When giant ice sheets that once covered the tropics melted, they produced what paleoclimatologists call a “plumeworld ocean”, for which we now have direct evidence. The plumeworld was relatively brief, as carbon dioxide levels skyrocketed and fell again after the ice was gone.
To the general public, references to “the last Ice Age” mean the event that finished around 12,000 years ago. However, when discussing a global Ice Age, many scientists mean the series of events also known as “Snowball Earth”. There’s still debate as to how life survived a time when glaciers were thick even at the equator, but somehow it did, and around the end of the Marinoan Ice Age, the first animals appeared.
Dr Tian Gan, then at Virginia Tech, was part of a team that explored what happened afterward using the ratio of lithium-6 to lithium-7 in carbonate rocks laid down as the ice melted. Their work supports a model of the process known as plumeworld.
Initially, Snowball Earth was self-reinforcing, with expanding ice reflecting sunlight back into space and keeping the planet cold. However, by blocking rocks’ exposure to water, the ice also prevented the weathering that removes carbon dioxide from the atmosphere. CO2, emitted from sources such as volcanoes, built up, and eventually warmed the planet enough to start melting the ice.
Inevitably, a pulse of freshwater was released, but uncertainty remains about the speed and geological effects. According to plumeworld, there should have been a phenomenal rebound in the weathering of continental rocks as they were exposed to rain for the first time in millions of years. Sediments deposited at the time that eventually became rocks should hold the clues, including in their lithium ratios.
Today, continental weathering produces lithium in riverbeds that deviates from standard lithium6/lithium7 ratios by 23 percent, but the figure is just 8.3 percent in the deep ocean. Other elements that could be studied in similar ways are affected by factors such as temperature, salinity, or absorption by microbes, complicating measurements – but lithium is much more straightforward. Measurements of the ratios are almost entirely dictated by how much water flows off the land.
Gan and colleagues measured the lithium isotope ratios in what is now South China, and found they change steadily the further one goes from what was the continental shoreline at the time.
This supports the theory that sea ice melted and formed a layer of freshwater on top of the saline oceans. This freshwater was then enhanced by plumes coming off the continents, with the freshwater most abundant close to the shore. The fresh and salty waters mixed slowly, but eventually, they did, leading to the incorporation of the minerals brought from the land into the sediments below, but there was less of this the further out to sea one went.
The authors identified three stages in the dolostones, the first laid down when the sea ice was the primary source of fresh water; the second as continental melting plumes reinforced it; and the third as salty and fresh waters mixed. In theory, the thickness of Stage II could show us how long plumeworld lasted, but the authors note; “The stratigraphic trends are not as robust as the spatial pattern.”
“Our results have important implications for understanding how Earth’s climate and ocean chemistry changed after the extreme conditions of the last global ice age,” Gan said in a statement.
The temperature swings of the era dwarfed those of today in size – going from -45 °C (-50 °F) to warmer than the modern world – but probably not in rate. Rapid as the warming was by geological standards, it still took hundreds of thousands or millions of years. There are few if any precedents for the change seen over the last 50 years’ rise in global temperatures in the geologic record. Nevertheless, it is possible that learning about periods like this could provide lessons for what to expect as humanity creates a new carbon dioxide pulse.
The study is published in the journal Proceedings of the National Academy of Sciences.
