Paleothermometer

Key takeaways

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

Reference excerpt

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 trace element ratios in biominerals and proxies from organic molecules.

Common paleothermometers

δ18O

Scientists use the isotopic ratio of 18O to 16O in ice, tree tissue, coral skeletons, speleothems, and sediments to calculate past temperature and/or estimate ice sheet volumes. Ocean water is mostly H216O, with small amounts of HD16O and H218O. Fractionation occurs during changes between condensed and vapor phases. The vapor pressure of heavier isotopes is lower, so vapor will have a lower ratio of heavy isotopes (it is more isotopically negative) than the pool from which it evaporated. Conversely, when the vapor condenses, the precipitation will have a higher ratio of heavy isotopes than the vapor from which it precipitated. As an air mass moves inland or from equator to poles, isotopically "heavy" water rains out, and the residual fraction is lighter (isotopically negative). Oxygen isotope values are often reported in terms of standard mean ocean water, or SMOW. The difference from SMOW is expressed as

δ O 18 = 1000 × [ O 18 O 16 / ( O 18 O 16 ) SMOW − 1 ] {\displaystyle \delta {\ce {^{18}O}}=1000\times \left[{\ce {{\frac {^{18}O}{^{16}O}}}}{\Bigg /}\left({\ce {{\frac {^{18}O}{^{16}O}}}}\right)_{{\ce {SMOW}}}-1\right]} ; and a similar formula for δD.

Trace Element Ratios in Biominerals Magnesium (Mg) is incorporated into the calcite shells (tests) of planktic and benthic foraminifera as a trace element. Because the incorporation of Mg as an impurity in calcite is endothermic, more is incorporated into the growing crystal at higher temperatures. Therefore, a high Mg/Ca ratio implies a high temperature, although ecological factors may confound the signal. Mg has a long residence time in the ocean, and so it is possible to largely ignore the effect of changes in seawater Mg/Ca on the signal. Mg/Ca ratios can sometimes underestimate seawater temperatures by way of the dissolution of foraminifer shells, which lowers Mg/Ca values.

Carbonate chemistry (which is pH-dependent) can also affect the incorporation of Mg into calcium carbonate. For this reason, researchers use boron isotopes (a proxy for ocean pH) or atmospheric CO2 values (which can be used to estimate ocean pH) from the period of interest to apply a correction to the Mg/Ca record. This correction isolates changes in Mg/Ca due solely to temperature variability, allowing for a more robust temperature reconstruction. Strontium (Sr) incorporates in coral aragonite, the mineral that corals precipitate for their skeletons. At higher temperatures, the incorporation of Sr impurities in the aragonite decreases, and the Sr/Ca ratio in the coral skeletons will be lower. Corals can provide exceptionally high resolution (sub-monthly) temperature records, particularly in taxa with a higher growth rate. High resolution records are necessary for reconstructing seasonal temperature variation, which is currently a source of uncertainty in climate models. Variability in the Sr/Ca ratio of seawater, particularly in near-coast areas with high river runoff, can introduce error to Sr/Ca temperature interpretations. Another source of error can come from "vital effects," or variations in geochemistry that come from the physiology of the coral rather than variation in temperature. For instance, seasonal variability in the coral growth rate may change the incorporation of Sr/Ca, adding noise to the geochemical record. Another potential source of error in Sr/Ca calibrations comes from diagenesis, in which part of the original aragonite mineral dissolves and a new mineral recrystallizes in its place. Because these new minerals are not forming under the same conditions as the original skeletal mineral, they can introduce false temperature anomalies to the time series. Sr/Ca temperature reconstructions require calibration based on a modern member of the same taxa sampled from the same location. This requirement can be limiting when modern analogues are unavailable. However, some researchers are exploring the potential of developing "universal" coral paleo-thermometers using multiple temperature-influenced trace element ratios in tandem. These elements include strontium, magnesium, boron (B), uranium (U), and lithium (Li). B/Ca and U/Ca have been found to vary with temperature, but are also strongly influenced by the pH and carbonate concentration in the coral's internal pool of calcifying fluid.

Proxies from Organic Molecules

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Paleothermometer

Start with the simplest possible case. Write down what Paleothermometer 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 Paleothermometer 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 Paleothermometer 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 Paleothermometer

In research: Paleothermometer 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 Paleothermometer 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: Paleothermometer is common in secondary-school and first-year university syllabi. It links to neighbouring topics Paleoclimatology, so understanding it makes those chapters shorter.
In everyday life: Look for Paleothermometer 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 Paleothermometer in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Paleothermometer 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 Paleothermometer out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Paleothermometer in simple terms?

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 trace element ratios in biominerals and proxies from organic molecules.

Why does Paleothermometer 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 Paleothermometer?

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 Paleothermometer.