Inventory-based evaluation of 210Po-210Pb-226Ra disequilibria in deep oceans and new insights on their utility as biogeochemical tracers: A global data synthesis of research over six decades

Abstract

Disequilibria between the long-lived parents and short-lived particle-reactive daughters have been extensively used over the past six decades as tracers and chronometers in aquatic systems. In particular, the particle-reactive progeny of 226Ra (210Pb, half-life, T1/2, = 22.3 years and 210Po, T1/2 = 138.4 days) have been widely utilized to quantify oceanic biogeochemical processes (e.g. elemental export fluxes, residence time and scavenging rate of particulate and dissolved 210Po and 210Pb, settling velocity of particles and remineralization rates of biogenic particulate matter) as well as analogs for stable Pb, key micronutrients (e.g., Cu, Zn, Fe), and lithogenic elements (e.g., Al, Th, Ti). Vertical profiles of 210Po and 210Pb from the deep-ocean water column have been published for over six decades that show disequilibrium between 210Po and 210Pb with 210Po/210Pb activity ratio (AR) of >1.1 and/or < 0.9 in discrete deep-water layers, although secular equilibrium with AR of 1.0 ± 0.1 is expected. To understand this discrepancy, a critical and in-depth review of 210Po-210Pb-226Ra published data was conducted, scrutinizing the sample size, methodology, reagent blanks, statistical analysis, and data reduction. For each of the published data sets for water depths >1000 m, the total water column inventories and the average activities of dissolved, particulate, and/or total (= particulate + dissolved) 210Po, 210Pb, and 226Ra were calculated. The total water-column-based average 210Po activities follow the pattern: Pacific Ocean > Indian Ocean > Antarctica > Atlantic Ocean > Arctic Ocean. The (210Po/210Pb) AR in small particles (1–51 μm) shows the following trend: N. Atlantic > E. Pacific > Arctic, whereas the AR in large particles shows the following trend: E. Pacific > N. Atlantic > Artic. The fractionation factors calculated using inventories of 210Po and 210Pb, exhibit the following pattern: Pacific Ocean > Indian Ocean > Antarctica > Atlantic Ocean > Arctic Ocean. The inventory-based distribution coefficients (Kd) of 210Po and 210Pb are comparable, as opposed to consistently higher Kd for 210Po reported in the literature. From deep water and whole column inventories of 210Pb and 226Ra, disequilibrium was found ubiquitously, with a highly varying residence time of 210Pb. Average surface water concentrations of 210Pb in the global oceans were found to correlate with latitude and show an overall decrease in concentration from mid-latitudes to poles, similar to 210Pb fallout.