TEX86

Key takeaways

TEX86 belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect TEX86 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of TEX86 from memory before moving on to harder problems.

Reference excerpt

TEX86 is an organic paleothermometer based upon the membrane lipids of mesophilic marine Nitrososphaerota (formerly "Thaumarchaeota", "Marine Group 1 Crenarchaeota").

Basics The membrane lipids of Nitrososphaerota are composed of glycerol dialkyl glycerol tetraethers (GDGTs) which contain 0-3 cyclopentane 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 moiety and a regio-isomer (cren'). 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 (TEX86) as a proxy for sea surface temperature (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:

GDGT ratio-2 = [ GDGT-2 ] + [ GDGT-3 ] + [ cren ′ ] [ GDGT-1 ] + [ GDGT-2 ] + [ GDGT-3 ] + [ cren ′ ] {\displaystyle {\text{GDGT ratio-2}}={\tfrac {[{\text{GDGT-2}}]+[{\text{GDGT-3}}]+[{\text{cren}}']}{[{\text{GDGT-1}}]+[{\text{GDGT-2}}]+[{\text{GDGT-3}}]+[{\text{cren}}']}}}

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:

GDGT ratio-1 = [ GDGT-2 ] [ GDGT-1 ] + [ GDGT-2 ] + [ GDGT-3 ] {\displaystyle {\text{GDGT ratio-1}}={\tfrac {[{\text{GDGT-2}}]}{[{\text{GDGT-1}}]+[{\text{GDGT-2}}]+[{\text{GDGT-3}}]}}}

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 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 isomers. Oxic 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 (~160Ma) and indicates relatively warm sea surface temperatures. TEX86 has been used to reconstruct temperature throughout the Cenozoic era (65–0 Ma) and is useful when other SST proxies are diagenetically altered (e.g. planktonic foraminifera) or absent (e.g. alkenones).

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with TEX86

Start with the simplest possible case. Write down what TEX86 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to TEX86 before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about TEX86 ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of TEX86

In research: TEX86 appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses TEX86 in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: TEX86 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Lipid methods, Paleoclimatology, so understanding it makes those chapters shorter.
In everyday life: Look for TEX86 outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study TEX86 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what TEX86 means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain TEX86 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is TEX86 in simple terms?

TEX86 is an organic paleothermometer based upon the membrane lipids of mesophilic marine Nitrososphaerota (formerly "Thaumarchaeota", "Marine Group 1 Crenarchaeota"). Basics The membrane lipids of Nitrososphaerota are composed of glycerol dialkyl glycerol tetraethers (GDGTs) which contain 0-3 cyclo…

Why does TEX86 matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study TEX86?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on TEX86.