Abstract
We collected surface ocean water, snow, grab ice, ice core, melt pond and ice-rafted sediment (IRS) from 5 ice stations during the Western Arctic US GEOTRACES cruise (USGCG Healy; August 10 – October 7, 2015) and analyzed for 210Po (T1/2 = 138.4 days) and 210Pb (T1/2 = 22.3 years) in dissolved and particulate phases (snow, grab ice, ice core, surface seawater) to investigate the 210Po:210Pb disequilibria in these matrices. Thirteen aerosol samples, using a large-volume aerosol sampler (PM10), from Dutch Harbor, AK to North Pole, were also collected and analyzed for 210Po/210Pb to quantify the atmospheric depositional input to the snow and surface waters. Falling snowfall is tagged with 210Po/210Pb ratio (AR) similar to that in the air column from the cloud condensation height to air-sea interface. From the measured AR in aerosol and snow, modeling the sources of 210Po and 210Pb input to the melt pond, and measured disequilibrium in ice core and ice-rafted sediment, we show 210Po/210Pb AR is a novel chronometer to date snow, ice core, melt pond, and IRS. The calculated mean ages of aerosol, snow, melt pond and IRS are 12 ± 7 (n = 13), 13 ± 11 (n = 6), 60 ± 14 (n = 4), and 87 ± 23 (n = 6) days, respectively. The average IRS age corresponds to an average drift velocity of sediment-laden ice of 0.18 ± 0.06 (n = 6) m s–1. We report highly elevated levels of 210Po and 210Pb in snow and melt pond compared to those in Arctic surface seawater and enrichment of 210Po compared to 210Pb onto particles extracted from snow, ice and melt ponds. The observed disequilibrium between 210Po and 210Pb in ice could serve as a quantitative tool in delineating multiple-year ice from seasonal ice as well as a metric in quantifying the speed of ice/snow melting and delay in autumn freeze.
Highlights
The Arctic is undergoing drastic environmental change which has manifested in decrease in the areal extent of sea ice cover, from 6.95 million km2 in 1980 to 3.95 million km2 in 2015, ∼45% decrease which is attributed to increase in sea surface as well as surface air temperature
For the first time, 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
We report one to two orders of magnitude higher 210Pb and 210Po activities in ice-rafted sediments (IRS) compared to benthic source sediments which indicate that the sea ice-sediment (i.e., IRS) is a powerful vector in the transport of land- and atmospherically delivered particlereactive contaminants from the coastal area to the deep Arctic
Summary
The Arctic is undergoing drastic environmental change which has manifested in decrease in the areal extent of sea ice cover, from 6.95 million km in 1980 to 3.95 million km in 2015, ∼45% decrease which is attributed to increase in sea surface as well as surface air temperature. Earlier melt of sea ice and later freeze in the Arctic shelves lead to a longer open-water season which has. Ice, Meltpond Dating impacted the biogeochemical cycling of key trace elements and isotopes due to wind-driven vertical mixing (Kipp et al, 2018; Rutgers van der Loeff et al, 2018; Grenier et al, 2019). Early retreat of sea ice edge is expected to result in higher wave action which in turn, expected to affect the amount of energy transferred from wind to surface water. Ice-free shelf waters are anticipated to result in higher wind-driven upwelling which in turn is expected to increase the amount of deeper waters onto the shelf (Carmack and Chapman, 2003). The residence time of snow, ice, and melt ponds is of great interest and has bearing on the changes in the biogeochemical cycling.
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