Controls on weathering zone thickness in a rapidly eroding mountain range, western Southern Alps/Ka Tiritiri o te Moana, New Zealand/Aotearoa

Abstract

Tectonic fracturing in uplifting mountains facilitates fluid-rock interactions, causing downward propagation of chemical weathering fronts. In contrast, erosion in uplifting mountains removes fractured and chemically altered bedrock, thinning the weathering zone. The interplay of these processes sets weathering zone thickness, but despite the disproportionate influence of chemical weathering in mountains on global biogeochemical cycles, it is unclear where within the weathering zone those chemical reactions predominantly occur. Here we present geochemical data from a 300 m-deep drill core and results from reactive transport modeling to assess weathering zone characteristics in the Southern Alps/Kā Tiritiri o te Moana of New Zealand/Aotearoa. Our findings indicate that soil is thin and chemical weathering fronts are shallow, with only apatite (and likely calcite) weathering extending below the soil-bedrock boundary. Simulations indicate that soil thickness is primarily controlled by porosity-generating plagioclase weathering and that simulated soil thicknesses are consistent with local precipitation and denudation rates. However, simulations also show that if all 6 m of annual precipitation infiltrated bedrock, chemical weathering fronts would extend substantially deeper than observed. We infer that the porosity contrast between soil and rock limits bedrock fluid flow, slowing the propagation of chemical weathering. Erosion and limited fluid-mineral interaction in deep fractures result in a thin weathering zone, suggesting that silicate weathering in uplifting mountains occurs primarily within soil, rather than bedrock. Our measurements suggest that oxidative weathering of petrogenic carbon has been overestimated previously, but, consistent with prior work, surface processes in the study area result in net consumption of atmospheric CO₂.