Mercury measurements in the ocean - GEOTRACES intercalibration exercises

Abstract:

Mercury is amongst the least concentrated trace metals in the ocean. We need to be able to measure, understand and interpret variability in mercury concentrations in seawater, which is often as low as some 10 percent. We organized international intcalibration exercises for total mercury and total methylmercury determination in seawater collected during the 2013 Dutch GEOTRACES MedBlack cruise (GA04- Black Sea), the 2014 French GEOTRACES GEOVIDE (GA01- North Atlantic Ocean) and the 2015 German GEOTRACES TranArc II cruise ( GN04 - Arctic Ocean). The exercises were intended to primarily evaluate the analytical performance of each participating laboratory. Therefore each laboratory received a single sample bottle of similar size, that has undergone the same cleaning procedure prior to sampling, and each sample was preserved in the same manner. The 2013 exercise was intended as a broader screening with a maximum number (25) of participating laboratories. Results indicated substantial disagreement between the participating laboratories, for both total mercury and methylmercury determinations. For the 2014 exercise we could only invite 10 laboratories of the 2013 exercise. Intercomparability of the second exercise was considerably better, but needs to be further improved in the years to come. The 2015 will be organized this summer. We will present the results of both intercalibration exercises in detail, attempt to explain causes for disagreements, and share our ideas about future developments to achieve traceable mercury measurements in the oceans. We will also make use of this occasion to plan the 2016 GEOTRACES intercalibration cruise.