Shelf-basin exchange times of Arctic surface waters estimated from 228Th/228Ra disequilibrium

The transpolar drift is strongly enriched in Ra-228 accumulated on the wide Arctic shelves with subsequent rapid offshore transport. We present new data of Polarstern expeditions to the central Arctic and to the Kara and Laptev seas. Because Ra-226 activities in Pacific waters are 30% higher than in Atlantic waters, we correct Ra-226 for the Pacific admixture when normalizing Ra-228 with Ra-226. The use of Ra-228 decay as age marker critically depends on the constancy in space and time of the source activity, a condition that has not yet adequately been tested. While Ra-228 decays during transit over the central basin, ingrowth of Th-228 could provide an alternative age marker. The high Th-228/Ra-228 activity ratio (AR = 0.8-1.0) in the central basins is incompatible with a mixing model based on horizontal eddy diffusion. An advective model predicts that Th-228 grows to an equilibrium AR, the value of which depends on the scavenging regime. The low AR over the Lomonosov Ridge (AR = 0.5) can be due to either rapid transport (minimum age without scavenging 1.1 year) or enhanced scavenging. Suspended particulate matter load (derived from beam transmission and particulate Th-234) and total Th-234 depletion data show that scavenging, although extremely low in the central Arctic, is enhanced over the Lomonosov Ridge, making an age of 3 years more likely. The combined data of Ra-2286 decay and Th-228 ingrowth confirm the existence of a recirculating gyre in the surface water of the eastern Eurasian Basin with a river water residence time of at least 3 years.