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Scientists know very little about conditions in the ocean when life first evolved, but new research published in Nature Geoscience has revealed how geological processes controlled which nutrients were available to fuel their development.
Researchers from the University of Cape Town (UCT) and the University of Oxford recreated ancient seawater in the laboratory. They found that greenalite, a mineral that is common in Archean rocks, forms rapidly and removes zinc, copper, and vanadium in the process. As greenalite was forming in early oceans, these metals would have been depleted from seawater, leaving it rich in other metals, such as manganese, molybdenum, and cadmium. Intriguingly, the metals they predict would have been most abundant in Archean seawater match the metals chosen by early lifeforms, explaining why these metals were favoured during early evolution.
Dr Rosalie Tostevin, a Senior Lecturer in the Department of Geological Sciences at UCT, said;
We were very excited when we noticed that our results match predictions from biologists who use a completely different approach. It is always reassuring when specialists in other fields are making similar findings.
Scientists agree that seawater in the Archean was very different to today, with more dissolved iron and silica, and little to no oxygen. However, there is little agreement about other aspects of seawater chemistry, such as the concentration of nutrients.
Dr Tostevin continued;
We can’t go back in time to sample seawater and analyse it, so reconstructing Archean conditions is quite a challenge. One approach is to look at the chemical make-up of sedimentary rocks, but the chemistry of very old rocks has sometimes been altered. We instead decided to create a miniature version of ancient seawater in the laboratory, where we could directly observe what was happening
Tostevin, and her colleague Imad Ahmed, recreated Archean seawater inside a special oxygen-free chamber, and watched as greenalite began to form. They observed dramatic changes in the metal concentrations in seawater as the minerals formed. They used X-Ray Absorption Spectroscopy at beamline B18 at Diamond Light Source synchrotron to prove that the metals were entering the minerals. In contrast, other metals were unaffected by this process and stayed at high levels in seawater.
Dr Tostevin said;
We know that greenalite was important on the early Earth, because we keep finding it in old rocks, such as the iron ore in the Northern Cape, South Africa, and similar rocks in Australia. We think this may have been one of the most important minerals in the Archean. But we don’t know exactly how greenalite was forming in nature. One possibility is that greenalite formed at hydrothermal vents, deep in the ocean. But it could also have formed in shallow waters, wherever there was a small change in pH.
Dr Tostevin and Ahmed decided to run their experiments under both types of conditions, and found that regardless of how greenalite forms, it removes metals in a similar way.
One question that concerned the researchers was whether the metals would be locked up for a long time or released back into seawater after several months or years. To test this, they heated the minerals to emulate what happens when they are buried and undergo crystallisation. The metals remained trapped in the mineral, suggesting this was a permanent sink for metals that would have had a profound impact on early seawater.
Giannantonio Cibin, former Principal Beamline Scientist for the B18 beamline during this research, now responsible for the SWIFT beamline for Diamond-II, concludes;
This work highlights how critical the information provided by X-ray Absorption Spectroscopy can be for environmental science research. XAS is a unique technique, which can be applied to complex systems where the important information is concealed in components that represent just a minor fraction of the material under study.
The publication from Tostevin and Ahmed is an excellent example of XAS abilities, as the experiment performed in B18 beamline at Diamond Light Source provided a detailed picture of the location of Ni inside minerals formed back in the Precambrian, a key result that could have not been obtained with other characterization methods.
Tostevin & Ahmed, 2023: 'Micronutrient availability in Precambrian oceans controlled by greenalite formation', Nature Geoscience, Doi: 10.1038/s41561-023-01294-0
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