Fluxes and pathways of carbon export associated with Himalayan erosion

Abstract

The evolution of the global climate is controlled through the cycling of carbon between its reservoirs on multiple timescales. The erosion and weathering of mountain ranges are major actors in the carbon cycle contributing significantly to the global carbon fluxes between the Earth’s surface and the Earth’s crust on timescales of millions of years. Different mechanisms acting as carbon sources or sinks to the Earth’s surface add up to a complex system controlling the erosional carbon budget of a mountain range. Enhanced soil erosion in rapidly eroding mountain ranges plays additionally a crucial role in climatic fluctuations on timescales of millennials and is important for the sustainability of agricultural soils. In this thesis, I address different aspects of the carbon fluxes associated with the erosion and weathering of the Central Himalaya. This tectonically active mountain range is characterized by high erosion rates and the large amounts of riverine sediments and solutes annually exported by rivers from the Nepalese Himalaya are of global importance. In the first part, I define the net carbon budget of the Central Himalayan erosion and evaluate the impact of a major earthquake on this budget. On a sample set of daily suspended sediment samples from a large Himalayan river, I quantify the export of biospheric Organic Carbon (OC) using radiocarbon signatures and total OC concentrations. Chemical weathering rates in the catchment are constrained by measurements of major ion concentrations on daily water samples from the same river. We find that the Central Himalayan erosion acts as a net carbon sink from the Earth’s surface and that coseismic landslides of the 2015 Gorkha earthquake (Mw 7.8) did not significantly influence the carbon fluxes. In the second part, I explore the application of brGDGTs (branched Glycerol Dialkyl Glycerol Tetraethers), a soil biomarker, as a tracer of the elevation at which soil OC is preferentially mobilized at the scale of large Himalayan catchments. Our results of brGDGT distributions in soils and river sediments show that soil organic matter entrained in fluvial sediments mostly reflects the mean elevation of the soil-covered catchments. Inverse modelling of the brGDGT dataset suggests that riverine soil OC export in the Himalaya occurs pervasively and is insensitive to anthropogenic perturbations at the catchment scale. Further, I investigate the structural diversity and main source region of petrogenic OC exported in Himalayan river sediments. The characterization of petrogenic OC by Raman spectroscopy reveals a large diversity of structural ordering, highlighting the importance of a systematic study of petrogenic OC material in river sediments. I show that petrogenic OC in suspended sediments of the Central Himalaya is mostly sourced from the upstream region's metasedimentary units and that in-river oxidation is likely negligible. Finally, the partitioning of chemical weathering pathways in 28 catchments across the Nepalese Himalaya is studied. I test the use of radiogenic and stable carbon isotopic compositions of dissolved inorganic carbon, sulfur isotopic signatures of dissolved sulfate, and major ion concentrations of river water samples for tracing chemical weathering reactions. Inverse modeling results suggest a strong lithological control on the weathering pathways and reveal the importance of sulfuric acid-driven weathering of both carbonates and silicates. In summary, this thesis shows that the Himalaya’s highly erosive environment acts as a carbon sink to the Earth’s surface. This carbon budget is not significantly impacted by extreme events such as major earthquakes. While the mobilization of soil organic matter happens mostly uniformly across the wide-spanning elevations of the Himalaya, petrogenic OC exported in river sediments is mostly sourced from upstream metasedimentary units. The chemical weathering regime of the Central Himalaya is mainly controlled by the bedrock lithology, revealing local changes in the relative contribution of different chemical weathering pathways.