|Actinium-227 als Tracer für Advektion und Mischung in der Tiefsee = Actinium-227 as a tracer for advection and mixing in the Deep-Sea|
Geibert, W. (2001). Actinium-227 als Tracer für Advektion und Mischung in der Tiefsee = Actinium-227 as a tracer for advection and mixing in the Deep-Sea. Ber. Polarforsch. Meeresforsch. 385: 1-112
In: Berichte zur Polar- und Meeresforschung = Reports on Polar and Marine Research. Alfred-Wegener-Institut für Polar- und Meeresforschung: Bremerhaven. ISSN 1618-3193, more
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|Document type: Project report|
The natural radionuclide Actinium-227 (227 Ac, half-life 21.77 years) is presented in this work as a tracer for mixing and advection of deep-sea water masses. Vertical profiles of 227 Ac were determined at 9 locations via a counting of its daughter 227Th. Previous to this work, three vertical profiles of 227 Ac existed, all of them in the Northern Pacific. Here I report 227 Ac data from the Central Arctic, the Southeast Pacific, the Antarctic Circumpolar Current (ACC) and the Weddell Gyre. For the first time, ratios of particulate and dissolved 227 Ac in the water column were obtained. Not more than 1.7% of 227 Ac were found on particles. 227 Ac was found to be depleted on particles with respect to its mother 231Pa. Only 11% of the 231pa activity were observed on particles. In the Southeast Pacific, high 227 Ac ex activities were found close to the seafloor. Due to its decay, no 227 Ac ex was found far above the seafloor. This finding is in agreement with earlier results. In contrast, for the Antarctic Circumpolar Current and the Weddell Gyre 227 Ac ex could be shown to reach the surface via mixing and advection. A location in the Central Arctic with a strong influence of shelfwater near the sea surface (ca. 100 dpm/m³ 228Ra) had only 0,17± 0.07 dpm/m³ 227 Acex. This indicates that shelf sediments are a minor source for 227 Acex. This observation is confirmed by model results. A model for the release of 227 Ac and 228Ra from sediments at different water depths is applied. The results indicate that deep-sea sediments release at least nine times more 227 Ac than shelf sediments. The fluxes of 227 Ac and 228Ra strongly depend on bioturbation rates. A comparison of model results and data revealed that inputs from deep-sea sediments and shelf sediments are similarly overestimated by the model. Based on water column measurements the total amount of 227 Acex in the ocean is estimated ta be 12.6 kg. Considering its source at the deep-sea floor, its half-life of 21.77 years, and its low particle reactivity, 227 Acex is an ideal tracer for water masses that were in contact with deep-sea sediments. Four possible applications of 227 Acex are presented. 1, The determination of coefficients for diapycnal mixing 2. The identification of inputs of nutrients and trace elements to the euphotic zone of the ACC by deep upwelling 3. The simultaneous determination of diapycnal mixing, isopycnal mixing and advection in global circulation models.4. The determination of upwelling velocities in the Weddel Gyre.For the eastern Weddell Gyre, the upwelling velocity is calculated to be about 55 m/year. This value is somewhat higher than earlier calculations for the entire Weddell Gyre. The possible influence of upwelling on particle preservation is investigated by a simplified model for particle sedimentation. The results indicate that upwelling can enhance particle dissolution in the water column of the Weddell Gyre in conjunction with other factors, e.g. small particle sizes. Altogether, 227 Acex turns out to be a promising tracer for several questions in marine geosciences.