Tracing Terrestrial Silica Cycling Using Ge/Si Ratios, Luquillo Mountains, Puerto Rico

Ge/Si ratios are fractionated by several processes in the weathering environment, potentially providing insight on silicate weathering processes and providing a tracer of Si sources in streamwater. We are analyzing Ge/Si ratios in soils, soilwaters, streams, and plants from the USGS Luquillo Water, Energy and Biogeochemical Budgets research watershed in Puerto Rico in an effort to apply this tracer system to granitoid weathering in a tropical environment. This system has many features in common with our previous work in the Hawaiian Islands, but the mineralogy here is more complex and more globally representative. Bedrock is a quartz diorite pluton with a Ge/Si ratio of 2.4 μ mol/mol. Soil and saprolite ratios range from 2.6 to 3.6 μ mol/mol. Soil Ge/Si ratios are lower than ratios measured in basaltic soils due to the accumulation of primary quartz with a low (0.5 μ mol/mol) Ge/Si ratio. Soil kaolinite has a Ge/Si ratio of 5.9 μ mol/mol demonstrating preferential partitioning of Ge into secondary soil clays. Nine common plant species were sampled from the Luquillo site to investigate the role of plants in the terrestrial silica cycle. Many plant species contain abundant opal phytoliths (as much as 4.4 wt% SiO₂ in aboveground biomass). Consistent with our work in Hawaii, plant phytolith opal at Luquillo has very low Ge/Si ratios (0.05 to 0.6 μ mol/mol). Recycling of phytolith opal likely explains surface (top 20 cm) maxima in soil-saprolite porewater [Si] profiles measured in lysimeter samples. Globally, most streams have Ge/Si ratios that vary with discharge and can be explained by mixing of a low-Ge/Si, high [Si] component, and a high Ge/Si, low [Si] component. Our prior work in Hawaii suggests that the low Ge/Si ratios commonly seen in streams reflect a contribution of plant-cycled Si that is particularly important at base flow. We will test this model with samples collected this fall by automatic streamwater samplers during storm events at two gauged stations in the Río Icacos basin.