Gernon
Active Research Projects
My foundation in classic field geology and interest in the ‘big picture’ — the fundamental puzzle of how the Earth works — has resulted in a diverse research portfolio. Recent, exciting research has ranged from investigating the coupling between global tectonic and geochemical cycles in ‘deep time’ (e.g. Snowball Earth), to the regional controls on induced seismicity today. I apply a range of different techniques, including fieldwork, experiments, modelling, and analysis, to address these problems. I am currently involved in projects studying the impact cratering record of Earth and the Moon, the evolution of volcanism and seismicity in a range of tectonic environments, and understanding global-scale geochemical trends through Earth history.
Below are general summaries of four exciting projects I have worked on.
ASTEROID IMPACT FLUX ON EARTH AND THE MOON
The number of asteroids colliding with the Earth and Moon has increased by up to three times over the past 290 million years, according to a major new study involving the University of Southampton. These findings, published in Science, challenge our previous understanding of Earth’s history. Scientists have tried to understand the rate that asteroids hit the Earth for decades, usually by studying the craters and dating the rocks around them. The problem with doing this is that many experts assumed that the earliest craters have worn away over time due to erosion and other geological processes.
Image by Ernest T Wright, NASA
However, researchers have now found that we can learn a lot about the Earth’s impact history by studying the Moon, because both bodies are hit in the same proportions over time. Further, the Moon is immune to many of the processes, like plate tectonics, that gradually destroy Earth’s craters. “The only obstacle to doing this has been finding an accurate way to date large craters on the Moon”, said William Bottke, an asteroid expert at the Southwest Research Institute in Boulder, Colorado and a co-author of the paper.
Dr TM Gernon, University of Southampton
The team studied the surface of the Moon using thermal data and images collected by NASA’s Lunar Reconnaissance Orbiter (LRO), to determine the ages of the lunar craters [see graphic]. The NASA spacecraft’s thermal radiometer, known as Diviner, showed the scientists how heat is radiating off the Moon’s surface – with larger rocks giving off more heat than finer, lunar soil. Paper co-author Rebecca Ghent, a planetary scientist at the University of Toronto and the Planetary Science Institute in Tucson, Arizona, calculated the rate at which Moon rocks break down into soil, and revealed a relationship between the amount of large rocks near a crater and the crater’s age. Using Ghent’s technique, the team compiled the ages of all lunar craters younger than about a billion years.
Younger craters tend to be covered by more boulders and rocks than older craters. This happens because the boulders ejected by an asteroid strike get ground down over hundreds of millions of years by a constant rain of tiny meteorites.
When the team compared the ages and numbers of craters on the Moon to those on Earth, they made the remarkable discovery that they are extremely similar, challenging the idea that Earth had lost so many craters. “This means that the Earth has fewer older craters on its most stable regions not because of erosion, but because the impact rate was lower prior to 290 million years ago,” said Bottke.
Dr Thomas Gernon, Associate Professor in Earth Science at the University of Southampton, and co-author on the study, said: “Proving that fewer craters on Earth meant fewer impacts—rather than loss through erosion—posed a formidable challenge”.
However, Gernon exploited an unlikely line of evidence to piece together the story—long-extinct diamond volcanoes called kimberlite pipes that stretch, in a carrot shape, a couple of kilometres below the surface. His research showed that kimberlite pipes formed over the past 650 million years in stable terrains are largely intact; indicating that large impact craters formed over the same period and in the same terrains should also be preserved. This explained the similarity of the Earth and Moon’s impact crater records, and helped the team establish that the sparsity of craters formed before 290 million years ago is because there were fewer asteroid strikes before then.
“It was a painstaking task, at first, to look through all of these data and map the craters out without knowing whether we would get anywhere or not,” said Sara Mazrouei, the lead author of the paper who collected and analysed all the data for this project at the University of Toronto.
Locations of all impact craters identified in the Planetary and Space Science Centre (PASSC) Earth Impact Database, scaled by size and colored by age. Kimberlite occurrences are also shown, denoting stable cratonic regions that have not been deeply eroded. Figure by Dr TM Gernon, University of Southampton; from Mazrouei et al. Science (2019).
Sonification by SYSYEM Sounds
The team’s work led to the discovery that the rate of crater formation over the last 290 million years has been two to three times higher than in the previous 700 million years.
The reason for this jump in the impact rate is unknown but it could be related to large collisions taking place more than 290 million years ago in the main asteroid belt between the orbits of Mars and Jupiter, the researchers noted. Such events can create debris that can reach the inner solar system.
The team’s findings related to Earth, meanwhile, have implications for the history of life—which is punctuated by major extinction events and rapid evolution of new species. Although extinction events could have many causes, the team points out that asteroid impacts are very likely to have played a major role. In particular, the dinosaurs proliferated about 250 million years ago, and “as a species were particularly vulnerable to large impacts from the get-go, more so than earlier animal groups”, says Gernon.
