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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