Rock solid evidence: Angola geology reveals prehistoric split between South America and Africa

An SMU-led research team has found that ancient rocks and fossils from long-extinct marine reptiles in Angola clearly show a key part of Earth's past -- the splitting of South America and Africa and the subsequent formation of the South Atlantic Ocean.

With their easily visualized "jigsaw-puzzle fit," it has long been known that the western coast of Africa and the eastern coast of South America once nestled together in the supercontinent Gondwana -- which broke off from the larger landmass of Pangea.

The research team says the southern coast of Angola, where they dug up the samples, arguably provides the most complete geological record ever recorded on land of the two continents moving apart and the opening of the South Atlantic Ocean. Rocks and fossils found date back from 130 million years ago to 71 million years.

"There are places that you can go to in South America, for instance, where you can see this part of the split or that part of it, but in Angola, it's all laid out in one place," said Louis L. Jacobs, SMU professor emeritus of Earth Sciences and president of ISEM. Jacobs is the lead author of a study published in The Geological Society, London, Special Publications.

"Before this, there was not a place known to go and see the rocks on the surface that really reflected the opening of the South Atlantic Ocean, because they're now in the ocean or eroded away," Jacobs said.

Angola rocks and fossils tell the whole story

Africa and South America started to split around 140 million years ago, causing gashes in Earth's crust called rifts to open up along pre-existing weaknesses. As the tectonic plates beneath South America and Africa moved apart, magma from the Earth's mantle rose to the surface, creating a new oceanic crust and pushing the continents away from each other. And eventually, the South Atlantic Ocean filled the void between these two newly-formed continents.

Scientists have previously found evidence of these events through geophysics and well cores drilled through the ocean floor.

But these tell-tale signs have never been found in one place, or been so clearly visible for anyone to see, said study co-author Michael J. Polcyn, research associate in the Huffington Department of Earth Sciences and senior research fellow, ISEM at SMU.

"It's one thing for a geophysicist to be able to look at seismic data and make inferences from that," he said. "It's quite another thing to be able to take a school field trip out to the rock formations, or outcrops, and say this is when the lava was spreading from eastern South America. Or this was when it was a continuous land."

Essentially, Angola presents the opportunity for someone to easily walk through each phase of this geologically significant chapter in Earth's history.

"That gives Angola major bragging rights," Jacobs said.

Jacobs, Polcyn and Diana P. Vineyard -- who is a research associate at SMU -- worked with an international team of paleontologists, geologists and others to analyze both the rock formations they found in eight different locations on the coast and the fossils within them.

Fieldwork in Angola's Namibe Province began in 2005. At that time, the research team recognized particular types of sediments, which gave them a good indication of what the western coast of Africa had been like at various stages millions of years ago. For instance, fields of lava revealed volcanic outpourings, and faults or breaks showed where the continents were being rifted apart. Sediments and salt deposits showed ocean flooding and evaporation, while overlying oceanic sediments and marine reptiles showed completion of the South Atlantic Ocean.

Paleontologists, meanwhile, discovered fossils in Angola from large marine reptiles that had lived late during the Cretaceous Period, right after the Atlantic Ocean was completed and while it grew wider.

By bringing together experts from a wide range of fields, "we were able to document when there was no ocean at all, to when there was a fresh enough ocean for those reptiles to thrive and have enough to eat," Vineyard said.

Many of the ancient fossils are currently on display at the Smithsonian's National Museum of Natural History "Sea Monsters Unearthed: Life in Angola's Ancient Seas" exhibit, which was co-produced with SMU -- a nationally-ranked Dallas-based private university.

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The last 12,000 years show a more complex climate history than previously thought

We rely on climate models to predict the future, but models cannot be fully tested as climate observations rarely extend back more than 150 years. Understanding the Earth's past climate history across a longer period gives us an invaluable opportunity to test climate models on longer timescales and reduce uncertainties in climate predictions. In this context, changes in the average surface temperature of the Earth during the current interglacial Epoch, the Holocene (approximately the past 12,000 years), have been thoroughly debated over the past decades. Reconstructions of past temperature seem to indicate that global mean temperature showed a maximum around 6,000 years ago and has cooled until the onset of the current climate crisis during the industrial revolution.

