POC export from ocean surface waters by means of 234Th/238U and 210Po/210Pb disequilibria: A review of the use of two radiotracer pairs

Abstract

Po/ 210 Pb disequilibrium may be used as an additional tool to examine particle export, given the direct biological uptake of 210 Po into cellular material. Differences in these two radioisotope pairs with regard to their half-lives, particle reactivity and scavenging affinity in seawater should provide complementary information to be obtained on the processes occurring in the water column. Here, we review eight different studies that have simultaneously used both approaches to estimate POC export fluxes from the surface ocean. Our aim is to provide a complete ''dataset'' of all the existing POC flux data derived from the coupled use of both 234 Th and 210 Po and to evaluate the advantages and limitations of each tracer pair. Our analysis suggests that the simultaneous use of both radiotracers provides more useful comparative data than can be derived from the use of a single tracer alone. The difference in half-lives of 234 Th and 210 Po enables the study of export production rates over different time scales. In addition, their different biogeochemical behaviour and preferred affinity for specific types of particles leads to the conclusion that 234 Th is a better tracer of total mass flux, whereas 210 Po tracks POC export more specifically. The synthesis presented here is also intended to provide a basis for planning future sampling strategies and promoting further work in this field to help reveal the more specific application of each tracer under specific water column biogeochemistries.

The influence of particle concentration and composition on the fractionation of 210Po and 210Pb along the North Atlantic GEOTRACES transect GA03

“…For example, the POC fluxes derived from 234 Th 48 were higher than those derived from 210 Po in the Polar Front region, where diatoms were 49 dominant in the phytoplankton (Friedrich and Rutgers van der Loeff, 2002), while 210 Po-POC 50 flux was greater than the 234 Th-POC flux in the Sargasso Sea where small phytoplankton 51 such as Prochlorococcus and Synechococcus dominated the community (Bibby et al, 2008;52 Buesseler et al, 2008). In addition, POC fluxes derived from 210 Po were usually higher than 53 234 Th-derived estimates in the middle of gyres (e.g., Sargasso Sea, and Hawaii), whereas the 54 opposite was generally true near the margins (Stewart et al, 2007a;Verdeny et al, 2009). 55…”

Particle-reactive radionuclides (<sup>234</sup>Th, <sup>210</sup>Pb, <sup>210</sup>) as tracers for the estimation of export production in the South China Sea

“…The F POC−Po based on the 210 Po-210 Pb system showed the largest range of POC export, ranging from 1.8 to 20.3 mmol-C m −2 d −1 , among the three pairs of disequilibria. Verdeny et al (2009) recently presented a review of previous studies using 234 Th and 210 Po as dual tracers to estimate the export production in the ocean. A discrepancy between the POC fluxes estimated by 234 Th-238 U and 210 Po- 210 Pb disequilibria was commonly found.…”

“…There is only a few studies that used 210 Pb to estimate the export production in the ocean, although a strong correlation of 210 Pb removal with the POC flux was previously demonstrated (Moore and Dymond, 1988). On the other hand, due to its high affinity with biological particles (Cherry et al, 1975), 210 Po is also used as a powerful tracer for particulate matter sinking out of surface ocean (Murray et al, 2005;Friedrich and Rutgers van der Loeff, 2002;Verdeny et al, 2009;Kim and Church, 2001;Buesseler et al, 2008).…”

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

“…Studies of particulate and dissolved 210 Po and 210 Pb in the marine environment have been used for quantifying the scavenging and removal of particle-reactive species including organic carbon integrated over sub-annual time scales not attainable by direct measurement synthesized in Verdeny et al (2009).…”

“…Studies of particulate and dissolved 210 Po and 210 Pb in the marine environment have been used for quantifying the scavenging and removal of particle-reactive species including organic carbon integrated over sub-annual time scales not attainable by direct measurement synthesized in Verdeny et al (2009).The GEOTRACES (Geochemical Trace Element/Isotope Studies) program has been initiated to study trace elements and isotopes (TEIs) and to quantify processes across major ocean boundaries. The 210 Po and 210 Pb radionuclide pair is a useful suite of radiometric TEIs that are both geochemically (e.g., Pb) and biochemically (e.g., Po) active.…”