Terrestrial paleorecords of Ge/Si cycling derived from lake diatoms

Abstract

Modern river studies support the generalization that surface waters in regions undergoing extensive chemical weathering have elevated dissolved germanium/silicon (Ge/Si) ratios compared to regions with less extensive chemical weathering, thought to be the result of Ge fractionation in 2:1 clays. Temporal variations in Ge/Si observed in marine diatoms have thus been linked to past global trends in terrestrial weathering. However, this relationship has not been adequately ground-truthed by a terrestrial-based paleoclimate study utilizing Ge/Si ratios in lake diatoms. To this end, a sediment core was extracted from Dry Lake (el. 2763 m) in the headwaters of the Santa Ana River of southern California in July 1996. The Dry Lake drainage basin is comprised of biotite–muscovite gneiss and granite, with sparse pine forests and relatively high relief. The core had a basal AMS 14C age of 8,350±60 ybp. We successfully separated diatom samples large enough for cleaning, dissolution and chemical analysis without contamination by detrital materials (confirmed by trace metal analyses). Two dissolved opal samples were analyzed for Ge and Si concentrations along with modern water samples collected from waters within the drainage basin. Diatoms obtained from 8000-year-old sediments near the bottom of the core had an opal Ge/Si of 0.79×10 −6 (mol/mol). A composite sample of diatoms deposited within the past 100 years yielded a significantly lower Ge/Si of 0.34×10 −6. Analysis of Ge/Si was also performed from a variety of other materials in the Dry Lake basin, including stream and lake water, unweathered bedrock, soils, and mineral separates. Together with sedimentologic records from the lake sediments and Ge/Si recorded in other basin materials, it appears that the high Ge/Si values recorded in the 8000-year opal sample were the result of preferential early weathering of high Ge/Si biotite and muscovite minerals from the slopes. Thus, the detailed examination of Ge/Si cycling in this isolated basin indicates that factors other than clay mineral transformations may drive Ge/Si paleorecords in some settings.