A new study has come up with a possible explanation for the puzzling reappearance of major iron formations long after the rise in atmospheric oxygen about 2.4 billion years ago.
Iron formations are unique sedimentary rocks with no modern analogue. Most iron formations were deposited in the oceans before free oxygen first accumulated in Earth’s atmosphere about 2.4 billion years ago (the so-called Great Oxidation Event).
However, the occurrence of major iron formations nearly 500 million years later (about 1.9 billion years ago) has been an enduring enigma because the build-up of oxygen in the atmosphere should have prevented iron formations from developing.
Professor Birger Rasmussen, ARC Professorial Fellow, Curtin’s Department of Applied Geology, said major iron formation of this age range occurred in North America and Australia.
But because the Australian iron formations were thought to be significantly younger, it was uncertain whether they provided information about the composition of the global ocean or conditions in a restricted or closed basin.
“We have dated volcanic ash beds in the Australian iron formations, showing that they were deposited at the same time as those in North America,” Professor Rasmussen said.
“These results suggest that the deposition of iron formations from two different continents was synchronous 1.9 billion years ago and therefore probably reflects the composition of the global ocean. Thus, it follows that seawater at this time was rich in dissolved iron and contained little or no oxygen below the surface water layer.”
He said the remarkable correlation in time between the deposition of major iron formations and a short-lived but intense interval global igneous activity was striking and suggested that geological processes deep beneath the Earth’s surface radically changed the chemistry of the global ocean.
“We suggest that extensive basaltic magmatism and hydrothermal alteration related to this major igneous event released vast volumes of iron and other reactive elements into the global ocean, overwhelming the supply of oxygen (and other oxidants) and promoting the deposition of iron formations across the world,” he said.
“The equally dramatic disappearance of iron formations some 40 million years later can be explained as a consequence of rapid waning igneous activity and hydrothermal alteration. Subsequently, the ocean became dominated by seawater oxidants until more than a billion years later when “snowball Earth” conditions once again favoured the return of iron formations.
“Our findings not only explain the sudden appearance and disappearance of iron formations circa 1.9 billion years ago, but also provide an explanation for the preservation of an oxygen-rich atmosphere above an oxygen-poor ocean.”
The research was carried out by Curtin University’s Professor Birger Rasmussen, Dr Ian Fletcher and Dr Courtney Gregory, Assistant Professor Andrey Bekker (University of Manitoba), Dr Janet Muhling (University of Western Australia) and Dr Alan Thorne (Geological Survey of Western Australia).
The study was funded by the Australian Research Council and the Western Australian State Government through the Royalties for Regions scheme.
The findings will be published in the science journal Nature this week.
Birger Rasmussen, Curtin University
Tel: 08 9266 9254, Email: B.Rasmussen@curtin.edu.au