Recent Circulation Changes at Intermediate Depths (Upper Polar Deep Water) in the Beaufort Gyre Inferred from Water Column Distribution of 230Th

Surface water circulation in the Arctic Ocean is known to vary with time in response to the Arctic Oscillation, and recent changes in circulation have been documented in the freshwater budget (Rabe et al., 2011) and in the penetration of 129I (Karcher et al., 2012). In the Atlantic Water Layer, the penetration of a warming anomaly, first observed 1990 in Fram Strait (Quadfasel et al., 1991) has been well documented. Much less is known of circulation changes at greater depths. We compare new 230Th data, including International Polar Year - GEOTRACES data, collected in 2007 in the Eurasian and Makarov Basins up to the Alpha Ridge, and in 2007-2009 in the southern Canada Basin, with previous data from the Arctic Basins. We observe lower 230Th activities in mid-depth waters (1000-2250m) at the Alpha Ridge compared to data from 1983, and increasing 230Th activities penetrating southward in the Beaufort Sea over the period 2007-2009 in the upper 1500 m, which is coincident with the arrival of the Atlantic temperature anomaly. We propose that the low-particle flux and high-230Th isolated water mass in the Alpha Ridge region described by Bacon et al. (1989) has been ventilated, resulting in decreased 230Th activities in Upper Polar Deep Water in this region in 2007, and included in an anti-cyclonic circulation that transported it to the southern region of the Beaufort Sea. Bacon, M.P, et al., 1989. Vertical profiles of some natural radionuclides over the Alpha Ridge, Arctic Ocean. Earth Planet. Sci. Lett. 95, 15-22. Karcher, M.J, et al., 2012. Recent changes of Arctic Ocean circulation revealed by 129 Iodine observations and modelling. Journal of Geophysical Research 117, C08007. Quadfasel, D., et al., 1991. Warming in the arctic. Nature 350, 385. Rabe, B., et al., 2011. An assessment of Arctic Ocean freshwater content changes from the 1990s to the 2006-2008 period. Deep Sea Research Part I: Oceanographic Research Papers 58, 173-185.