Lignin phenols and BIT index distributions in the Amur River and the Sea of Okhotsk: Implications for the source and transport of particulate terrestrial organic matter to the ocean

Delta and coastal regions play a key role in the global carbon cycle as the main repository of inputs of terrestrial organic matter, delivered by rivers to marine sediments. The Amur River system is one of the largest in Asia and supplies organic matter to the Sea of Okhotsk and the North Pacific Ocean. We measured lignin phenols (a proxy for terrestrial plant derived organic matter) and the branched and isoprenoidal tetraether (BIT) index (a proxy for riverborne soil organic matter), in soils and river bed sediments from the Amur River basin and surface sediments from the Sea of Okhotsk, to study the source, transport, and deposition of fluvial particulate organic matter to the ocean. Concentrations of lignin phenols are relatively high in sediments from the shallow continental shelf compared to that of the deep offshore basin. For the first time we report a spatial distribution of the BIT index which shows a similar pattern to that of lignin phenol concentrations, with relatively high values in the coastal sites. The highest values of both lignin concentrations and BIT values are observed in a site proximal to mouth of Amur River. These results indicate that terrestrial organic matter deposited in the Sea of Okhotsk mostly originates from soil organic matter supplied from Amur River. Spatial distributions of lignin phenols/TOC (LP/TOC) and the BIT index in the Sea of Okhotsk and comparison with S/V (a contribution index of angiosperm vs. gymnosperms) and C/V ratios (indicative of the relative contribution of non-woody tissue vs. woody tissue), between the Amur River basin and Okhotsk Sea samples, suggest that Amur River fluvial organic matter is supplied and deposited to the northwestern continental shelf. Furthermore, S/V values from our Amur River and Okhotsk Sea continental shelf sediment samples are consistent with the reported S/V ratio (0.32) of particulate organic matter in the North Pacific Intermediate Water (NPIW) layer (750 m depth). Hence, our study supports previous studies from the NW Pacific Ocean, which suggest inputs of terrestrial organic matter from the Amur River via the Sea of Okhotsk, as a major source of particulate organic matter to the North Pacific Ocean.