Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

The data include measurements of total dissolved selenium and particulate selenium with depth for 9 stations occupied in the tropical Pacific Ocean in 2011, and total dissolved selenium in surface waters collected while the ship was underway between stations. 

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.

BCO-DMO Processing Notes:

• Combined two excel spreasheets into one dataset with all observations organized by station and depth
• added conventional header with dataset name, PI name, version date
• modified parameter names to conform with BCO-DMO naming conventions
• added the station identifier "Underway" and depth "surface" for the underway observations.

NSF Award Abstract:
Researchers from the University of Connecticut, Woods Hole Oceanographic Institution, and Harvard University plan to address three questions related to the global biogeochemical mercury (Hg) and selenium (Se) cycles, namely (1) what are the abiotic and biotic mechanisms for formation of methylated Hg and Se compounds in the upper ocean?; (2) what is the role of photochemical reactions in air-sea exchange of Hg and Se?; and (3) how are the biogeochemical cycles of Hg and Se related? To attain their goal, the scientists will participate in a cruise of opportunity to the Tropical North Pacific, as well as carry out laboratory culture and controlled incubation experiments. Samples collected during the cruise will be used to determine the speciation of Hg and Se, as well as obtain measurements of photochemical status (i.e., UV, ozone, light levels, chemical (i.e., natural organic matter, redox metals), and biological (i.e., chlorophyll a, phytoplankton composition,proteomics, estimates of carbon mineralization) properties. The laboratory culture and controlled incubation experiments will be used to determine the specific pathways for Hg and Se compound formation and degradation, especially the role of photochemical transformations, as well as assess the importance of Se as a binding ligand for Hg in the marine environment. Lastly, the researchers will continue to develop the oceanic sub-model of the GEOS-Chem global biochemical Hg model to include the cycling of Se and will use the model to ascertain the importance of various processes of conversion and evasion in the global cycles of these two elements.

As regards broader impacts, this study has societal benefits because it would improve our understanding on how mercury enters seafood which impacts human health. Results from the research would be included in curriculum material. One graduate student from the University of Connecticut, one postdoc from Harvard University, and one graduate student from Woods Hole Oceanographic Institution would be supported and trained as part of the project.