Reply on RC2

Abstract

Arctic rivers will be increasingly affected by the hydrological and biogeochemical consequences of thawing permafrost. During transport, permafrost-derived organic carbon (OC) can either accumulate in floodplain and shelf sediments or be degraded into greenhouse gases prior to final burial. Thus, the net impact of permafrost OC on climate will ultimately depend on the interplay of complex processes that occur along the source-to-sink system. Here, we focused on the Kolyma River, the largest watershed completely underlain by continuous permafrost, and marine sediments of the East Siberian Sea as a transect to investigate the fate of permafrost OC along the land-ocean continuum. Three pools of riverine OC were investigated for the Kolyma main stem and five of its tributaries: dissolved OC (DOC), suspended particulate OC (POC), and riverbed sediment OC (SOC) and compared to earlier findings in marine sediments. Carbon isotopes (δ13C, Δ14C), lignin phenol, and lipid biomarkers show a contrasting composition and degradation state of these different carbon pools. Dual isotope source apportionment calculations imply that old permafrost-OC is mostly associated with sediments (SOC; contribution of 68 ± 10 %), and less dominant in POC (38 ± 8 %), while autochthonous primary production contributes around 44 ± 10 % to POC in the main stem and up to 79 ± 11 % in tributaries. Biomarker degradation indices suggest that Kolyma DOC is relatively degraded, regardless of its generally young age shown by previous studies. In contrast, SOC shows the lowest Δ14C signal (oldest OC), yet relatively fresh compositional signatures. Furthermore, decreasing mineral surface area-normalised OC- and biomarker loadings suggest that SOC is reactive along the land-ocean continuum supporting the idea that floodplain and shelf sediments are efficient reactors. A better understanding of DOC and POC dynamics in Arctic rivers is still necessary, however, this study highlights that sedimentary dynamics play a crucial role when targeting permafrost-derived OC in aquatic systems. Chemical and physical processes (e.g. degradation, sorption) along fluvial-marine transects will determine to what degree thawed permafrost OC may be destined for long-term burial, therewith attenuating further global warming.