LI/CA A NOVEL PALEO THERMOMETER: UNLOCKING DEEP-TIME CLIMATE PERTURBATIONS

Brachiopods are important archives because of their wide distribution in modern and deep-time oceans. Their low-Mg calcite shell mineralogy is quite resistant to the voracities of diagenesis, and thus, their proxies are excellent recorders of ambient environmental conditions. Several studies have attempted to correlate their Li/Ca ratios to environmental temperatures of formation with mixed success.

Here, we report on a large and diverse database of modern articulated brachiopods collected from all oceans and seas. Our detailed investigation reveals that the primary layer calcite of all articulated brachiopods does not follow a systematic Li/Ca regime with the ambient environment. In contrast, the biomineralization process of the secondary and tertiary (if present) layer calcite is quite complex but well-documented in the literature. Unlike, the traditional stable isotopes, the Li/Ca ratios of the fibres of the secondary layer and the prisms of the tertiary layer exhibit significant differences and behaviours in their content. The calcite prisms of the tertiary layer show a subdued relationship between their Li/Ca ratio and ambient water temperature and crystallization order. In contrast, the fibres of the secondary layer in articulated brachiopods show a strong relationship between their Li/Ca ratios and ambient seawater temperature. This relationship is not impeded by variations in seawater salinity and pH, and furthermore, it maintains this characteristic in all specimens collected from the global oceans.

The brachiopod-Li/Ca ratio is independent of seawater Li and Ca contents, and thus, is exclusively controlled by their ambient seawater temperature. These observations and evaluations allow us to present a novel paleothermometer based on the Li/Ca ratio stored in the secondary-layer fibres of a large and comprehensive brachiopod population. This attribute makes deep-time brachiopods coupled with this novel paleothermometer an invaluable archive in describing diversifications, mass extinctions and other geologically important events associated with changes in past climates and associated perturbations.