Calculating ancient temperatures using oxygen

 
What are isotopes?

The term isotope was coined by English radiochemist Fredrick Soddy (1913) to distinguish chemical elements with different atomical mass, different numbers of neutrons, but which occupy the same position of the periodic table, i.e. have the same numbers of protons and electrons. The term isotope is derived from iso meaning equal and the Greek topos meaning place in reference to the same placement in the periodic table.

Concentrations of isotopic species are represented by the relative abundance ratios of the heavy to the light isotopes in each sample to the ratio of a given standard. The values of the 𝛿 can be either positive, showing an enrichment of the heavier isotopes relative to the standard, or negative, showing depletion of the heavier isotopes relative to the standard.

Introduction to oxygen

The isotope used most often as a palaeothermometer is oxygen. Oxygen (O) has three stable isotopes: ¹⁶O, ¹⁷O, and ¹⁸O. It is the ratio of ¹⁸O to ¹⁶O that is usually used in palaeoclimatological studies. The ratio of ¹⁸O and ¹⁶O in biomineralised elements (that is bones, shells and teeth) is temperature dependent. During cool periods, such as glaciations, the lighter ¹⁶O isotope will evaporate at a much faster rate compared to the heavier ¹⁸O which is left behind in the water and is thus incorporated into shells and tests.

The use of oxygen isotope ratios as a palaeothermometer in carbonate minerals is based on the thermodynamic fractionation between ¹⁶O and ¹⁸O that happens during precipitation. Fractionation alters the 𝛿¹⁸O relative to temperature logarithmically over an oceanic temperature range of -2°C to 30°C of between -0.20‰ and -0.27‰ per °C in agreement with thermodynamic predictions. Because the oxygen isotopic proxy is based on thermodynamic principles, we expect it to be relatively unaffected by secondary kinetic factures. However, there are reasons to suspect that non-temperature dependent factors such as ontogenetic variations and seawater carbonate ions can affect the isotope ratios. Despite such issues, oxygen isotopic ratios are the most widely used and reliable palaeotemperature proxies.

The ratio of ¹⁶O to ¹⁸O decreases as temperatures and evaporation rates increase. In general, a 𝛿¹⁸O increase of 1.0‰ is equivalent to approximately 4°C of cooling. The reverse is true when water condenses. Isotope ratios are tracked using a Delta Notion with the Standard Mean Ocean Water (SMOW) as a reference.

Fossils

Some fossils groups; principally foraminifera, brachiopods and belemnites can be used to directly measure, rather than interpret, palaeotemperatures and ice volumes. Geochemical proxies from foraminiferal tests have been used to reconstruct global ice volumes and temperatures from various water depths.

Organisms with hard parts made of calcium carbonate (CaCO₃) combine dissolved oxygen from the water with carbon and calcium to form their shells. This ratio, so long as the material hasn’t been alternated during diagenesis, survives in the geological record.

A commonly used fossil during the Mesozoic (251-66 million years ago) are belemnites, a prehistoric squid-like animal. They make an ideal paleothermometer since their low-Mg calcite shells (called rostrum) are relatively resistant to diagenetic alternations. Being relatively large and having well organised growth lines means that details such as seasonal changes can be reconstructed (Ullmann et al. 2015).

The most commonly used fossil for the Cenozoic is foraminifera, a type of unicellular marine protist. Measurements of oxygen and carbon isotopes, and Mg/Ca ratios from calcite from foraminifera have been one of the most reliable methods of reconstructing past oceanic and climatic conditions. For example, δ¹⁸O values taken from foraminifera show a shift to glacial conditions in Antarctica near the Eocene – Oligocene boundary 34 million years ago (Lear et al., 2008).

Ice cores

When glacier ice is formed by the compression of snow, gas bubbles containing small amount of ancient atmospheric air are preserved. Isotopic measurements are taken from ice cores to measure the δD (deuterium – an isotope of hydrogen) and δ¹⁸O ratios. The oldest ice core sample comes from the Allan Hills of Antarctica and is 2.7 million years old (Yan et al. 2019).

Isotopes recovered from ice core are lighter than ocean water due to path fractionation – the process by which heavier isotopes are “rained out” so by the time they precipitate over higher latitudes there is a light isotope bias. Most of what we know about climate change over the last 2 million years comes from ice core samples from the north and south poles.

Summary

This article aimed to present a brief overview of the use of oxygen isotopes in calculating ancient temperatures. Temperature is the most primary representation of the state of the climate system. This is true both in the geological past and now. Thus, understanding how the climate responded in the past is fundamental to predicting and preparing for future climate change.

About this author

Jack Wilkin is a graduate researcher at the Camborne School of Mines (University of Exeter) in the United Kingdom. His research focuses on isotopic analysis of belemnites for palaeoclimate studies.