NASA Goddard's press video:
Thanks to Ernest T Wright, NASA
NEOPROTEROZOIC SNOWBALL EARTH
Around 720-640 million years ago, much of the Earth’s surface was covered in ice during a glaciation that lasted millions of years. My group has shown that explosive underwater volcanic eruptions, and deep glacial erosion were major features of this ‘Snowball Earth’. I use a combination of field geology, geochronology and computational modelling to address the complex challenges associated with rocks of this age. Below are two key studies which I have led and co-authored.
Unzipping of Rodinia
Many aspects of Snowball Earth remain uncertain, but it is widely thought that the breakup of the supercontinent Rodinia (see the figure, below) resulted in increased river discharge into the ocean. This changed ocean chemistry and reduced atmospheric CO2 levels, which increased global ice coverage and propelled Earth into severe icehouse conditions.
Because the land surface was then largely covered in ice, continental weathering effectively ceased. This locked the planet into a ‘Snowball Earth’ state until carbon dioxide released from ongoing volcanic activity warmed the atmosphere sufficiently to rapidly melt the ice cover. This model does not, however, explain one of the most puzzling features of this rapid deglaciation; namely the global formation of hundreds of metres thick deposits known as ‘cap carbonates’, in warm waters after Snowball Earth events.
The Southampton-led research, published in Nature Geoscience, now offers an explanation for these major changes in ocean chemistry.
Lead author of the study Dr Thomas Gernon, Associate Professor in Earth Science at the University of Southampton, said: “When volcanic material is deposited in the oceans it undergoes very rapid and profound chemical alteration that impacts the biogeochemistry of the oceans. We find that many geological and geochemical phenomena associated with Snowball Earth are consistent with extensive submarine volcanism along shallow mid-ocean ridges.”
Evolution of spreading-ridge systems during the late Neoproterozoic, showing the importance of explosive eruptions in shallow basins during the onset of mid-ocean ridge formation. Figure by Dr TM Gernon, University of Southampton; from Gernon et al. (2016).
During the breakup of Rodinia, tens of thousands of kilometres of mid-ocean ridge were formed over tens of millions of years. The lava erupted explosively in shallow waters producing large volumes of a glassy pyroclastic rock called hyaloclastite. As these deposits piled up on the sea floor, rapid chemical changes released massive amounts of calcium, magnesium and phosphorus into the ocean.
Dr Gernon explained: “We calculated that, over the course of a Snowball glaciation, this chemical build-up is sufficient to explain the thick cap carbonates formed at the end of the Snowball event.
“This process also helps explain the unusually high oceanic phosphorus levels, thought to be the catalyst for the origin of animal life on Earth.”
An artist's impression of the shallow ridge hypothesis for Snowball ocean chemistry proposed by Gernon et al. (2016). Illustrated by Gary Hincks.
Deep Cryogenian erosion
The Earth’s surface experienced the largest crustal erosion event in Earth’s history some 700 million years ago, paving the way for animal life to develop, according to a major new study involving the University of Southampton.
A team of scientists present compelling new evidence for scouring of three to five kilometres across all the continents during the Neoproterozoic Era (one billion to 550 million years ago) which would have seen the Earth’s crust washed into the oceans in unprecedented volumes. The research is published in the journal Proceedings of the National Academy of Sciences of the United States of America.
The discovery provides the strongest explanation yet for the origin and extent of the ‘Great Unconformity’ – a profound gap in the Earth’s rock record – exposed most dramatically in the Grand Canyon in the United States. Here, sedimentary rocks from the Cambrian era, which began 550 million years ago, were deposited directly on top of rocks from the Mesoproterozoic era, which ended one billion years ago.
The Grand Canyon. Image by Murray Foubister, Creative Commons Attribution-Share Alike 2.0 Generic license
The erosion happened when most of the Earth’s surface was covered in ice during a severe glaciation, dubbed ‘Snowball Earth’, that lasted over 50 million years.
Dr Thomas Gernon, Associate Professor in Earth Science at the University of Southampton, and co-author on the study, said: “The findings help explain a fundamental enigma of Earth's history, whilst also having profound implications for mineral exploration in ancient terrains worldwide.
“A massive proportion of ancient rocks were simply scraped off the surface of the Earth in an abrupt event like no other in Earth’s history.”
The Great Unconformity, as exposed in the Grand Canyon. Photo courtesey of Dr Brenhin Keller
Whilst preserved rocks from this era are sparse, the scientists were able to study a database of 30 thousand zircon crystals formed in magmas – essentially ‘time capsules’ that preserve vital information on the chemical conditions that prevailed on Earth when they crystallised. These tiny inclusions were critical to unlocking the evidence for massive recycling of sediment into the interior of the Earth in a process known as subduction.
