A speleothem record of the Mid-Brunhes Transition from southern Europe

Antarctic ice cores and ocean-sediment records preserve evidence for an increase in the amplitude of glacial-interglacial cycles at around 430 ka, known as the Mid-Brunhes Transition (MBT). However, similar evidence from non-polar terrestrial environments is rare, casting some doubt on the global extent of this transition. Here we present a multi-proxy speleothem record from Corchia Cave (Alpi Apuane, Italy) that spans the MBT. It comprises a stacked d¹⁸O and d¹³C time series from multiple stalagmites anchored in time by U-Th and U-Pb ages; and trace element, ⁸⁷Sr/⁸⁶Sr, and d¹⁸O and d¹³C profiles from a subaqueous calcite deposit (CD3) that has grown continuously from 970 ka to the present. We anchored the CD3 record to the chronology of a stalagmite stack by synchronisation of their respective d¹⁸O and d¹³C profiles.

 

CD3 is well suited to this study because it yields a suite of proxies from just a single specimen that covers multiple glacial-interglacial cycles either side of the MBT. In particular, its d¹³C profile provides a reference for comparing the amplitude of glacial-interglacial temperature changes at Corchia to globally integrated ice-volume (LR04 benthic ¹⁸O/¹⁶O stack) and greenhouse gas (ice-core CO₂ and CH₄)time series. The CD3 temperature record builds on a previous trace element study, which revealed that the Mg/Ca in this speleothem is strongly influenced by mineralisation temperature (a proxy for external air temperature at the cave site). This is supported by subsequent clumped-isotope palaeothermometry. We thus developed a continuous palaeotemperature time series for CD3 extending to ~650 ka via a Mg-D47 transfer function.

 

The temperature profile reveals compelling evidence for a shift in glacial-interglacial amplitude across the MBT. Temperatures during the interglacials of MIS15e, 15a and 13a are lower in Corchia compared to those of MIS11c, 9e, 5e and the Holocene; temperatures during MIS7e and 7c are the exception, only reaching the levels of the pre-MBT interglacials. Minimum glacial temperatures for MIS16 and 14 are warmer in Corchia than those of the subsequent glacial maxima, and the MIS12 and 6 glacials are the coldest of the last 650 kyr. All of these patterns are consistent with existing global ice-volume and greenhouse gas records but provide a rare and important terrestrial perspective. This finding confirms previous assessments that the MBT was global in extent.