New study finds oxygenic photosynthesis 3.5 billion years ago
A new study published in Nature Geoscience by Masamichi Hoashi at Kagoshima University, and coworkers, proposes that oxygenic photosynthesis may have been going on 3.5 billion years ago… by far the oldest implication of cyanobacterial O2 production ever posited. The new study bases its conclusions on hematite (Fe2O3) crystals obtained from marine sediments laid down during the early Archean Eon, about 3.46 billion years ago.
The new study examines layered sediments from Archean rocks found in Australia, and proposes that seafloor hot spring activity delivered iron to ancient sea water as dissolved ferrous ions… which also would have been the dominant form of dissolved iron in the oceans back then, before free oxygen was present in the atmosphere. According to the authors, ferrous iron oxidized on the spot as it was emitted from hot springs, by oxygen gas permeating the water. Where would such oxygen gas come from? According to Hoashi et al., it came from oxygenic photosynthesizers – microbes capable of using sunlight energy for growth by taking in water and converting it to waste oxygen. The only problem with this model is that oxygenic photosynthesis probably didn’t appear on Earth prior to about 2.7 billion years ago. Widespread cyanobacteria didn’t start adding measurable amounts of O2 to Earth’s air until about 2.2 billion years ago… more than a billion years after Hoashi’s rocks were laid down.
What else could have oxidized all that ferrous iron, 3.5 billion years ago? Sunlight itself. Cyanobacteria use sunlight to work some complicated chemistry that results in O2 production, and O2 can rust iron. But sunlight can do the job directly, too. Ultraviolet light from the Sun can kick electrons from dissolved ferrous ions in water, forcing them to oxidize to ferric iron. Ferric is highly insoluble, and in Earth’s ancient oceans any ferric iron that formed would have sedimented out almost instantly as ferric oxides… which rapidly recrystallize to hematite in bottom sediments.
On the ancient Earth, an atmosphere without O2 meant a stratosphere without ozone, so the UV flux to the Earth’s surface would have been harsher than today. Surface ocean water – containing 100s of parts per million dissolved ferrous iron – irradiated by solar UV would have produced a constant slow trickle of ferric oxide rust, which would have drifted down through the water column to collect on the sea floor. In spots on the Earth’s sea surface where extra ferrous was coming in, say from sea floor hydrothermal activity below, hematite production at the surface would have been concomittantly more substantive. So why did the authors skip photolysis and jump straight to cyanobacteria?
Depth. The authors noted that sedimentary layers in their cores lacked any evidence of wave action or ripple marks that might occur in shallow sand and mud. Based on that – and apparently on that alone – the authors concluded that ferrous iron was belching into ancient seas filled with cyanobacteria, whose waste O2 also filled ancient sea water and quickly oxidized the ferrous to hematite.
Not so fast. I have serious doubts about this article, primarily because the authors appear to have leapt over a few logical steps in their rush to arrive at a flashy headline. If one stipulates that the authors’ observations are accurate, and no shallow-water sedimentary structures are apparent in their cores, that still doesn’t mean the sediments were laid down in deep water. On Earth today there are many marine settings where wave action is minimal and where waters are calm… such as behind barrier islands and sand bars, or within sheltered estuaries. There are simply too many options for sediments to avoid ripple marks, to justify a claim that the absense of such marks necessarily connotes depth.
In addition, even if the sediments derived from deep water there’s no reason to assume hematite had to form right there. Ferrous-iron photolysis occurs only in shallow water where UV can penetrate – typically in the upper few meters of water – but have you ever noticed what happens to ink dropped into water? It mixes. Ferrous iron delivered into 200 m deep ocean water by hot springs will naturally circulate with the water, mixing through the water column by advection and convection… until it passes through the upper few meters and gets oxidized by UV rays, at which point it forms ferric oxide solids and begins to sink back to the bottom. Hematite can build up in bottom sediment even if it gets oxidized in the photic zone, hundreds of meters above, simply through the action of gravity… with no anomalous, time-traveling cyanobacteria needed as an explanation.
This new paper by Hoashi et al. is an interesting study that provides some highly intriguing data… but only a weak argument that cyanobacteria evolved 800 million years earlier than we thought. In fact, if I were one of the peer-reviewers on the manuscript, I’d probably have sent this one back for revision, based on the objections I describe above. I don’t think the authors have made their case.