Composition and sources of suspended particles in the Pacific Arctic region.

During the 6th (2014) and 7th (2016) Chinese Arctic Expedition (CHINARE), samples of suspended particulate matter (SPM) were collected from both surface (depth: <1.0 m) and subsurface (depth: approximately between 10 and 150 m) waters over the northern shelf of the Bering Sea and in the western Arctic Ocean. To investigate the distribution and sources of organic matter in both the surface water and the vertical profile, the concentration and stable carbon isotopic composition of SPM, particulate organic carbon (POC), and particulate nitrogen (only in surface water samples) were determined, and some particle samples were selected for examination using scanning electron microscopy. Results showed apparent geographical partitioning and temporal variation in both the concentration and the composition of SPM. Higher SPM concentrations were observed in nearshore, shelf break, and sea ice edge areas; the distribution of POC concentration displayed a similar pattern, with higher values found from the northern part of the Bering Shelf to southern parts of the Chukchi Shelf. In surface water, SPM mainly comprised clay and detrital minerals with higher POC contents, lighter δC values, and higher POC/PN ratios, indicating organic matter predominantly derived from terrestrial sources in areas south of St. Lawrence Island and north of 73°N. The downward trend of heavier δC values, together with reduction in clay and detrital minerals, suggests that vertical transport of SPM is hindered by stratification, resulting in transport of terrestrial materials toward northern basin areas. In the Chukchi Slope and Canada Basin areas, extremely light δC values (as low as -33.41‰ PDB) were mainly observed at depths of 20-60 m, where the Polar Mixed Layer (PML) intersects with the Upper Halocline Layer (UHL). Under the condition of low sea ice extent in 2016, the POC-δC values were heavier in the PML than in the UHL in the Chukchi Slope and Canada Basin areas. These findings provide insights into the sources, transport, and fate of organic matter in the Pacific Arctic region, which have important implications for understanding the biogeochemical cycles and ecosystem dynamics in this remote and rapidly changing environment.