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1. ²¹⁰ Pb and ⁷ Be as Coupled Flux and Source Tracers for Aerosols in the Pacific Ocean

   https://doi.org/10.1029/2022GB007378  

   Wei, Ziran; Cochran, J. Kirk; Horowitz, Evan; Fitzgerald, Patrick; Heilbrun, Christina; Kadko, David; Stephens, Mark; Marsay, Chris M.; Buck, Clifton S.; Landing, William M. (August 2022, Global Biogeochemical Cycles)

   Abstract Deposition of aerosols to the surface ocean is an important factor affecting primary production in the surface ocean. However, the sources and fluxes of aerosols and associated trace elements remain poorly defined. Aerosol²¹⁰Pb,²¹⁰Po, and⁷Be data were collected on US GEOTRACES cruise GP15 (Pacific Meridional Transect, 152°W; 2018).²¹⁰Pb fluxes are low close to the Alaskan margin, increase to a maximum at ∼43°N, then decrease to lower values. There is good agreement between²¹⁰Pb fluxes and long‐term land‐based fluxes during the SEAREX program (1970–1980s), as well as between GP15 and GP16 (East Pacific Zonal Transect, 12°S; 2013) at adjacent stations. A normalized fractionf(⁷Be,²¹⁰Pb) is used to discern aerosols with upper (highf) versus lower (lowf) troposphere sources. Alaskan/North Pacific aerosols show significant continental influence while equatorial/South Pacific aerosols are supplied to the marine boundary layer from the upper troposphere. Lithogenic trace elements Al and Ti show inverse correlations withf(⁷Be,²¹⁰Pb), supporting a continental boundary layer provenance while anthropogenic Pb shows no clear relationship withf(⁷Be,²¹⁰Pb). All but four samples have²¹⁰Po/²¹⁰Pb activity ratios <0.2 suggesting short aerosol residence time. Among the four samples (²¹⁰Po/²¹⁰Pb = 0.42–0.88), two suggest an upper troposphere source and longer aerosol residence time while the remaining two cannot be explained by long aerosol residence time nor a significant component of dust. We hypothesize that enrichments of²¹⁰Po in them are linked to Po enrichments in the sea surface microlayer, possibly through Po speciation as a dissolved organic or dimethyl polonide species. 

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2. 210Po and 210Pb distributions during a phytoplankton bloom in the North Atlantic: Implications for POC export.

   Horowitz, EJ; Cochran, JK; Bacon, MP; Hirschberg, DJ (June 2020, Deep sea research)

   null (Ed.)

   During the North Atlantic Bloom Experiment (NABE) of the Joint Global Ocean Flux Study (JGOFS), water column sampling for particulate and dissolved 210Po and 210Pb was performed four times (26 April and 4, 20, 30 May 1989) during a month-long Lagrangian time-series occupation of the NABE site, as well as one-time samplings at stations during transit to and from the site. There are few prior studies documenting short-term changes in 210Po and 210Pb profiles over the course of a phytoplankton bloom, and we interpret the profiles in terms of the classical “steady-state” (SS) approach used in most studies, as well as by using a non-steady state approach suggested by the temporal evolution of the profiles. Changes in 210Po profiles during a bloom are expectable as this radionuclide is scavenged and exported. During NABE, 210Pb profiles also displayed non-steady state, with significant increases in upper water column inventory occurring midway through the experiment. Export of 210Po from the upper 150 m using the classic “steady- state” model shows increases from 0.1 ± 1.7 dpm m-2 d-1 to 84.8 ± 6.2 dpm m-2 d-1 over the ~1 month occupation. Application of a non-steady state model, including changes in both 210Pb and 210Po profiles, gives higher 210Po export fluxes. Detailed depth profiles of particulate organic carbon (>0.8 μm) and particulate 210Po (>0.4 μm) are available from the 20 and 30 May samplings and show maxima in POC/Po at ~37 m. Applying the POC/210Po ratios at 150 m to the “steady state” 210Po fluxes yields POC export from the upper 150 m of 8.5 mmol m-2 d-1 on 20 May and 6.3 mmol C m-2 d-1 on 30 May. The non-steady state model applied to the interval 20 to 30 May yields POC export of 24.3 mmol C m-2 d-1. The non-steady state (NSS) 210Po-derived POC fluxes are comparable to, but somewhat less than, those estimated previously from 234Th/238U disequilibrium for the same time interval (37.3 and 45.0 mmol m-2 d-1, depending on the POC/Th ratio used). In comparison, POC fluxes measured with a floating sediment trap deployed at 150 m from 20 to 30 May were 11.6 mmol m-2 d-1. These results suggest that non-steady state Po-derived POC fluxes during the NABE agree well with those derived from 234Th/238U disequilibrium and agree with sediment trap fluxes within a factor of ~2. However, unlike the 234Th-POC flux proxy, non-steady stage changes in profiles of 210Pb, the precursor of 210Po, must be considered. 

