From soil to sea: transport of organic matter in monsoon-fed rivers
Authors:   , Kirkels Frederique Marie Sophie Anne 
Published: January 2025

Rivers play an important role in the global carbon cycle by transporting organic matter (OM) from soil to sea, where upon burial in marine sediments it may form a long-term sink of photosynthetically fixed CO2. Climate change has increased the probability of extreme events such as rainfall and droughts, which, next to impacting societies and ecosystems, can cause large fluctuations in river discharge and thus soil mobilisation and downstream transport. However, where, when, and to what extent OM is mobilised, processed enroute, or transferred downstream and exported to the ocean often remains enigmatic. This research aims to identify the sources, pathways and fate or OM transported in the monsoon-fed Madre de Dios (a tributary of Amazon River, Peru) and Godavari River (India) basins, using diverse molecular, isotopic and geochemical tracers. In addition to identifying different OM sources, the comparison of tracers in plants, soils, suspended or riverbed sediments and in estuarine and marine sediments allows us to assess soil-to-sea transport and test the applicability of these tracers along this continuum. In the Madre de Dios River, branched glycerol dialkyl glycerol tetraethers (brGDGTs) indicate transport of Andean-derived soil OM in the wet season, but this signal was overprinted by contributions of in-river production in the dry season and/or by input of local soils towards the lowlands. In the Godavari River, downstream transport of brGDGTs occurs primarily from the northern and eastern tributaries in the wet season, in response to spatial rainfall patterns. However, in-river production prevails in the dry season and the upper basin year-round due to dams hampering sediment transport. Upon discharge to the marine realm, the soil-derived brGDGT signal is rapidly lost, potentially due to marine production, mineralogy or priming. Hence, further research on the fate and behaviour of brGDGTs during fluvial transport is needed, in particular at the river-sea transition. The occurrence and distribution of the more recently discovered branched glycerol monoalkyl glycerol tetraethers (brGMGTs) in the Godavari basin was assessed as alternative tracer. BrGMGTs are mostly absent in mineral soils, and primarily occur at sites with low oxygen and/or high nutrient levels. In addition, based on their distinctly different composition in marine sediments compared to on land, brGMGTs are likely also produced in the marine realm, making them unsuitable as tracer for land-sea soil OM transfer. Finally, the carbon isotopic composition (d13C) of an extensive collection of C3 and C4 plants in the Godavari basin indicates a strong influence of moisture availability on C3 plant d13C as well as an offset between regional and global vegetation d13C values, which should be considered in vegetation reconstructions. Taken together, this thesis emphasises the importance of evaluating tracers along the whole soil-to-sea continuum, and investigating their spatial and seasonal patterns and limitations. The findings in this thesis advance our knowledge on the applicability of tracers for soil-to-sea transport as well as on the impact of extreme rainfall and drought conditions on fluvial OM transport.