Sources and Behavior of Particulate Organic Carbon in the Yellow Sea and the East China Sea Based on 13C, 14C, and 234Th

Abstract

Thorium-234 (234Th; half life = 24.1 days) has been used as an excellent tracer of element scavenging, particle settling, and the export of particulate organic carbon (POC) in the ocean. We measured the vertical and horizontal distributions of particulate and total phases of 234Th, POC and its carbon isotopes (13C and 14C), and other oceanographic parameters in the continental shelf (~ 80 m depth) of the Yellow Sea and the East China Sea in summer. Changjiang Diluted Water (CDW) with the salinity range from 26 to 31 was present with higher POC and chlorophyll-a contents, in the surface layer. Interestingly, 234Th activities were extremely low compared to the values observed in the open ocean. The disequilibrium of dissolved and total 234Th relative to its parent 238U were highest in the CDW stations, with short residence times 234Th for the upper 50 m being shorter than 10 days. We propose rapid settling of particles in this shelf region, especially in CDW-affected stations. POC export flux to the seafloor estimated using 234Th was 60±50 mmol m-2 d-1, which is an order of magnitude higher than that in the sediment column. The rapid scavenging of particulate organic matter (POM) implies effective degradation of POM to inorganic or dissolved species in the benthic boundary layer.