Sedimentological and biogeochemical composition of fractionated permafrost- and sediment organic carbon of the nearshore zone of Herschel Island (Yukon, Canada)

This study aims to give insight into the processes affecting permafrost organic carbon (OC) during transport from its source to its sink, through a study on three sediment fractions along a land-to-ocean transect. Material was followed from thawing permafrost, through a dynamic 'disturbed zone' and the nearshore zone, to an enclosed basin offshore Herschel Island - Qikiqtaruk, to assess sorting and degradation processes on specific fractions of sediment OC. Sediment, soil and permafrost samples were taken along a transect from the source (undisturbed active layer and permafrost), via two transitional zones (a terrestrial disturbed zone, i.e. the 'scar zone' of the RTS, and the marine nearshore zone up to a water depth of 5 m), to sink (basin sediment, water depth >20 m) at the coast of Herschel Island – Qikiqtaruk and the (semi-enclosed) Herschel Basin in Yukon, Canada, just west of the Mackenzie River delta. Samples were taken in May 2016 and July 2017. Sample material was fractionated with an aqueous (MilliQ) solution of sodium polytungstate (SPT; Na6[H2W12O40]), with a density of 1.8 g cm³, followed by wet-sieving over a 63 µm mesh, thus separating loose OC from mineral-associated OC. Each fraction was analysed for element content (TOC, TN), carbon isotopes (δ¹³C, Δ¹⁴C), molecular biomarkers (n-alkanes, n-alkanoic acids, lignin phenols, cutin acids), and mineral surface area. The relative abundance of specific biomarkers can also be used as indicator for degradation of organic carbon. Furthermore, the OC 'loading' (concentration of OC normalised to mineral surface area; in mgOC/m²) and terrestrial biomarker loading (µgOC/m²) can be used to assess loss of (permafrost) OC from mineral particles. The combination of these methods allows us to disentangle sorting processes from degradation of OC along the land-to-ocean continuum, and provides a detailed insight into the fate of thawed and eroded permafrost OC.