| Summary: | Global climate is strongly influenced by fluctuations in atmospheric CO₂ concentrations. Weathering of silicate rocks consumes CO₂, transports cations to the oceans, and thus plays a critical role in both seawater chemistry and climate. A major product of silicate weathering is magnesium, which in the oceans is homogeneous in both concentration and isotopic composition (δ26Mg[subscript SW] = -0.82‰). This homogeneity reflects a balance between continental weathering input by rivers (δ26MgRiver ~ -1.09‰) and groundwaters, and removal by high-temperature hydrothermal oceanic crust alteration (δ²⁶Mg[subscript SW] - δ²⁶Mg[subscript HT] ~ 0.0‰), dolomite formation, and authigenic alumino-silicate clay formation during low-temperature alteration of the oceanic crust. Since the oceanic residence time of Mg is significantly longer than the ocean mixing time, temporal variations in Mg isotopic composition of seawater (δ²⁶Mg[subscript SW]) recorded by marine calcites, such as foraminifera, can reflect a global picture of Cenozoic climate that is driven by an imbalance between the source and sink. A review of previous geochemical proxies and the terrestrial processes of magnesium are discussed in Chapter 1. In Chapter 2, we present an improved method for trace level Mg measurements in natural carbonates. The method was developed for analysis of planktonic foraminifera samples in order to generate a Mg isotopic record for Cenozoic seawater, however it can also be applied to seawater and other natural samples with a high ionic strength sample matrix. There were three major analytical challenges that needed to be overcome: 1. Maintain a precision of ±0.1‰ with a mass consumption as low as ~10 ng of Mg; 2. Maintain minimal blanks (≤0.1 ng) on Mg and other matrix elements (Na, K, Ca, etc.); 3. Purify samples to have complete separation of Mg from matrix elements with 100% recovery. We achieved this through cation-exchange column chromatography and analysis on a Multi-Collector Inductively-Coupled Plasma Mass Spectrometer ...
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