Carbon in an Arctic fjord: sea ice carbon transformations and CO2 linkages

Introduction The biogeochemical processes underlying carbon cycling in Arctic coastal systems are rapidly evolving due to intensified ice loss. (Aim) This study examined the distinct contributions of dissolved organic carbon (DOC) and particulate carbon from sea ice in Kongsfjorden, Svalbard (Methods) focusing on the optical characteristics of coloured dissolved organic matter (CDOM) to trace its fate. Results Our results reveal that sea ice melt delivers a complex mixture: specific types of CDOM and a dominant load of total particulate carbon (TPC) that was identified as being primarily particulate inorganic carbon (PIC). The fate of the dissolved fraction was clearly traced by Gaussian decomposition. Discussion Sea ice delivered nitrogen-rich organic components, creating spatial hotspots of aCDOM275 at the innermost site and of aCDOM330 at the outermost site, with a strong correlation with CO 2 . At the surface, photodegradation breaks down high-molecular-weight (HMW) (low S275–295) dissolved organic matter (DOM) into low-molecular-weight (LMW) fractions (high S275–295). Below the surface, microbial degradation further transforms this organic carbon, promoting remineralisation processes and releasing dissolved inorganic carbon (DIC) and CO 2 . Higher N:P and Si:P ratios and nutrients in these layers indicated enrichment by meltwater (sea ice/glacial) and microbial organic matter (OM) degradation, supported by shifts in CDOM spectral properties (SR, S275–295, and S350–400) and higher CO 2 . In contrast, the PIC-dominated TPC pool was decoupled from these biological transformations. Given the accelerating rate of Arctic warming, the impacts of sea ice and glacial melting on carbon dynamics in fjords like Kongsfjorden are likely to intensify, with potential positive feedback in the Arctic.