Dynamics of particulate black carbon in the South China Sea: Magnitude, resident timescale, sinking speed, and flux.

The dynamics of particulate black carbon (PBC) in marine environments are poorly understood. Here, radioactive 234Th was used to constrain the resident timescale, settling speed, and sinking flux of PBC (soot) in the coastal Northeastern South China Sea (NSCS). The PBC concentration varied from 0.013 μg-C L-1 to 4.340 μg-C L-1. Spatially, PBC showed an exponential decrease offshore, with a coefficient of 0.030 ± 0.004. Compiling available data, an empirical formula of PBC = a e-0.032x (x is the distance offshore) was proposed for predicting the descent of PBC offshore in coastal seas. Residence times of 0.8-13 d indicate that PBC is retained for days, implying its limited dispersal to the open sea. For the first time, the settling speed of PBC was evaluated in seawater, which averaged 8.8 ± 7.1 m d-1. These results highlight that bottle-sampled PBC falls mainly into the slow-sinking particle continuum in marine environments, due to its fine size. The sinking flux of PBC averaged 4.57 mg-C m-2 d-1 in the coastal NSCS. Using the sinking speed, the preliminarily estimated sinking rate of PBC was 23.8-1800 Tg-C yr-1 on global shelves. The crucial dynamic parameters of PBC provide insights into its internal cycling in coastal seas and can be used as model parameters for assessing global PBC.