Particulate Silver Flux to the Seafloor and its Link to Marine Productivity

Abstract:

A strong positive correlation between the concentration of silver (Ag) and Ba/Al ratios in continental slope surface sediments from Eastern Pacific Ocean led to the hypothesis that Ag, like Ba, accumulates in settling organic-rich particles, and that Ag might be useful as a paleoproductivity proxy (McKay and Pedersen, 2008). To test this hypothesis Ag concentrations were measured for sediment trap and surface sediment samples from the Equatorial Pacific, Western Arabian Sea, and Oregon Margin. These data were then compared to the concentrations and fluxes of various productivity proxies (organic carbon, biogenic opal, Ba/Al). Sediment trap data show that in highly productive regions, both coastal and open ocean, the flux of Ag increases with water depth down to ~3000 m; however, the trend reverses itself below this. These results suggest scavenging of Ag down to intermediate water depths and remineralization of this scavenged Ag in the deep ocean. Where scavenging of Ag is observed, there is a positive correlation between Ag flux and the fluxes of both organic carbon and biogenic opal. It is therefore no surprise that the concentration of Ag in surface sediments generally reproduce the trends observed for marine productivity in the surface ocean, just as the traditional productivity proxies do. While more work is required to understand exactly how Ag is scavenged and why remineralization occurs, these results clearly show that Ag flux to the seafloor is linked to marine productivity. Thus, Ag should be useful as a paleoproductivity proxy, at least in continental margin locations.