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

摘要

The contrasting geochemical behavior of two long-lived progeny of Rn-222 (daughter product of Ra-226), Po-210 and Pb-210, 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 P-210 and Pb-210 as well as a suite of aerosol samples for Pb-210 along the US GEOTRACES East Pacific Zonal Transect (GP16). Calculated atmospheric depositional flux of Pb-210 from the measured Pb-210 activity in aerosols exhibits an overall decrease from east to west along the cruise track. The inventories of Po-210 and Pb-210 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 Po-210 and Pb-210 in 500-m layers (maximum penetration depth of atmospherically-delivered Pb-210 was reported to be upper 500 m) for all stations indicates clearly discernable disequilibrium between total (= particulate + dissolved) Po-210 and total Pb-210, with Po-210/Pb-210 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 ziopo/ziopb 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 Po-210/Pb-210 AR, calculated from the inventory of total Po-210 and total Pb-210, 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 Po-210 and Pb-210 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 Po-210 and Pb-210 in the water column due to remineralization of biogenic particulate matter and preferential scavenging of Po-210 over Pb-210. We observed intense scavenging of Po-210 and Pb-210 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.