Establishing a new palaeothermometer from the speleothem archive of phosphate-oxygen isotopes

Temperature records are critical for understanding past and future climate. However, reconstructing past temperature dynamics is incredibly difficult. Of the currently available terrestrial archives of past temperature, these are often spatially limited, suffer from ambiguity around calibration, or require large sample sizes. These issues have prevented the development of a high resolution, high density network of terrestrial temperature records. This is now often considered the single most significant gap in the palaeoclimate archive. Here, we seek to provide a breakthrough in the field of temperature reconstruction by developing a new palaeothermometer. For this, we use speleothems (cave stalagmites). Speleothems grow in layers, which can be dated like the rings in a tree. The chemistry in each layer offers an unprecedented resolution of environmental information, constrained by an absolute age model over 500,000 years. At the Lancaster Environment centre, we have recently developed a technique which allows phosphate to be extracted from the stalagmite layers. This is a critically important advance in the research field, as phosphate-oxygen isotopes are known to be controlled by temperature dynamics. Our first measurements of the phosphate-oxygen isotope composition in cave drip waters and modern cave calcite provide clear evidence that the cave temperature signal can be captured and stored within the speleothem record. As the internal temperature of shallow cave systems are known to reflect the external average air temperature (plus or minus localised effects), this provides an exciting opportunity through which a truly independent terrestrial temperature record may be built. This research aims to build and test a modern-day calibration between cave temperature and speleothem phosphate-oxygen isotopes. This will enable a platform from which precisely dated, well preserved, independent temperature records can be confidently obtained from the global archive of speleothems at a spatial and temporal scale hitherto unprecedented.