New insights into the geochemistry of the Critical Zone in rapidly uplifting mountains (Southern Alps, New Zealand)

Abstract:

Rapidly uplifting mountains at convergent plate margins are currently not represented in the CZO network. Despite the potential impact on global carbon sequestration and biogeochemical cycling, the critical zone’s response to processes associated with high tectonic uplift is poorly defined. The western Southern Alps constitute an ideal laboratory of such conditions with up to 1 cm y^-1 tectonic uplift, >10 m y^-1 precipitation and up to 2.5 mm y^-1 denudation/ soil production. To date, most studies have utilised river data to make inferences about weathering but little work has been done on the critical zone directly. Here we present new river, soil, groundwater and bedrock geochemical data and discuss their implications. River water composition followed the known trend of high Ca/Na molar ratios in the region (=2.7, n=6) but also revealed positive and negative correlations of flow-rate with ^87/86Sr and Ca/Na molar ratios, respectively. High flow rates are also associated with elevated Fe, Al and Mn concentrations. Maximum Fe, Al and Mn concentrations were observed in soil water that also had the lowest measured Ca/Na molar ratios of <0.1. Groundwater typically featured the highest Ca/Na ratios of >3.9, and had the highest concentrations of Ca, Mg, Si, and Sr. Phosphorus fractions used as tracers for chemical modification indicate that 50% of the total P in bedrock at the soil base has been already transformed from a primary mineral form into various secondary forms, further increasing to >90% in soils. Referenced to 60-80 m deep rock and Zr as refractory element, in-situ bedrock weathering contributes 72-86% Ca, 63-82% Mg, 39-81% P and 26-55% Si to the total depletion observed in the soil. We conclude 1) the critical zone is compartmentalised into areas of characteristic weathering processes with their respective imprints on the catchment discharge at different hydrological states, and 2) deep bedrock, like soil, is an important domain of chemical alteration and depletion.