Undocumented cadmium, zinc and copper sink in oxygen minimum zones

Cadmium (Cd) is a micronutrient for marine algae and has a marine distribution similar to the macronutrients nitrate and phosphate. The use of sedimentary microfossil records of Cd, thus, allow reconstructions of past ocean nutrient distributions that facilitate the understanding of the role of the oceans in the carbon cycle and climate change. However, this proxy is limited by the incomplete knowledge of processes that control the addition and removal of Cd in the ocean, and Cd’s variability relative to major nutrients.

Janssen and co-authors (2014, see reference below) present coupled data of Cd concentration and isotopic composition in seawater and suspended marine particles. They found that in oxygen-deficient waters, Cd is removed directly via coprecipitation with sulfide. They also underline that, together with Cd, concurrent decoupling of zinc (Zn) and copper (Cu) from corresponding macronutrients are observed in the northeast Pacific Ocean. These results suggest that the marine Cd cycle (but also Zn and Cu ones) may be highly sensitive to the extent of global oceanic oxygen depletion.

Figure: This figure shows particulate cadmium and phosphorus concentrations and cadmium stable isotope ratios (δ114Cd) from the US GEOTRACES North Atlantic Transect (GA03), along with oxygen concentrations and fluorescence which is an estimate of algal biomass.  In blue, station USGT 11-14 from the central North Atlantic (27.6°N, 49.6°W) and, in purple, station USGT 10-09 from the Mauritanian upwelling (17.4°N, 18.25°W). The upper panel shows the depth range of 0-2000 m while the lower panel focuses on the 0-500 m (oxygen deficient zone). Click here to view the figure larger.

In both the oxygen depleted waters of the North Atlantic (purple) and the higher dissolved oxygen waters of the central basin (blue), particulate phosphorus concentrations show a decreasing trend from the near surface waters to deep waters, typical of nutrient type elements such as phosphorus and cadmium in particles. In the central basin (blue), particulate cadmium and the particulate cadmium to phosphorus ratio show low and nearly constant values with depth; however there is a pronounced subsurface increase in particulate cadmium and the cadmium to phosphorus ratio in particles from the Northwestern Atlantic oxygen deficient waters (in purple, at a depth of 100 – 500 m).

The oxygen deficient zone particulate cadmium maximum near 200 m depth is enriched in light stable cadmium isotopes (there is a low particulate δ114Cd at this depth). Furthermore the oxygen deficient zone particles have a lighter isotopic signature than both the dissolved stable cadmium isotopes at this depth and particles in the chlorophyll maximum near 50 m depth. Therefore a non-biological formation of particulate cadmium is occurring in these oxygen deficient waters, with stable isotope fractionation resulting in isotopically lighter particulate cadmium. This is consistent with the formation of insoluble cadmium sulfides and suggests that sulfide formation in the water column is acting to remove dissolved cadmium to the particulate pool.

Reference:

Janssen, D. J., Conway, T. M., John, S. G., Christian, J. R., Kramer, D. I., Pedersen, T. F., & Cullen, J. T. (2014). Undocumented water column sink for cadmium in open ocean oxygen-deficient zones. Proceedings of the National Academy of Sciences of the United States of America, 111(19), 6888–93. doi:10.1073/pnas.1402388111. Click here to access the paper.