Shells, Corals, and other Carbonate Samples: Analysing U-Th, C-14, Strontium, Boron, and Oxygen Isotopes

Radiocarbon (¹⁴C) analysis is another option to date shell, coral and carbonate samples based on fitting the ¹⁴C level of the sample on the radiocarbon calibration curve. This method allows dating from the present day until approximately 40,000 years before present. However, shells, corals and other marine-based carbonates are subject to the “reservoir effect” and recrystallisation – both of which can impact the accuracy of ¹⁴C dating. As a result, careful site-based calibration and subsampling is required. Furthermore, in archaeological studies, the “old shell” effect should be considered if old shells may have been used for beads or decor (Rick et al. 2005). (Read more about radiocarbon dating shells and carbonates).

Strontium isotopes (⁸⁷Sr, ⁸⁶Sr) in seawater have varied through time in the geological past. Shells, corals, marine carbonates and other organic matter present in the ocean takes up strontium isotopes, resulting in a snapshot of oceanic strontium during their lifetime. By measuring the ⁸⁷Sr/⁸⁶Sr ratio in the sample and placing it on the standard seawater Sr-curve the age of the sample can be determined. However, issues with homogeneity of seawater strontium make sample calibration difficult (Meknassi et al. 2018).

Beyond Sr-curve aging, recent studies have shown that Sr uptake of certain shells is a function of local seawater temperature (Toland et al. 2000), thus Sr may also be used as a paleothermometer in combination with calcium (Ca) analysis. (Read more about Strontium Isotopes).

Boron isotopes (δ¹¹B) are a function of seawater pH and thus can be measured in shells, corals and carbonates to decipher changes in pH and ocean acidification through time which reflects past atmospheric CO2 concentrations. Shells, corals and other marine carbonates are particularly vulnerable to ocean acidification and thus numerous studies are currently underway to assess how pH level has varied through time and what implications this has for the vitality of these organisms (Rae et al. 2011; Rae 2018). (Read more about Boron Isotopes).

Oxygen isotopes (δ¹⁸O) vary in response to temperature, humidity, precipitation and other climate parameters. Similar to boron and strontium, marine organisms take up oxygen in their growth structures, providing insight into oxygen variability – and thus climate variability – through time. Oxygen isotopes in corals and shells can be used to reconstruct sea surface temperature and in some cases, precipitation (Rollion-Bard et al. 2011; Leng & Lewis, 2016). (Read more about Oxygen Isotopes).

The reconstruction of environmental changes through time using corals, shells and marine carbonates requires a robust chronology using methods such as  U-Th or ¹⁴C dating. Once dated, these samples can be further analyzed to provide many paleoenvironmental reconstructions, including: wet/dry phases (Yehudai et al. 2017), mortality/recovery of coral reef systems (Clark et al. 2017), seawater temperature (DeCarlo et al. 2016) and pH (Stewart et al. 2016; Pauly et al. 2015). For further information about how you can integrate multi-proxy analysis of your shell, coral or carbonate samples, contact us!