Organic carbon and nitrogen isotopes in surface sediments from the western Arctic Ocean and their implications for sedimentary environments

Surface sediments from the Chukchi Sea and adjacent arctic deep sea were investigated for organic carbon and nitrogen isotopes (in δ~ 13 C_ org and δ~ 15 N_ org ) as well as biogenic silica (BSiO_ 2 ). δ~ 13 C_ org and δ~ 15 N_ org values of surface sediments in the study area fall between the end-member values of marine and terrestrial organic matter from the surrounding lands and seas, their variations reflect the changes of marine productivity and terrestrial supply in the study area. BSiO_ 2 shows a similar distribution pattern with δ~ 13 C_ org and δ~ 15 N_ org , and can be used as an indicator of marine productivity. In the central-west Chukchi Sea and the Chukchi Rise, sediments have higher δ~ 13 C_ org , δ~ 15 N_ org and BSiO_ 2 values, indicating the region has high marine productivity influenced by the nutrient-rich branches of the Pacific waters. In the coastal zone off northwestern Alaska, δ~ 13 C_ org and δ~ 15 N_ org values become lighter, indicating a weakening marine productivity and an increasing terrigenous supply due to the effects of the least nutrient-rich branch of the Pacific waters. In the north and the northeast of the study area (including the Chukchi Plateau, the Canada Basin and the Beaufort shelf), δ~ 13 C_ org , δ~ 15 N_ org and BSiO_ 2 have the lowest values, and the terrigenous organic matter becomes dominant in surface sediments because this region has the longest ice-covered duration, the least nutrient-rich seawater and the increasing supply of terrestrial materials from the Mackenzie River and the northern Alaska under the action of the clockwise Beaufort gyre. Because the subarctic Pacific waters are continuously discharged into the central basin of the Arctic Ocean through the study area, the nutrient pool in the Chukchi Sea can be considered as a typical open system, the ratio of δ~ 15 N to BSiO_ 2 content show some tracers that the level of nutrient utilization is contrary to nutrient supply and marine productivity formed in seawater.