palaeothermometers

Palaeothermometers are proxies used to estimate past temperatures from geological and paleontological materials. They rely on signals that change with temperature during formation or subsequent burial, and they are interpreted through calibrations against modern analogues. By combining measurements with environmental context, researchers reconstruct temperature histories for oceans, lakes, and terrestrial environments.

Common palaeothermometers include stable oxygen isotopes in carbonates and biogenic materials (foraminifera, corals), where the delta-18O value depends on both temperature and the isotopic composition of seawater. Magnesium-to-calcium ratios in foraminiferal calcite (Mg/Ca) generally increase with temperature and can provide quantitative seawater temperatures. Alkenone unsaturation indices (UK'37) derived from coccolithophore lipids yield sea-surface temperature estimates, while TEX86 uses branched and isoprenoid glycerol dialkyl glycerol tetraether lipids to extend temperature estimates to warmer oceans. Clumped-isotope thermometry (Delta47) analyzes the bonding of heavy isotopes within carbonate or phosphate minerals to infer temperature directly. Oxygen isotopes in phosphate from vertebrate bone or tooth enamel can reflect body-water temperatures in certain ecological settings. Terrestrial proxies include stomatal indices and leaf-margin analysis from fossil leaves, as well as soil- and peat-derived lipid biomarkers that track temperature and humidity.

Interpretation relies on modern calibrations and corrections for factors such as ice-volume, seawater composition, diagenesis, and regional variability. Proxies may reflect different timescales or habitats, so multi-proxy approaches are common to improve robustness. Limitations include diagenetic alteration, reservoir effects, dating uncertainties, and the need for careful contextual information to distinguish local from global temperature signals.