Trace element cycling in the ocean plays an important role on marine ecosystem functioning and the carbon cycle. Pb-210 and Po-210 are the progeny of Rn-222 in the U-238-decay chain and has contrasting biogeochemical behavior in marine system. Po-210 preferentially bioaccumulates within organic tissues, in addition to the removal by terrigenous and biogenic particulate matter. Pb-210 is predominantly removed by terrigenous particulate matter. Their overall geochemical behavior in marine system is relevant to export of particulate organic carbon and other key micro-nutrient elements from the upper ~200 m, export fluxes of sinking particulate matter, sediment mixing and deposition rates. As a part of our group (Gillian Stewart, Queens College – particulate phase both 0.8-51 and >51 um), we measured particulate and dissolved activities of Po-210 and Pb-210. The primary objective is to improve our understanding of processes that control the distribution of Po-210 and Pb-210 nuclide cycles in the East Pacific and its interfaces. We collected and analyzed 135 seawater samples from 6 super stations from east to west along a transect from Peru to Tahiti, covering a distance of over 8,400 km. The contrasting characteristics in these six stations include: most productive shelf waters off of Peru where upwelling is one of the highest, and water with a strong oxygen minimum zone, East Pacific Rise (EPR) hydrothermal plume and most oligotrophic waters. The uniqueness of this program is that we have access to a large amount of ancillary data (e.g. concentrations of pigments, nutrients, key micro-nutrient trace metals, suspended particle concentration, Ra-226, stable Pb, etc) to interpret our data in light of other key parameters. Based on our results, we report the following: i) There is a wide-spread (64 % of all 500-m integrated layers) disequilibrium between 210Po and 210Pb in the whole water column that we have analyzed. While radioactive disequilibria between these two nuclides were not uncommon (from earlier published results) in discrete depths, disequilibrium at 500-m integrated layer indicates varying extents of preferential removal of 210Po and remineralization of 210Po-enriched biogenic particulate matter affecting the 210PoT/210PbT activity ratio. However, 210PoT/210PbT ratio over the whole water column varied between 0.97 and 1.09 (3% deficiency to 9% excess of 210Po), due to preferential removal of 210Po or more extensive remineralization of 210Po in the water column; ii) From a comparison of the inventories of 210Po and 210Pb and the nutrients in the upper and bottom 300 m of the biologically most productive (ST-1) and least productive waters, we propose that the whole water column activity ratio of (210Po/210Pb)T can be utilized as a metric for the quantification of biological productivity of a given station; iii) The residence time of 210Pb in 500-m layers vary widely. Overall, the residence time of 210Pb is much longer (71 y to >600 y) in the most oligotrophic station (ST-36) compared to previously reported values for the deep ocean, but the residence time at 2,500-3,000 m at ST-26 is the lowest, 14 y. There is an overall decrease in residence time from 1000 m to the sediment-water interface in the most productive margin waters off of Peru; iv) The atmospheric depositional flux calculated using 210Pb concentration in aerosols decreases from the station off of Peru to the farthest oceanic station, indicating the contrasting differences in the sources of air masses as a source of atmospheric 210Pb at the study sites; v) There is strong correlation between inventories of total 210Pb and particulate Al, but there is no correlation between 210Po and particulate Al, suggesting much less removal of 210Po by lithogenic material compared to 210Pb; vi) There is an intense scavenging of 210Po in the hydrothermal vent waters compared to the most oligotrophic waters, resulting in the lowest residence time of 210Po in the water column. A 35% excess of 210Po over 210Pb at 1,000-1,500 m indicates much active remineralization in that layer. The whole water column scavenging of 210Po and 210Pb in the EPR site is affected by the Fe and Mn released from the vent which is transported far away distances, both horizontally and vertically; and vii) There is a strong coupling between the upper and bottom 300 m in terms of scavenging intensities, as observed both in the EPR and most oligotrophic stations. Last Modified: 01/07/2017 Submitted by: Mark Baskaran
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