Ocean Deoxygenation Conference | Kiel 2018

Benthic trace metal fluxes in the oxygen minimum zone off Peru

A. PLASS1*, F. SCHOLZ1, C. SCHLOSSER1, A. W. DALE1, E. P. ACHTERBERG1, S. SOMMER1

1GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstraße 1-3, 24148 Kiel, Germany
(*correspondence: aplass@geomar.de)

Iron and several other trace metals (e.g. cobalt, zinc, cadmium) are essential micronutrients, required for the growth of marine organisms. Their availability in the oceans can (co-)limit primary productivity and thus affect the cycling of macronutrients and the biological pump [e.g. 1]. Marine sediments in oxygen-deficient regions are proposed to be an important source or sink for these trace metals to the ocean. In the Peruvian oxygen minimum zone (OMZ), substantial fluxes of reduced iron across the sediment-bottom water interface have been documented [e.g. 2] and inferred for other trace metals such as cobalt [e.g. 3]. However, the key biogeochemical processes that control the sedimentary release or burial of trace metals are poorly constrained.
We present iron and other trace metal data for in situ benthic chamber incubations, sediment pore waters, near-bottom water profiles and ex situ experiments from a transect across the Peruvian continental margin at 12 °S. During our sampling campaign (austral autumn 2017), the water column featured steep biogeochemical gradients. While the upper shelf was oxygenated, anoxic conditions prevailed on the deeper shelf and slope, with a nitrogenous OMZ core expanding from around 150 m to 400 m water depth. Close to the upper rim of the OMZ core, the highest in-situ benthic fluxes of iron and the steepest iron gradients in the bottom water were found. In some incubations and ex-situ experiments, the release of iron coincided with nitrate and nitrite depletion, suggesting that reductive processes in the nitrogen cycle exert an important control on the benthic iron release. The distribution of mat-forming sulfide-oxidizing bacteria on the seafloor also seems to affect benthic trace metal fluxes. These bacteria regulate sulfide concentrations in the surface sediment and, thus, the extent of trace metal retention through their precipitation sulfides. Further, the decrease of cadmium concentrations during benthic chamber incubations suggests that oxygen minimum zone sediment represents an effective sink for cadmium. Our findings can help to predict how ocean deoxygenation will impact benthic trace metal fluxes in the future.

[1] Moore et al. (2013). Nature Geoscience, 6(9), 701
[2] Noffke et al. (2012). Limnology and Oceanography, 57(3), 851-867
[3] Hawco et al. (2016). Biogeosciences, 13(20), 5697