River Fluxes of Dissolved Silica to the Ocean Were Higher during Glacials: Ge/Si In Diatoms, Rivers, and Oceans

Abstract

Two centric marine diatom species, Thalassiosira oceanica and Thalassiosira antarctica, were grown in batch cultures to determine the incorporation of germanium (Ge) and silicon (Si) into siliceous shells (opal). The results were modeled as Ge/Si “isotope” fractionation. During exponential growth, diatoms take up and incorporate Ge/Si from solution without major discrimination against Ge. During stationary phase growth near silica limitation, the Antarctic species (T. antarctica) discriminates slightly against Ge but integrated (Ge/Si)opal produced over the latter portion of the growth cycle is indistinguishable from the initial solution ratio. These results confirm experiments using radioactive 68Ge that showed absence of fractionation during diatom silica uptake (Azam and Volcani, 1981), in contrast to two‐box ocean models that invoke 50% Ge discrimination by diatoms to explain the observed “excess” surface ocean germanium concentration (Murnane and Stallard, 1988; Froelich et al., 1989) and late Pleistocene ocean sediment (Ge/Si)opal records (Mortlock et al., 1991). Runs of a 10‐box ocean Ge and Si model (PANDORA) with 50% discrimination reproduce the excess surface ocean Ge but introduces curvature into the deep ocean Ge versus Si relationship that is not observed in the oceans. Thus 50% fractionation is not supported by either cultures or models. If diatoms do not fractionate Ge/Si, then late Pleistocene (Ge/Si)opal variations in piston cores are caused not by changes in local biosiliceous production and silica utilization (Mortlock et al, 1991) but rather by whole ocean changes in (Ge/Si)seawater. The marine (Ge/Si)opal record of the last 450 kyr can be modeled as transient oceanic responses to instantaneous continental climate transitions consistent with the chemical weathering model of Murnane and Stallard (1990). Glacial periods are characterized by lower continental weathering intensity, lower (Ge/Si)riv, and two fold higher dissolved silica river fluxes. Marine (Ge/Si)opal records thus contain a history of ocean silica chemistry that reflect rapid global changes in continental weathering.

Modelled glacial and non-glacial HCO3−, Si and Ge fluxes since the LGM: little potential for impact on atmospheric CO2 concentrations and a potential proxy of continental chemical erosion, the marine Ge/Si ratio

“…Chillrud et al (1994) have published the only glacial meltwater Ge/Si ratio to date. A maximum ratio of 1.35 pmol/Amol is given, which is more than double that of the global average riverine Ge/Si of 0.6 pmol/Amol (Froelich et al, 1992). With an increase in the runoff of glacial meltwater during the main period of deglaciation (7 -12 ka BP), there is a possibility that these higher ratios would increase the corresponding ratios within the oceans.…”

“…Ge/Si ratios within opals derived from deep ocean cores (Mortlock et al, 1991) vary on glacial to interglacial time scales. Froelich et al (1992) and Bareille et al (1998) have shown that these ratios within the opal faithfully record those of the ocean at the time they were formed. Primary controls on Ge/Si ratios within the oceans may include terrestrial chemical weathering intensity (Froelich et al, 1992), but more recent work suggests that diagenetic effects (Hammond et al 2000;King et al, 2000) and the preferential weathering of minerals with high Ge/Si ratios, particularly biotite (Filippelli et al, 2000), are probably more important.…”

“…Froelich et al (1992) and Bareille et al (1998) have shown that these ratios within the opal faithfully record those of the ocean at the time they were formed. Primary controls on Ge/Si ratios within the oceans may include terrestrial chemical weathering intensity (Froelich et al, 1992), but more recent work suggests that diagenetic effects (Hammond et al 2000;King et al, 2000) and the preferential weathering of minerals with high Ge/Si ratios, particularly biotite (Filippelli et al, 2000), are probably more important. Murnane and Stallard (1990) argue that the Ge/Si ratio of riverine solute [(Ge/Si) riv ] is lower than that of the underlying bedrock due to the preferential retention of Ge in secondary mineral phases (e.g.…”

“…The amount of Ge retained within clays is dependent on weathering intensity: the lower the weathering intensity, the higher the Ge/Si ratio in the secondary mineral phases, and consequently, the lower the Ge/Si ratio in the riverine water. Froelich et al (1992) suggested that the variations in (Ge/Si) seawater are driven by large glacial to interglacial changes in continental weathering intensity, resulting in a two-fold increase in the amount of dissolved silica and a 20% reduction in the (Ge/Si) riv ratio. Chillrud et al (1994) have published the only glacial meltwater Ge/Si ratio to date.…”

What caused the glacial/interglacial atmospheric pCO2 cycles?

“…The ocean inventory of H4SiO 4 is determined by the balance between influx by weathering and removal by burial. There has been a suggestion, based on the germanium to silica ratio of sedimentary diatoms, that the global rate of silica delivery to the oceans from chemical weathering might have been higher during glacial time, perhaps because of the abrasive exposure of igneous rocks by ice sheets [Froelich et al, 1992]. Another possibility is that the riverine flux of H4SiO 4 is better able to reach the ocean during glacial time because of the lack of estuaries.…”