University of Oxford
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Ocean circulation is a fundamental component of the modern climate system but may have operated very differently in the geological past, especially when palaeogeography was different, climates were much warmer and sea-levels were considerably higher. During the Late Mesozoic, epeiric or epicontinental (shelf) seaways acted as important conduits connecting different ocean basis and were major sinks for organic and inorganic carbon. Therefore, these seaways likely contributed to global heat transport and carbon cycling. However, the evolution, dynamics and oceanography of these seaways is, in many cases, not well known, with little knowledge of how circulation in the seaways related to global climatic, tectonic and oceanographic events.
During the Late Cretaceous sea-levels were extremely high and a vast ‘Chalk Sea’ covered much of NW Europe, resulting in the deposition of carbonate-rich pelagic sediments, such as the English Chalk. Climate variations during the Late Cretaceous occurred over both long and short time scales, yet the dynamic responses of the Chalk Sea to these changes are poorly known. Previous work in Oxford has started to unravel aspects of such changes at single localities (e.g. Zheng et al., 2013) but a broader spatial and temporal reconstruction of ocean circulation is required to fully understand the significance of the local records.
Neodymium (Nd) has a residence time in seawater that is comparable to the mixing time of the modern ocean and, consequently, seawater Nd-isotopes are very heterogeneous, reflecting the geology of the area in which a water mass formed, particle exchange processes and local weathering inputs (e.g. Goldstein and Hemming, 2003). The spatial heterogeneity allows Nd-isotopes in seawater to act as semi-conservative tracers of different water masses. This principle can be applied to the geological record by measuring the Nd-isotopes in phosphatic fish debris (biogenic apatites, which incorporate Nd at the sea floor) and Fe-Mn oxyhydroxide coatings. Such techniques have been used to reconstruct watermass circulation and patterns during the Mesozoic and early Cenozoic, particularly in the deep sea (e.g. Robinson et al., 2010), but with relatively little work on shelf seas.
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