Dataset: Selenium
Deployment: KM1128

Water samples were collected using a trace metal clean rosette from eight stations occupied in the tropical North and South Pacific during the Metzyme cruise from Hawaii to Samoa (1 to 24 October 2011; 20°N to 15°S). Surface water samples were obtained using water collected at 5-10 m depth from the ship’s underway sampling system. Particulate samples were collected using McLane in-situ pumps from six stations occupied during the cruise at depths up to 900 m, with ~1000 L filtered per deployment. Water was filtered on board (0.2 μm), acidified to 0.5% HCl (trace metal grade) and transported to the University of Connecticut for analysis.  All samples were stored refrigerated (4C) and under dark conditions prior to analysis.
Dissolved Se speciation was determined within six months of collection following established methods (Cutter and Cutter, 2001; Cutter and Bruland, 1984). The method relies on hydride generation of hydrogen selenide using sodium borohydride. Total dissolved Se was determined by boiling samples in 4M HCl for 15 min to convert all forms to Se(IV), followed by hydride generation step using sodium borohydrate. A PS Analytical Millennium Excalibur instrument equipped with a specific high discharge Se lamp that relies on atomic fluorescence was used for detection. The particulate samples were quantified after microwave digestion (800 W microwave; 20 min at 10% power, then 20 min at 20% power to slowly increase temperature). Filters were placed in Teflon bombs with 5 mL of a 10:1 concentrated HNO3:H2SO4 mixture (Zhao et al., 2010). The volume filtered was determined from the total volume filtered by the in situ pump and the fraction of the filter that was analyzed.