Why was the earth so warm three billion years ago, even though the young sun was still shining very weakly? Researchers could now have found an explanation for this paradox: microbes in the iron-rich seas at the time intensively released the greenhouse gas methane into the atmosphere, thus causing warming. Research results from an Indonesian lake support this scenario, at the bottom of which the conditions are similar to those in the seas three billion years ago. The analyzes showed that microbes release surprisingly large amounts of methane there.
It is referred to as the “paradox of the young, weak sun”: Because of the still low radiance of our star, most of the earth’s oceans should have been frozen three billion years ago. But geological evidence suggests that this was not the case. This inconsistency was already shown in the 1970s. A possible explanation was already that a strong greenhouse effect must have been responsible for increased temperatures in the earth’s atmosphere at that time. But how such high concentrations of greenhouse gases could have come about remained a mystery.
What heated the earth three billion years ago?
The conditions in the oceans of that time seemed to speak against intensive production of the particularly strong greenhouse gas methane by the microorganisms that already existed at the time. Because they hardly contained any free oxygen and the sediments consisted to a large extent of iron minerals. Today the so-called band iron ore deposits – the most important iron ore deposits on earth – testify to this. Given these biochemical conditions, metabolic processes would be more likely to produce carbon dioxide, which, compared to methane, would have had a significantly lower greenhouse effect.
In order to gain more precise information about the microbial processes as they could have happened in the oceans three billion years ago, an international team of researchers has now devoted itself to the investigation of a body of water with unusual conditions: Lake Towuti in the jungle of the island of Sulawesi has depths from more than 130 meters of oxygen-free water that is rich in dissolved iron. Iron minerals, especially iron oxides, also make up more than a third of the sediment material. The researchers explain that the conditions in Lake Towuti are very similar to those in the oceans around three billion years ago. For their investigations, they extracted sediment drill cores from the depths of the lake. Samples were taken from them, which were then sent to the laboratories of the participating research groups. There the scientists then carried out mineralogical and biochemical studies of the microbial metabolism.
Surprising methane production
As the researchers explain, they actually expected that in the absence of oxygen, the microbes would mainly use the iron oxides in the sediment to process organic matter, releasing carbon dioxide in the process. In fact, they found that the organisms in the sediment of Lake Towuti instead use metabolic processes that produce methane. “Since microorganisms prefer to use iron oxide minerals in laboratory studies, these studies have so far formed the basis of our knowledge for such a metabolism on early Earth,” says first author Kohen Bauer from the University of British Columbia in Vancouver. “However, our findings from Lake Towuti now force us to rethink the processes that contributed to the regulation of the Earth’s early climate.”
As the researchers explain, the methane that rises from the sediment of Lake Towuti is now quickly converted to carbon dioxide through reactions with the oxygen dissolved in the upper water layers. However, due to the lack of oxygen three billion years ago, these processes did not exist and so the gas could have passed into the atmosphere in considerable quantities. Since methane is a much stronger greenhouse gas than carbon dioxide, such an enrichment process may have led to a significant warming of the world climate at that time, say the scientists.
“Our results from Lake Towuti challenge current knowledge of microbial processes in iron-rich environments. They are completely different than expected. In this way we can help solve the mystery of atmospheric evolution three billion years ago, ”says senior author Jens Kallmeyer from the German Research Center for Geosciences Potsdam. “Due to its special chemistry, we see Lake Towuti as an ideal model system for this time. However, we are only slowly beginning to understand the characteristics. ”The geomicrobiologist is now hoping for further exciting insights from this mysterious body of water deep in the jungle of the island of Sulawesi.
Source: Helmholtz Center Potsdam – GFZ German Research Center for Geosciences. Articles: Nature Communications, doi: 10.1038 / s41467-021-22453-0