The last deglaciation (~20 to 11 thousand years ago (ka)) was a period of dramatic natural warming on Earth. During this time, North America experienced the most extensive ice-sheet melting on the planet, which profoundly reshaped its climate and water cycle.
But when scientists look at oxygen isotopes in stalagmites—a key tool for reconstructing past climate—the signals from North America have been hard to interpret. A new study now provides a physical explanation for those puzzling patterns.
The study, published in Atmospheric and Oceanic Science Letters, was led by researchers from Nanjing Normal University and Nanjing University, China. The team used climate simulations to decode how water isotopes evolved across North America during the last deglaciation.
From the Last Glacial Maximum (~ 20 ka) to the early Holocene (~ 11 ka), water isotopes became more enriched across the continent. But the enrichment was strong in the north (above 50°N) and remarkably weak in the south (15°–50°N).
IMAGE: North–south water isotope divide: ice-sheet warming enriches the north; weak evaporation and remote moisture mute the south. Credit: Xiaoqing Wang.
"If we try to explain this with the simple rule of “warmer temperature, more enriched isotopes”, it works well in the north but fails in the south,” explains Xiaoqing Wang, the lead author and a Ph.D. candidate at Nanjing Normal University. “In the south, it explains only about 22% of the variance. Something else is at play.”
In the north, the enrichment happened mainly in winter. As the massive Laurentide Ice Sheet melted away, several key processes changed. First, winter warming weakened local condensation fractionation, so precipitation became less depleted in heavy oxygen. Second, the temperature difference between the moisture source and the rainfall site shrank, reducing the depletion of heavy isotopes along the transport path. The shrinking ice sheet also lowered its elevation, removing the altitude effect that had previously caused strong rainout of heavy isotopes.
In the south, the muted response comes from a different set of processes. Using water-tagging experiments, the team found that moisture from the subtropical North Pacific dropped by about 62% during the deglaciation. At the same time, moisture from the northern North Pacific and the Atlantic traveled much longer distances to reach the south, which made the incoming vapor more depleted.
Weaker local evaporation in the south further limited the chance to enrich isotopes through moisture recycling. As a result, the warming-driven enrichment was largely offset.
“The south tells us that future greenhouse?gas?driven warming may influence midlatitude hydroclimate not just through local temperature, but through major reorganizations of atmospheric circulation and moisture sources,” concludes Professor Jian Liu of Nanjing Normal University, corresponding author of the study.
Citation:
Xiaoqing Wang, Weiyi Sun, Jian Liu, Liang Ning, Yuntao Bao, Mi Yan, 2026. Simulation studies of climate and water isotopes in North America during the Last Deglaciation. Atmospheric and Oceanic Science Letters, https://doi.org/10.1016/j.aosl.2026.100849.
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