Environmental and hydrologic controls on sediment and organic carbon export from a subalpine catchment: insights from a time series

Abstract

Abstract. Studies engaging in tracking headwater carbon signatures downstream remain sparse, despite their importance for constraining transfer and transformation pathways of organic carbon (OC) and developing regional-scale perspectives on mechanisms influencing the balance between remineralization and carbon export. Based on a 40-month time series, we investigate the dependence of hydrology and seasonality on the discharge of sediment and OC in a small (350 km2) Swiss subalpine watershed (Sihl River basin). We analyze concentrations and isotopic compositions (δ13C, F14C) of particulate OC and use dual-isotope mixing and machine learning frameworks to characterize and estimate source contributions, transport pathways, and export fluxes. The majority of transferred OC is sourced from plant biomass and soil material. The relative amount of bedrock-derived (petrogenic) OC, abundant in headwater streams, progressively decreases downstream in response to a lack of source material and efficient overprinting with biospheric OC, illustrating rapid organic matter alteration over short distances. Large variations in OC isotopic compositions observed during baseflow conditions converge and form a homogenous mixture enriched in OC and characterized by higher POC-F14C values following precipitation-driven events. Particulate OC isotopic data and model results suggest that storms facilitate surface runoff and the inundation of riparian zones, resulting in the entrainment of loose plant-derived debris and surficial soil material. Although particle transport in the Sihl River basin is mainly driven by hydrology, subtle changes in bedrock erosivity, slope angle, and floodplain extent likely have profound effects on the POC composition, age, and export yields.

Radiocarbon signatures of carbon phases exported by Swiss rivers in the Anthropocene

“…The elevated discharge during our sampling meets these conditions. Above-average carbon fluxes and uniform radiocarbon signatures likely reflect carbon supply predominantly via surface runoff and soil erosion that would contribute to higher 14 C values [17,43,66,69,70]. Thus, our snapshot dataset may disproportionately reflect specific endmembers and lead to a radiocarbon signature that is biased towards higher 14 C values with the magnitude of this bias dependent upon catchment characteristics.…”

“…Other studies of similar river systems outside Switzerland support this, especially during the wet season or with the onset of the spring thaw [ 64 – 71 ]. With respect to radiocarbon, according to recent studies of a sub-alpine Swiss catchment, riverine POC radiocarbon signatures are more variable under low flow conditions but tend to cluster around higher values during above-average river discharge conditions [ 17 , 43 ]. This implies that samples collected during high flow conditions are likely to be more representative, while also accounting for the majority of carbon export.…”

“…A total of 19 sampling stations were chosen to align with those of the NADUF programme (figure 1b). In addition to the NADUFstations, two additional rivers-the Sihl River (at Brunau, station 10) and the Maggia River (at Locarno, station 17)-were chosen, the former (and its headwaters) being the focus of prior indepth investigations [17,43]. Land-use within the different river basins differs considerably (figure 1c).…”

“…lower 14 C values) in the headwaters to young in the lowland rivers [8,28]. The latter may reflect dilution or remineralization processes whereby ancient (radiocarbon-free) petrogenic carbon is superseded by carbon from fresh vegetation both from land and in-stream production during downstream transport [8,17,28]. The radiocarbon characteristics of riverine dissolved inorganic carbon (DIC) are influenced by gas exchange with the atmosphere, organic matter remineralization processes and bedrock chemical weathering pathways.…”

“…Our understanding of the role of rivers has evolved from the concept of a simple pipeline to a more reactive system interacting with its surroundings [13]. In general, riverine carbon dynamics vary by catchment characteristics, such as lithology, geomorphology, climate and hydrology [8,[14][15][16][17][18]. Extreme hydrologic events (e.g.…”