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A test of (Ge/Si)_opal as a paleorecorder of (Ge/Si)_seawater

¹ Département de Géologie et Océanographie, CNRS URA 197, Université de Bordeaux I, Avenue des Facultés, 33405 Talence Cedex, France
² Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964
³ Max-Planck Institut für Meteorologie, Bundesstrasse 55, 20146 Hamburg, Germany
⁴ School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332-0340

Late Pleistocene variations of germanium to silicon ratios in marine diatom shells from sediment cores, (Ge/Si)_opal, are coherent with the global isotope record of glacial to interglacial climate change. These variations are thought to reflect changes in (Ge/Si)_seawater driven by climate-modulated alterations in oceanic Ge/Si sources and sinks. However, an important criterion for interpreting (Ge/Si)_opal as a monitor of whole ocean (Ge/Si)_seawater is that the opal burial ratio be insensitive both to local diatom production and surface ocean silica concentrations (so-called biological fractionation effects) and to differential dissolution artifacts (so-called diagenesis offsets). Here we test these assumptions by comparing model ocean sediment (Ge/Si)_opal distributions with data from Holocene and glacial sediments across the high-latitude Indian-Antarctic Ocean siliceous ooze belt. In contrast to the model, the data show no gradients in either Holocene or glacial (Ge/Si)_opal values across productivity zones displaying dramatic changes in biosiliceous production, opal burial, and dissolution. This evidence supports the contention that fractionation effects are small and that observed down-core variations in (Ge/Si)_opal faithfully record secular changes in (Ge/Si)_seawater.

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