Intercalibration

Intercalibration of analytical methods is vital to ensure that the comparison of data generated in different labs is meaningful. This is especially important when you consider the global nature of the GEOTRACES community. 

 

Intercalibration is a key component of the international GEOTRACES programme with the overall aim of achieving the best accuracy possible for sampling and analysis (lowest random and systematic errors) for the full suite of Trace Elements and Isotopes (TEI). This is essential as many nations have and will be involved in carrying out GEOTRACES section cruises throughout the world’s oceans and therefore accurate results are required to compare between the sections. To achieve this aim a two-pronged approach was devised, which involved (i) intercalibration cruises, and (ii) to have at least one crossover station on each GEOTRACES section where results can be compared with another nations section (Figure 1). Further information can be found at http://www.geotraces.org/science/intercalibration

The two intercalibration cruises took place in the Atlantic close to the Bermuda Atlantic Time-series Station (BATS) in 2008, and in the oligotrophic Pacific Ocean in 2009. The aim of these cruises was to evaluate and develop sample acquisition, handling and storage protocols. In addition large volumes of homogenised surface and deep seawater were collected to provide 0.5 L reference samples for dissolved trace elements. These samples have been analysed by a large number of international laboratories and results are available on the international GEOTRACES website (see link above). These samples can still be requested by sending an e-mail to bruland@ucsc.edu. The two cruises were also used to collect large volumes of homogenised surface and deep seawater for analyses of a number of isotope systems that require large volumes of seawater (i.e., between 2 and 100L) such as Pb isotopes, Nd isotopes, Pa/Th ratios, and man-made radionuclides (e.g., Pu isotopes). For some isotope systems a small number of archive samples are still available, and can be requested by sending an e-mail to tina.vandeflierdt@imperial.ac.uk. Results for the international intercalibration exercise have been published in a special issue of Limnology & Oceanography Methods (http://www.aslo.org/lomethods/si/intercal2012.html).

 

Figure 1: Crossover stations from GEOTRACES sections, which have been completed to date in the Atlantic Ocean a) US and Netherlands also station from 1st intercalibration cruise, b) Netherlands and Germnay, c) UK and Netherlands, d) French and Germany, e) UK and French, f) UK and Germany, g) US and Germany

The UK played a large role in these intercalibration cruises. Dr. Tina van der Flierdt participated in the first intercalibration cruise, and was the co-coordinator for the intercalibration of Nd isotopes and rare earth elements (REE) http://www.geotraces.org/science/intercalibration/217-intercalibration-coordinators. Dr. Maeve Lohan also participated in these cruises, being responsible for on-board iron concentration measurements to ensure the samples were collected cleanly. Dr Maeve Lohan is a member of the GEOTRACES Standards and Intercalibration committee http://www.geotraces.org/science/intercalibration/88-the-geotraces-standards-and-intercalibration-committee


Figure 2. Silicate and dissolved zinc concentrations from crossover station c (UK and Netherlands). Silicate data from Malcolm Woodward (UK) and the GEOTRACES team (NL), dissolved Zn data from Neil Wyatt and Maeve Lohan (UK) and Rob Middag and Hein de Baar (NL).

After completing the initial intercalibration phase, crossover stations will now become the key component of the ongoing intercalibration effort. Figure 1 shows all the crossover stations for cruises, which have taken place in the Atlantic Ocean. The international GEOTRACES Standards and Intercalibration committee collects the data from all crossover stations to ensure that intercalibration of sampling and analysis can be achieved and that the results between cruises are comparable. The UK GEOTRACES group is actively working with both, French and Dutch scientists (crossover stations e and c respectively), to compare our data and resolve any discrepancies. An excellent example for intercalibration at a crossover station is provided by the Dutch and UK data for dissolved zinc (Figure 2). The two laboratories involved in determining these values have also undertaken analysis on GEOTRACES reference materials and reported the values to Prof. Ken Bruland.

Intercalibration is an absolute requirement for all GEOTRACES cruises. Furthermore, all individuals involved with UK funding for GEOTRACES are committed to taking part in International Intercalibration.