TEX-86
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TEX86 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 trace element ratios in bio ...
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 (about 99 °F). The term is mainly applied ...
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 Thermopr ...
(formerly "Thaumarchaeota", "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 Thermopr ...
are composed of glycerol dialkyl glycerol tetraethers (GDGTs) which contain 0-3
cyclopentane Cyclopentane (also called C pentane) is a highly flammable alicyclic compound, alicyclic hydrocarbon with chemical formula C5H10, C5H10 and CAS number 287-92-3, consisting of a ring of five carbon atoms each bonded with two hydrogen atoms above and ...
moieties (commonly annotated as GDGT-''n'' where ''n'' = numbers of cyclopentane moieties). Nitrososphaerota also synthesise crenarchaeol (cren) which contains four cyclopentane moieties and a single
cyclohexane Cyclohexane is a cycloalkane with the molecular formula . Cyclohexane is non-polar. Cyclohexane is a colourless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used). Cyclohexan ...
moiety and a regio-isomer (cren'). The cyclohexane and cyclopentane rings, formed by internal cyclisation of one of the
biphytane Biphytane (or bisphytane) is a C40 Terpenoid, isoprenoid produced from glycerol dialkyl glycerol tetraether (GDGT) degradation. As a common lipid membrane component, biphytane is widely used as a biomarker for archaea. In particular, given its ass ...
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 (TEX86) as a proxy for
sea surface temperature Sea surface temperature (or ocean surface temperature) is the ocean temperature, temperature of ocean water close to the surface. The exact meaning of ''surface'' varies in the literature and in practice. It is usually between and below the sea ...
(SST). GDGT-0 is excluded from the calibration as it can have multiple sources 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. The most recent TEX86 calibration invokes two separate indices and calibrations: TEX86H uses the same combination of GDGTs as in the original TEX86 relationship: :\text=\tfrac 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 ±2.0 °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 cren' entirely: :\text=\tfrac 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.


Caveats

There are several caveats to this proxy and this list is by no means exhaustive. For more information, consult Schouten ''et al.'' 2013.


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. The BIT index is based upon the premise that 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 TEX86-based SST estimates. 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. 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 Methanobacteriota is a phylum in the domain Archaea. Taxonomy The phylum ''Methanobacteriota'' was introduced to prokaryotic nomenclature in 2023. It contains following classes: *Archaeoglobi Garrity & Holt (2002) *Halobacteria Grant ''et al ...
in settings characterised by diffuse methane flux and anaerobic oxidation of methane (AOM). 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 characte ...
isomers In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the existence or possibili ...
.
Oxic Hypoxia (''hypo'': 'below', ''oxia'': 'oxygenated') refers to low oxygen conditions. Hypoxia is problematic for air-breathing organisms, yet it is essential for many anaerobic organisms. Hypoxia applies to many situations, but usually refers t ...
degradation, which is a selective process and degrades compounds at different rates, has been shown to affect TEX86 values and can bias SST values by up to 6 °C.


Application

The oldest TEX86 record is from the middle
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
(~160Ma) and indicates relatively warm sea surface temperatures. TEX86 has been used to reconstruct temperature throughout the
Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...
era (65–0 Ma) and is useful when other SST proxies are diagenetically altered (e.g. planktonic
foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...
) or absent (e.g.
alkenones Alkenones are long-chain unsaturated methyl and ethyl ''n''-ketones produced by a few phytoplankton species of the class Prymnesiophyceae. Alkenones typically contain between 35 and 41 carbon atoms and with between two and four double bonds. Uniq ...
Bijl, P. K., Schouten, S., Sluijs, A., Reichart, G.-J., Zachos, J. C., and Brinkhuis, H., 2009, Early Palaeogene temperature evolution of the southwest Pacific Ocean: Nature, v. 461, no. 7265, p. 776-779.).


Eocene

TEX86 has been extensively used to reconstruct
Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
(55-34Ma) SST. 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 Alkenones are long-chain unsaturated methyl and ethyl ''n''-ketones produced by a few phytoplankton species of the class Prymnesiophyceae. Alkenones typically contain between 35 and 41 carbon atoms and with between two and four double bonds. Uniq ...
, CLAMP,
Mg/Ca 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 trace element ratios in bio ...
). During the middle and late Eocene, high southern latitude sites cooled while the tropics remained stable and warm. Possible reasons for this cooling include long-term changes in
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
and/or changes in gateway reorganisation (e.g.
Tasman Gateway Tasman most often refers to Abel Tasman (1603–1659), Dutch explorer. Tasman may also refer to: Animals and plants * Tasman booby * Tasman flax-lily * Tasman parakeet (disambiguation) * Tasman starling * Tasman whale People * Tasman (name) ...
,
Drake Passage The Drake Passage is the body of water between South America's Cape Horn, Chile, Argentina, and the South Shetland Islands of Antarctica. It connects the southwestern part of the Atlantic Ocean (Scotia Sea) with the southeastern part of the Pa ...
).


References


Further reading

{{Wikibooks, Historical Geology, TEX86 Lipid methods Paleoclimatology