High Throughput Technique for Screening Archaeal Tetraether Lipid Cores in Sediments as Probes of Palaeo Surface-water Temperature

Archaea, one of the three domains of life, were long considered to be limited to environments that exhibit extremes of temperature, pH, salinity and anoxia. Since the recognition, in the early 1990's, of archaeal gene sequences in the oceans, non-extreme archaea have been shown to be prevalent in the oceans where they fulfill important roles in the carbon and nitrogen cycles. The long term contribution of these organisms to marine element cycling is evident from the recognition of lipid structures, that are specific to the archaea and derive from their membranes, in sediments deposited around 90 million years ago. Among the lipids produced only by the archaea are structures that differ in the number of five-membered rings contained within these membrane lipid-derived components. The differences in the number of rings are believed to regulate the organisation of the membrane, allowing the organisms to adapt to different conditions, e.g. temperature, under which they grow. A particular ratio (the TEX86 index) computed from the proportions of structures with different numbers of five-membered rings that are present in marine sediments was shown to correlate very strongly with the surface water temperature of the overlying ocean, providing the potential to derive sea surface water temperatures of oceans in times past from analysis of the archaeal lipid residues preserved within sediment cores. Further investigations have identified situations where the TEX86 index shows linear trends with surface water temperature, but the values are offset by a number of degrees C. We have previously demonstrated a method using tandem mass spectrometry that differentiates different archaeal lipid structures and reveals that some of the chromatographic peaks measured for use in the TEX86 index contain isomeric structures. We plan to differentiate individual lipid structures, including isomers, using a combination of improved separation and application of tandem mass spectrometry. Hence, we will obtain much improved measurements of TEX86. At the same time, we plan to address points in the analytical approach where new strategies can be introduced to improve both the capability to detect compounds at very low levels and the speed of the overall technique for deriving the TEX86 values. Thus, we plan to develop a high throughput technique that will deliver the capacity to assess sea surface water temperatures in the past using very small sediment sample sizes and with very short analysis times. This will enable the temperature records to be generated at very high depth resolution, allowing high time resolution in the data and thereby revealing changes in temperature even over short intervals of time. The ability to generate intensive records of temperature change will be of considerable benefit to researchers concerned with understanding the nature of the marine environment in times past and its response to environmental stresses, and in developing clearer records that relate to climate change over the recent geological past (Quaternary). The method may also have application to ancient sediment sequences subject to assessment of the longer-term stability of the lipids.