Apparent impact cratering rate per unit bedrock area. From Keller et al. (2019)
The team also made the remarkable observation that Earth’s largest asteroid impact craters were largely missing in rocks older than 700 million years, supporting the idea of deep global erosion. This process reconfigured the Earth’s surface and paved the way for the origin of animal life during the Cambrian era, known as the ‘Cambrian Explosion’, by changing the shape and chemical composition of the oceans, giving animals the environments and nutrients they needed to evolve.
Dr. Brenhin Keller, an Assistant Professor of Earth Sciences at Dartmouth College, who led the study, said: “Our study unifies a diverse set of geological observations and may prompt a fundamental reassessment of the relationship between erosion, sedimentation and sea level, on billion-year timescales.”
Satellite image of Arkaroola Wilderness Sanctuary, South Australia (from SENTINEL).
Fieldwork in "Tillite Gorge" within the Arkaroola Wilderness Sanctuary (shown in the satellite image above), South Australia, 2016. Prof Paul Hoffman for scale (in the foreground). Photo by Dr TM Gernon, University of Southampton. Here, our team led by Ross Mitchell documented the presence of glacial incisions several kilometres deep.
UNDERSTANDING INDUCED EARTHQUAKES
A huge increase in the number of man-made earthquakes in Oklahoma, USA, is strongly linked to the depth at which wastewater from the oil and gas industry is injected into the ground, according to a new study involving the University of Southampton.
Oklahoma has been a seismic hotspot for the past decade, with the number of damaging earthquakes – including the magnitude 5.8 Pawnee earthquake in 2016, the strongest in state history – regularly impacting on the lives of residents, leading to litigation against well operators.
Enbridge tank farm, Cushing, Oklahoma, Credit: Roy Luck, CC-BY-2.0
The ‘induced’ earthquakes also pose an increased risk to critical infrastructure such as a major commercial oil storage facility at Cushing (above), making them a national security threat.
The connection between ‘seismicity’ – the frequency of earthquakes – and deep fluid injection into underground rock formations is well established. However scientists, policymakers, and the oil and gas industry have been bewildered by the unprecedented surge in the number of earthquakes in Oklahoma, which has soared from two in 2008 to about 900 in 2015.
Oklahoma’s well operators have injected, on average, 2.3 billion barrels of fluids per year into the ground since 2011. Saltwater is injected deep underground as part of the oil and gas extraction process. The resulting wastewater is then routinely disposed of below ground, typically at depths 1 km to 2 km – well below the level of fresh ground water supplies, in order to avoid contamination.
Now a major study led by the University of Bristol and involving the University of Southampton, Delft University of Technology and Resources for the Future, published in the journal Science, shows conclusively that Oklahoma’s seismicity is strongly linked to fluid injection depth.
Earthquakes and wastewater injection in Oklahoma, from Hincks et al. (2018); Dr TM Gernon
Using a powerful, specially developed computer model incorporating injection well records and earthquake data from the US Geological Survey, the team examined the links between injection volume, depth and location, as well as geological features, over a six-year period.
The team found that the joint effects of depth and volume are critical, and that injection volume becomes more influential – and more likely to cause earthquakes – at depths where layered sedimentary rocks meet crystalline basement rocks. This is because deeper wells allow easier access for fluids into fractured basement rocks that are much more prone to earthquakes.
The volume of fluids injected under the ground has increased from 1.85 billion barrels in 2009 (the first year for which the team has data) to 2.75 billion barrels in 2014.
Study co-author Dr Thomas Gernon, Associate Professor in Earth Science at the University of Southampton, said: “The underlying causes of Oklahoma’s induced earthquakes are an open and complex issue, not least because there are over 10,000 injection wells, with many different operators and operating characteristics, all in an area of complex geology.
“Thanks to an innovative model capable of analysing large and complex data sets, our study establishes for the first time a clear link between seismicity and fluid injection depth.”
Simulated impact of raising the injection well level or capping monthly injection volume.
An infographic about our study to download
The study also shows how raising injection well depths to above the basement rocks in key areas could significantly reduce the annual energy released by earthquakes – thereby reducing the relative likelihoods of larger, damaging earthquakes. Current regulatory interventions include requiring operators to either reduce injection or raise wells above the basement, often by an unspecified amount.
Lead author of the study, Dr Thea Hincks, Senior Research Associate at the University of Bristol, said: “Our new modelling framework provides a targeted, evidential basis for managing a substantial reduction in induced seismicity in Oklahoma, with extensive possibilities for application elsewhere in the world. This marks a step forward in understanding the evolution of seismicity in the Oklahoma region.”
Professor Willy Aspinall, of the University of Bristol, who initiated the study, added: “This new diagnostic finding has potential implications for scientists, regulators and civil authorities concerned about induced seismicity, both in the US and internationally. The research addresses a growing need for a broader understanding of how operational, spatial and geologic parameters combine to influence induced seismic risk.
