Although knowledge of past temperatures on Earth informs modern regional responses to climate change, a lack of reliable terrestrial palaeotemperature proxies limits our ability to construct a global picture of past temperatures. The chemical composition of speleothems — calcium carbonate deposits in caves — has been used to investigate palaeoclimates, but signals are often dominated by changes in hydrological conditions rather than temperature.
Russell Drysdale from the University of Melbourne, Australia, and colleagues investigated Mg concentrations in an underwater speleothem as a terrestrial palaeotemperature proxy. A core was extracted from a submerged speleothem within the Antro del Corchia cave system, Italy, and Mg concentrations were measured at 1 mm intervals. The results, which span the past ~350,000 years and numerous glacial–interglacial cycles, correlate with regional sea-surface temperatures determined from ocean sediment cores in the North Atlantic. Higher Mg concentrations are associated with warmer periods, and vice versa, consistent with the temperature dependence of Mg uptake into calcium carbonate. Moreover, measurements of the Mg concentration at a higher temporal resolution more accurately capture rapid temperature changes associated with the penultimate glacial termination ~136,000–129,000 years ago, revealing the temperature sensitivity of this proxy. The Mg concentration is also a more reliable record than are stable isotope ratios (δ18O and δ13C) measured using the same speleothem. Comparisons with a nearby stalagmite, for which Mg variations reflect changes in hydrology, suggest that the temperature sensitivity of the submerged speleothem is due to the depositional conditions of its underwater environment.
These results indicate that Mg content in a submerged speleothem formed under similar conditions can function as a ‘palaeothermometer’. As cave systems with submerged speleothems can be found throughout the world, this palaeothermometer has the potential to extend our knowledge of past terrestrial temperatures beyond the poles.
Drysdale, R. et al. Magnesium in subaqueous speleothems as a potential palaeotemperature proxy. Nat. Commun. 11, 5027 (2020)
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Ashworth, C. A new palaeothermometer. Nat Rev Earth Environ 1, 556 (2020). https://doi.org/10.1038/s43017-020-00113-z