Late Cenozoic Ge/Si record of marine biogenic opal: Implications for variations of riverine fluxes to the ocean

Abstract

We have determined germanium/silicon ratios in purified diatoms and radiolarians from siliceous sediments in Holocene core tops, one late Pleistocene piston core, and four high‐latitude Southern Ocean Deep Sea Drilling Project (DSDP) sites ranging in age from Holocene to Oligocene. High‐latitude Holocene diatoms in the 10‐ to 38‐µm size fraction ([Ge/Si]opal = 0.693 ± 0.039 × 10−6) faithfully record the present oceanic ratio ([Ge/Si]seawater = 0.699 ± 0.004 × 10−6). This confirms that diatoms from siliceous oozes are monitors of seawater Ge/Si and may record past changes in rates of delivery of weathering products to the sea from continental and seafloor weathering. Large diatoms and radiolarians display ratios much lower than (Ge/Si)seawater, suggesting that (Ge/Si)opal, to some extent, displays the effects of biological fractionation. From 0 to 2.5 m.y.B.P., (Ge/Si)opal in diatoms from DSDP sites ranges from about 0.54 to 0.70 × 10−6, similar to the range observed in one late Pleistocene piston core over the last 25 kyr. From 2.5 to 6 m.y.B.P., the ratio is more variable, while prior to 10 m.y.B.P., ratios are clearly higher, averaging about 0.9 × 10−6 in the early to middle Miocene. Low values of the ratio are consistent with global weathering regimes dominated by river silica input to the sea, while higher ratios suggest periods of enhanced hydrothermal input or reduced fluvial contribution. Nevertheless, an oceanic one‐box model with variable river and hydrothermal inputs and biogenic opal output cannot adequately account for the magnitude or the timing of changes observed in the record, suggesting the existence of other sources/sinks for inorganic germanium and/or silicon to or from the sea.