Magnesium Isotope Geochemistry of Carbonate Sediments

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Funds are provided to reconstruct the δ26Mg of seawater by measuring it the δ26Mg of pelagic carbonates across the Cenozoic. PI's preliminary results show similar patterns in δ26Mg at sites in both the Pacific and Atlantic Ocean sites. However, some of the observed changes in δ26Mg are too fast and too large to be compatible with the current understanding of the geochemical magnesium cycle. To refine understanding of magnesium isotope variability in pelagic carbonates the PI will also make magnesium isotope measurements on individual species of planktonic foraminifera and other size fractions of the bulk carbonate sediment as well as in pore fluids. These measurements will provide important new constraints on the potential effects of diagenesis. The PI will also pursue alternative approaches to reconstructing the δ26Mg of seawater using fluid inclusions in evaporite minerals. The work will help elucidate the processes that control the major element chemistry of seawater, and therefore the global carbon cycle and climate, on geologic timescales. The research has the potential to make a broad impact on our understanding of the chemistry of the ocean, particularly with respect to the geochemical cycles of magnesium, calcium, and carbon. In addition, the proposed activities may help elucidate a better understanding of the causes of climate change over the Cenozoic by better constraining factors that affect the carbon cycle over this timescale. The research will support the training of a graduate student who will be exposed to a range of approaches including mass spectrometry and numerical modeling.