TEX86

TEX86 is an organic paleothermometer based upon the membrane lipids of mesophilic marine Thaumarchaeota (formerly Marine Group 1 Crenarchaeota). TEX86 is an organic paleothermometer based upon the membrane lipids of mesophilic marine Thaumarchaeota (formerly Marine Group 1 Crenarchaeota). The membrane lipids of Thaumarchaeota are composed of glycerol dialkyl glycerol tetraethers (GDGTs) which contain 0-3 cyclopentane moieties. Thaumarchaeota also synthesise crenarchaeol which contains four cyclopentane moieties and a single cyclohexane moiety and a regio-isomer. The cyclohexane and cyclopentane rings, formed by internal cyclisation of one of the biphytane chains, have a pronounced effect on the thermal transition points of the Thaumarchaeotal cell membrane. Mesocosm studies demonstrate that the degree of cyclisation is generally governed by growth temperature Based upon the relative distribution of isoprenoidal GDGTs, Schouten et al. (2002) proposed the tetraether index of 86 carbon atoms (TEX86) as a proxy for sea surface temperature (SST). GDGT-0 is excluded from the calibration as it can have multiple sources while GDGT-4 is omitted as it exhibits no correlation with SST and is often an order of magnitude more abundant than its isomer and the other GDGTs. The most recent TEX86 calibration invokes two separate indices and calibrations: TEX86H uses the same combination of GDGTs as in the original TEX86 relationship: