Radiocarbon Constraints On Carbon Dynamics In River Basins

Abstract

Constraining inputs and processes that influence organic carbon (OC) storage and export in fluvial systems is crucial for understanding the fate of carbon during its transfer from the terrestrial biosphere to the marine sedimentary sink. However, given the heterogeneous and dynamic nature of carbon, in-depth knowledge of spatial and temporal scales over which carbon cycles through river systems is required. Pathways and timescales of carbon turnover and transport are known to vary with environmental conditions (temperature, humidity, topography etc.; e.g., Carvalhais et al., 2014; Galy et al., 2015) and are sensitive to anthropogenic disturbances (e.g., Butman et al., 2015; Grill et al., 2019; Raymond et al., 2008; Regnier et al., 2013). However, the relative importance of these factors for different types of fluvial systems remains the subject of debate (e.g., Marwick et al., 2015) and much work needs to be done to develop a comprehensive understanding of underlying processes and robust predictive capabilities regarding the nature, magnitude, and pace of future carbon cycle change. Acquisition of information on carbon cycle processes over appropriate spatial and temporal scales is crucial to quantitatively assess and upscale observations from watershed to regional and global biogeochemical scales. This thesis aims to further our understanding of controls impacting riverine OC export over various spatial and temporal scales under differing climatic and geomorphic settings. Sedimentological and organic geochemical tools are applied to a multi-annual time-series, located in a moderately steep, subalpine Swiss river basin, to assess the influence of short-term hydrological and environmental variability (seasonality, storm events) on exported carbon and plant-derived biomarkers (chapters 2 and 3). By augmenting pre-existing data with a comprehensive suite of new sedimentological and OC measurements, we constrain the inputs and propagation of particulate (POC) and dissolved (DOC) organic carbon in the Arctic Mackenzie River basin and evaluate potential changes in OC dynamics along the land-ocean continuum in response to regional climate change (chapters 4 to 6). In Scotland, we investigate eight peatland-dominated watersheds and their link between POC, DOC, and land-use forms. The isotopic composition of POC in the Sihl River suggests that plant debris and surface soils are the main sources of organic matter. Sediments and OC are primarily entrained to the Sihl River via surface runoff, with the majority of particulate material transported during short-lived storm events. Although headwater streams receive a substantial proportion of fossil carbon from underlying bedrock, the petrogenic carbon signature is masked or diluted by the input of fresh biospheric OC. Our findings indicate that even local processes might likely significantly impact the concentration and composition of exported POC. Similar to the bulk OC, the main export of vascular plant wax lipids occurs during high-discharge events. Plant-derived biomarker abundances and compound distributions transported in the Sihl River are controlled by seasonal cycles of plant growth, senescence, and environmental factors. Higher plant leaf wax lipid classes (i.e., alkanes and fatty acids) display a temporal lag in riverine export that varies according to their physicochemical characteristics influencing the leaching from plant litter and surface soils. Leaf wax lipids are faithful tracers recording the effects and the response of vegetation to extreme, climatic events such as droughts (2018). Hydrodynamic sorting controls the transfer and distribution of OC over channel to basin-scales. In the Mackenzie River, the most 14C-depleted signature is encountered in fine-grained sediments transported in surface waters. In contrast, the 14C content increases (i.e., 14C age decreases) with increasing water depth and particle coarsening, corresponding to the enrichment of relatively fresh plant debris near the river bed. This concentration of water-logged woody material in the near-bottom suspended load reveals a hitherto poorly known mechanism for the export of coarse biospheric carbon to deltaic and adjacent shelf margins. The significance of this transport mechanism with respect to terrestrial OC burial and its impact on atmospheric CO2 in the Mackenzie River and other fluvial systems remains poorly constrained and warrants further study. Based on the persistence of mineral surface area-normalized loadings of OC and vascular plant leaf wax biomarkers, POC appears to be resilient against extensive remineralization during its transit through the Mackenzie River Delta. Higher plant-derived n-fatty acid 14C values indicate that the Mackenzie River and its tributaries supply extensively pre-aged biospheric OC to the Mackenzie River Delta that has hitherto been sequestered in permafrost soils. The terrestrial OC is further subjected to intermediate storage and suspension cycles. These processes, together with the entrainment and mixing of OC derived from channel bank and levee deposits, result in a systematic “aging” during transit through the delta. While thermo-mechanical erosion of aged permafrost soils regulates the import of POC to the Mackenzie River, mobilization of DOC is controlled by the hydrological interconnectivity of seasonal thawed soil layers. In 2018, we observed an unprecedented export event of 14C-depleted DOC in the Mackenzie River basin, preceded by warm winter and summer seasons. The release of aged DOC is attributed to the lateral flow of groundwater through perennial thaw zones (taliks), causing the destabilization and mobilization of previously frozen organic matter. Despite the apparent uniqueness of this mobilization event, this finding highlights the potential for widespread and dramatic shifts in carbon release from permafrost with continuing climate warming. Riverine POC and DOC contents and isotopic compositions in Scottish rivers are strongly influenced by land-use forms. While croplands and woodlands display no significant impact on the nature of the OC export, we observe positive relationships with soil organic carbon contents, peatland coverage, and a negative correlation with grasslands. The age of DOC increases with population density, likely caused by the entrainment of 14C-depleted wastewater. This thesis defines pathways and constrains dynamics of OC transfer through river systems shedding new light on underlying mechanisms and their sensitivity to natural and anthropogenic forcing. The findings bear upon our understanding of the role of fluvial systems within the global carbon cycle and the interpretation of signals exported from river basins and preserved in sedimentary records. Conclusions and implications are based on new data sets with extensive spatial and temporal coverage and are derived from the intensive use of 14C analysis at the bulk and molecular level providing novel insights into the sensitivity of local and regional carbon cycles.