Investigating controls on the variation in sediment and carbon export from the Fraser River Basin during the Anthropocene
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2025
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Master Thesis
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Abstract
Understanding the factors controlling sediment and organic carbon (OC) transport from land to ocean in source-to-sink systems is essential for reconstructing the global carbon cycle over geological timescales. While OC-mineral associations play a key role in terrestrial OC delivery to river systems, most studies have examined different carbon pools in isolation. As a result, the spatial evolution and controlling mechanisms of OC-mineral interactions during source-to-sink transport remain poorly understood. The Fraser River Basin, Canada’s fourth-largest river system, is characterized by high sediment flux, diverse lithologies, and increasing human activities such as flood control and agricultural land use. Despite its significance, the Fraser River's role as a carbon source or sink in the global carbon cycle remains debated. This study investigates the influence of hydraulic sorting and seasonal sediment source variability on OC-mineral association during transport and explores how depositional processes impact this association over time at depositional sink.
To achieve this, six sediment types were investigated along the source-to-sink system: soils, riverbank sediments, river suspended sediments, as well as sinking particles, core top sediments, and a sediment core (IOS) from the Strait of Georgia. Core IOS was analyzed for density and magnetic susceptibility using a Geotek multi-sensor core logger, while 210Pb and 137Cs measurements for the entire core IOS provided insights into sedimentation dynamics. Grain size (GS) and surface area (SA) analyses were conducted to assess the effects of hydraulic sorting, and total organic carbon (TOC) content was also measured across all sediment types. The Soli-TOC method was applied to core IOS, soil, and core tops to differentiate OC contributions from various sources. OC loading, a measure of OC-mineral association capacity, was compared both spatially and temporally. Radiocarbon (Δ14C) measurements from core IOS (via Accelerator Mass Spectrometry) were combined with previous Δ14C and δ13C data to assess temporal changes in OC contributions using a dual-isotope mass balance model. Additionally, previous and new measurements of Sr-Nd isotopic compositions (measured via MC-ICPMS) were analyzed for riverbank sediments, suspended sediments, core IOS, and core tops to trace terrestrial detrital sediment provenance and dispersal during transport and within the Strait of Georgia.
This study reconstructs the evolution of OC-mineral associations along the Fraser River, demonstrating that terrestrial biospheric OC inputs are predominantly derived from C3 vegetation. Bedrock lithology controls OC-mineral associations at the source, while hydraulic sorting exerts a greater influence along the transport pathway. At the depositional sink, terrestrial OC is mixed with marine OC and preserved in the sediment record. The IOS core captures OC variability over the past century, exhibiting higher sedimentation and mass accumulation rates compared to older cores from the Strait of Georgia. Over time, aged terrestrial OC has been progressively replaced by younger OC and increasing marine OC contributions. Sr-Nd isotopic data confirm that headwater signal is detectable during freshet at the river mouth, while Coastal Range lithology dominates sediment in the lower Fraser River and offshore, with terrestrial sediments transported northwestward by river outflow and tidal currents.
Future work should focus on refining OC source contributions to the depositional sink. While sediment granulometry and provenance remain stable, OC loading patterns and SA evolution require further investigation. An improved age model, based on the ongoing 210Pb and 137Cs analyses, will enhance the understanding of long-term depositional characteristics. Additional monitoring of Sr-Nd isotopic variations in tributaries is also needed to refine interpretations of sediment transport dynamics.
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ETH Zurich
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source to sink; Biogeochemistry; marine biogeochemistry; Global carbon cycle
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03868 - Eglinton, Timothy I. (emeritus) / Eglinton, Timothy I. (emeritus)
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215163 - Climate and Anthropogenic PerturbationS of Land-Ocean Carbon TracKs+ (CAPS-LOCK+) (SNF)