Abstract:
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.
