Comparing oxygen isotope records from Asian stalagmites and Greenland ice cores, alongside deuterium excess records from Antarctic ice cores, during abrupt warming events of the last glacial period.

A research team from Xi'an Jiaotong University (XJTU) has made a breakthrough in understanding abrupt climate changes, according to a research article published in the journal Science on Nov 6.

The paper, titled Glacial abrupt transitions in ocean circulation were much slower than ice-core records imply, lists XJTU as the first institutional affiliation and a co-corresponding institution.

Assistant Professor Dong Xiyu of XJTU is the paper's first author. Professor Cheng Hai of XJTU and Professor Zhang Xu, a chair professor at XJTU, are co-corresponding authors. This marks the third paper published in Nature or Science by Professor Cheng's team with XJTU as the lead institution.

With the support of scientists from various institutions, the team combined climate records and climate-model simulations to investigate a series of abrupt climate shifts during the last glacial period, known as Dansgaard-Oeschger (DO) events.

For the study, researchers analyzed two annually laminated stalagmites from Cherrapunji Cave in northeastern India and Shuixi Cave in southwestern China. Using the isotope laboratory at XJTU's Isotope Laboratory at Institute of Global Environmental Change and the university's State Key Laboratory for Manufacturing System Engineering, they conducted uranium-thorium (U-Th) dating and annual layer counting.

Together with previously published records from Zhangjia Cave, the team reconstructed high-resolution stalagmite δ¹⁸O records covering DO-1 through DO-8 events, allowing them to precisely determine the duration of transitions into DO warm periods.

These stalagmite records feature densely spaced, highly precise U-Th dating, high temporal resolution and annual-layer constraints, making them a new generation of high-quality records long sought by the scientific community.

The findings show that transitions into DO warm periods recorded in stalagmites all lasted more than 100 years, with an average duration of approximately 180 years, although the length varied from event to event. This is considerably longer than the corresponding transitions recorded in Greenland ice-core δ¹⁸O data, which lasted less than 70 years.

By combining these findings with climate-model simulations that incorporate isotope data, the researchers further identified a two-stage process of strengthening in the Atlantic Meridional Overturning Circulation (AMOC).

In the first stage, the retreat of sea ice in the northeastern North Atlantic triggered a rapid shift in regional atmospheric circulation from glacial to interglacial conditions, causing a sharp rise in Greenland's δ¹⁸O values.

In the second stage, the AMOC continued to strengthen, while sea ice in the northwestern North Atlantic retreated further. However, atmospheric circulation patterns and moisture transport pathways remained largely unchanged, resulting in no significant further fluctuations in Greenland's δ¹⁸O values.

The simulations indicate that stalagmite δ¹⁸O records from the Asian monsoon region capture the full process of AMOC strengthening. Meanwhile, the Southern Hemisphere westerly wind belt gradually shifted northward over centuries, with its transition broadly coinciding with the strengthening of the AMOC.

These findings suggest that the full process of interhemispheric thermal adjustment unfolded gradually over centuries, rather than occurring as rapidly as Greenland ice-core δ¹⁸O records alone might suggest. The study offers new insights into the pace and mechanisms of abrupt climate transitions during the last glacial period.