Dissolved Barium and particulate Rare Earth Elements as tracers for shelf-basin interaction in the Arctic Ocean

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Roeske, T. (2011): Dissolved Barium and particulate Rare Earth Elements as tracers for shelf-basin interaction in the Arctic Ocean , PhD thesis, Alfred-Wegener Institute for Polar and Marine Research.
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This study gives an outlook on the usability of dissolved Ba and particulate REE as oceanographical tracers in the Arctic Ocean. From Polarstern expedition ARK-XXII/2 in summer 2007, dissolved Ba and particulate rare earth elements (REE) were analyzed in the Barents, Kara, Laptev seas, and the Eurasian Basins as well as the Makarov Basin up to and beyond the Alpha and Mendeleyev Ridges. Data on particulate REE from the Arctic Ocean are discussed in a wider context including samples from the East Atlantic and the Atlantic sector of the Southern Ocean. In chapter 2 I discuss Ba as a tracer in discerning Eurasian from North American runoff together with well established water mass tracers (salinity, δ18O, nutrients). Prolonged ice-free conditions promote phytoplankton growth and thereby are influencing microbiogeochemical cycling of trace elements in the water column. Changing climate is expected to have an impact on trace element distributions in the Arctic Ocean. It has to be questioned to what extent tracers like Ba involved in biological cycles can be used in the future dependent on those changes. In the light of this question, it is shown in chapter 2 that in the Arctic Ocean Ba is involved in microbiogeochemical cycling. Ba is removed from surface waters and released to deep waters. This process masks the presence of North American runoff in surface waters and leads to an overestimation of these waters in sub-surface layers of the Makarov Basin. In chapter 3 I investigate how dissolved Ba of intermediate and deep waters may be used together with Al and silicate to describe exchange of waters from the shelf seas with the interior basins. The vast Arctic shelves are the main source of these elements to the central Arctic Ocean. Distribution of these elements indicates deep shelf convection into the Nansen Basin north of Severnaya Zemlya. Moreover in the Makarov Basin, inflow from the Canadian Basin and overflow from the Amundsen Basin at 2000 m depth at the Lomonosov Ridge are required to explain the composition of bottom waters. This supports previous studies on the renewal of bottom waters in the Arctic basins. Furthermore, in chapter 4 I describe REE patterns in suspended particulate matter (SPM) from surface water. REE patterns are supposed to help understanding the transport ways of particles to the water column. Input of terrigenous particles plays an important role in contributing (micro-)nutrients to the oceans and in this way is controlling biological productivity. A unique set of particulate REE from different oceanographic regimes between 88°N and 72°S has been sampled following the same sampling protocol so that comparison of REE patterns becomes possible. In the East Atlantic Ocean a clear transition can be seen from strong inputs from Saharan dust in the north of the intertropical convergence zone (ITCZ) to low terrigenous supply in the south. In the Southern Ocean, a much stronger impact of exchange with seawater can be seen. This illustrates the contrast between the remote Southern Ocean where REE patterns are dominated by adsorption from seawater and the nearly land locked Arctic Ocean where REE patterns are fully terrigenous. The homogeneity of REE patterns in SPM from the Arctic Ocean leads to the conclusion that sources other than continental runoff, i.e., dissolved REE from seawater or REE resuspended from shelf sediments, are of minor importance regarding surface water SPM.

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