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Life on Earth began 3.5 billion years ago. But the life as we know it was shaped in the Cenozoic epoch, which began only 66 million years ago and continues to this day.
During this period, mammals, insects, birds and flowering plants flourished on land, while fish, corals and molluscs thrived in the ocean. Earth transitioned from a hothouse with no glaciers to a cooler one with ice sheets at the poles.
However, between 14 and 17 million years ago, known as the Miocene Climatic Optimum (MCO) period, temperatures soared (around 10°C higher than today) and carbon dioxide (CO2) levels rose to 1,000 parts per million (PPM) against the current 419 PPM, leading to the disappearance of glacial masses and several species.
Several such episodes of global warming in the past have always baffled scientists. Humans were not responsible for this warming as they arrived about 15 million years after MCO, Timothy Herbert, a scientist at Brown University, told Down To Earth (DTE).
So what triggered these changes? Understanding these factors is important at a time when human activities are already spewing greenhouse gases into the atmosphere, pushing the planet towards a climate tipping point.
For the past seven years, Herbert and his fellow researchers in the US and Hong Kong have been trying to solve this riddle by peering into underwater volcanoes that are spread across the seabed for 75,000 km along divergent plate boundaries, where tectonic plates drift apart.
These volcanoes, known as the mid-ocean ridge system, spew molten magma from the Earth’s interiors, which gradually moves away from the ridge and cools down to form rocks.
Since new ocean floor or crust is created during such seafloor spreading, to maintain the planetary balance, the Earth returns a similar area into the deep mantle elsewhere by pushing the older seafloor towards subduction zones, where the heavier tectonic plate sinks below a lighter one into the Earth’s interiors.
Scientists have long known that seafloor spreading rates impact CO2 levels. Faster spreading plates have more volcanic activity and inject more CO2 into the water, some of which eventually ends up in the atmosphere.
They also influence sea levels. “When plates spread rapidly, the entire base of the seafloor rises, as do sea levels. But during slow movement, the base and sea levels fall as the crust material cools,” Herbert said.
Using magnetic records of the seafloor, available in their complete forms for last 19 million years, Herbert and his team have mapped the spread rates of 18 major mid-ocean ridges.
In a study published in Geophysical Research Letters in March this year, they write that the spreading has slowed by 35 per cent. Some 15 million years ago, the rate of seafloor spreading was 200 mm a year, whereas now, on average, it is 140 mm a year.
“Many scientists are convinced that plate behaviour is extremely slow. But our results show that it was significantly faster during the MCO. This period might seem too long ago, but not for geologists,” Herbert said.
But not all ridges moved alike; while some sped up, others slowed down.
Douglas Wilson, a research geophysicist at Marine Science Institute, University of California, US, who is part of the study, told DTE that the fastest known spreading rate of a tectonic plate is 210-220 mm a year — roughly the rate of the growth of human hair.
Slow plates spread less than one-tenth as much as their faster counterparts. Ridges along the eastern Pacific has such plates whose spread rates are nearly 100 mm a year slower compared to 19 million years ago, lowering the world’s average.
The reduced rates in this region could be because the Pacific Ocean is getting smaller while the Atlantic and Indian Oceans are getting bigger, Wilson pointed out. Overall, 15 of the 18 ridges slowed down, Colleen A Dalton, associate professor of geological sciences at Brown University, US, who led the research, wrote in the study.
The reason for this slowdown is unknown, Herbert said, adding that mantle circulation may be driving it. “This is similar to how the water moves when you boil it on a stove. As circulation slows, it changes spreading rates,” he said.
Studying seafloor spreading rates will show how tectonic forces contribute to the global carbon budget. Tectonic plates are known to recycle carbon. During volcanic eruptions at the ocean ridges, CO2 trapped in the lava escapes into the atmosphere, Herbert said.
At subduction zones, the gas is removed from the surface when organisms such as corals and plankton die and sink to the bottom of the seafloor. Their shells, made of calcium carbonate, combine with sediments to form limestone that ferry the trapped carbon into the mantle.
However, even here, some of the carbon escapes into the atmosphere when rocks melt at subduction zones. The CO2 in the interiors then charts its journey back into the atmosphere at mid-ocean ridges, continuing the cycle.
“We do not know whether the CO2 release is more at mid-ocean ridges or at the subduction zones,” Herbert said. “We will study this over the next few decades,” he added.
In another study published in the journal Science in July 2022, Herbert, Dalton and Wilson focus on the link between seafloor spreading rates and climatic conditions. They find that faster seafloor spreading are linked with higher CO2 levels during MCO period.
“Think of the interior of the Earth (containing CO2) as a carbonated bottle of soda, and the plates as the bottle cap. The more the cap is opened — or in other words, the faster the plate spread — the more CO2 is released,” Herbert said.
During the MCO period, magnetic records show that total new crust production rate was 3.5 sq km per year due to fast-spreading plates. Since then, the new crust production rate has dropped to a little above 2.5 sq km a year.
To estimate the levels of CO2, the team analysed the ratio of boron isotopes found in fossils of foraminifera, a single-celled organism that builds complex shells using minerals in the seawater.
Once it dies, it sinks to the bottom of the seafloor and get trapped in layers of sediment. The analysis shows that CO2 levels varied between 500-1,000 PPM during the MCO.
“This does not prove that CO2 contributed to the speed of the spread, but there is a strong link between the two,” Herbert said. “The idea is that the amount of CO2 released from the underwater volcanoes should be approximately proportional to the amount of seafloor generated at the mid-ocean ridge,” he added.
Researchers from Australia have gathered similar evidence on the link between CO2 levels and plate tectonics.
Researchers led by Dietmar Müller and Ben Mather from the University of Sydney’s School of Geosciences modelled Earth’s carbon cycle over the last 250 million years, “We show a strong correlation between high concentrations of atmospheric carbon and plate tectonic cycles,” Mather said.
They find that during the Cretaceous Period 145-66 million years ago, when dinosaurs dominated land, atmospheric CO2 levels shot above 1,000 PPM, taking the mean annual temperatures up to 10°C higher than today.
They published their findings in Nature in May 2022. The scientists used a computer model to simulate this hothouse world and found that the tectonic plates were moving quickly during this period, which doubled CO2 release from mid-ocean ridges.
The study said 66 million years ago, when Earth entered the Cenozoic era, CO2 levels dropped to 300 PPM and the seafloor spreading slowed down.
Herbert and his team plan further research on the factors that drive this slowdown and increase of spreading rates, and whether there is a pattern to be seen. Magnetic records, however, become increasingly incomplete farther back in time as they are destroyed at subduction zones.
“It is not hopeless to go back to 30 or 40 million years ago, but we have not done it carefully yet,” Wilson said.
This was first published in the 1-15 September, 2022 edition of Down To Earth