Trace metal cycling in the deep water of the South China Sea: The composition, sources, and fluxes of sinking particles

Moored sediment traps were deployed for 1 yr at depths of 120, 600, and 3500 m in the water column to investigate the trace metal (M) (Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) composition, sources, and fluxes of deep-water sinking particles. Vertical transport for most of the metals at 120 m was strongly associated with organic matter production, and the major part of many of the metals in the sinking particles was of anthropogenic origin. Although the lithogenic fraction in deeper waters greatly increased with depth, significant proportions of the nonlithogenic and nonintracellular fractions were still observed when using M: Al or M: P ratios as indicators, particularly for Zn, Cu, Ni, Co, and Mn. Because particles from horizontal transport and sediment resuspension are known to be lithogenic in the deep basin, the trace metals in sinking particles in the twilight zone and deep water, which feature significantly elevated M: Al ratios, originate from the downward transport of anthropogenic or authigenic origins. With increasing inputs from anthropogenic aerosols over large oceanic regions globally, the coupling and transport of some trace metals originating from anthropogenic aerosols, such as Zn, Cu, Co, and Ni, with sinking organic particles has become an important pathway for trace metal cycling not only in the euphotic zone, but also in the twilight zone and deeper waters.