Sulfur-using bacteria are one of Earth’s most ancient and widespread life forms, arising at a time when oxygen levels in the atmosphere were less than one-thousandth of what they are now.There is a vigorous debate about the evolution of sulfur-dependent bacteria. Living in ocean waters, they breathed sulfate instead of oxygen. But how did that sulfate reach the ocean, and when did it become abundant enough for living things to use it?
Rocks 2.5 billion years old, formed prior to the Great Oxygenation Event, are extremely rare, so geologists’ understanding of this Neoarchaean Eon are based on a handful of samples from a few small areas, such as Western Australia, South Africa and Brazil. Most of the Neoarchaean rocks studied are from Western Australia and South Africa and are black shale, which forms when fine dust settles on the sea floor. The Brazilian sample contains plenty of black shale and a band of carbonate rock, formed below the surface of shallow seas. Black shale usually contains sulfur-bearing pyrite, but carbonate rock typically does not, so geologists have not focused on sulfur signals in Neoarchaean carbonate rocks until now.
Prof. James Farquar and his team from the University of Maryland worked with flagship engineer, Dr John Cliff in the AMMRF at the University of Western Australia, to take advantage of a new ion probe method for in-situ sulfur (S) isotope analysis developed by Dr Cliff. His technique is able to measure very low concentrations of the minor sulfur isotopes 33S and 36S in-situ at the scale of a few tens of micrometres – a finer scale than previously possible. They applied the technique to 2.5 billion-year-old carbonate rocks from Brazil.
The key aspect was the measurement of the minor sulfur isotopes, which account for only 0.75% and 0.01% of naturally occurring sulfur. Their precise ratio, however, gave a very strong signal that sulfur compounds were consumed and altered by living organisms and that the sulfur had originated in the atmosphere. Carbonates were not thought to hold this kind of information. The researchers also found that the amount of sulfate in the oceans was very much lower than previously thought.
This research was published in today’s issue of the journal Science and shows that bacteria dependent on sulfate were plentiful in some parts of the Neoarchaean ocean, even though sea water typically contained about 1,000 times less sulfate than it does today. It seems that the bacteria lived in shallow regions, where evaporation may have been high enough to concentrate the sulfate, making it abundant enough to support the bacteria. These bacteria were plentiful in the sediment, respiring sulfate and emitting hydrogen sulfide—the same process that goes on today as bacteria recycle decaying organic matter into minerals and gases.
The team is now analysing carbonate rocks of the same age from Western Australia and South Africa, to see if the pattern holds true for rocks formed in other shallow water environments. If it does, the results may change scientists’ understanding of one of Earth’s earliest biological processes.
Ref.: Iadviga Zhelezinskaia, Alan J. Kaufman, James Farquhar and John Cliff, Science Nov. 7, 2014.