As meteoric waters percolate into the subsurface through soils, fractures, micro-fractures, and rock matrix pores, dilute waters bring O2 and dissolved organic matter to depth, promoting the conversion of unweathered bedrock to regolith. The transition from unweathered to weathered rock occurs in the shallow subsurface as well as along deeper fractures where high permeability conduits transmit reactive fluids into the rock. As weathering progresses, the system shifts from almost-closed to fully open. However, the question remains, how does the first reactant enter essentially closed rock and at what depth does weathering shift from biologically controlled to lithogenic controlled?
We evaluated weathering granitoid and diabase cores (2 each) from ridgetops within the VA and PA Piedmont for bulk chemistry, total CaCO3, total S, δ13C, and Fe(II) from unweathered bedrock to the surface. At all sites, the base of the weathering reaction front—defined by either oxidation or depletion of Ca-bearing minerals—is marked by an increase in CaCO3 and S concentrations relative to the parent rock. The increase, attributed to secondary precipitation, is limited to a narrow depth interval. δ13C values are most depleted at the weathered rock-unweathered rock boundary in every profile consistent with organic derived C as the C source for secondary calcite. In a nearly closed system where O2 and CO2 are depleted, biota can utilize Fe(III) or SO4 as electron acceptors to oxidize organic material that may be present. Reduced S can precipitate with Fe(II) to form iron sulfide.
Reactive-transport models run in batch mode for either simulated biotic or abiotic conditions document sulfate-reducing bacteria (SRB) can lead to the formation of secondary calcite and FeS during incipient weathering of bedrock. Furthermore, δ13C-depleted calcite forms only when biota are included. These data and batch models are consistent with SRB thriving near the weathered rock-unweathered rock boundary and lead to formation of calcite and FeS even in temperate climates. We infer that concurrent formation of secondary calcite and FeS mark the transition from an effectively closed to an open system moving towards the surface and that measurement of isotopes in these secondary minerals can be used to indicate the deepest biotic reactions in a weathering system.
Virginia Marcon*, Hang Wen, Li Li, Susan Brantley (2018): Co-precipitation of calcite and sulfur at depth signifies biotic activity at the onset of weathering in crystalline rocks from temperate climates. Abstract B23H-2625 presented at 2018 AGU Fall Meeting, Washington, D.C., 10-14 Dec .
This Paper/Book acknowledges NSF CZO grant support.