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Shelfwater input as a possible transport path for iron into the Atlantic Sector of the Southern Ocean: Indications from Ra-228 and Th-228

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Hanfland, C. , Walter, H. J. and Rutgers v. d. Loeff, M. (2000): Shelfwater input as a possible transport path for iron into the Atlantic Sector of the Southern Ocean: Indications from Ra-228 and Th-228 , EOS, Transactions AGU .
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Iron is known to be a growth-limiting factor for phytoplankton in the productive regions of the Atlantic Sector of the Southern Ocean. Yet there is still a debate about the exact role of iron in controlling the occurrence of phytoplankton blooms in this region. Principle iron sources are: aeolian inputs from southern South America and the Antarctic Peninsula, advective inputs from their respective continental shelves, and upwelling of deep waters.Ra-228 (half-life 5.75 years) is a well-known tracer for shelfwater input into the open ocean.Thus, any signal of Ra-228 in open ocean waters reflects a recent contact of the water mass with continental shelf sediments. Here we present first results from the 1999 cruise with RV POLARSTERN (ANT XVI/3). On a N-S-transect from Cape Town to Neumayer Base, about 50 surface water samples have been taken for the analysis of dissolved radium. Sampling was done with MnO2-coated cartridges connected to the ships sea-water supply and was run parallel to the sampling of suspended particulate matter (Walter et al., this volume). In order to get a three-dimensional picture of the radium distribution across the ACC, in situ-pumps have been deployed on eight deep stations (upper 1000m). The sampling was done in conjunction with Fe-measurements by the NIOZ group, Texel.Due to extremely low Ra-228 activities, analysis of all samples for Ra-228 will be done by the so-called Th-228 ingrowth method. However, the direct analysis of the daughter product Th-228 (half-life 1.92 years) can give an approximation of the final Ra-228 activities. First results come from on-board gamma counting of the ashed cartridges and measurement of Th-228, both indicating higher Ra-228 concentrations in surface waters close to Africa (samples taken north of 44° S) and around 49° S. Activities of Th-228 in the ACC south of the polar front are uniformly low, only increasing close to the Antarctic continent (69° S). This pattern corresponds well with a similar transect of Th-228 measured in surface water samples during ANT XV/2. We interpret the signal at 49° S as shelfwater input, probably from the Argentinian shelf, transported eastward with the fast flowing jet stream along the polar front. Higher values at 69° reflect a shelf water signal from the Antarctic continent. Together with the analysis of suspended particulate matter and dissolved iron (NIOZ group, Texel) in relation to biological data like chlorophyll a, we hope to get a better understanding to what extent shelf-derived(iron-) inputs at the polar front can be linked to enhanced productivity.

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