Semi-enclosed bays serve as hotspots for black carbon burial: A case study in Jiaozhou Bay, western Yellow Sea.

The provenance of black carbon (BC) and its role in affecting contaminant cycling in both the atmosphere and aquatic environments have been extensively studied. However, the fate and cycling dynamics of BC, particularly in marine environments, are poorly understood. Herein, soot BC was determined in the semi-enclosed Jiaozhou Bay to examine the seasonal variability, residence timescale in seawater, and settling flux to sediments, together with particle-reactive Th. Soot BC ranged from 0.39 to 4.26 μmol-C L. On average, spring produced the highest value of 1.88 ± 0.31 μmol-C L, followed by winter (1.59 ± 0.18 μmol-C L), summer (0.94 ± 0.09 μmol-C L), and autumn (0.90 ± 0.09 μmol-C L). The seasonality of soot BC was similar to the activity concentration of particulate Th (i.e., Th). The close relationships between soot BC and Th (p < 0.01) provide the basis for the application of Th to trace the fate of soot BC. Based on the Th deficit with respect to U, the residence times of soot BC were estimated to be 41 ± 6 d and 36 ± 5 d for May-August and August-November, respectively. The shorter residence times of soot BC than that of seawater indicated that soot BC delivered to Jiaozhou Bay settled in the local sediments. Furthermore, soot BC concentrations were higher in the inflow seawater from the Yellow Sea than the outflow water from Jiaozhou Bay, implying a net input of soot BC from the Yellow Sea to Jiaozhou Bay. The soot BC fluxes were 0.266 ± 0.035 mmol-C m d and 0.0472 ± 0.0065 mmol-C m d for May-August and August-November, respectively. From the bay-scale perspective, Jiaozhou Bay had buried 0.101 ± 0.010 Gg of soot BC each year. These results indicate that the semi-enclosed Jiaozhou Bay acts as an effective trap for soot BC and particle-reactive contaminants.