Geological respiration of a mountain belt revealed by the trace element rhenium

Robert G Hilton,Jérôme Gaillardet,Damien Calmels,Jean-Louis Birck

doi:10.1016/j.epsl.2014.06.021

Abstract

Oxidation of rock-derived, petrogenic, organic carbon (OCpetro) during weathering of sedimentary rocks is a major source of carbon dioxide (CO2) to the atmosphere. This geological respiration is thought to be enhanced by physical erosion, suggesting that mountain belts could release large amounts of CO2 to counter the CO2 sequestration achieved by the erosion, riverine transfer and oceanic burial of organic carbon from the terrestrial biosphere. However, OCpetro oxidation rates in mountain belts have not been quantified. Here we use rhenium (Re) as a proxy to track OCpetro oxidation in mountain river catchments of Taiwan, where existing measurements of physical erosion rate allow the controls on OCpetro oxidation to be assessed. Re has been shown to be closely associated with OCpetro in rocks and following oxidation during chemical weathering forms a soluble oxyanion (ReO4−) which contributes to the dissolved load of rivers. Soils on meta-sedimentary rocks in Taiwan show that Re loss is coupled to OCpetro loss during weathering, confirming previous observations from soil profiles on sedimentary rocks elsewhere. In Taiwan rivers, dissolved Re flux increases with the catchment-average sediment yield, suggesting that physical erosion rate is a major control on OCpetro oxidation. Based on our current understanding of Re mobility during weathering, the dissolved Re flux can be used to quantify an upper bound on the OCpetro oxidation rate and the associated CO2 transfer. The estimated CO2 release from this mountain belt by OCpetro oxidation does not negate estimates of CO2 sequestration by burial of biospheric OC offshore. The findings are compared to OC transfers estimated for the Himalaya, where OCpetro oxidation in the mountain belt remains unconstrained. Together, these cases suggest that mountain building in the tropics can result in a net sink of OC which sequesters atmospheric CO2.

Highlights

Organic matter within sedimentary rocks constitutes a vast stock of carbon that was sequestered from the atmosphere in the geological past, containing ∼15 × 1015 gC which is ∼25 000 times the carbon content of the pre-industrial atmosphere (Sundquist and Visser, 2005)
Together with our new data, these findings suggest that in mountain belts located in the subtropics, the CO2 sequestration achieved by OCbiosphere export and burial is greater than the CO2 emissions by geological respiration during oxidative weathering of OCpetro (Fig. 5)
In solid weathering products from Taiwan, Re loss was coupled to OCpetro loss (Fig. 3), confirming previous work from soil profiles on OCpetro-rich rocks

Summary

Introduction

Organic matter within sedimentary rocks constitutes a vast stock of carbon that was sequestered from the atmosphere in the geological past, containing ∼15 × 1015 gC which is ∼25 000 times the carbon content of the pre-industrial atmosphere (Sundquist and Visser, 2005). Oxidation of this rock-derived, or ‘petrogenic’, organic carbon (OCpetro) during weathering at Earth’s surface is a major source of CO2 to the atmosphere and sink of O2 (Berner and Canfield, 1989; Derry and France-Lanord, 1996). In order to assess the net impact of mountain building on CO2 fluxes to and from the atmosphere (Caldeira et al, 1993; Derry and France-Lanord, 1996; Gaillardet and Galy, 2008), the rates of OCpetro oxidation during weathering in mountain belts must be quantified

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Discussion

Conclusion