High-resolution Cd isotope systematics in multiple zones of the Southern Ocean from the Antarctic Circumnavigation Expedition

Abstract

The Southern Ocean plays a major role in determining the global distribution of trace metals such as cadmium (Cd). Here, we present 17 high-depth-resolution profiles of dissolved Cd and its stable isotope composition (δ114Cd) over the top 1000 m of the Pacific and Atlantic sectors of the Southern Ocean, collected during the Antarctic Circumnavigation Expedition. Our dataset reinforces the view that Cd and its isotopes are dominated by shallow biological cycling in this region. A close examination of variations in Cd cycling across the different zones of the Southern Ocean reveals how the interplay between uptake and regeneration, seasonal mixing, and upwelling controls both Cd and δ114Cd in this region. The only deviations from these systematics are due to the influence of local processes such as continental influence or Fe-fertilization, close to the Mertz Glacier and the Balleny Islands, respectively. Deep convection during winter incorporates the Southern Ocean Cd isotope signatures into Subantarctic Mode Water and Antarctic Intermediate Water during water mass formation. Incorporating published data, we present the first complete picture of how Cd is cycled through the entire Pacific Ocean, revealing the manner in which the Southern Ocean controls the global cycling of Cd and δ114Cd; analogous to Si or Zn, we propose that Southern Ocean processes, in combination with global ocean circulation, cause a division into two separate Cd regimes, a Cd-depleted surface ocean above a Cd-rich deep ocean loop. Therefore, the relationship between Cd and PO4 on a global scale is largely a result of these processes in the Southern Ocean, rather than a local correlation between the two elements.