Biogeochemical Weathering Processes in a California Marine Chronosequence as Implicated by Pore-Water Major-Constituent Concentrations and Germanium-Silicon Fractionation

Silicate weathering processes dominate weathering rates from terraced grassland surfaces in the drainage basin of Wilder Creek (Wilder Ranch State Park, Santa Cruz County, California). This study uses Si, Na, and Ge concentrations from soil pore-water samples taken from a flight of five marine terrace locations in the watershed. A sample-concentration procedure preceded ICP-MS instrumental analysis in order to determine Ge at parts-per-trillion levels. At two sites (Terraces 3 and 4), higher Si and Na concentration levels at the surface or near-surface depths showed the effect of evapo-transpiration. Minimum levels of these two elements occurred from 15 cm to 90 cm deep, and for each the concentration increased gradually with depth. These increases showed that plagioclase dissolution controlled pore-water Si and Na deep in the respective soil profiles. Samples from both Terrace 3 (137 ka in age) and Terrace 4 (139 ka) had Ge/Si ratio values ranging from 0.3 x 10^-6 to 5 x 10^-6. For both terrace sites, larger ratios occurred in samples taken from shallower depths. The ratio values progressively decreased with increasing depth, down to about 1 to 2 meters below the surface. Larger Ge/Si values provided evidence of uptake and incorporation of Si into phytoliths in leaf structures by prevailing native grasses that depleted Si at shallower depths. As the end of the annual growth cycle for the grasses coincides with the onset of seasonal rains, the phytolithic Si becomes readily available for fluvial transport. These results have important ramifications in interpreting Si fluxes for the estimation of weathering rates in watersheds.