Assessing the use of non-traditional stable isotopes as tracers of weathering processes: with evidence from the Southern Alps, South Island, New Zealand

Abstract

Concentrations of carbon dioxide (CO2) in the Earth’s atmosphere have increased by >100 ppm since pre-industrial times due to the burning of fossil fuels for energy and it is now clear that the consequent warming of the climate system will have widespread impacts on human and natural systems. Chemical weathering of silicate rocks draws down CO2 from the atmosphere, but the significance of this process and the mechanisms which control weathering rates remain poorly constrained. New information with regards to chemical weathering processes, linkages to physical denudation rates, and the effects of certain rock types on global chemical budgets, are all required.
This study utilises elemental concentrations together with lithium (?7Li) and magnesium (?26Mg) isotopic values of river waters and weathering products to determine the parameters that regulate weathering in a terrestrial environment, and assesses the influence of progressive metamorphism, glacial activity, rainfall patterns, rapid tectonic uplift, climate and geothermal fluid flow. Samples were collected from the Southern Alps on South Island, New Zealand. The Southern Alps represent a relatively pristine environment and a natural laboratory to examine the climatic and tectonic controls on chemical weathering of a largely lithologically homogenous metasilicate terrane with minor metamorphic carbonate, in a temperate, maritime environment.
Chemical weathering and atmospheric CO2 consumption rates, calculated from riverine elemental data, demonstrate that CO2 consumed by silicate weathering is relatively low compared to rivers globally. High overall chemical weathering rates (3.1 x 107 g·km-2·yr-1 in the west vs. 1.8 x 107 g·km-2·yr-1 in the east) are associated with high uplift and erosion rates, and high rainfall on the western side of the Southern Alps, where chemical erosion of metamorphic carbonates is more prevalent. However, higher rates of atmospheric CO2 consumption due to silicate weathering were found on the eastern side of the Southern Alps (6.4 x 104 mol·km-2·yr-1 in the west vs. 7.7 x 104 mol·km-2·yr-1 in the east), where uplift and erosion rates are lower. This indicates that uplift accelerates weathering rates of metamorphic carbonates, but has little effect of rates of silicate weathering, which regulates CO2 drawdown from the atmosphere on long timescales.
The mechanisms that moderate Li and Mg isotopic fractionation in the Southern Alps were thoroughly investigated. Protolith lithology and metamorphic grade have little effect upon the ?7Li and ?26Mg composition of the bedrock. Secondary clay formation (e.g. illite, kaolinite and smectite) during weathering is the dominant process by which Li and Mg isotope fractionation occurs, and climate only has an indirect influence. The residence time of water-rock interaction imposes an important control upon the ?7Li composition of rivers. However, the effect of this control upon the ?26Mg composition of rivers is less clear. The evidence for coupling between riverine ?7Li and ?26Mg values during chemical weathering is weak with respect to results from this study and global studies. This suggests that the behaviour of these isotopes varies between different weathering environments, adding to the complexity of extrapolating local studies to global interpretations.

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ORCID for Rachael James: orcid.org/0000-0001-7402-2315

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