Trace metals such as iron (Fe) exist in seawater at picomolar to nanomolar levels, but they play essential roles in marine ecosystems and are closely linked to marine biological productivity and the oceanic carbon cycle (Morel et al., 2003). Therefore, trace metals are key parameters for understanding marine biogeochemical processes.
In the water column of deep trenches, where sediment resuspension is vigorous,  the distribution and behavior of trace metals remain poorly constrained. Physical and chemical properties suggest that trench waters are relatively well mixed and stable (Mantyla and Reid, 1978). However, trace metal dynamics in these environments may be strongly influenced by local processes such as sediment resuspension, which may cause very different biogeochemical cycles of trace metals among trenches.

 In this study, we investigated the behavior of particulate trace metals in the North Pacific trench regions. We revealed the vertical distributions of dissolved and total dissolvable trace metals in the Ryukyu and Aleutian trenches to elucidate the biogeochemical processes controlling trace metal cycling within the trenches.
 Vertical distributions of dissolved (D) trace metals were similar among the three trenches. In contrast, labile particulate (Lp)Fe, Mn, and Co concentrations were higher in the Aleutian Trench than those in the Ryukyu Trench. In both trenches, LpFe concentrations increased toward the bottom layer. In the Ryukyu Trench, the increase in sediment-derived LpFe was accompanied by a decrease in DFe, suggesting that resuspended particles act as a sink for DFe. LpMn and LpCo were enriched in the bottom waters of the Aleutian Trench and exhibited a clear positive correlation, whereas no such correlation was observed in the Ryukyu Trench. These results suggest that diagenetic Mn enrichment at the sediment surface is more active on the slopes of the Aleutian Trench. Overall, the contrasting behaviors of LpMn and LpCo between the two trenches are probably controlled by differences in redox conditions at trench-slope sediments.

trace metals,the North Pacific trench regions,chelating resin preconcentration,ICP-MS