Abstract

It is well known that chemical weathering and porosity development in rocks are coupled. It is less understood how these coupled processes, which operate at the mineral grain-scale, impact larger-scale phenomena such as watershed fluxes and regolith and landscape development. The Bisley watershed in the Luquillo Critical Zone Observatory (Puerto Rico) is formed on meta-volcaniclastic bedrock that is blanketed by thick regolith comprised of thin soil overlying saprolite embedded with corestones. Saprolites here are highly leached; as a result, most chemical weathering fluxes are attributed to weathering at the bedrock-saprolite interface. However, corestone surfaces and fractures represent multiple “bedrock-saprolite interfaces” distributed throughout the regolith profile. Here we compare macro-scale weathering profiles (m’s thick) in saprolite to micro-scale (mm’s thick) profiles across corestone weathering rinds. Buried corestones were sampled by drilling two boreholes (27 and 37 m deep).
Weathering fronts are most dramatic in rinds on corestone fractures and surfaces. For example, ~40% of the protolith Mg is lost over ~3 mm of rind. In saprolite hand-augered to a corestone at 9.3 m, the final 20% of protolith Mg is lost over ~8 m. These fronts may reflect different weathering mechanisms as well as different weathering rates.
Pyrite crystals, which are of low abundance in these rocks and not detected by powder XRD, were identified by SEM in many of the thin sections and appear to be the first mineral to weather. In thin sections containing fracture surfaces, pyrite is associated with increased porosity and dissolving plagioclase and amphibole grains. These associations are observed even cm’s inboard of the visible weathering rinds. Although present only in trace amounts, pyrite appears to play a controlling role in secondary porosity development and weathering of the major minerals by releasing sulfate during oxidative dissolution, creating micro-environments of highly reactive, low pH fluid within the corestones. Where the bedrock contains more pyrite, corestones may split and shrink more readily, affecting mineral weathering rates and larger-scale phenomena such as the size and distribution of corestones within the regolith profile.

Citation

Buss, H.L., Moore, O.W., Chapela Lara M., Schulz, M., White, A.F (2013): Relating Grain-Scale Weathering Observations to Catchment-Scale Critical Zone Morphology. Goldschmidt2013 Conference Abstracts. Florence Italy..

This Paper/Book acknowledges NSF CZO grant support.