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

• Published in: Journal of Geophysical Research: Oceans Vol. 117; no. C3
• Main Authors: Rutgers van der Loeff, M., Cai, P., Stimac, I., Bauch, D., Hanfland, C., Roeske, T., Moran, S. B.
• Format: Journal Article
• Language: English
• Published: 01-03-2012
• ISSN: 0148-0227
• DOI: 10.1029/2011JC007478

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.
Key Points
228Ra on Arctic shelves may not be sufficiently constant to serve as age marker
228Th ingrowth provides independent age information
High 228Th/228Ra age infers a recirculating gyre above the Gakkel Ridge