Do glendonites provide faithful records of bottom water temperatures

Understanding Earth's history of temperature is a key element in constraining climate sensitivity to a host of different forcing factors. Yet, in deep time, very few geochemical signatures of temperature remain pristine, and we are forced to rely on sedimentary clues of ice presence such as drop stones, tillites, and glendonites to identify times of major climatic shifts. Glendonites are not minerals but form as a result of a transformation process in which ikaite, a carbonate mineral containing structural water (CaCO3.6H2O,) is replaced by calcite (CaCO3) a carbonate mineral containing no structural water, while retaining the original crystal morphology. Ikaite and glendonite are often found as an iconic features of sediments, being laid down both in the present day and throughout the past, appearing as stellate clusters crystals or large euhedral single crystals, up to ~1m in length. It has been proposed that ikaite formation is limited by a stability field of near-freezing water temperatures and thus glendonites have come to represent an important climate proxy for glacial conditions. Earth's climate has changed throughout the billions of years of our planet's geological history. Evidence for past climates can be found in the rocks around us and many rock-forming environments are directly influenced by climate. In this way, glendonites, in association with other indicators in sedimentary strata, have the potential to show whether, when and where colder intervals with more/less widespread glaciation occurred through the geological record. Glendonites have an added benefit as the distribution of oxygen stable isotopes in its calcium carbonate structure, has the potential to recorde the bottom water temperatures when the original mineral, ikaite, formed. This potential has been realised without properly testing particular assumptions of the method. We will test these assumptions thorough two objectives. 1] That the distribution of oxygen stable isotopes in ikaite varies with temperature in an identical manner to that already shown for calcite. 2] That the distribution of oxygen stable isotopes in ikaite is maintained as it transforms to calcite. These assumptions have never been tested and this is what we aim to do in our proposal. We will grow ikaite crystals under well constrained laboratory conditions that represent the natural environment. We will examine the distribution of oxygen stable isotopes of the ikaite crystals with respect to the temperature at which the crystals grew and with respect to different distributions of oxygen stable isotopes in the solution from which the ikaite precipitated. Afterwards, we will take the various ikaite crystals that we have synthesised, and expose them to warming conditions so that the ikaite will decompose and calcite (as glendonite) form in its wake. By determining the isotope composition of both minerals and solutions during these processes we will be able to say whether the assumptions outlined previously and currently used to examine the bottom water temperatures recorded in glendonite, are correct. The outcome of this research will directly benefit scientists who are using glendonites to try and determine the timing and location of freezing (glacial) conditions in past oceans. It will also benefit other scientists who are interested in ikaite where it forms in other natural settings such as terrestrial springs, sea ice and maybe on other planets in our solar system. We will employ a graduate student to work in our laboratory for two months and organize an 'experience day' to exchange our ideas with non-specialists about the use of glendonites as climate proxies for glacial conditions.