Selenium Cycling in Low Oxygen Waters of the Equatorial Pacific Ocean

Low-oxygen concentrations, ranging from suboxic to anoxic, in the world’s oceans and coastal seas have become hot topics due to their increasing areal extent. The fate and transport of trace elements may be affected by low oxygen concentrations via changes in chemical speciation and solubility. The trace element of focus in this work is selenium, which can be both toxic and essential to organisms depending on its concentration and speciation. Dissolved selenium can be found as selenite (SeIV), selenate (SeVI), and organic selenides (Se-II). Dissolved selenium was first reported to undergo redox conversions by Measures et al. (1981), where the loss of selenate in low oxygen waters was termed “deselenification,” much like denitrification. Subsequent research found selenite and selenate depletions under suboxic to anoxic conditions and the reduced product to be insoluble elemental selenium (Se0). In addition to affecting the Se cycle its speciation could function as a useful redox indicator for the more reducing part of suboxic respiration. The 2013 US GEOTRACES transect from coastal Peru to Tahiti passed through the widespread suboxic waters off Peru. Selenium speciation was determined by selective hydride generation/atomic absorption spectrometry. Total dissolved selenium (sum of all Se species) is depleted in surface waters and increases with depth like other nutrient elements. Both selenite and selenate also increase with depth except in the suboxic waters between 80-600 m; here both show depletion below anticipated concentrations. This decrease reflects in situ reduction of selenium to particulate elemental selenium. Suspended particulate Se(0) was determined in depth profiles along the transect to verify this mechanism. Other aspects of selenium cycling in the Equatorial Pacific will also be discussed.