Adsorption–Desorption Reactions in the Weathering Zone: Implications for Silicate Weathering & the Carbon Cycle

The carbonic acid-driven chemical weathering of silicate minerals removes carbon dioxide (CO2) from Earth's atmosphere. The tectonic processes of uplift, exhumation, and erosion modulate the negative feedback relationship between climate, silicate weathering rates, and atmospheric CO2 removal rates by controlling the exposure rate of unweathered minerals at the Earth's surface. However, the extent of this modulation is debated and critically depends on field-based estimates of silicate weathering. Constraining the rate of silicate mineral dissolution and its dependence on climatic factors (such as temperature and rainfall) in highly erosive regions is therefore crucial for understanding how Earth has remained a habitable planet for billions of years. Field-based estimates of silicate mineral dissolution rates are often derived from measurements of dissolved reaction products (solutes) transported by large rivers. However, this approach may overlook ions chemically bound to the surfaces of sediments, which can be significant in highly erosive regions. To better understand the contribution of the adsorbed phase to chemical weathering budgets, a combination of laboratory and field studies have been conducted. The field study focuses on the Himalayas, where uplift is postulated to have driven the decrease in atmospheric CO2 concentrations over the Cenozoic by increasing silicate weathering rates. Between 2015 and 2018, paired river water and suspended sediment samples were collected from three Himalayan tributaries of the Ganges-Brahmaputra River system. A total of 72 samples were analysed for their sediment-adsorbed and dissolved cation concentrations. This was supplemented by a larger time series of dissolved phase data to provide insights into seasonal variations in adsorbed cation flux dynamics. Laboratory batch experiments were conducted to complement the field study and investigate the relative selectivity of alkali and earth-alkaline elements for the riverine adsorbed phase. These experiments utilised environmentally relevant adsorbent minerals and surface waters. By systematically varying the mineral-to-water ratio, reaction duration, and water chemistry, the experiments aimed to elucidate the role of adsorption-desorption reactions in controlling the transport of fluid-mobile elements in rivers and estuaries. Additionally, stable barium isotope ratios were explored as potential tracers of terrestrial weathering and erosion processes. A systematic partitioning pattern was observed for fluid-mobile cations in rivers. Ions with a more negative enthalpy of hydration, such as barium, tended to partition more strongly into the adsorbed phase relative to ions with a less negative enthalpy of hydration, such as sodium, which were primarily found in the dissolved phase. The significance of the adsorbed phase in transporting chemical weathering products from highly erosive regions was largely influenced by seasonal precipitation-driven changes in sediment fluxes. Seasonal variations in the cation exchange capacity (CEC) of the sediment provided a secondary control. Measurements of stable barium isotope ratios from both field samples and laboratory experiments confirmed that during adsorption-desorption reactions, the lighter isotopes are preferentially enriched in the adsorbed phase, while the dissolved phase is enriched with the heavier isotopes. The degree of enrichment depended on the mineral used, with greater enrichment observed for iron oxyhydroxides compared to clay minerals. These findings, combined with the observed strong affinity of barium for the adsorbed phase, suggest that barium isotope ratios are likely to serve as a unique tracer of terrestrial weathering and erosion processes. Accounting for the desorption of sodium from uplifted siliclastic sediment in mountainous regions reveals that silicate weathering rates in the study catchments may have been overestimated by up to 79%. These findings suggest that silicate weathering rates in highly erosive regions need re-evaluation. Additionally, reactive transport models of adsorption-desorption reactions highlight another potential bias: the differential retardation of ions within regolith following a chemical perturbation. The extent of this bias depends on the perturbation magnitude and the relative selectivity of the elements for the adsorbed phase. Overall, this study emphasises the necessity of including adsorption-desorption reactions in chemical weathering studies to accurately define the interplay between tectonic processes, climate, and silicate weathering rates.