Tracing riverine dissolved organic carbon and its transport to the halocline layer in the Chukchi Sea (western Arctic Ocean) using humic-like fluorescence fingerprinting

Abstract

Dissolved organic carbon (DOC) and the fluorescence properties of dissolved organic matter (FDOM) were investigated using parallel factor analysis (PARAFAC) for seawater samples collected in the Chukchi Sea (65°N–78°N, 170°E–160°W) during summer 2017. River water (friver) and sea-ice meltwater (fsea ice melt) fractions were also derived using oxygen isotopes ratios (δ18O) to examine the influence of sea ice on riverine DOM. The spatial distributions of friver, riverine DOC, and the humic-like fluorescent component (C1) showed an overall south-north gradient, with higher values in the northern Chukchi Sea in summer. Pronounced accumulation of river water and riverine DOM was also observed in the anticyclonic Beaufort Gyre at the eastern stations of the northern Chukchi Sea in association with a long water residence time. Estimated riverine DOC in the surface layer accounted for 27 ± 9% (range: 17–47%) of the total DOC in the southern Chukchi Sea, and 39 ± 6% (range: 32–49%) and 31 ± 4% (range: 25–37%) for the eastern and western stations of the northern Chukchi Sea, respectively. Humic-like C1 showed negative and positive relationships with sea-ice meltwater-corrected salinity (Ssim_corrected) and friver, respectively. However, Arctic river waters with distinct humic-like C1 characteristics were likely mixed in the northern Chukchi Sea. The vertical distributions of riverine DOC, humic-like C1 fluorescence, and friver generally decreased with water depth, reflecting the strong influence of riverine DOM in the surface layer. Although riverine DOM and friver were dominant in the upper 50 m of the water column, they were also pronounced in the upper halocline (50–200 m), in which fsea ice melt dropped below zero. Our results indicated the existence of brine rejected from growing sea ice, and that sea-ice formation was a key factor for the transport of riverine DOM to the upper halocline layer in the northern Chukchi Sea.