Rising sea levels from melting land ice caused an abrupt transition from dry to wet conditions in a subregion of mainland Southeast Asia some 14,000 years ago, accounting for the unique autumnal monsoons in this area. Researchers at the University of California, Irvine and other international institutions found evidence of this shift in the geochemical signals stored in a stalagmite taken from a remote cave in central Vietnam.
The researchers used a previously broken 12-foot-long speleothem to create a hydroclimate history of the region dating between 4,000 and 45,000 years in the past. In a Proceedings of the National Academy of Sciences study, the UCI-led team found dry conditions occurred in central Vietnam at the peak of the last ice age, when 25 percent of Earth's land regions were covered in glacial ice. As the ice sheets melted during the end of this period, rising oceans inundated previously dry land, and rainfall abruptly increased, creating a climate much more similar to what is observed today.
"Through our geochemical analysis of stalagmite minerals and climate model simulations, we have been able to confirm that changes in sea level contributed to variations in autumn rainfall amount in central Vietnam," said lead author Elizabeth Patterson, UCI Ph.D. candidate in Earth system science. "This previously unknown influence on mainland Southeast Asia hydroclimate demonstrates that sea level has been a dominant driver of monsoon rainfall variability over the last 45,000 years."
Patterson said that when more of Earth's water was held as ice and global sea level was low, the Gulf of Tonkin to the east of Vietnam and the South China Shelf was dry land, causing less evaporated water vapor to reach central Vietnam. As ice sheets melted, exposed terrain began to be covered with water, increasing regional convection and moisture transport.
More than 650 million people living in Southeast Asia are impacted by heavy rainfall during the region's summer monsoon months. But in topographically isolated central Vietnam – where water vapor bumps up against the Truong Son Mountains that run north to south along the Vietnam-Laos border – the rainy season comes in September to November.
This distinction inspired the UCI-led team to explore caves in Phong Nha-Ke Bang National Park to look for evidence of the link between sea level rise and autumn monsoons in this unique region. The nearly 4-meter-long stalagmite they found grew continuously between 4,000 and 45,000 years ago in the remote Hoa Huong cave. The UCI-led team collected this sample in March 2020 on an overnight expedition to the remote cave in partnership with researchers and managers at Phong Nha-Ke Bang National Park, a UNESCO world heritage site.
The researchers relied on their understanding of cave hydrology and geochemistry, as well as stalagmite formation processes, to construct a climate history based on isotopic and elemental variations within the stalagmite layers. As water flows through the limestone rock above the cave, certain elements and isotopes accumulate in the drip water that ultimately forms the stalagmite. The relative concentration of these elements and isotopes change depending on whether the climate is wet or dry.
"Stalagmites grow drip by drip; calcite precipitates out of these droplets, depositing new layers through time," she said. "We measure stable oxygen and carbon isotopes and trace elements, such as magnesium, from these layers and see how they change through time. These measurements allow us to reconstruct how rainfall amount has changed in Vietnam over the last 45,000 years."
According to senior co-author Kathleen Johnson, UCI professor of Earth system science, this study demonstrates the power of using Earth's own natural climate archives – in this case the stalagmite minerals that accumulated slowly over thousands of years – to study the timing and mechanisms of past climate changes.
"Our comparison with climate model data confirms that the models are accurately capturing the influence of deglacial sea level rise and abrupt climate changes on Vietnam hydroclimate," she said. "By validating the models in this way, we can be more confident in projections of future hydroclimate in this sensitive region as we go deeper into the anthropogenic climate change era."
Source: https://uci.edu/