Weifeng Yang
Jinzhou Du
Laodong Guo- 1State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, China
- 2State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, China
- 3School of Freshwater Sciences, University of Wisconsin-Milwaukee, Milwaukee, WI, United States
Editorial on the Research Topic
Geochemical Cycling of 210Po and 210Pb in Marine Environments
Introduction
The radioactive isotope 210Po (T1/2 = 138.4 days) and its grandparent 210Pb (T1/2 = 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.
Geochemical Behaviors of 210Po and 210Pb
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.
Trace Particle Cycling Using 210Po and 210Pb
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.
Novel Application of The 210Po/210Pb Pair
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.
Conclusion
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.
Author Contributions
All authors listed have made intellectual contributions to this Research Topic and approved it for publication.
Funding
This work was supported by the National Natural Science Foundation of China (42076030 and 41476061).
Conflict of Interest
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.
Publisher's Note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
Acknowledgments
We thank all the authors, reviewers, and editors for their contributions to this Research Topic.
References
Friedrich, J., and Rutgers van der Loeff, M. M. (2002). A two-tracer (210Po-234Th) approach to distinguish organic carbon and biogenic silica export flux in the Antarctic Circumpolar Current. Deep-Sea Res. I 49, 101–120. doi: 10.1016/S0967-0637(01)00045-0
Stewart, G., Cochran, J. K., Miquel, J. C., Masqué, P., Szlosek, J., Baena, A. M. R., et al. (2007). Comparing POC export from 234Th/238U and 210Po/210Pb disequilibria with estimates from sediment traps in the northwest Mediterranean. Deep-Sea Res. I 54, 1549–1570. doi: 10.1016/j.dsr.2007.06.005
Verdeny, E., Masqué, P., Garcia-Orellana, J., Hanfland, C., Cochran, J. K., and Stewart, G. M. (2009). POC export from ocean surface waters by means of 234Th/238U and 210Po/210Pb disequilibria: a review of the use of two radiotracer pairs. Deep-Sea Res. II 56, 1502–1518. doi: 10.1016/j.dsr2.2008.12.018
Keywords: polonium-210, lead-210, biological pump, radiochronology, particulate organic carbon (POC)
Citation: Yang W, Du J and Guo L (2022) Editorial: Geochemical Cycling of 210Po and 210Pb in Marine Environments. Front. Mar. Sci. 9:852558. doi: 10.3389/fmars.2022.852558
Received: 11 January 2022; Accepted: 27 January 2022;
Published: 17 February 2022.
Edited and reviewed by: Pere Masque, IAEA International Atomic Energy Agency, Monaco
Copyright © 2022 Yang, Du and Guo. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Weifeng Yang, wyang@xmu.edu.cn; Jinzhou Du, jzdu@sklec.ecnu.edu.cn; Laodong Guo, guol@uwm.edu