The Long chain Diol Index: A marine palaeotemperature proxy based on eustigmatophyte lipids that records the warmest seasons

Long chain 1,13- and 1,15-diols are lipids which are omnipresent in marine environments, and the Long chain Diol Index (LDI), based on their distributions, has previously been introduced as a proxy for sea surface temperature. The main biological sources for long chain 1,13- and 1,15-diols have remained unknown, but our combined lipid and 23S ribosomal RNA (23S rRNA) analyses on suspended particulate matter from the Mediterranean Sea demonstrate that these lipids are produced by a marine eustigmatophyte group that originated before the currently known eustigmatophytes diversified. The 18S rRNA data confirm the existence of early-branching marine eustigmatophytes, which occur at a global scale. Differences between LDI records and other paleotemperature proxies are generally attributed to differences between the seasons in which the proxy-related organisms occur. Our results, combined with available LDI data from surface sediments, indicate that the LDI primarily registers temperatures from the warmest month when mixed-layer depths, salinity, and nutrient concentrations are low. The LDI may not be applicable in areas where Proboscia diatoms contribute 1,13-diols, but this can be recognized by enhanced contributions of C-28 1,12 diol. Freshwater input may also affect the correlation between temperature and the LDI, but relative C-32 1,15-diol abundances help to identify and correct for these effects. When taking those factors into account, the calibration error of the LDI is 2.4 degrees C. As a well-defined proxy for temperatures of the warmest seasons, the LDI can unlock important and previously inaccessible paleoclimate information and will thereby substantially improve our understanding of past climate conditions.

Automated summary

Long chain 1,13- and 1,15-diols are lipids found in marine environments and can be used as a proxy for sea surface temperature. This study reveals that early-branching marine eustigmatophytes are the main biological source of these lipids. The Long chain Diol Index (LDI) primarily reflects temperatures from the warmest month. Factors such as diatoms and freshwater input can affect the accuracy of LDI, but can be identified and corrected for.