Climate model simulations, on the other hand, suggest continuous warming since the start of the Holocene. In 2014, researchers named this major mismatch between models and past climate observations the "Holocene Temperature Conundrum."

In this new study, scientists used the largest available database of past temperature reconstructions extending back 12,000 years to carefully investigate the geographic pattern of temperature change during the Holocene. Olivier Cartapanis and colleagues find that, contrary to previously thought, there is no globally synchronous warm period during the Holocene. Instead, the warmest temperatures are found at different times not only in different regions but also between the ocean and on land. This questions how meaningful comparisons of the global mean temperature between reconstructions and models actually are.

According to the lead author Olivier Cartapanis, "the results challenge the paradigm of a Holocene Thermal Maximum occurring at the same time worldwide." And, while the warmest temperature was reached between 4,000 and 8,000 years ago in western Europe and northern America, the surface ocean temperature cooled since about 10,000 years ago at mid-high latitudes and remained stable in the tropics. The regional variability in the timing of maximum temperature suggests that high latitude insolation and ice extent played major roles in driving climate changes throughout the Holocene.

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Earliest record of wildfires provide insights to Earth's past vegetation and oxygen levels

While wildfires over recent years have raged across much of the western United States and pose significant hazards to wildlife and local populations, wildfires have been a long-standing part of Earth's systems without the influence of humans for hundreds of millions of years.

"Wildfire has been an integral component in earth-system processes for a long time and its role in those processes has almost certainly been underemphasized," said Ian Glasspool, lead author of a study published yesterday in Geology that describes the earliest record of wildfire found yet to date.

In the study, Glasspool and co-author Robert Gastaldo document 430-million-year-old charcoal produced by wildfires found in samples from Wales and Poland. Their discovery pushes back the earliest record of wildfire by an additional 10 million years.

Glasspool explained that wildfire has three essential ingredients: a source of fuel, a source of ignition (which comes in the form of lightning strikes), and sufficient atmospheric oxygen.

"It looks now as though our evidence of fire coincides closely with our evidence of the earliest land plant macrofossils. So as soon as there's fuel, at least in the form of plant macrofossils, there is wildfire pretty much instantly," said Glasspool.

However, the types of plants that existed 430 million years ago during the Silurian period would have looked starkly different from the plants we see and are familiar with today. Instead of grasses, trees, and flowers, flat-lying plants barely even an inch tall would have covered much of the landscape, with the occasional waist-height or knee-height plant. In contrast to much of the diminutive plant cover, the ancient fungus Prototaxites would have stood nearly 30 feet (9 meters) tall, towering over the landscape. These Silurian plants would have been strongly dependent on water for their reproduction and likely would not have been found in seasonally dry areas.

"The Silurian landscape had to have enough vegetation across it to have wildfires propagated and to leave a record of that wildfire," said Gastaldo. "At points in time that we're sampling windows of, there was enough biomass around to be able to provide us with a record of wildfire that we can identify and use to pinpoint the vegetation and process in time."

In addition to a sufficient source of fuel, which Silurian plant life was able to provide, the other crucial factor in producing early wildfires is atmospheric oxygen levels. At the present day, oxygen makes up approximately 21% of the gasses in the planet's atmosphere. Atmospheric oxygen levels have changed greatly over Earth's history, with essentially zero oxygen in Earth's atmosphere for the first part of the planet's history.

As the research study describes, modern burn experiments indicate that wildfires are unlikely to occur below levels of 16% atmospheric oxygen.

"If you drop below that level you might initiate a fire but it's not going to propagate," said Glasspool. "So when you look at the probability of finding charcoal in the record, you're really only going to find charcoal if that fire was able to propagate, and you can put a minimum threshold value on atmospheric oxygen when you find charcoal."

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