Freshwater has covered the ocean: a previously overlooked period in Earth's history has been discovered.

The last glacial period in Earth's history ended around 12,000 years ago. Scientists believe that during this time, our planet resembled a Snowball Earth, with glaciers covering much of its surface, even thickly at the equator. This period is well-studied by scientists, yet they still do not fully understand what happened afterward, which led to the emergence of the first animals, as reported by IFLScience.

As the massive ice sheets that once covered the tropics melted, they created what paleoclimatologists today refer to as the "plumeworld ocean." Now, it appears that scientists have, for the first time in history, found direct evidence of its existence. The plumeworld period was relatively short, as carbon dioxide levels rose sharply and then fell again after the ice melted.

According to co-author of the study, Dr. Tian Gan, he and his colleagues utilized the ratio of lithium-6 to lithium-7 in carbonate rocks formed during the ice melt. Their work supports the model of a process known as plumeworld.

Initially, Snowball Earth was self-reinforcing, with expanding ice reflecting sunlight back into space, keeping the planet cold. However, by blocking water from interacting with rocks, the ice also prevented weathering, which removes carbon dioxide from the Earth's atmosphere. As a result, CO2 emitted from sources like volcanoes accumulated and eventually heated the planet enough to start melting the ice.

This inevitably led to the release of freshwater, but scientists are still uncertain about the speed and geological effects of this process. According to the plumeworld model, there was supposed to be a phenomenal rebound in the weathering of continental rocks, as they were exposed to rain for the first time in millions of years. Consequently, the sediments deposited during that time eventually formed rocks and should contain clues, including their lithium ratios.

It is noteworthy that today, continental weathering produces lithium in riverbeds, deviating from the standard lithium-6/lithium-7 ratios by 23%. However, in the deep ocean, this figure is only 8.3%. Researchers point out that other elements that could be studied similarly depend on a variety of factors, including:

• temperature;
• salinity;
• microbial absorption.

All of this significantly complicates measurements, but lithium, according to scientists, is much simpler. The ratios are almost entirely determined by how much water drains from the land.

In the new study, Gan and his colleagues measured lithium isotope ratios in an area that is now Southern China and found that they change relentlessly as one moves away from what was then the continental coastline.

In fact, the results provide evidence that sea ice melted and formed a layer of freshwater above the salty oceans. After that, freshwater was enhanced by plumes coming from the continents, with significantly more freshwater near the shore. Fresh and saltwater mixed slowly, but eventually, this occurred, leading to the inclusion of minerals brought from land into the sediments below, although they became less abundant the further out to sea.

The authors of the study identify three stages in the dolomites:

1. the first, when sea ice was the primary source of freshwater;
2. the second, when continental melting plumes enhanced it;
3. the third, when salt and freshwater mixed.

Researchers note that the second stage essentially indicates how long the plumeworld existed, but there is one caveat: stratigraphic trends are not as reliable as the spatial picture.