Particulate Trace Metal Fluxes in the Western Arabian Sea and Their Links to Marine Productivity Public Deposited

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  • Many trace elements (e.g., Zn, Cd, Mo) are essential phytoplankton micronutrients, making them crucial to the marine ecosystem and ultimately the carbon cycle. Because of this association trace metals are also utilized in paleoceanographic studies (e.g., Mo, Cd). However, not much is known about what controls the cycling of these trace metals in seawater. The primary goal of this study was to improve our understanding Ag, Mo and Cd cycling, focusing on the particulate phase, so that we can understand what influences their delivery to the seafloor.The concentrations of trace metals (i.e., Ag, Mo, and Cd) and minor elements (i.e., Al, Ba, Mn, Zn) were measured in sediment trap samples collected at three sites in the western Arabian Sea; two high productivity nearshore sites (MS2 and MS3) and one oligotrophic gyre site (MS5). The concentrations of these metals were converted to fluxes and compared to four commonly utilized productivity proxies; particulate organic carbon (POC), total barium (Ba), opal, and carbonate (CaCO₃). Particulate Ag flux increases with increasing trap depth at both MS2 and MS3, but decreased with increasing trap depth at MS5. Ag flux also shows a strong positive correlation with particulate organic carbon (POC), as well as opal and Ba. These data suggest that at the high productivity sites, Ag is accumulates in settling, organic-rich particles. There is no evidence that the intense Oxygen Minimum Zone, which characterizes the Arabian Sea, is influencing Ag accumulation. Nor is Ag simply related to the opal flux, refuting the idea that Ag is delivered to the seafloor by diatom frustules. Instead, dissolved Ag concentrations and POC and or total particle flux are probably the main controls on particulate Ag formation. Particulate Mo flux also positively correlates well with POC flux, suggesting it might be scavenged onto particulate organic matter. There is also evidence of Mo scavenging by Mn oxyhydroxides in the deep trap. Therefore particulate Mo flux to the seafloor seems to be controlled by both POC flux and Mn oxyhydroxide formation. Particulate Cd is organic, despite the fact that Cd and POC fluxes do not correlate particularly well due to the unusual behavior of POC.In summary, all three of the trace metals studied show evidence of a link to organic carbon flux. Particulate Cd and Mo, as well as Ag at MS5, are found within, or possibly absorbed on to, the organic matter. They are cycled with this organic matter and thus their fluxes decrease with water depth. Very little of these metals will make it to the seafloor in deep ocean. Particulate Ag at the high productivity stations (MS2 and MS3) behaves differently. It is scavenged by sinking organic-rich particles and fluxes increase with water depth. Therefore, large amounts of particulate Ag could reach the seafloor, at least in regions characterized by high productivity.
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