Long-term variability of ocean chemistry: Implications for deep-time palaeothermometry and the carbon cycle
David Evans, Wolfgang Müller, Jens Fiebig, Silke Voigt (Goethe University Frankfurt), Laura Cotton (Bristol University), James Rae (University of St Andrews)
PhD student in Palaeoclimatology
The major ion and isotopic composition of seawater is known to have undergone large secular variations over geological time [e.g. Horita et al., 2002; Evans et al., 2018]. A thorough understanding of these past changes is important for two reasons: i) A precise and accurate knowledge of seawater chemistry is a prerequisite of producing informative palaeoclimate reconstructions from trace element incorporation in biogenic carbonate minerals, and ii) Ocean chemistry is in itself an archive of the factors that drive climatic change, especially temperature-controlled weathering rates, the efficiency of biological and inorganic carbonate and silicate mineralisation and burial, and the interaction of seawater with ocean crust [e.g. Dunlea et al., 2017]. Therefore, understanding secular variations in seawater chemistry will not only directly result in more accurate palaeoclimate reconstructions, but would also facilitate an understanding of what drives natural climate change over geologic time.
However, only sparse reconstructions of seawater chemistry are available for limited intervals, for example, data regarding the ionic composition of seawater are lacking for large portions of the last 100 million years. As such, we lack a good understanding of the degree to which there is a feedback between seawater chemistry, (reverse) weathering, and climate.
This project will use novel geochemical methods to produce new, precise reconstructions for the Cenozoic and early Paleogene; e.g. coupled clumped isotopes and Mg/Ca ratios in foraminifera, the Na/Ca ratio of marine calcites, and Li, Mg, and/or K isotope analysis. These methods have recently been demonstrated to be an extremely promising tool in reconstructing changes in (e.g.) weathering, fluid-rock interaction [e.g. Ramos et al., 2018; Evans et al., 2018], but are yet to be widely applied to fossil samples in order to produce a continuous record of seawater chemistry over the Cretaceous-Paleogene. This project will focus on the application of these techniques to a suite of exceptionally-preserved Cretaceous and early-mid Paleogene shallow-dwelling large benthic foraminifera (LBF) from comparative NW European (Paris, Belgium and Hampshire Basins) and tropical sites (Kutch, India and Java, Indonesia).