Intercalibration studies of short-lived thorium-234 in the water column and marine particles

Intercomparision of 234Th measurements in both water and particulate samples was carried out between 15 laboratories worldwide, as a part of GEOTRACES inter-calibration program. Particulate samples from four different stations namely BATS (both shallow and deep) and shelf station (shallow) in Atlantic and SAFE (both shallow and deep) and Santa Barbara station (shallow) in Pacific were used in the effort. Particulate intercalibration results indicate good agreement between all the participating labs with data from all labs falling within the 95% confidence interval around the mean for most instances. Filter type experiments indicate no significant differences in 234Th activities between filter types and pore sizes (0.2–0.8 μm). The only exception are the quartz filters, which are associated with 10% to 20% higher 234Th activities attributed to sorption of dissolved 234Th. Flow rate experiments showed a trend of decreasing 234Th activities with increasing flow rates (2-9 L min–1) for > 51 μm size particles, indicating particle loss during the pumping process. No change in 234Th activities on small particles was observed with increasing flow-rates. 234Th intercalibration results from deep water samples at SAFe station indicate a variability of < 3% amongst labs while dissolved 234Th data from surface water at Santa Barbara Station show a less robust agreement, possibly due to the loss of 234Th from decay and large in-growth corrections as a result of long gap between sample collection and processing. *Corresponding author: E-mail: [email protected] Acknowledgments Full text appears at the end of the article. DOI 10.4319/lom.2012.10.631 Limnol. Oceanogr.: Methods 10, 2012, 631–644 © 2012, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODS pair of intercalibration cruises in 2008 and 2009. Our scientific interests in this effort centered on the determination and intercalibration of the short-lived isotope, Thorium-234. The use of 234Th as a flux proxy was recently reviewed and synthesized at an international workshop held in August 2004 (Benitez-Nelson and Moore [2006] and other manuscripts in Marine Chemistry volume 100[3-4]). In essence, 234Th is a relatively, short-lived (t1/2 = 24.1 d) particle reactive tracer that is produced at a constant rate from its conservative and longlived parent, Uranium-238 (t1/2 = 4.5 ¥ 10 9 y). In surface waters where the vertical flux of 234Th on sinking particles is large, there is a 234Th deficit (i.e., total 234Th < 238U). Conversely, if the sinking particles are remineralized at depth, one can find “excess” 234Th (i.e., 234Th > 238U) (Bhat et al. 1969; Coale and Bruland 1985; Buesseler et al. 1992). In most applications, 234Th is primarily used to quantify shallow export rates of sinking particles, however it is becoming increasingly evident using higher vertical resolution sampling that shallow layers of excess 234Th may be used to investigate particle remineralization rates as well (Maiti et al. 2010). Such layers have been observed in a range of environments, including regions of low particle flux, such as just below the deep chlorophyll maximum layer in the Sargasso Sea (BenitezNelson et al. 2001b; Buesseler et al. 2008; Maiti et al. 2010). 234Th may also be used as a sinking flux proxy of specific elements. Whereas the most common application is for examining the export flux of particulate organic carbon (POC) (reviewed in Buesseler et al. 2006), other uses include the export flux of compounds such as surface associated organics (PAHs; PCBsGustafsson et al. 1997a, 1997b) and trace metals (Weinstein and Moran 2005). When used in this manner, the 234Th activity balance is integrated from the surface to a given depth, and net export, P, is calculated on a dpm m–2 d–1 basis. Elemental fluxes are subsequently determined when the 234Th flux, PTh, is multiplied by its ratio to other elements (E) on sinking particles, i.e., Flux PE = PTh ¥ E/ 234Th on particles, thereby providing direct information on the vertical fluxes and remineralization rates on time scales of days/weeks. When properly applied, comparisons between C flux derived from 234Th and other independent estimates (mass balances, new production) show good agreement when integrated over appropriate time and space scales (Buesseler et al. 2006). Given the widespread and increasing use of 234Th as a particle flux tracer (Benitez-Nelson and Moore 2006, Waples et al. 2006) and the application of 234Th on GEOTRACES cruises to estimate fluxes of different GEOTRACES Trace Elements and Isotopes (TEIs), it is essential that the various groups involved in 234Th measurements have a comprehensive understanding of how their measurements compare. The additional information that we have gathered on the strengths and weaknesses of 234Th analysis throughout this manuscript will further improve the way 234Th is used to determine vertical fluxes and remineralization rates of biogeochemically relevant materials that vary on time scales of days to weeks. The major objectives of the GEOTRACES program are to (1) determine global ocean distributions of selected trace elements and isotopes, including their concentration, chemical speciation, and physical form, and (2) evaluate the sources, sinks, and internal cycling of these species in order to characterize more completely the physical, chemical, and biological processes regulating their distributions (Henderson et al. 2007). These objectives are being met by national and international sampling efforts carried out along a number of sections in all the major ocean basins. Since GEOTRACES is an international program, many countries are, or will be, conducting their own GEOTRACES cruises. However, before this major effort can begin in earnest, it is essential that intercalibration activities take place to ensure the accuracy of all the collected data. Materials and procedures Intercalibration is defined by the GEOTRACES Science Plan as “The process, procedures, and activities used to ensure that the several laboratories engaged in a monitoring program can produce compatible data. When compatible data outputs are achieved and this situation is maintained, the laboratories can be said to be intercalibrated” (Taylor 1987). Thus, intercalibration is an active process between laboratories that includes all steps, from sampling to analyses, with the goal of achieving the same accurate results regardless of the method or lab. The ultimate goal for the Thorium-234 intercalibration component of GEOTRACES is to achieve the best precision and accuracy possible (lowest random and systematic errors) as a prelude to, and continuing effort throughout, the sampling and analysis program. An outcome of a relatively recent workshop on “Future Applications of 234Th in Aquatic Environments” (Benitez-Nelson and Moore 2006; Waples et al. 2006; Rutgers van der Loeff et al. 2006) was an explicit need for a particulate 234Th intercalibration and standards. One option would be to create an artificial 238U-234Th enriched particle/filter matrix as a standard, but the difficulty in making such as standard and the issues associated with correcting 234Th activities for absorption of b particles by differing materials would not be well addressed. An alternative solution is to collect evenly distributed natural marine particles on filters with different loading and particle characteristics from surface and deep waters as well as oligotrophic and mesotrophic regions. The latter option was thus used in the GEOTRACES 234Th intercalibration effort. To accomplish this goal, particulate 234Th material was collected using two size classes (>51 μm and 1–51 μm), as recommended by Rutgers van der Loeff et al. (2006). On the >51 μm particles, samples were collected using two different filter types: 1 μm nominal quartz depth filters (QMA) filters and 0.45 μm polyethersulfone membrane filters (SUPOR). A number of tests were also conducted to understand the variability associated with collection of large particles. Finally, small volume water samples (2–8 L) were also collected for intercalibration of methods used to measure dissolved and total 234Th measurements. Maiti et al. Intercalibration studies of Thorium-234