High-resolution analysis of the terrestrial influence on DOC and POC export in a Siberian Arctic River during the spring freshet

Abstract:

Arctic Rivers transport vast amounts of terrestrial organic material (TOM) to the Arctic Ocean. The Yenisei River delivers ~18% of total dissolved organic carbon (DOC) exported to the Arctic Ocean each year during peak river discharge (May-June), known as the spring freshet. Previously published DOC fluxes for the freshet period extrapolate from relatively few data points, due to the uniquely difficult sampling conditions during this dynamic period. Here, we present new high resolution data collected from an extensive sampling campaign from April-July 2014 using a reverse osmosis system for DOC isolation. The similarity between the calculated DOC load delivered during the 2014 freshet (2.94 TgC) and the ten-year average from the previous decade (2.92 TgC for 1999-2008) validates the methodological approach used here. In contrast, the total measured load of polymeric lignin phenols (∑8: 643 Gg), an indicator of TOM input, was one order of magnitude higher than a previously estimated load (42 Gg) for May-June. Hence, we may need to re-evaluate the magnitude of terrestrial carbon exported, including the efficiencies of different sampling methods. Additionally, we present the first simultaneous particulate and dissolved lignin analyses in a major Arctic river. Approximately 30% of the total lignin flux in the Yenisei River is delivered in the particulate phase. Particulate lignin export is decoupled from dissolved lignin during the freshet; the initial flush is dominated by dissolved lignin, while the latter portion of the freshet is dominated by particulate lignin. The chemical signatures of lignin in both phases are similar throughout the freshet, suggesting a mobilization of the same source of carbon each spring. This conclusion is at odds with reported isotopic sources signatures (¹⁴C age) of bulk organic matter and lignin in these rivers, requiring a multi-faceted approach to fully understand the sources and ages of terrestrial organic matter in Arctic rivers.