Dynamics of dissolved organic carbon in the western Arctic Ocean

Abstract

Dissolved organic carbon (DOC) is an essential component for understanding the regional carbon budget and the global carbon cycle. As the amount of river discharge continues to increase, along with increasing DOC export due to climatic warming and permafrost thawing, the remineralization of terrigenous organic marine-origin matter in the Arctic Ocean can reduce the Arctic Ocean's ability to absorb atmospheric carbon dioxide (CO2). Thus, a complete understanding of the terrigenous and marine-origin DOC dynamics is required. To investigate behavior of DOC and sources of DOC, seawater sampling was carried out over in the Chukchi Sea, using a CTD/rosette sampler holding 24-10 L Niskin bottles during Korea research ice breaker R/V Araon cruises (ARA06B, August 122, 2015; ARA07B, August 619, 2016). δ18O and salinity were used to estimate DOC inputs by Arctic rivers and sea ice melt, allowing the marine portion of the DOC pool. Concentration of DOC ranged from 34116 μM. High DOC concentration was observed in the surface layer, suggesting the strong influence of terrigenous DOC derived from Arctic rivers. However, low-salinity water from ice melt diluted DOC concentration in the surface layer. In 2015, DOC concentrations observed in the northern Chukchi Sea showed a positive relationship with heterotrophic bacterial abundance, suggesting that DOC was bioavailable and used by bacteria for their growth. In contrast, in 2016, bacterial abundances were lower than those in 2015 although there was a positive relationship between bacterial abundance and DOC concentrations, suggesting that DOC observed in 2016 was more refractory. The penetration depth of brine, rejected during sea ice formation, was observed from the surface layer to 200 m depth, where the contribution of riverine DOC was more than 50%. Our result revealed that sea ice formation, which injects brine into the underlying seawater, is a key mechanism for delivering riverine DOC into the deeper layer.