TEX86
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:
GDGT ratio-2 is correlated to SST using the calibration equation: TEX86H = 68.4*log(GDGT ratio-2) + 38.6). TEX86H has a calibration error of ±2.5 °C and is based upon 255 core-top sediments.
TEX86L employs a combination of GDGTs that is different from TEX86H, removing GDGT-3 from the numerator and excluding GDGT-4’ entirely:
GDGT ratio-1 is correlated to SST using the calibration equation:(TEX86L=67.5*log(GDGT ratio-1)+46.9). TEX86Lhas a calibration error of ±4 °C and is based upon 396 core-top sediment samples.
Other calibrations exist (including 1/TEX86, TEX86' and pTEX86) and should be considered when reconstructing temperature.
There are several caveats to this proxy and this list is by no means exhaustive. For more information, consult
The branched vs isoprenoidal tetratether (BIT) index can used to measure the relative fluvial input of terrestrial organic matter (TOM) into the marine realm (Hopmans et al., 2004). The BIT index is based upon the premise that GDGT-4 (also known as crenarchaeol) is derived from marine-dwelling Thaumarchaeota and branched GDGTs are derived from terrestrial soil bacteria. When BIT values exceed 0.4, a deviation of >2 °C is incorporated into TEX86 SST estimates. However, isoprenoidal GDGTs can be synthesised in the terrestrial environment and can render BIT values unreliable (Weijers et al., 2006; Sluijs et al., 2007; Xie et al., 2012). A strong co-variation between GDGT-4 and branched GDGTs in modern marine and freshwater environments also suggests a common or mixed source for isoprenoidal and branched GDGTs (Fietz et al., 2012).
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