Contrasts in dissolved, particulate, and sedimentary organic carbon from the Kolyma River to the East Siberian Shelf

[摘要] Arctic rivers will be increasingly affected by thehydrological and biogeochemical consequences of thawing permafrost. Duringtransport, permafrost-derived organic carbon (OC) can either accumulate infloodplain and shelf sediments or be degraded into greenhouse gases prior tofinal burial. Thus, the net impact of permafrost OC on climate willultimately depend on the interplay of complex processes that occur along thesource-to-sink system. Here, we focus on the Kolyma River, the largestwatershed completely underlain by continuous permafrost, and marinesediments of the East Siberian Sea, as a transect to investigate the fate ofpermafrost OC along the land–ocean continuum. Three pools of riverine OCwere investigated for the Kolyma main stem and five of its tributaries:dissolved OC (DOC), suspended particulate OC (POC), and riverbed sediment OC(SOC). They were compared with earlier findings in marine sediments. Carbon isotopes( δ 13 C , Δ 14 C ), lignin phenol, and lipid biomarkerproxies show a contrasting composition and degradation state of thesedifferent carbon pools. Dual C isotope source apportionment calculationsimply that old permafrost-OC is mostly associated with sediments (SOC;contribution of 68±10  %), and less dominant in POC ( 38±8  %), whereas 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 might be relativelydegraded, regardless of its generally young age shown by previous studies.In contrast, SOC shows the lowest Δ 14 C value (oldest OC), yetrelatively fresh compositional signatures. Furthermore, decreasing mineralsurface area-normalised OC- and biomarker loadings suggest that SOC might bereactive along the land–ocean continuum and almost all parameters weresubjected to rapid change when moving from freshwater to the marineenvironment. This suggests that sedimentary dynamics play a crucial rolewhen targeting permafrost-derived OC in aquatic systems and support earlierstudies highlighting the fact that the land–ocean transition zone is an efficientreactor and a dynamic environment. The prevailing inconsistencies betweenfreshwater and marine research (i.e. targeting predominantly DOC and SOCrespectively) need to be better aligned in order to determine to what degreethawed permafrost OC may be destined for long-term burial, therebyattenuating further global warming.

[学科分类] 大气科学