Widespread anthropogenic nitrogen in the northwestern Pacific Ocean sediment

Abstract

Sediment samples from the East China and Yellow seas collected adjacent to continental China were found to have lower delta N-15 values (expressed as delta N-15 = [N-15:N-14(sample)/N-15:N-14(air) - 1] x 1000; the sediment N-15:N-14 ratio relative to the air nitrogen 15N:14N ratio). In contrast, the Arctic sediments from the Chukchi Sea, the sampling region furthest from China, showed higher delta N-15 values (2-3 higher than those representing the East China and the Yellow sea sediments). Across the sites sampled, the levels of sediment delta N-15 increased with increasing distance from China, which is broadly consistent with the decreasing influence of anthropogenic nitrogen (N-ANTH) resulting from fossil fuel combustion and fertilizer use. We concluded that, of several processes, the input of N-ANTH appears to be emerging as a new driver of change in the sediment delta N-15 value in marginal seas adjacent to China. The present results indicate that the effect of N-ANTH has extended beyond the ocean water column into the deep sedimentary environment, presumably via biological assimilation of N-ANTH followed by deposition. Further, the findings indicate that N-ANTH is taking over from the conventional paradigm of nitrate flux from nitrate-rich deep water as the primary driver of biological export production in this region of the Pacific Ocean.