Millennial-scale climate events are the best example of naturally occurring rapid climate change. However, the absolute timing and underlying cause of these events remains unknown, largely due to the absence of long continuous records of the events that are anchored in radiometric time. Speleothems (cave deposits such as stalagmites) can preserve long, continuous and precisely dated records of millennial-scale climate events, thus providing vital information on their timing and underlying cause. A subaqueous speleothem from Corchia Cave, NW Italy, contains a palaeoclimate record spanning the last 1 million years, presenting an unprecedented opportunity to investigate the long-term pattern of millennial-scale climate events.
The pilot study presented here tests the potential of this speleothem to record millennial-scale climate events by focusing on the last glacial period (120-12 ka), a period already well captured in Greenland ice cores (the current reference record of the events). Due to its slow growth rate, microanalytical techniques were applied to the speleothem to measure three potential climate proxies: Mg (a proxy for temperature), Zn (a proxy for cold climate intervals) and 5180 (a proxy for rainfall amount). Uranium-thorium (U-Th) dating was undertaken to provide the temporal framework.
Mg and Zn provide an impressive record of fluctuations consistent with millennial time scales, whilst the 8180 analysis is limited by the resolution and precision of the technique applied. Unexpectedly, the U-Th results reveal an age offset, whereby the timing of high-amplitude regional climate events is at least 2 kyr too old. This age offset, possibly due to a process known as `thorium scavenging', casts serious doubt over the integrity of the chronology, precluding any robust comparison with the Greenland ice-core record. To partly overcome the age offset issue, a first-order correction was performed by tying the speleothem series to well-constrained Corchia Cave stalagmite records. This correction is only possible over a short interval (100-60 ka), which contains few millennial-scale climate events. However, based on this correction, fluctuations in Zn and Mg are broadly in alignment with the higher-magnitude events recorded in Greenland ice, suggesting that the speleothem does record evidence of millennial-scale climate events, and, with further analysis, could yield a 1 Myr record of such events.
Meanwhile, the dating problem could potentially resolve a long-standing debate over the chronology of a high-profile palaeoclimate record published in the 1980s and 1990s from Devils Hole (USA), and has important implications for future studies focusing on subaqueous speleothems
