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

T. J. Shaw; K. L. Smith; C. R. Hexel; Rebekkah Dudgeon; Alana D. Sherman; M. Vernet; R. S. Kaufmann

doi:10.1016/j.dsr2.2010.11.019

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

T. J. Shaw, K. L. Smith, C. R. Hexel, Rebekkah Dudgeon, Alana D. Sherman, M. Vernet, R. S. Kaufmann

Research output: Contribution to journal › Article › peer-review

16 Scopus citations

Abstract

The impact of free-drifting icebergs on the efficiency of carbon export from the upper water column was measured using the disequilibrium of ²³⁴Th and its parent ²³⁸U. The study addressed the null hypothesis that free-drifting icebergs do not alter ²³⁴Th deficiency and carbon export compared to surrounding waters. Upper-water-column inventories of ²³⁴Th 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 0.3km to <20km of the edge of a large free-drifting iceberg (C-18a) and two were sampled at distances >60km from C-18a. Temperature and salinity anomalies indicated enhanced upwelling and turbulent mixing extending downstream of the iceberg to a minimum of ~20km 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 ²³⁴Th were measured in close proximity and downstream of the iceberg and extending to below 100m depth. A steady-state model was used to estimate the export of ²³⁴Th 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 20km of the iceberg) and far-field (greater than 60km) 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 600m indicate that ~90 percent of the exported material is regenerated by 600m 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.

Original language	English
Pages (from-to)	1384-1391
Number of pages	8
Journal	Deep-Sea Research Part II: Topical Studies in Oceanography
Volume	58
Issue number	11-12
DOIs	https://doi.org/10.1016/j.dsr2.2010.11.019
State	Published - Jun 2011
Externally published	Yes

Funding

We are grateful to personnel from Raytheon Polar Services and Edison Chouest Offshore, as well as the captain and crew of the RVIB Nathaniel B. Palmer for their support over the course of this study. Dr. Claudia Benitez-Nelson provided valuable support during the setup, calibration, and operation of the Risø counters as well as numerous discussions during the preparation of this manuscript. Dr. Justina Burns and the Department of Chemistry and Biochemistry Mass Spectrometry Facility at the University of South Carolina provided support for the analysis of samples. Gordon Stephenson and Dr. John Helly provided support for interpretation and presentation of the physical data and mapping. Two anonymous reviewers contributed a number of useful suggestions and comments on the manuscript. Scott Kindelberger provided assistance in the collection and processing of CTD samples. This research was supported by NSF grant OPP-0636319 and the David and Lucile Packard Foundation .

Funders	Funder number
National Science Foundation	OPP-0636319
David and Lucile Packard Foundation

Keywords

Atmospheric carbon drawdown
Carbon export efficiency
Carbon sequestration
Iron fertilization

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10.1016/j.dsr2.2010.11.019

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title = "234Th-Based Carbon Export around Free-Drifting Icebergs in the Southern Ocean",

abstract = "The impact of free-drifting icebergs on the efficiency of carbon export from the upper water column was measured using the disequilibrium of 234Th and its parent 238U. The study addressed the null hypothesis that free-drifting icebergs do not alter 234Th deficiency and carbon export compared to surrounding waters. Upper-water-column inventories of 234Th 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 0.3km to <20km of the edge of a large free-drifting iceberg (C-18a) and two were sampled at distances >60km from C-18a. Temperature and salinity anomalies indicated enhanced upwelling and turbulent mixing extending downstream of the iceberg to a minimum of ~20km 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 234Th were measured in close proximity and downstream of the iceberg and extending to below 100m depth. A steady-state model was used to estimate the export of 234Th 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 20km of the iceberg) and far-field (greater than 60km) 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 600m indicate that ~90 percent of the exported material is regenerated by 600m 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.",

keywords = "Atmospheric carbon drawdown, Carbon export efficiency, Carbon sequestration, Iron fertilization",

author = "Shaw, {T. J.} and Smith, {K. L.} and Hexel, {C. R.} and Rebekkah Dudgeon and Sherman, {Alana D.} and M. Vernet and Kaufmann, {R. S.}",

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AU - Shaw, T. J.

AU - Smith, K. L.

AU - Hexel, C. R.

AU - Dudgeon, Rebekkah

AU - Sherman, Alana D.

AU - Vernet, M.

AU - Kaufmann, R. S.

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AB - The impact of free-drifting icebergs on the efficiency of carbon export from the upper water column was measured using the disequilibrium of 234Th and its parent 238U. The study addressed the null hypothesis that free-drifting icebergs do not alter 234Th deficiency and carbon export compared to surrounding waters. Upper-water-column inventories of 234Th 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 0.3km to <20km of the edge of a large free-drifting iceberg (C-18a) and two were sampled at distances >60km from C-18a. Temperature and salinity anomalies indicated enhanced upwelling and turbulent mixing extending downstream of the iceberg to a minimum of ~20km 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 234Th were measured in close proximity and downstream of the iceberg and extending to below 100m depth. A steady-state model was used to estimate the export of 234Th 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 20km of the iceberg) and far-field (greater than 60km) 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 600m indicate that ~90 percent of the exported material is regenerated by 600m 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.

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

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