Radium Isotopes Across the Arctic Ocean Show Time Scales of Water Mass Ventilation and Increasing Shelf Inputs

The first full transarctic section of Ra-228 in surface waters measured during GEOTRACES cruises PS94 and HLY1502 (2015) shows a consistent distribution with maximum activities in the transpolar drift. Activities in the central Arctic have increased from 2007 through 2011 to 2015. The increased Ra-228 input is attributed to stronger wave action on shelves resulting from a longer ice-free season. A concomitant decrease in the Th-228/Ra-228 ratio likely results from more rapid transit of surface waters depleted in Th-228 by scavenging over the shelf. The Ra-228 activities observed in intermediate waters (<1,500 m) in the Amundsen Basin are explained by ventilation with shelf water on a time scale of about 15-18 years, in good agreement with estimates based on SF6 and I-129/U-236. The Th-228 excess below the mixed layer up to 1,500m depth can complement Th-234 and Po-210 as tracers of export production, after correction for the inherent excess resulting from the similarity of Ra-228 and Th-228 decay times. We show with a Th/Ra profile model that the Th-228/Ra-228 ratio below 1,500m is inappropriate for this purpose because it is a delicate balance between horizontal supply of Ra-228 and vertical flux of particulate Th-228. The accumulation of Ra-226 in the deep Makarov Basin is not associated with an accumulation of Ba and can therefore be attributed to supply from decay of Th-230 in the bottom sediment. We estimate a ventilation time of 480 years for the deep Makarov-Canada Basin, in good agreement with previous estimates using other tracers.