Trace Metal Consensus Seawater - GoM

Jessica Fitzsimmons, James Moffett, and Nicholas Hawco propose to collect four large volume (1000 L) seawater samples to develop an archive of consensus materials for trace metal analysis. Samples will be collected from the surface and 1000m at the Station ALOHA, and from two stations in the Gulf of Mexico influenced by the Mississippi River outflow. Samples will be collected and filtered using trace metal clean techniques into large-volume tanks, homogenized, and then dispensed into 500mL bottles for archiving and distribution to the community. Samples will be analyzed by groups worldwide for a suite of dissolved trace metals. Their data will be analyzed in collaboration with Antonio Possolo, of the National Institute of Standards and Technology (NIST). Consensus values derived from this synthesis for each element will be reported on the US GEOTRACES Web Site and in a peer-reviewed publication. The majority of samples will be archived for distribution, replacing the exhausted archives of previous intercomparison projects. The last two decades have seen great advances in our knowledge about the distribution and biogeochemical cycles of trace elements in the ocean. This was possible because laboratories worldwide had confidence that their analyses were producing consistent results. The establishment and distribution of consensus standards, from the SAFe (2004) and GEOTRACES (2008-2009) programs, were critical in this development. We propose to collect samples from Station ALOHA, where water chemistry will be very similar to the valuable samples collected in those former consensus-building programs. Those earlier efforts also revealed that there were some intercalibration issues that need to be addressed. For example, copper displays surprising variability in reported values that did not appear to arise from contamination, but could reflect artifacts arising from Cu complexation by organic matter for specific protocols. Cobalt shows similar issues, while vanishingly low surface oligotrophic concentrations of cadmium, zinc, and iron can prevent strong consensus against significant blanks at the picomolar level. These issues and others will be explored by Fitzsimmons, Moffett, Hawco, and other participants in the course of consensus building. Recent field campaigns in margins and estuaries have underscored the importance of a consensus standard in waters containing high concentrations of dissolved organic matter, which may create analytical artifacts for copper and other metals. It also needs to be determined what the constraints on long term storage are for such samples. This is the rationale for collecting a high-organics, lower salinity sample in the Gulf of Mexico.