Sea water – shelf sediment interactions: Tracing Neodymium export and water/particle exchange with radiogenic isotopes

Sammanfattning: Ocean circulation is a key component for the Earth climate system. Driven by wind and density distribution, governed by temperature and salinity, the ocean currents are redistributing heat from low to high latitudes. To track ocean circulation and its variation through time require tracers that can be linked to the present conditions and also stored in sedimentary archives. Neodymium has the potential being a key proxy for tracing water masses and by analysing marine sediments studying the circulation in the paleocean. Due to the low Nd concentration in sea water large volume (about 5 L) needs to be pre-concentrated before analysis. In this work we present a new method of preconcentration Nd from a sea water matrix. The method uses a chelating resin, Nobias PA1 from Hitachi High-Technologies, and is tested using artificial solutions as well as natural brackish water and sea water samples. The described preconcentration method, which can be used in the field, is easy, fast and reliable for preconcentration of Nd from a sea water matrix.A key component for any tracer’s ability to be used as proxy in the sedimentary record is knowledge of the behaviour in the present system. One of the issues that must be solved before Nd can evolve as a proxy is the Nd paradox. A large flux or process that contributes to the Nd distribution in the oceans is unknown. Here the interaction between Nd in suspended particular matter (SPM), sediments and sea water is studied. The results show that there is an exchange between the leachable fractions and the dissolved phase. The most important carrier of Nd in SPM and sediment is Fe-oxyhydroxide with 50- 90 % of the leachable fraction. The exchange between particulate and dissolved Nd shifts the isotopic composition towards more radiogenic values in the estuary.