Rainy day ice age in the global south

An international study of the mineral deposits in stalactites in South Australia’s Naracoorte Caves, has shed new light on climate conditions in the Southern Hemisphere during ice ages.

Research led by Melbourne University and including Naracoorte Caves and fossil forensics expert, Dr Liz Reed from the University of Adelaide, has turned the established theory about conditions during glacial periods on its head.

“What we have found is, that at least in the sub-tropical regions of the Southern Hemisphere, parts of the ice ages were even wetter than what we experience today,” Dr Reed says.

“Where other research has suggested the ice ages were uniformly dry, dusty and inhospitable, in our samples collected from speleothems in two Australian cave regions, the Naracoorte caves and the Leeuwin-Naturaliste caves, we discovered evidence that we did not expect.

“Using a dating technique based on the decay of naturally occurring uranium, we determined the age of more than 300 individual speleothem fragments from the caves and then produced a precipitation record spanning the past 350,000 years.”

One way to understand how wet it was in the past is to look at mineral deposits called speleothems, found in underground caves. These deposits, which include stalagmites and stalactites, build up over time as rainwater filters down through soil and limestone into the cave.

The extent of speleothem growth over time reflects changes in water availability. More speleothem growth broadly indicates wetter conditions, while less growth suggests a drier environment.

“It is broad, but the pattern we found was clear - wetter times occurred within the cooler, glacial periods, while interglacials (warmer periods) were consistently dry,” Dr Reed says.

“There was also fossil pollen trapped within the same speleothems which allowed us to compare glacial and interglacial periods.”

The team found that despite low levels of atmospheric carbon dioxide in glacial periods, the research showed moisture-demanding herbs and shrubs thrived.

The research team benchmarked their results against other public records for sub-tropical areas in South Africa and South America and found consistent evidence that parts of glacial periods had higher rainfall.

“This has been a really exciting project because it challenged the things we thought we knew about climate conditions during the glacial periods and that challenge leads us to rethink how we interpret the movement and expansion of plants, animals and even humans in the past,” Dr Reed says.