Zn elemental and isotopic features in sinking particles of the South China Sea: Implications for its sources and sinks

We determined the elemental and isotopic composition of Zn in sinking particles collected in the deep water of the northern South China Sea (NSCS) to investigate the relative contribution of various sources and assess their isotopic signatures. Using differentiable elemental ratios and δ66Zn of the potential sources, a mass balance approach estimates that anthropogenic aerosol Zn accounted for 64 ± 10% of the total Zn in sinking particles for more than 50% of the sampling period, indicating that anthropogenic aerosol Zn has become a dominant form of Zn source in the deep water. A relatively large discrepancy between the estimated and measured δ66Zn is observed during the high productivity season, which can be attributed to the elevated contribution of the biogenic hard parts or scavenging Zn on organic materials. Elevated δ66Zn values were observed at 3500 m during autumn which may be caused by the influence of authigenic particles during the lowest flux period. We found that the averaged measured output δ66Zn value, +0.35 ± 0.12‰, is significantly lighter than most of the output values proposed in previous studies. Due to recent findings highlighting the importance of anthropogenic aerosol Zn in the ocean, we have re-evaluated the solubility and fluxes of aerosol Zn in the ocean and found that the flux has been significantly underestimated in previous studies. The updated global aerosol Zn input to the ocean, ranging from 0.3 to 3.0 Gmol yr−1, is comparable to the output magnitude from hydrothermal and riverine sources. The updated Zn residence time would then be down to 1400 years on average. In addition to organic decomposition, the sinking particle data indicate that particle-associated removal and release processes play important roles in controlling Zn cycling in the water column. How anthropogenic aerosol deposition influences Zn fluxes and cycling in other oceanic regions deserves further investigation.