VARIATIONS IN SAPROLITE CHEMICAL WEATHERING SIGNATURES

Coupling chemical weathering with physical weathering processes is key to understanding landscape development. Point measurements used to infer area-wide or catchment-averaged chemical weathering signatures assume local homogeneity of bedrock and its weathering products. Excavation of a profile from ground surface through the soil-saprolite boundary usually reveals variability in both soil thickness and the extent of saprolite weathering. Here we present initial results from a study examining the spatial variability of saprolite from just beneath the soil/saprolite interface. This study seeks to test the assumption of spatial homogeneity.

We dug two 2-meter square soil pits at two fields sites, Frog’s Hollow and Nunnock River, in southeastern New South Wales, Australia, and one in Point Reyes National Seashore, California. The floor of each pit extended beyond the soil column and into the saprolite. We gridded and sampled the floor of each pit on a half-meter interval, which yielded twenty-five saprolite samples per pit. Each saprolite sample was analyzed by ICP and XRF for trace metals and major oxides and by XRD for secondary mineral content. All pits are on convex hillslopes underlain by granitic bedrock. Using three well studied field sites with different climatic and tectonic regimes also enables comparison of the relative roles of chemical and physical processes between sites.

The weight-averaged coefficients of variation at all three sites are similar and quite low for major rock forming minerals accounting for 95% of sample mass: between 0.06 and 0.09. XRD and XRF data from Nunnock River and Point Reyes indicate very homogenous saprolite across the respective local field areas with few outliers among other major oxides and trace metals. At Frog’s Hollow, however, there is considerable variability in saprolite, from clay-rich to rough crystalline texture and density, as well as local variability in soil column thickness that is between 20 and 75 cm.