“Our analysis allows regulatory actions to be evaluated on a rational, quantitative basis in terms of seismic effects.”
KIMBERLITE VOLCANISM
Early in my career, I made a number of important contributions to understanding the dynamics of kimberlite eruptions, which carry diamonds from Earth's deep interior to the surface. These contributions range from developing a fundamental understanding of the processes that occur inside kimberlite volcanoes, and that unfluence diamond distributions in mines, to predicting diamond grades in geographic clusters of kimberlite bodies.
Jwaneng Diamond Mine (operated by Debswana), Republic of Botswana, is the richest diamond mine in the world (photographed in 2005 by Dr TM Gernon).
Processes inside diamond volcanoes
During my Ph.D. (2004-2007), I developed analogue laboratory experiments to characterise the fluid dynamical behaviour of pyroclastic material confined to diverging volcanic conduits (funded by De Beers). These experiments revealed an important, previously unrecognised physical process that operates in volcanic vents and generates a “conveyor-belt”-type mechanism of particle transport (up the centre and down the sides, see the figure below). These results demonstrate how fluctuations in gas velocity can produce steep boundaries between pyroclasticc units in volcanic vents, and exert a controlling influence on diamond size and grade distributions. The work provides a first-order framework for understanding the sequences of pyroclastic rocks infilling volcanic conduits, and has resulted in important changes to the community's perception of how eruptive regimes and key related parameters (e.g. gas velocity) are influenced by vent geometry. There is now general consensus in the diamond industry that this is an important process in kimberlite systems, exerting a considerable influence on diamond size and grade distribution, and impacting on economic forecasts. This model has been used to explain observed concentrations of large stones near the margins of kimberlites, and the homogeneous diamond grades found in massive, central regions of pipes inferred to have undergone vigorous fluidisation. There is also evidence that this process extends to other volcanic systems including basaltic vents and even subglacial volcanoes. The research was featured in Nature (Research Highlights).
Above: My experimental work recognised that in a tapered container there is strong fluidisation and mixing up the centre of the container, while there is downward motion of unfluidised material at the sidewalls, creating a convective system. This physical picture explains many of the key geological relationships in kimberlite pipes, as well as identifying for the most time a new and likely important regime of processes in volcanic conduits more generally.
Deciphering ancient diamond pipes
During my Ph.D., I also made contributions to understanding sedimentological processes that operate within open volcanic craters, and for the first time demonstrated how contamination of diamondiferous volcanoes by extraneous flows could exert a first-order influence on diamond grade and distribution in mines. This research demonstrates how knowledge of the physical volcanology (for example fabric analysis in pyroclastic flow deposits) can be used to constrain the flow direction and therefore identify the ultimate source of potentially high-grade diamondiferous material. As a consequence, the contribution demonstrates the feasibility of using detailed geological observations (and reconstructing the sequence of events) to guide mine economics and planning, and potentially even target new resources within a kimberlite cluster. These compelling findings have important implications for predicting diamond grade, and thus have been utilised by De Beers and the Debswana Diamond Company of Botswana, and prompted reconsideration of the volcanic geology of other major diamond mines. The work also made a difference to the wider community, including palaeontologists studying exceptionally preserved fauna and flora within this unique site of Cretaceous sedimentation.
An artist's impression of a kimberlite pipe, based on Gernon's Ph.D. thesis (illustrated by Gary Hincks)
International impact on diamond mining
Gernon's contributions to the diamond industry (mining and exploration) are summarised in a UK Research Excellence Framework Impact Case Study submission from the University of Bristol (2014). Referring to the scientific results of his work, a senior member of De Beers management stated “there are a number of important changes that have happened in the industry because of [this] contribution" . One example is “the sampling programme for the world class Orapa kimberlite mine was changed because of the geological mapping and interpretation conducted by Bristol researchers (see the papers by Gernon et al., 2009a,b). In the strictest confidence I can confirm that this change in sampling strategy has had a major positive influence on the life of this mine. This mine contributes roughly 10% of the world’s annual diamond supply. This change will significantly increase that proportion”. Furthermore, “Research conducted at Orapa mine by the Bristol team [Gernon et al., 2009a,b] showed the current reserve being mined may not continue below a certain depth level, and therefore the sampling programme to evaluate the deeper levels was tailored to ensure that significant diamonds are recovered from the deeper regions to test whether the diamond grades and qualities may have changed”. A second example is that “Mapping at Jwaneng Mine provided a clearer understanding of the geological continuity of facies in this mine, and this provided confidence that allowed management to approve changing the sampling methodology for grade determination”.
A high-quality macle diamond (10 ct) found by Dr TM Gernon in Snap Lake Diamond Mine, Arctic Canada.
