Earth’s climate is influenced by a suite of dynamic interactions between the marine, terrestrial and atmospheric environments. Heat exchange between the oceans and the atmosphere are particular important and is reflected, in part, by sea surface temperature (SST). As a result, SST is used as a key parameter for assessing changes to Earth’s climate in the past and is also a central feature when predicting future change.
The determination of past SST is traditionally carried out using so-called proxy methods applied to geological archives of Earth’s history, many of which are recorded in marine sediments underlying the world’s oceans. Of the many proxy methods for SST reconstruction, those based on the distributions of lipid chemicals made by marine organisms are some of the most developed and commonly used. The basic principle is that lipid distributions are controlled by the temperature in which their host organisms grow. At the end of their life cycle, such organisms are deposited into underlying ocean sediments where their fossilised remains, including their lipids, are preserved. As such, the original SST is preserved in the geological record and can be reconstructed by analysis of relevant lipids in well-dated sediment cores. A number of lipid-based SST proxies exist, and have been used with great success over the last four decades or so to reconstruct SST for temperate-tropical regions over timescales spanning Earth’s history; however, no existing methods work for the characteristically low temperatures (typically <5 degrees Celcius) of the Arctic and Antarctic, which represent approximately 25% of the world’s oceans. The reasons for this failure are (1) A general absence of the source organisms at high latitudes; (2) Arctic and Antarctic SST are below the accepted calibration ranges of traditional lipid-based SST proxies. Both limitations will be addressed in PAST by investigating a novel SST proxy based on some so-called highly branched isoprenoid (HBI) lipids (HBIs III&IV) made by certain phytoplankton common to the Arctic and Antarctic, and whose distribution is hypothesised to be controlled by chemical kinetics, rather than biological function, so should follow a strict temperature dependence over all temperatures. This hypothesis, although unusual for biological systems, is nonetheless supported by some preliminary data, which provide the platform for further investigation in PAST. There are two related approaches to be taken in PAST. The first is to test the aforementioned temperature control over HBI lipid distributions through the analysis of Arctic and Antarctic phytoplankton samples for which temperature data are also available. Findings from these data will be used to calibrate the temperature-lipid distribution relationship within the source environment. The second approach is to test the same temperature-lipid distribution relationship in the recent geological record through the analysis of surface and short core marine sediments from the Arctic that represent timeframes (recent decades to centuries) for which temperature data are also available. To broaden the scope of the new SST proxy, water column samples from a temperate location (Western English Channel; Temperature range = 8-16 degrees Celcius) will also be analysed. Once calibrated/tested, the new SST proxy will thus enable scientists, for the first time, to accurately reconstruct Arctic SSTs within Earth’s history, and to determine complementary SST data for other regions. Such data are critical for the improvement of climate hindcasting and prediction models such as those used by the Intergovernmental Panel on Climate Change (IPCC).