![Molecular structures and HPLC detection of GDGTs](https://upload.wikimedia.org/wikipedia/commons/2/29/Molecular_structures_and_HPLC_detection_of_GDGTs.jpg)
TEX
86 is an organic
paleothermometer
A paleothermometer is a methodology that provides an estimate of the ambient temperature at the time of formation of a natural material. Most paleothermometers are based on empirically-calibrated proxy relationships, such as the tree ring or TEX ...
based upon the membrane lipids of
mesophilic
A mesophile is an organism that grows best in moderate temperature, neither too hot nor too cold, with an optimum growth range from . The optimum growth temperature for these organisms is 37°C. The term is mainly applied to microorganisms. Organi ...
marine
Nitrososphaerota
The Nitrososphaerota (syn. Thaumarchaeota) are a phylum of the Archaea proposed in 2008 after the genome of ''Cenarchaeum symbiosum'' was sequenced and found to differ significantly from other members of the hyperthermophilic phylum Thermoprote ...
(formerly Marine Group 1
Crenarchaeota).
Basics
The membrane lipids of
Nitrososphaerota
The Nitrososphaerota (syn. Thaumarchaeota) are a phylum of the Archaea proposed in 2008 after the genome of ''Cenarchaeum symbiosum'' was sequenced and found to differ significantly from other members of the hyperthermophilic phylum Thermoprote ...
are composed of
glycerol dialkyl glycerol tetraethers Glycerol dialkyl glycerol tetraether lipids (GDGTs) are a class of membrane lipids synthesized by archaea and some bacteria, making them useful biomarkers for these organisms in the geological record. Their presence, structure, and relative abundanc ...
(GDGTs) which contain 0-3
cyclopentane
Cyclopentane (also called C pentane) is a highly flammable alicyclic hydrocarbon with chemical formula C5H10 and CAS number 287-92-3, consisting of a ring of five carbon atoms each bonded with two hydrogen atoms above and below the plane. It occur ...
moieties. Nitrososphaerota also synthesise crenarchaeol which contains four cyclopentane moieties and a single
cyclohexane
Cyclohexane is a cycloalkane with the molecular formula . Cyclohexane is non-polar. Cyclohexane is a colorless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used). Cyclohexan ...
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 Nitrososphaerota cell membrane. Mesocosm studies demonstrate that the degree of cyclisation is generally governed by growth temperature.
Calibrations
Based upon the relative distribution of isoprenoidal GDGTs, Schouten et al. (2002) proposed the tetraether index of 86 carbon atoms (TEX
86) as a proxy for
sea surface temperature
Sea surface temperature (SST), or ocean surface temperature, is the ocean temperature close to the surface. The exact meaning of ''surface'' varies according to the measurement method used, but it is between and below the sea surface. Air mass ...
(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 TEX
86 calibration invokes two separate indices and calibrations: TEX
86H uses the same combination of GDGTs as in the original TEX
86 relationship:
:
GDGT ratio-2 is correlated to SST using the calibration equation:
:TEX
86H = 68.4×log(GDGT ratio-2) + 38.6.
TEX
86H has a calibration error of ±2.5 °C and is based upon 255 core-top sediments.
TEX
86L employs a combination of GDGTs that is different from TEX
86H, removing GDGT-3 from the numerator and excluding GDGT-4’ entirely:
:
GDGT ratio-1 is correlated to SST using the calibration equation:
:TEX
86L = 67.5×log(GDGT ratio-1) + 46.9.
TEX
86Lhas a calibration error of ±4 °C and is based upon 396 core-top sediment samples.
Other calibrations exist (including 1/TEX
86, TEX
86' and pTEX
86 ) and should be considered when reconstructing temperature.
Caveats
There are several caveats to this proxy and this list is by no means exhaustive. For more information, consult
Terrestrial input
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 Nitrososphaerota and branched GDGTs are derived from terrestrial soil bacteria. When BIT values exceed 0.4, a deviation of >2 °C is incorporated into TEX
86 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).
Anaerobic Oxidation of Methane (AOM)
The Methane Index (MI) was proposed to help distinguish the relative input of methanotrophic
Euryarchaeota
Euryarchaeota (from Ancient Greek ''εὐρύς'' eurús, "broad, wide") is a phylum of archaea. Euryarchaeota are highly diverse and include methanogens, which produce methane and are often found in intestines, halobacteria, which survive extre ...
in settings characterised by diffuse methane flux and anaerobic oxidation of methane (AOM) (Zhang et al., 2011). These sites are characterised by a distinct GDGT distribution, namely the predominance of GDGT-1. -2 and -3. High MI values (>0.5) reflect high rates of gas-hydrate-related AOM.
Degradation
Thermal maturity is only thought to affect GDGTs when temperature exceed 240 °C. This can be tested using a ratio of specific
hopane
Hopane is a natural chemical compound classified as a triterpene. It forms the central core of a variety of other chemical compounds which are collectively known as hopanoids. The first compound of the hopane family to be isolated and characteri ...
isomers
In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers.
Iso ...
.
Oxic
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