Award: PLR-1434578

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. Most of the Pb-210 in the surface ocean are derived from atmospheric deposition of Pb-210 which is derived from the decay of Rn-222. Since the average residence time of water vapor in the atmosphere is ~ 10 days, generally the radon progeny is also expected to have comparable residence time. When there are no anthropogenic (e.g. fossil fuel combustion, use of phosphate) or natural (e.g. volcanic eruption, forest fire) sources of polonium, and all the atmospheric polonium is derived from atmospheric Pb-210, then, the Po-210/Pb-210 activity ratio (Po/Pb AR) is expected to be about 0.1 or less. The initial Po/Pb AR in a given rain/snow precipitation is similar to that of atmospheric aerosols at that site and the in-growth of Po-210 from the decay of Pb-210 with time in snow, provides time elapsed since the incorporation of Pb-210 into the snow. During storm events in early autumn, atmospherically-delivered Po-210 and Pb-210 in the coastal Arctic, with Po/Pb AR of < 0.1, are scavenged by finer resuspended sediments and eventually are incorporated into coastal sea ice which are subsequently transported to the interior Arctic. The in-growth of 210Po increases with time and the disequilibrium between Po-210 and Pb-210 in the ice-rafted sediment is utilized to determine the time elapsed since its incorporation (?age?). Melt water from ice and snow in conjunction with a fractional amount of surface seawater are the sources of water for the melt ponds. From an independent determination of the fractional amounts of water from these three sources and knowing the Po-210 and Pb-210 activities in these three fractions, the ?age? of the melt pond can be determined. We collected and analyzed for particulate and dissolved phases of Po-210 and Pb-210 from a total of 43 water samples (2 super stations), 14 different aerosol samples, 6 ice cores (31 split sections), 5 melt pond water samples, 6 snow samples, 6 under ice samples, 12 ice-rafted sediments and 2 surface water samples for inter-calibration. Based on our results, we report the following: i) from the measured Po-210 and Pb-210 activities in aerosols, collected from 13 from different stations covering the entire 10 weeks Western Arctic GEOTRACES cruise track from Dutch Harbor, AK to the North Pole, we show that the mean Po/Pb AR of 0.039?0.026 (n=13) which corresponds to a residence time of 12.9?7.0 days; ii) from the measured Po/Pb AR in snow and using the mean AR in aerosol as the initial AR in snow, the calculated snow age for 6 different ice stations varied between 1.7 and 34 days (mean: 13 days); iii) the age of 5 different melt ponds, based on the measured Po/Pb AR and three end-member mixing model, ranged between 18 and 79 days; iv) the measured activities of Po-210 and Pb-210 in melt-ponds is about 10 times higher compared to the surface seawater in the Arctic Ocean; v) The transport velocity of the ice-rafted sediment in the shelf and interior Arctic Ocean, estimated based on the calculated age using Po/Pb AR, 0.08 and 0.17 m/s agree with the data obtained using buoys and Acoustic Doppler current profiler; vi) we report highly elevated levels of 210Po in biogenic particulate matter in snow and melt pond compared to 210Pb indicating biogeochemical cycling of biogenic elements such as polonium is different in the Arctic; vii) analysis of the water column samples from two super stations (ST-30 and 43) indicate gross disequilibrium between total 210Po and 210Pb throughout the water column in the central Arctic basin suggesting the preferential removal of 210Po taking place in the water column. This is the first observation reporting Po-210 deficiency for the whole column which we did not find in any of the superstations in the North Atlantic or East Pacific GEOTRACES cruises; and viii) The residence time of 210Pb in the Makarov Basin seems to be the longest compared to all other published deep water stations from the Arctic Ocean. The implication of this study for the future is that the routine identification of multi-year (those that survived more than one melt cycle) ice is now possible using Po/Pb AR as a metric. Furthermore, long-term longitudinal study on the annual variations in the age of snow and melt pond in a particular region caused by climate change in the Arctic may enable us to quantify the radiation balance changes caused by faster melting of snow and later freeze. Last Modified: 02/01/2020 Submitted by: Mark Baskaran