Colluvial deposits as a possible weathering reservoir in uplifting mountains

Sébastien Carretier,Javier Martinez,Pierre Martinod,Yves Goddéris,Martin Reich

doi:10.5194/esurf-6-217-2018

Abstract

Abstract. The role of mountain uplift in the evolution of the global climate over geological times is controversial. At the heart of this debate is the capacity of rapid denudation to drive silicate weathering, which consumes CO2. Here we present the results of a 3-D model that couples erosion and weathering during mountain uplift, in which, for the first time, the weathered material is traced during its stochastic transport from the hillslopes to the mountain outlet. To explore the response of weathering fluxes to progressively cooler and drier climatic conditions, we run model simulations accounting for a decrease in temperature with or without modifications in the rainfall pattern based on a simple orographic model. At this stage, the model does not simulate the deep water circulation, the precipitation of secondary minerals, variations in the pH, below-ground pCO2, and the chemical affinity of the water in contact with minerals. Consequently, the predicted silicate weathering fluxes probably represent a maximum, although the predicted silicate weathering rates are within the range of silicate and total weathering rates estimated from field data. In all cases, the erosion rate increases during mountain uplift, which thins the regolith and produces a hump in the weathering rate evolution. This model thus predicts that the weathering outflux reaches a peak and then falls, consistent with predictions of previous 1-D models. By tracking the pathways of particles, the model can also consider how lateral river erosion drives mass wasting and the temporary storage of colluvial deposits on the valley sides. This reservoir is comprised of fresh material that has a residence time ranging from several years up to several thousand years. During this period, the weathering of colluvium appears to sustain the mountain weathering flux. The relative weathering contribution of colluvium depends on the area covered by regolith on the hillslopes. For mountains sparsely covered by regolith during cold periods, colluvium produces most of the simulated weathering flux for a large range of erosion parameters and precipitation rate patterns. In addition to other reservoirs such as deep fractured bedrock, colluvial deposits may help to maintain a substantial and constant weathering flux in rapidly uplifting mountains during cooling periods.

Highlights

Since the contribution of Walker et al (1981), the chemical weathering of continental silicate rock is known to be at the heart of the geological regulation of the carbon cycle and climate, through the existence of a negative feedback between climate and silicate weathering (Berner et al, 1983; François and Walker, 1992)
By linking weathering processes on the mineral, hillslope, and river scales, we provide new insights into the effect of valley widening and the associated colluvial deposits on weathering rates in uplifted areas
CIDRE does not model the precipitation of secondary minerals or variations in the pH, pCO2, and changes in the chemical equilibrium related to the water–rock interaction (Oelkers et al, 1994; Brantley et al, 2008; Maher et al, 2009; Lebedeva et al, 2010)

Summary

Introduction

Since the contribution of Walker et al (1981), the chemical weathering of continental silicate rock is known to be at the heart of the geological regulation of the carbon cycle and climate, through the existence of a negative feedback between climate and silicate weathering (Berner et al, 1983; François and Walker, 1992). Since the pioneering work of Walker et al (1981), numerous studies have investigated the role of parameters other than climate on the silicate weathering efficiency. Those parameters include the key role of the vegetation cover (Berner, 1994; Drever, 1994; Le Hir et al, 2011), of the lithology (Bluth and Kump, 1994; Dessert et al, 2003; Ibarra et al, 2016; Caves et al, 2016), and of the paleogeography

Results

Discussion

Conclusion