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 oceanicaandThalassiosira 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)opalproduced over the latter portion of the growth cycle is indistinguishable from the initial solution ratio. These results confirm experiments using radioactive68Ge 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)opalrecords (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)opalvariations 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)opalrecord 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)opalrecords thus contain a history of ocean silica chemistry that reflect rapid global changes in continental weathering.