Towards calibration of the TEX86 palaeothermometer for tropical sea surface temperatures in ancient greenhouse worlds

Marine Crenarchaeota from tropical Indian Ocean water were incubated at temperatures ranging from 25 to 40 °C to study the changes in glycerol dibiphytanyl glycerol tetraether (GDGT) membrane lipid composition. The results show that Crenarchaeota were able to thrive at temperatures up to 40 °C. Archaeal 16S ribosomal DNA analysis revealed that different species proliferated with moderate (25 °C) and high (36 °C) optimum growth temperatures. Analysis of the GDGT distribution shows a similar linear correlation of TEX86 with incubation temperature as demonstrated previously for incubation experiments at lower temperatures [Wuchter, C., Schouten, S., Coolen, M.J.L., Sinninghe Damsté, J.S., 2004. Paleoceanography 19, PA4028, doi:10.1029/2004PA001041]. Our results show that Crenarchaeota can thrive at temperatures warmer than present day tropical sea surface temperatures, such as the high temperature (up to 40 °C) inferred for tropical oceans in ancient greenhouse worlds. Our results also imply that the TEX86 is still applicable in this regime. However, the crenarchaeol regioisomer in the GDGT distribution obtained from the incubation experiments is substantially less than in sediments deposited in exceptionally warm oceans of the geological past, so our laboratory results cannot be directly used to convert TEX86 values from these sediments into temperature.