Open AccessShelf‐basin exchange times of Arctic surface waters estimated from 228 Th/ 228 Ra disequilibrium
Author(s) -
Rutgers van der Loeff M.,
Cai P.,
Stimac I.,
Bauch D.,
Hanfland C.,
Roeske T.,
Moran S. B.
Publication year - 2012
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2011jc007478
Subject(s) - scavenging , ocean gyre , geotraces , arctic , ridge , geology , oceanography , advection , radiogenic nuclide , atmospheric sciences , seawater , physics , chemistry , geophysics , paleontology , subtropics , biology , biochemistry , fishery , mantle (geology) , antioxidant , thermodynamics
The transpolar drift is strongly enriched in 228 Ra 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 226 Ra activities in Pacific waters are 30% higher than in Atlantic waters, we correct 226 Ra for the Pacific admixture when normalizing 228 Ra with 226 Ra. The use of 228 Ra 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 228 Ra decays during transit over the central basin, ingrowth of 228 Th could provide an alternative age marker. The high 228 Th/ 228 Ra 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 228 Th 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 234 Th) and total 234 Th 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 228 Ra decay and 228 Th 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.