The radioactive isotope ²¹⁰Po (T_1/2 = 138.4 days) and its grandparent ²¹⁰Pb (T_1/2 = 22.3 years) have been increasingly used to trace particle dynamics and the biogeochemical cycling of chemical species in aquatic environments over recent decades (Verdeny et al., 2009). This Research Topic provides a set of new studies focusing on the biogeochemical cycling of both ²¹⁰Po and ²¹⁰Pb in the marine environments. A total of 11 articles were published on this Research Topic, covering the biogeochemical behaviors of ²¹⁰Po and ²¹⁰Pb in various oceanic settings, their ability to bind with diatom- and coccolithophore-associated biopolymers, the utilization of ²¹⁰Po/²¹⁰Pb to quantify the sinking flux of particulate organic carbon and the residence times (or ages) of particulate matter in a variety of environmental settings, and the coupled application with other radionuclides and soot to expand their utilities as biogeochemical proxies.

An understanding of the geochemical behaviors of ²¹⁰Po and ²¹⁰Pb is the foundation for their applications in constraining particle dynamics. In studying this Research Topic, Seo et al. observed contrasting behaviors of ²¹⁰Po and ²¹⁰Pb over the productive East China Sea Shelf, showing a net addition of ²¹⁰Po and a net removal of ²¹⁰Pb from the water column. The regeneration of ²¹⁰Po from organic matter in the sinking particles and sediments was suggested to explain the difference in behavior, which is also supported by the observation that ²¹⁰Po was mainly bound to more hydrophobic (high protein to carbohydrate ratio) nitrogen/sulfur-enriched organic moieties, whereas the strongest ²¹⁰Pb binding agents were phosphate-containing molecules based on coccolithophore (Emiliania huxleyi)- and diatom (Phaeodactylum tricornutum)-associated biopolymers (Lin P. et al.). These results highlight the role of marine organisms in affecting the disequilibrium between ²¹⁰Po and ²¹⁰Pb and lend support for the potential application of ²¹⁰Po as a proxy for sulfur group elements (S, Se, and Te) (Seo et al.) and nitrogen cycling. Based on a direct assessment of ²¹⁰Po and ²¹⁰Pb in marine organisms, Sun et al. reported bioconcentration factors (BCFs) up to 3–4 orders of magnitude higher for ²¹⁰Po than for ²¹⁰Pb, indicating that bioconcentration might, particularly in productive waters, affect the disequilibrium between ²¹⁰Po and ²¹⁰Pb. The close relationship between the ²¹⁰Po deficit and the dissolved silicate concentration in the upper 200 m of the water column also highlights the influence of organisms (i.e., phytoplankton growth) on the disequilibria between ²¹⁰Po and ²¹⁰Pb (Ma et al.). All these results confirm the important roles of marine organisms in affecting the deficit of ²¹⁰Po in the upper ocean.

One of the applications of ²¹⁰Po and ²¹⁰Pb is the quantification of the sinking fluxes of various particulate components such as particulate organic carbon (POC; Stewart et al., 2007; Verdeny et al., 2009), particulate nitrogen (PN; Yang et al., 2011), and biogenic silica (BSi; Friedrich and Rutgers van der Loeff, 2002). In studying this Research Topic, Bam et al. reported the highest fluxes of POC and PN in rarely studied ice-covered areas near the North Pole compared with other non-permanent ice-covered areas, based on ²¹⁰Po-²¹⁰Pb disequilibria. In addition, extremely low BSi and particulate inorganic carbon (PIC) fluxes were observed, suggesting the absence of ballast effects in the Arctic Ocean. Such a high POC flux scenario, independent of the ballast effect, indicates differences in the biological carbon pump below the sea ice compared with other oceanic environments. Hu et al. found a significant positive correlation between POC and the partitioning of ²¹⁰Po between particles and seawater, lending support for the application of ²¹⁰Po-²¹⁰Pb disequilibrium in evaluating the POC fluxes in Prydz Bay, Antarctica. A similar investigation was conducted in the western North Pacific Ocean (Zhong et al.), which showed enhanced POC export fluxes near the continental shelf corresponding to a moderate biological carbon pump efficiency, compared with the high-latitudinal Arctic and Southern Ocean (Bam et al.; Hu et al.). In addition, Lin F. et al. evaluated organic carbon transport from the surface to deeper sediment by benthos using excess ²¹⁰Pb relative to supported ²¹⁰Pb (²²⁶Ra; i.e., the difference between total ²¹⁰Pb and supported ²¹⁰Pb from ²²⁶Ra) in the sediment of the Tropical Northwest Pacific, highlighting the driving relation between POC flux and the benthic ecosystem.

The expansion of ²¹⁰Po and ²¹⁰Pb in constraining geochemical processes is of great importance to the field of isotopic marine chemistry. In studying this Research Topic, Baskaran and Krupp proposed a novel application of the ²¹⁰Po-²¹⁰Pb pair as a chronometer to date the age of snow, the formation time of ice cores and melt ponds, and the residence time of ice-rafted sediment in the Arctic Ocean. These timescales constrained a series of crucial parameters for certain geochemical processes, e.g., the age of snow and the elapsed time of ice-rafted sediment after incorporation into ice. Yang et al. quantified the laterally contributed sinking flux of soot (the refractory fraction of black carbon) using ²¹⁰Po-²¹⁰Pb disequilibria and discriminated locally settled POC fluxes from those contributed by sediment resuspension coupled with lateral transport over the slope region of the northern South China Sea, which enables the ²¹⁰Po-²¹⁰Pb disequilibrium method to quantify the efficiency of the biological carbon pump in marginal seas with intensive cross-shelf material exchange. In combination with ⁷Be, Schmidt et al. used excess ²¹⁰Pb to estimate the residence times of total suspended sediment (TSS) in Galveston Bay, thus constraining the cycling of TSS within shallow, dynamic marine environments.

This Research Topic expands our knowledge of the biogeochemical cycling of ²¹⁰Po and ²¹⁰Pb and their applications to understanding different biogeochemical processes in the marine environments. Overall, the collected articles enhance the understanding of the behaviors of ²¹⁰Po and ²¹⁰Pb and the potential roles of various organic components and marine organisms in affecting the scavenging and phase partitioning of ²¹⁰Po and ²¹⁰Pb, validate the applicability of ²¹⁰Po/²¹⁰Pb disequilibrium in quantifying POC fluxes in various oceanic settings, and apply the pair in the dating of snow, ice, and ice-rafted sediment. Still, future studies are needed to improve our understanding of the biogeochemical behaviors of ²¹⁰Po and ²¹⁰Pb and to expand their applications.

All authors listed have made intellectual contributions to this Research Topic and approved it for publication.

This work was supported by the National Natural Science Foundation of China (42076030 and 41476061).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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