Intercalibration studies of 210Po and 210Pb in dissolved and particulate seawater samples

Documented is an intercalibration (IC) exercise for both 210Po and 210Pb in seawater aliquots distributed between up to eight international laboratories that followed individual protocols. Dissolved and particulate samples were provided by GEOTRACES during two IC cruises at baseline stations in the North Atlantic and North Pacific oceans. Included were surface and/or deep dissolved and particulate samples at each site, plus complete profiles analyzed by the laboratory of the lead author. An unspecified solid phase standard was also distributed with 210Po and 210Pb in secular equilibrium to confirm spike calibrations. The 210Po activities reported n = 8) for the standard were very similar with a relative standard deviation (RSD) of 3.6% and mean value indistinguishable from the certified value, confirming accurate calibration of Po spikes. For seawater samples, the agreement was strongly dependent for both nuclides on the activity of the samples. The agreement was relatively good for dissolved seawater samples (RSD = 9% to 29%, n = 4), moderate for the particulate samples (RSD = 12% to 80%, n = 8), and poor for particulate dip blanks (RSD = 50% to 200%, n = 8). Noted is the higher apparent affinity of 210Po versus 210Pb for polysulphone filter material. Some lack of reproducibility between labs may have been caused by unspecified differences in individual lab protocols and calculations. A minimum sample activity of 0.1 dpm for both nuclides is recommended for an adequate reproducible sample activity. It is suggested that a consistent set of procedures and calculations be used to optimize future 210Po and 210Pb analyses in seawater samples. *Corresponding author: E-mail: [email protected]; phone: (302) 831-2558; fax: (302) 831-4575 Acknowledgments Full text appears at the end of the article. DOI 10.4319/lom.2012.10.776 Limnol. Oceanogr.: Methods 10, 2012, 776–789 © 2012, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODS orities of the U.S. GEOTRACES Intercalibration Initiative” (www.geotraces.org). To use the 210Po/210Pb radionuclide pair under the international auspices of the GEOTRACES program, a thorough intercomparison was deemed necessary leading to intercalibration across participant laboratories. This work is also intended to assess the protocols suggested in the methods manual prepared for the program. This process will allow adjusting the methods so that reliable and consistent data can be generated along multiple transects sponsored by the international community and across the variety of interfaces being targeted. Why intercalibration is necessary: In general and for GEOTRACES The 210Po/210Pb disequilibrium in the surface ocean is evidenced as a deficiency of 210Po in the dissolved phase relative to 210Pb (210Po/210Pb < 1.0) and corresponding excess in particulates (210Po/210Pb > 1.0). This is largely recognized as due to preferential extraction of Po by phytoplankton and regeneration in the subsurface reviewed in Stewart et al. (2008). This disequilibrium is one means for modeling organic carbon export e.g., Stewart et al. (2007) and to derive vertical eddy diffusion coefficients to model the transport of new nitrogen production in Sarin et al. (1994). Such modeling demands relatively high precision for analyzing this granddaughter/parent pair by any given laboratory, and accuracy between laboratories conducting contiguous sampling across oceanic regimes. Previously, the global distribution of the 210Po and 210Pb pair was studied during the GEOSECS (Geochemical Ocean Section Study) program by 3 groups that identified sampling and analytical problems with the 210Po/210Pb measurements (Scripps, Yale and Woods Hole; Cochran et al. 1983; Chung and Craig 1983). The causes of differences between results included problems with water sampling materials (e.g., preferential loss of 210Pb on stainless steel or PVC) and filtration apparatus to accurately differentiate the dissolved fractions. These discrepancies were largely reconciled during a final intercomparison representing the only prior attempt for a thorough intercomparison for dissolved 210Pb. This was made toward the end of GEOSECS at Station 500 (28°27’N and 122°11’W) to help resolve the differences outlined above Chung et al. (1983). At that time there was good agreement between the two extraction methods being used by the YaleWHOI groups (Co-APDC) and Scripps (Fe-OOH) for 210Pb (5% to 10% at all depths). Other artifacts including different methods of filtration had been resolved, and residual 210Pb carry over between sampling bottles minimized to a few percent (Chung and Craig, 1983). The particulate burden was so minor as to not be resolved unless large volume samples of 6-20 L are obtained (Bacon et al. 1976). However the corresponding 210Po data were either not completed or not reported. This supports the rationale for an intercalibration of 210Po and 210Pb in future studies as well as for the employment of submersible pumps to collect particulate nuclides of sufficient activities. Oceanographic perspective for the intercalibration Observations and discussions continue on the exact nature of the 210Po and 210Pb granddaughter/parent distribution, including the cause of reported large scale deficiency of 210Po (210Po/210Pb activity ratios < 1) in both pelagic and meso-pelagic waters of some oceanic regimes (e.g., western and eastern Indian Ocean: Chung and Finkel 1988 and Cochran et al. 1983, respectively; East China and Philippine Seas: Nozaki et al. 1990; Equatorial Pacific and Bering Sea: Nozaki et al. 1997; Sargasso Sea: Kim and Church 2001; South China Sea: Chung and Wu 2005; Antarctic Circumpolar Current: Friedrich and Rutgers van der Loeff 2002). Proposed processes in benthic waters include preferential extraction of 210Po onto the suspended particulate phase versus adsorption of 226 Ra supported 210Pb at the sediment water interface (Bacon et al. 1976; Cochran et al. 1983). However to reach such conclusions, an ideal analytical uncertainty of less than 10% needs to be obtained. Thus, a thorough intercalibration for both 210Po and 210Pb is timely to ascertain whether mid-waters and deeper 210Po deficiencies are accurate and not due to inadequacy in the analytical techniques. Furthermore, if such deficiencies are shown to exist, they could reveal those meso-pelagic processes that might be operational and modeled on the sub-annual time scales commensurate to that of the 210Po half-life (Turekian and Nozaki 1980; reviewed in Rutgers van der Loeff and Geibert 2008). Approach to intercalibration and participation The present article reports an inter-laboratory comparison for the assay of 210Po and 210Pb conducted on identical dissolved and particulate aliquots using lab specific protocols and calculations. The international laboratories with published peer-reviewed articles on the 210Po and 210Pb nuclide pair were invited to participate and those responded received samples. Not all laboratories elected to participate and those reporting partial data (i.e., only 210Po without a subsequent 210Pb ingrowth measurement) were excluded from this manuscript. The approach to the intercalibration exercise was to first distribute a solid standard of higher activity (IAEA-RGU-1), and also to collect and distribute common aliquots of dissolved and particulate marine samples at lower natural activity. The natural intercalibration samples were taken at the GEOTRACES baseline station (BATS) in the North Atlantic at (31°40¢N; 64°10¢W) as well as a coastal (SHELF) site on the slope off Chesapeake Bay (37.0°N, 74.4°W, depth 1655 m), and at the baseline station (SAFe) in the North Pacific at (30°00¢N; 140°00¢W). The present article basically reports the inter laboratory comparison of the same dissolved and particulate aliquots using their own protocols and calculations. Materials and procedures Intercalibration solid standard The certified reference material (IAEA-RGU-1) was distributed to the participating groups as an unidentified solid. The participating laboratories were not informed of the activity in the certified material, thus we do not anticipate the labs readChurch et al. Intercalibration of seawater 210Po and 210Pb