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

E. D. Erlanger,S. D. Willett,V. Picotti,A. Bufe,J. K. C. Rugenstein

doi:10.1029/2021jf006064

Abstract

AbstractMixed 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.

Highlights

Active orogens are responsible for the majority of sediments and solutes delivered annually to the world's oceans (Larsen et al, 2014; Milliman & Farnsworth, 2011)
The results from this study reveal four key characteristics of denudation partitioning in mixed-lithology active orogens: (a) physical erosion dominates the total denudation flux in the Northern Apennines, and the relative contribution of carbonates and silicates to the physical erosion flux appears to be lithologically controlled, (b) denudation fluxes are decoupled from both silicate and carbonate weathering, likely due to kinetic limitations on silicate weathering and acid-supply limits on carbonate weath
Catchment-wide percent carbonate sand varies from 17% to 76% (Figure 2, Table S2) and values observed here are consistent with point counts of lithic carbonate sand grains (Lc) observed in river or beach sands derived from these catchments (Garzanti et al, 1998, 2002) (Table S2)

Summary

Introduction

Active orogens are responsible for the majority of sediments and solutes delivered annually to the world's oceans (Larsen et al, 2014; Milliman & Farnsworth, 2011). Runoff transports these sediments and solutes to the surrounding floodplains and to the ocean. This process is so fundamental to the cycling of rocks, nutrients, and carbon on Earth that changes in the activity or spatial distribution of active orogens is frequently invoked to ERLANGER ET AL. Periods of icehouse and greenhouse climates have been attributed to tectonic activity, because the uplift and erosion of rock generates fresh mineral surfaces that become available for silicate weathering, thereby modulating atmospheric CO2 concentrations (Berner & Raiswell, 1983; Caves Rugenstein et al, 2019; Goddéris et al, 2017; Raymo & Ruddiman, 1992)

Methods

Results

Discussion

Conclusion