability over a range of test sizes for two surface‐dwelling foraminifers (Globigerinoides sacculifer and Globigerinoides ruber). Measurements of δ11B in G. ruber show no significant relationship with test size in either Atlantic or Pacific sites and appear to provide a robust proxy of surface seawater pH. Likewise there is no significant variability in the δ11B of our Atlantic core top G. sacculifer, but we find that δ11B increases with increasing test size for G. sacculifer in the Pacific. These systematic differences in δ11B are inferred to be a consequence of isotopically light gametogenic calcite in G. sacculifer and its preferential preservation during postdepositional dissolution. The trace element ratio proxies of ocean carbonate equilibria, U/Ca and B/Ca, show systematic increases in both G. ruber and G. sacculifer with increasing test size, possibly as a result of changing growth rates. This behavior complicates their use in paleoceanographic reconstructions. In keeping with several previous studies we find that Mg/Ca ratios increase with increasing size fraction in our well‐preserved Atlantic G. sacculifer but not in G. ruber. In contrast to previous interpretations we suggest that these observations reflect a proportionally larger influence of compositionally distinct gametogenic calcite in small individuals compared to larger ones. As with δ11B this influences G. sacculifer but not G. ruber, which has negligible gametogenic calcite.

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Paleoceanography

Tập 22 Số 3

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