Variability of Dissolved Organic Matter Sources in the Upper Eurasian Arctic Ocean

Chromophoric dissolved organic matter (CDOM) is a ubiquitous component in marine environments, and substantial changes in its sources and distribution, related to the carbon cycle in the Arctic Ocean, are expected due to Arctic warming. In this study, we present unique CDOM data in the Eurasian Arctic Ocean derived from the year-round MOSAiC expedition. We used CDOM absorbance spectra and fluorescence excitation-emission matrices in combination with parallel factor analysis to characterize differences in DOM sources and composition. Our results suggested that terrestrial DOM was less sensitive to seasonal changes but controlled by regionality in hydrography. Elevated dissolved organic carbon (DOC) levels in polar surface water were primarily derived from terrigenous sources as identified by CDOM absorption and fluorescence characteristics. In the Amundsen Basin and western Fram Strait surface waters, to which terrestrial DOM is primarily transported by the Transpolar Drift, we found, on average, a 188% larger meteoric water fraction and a 40% higher DOC concentration compared to the Atlantic water that dominated western Nansen Basin and Yermak Plateau. In the Amundsen Basin, the DOC concentration in summer of surface water was only 13% higher compared to winter season. Additionally, autochthonous DOM and chlorophyll-a concentrations were relatively low in surface water and exhibited significant differences compared to those observed in summer, while there were significant differences between autochthonous DOM and chlorophyll-a. We also observed that sea ice melt contributed to autochthonous DOM in summer, while storms in winter affected the vertical distribution of terrestrial and autochthonous DOM in the subsurface. In the Arctic Ocean, dissolved organic matter (DOM) is important because it represents a large carbon reservoir that cycles through the environment and supports the growth of marine life in the Arctic. DOM originates from various sources and its composition changes depending on factors like location and time of year, largely influenced by ocean currents. Microbes in the water break down DOM, but this process is hindered during the winter months and in areas with limited sunlight. Due to the very limited and challenging accessibility of the Central Arctic Ocean, there is a lack of DOM data, especially during winter in the Eurasian Arctic Ocean. Scientists conducted a comprehensive year-round study during the MOSAiC expedition and discovered that much of the DOM in the Arctic comes from land in Siberia. Additionally, seasonal changes were observed to alter the composition of locally produced chromophoric DOM (CDOM) in surface waters of the Amundsen Basin. In contrast, terrestrial CDOM is less responsive to seasonal shifts but more influenced by regional variations in hydrography. This highlights the substantial role of regional hydrographic differences in shaping the characteristics of DOM in surface water across the Eurasian Arctic Ocean. Elevated dissolved organic carbon levels in polar surface water are primarily derived from terrigenous sources in the Eurasian basin Contrasting dissolved organic matter characteristics in Amundsen and Nansen basins caused by Transpolar Drift waters Autochthonous dissolved organic matter in polar surface water is low in winter and significantly different from that in summer