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

Abstract

Lateral carbon transport through the land-to-ocean-aquatic-continuum (LOAC) represents a key component of the global carbon cycle. This LOAC involves complex processes, many of which are prone to anthropogenic perturbation, yet the influence of natural and human-induced drivers remains poorly constrained. This study examines the radiocarbon (¹⁴C) signatures of particulate and dissolved organic carbon (POC, DOC) and dissolved inorganic carbon (DIC) transported by Swiss rivers to assess controls on sources and cycling of carbon within their watersheds. Twenty-one rivers were selected and sampled during high-flow conditions in summer 2021, a year of exceptionally high rainfall. Δ¹⁴C values of POC range from -446‰ to -158‰, while corresponding ranges of Δ¹⁴C values for DOC and DIC are -377‰ to -43‰ and -301‰ to -40‰, respectively, indicating the prevalence of pre-aged carbon. Region-specific agricultural practices seem to have an influential effect on all three carbon phases in rivers draining the Swiss Plateau. Based on Multivariate Regression Analysis, mean basin elevation correlated negatively with Δ¹⁴C values of all three carbon phases. These contrasts between alpine terrain and the lowlands reflect the importance of overriding ecoregional controls on riverine carbon dynamics within Switzerland, despite high spatial variability in catchment properties. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.

Keywords: Anthropocene; Switzerland; global carbon cycle; land-to-ocean-aquatic-continuum; radiocarbon; rivers.

Figure 1.

(a) Map of Switzerland and its five ecoregions. Major rivers and lakes are indicated in blue. (b) Map of Switzerland with 21 river sampling stations indicated as numbered points. Different colours indicate different watersheds. Station locations are coloured according to the colour of the corresponding watershed. Major rivers and lakes are indicated in blue. (c) Map of Switzerland where different colours indicate different types of land use. Yellow is intensive agriculture and orange is alpine agriculture. The watershed area of each station is indicated with a dark grey line. Major lakes are indicated in blue. (d) Map of Switzerland where different colours indicate different types of underlying bedrock lithology.

Figure 2.

Violin plots of radiocarbon phases (DI¹⁴C, DO¹⁴C, PO¹⁴C) with boxplots showing median values of Δ¹⁴C. Red dots indicate the mean values, whereas the thick line within the boxplot indicates the average value. Swiss ¹⁴C phases are compared against the global compilation from [28]. Radiocarbon values are expressed in Δ¹⁴C notation. Coloured dots indicate the range and variability of single measurements.

Figure 3.

Matrix of Pearson correlation coefficients (r-values) between land use, lithology, topography and water parameters (controls) and radiocarbon values (responses). Circle sizes and colours correspond to the strength of the correlation. Correlations that are significant at the p = 0.05, p = 0.01 and p = 0.001 level are outlined with a thin, regular and thick black border, respectively. Variables with an ‘a’ at the end indicate annual averages from the past decade extracted from the NADUF dataset (2012–2020).

Figure 4.

RDA plot showing the RDA1 and RDA2 canonical axes. Environmental control variable loadings are plotted as black arrows where significant vectors are illustrated in red (basin_elevation), PO¹⁴C, DO¹⁴C and DI¹⁴C response variable loadings are plotted in red, and individual sampling stations (table 1) are plotted as blue numbers. Variables annotated with an ‘_a’ correspond to annual average values of the past decade (2012–2020).