Spatio-temporal evolution of organo-mineral associations during fluvial transport across the land-ocean aquatic continuum

Rivers are important transport agents, transferring organic carbon (OC) from terrestrial carbon pools to ocean sediments, which act as an important OC sink. Research has shown that OC-mineral associations exert a key influence on OC dynamics by influencing the stability and thus the timescales of carbon storage and transport within drainage basins. Although evidence is mounting that these interactions develop in soils, observations of how the partnership evolves over the land-ocean aquatic continuum are scarce. This thesis examines two natural mountainous river systems with differing climates, lithologies, and tectonic settings, and the results further reinforce the crucial role played by OC-mineral associations in the preservation of organic carbon. The results of this thesis show an irregular radiocarbon pattern, where riverine dissolved organic carbon (DOC) was older than the corresponding particulate organic carbon (POC), challenging the paradigm that POC is generally older than DOC in global rivers. This age inversion is attributed to differing mobilization and transport patterns of POC and DOC. Younger POC might be transported by aeolian processes or rapidly eroding surface soils, while aged DOC could result from the pre-aging of OC in soils, and a subsequent release into the soil solution during the transformation of short-range order (SRO) minerals into crystalline clays. Volcanic soils, such as Andosols formed from tephra, are rich in high surface area minerals, which facilitate the formation of metal-humus complexes and amorphous SRO minerals, which over time transform into crystalline clays like smectite. The insights from Icelandic rivers, applied to global rivers, suggest that volcanic mountainous regions in wet climates (e.g., Chile, Japan, New Zealand, the Azores, and the Philippines) could be hotspots for carbon stabilization through high surface area minerals. This study continued with a detailed analysis of inorganic and organic tracers across the Fraser River basin in British Columbia, Canada, which provided ideal conditions to trace inorganic (87Sr/86Sr, εNd) and organic (leaf wax δ2H) detrital loads from their source to the ocean. An increase in εNd from the headwaters to the Strait of Georgia indicated a transition from older (lower Sr/ Sr, higher εNd). Similarly, leaf wax δ H values increased from source to sink ( : 45‰), reflecting environmental gradients (climate, elevation and distance from coast) between the Rockies and the Coast. A two-component mixing model revealed coupled transport throughout most of the river, with loss and replacement confined to the lower floodplain. crustal material in the Rocky Mountains (higher 87Sr/86Sr, lower εNd) to younger, mantle-like material in the Coast Range 87 86 2 Seasonal suspended sediment sampling across a river depth profile, combined with a year-long biweekly marine sediment trap records, provided insights into the temporal evolution of OC-mineral associations throughout a year. Isotope ratios of Sr and Nd in the sediment indicated the Coast Range as the predominant source of the Fraser’s inorganic load. While trap sediments showed only small temporal variations in 87Sr/86Sr and εNd, leaf wax δ2H varied significantly through the year, tending lower during Autumn which could be due to floodplain contributions replacing mountain-derived leaf waxes. During early freshet, both radiogenic 87Sr/86Sr and low leaf wax δ2H pointed to the Rocky Mountain headwaters as the primary sediment source. A significant transient increase in 87Sr/86Sr during a 2010 storm event provides further evidence for a rapid transport from source to sink during periods of high flow. The geomorphology of the Fraser, characterized by narrow canyons and its undammed nature throughout the river main stem promotes pipe-like behaviour, limiting internal sediment accumulation apart from the lower floodplain, which is now significantly restricted by dikes. Climate change induced warming lead to intensified rainstorms in the Pacific Northwest potentially triggering more severe floods and landslides. Furthermore, summer droughts and mountain pine beetle infestations left the region vulnerable to wildfires. The associated soil loss could increase terrestrial carbon export including recalcitrant black carbon and subsequent burial in the Salish Sea and thereby enhance long-term CO2 drawdown. These findings underscore the role of mountainous rivers as conduits for transport of terrestrial organic carbon to floodplains. A better understanding of riverine OC dynamics will facilitate the interpretation of sedimentary archives, which are an important window into the past terrestrial carbon cycle and are key to understanding and predicting impacts of current anthropogenic perturbations.

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