Controls on Physical and Chemical Denudation in a Mixed Carbonate‐Siliciclastic Orogen

• Published in: Journal of geophysical research. Earth surface Vol. 126; no. 8
• Main Authors: Erlanger, E. D., Rugenstein, J. K. C., Bufe, A., Picotti, V., Willett, S. D.
• Format: Journal Article
• Language: English
• Published: 01.08.2021
• Subjects: carbonate precipitation; chemical Weathering; erosion; Italy; lithology; mountain landscapes
• ISSN: 2169-9003; 2169-9011
• DOI: 10.1029/2021JF006064

Mixed siliciclastic‐carbonate active orogens are common on Earth's surface, yet most studies have focused on erosion and weathering in silicate‐rich landscapes. Relative to purely siliciclastic landscapes, the response of erosion and weathering to uplift may differ in mixed‐lithology regions. However, our knowledge of weathering and erosion in mixed carbonate‐silicate lithologies is limited and, thus, so is our understanding of the mechanistic coupling between uplift, weathering, and the carbon cycle. Here, we partition denudation fluxes into erosion and weathering fluxes of carbonates and silicates in the Northern Apennines—a mixed carbonate‐siliciclastic active orogen—using dissolved solutes, the carbonate sand fraction, and existing 10Be denudation rates. Erosion generally dominates total denudation fluxes relative to weathering by an order of magnitude. Carbonate and silicate contributions to erosion vary between lithologic units, but weathering fluxes are systematically dominated by carbonates. Silicate weathering may be kinetically limited, whereas carbonate weathering may be limited by acid supply. Carbonate re‐precipitation estimated by comparing ion ratios (Sr, Ca, Na) from rivers and bedrock suggests that up to 90% of dissolved Ca2+ is lost from carbonate‐rich catchments. Corresponding [Ca2+] estimates for the weathering zone are high, likely driven by high soil CO2 partial pressures (pCO2); however, re‐equilibration with atmospheric pCO2 in rivers converts solutes back into grains that become part of the physical denudation flux. Weathering limits in this landscape therefore differ between the subsurface weathering zone and riverine exports, and our findings suggest that carbon cycle models may overestimate the sensitivity to erosion of solute exports (Ca2+ and HCO3−) derived from carbonate weathering. Plain Language Summary Erosion moves sediment across the surface and controls how natural resources (e.g., sand and gravel) are generated. Conversely, the dissolution of rock in water (weathering) is the source for nutrients and carbon transported in rivers. Understanding how total surface lowering (denudation) is divided into weathering and erosion is important for establishing the link between mountain‐building processes and the generation of sediments and dissolved material. Existing studies on denudation in mountainous ranges have primarily focused on landscapes comprised of silicate rocks. However, many mountain ranges are characterized by mixed silicate‐carbonate rocks, and the processes that influence denudation of these landscapes may differ relative to silicate‐rich landscapes. In this study, we separate measurements of denudation into erosion and weathering for the Northern Apennine Mountains of Italy, a mixed‐lithology mountain range. Similar to silicate‐rich landscapes, erosion is the dominant process here. Carbonate weathering dominates the total weathering signal, and rock type is an important control on the amount of eroded carbonate delivered to river channels. Most rivers are oversaturated in carbonate, therefore limiting the amount that can be dissolved in rivers. This has resulted in the transformation of dissolved material back into carbonate rock that is once again available to be eroded. Key Points Quantify chemical weathering and physical erosion fluxes for a mixed‐lithology mountain range Physical erosion dominates the denudation signal and carbonate weathering dominates the weathering signal Up to 90% of chemically weathered Ca2+ is precipitated as solid secondary carbonate that becomes part of the physical denudation flux