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3. Flux of Particulate Elements in the North Atlantic Ocean Constrained by Multiple Radionuclides

   https://doi.org/10.1029/2018GB005994  

   Hayes, Christopher T.; Black, Erin E.; Anderson, Robert F.; Baskaran, Mark; Buesseler, Ken O.; Charette, Matthew A.; Cheng, Hai; Cochran, J. Kirk; Edwards, R. Lawrence; Fitzgerald, Patrick; et al (December 2018, Global Biogeochemical Cycles)

   Abstract Sinking particles strongly regulate the distribution of reactive chemical substances in the ocean, including particulate organic carbon and other elements (e.g., P, Cd, Mn, Cu, Co, Fe, Al, and²³²Th). Yet, the sinking fluxes of trace elements have not been well described in the global ocean. The U.S. GEOTRACES campaign in the North Atlantic (GA03) offers the first data set in which the sinking flux of carbon and trace elements can be derived using four different radionuclide pairs (²³⁸U^:234Th^;210Pb:²¹⁰Po;²²⁸Ra:²²⁸Th; and²³⁴U:²³⁰Th) at stations co‐located with sediment trap fluxes for comparison. Particulate organic carbon, particulate P, and particulate Cd fluxes all decrease sharply with depth below the euphotic zone. Particulate Mn, Cu, and Co flux profiles display mixed behavior, some cases reflecting biotic remineralization, and other cases showing increased flux with depth. The latter may be related to either lateral input of lithogenic material or increased scavenging onto particles. Lastly, particulate Fe fluxes resemble fluxes of Al and²³²Th, which all have increasing flux with depth, indicating a dominance of lithogenic flux at depth by resuspended sediment transported laterally to the study site. In comparing flux estimates derived using different isotope pairs, differences result from different timescales of integration and particle size fractionation effects. The range in flux estimates produced by different methods provides a robust constraint on the true removal fluxes, taking into consideration the independent uncertainties associated with each method. These estimates will be valuable targets for biogeochemical modeling and may also offer insight into particle sinking processes. 

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4. Measuring bismuth‐210, its parent, and daughter in aquatic systems

   https://doi.org/10.1002/lom3.10352  

   Waples, James T. (February 2020, Limnology and Oceanography: Methods)

   Abstract ²¹⁰Bi (t_1/2: 5.01 d)—the daughter of²¹⁰Pb and parent of²¹⁰Po—has rarely been measured in aquatic systems, and its behavior in the water column is poorly understood. In this article, I present a method for quickly measuring²¹⁰Pb,²¹⁰Bi, and²¹⁰Po in aquatic samples, where (1)²¹⁰Bi and²¹⁰Po are scavenged onto an anion solid‐phase extraction disk within 15 min of pretreating the sample; (2) beta decay of²¹⁰Bi is counted on the disk immediately thereafter; (3)²¹⁰Po is subsequently removed from the disk and redeposited on a copper plate for α‐spectroscopy; and (4)²¹⁰Pb is determined via the ingrowth of²¹⁰Bi. I present decay‐corrected calculations for total, dissolved, and particle‐bound fractions of each nuclide and conclude with an analysis of²¹⁰Pb,²¹⁰Bi, and²¹⁰Po activities in rain,dreissenid(quagga) mussels, and water samples from the Milwaukee Inner Harbor in Lake Michigan. Results show that the loss of lead on the anion solid‐phase extraction disks was negligible (0.2% ± 2.1%; ± 1 SD,n= 4), and the sorption of bismuth was complete (99% ± 2%; ± 1 SD,n= 16). Relative mean absolute deviations of duplicate sample analyses of lake water were 2.4% ± 1.9% for²¹⁰Pb (geometric mean of total sample activity: 3.0 disintegrations per minute [dpm],n= 6), 7.7% ± 5.8% for²¹⁰Bi (geometric mean of total sample activity: 2.6 dpm,n= 8), and 2.7% ± 1.7% for²¹⁰Po (geometric mean of total sample activity: 1.4 dpm,n= 8). 

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5. Quantifying 210Po/210Pb disequilibrium in seawater: A Comparison of two precipitation methods with differing results.

   https://doi.org/doi:10.3389/fmars2021.684484  

   Roca-Martí, M; Puigcorbé, V; Castrillejo, M; Casacuberta, N; Garcia-Orellana, J; Cochran, JK; Masqué, P (June 2021, Frontiers in marine science)

   null (Ed.)

   The disequilibrium between lead-210 (210Pb) and polonium-210 (210Po) is increasingly used in oceanography to quantify particulate organic carbon (POC) export from the upper ocean. This proxy is based on the deficits of 210Po typically observed in the upper water column due to the preferential removal of 210Po relative to 210Pb by sinking particles. Yet, a number of studies have reported unexpected large 210Po deficits in the deep ocean indicating scavenging of 210Po despite its radioactive mean life of ~200 days. Two precipitation methods, Fe(OH)3 and Co-APDC, are typically used to concentrate Pb and Po from seawater samples, and deep 210Po deficits raise the question whether this feature is biogeochemically consistent or there is a methodological issue. Here, we present a compilation of 210Pb and 210Po studies that suggests that 210Po deficits at depths >300 m are more often observed in studies where Fe(OH)3 is used to precipitate Pb and Po from seawater, than in those using Co-APDC (in 68 versus 33% of the profiles analyzed for each method, respectively). In order to test whether 210Po/210Pb disequilibrium can be partly related to a methodological artifact, we directly compared the total activities of 210Pb and 210Po in four duplicate ocean depth-profiles determined by using Fe(OH)3 and Co-APDC on unfiltered seawater samples. While both methods produced the same 210Pb activities, results from the Co- APDC method showed equilibrium between 210Pb and 210Po below 100 m, whereas the Fe(OH)3 method resulted in activities of 210Po significantly lower than 210Pb throughout the entire water column. These results show that 210Po deficits in deep waters, but also in the upper ocean, may be greater when calculated using a commonly used Fe(OH)3 protocol. This finding has potential implications for the use of the 210Po/210Pb pair as a tracer of particle export in the oceans because 210Po (and thus POC) fluxes calculated using Fe(OH)3 on unfiltered seawater samples may be overestimated. Recommendations for future research are provided based on the possible reasons for the discrepancy in 210Po activities between both analytical methods. 

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