Water mass mixing and silicate utilization in the Atlantic Ocean

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The central goal of this proposed study is a detailed investigation of the distribution of dissolved neodymium (Nd), hafnium (Hf) and silicon (Si) concentrations and isotope compositions in the water column of the tropical Atlantic Ocean. Radiogenic Nd and Hf isotopes are powerful geochemical tracers of water mass flow and mixing, as well as of continental inputs and weathering regimes, but there are significant uncertainties concerning the quantitative importance of input sources such as Saharan dust, rivers and exchange with shelf sediments, which currently prevent the use of these isotope systems as quantitative proxies for present and past ocean circulation. Silicon isotopes are a promising tracer for Si utilization by biological activity the surface waters, but the Si isotope distributions are also influenced by water mass mixing. For all these isotopic tracers there is a severe lack of detailed data on their distribution in critical areas of the present day Atlantic Meridional Overturning Circulation (AMOC) in the northwestern and tropical Atlantic, which severely restricts their reliable application as proxy tracers. To remove this gap, full water column profiles were sampled at high resolution and under trace metal clean conditions along the sections of expedition Meteor M81/1 (GEOTRACES Cruise A11, Canaries – Port of Spain, Trinidad and Tobago in February/March 2010) and during RV Pelagia expedition 64PE319/321 (GEOTRACES Cruise A02, Scrabster, Scotland – Fortaleza, Brasil, April - July 2010) in the tropical and western North Atlantic, which will serve as material for the proposed study. The systematic measurement of the isotope composition and concentration of Nd, Hf, and Si will for the first time characterize the factors controlling their water mass signatures and inputs in the tropical Atlantic Ocean and will significantly improve their applicability as proxy tracers for ocean circulation, weathering inputs, and silicate utilisation in the present and past ocean.