TEX86

The cyclohexane and cyclopentane rings, formed by internal cyclisation of one of the biphytane chains,[4] have a pronounced effect on the thermal transition points of the Nitrososphaerota cell membrane.

[5] Based upon the relative distribution of isoprenoidal GDGTs, Schouten et al. (2002)[6] 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 [7] while cren 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.

[12] The branched vs isoprenoidal tetratether (BIT) index can used to measure the relative fluvial input of terrestrial organic matter (TOM) into the marine realm.

However, isoprenoidal GDGTs can be synthesised on land (by terrestrial archaea) and can render BIT values unreliable; isoGDGT becomes more abundant with higher soil pH.

During the early Eocene, TEX86 values indicate warm high southern hemisphere latitude SSTs (20-25 °C) in agreement with other, independently derived proxies (e.g. alkenones, CLAMP, Mg/Ca).