Carbonate clumped-isotopic constraints on marine temperatures during the Cretaceous

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A fundamental question we can ask about the Earth's past climate is how does surface temperature respond to external forcing? Evidence suggests that the Earth's climate was warmer and more equable during the Cretaceous Period (circa 146 to 65 million years ago) than it is today - an interpretation based on the distribution of thermophylic (cold intolerant) animals, plants and geochemical proxies.This warmth during the Cretaceous has been linked to high atmospheric CO2 concentrations. Although most evidence indicates a significantly warmer Earth, with warm polar regions (including carbonate clumped-isotope data, a new tool for reconstructing temperatures), some research has suggested a negative relationship (or decoupling) between temperature and CO2 during this period. Although the Mesozoic is not a direct analogue for future greenhouse warming such intervals in Earth history provide important insights into processes operating in the climate system. Hence the relationship between atmospheric CO2 and temperature has added significance as we try to place the present climate in the context of predicted future scenarios. Warm polar temperatures during the Cretaceous indeed challenge our understanding of how the ocean-atmosphere system operated as they are significantly warmer than General Circulation Models can reproduce. This has important implications for the prediction of future climates as it implies we may be underestimating future climate change in such regions. Despite the intensive study of Cretaceous marine temperatures an equator-to-pole temperature profile for the Cretaceous greenhouse world remains poorly constrained. Problems stem from the fact that traditional palaeoproxies like oxygen isotopes requires an estimate of the isotopic composition of seawater - particularly difficult to constrain at high latitudes. A more recent approach has been to sidestep this issue by using the TEX86 palaeothermometer (a temperature proxy), although with respect to the Cretaceous this technique is limited by the distribution of suitably preserved sediments, particularly at high latitudes.This proposal aims to address this significant gap in knowledge by undertaking the first quantitative and systematic study of early Cretaceous (Valanginian-Hauterivian) marine temperatures obtained from fossil molluscs using the novel clumped-isotope palaeothermometer. These data will be integrated with new temperature data arising from other techniques. In addition to generating marine temperatures for the Cretaceous tropics, temperate and polar regions we will be able to constrain the isotopic composition of seawater and hence will also be able to provide data concerning the debate about what is the most likely mechanisms to increase the transfer of heat from the equator towards the poles. Our novel clumped-isotope derived temperatures are also expected to be of benefit to the wider community of palaeoceanographers and climate modellers by providing extensive, new data against which to test model outputs.