Online measurements of δ¹⁸O and δ²H can be used to reveal more information about past climates than current offline methods. In this thesis I present work carried out developing the online Differential Thermal Isotope Analysis (DTIA) method, including demonstrations of the method on gypsum samples and clay samples, and the application of DTIA to the ongoing research into climate conditions during the Paleocene-Eocene Thermal Maximum (PETM).

Measurements of gypsum and clay samples demonstrate the ability of DTIA to separate out different dehydration steps for individual measurement, both for minerals with multiple water environments, and for minerals with multiple-step dehydrations. The gypsum results are also used to examine the dehydration of gypsum to anhydrite, via the intermediate bassanite. I show that this dehydration reaction is highly sensitive to sample grain size and the partial pressure of water, and crucially, that the two-step dehydration of gypsum does not result from the presence of multiple water environments that are preferentially dehydrated at different temperatures, but rather from kinetic factors upon dehydration.

DTIA is also applied to a series of clays buried in the North Sea Basin across the PETM. The results from the hydroxyl isotopic composition of the clays show a trend of slowly decreasing δ²H prior to the PETM, followed by abrupt decreases in δ²H at the onset of the PETM, indicating increased precipitation intensity and weathering, and implying an enhanced hydrologic cycle response to global warming, particularly at the early stages of the PETM. These results are consistent with other research indicating higher precipitation rates during the PETM. Our results are presented alongside consistent evidence from the measurements of clay composition and plankton species concentration undertaken by previous researchers at this section, demonstrating how DTIA can be used alongside other methods.

This thesis shows the potential DTIA has to aid palaeoclimate reconstruction in a number of geological settings. DTIA can be applied both to isolated hydrated minerals and to assemblages of hydrated minerals to better understand the formation environments of these minerals, and thus gain insight into the palaeoclimate conditions under which geological deposits form.