S. B. Moran

234Th/238U disequilibrium in the central Arctic Ocean: implications for particulate organic carbon export

Abstract 234 Th (t1/2 = 24.1 days) was used as a tracer to investigate scavenging and particle export in the upper ocean along a transect from the Chukchi Sea to the North Pole during the 1994 Arctic Ocean Section (August–September). Large volume water samples (200–6001) were collected using in situ pumps for the determination of dissolved and particulate 234 Th activity and POC and PON concentrations. The lowest total 234 Th/ 238 U activity ratio (0.14) was observed in the Chukchi Sea, consistent with enhanced scavenging in these particle-rich shelf waters. 234 Th/ 238 U disequilibrium in the interior Arctic is attributed to local biological production and the associated particle export. Estimates of the export flux of POC and PON from the upper 20–30 m were made using the 234 Th, POC and PON results. POC export ranged from 38 mmol C m −2 d −1 on the Chukchi Shelf to 0.3–7 mmol C m −2 d −1 (mean=3±2 mmol C m −2 d −1 ) within the ice-covered interior Arctic Ocean. PON export ranged from 4.8 mmol N m −2 d −1 on the Chukchi Shelf to 0.05-1.1 mmol N m −2 d −1 (mean = 0.36 ±0.39 mmol N m −2 d −1 ) in the central Arctic Ocean.

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

The transpolar drift is strongly enriched in 228Ra 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 226Ra activities in Pacific waters are 30% higher than in Atlantic waters, we correct 226Ra for the Pacific admixture when normalizing 228Ra with 226Ra. The use of 228Ra 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 228Ra decays during transit over the central basin, ingrowth of 228Th could provide an alternative age marker. The high 228Th/228Ra 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 228Th 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 234Th) and total 234Th 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 228Ra decay and 228Th 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.