Classical work by Bertine and Turekian (1973) divided the removal of molybdenum (Mo) in the oceans into two groups - Mo removed to Mn-rich deep sea sediments (the oxic sink) and Mo removed ‘near shore reducing’ environments. Near shore reducing areas remove Mo nearly 500 times more efficiently per unit area than their oxic counterparts. In other words, an expansion in OMZs, even a small one, will have a severe effect on the Mo concentration of the oceans. To better understand removal/cycling processes in a continental margin setting, where sulfide may only be present in the pore waters and not in the water column, Mo was studied in an array of marine settings off the Namibian coast.

Sulfur and oxygen systematics of sulfate reduction

Sulfate reducing bacteria cause apparent oxygen isotope exchange between sulfate and water by the back-reaction of intermediate sulfur species. This metabolic process causes the oxygen isotopic composition of sulfate to approach an equilibrium value close to 29‰ at 5º C (Fritz et al., 1989). However, the full expression of this equilibrium value is controlled by the balance between forward and backward reactions during sulfate reduction.