Comparison of the scavenging intensity, remineralization and residence time of ²¹⁰Po and ²¹⁰Pb at key zones (biotic, sediment-water and hydrothermal) along the East Pacific GEOTRACES transect

Highlights: • Wide-spread (64% of 500-m layers) ²¹⁰Po-²¹⁰Pb disequilibrium in open ocean. • For the whole water-column, ²¹⁰Po/²¹⁰Pb activity ratio >0.9 • Redistribution of ²¹⁰Po and ²¹⁰Pb in the whole water column caused by remineralization. • Coupling between scavenging of Po and Pb in upper and bottom 300 m of water column. - Abstract: The contrasting geochemical behavior of two long-lived progeny of ²²²Rn (daughter product of ²²⁶Ra), ²¹⁰Po and ²¹⁰Pb, provide valuable insights on the extent of recycling of biogenic particulate matter and their preferential removal from the water column. We collected and analyzed 135 water samples from six vertical profiles for ²¹⁰Po and ²¹⁰Pb as well as a suite of aerosol samples for ²¹⁰Pb along the US GEOTRACES East Pacific Zonal Transect (GP16). Calculated atmospheric depositional flux of ²¹⁰Pb from the measured ²¹⁰Pb activity in aerosols exhibits an overall decrease from east to west along the cruise track. The inventories of ²¹⁰Po and ²¹⁰Pb and residence times in key zones that include biotic (upper 60 and 300 m), sediment-water (bottom 300 m) and volcanogenic (bottom 300 m at the East Pacific Rise (EPR) station) were calculated and compared. A comparison of inventories of ²¹⁰Po and ²¹⁰Pb in 500-m layers (maximum penetration depth of atmospherically-delivered ²¹⁰Pb was reported to be upper 210Po and total ²¹⁰Pb, with ²¹⁰Po/²¹⁰Pb activity ratios less than 0.95 or greater than 1.05, in 63% (26 out of 41) of the 500-m layers. The observed large variations in the ²¹⁰Po/²¹⁰Pb activity ratio (AR) in 500 m layers (range: 0.79–1.32) of the whole water column is by far the largest disequilibrium in open ocean water reported when integrated over 500 m depths. However, the whole water-column ²¹⁰Po/²¹⁰Pb AR, calculated from the inventory of total ²¹⁰Po and total ²¹⁰Pb, varied between 0.97 ± 0.02 and 1.06 ± 0.02 (except in the EPR site, with 1.09 ± 0.02), indicating at or near-equilibrium for the whole water column in all five deep water stations. This contrasts with earlier claims of gross disequilibrium between ²¹⁰Po and ²¹⁰Pb in water depths >1000 m and we contend that most of these observations do not take in to account the variable extent of redistribution of ²¹⁰Po and ²¹⁰Pb in the water column due to remineralization of biogenic particulate matter and preferential scavenging of ²¹⁰Po over ²¹⁰Pb. We observed intense scavenging of ²¹⁰Po and ²¹⁰Pb in the bottom 300 m of the EPR site (attributed to discharge of high amounts of dissolved and colloidal Fe and Mn from the hydrothermal vent) and lower scavenging in the upper and bottom 300 m at the most oligotrophic station (ST-36), indicating that the scavenging intensities in bottom waters are likely coupled to the surface waters.