Earth's oldest soil could be tucked away in an ancient rock outcrop in Greenland, according to new research. Dating back some 3.7 billion years, the suspected soil — exposed underneath a retreating ice cap — could potentially contain fossilised traces of primordial life.
The new study, published this week in the awkwardly named science journal Palaeogeography, Palaeoclimatology, Palaeoecology, opens with: "Soil formation is a combination of physical, chemical, or biological processes important for regulating planetary atmospheres, and the ultimate source of essential nutrients such as phosphorus for the nutrition and origin of life."
Indeed, soil — unlike sterile bits of rock or sand — serves as a natural medium for the growth of land plants. Identifying our planet's oldest soils, therefore, is of critical importance to scientists who study Earth's formative period and the emergence of our planet's first organisms.
The new study, led by University of Oregon geologist Greg Retallack and Old Dominion University geologist Nora Noffke, describes a tantalising new rock outcrop in the Isua Greenstone Belt of southwestern Greenland that, quite possibly, contains our planet's oldest dirt, and by consequence, the oldest traces of life on the planet.
It's not actually dirt — it's a substance known as palaeosol, former soil that's been packed tightly into solid rock. During a recent helicopter survey, Noffke saw the outcrop sticking out from underneath a receding ice cap. After collecting the rocks and analysing them in the lab, the researchers dated the samples at 3.7 billion years old — which, if confirmed, would make it the oldest known dirt on the planet.
Prior, the oldest known palaeosol was found in Australia's 3.5 billion-year-old Panorama Formation.
"This area of Greenland keeps throwing up new outcrops as the ice sheets retreat — I guess global warming isn't all bad," said Matthew Dodd, a geologist from University College, London, who was not involved with the new study. "They make a good first case for rocks being ancient soils. As for ancient life, if true then it trumps the current record for oldest life on land, which is around 3.5 billion years ago. This is interesting because the surface world would have been a nasty place to live on the early Earth without protection from UV radiation."
No doubt, the rock sample is providing a fascinating glimpse back into time. Chemical, microscopic, and physical analyses of the twisted metamorphic rock and the graphite tucked within it suggests a climate with moderate humidity and temperatures averaging around 12C. The topographic setting where this soil formed (if that's indeed what it is) was a narrow coastal plain with braided streams tucked between a continental shelf and some mountains, the authors write in the study.
The palaeosols "accumulated sulphate and sand crystals, of a size found in modern well-drained soils over periods of several thousand years," the authors write. Fragments taken from the outcrop display distinctly soil-like characteristics, including mudcracks and sand crystals.
Retallack said the sample provided an unprecedented view of the conditions on land at the time.
"The Earth would have been uninhabitable by humans or other animals, because the minerals in the soil show that there was very little oxygen in the air," he said in a statement. "Weathering back then was also odd, because it was more like acid-sulphate weathering of desert crusts than modern weathering by rain and plants. Such acid sulphate palaeosols have also been found on Mars, where they are also about 3.7 billion years old."
In all, eight different tests were done on the samples, none of which, the authors claim, were able to decisively rule out the palaeosol hypothesis. As for the claim that this soil may have harboured early microbial life, the researchers say this evidence comes in the form of light carbon isotopes with signatures consistent with photosynthesis. Obviously, where there's photosynthesis, there's life.
"The characteristic isotopic ratios of carbon throughout the palaeosol are tantalising indications of life on land much further back in time than previously thought," said Retallack. "Although the origin of life has been envisaged in warm little ponds or scalding hot submarine springs, this discovery encourages those who think that life originated in soil."
The discovery of specific carbon isotopes within the samples is great, but as Retallack himself admitted, the "evidence for life is circumstantial." Further evidence would be required to pull more conclusive biosignatures from the rock.
"A lot more work is needed to convince me that the graphite is biomass," Dodd told Gizmodo. "There are very small amounts of graphite, and it is tricky to rule out a meteoritic origin for the graphite at present."
To be clear, Retallack and Noffke are not saying the samples are most definitely chunks of palaeosol. And in fact, these researchers went so far to express their trepidation by phrasing the title of the study in the form of a question: "Are there ancient soils in the 3.7 [billion year old] Isua Greenstone Belt, Greenland?" But what Retallack and Noffke are saying is that they've found something super interesting, and that further investigations of this outcrop are most definitely warranted.