Oceanic germanium/silicon ratios: Evaluation of the potential overprint of temperature on weathering signals Public Deposited

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  • Diatom remains indicate the oceanic Ge/Si ratio (μmol/mol) has varied temporally, ranging from Miocene values of 0.9 to Last Glacial Maximum (LGM) values of 0.55, with the present value of 0.69. These ratios lie between those for the primary sources for these elements: continental weathering (Ge/Si = 0.5) and hydrothermal fluids (Ge/Si = 11). Previous explanations for temporal variation have focused on variations in the relative strength of the primary inputs. Alternatively, the cause may be temporal variation in the relative strength of the two sinks for Ge, opal, and a nonopal sink found in margin sediments. The importance of the latter depends on the rate of opal rain to the seafloor in margin environments. As temperature decreases, opal dissolution rates in the water column decrease, permitting a larger fraction of the opal rain to reach the seafloor before dissolving. This increases the rate of Ge uptake into nonopal phases. A model calculation predicts that a decrease of 3.5°C in LGM ocean temperature should increase opal rain to the margin seafloor by a factor of 1.5; an increase of 3.5°C in Miocene temperature should decrease rain by a factor of 0.6. The predicted changes for Ge uptake in the nonopal sink can account for essentially all of the temporal Ge/Si variation recorded in diatoms. The absence of larger changes constrains silicate weathering rates relative to the present as 106 ± 16% during the LGM and 88 ± 12% during the mid-Miocene (15–20 Ma). However, additional uncertainty exists because some factors of potential importance remain unconstrained.
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  • Hammond, D. E., J. McManus, and W. M. Berelson (2004), Oceanic germanium/silicon ratios: Evaluation of the potential overprint of temperature on weathering signals, Paleoceanography, 19, PA2016.
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