GEOTRACES: Our participation on the GEOTRACES Intercalibration cruises focused on ensuring the clean collection and accurate analyses of samples for dissolved trace metals. One of the key aspects of the initial U.S. GEOTRACES Intercalibration cruise in the North Atlantic at a station close to the BATS station off Bermuda was to test the ability of the new U.S. GEOTRACES clean sampling system consisting of a 24 position “trace-metal-clean” rosette/winch and a dedicated clean van for processing 24, twelve-liter, Teflon™-coated, GO-Flo samplers to collect uncontaminated samples for the suite of key GEOTRACES trace metals and isotopes. During the Atlantic GEOTRACES Intercalibration cruise, this sampling system was compared with a variety of other proven sampling systems, including 30 L, Teflon-coated, GO-Flo samplers (Bruland et al., 1979), MITESS samplers (Boyle, MIT), and UAF Vane samplers (Wu, University of Alaska, Fairbanks) deployed on Kevlar line, and the CLIVAR ”trace-metal-clean” rosette/winch system (Landing, FSU; Measures, UH) that the GEOTRACES sampling system was based upon. A number of experienced shipboard analysts were on board to assist in this effort: Dissolved Fe - Maeve Lohan (Plymouth, UK), Ana Aguilar-Islas (UAF), Max Grand and Mariko Hatta (UH); Dissolved Al - Matt Brown (UCSC), Max Grand and Mariko Hatta (UH); and dissolved Zn - Jay Cullen (U of Victoria) and Sherry Lippiatt (UCSC). With the invaluable help of these shipboard analysts, approximately 30 out of 52 GO-Flo samplers were selected as capable of collecting uncontaminated samples (the "A- team"). The A-team of GO-Flo bottles were then used on the U.S. GEOTRACES rosette during the remainder of the cruise to obtain a practice profile and then eventually a baseline profile for trace metals and an additional baseline profile for trace metal isotopes. Samples from the baseline profiles also were collected for a variety of national and international labs for shore-based analyses. Representative shipboard results for dissolved Fe and Al from the practice profile are shown below in Figure 1a and 1b.
Figure 1a. Dissolved Fe on the GEOTRACES Atlantic practice profile with the “A-team” of GO-Flo samplers – all 24 samplers – after pre-cruise cleaning and multiple conditioning casts. Samples were filtered through an Osmonics capsule filter. With the exception of the Fe minimum at ~120 m depth, there was good agreement between the various shipboard analysts. Both Maeve Lohan and Ana Aguilar-Islas used the SAFe S and SAFe D2 reference samples as accuracy checks in their analyses of the profile.
Figure 1b. Dissolved Al on the GEOTRACES Atlantic practice profile with the “A-team” of GO-Flo samplers – 22 of 24 samplers – after multiple conditioning casts. Samples were filtered through an Osmonics capsule filter. There was good agreement between the shipboard analysts. Brown & Bruland used the SAFe S and SAFe D2 reference samples as accuracy checks in their analyses of the profile.
An additional aspect of the U.S. GEOTRACES Intercalibration cruises was to collect and provide trace element and isotope seawater reference samples for the international GEOTRACES community. As mentioned previously, Ken Bruland and Geoffrey Smith (UCSC) continue to provide SAFe reference samples to the international community to be assayed for the suite of key trace metals in GEOTRACES programs. In addition to this, Bruland and Smith used the SAFe tank system on the GEOTRACES Intercalibration cruises to collect an additional set of Atlantic GEOTRACES reference samples from the surface and from a depth of 2000 meters. This time the surface reference sample (GS) was collected with the GeoFish sampling system in the same manner as for SAFe, while the 2000 m deep reference sample (GD) was collected with 5 separate casts of the GEOTRACES rosette to rinse and fill a tank. During the Pacific GEOTRACES Intercalibration cruise, Bruland and Smith used the GeoFish/SAFe tank system to collect additional Pacific surface water from the SAFe site to be used as GEOTRACES surface Pacific (GSP) reference samples (~300 0.5 L bottles). Additionally, Bruland and Smith collected a surface tank from the highly productive surface waters of the Santa BarbaraBasin coastal site on the Pacific Intercalibration cruise and aliquoted ~600 0.5 L samples of this water as a coastal SAFe/GEOTRACES coastal surface reference sample, “GCS”. Results of trace metal concentrations from analyses of all these reference samples and the baseline profiles are being reported to Ken Bruland (). Thus, there is a range of reference samples available with different concentrations and chemistries representative of what one encounters in the real ocean. As previously mentioned, results and “consensus values” of the SAFe reference samples are now available on line at the GEOTRACES website and Bruland’s website for a suite of GEOTRACES trace metals.
The GEOTRACES reference samples, however, are still being treated as "unknown or blind" samples until more data is gathered. Once a researcher reports their data, Bruland then shares what other data he has gathered for the metal of interest on the relevant reference samples with that investigator and they try to further investigate any problems that may exist. This stage is a confidential process and is an important component of the intercalibration exercises and part of the international “learning curve” to help raise the level of expertise of the U.S. and international trace metal community.