234Th-Based Carbon Export around Free-Drifting Icebergs in the Southern Ocean

The impact of free-drifting icebergs on the efficiency of carbon export from the upper water column was measured using the disequilibrium of Th-234 and its parent U-238. The study addressed the null hypothesis that free-drifting icebergs do not alter Th-234 deficiency and carbon export compared to surrounding waters. Upper-water-column inventories of Th-234 were measured at six stations in the Weddell Sea concurrently with four deployments of Lagrangian Sediment Traps (LSTs) during a cruise in March/April 2009. Four stations were sampled ranging from 0.3 km to <20 km of the edge of a large free-drifting iceberg (C-18a) and two were sampled at distances > 60 km from C-18a. Temperature and salinity anomalies indicated enhanced upwelling and turbulent mixing extending downstream of the iceberg to a minimum of similar to 20 km from the iceberg edge. Separate studies of the impact of C-18a on water column physical properties were used to define the extent of the iceberg's influence on surrounding waters. The largest upper-water-column deficiencies in the inventories of Th-234 were measured in close proximity and downstream of the iceberg and extending to below 100 m depth. A steady-state model was used to estimate the export of Th-234 from the upper water column. Organic carbon export was calculated using C/Th from the concurrent LST collections. Comparison of stations within the iceberg's influence (close proximity and downstream to within 20 km of the iceberg) and far-field (greater than 60 km) measurements showed a factor of 3 increase in organic carbon export near the iceberg. The factor correlated well with the results from the near- and far-field LST measurements. Differences in the magnitude of carbon export at 100 and 600 m indicate that similar to 90 percent of the exported material is regenerated by 600 m depth. This study confirms that the increased abundance of large free-drifting icebergs in the Southern Ocean can contribute to the drawdown of atmospheric CO2 through increased organic carbon export. (C) 2011 Elsevier Ltd. All rights reserved.