New research shows that the formation of epidote in ancient oceans played a crucial role in determining the availability of certain metals, such as manganese and molybdenum, that were crucial to early life forms. The discovery, made by recreating Archean seawater in the laboratory, provides new insights into how early life evolved.
Scientists know very little about the ocean conditions when life first evolved, but new research published in the journal Nature Geoscience reveals how geological processes control which nutrients are available to fuel life.
All life uses nutrients such as zinc and copper to form proteins. The oldest living organisms evolved in the Archean Eon, about 3.5 billion years before dinosaurs first appeared. These microorganisms favored metals such as molybdenum and manganese compared to more recent microorganisms. This preference is thought to reflect the availability of metals in the ocean at the time.
Researchers from the University of Cape Town (UCT) and the University of Oxford have reconstructed ancient seawater in the laboratory. They found that chlorite, a mineral commonly found in Archean rocks, formed quickly and removed zinc, copper and vanadium in the process. Because epidote formed in early oceans, these metals were removed from the seawater, leaving it rich in other metals such as manganese, molybdenum and cadmium. Intriguingly, the metals they predicted would be most abundant in Archean seawater match those selected by early life forms, explaining why these metals were favored during early evolution.
Researcher Dr Rosalie Tostevin, who was based at the University of Oxford at the time of the study and is now a senior lecturer in the Department of Geological Sciences at UCT, said: "We were very excited when we noticed that our results matched the predictions of biologists using completely different methods. Experts in other fields have made similar findings, which is always reassuring."
Scientists agree that Archean seawater was very different from today's seawater, with more dissolved iron and silica and almost no oxygen. However, opinions are less consistent about other aspects of seawater chemistry, such as nutrient concentrations.
"We can't go back in time to sample and analyze seawater, so reconstructing the conditions of the Alcene Epoch is quite a challenge. One way is to look at the chemistry of sedimentary rocks, but the chemistry of very old rocks can sometimes be altered," Tostevin said. "We decided to create a miniature version of ancient seawater in the laboratory, where we could directly observe what was going on."
Tostevin and her colleague Imad Ahmed recreated Archean seawater in a special oxygen-free chamber and watched as epidote began to form. They observed dramatic changes in metal concentrations in seawater as minerals formed. They used the X-ray adsorption spectrometer at the Diamond Light Source Synchrotron to demonstrate that metals are moving into minerals. In contrast, other metals are not affected by this process and remain at higher levels in seawater.
"We know that chlorite was important in the early Earth because we keep finding it in ancient rocks, such as iron ore from the Northern Cape in South Africa and similar rocks in Australia," Tostevin said. "We think it may have been one of the most important minerals in the Archean Eon. But we don't know exactly how epidote formed in nature. One possibility is that epidote formed in hydrothermal vents deep in the ocean. But it could also have formed in shallow waters with small changes in pH."
Tostevin and Ahmed decided to experiment under both conditions and found that regardless of how chlorite was formed, it removed metals in a similar way.