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Radium-226 and Radium-228 in the Atlantic Sector of the Southern Ocean

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Citation:
Hanfland, C. (2002): Radium-226 and Radium-228 in the Atlantic Sector of the Southern Ocean , PhD thesis, Universität Bremen.
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Abstract:

This study investigates the distribution and the biogeochemical behaviour of 226Ra and 228Ra (half-lifes 1600 and 5.75 years, respectively) in the Atlantic sector of the Southern Ocean. Both are important tracers in oceanographic issues on time-scales from months to years. 226Ra with a deep-sea source has been suggested as a tracer for ocean mixing processes. 228Ra gets enriched in shallow water regions and represents a suitable tracer for advection of shelfwater into the open ocean. In the context of iron as a growth-limiting factor for primary productivity in the Southern Ocean, 228Ra is used in this study to investigate the role of iron input from coastal regions into the Atlantic sector. For a better understanding of the biogeochemical behaviour of radium in circumpolar waters, the distribution of 226Ra was compared with Si concentrations.During six cruises, 226Ra and 228Ra have been measured in high resolution in surface waters of the Antarctic Circumpolar Current (ACC), the Weddell Gyre, on the continental shelves and on a vertical transect across the ACC at 20° E. 226Ra and 228Ra samples with high activities were analyzed by g-spectrometry. Determination of the open ocean 228Ra activities was done by the 228Th-ingrowth method via a-spectrometry.226Ra activities approximately double from north to south across the ACC. Highest activities (up to 18 dpm/100kg) are found in the southeastern corner of the Weddell Gyre where upwelling of Circumpolar Deep Water occurs. A correlation between 226Ra and Si yields best results for surface water samples south of the Polar Front (PF) and for intermediate water masses. The correlation does not hold north of the PF, where 226Ra depletion continues when Si is already exhausted. Acantharians, SrSO4-building microzooplankton, are proposed as an important carrier phase in the marine biogeochemistry of radium.228Ra activities have been determined for the first time on both sides of the Antarctic Peninsula and on the Argentinean shelf. On the continental shelves in the Weddell Sea and along the Antarctic Peninsula, 228Ra activities range from 0.2 to 2 dpm/100kg. Highest values have been determined on the Argentinean Shelf (3.7 dpm/100kg). Associated residence times for water masses on the Antarctic and Argentinean shelves vary between 2 and 10 months. In the open ACC, 228Ra activities have a mean of 0.1 dpm/100kg but are below the detection limit within the Weddell Gyre. On two N-S-transects, extremely high 228Ra signals of 4.3 dpm/100kg occur and coincide with the approximate position of the PF. They are suggested to originate from a temporary merging or a close proximity of the Subantarctic and the Polar Front at 40° W. Increased activities in the Argentine Basin and south of Africa could be related to the Brazil and the Agulhas Current, respectively. Satellite altimetry enabled the correlation of 228Ra signals with cyclonic and anticyclonic eddies spawned from the Agulhas Retroflection Area. The subtropical intrusions containing 228Ra enriched water could be traced as far as 45° S.Vertical 228Th/230Th AR have been determined in the upper 1000 m along 20° E. It could be shown that the 228Th/230Th AR can be used as a qualitative indicator of 228Ra activities. The depth profiles showed that 228Ra enrichment is limitated to a shallow surface layer north of the PF.The distribution of 228Ra was set in context to the distribution of tracers for iron input via terrigenous input (Al and the isotopic composition of neodymium) and deep upwelling (227Ac) that had been determined in associated studies. Input of shelfwater seems to occur rather sporadically in restricted areas associated with the PF and the Southern ACC Boundary as elevated 228Ra are not a regular feature of the frontal jets in the ACC.A compilation of world-wide data from produced waters released during oil and gas exploitation reveals extremely high activities of 226Ra and 228Ra in these effluents. Several large hydrocarbon fields in operation are located in the influence of the Falkland or the Brazil Current on the continental shelf or slope of South America. It must be assumed that these discharges are subject to the same transport processes as 228Ra released from shelf sediments and may complicate the distinguishment of the two sources.

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