Increased seafloor spreading may accelerate global warming
Seabed spreading, caused by upwelling magma, has led to episodes of global warming in the geological past; the rate of spread has slowed over the past 19 million years, but may gain momentum
Life on Earth began 3.5 billion years ago. But life as we know it was shaped in the Cenozoic era, which began just 66 million years ago and continues to this day.
During this period, mammals, insects, birds, and flowering plants thrived on land, while fish, corals, and molluscs thrived in the ocean. The Earth has gone from a greenhouse without glaciers to a colder greenhouse with polar ice caps.
However, between 14 and 17 million years ago, known as the Miocene Climatic Optimum (MCO) period, temperatures rose (about 10°C warmer than today) and Carbon dioxide (CO2) levels have reached 1,000 parts per million (PPM) from the current 419 PPM, leading to the disappearance of glacial masses and several species.
Many of these episodes of global warming in the past have always baffled scientists. Humans were not responsible for this warming because they arrived about 15 million years after MCO, said Brown University scientist Timothy Herbert. Down to earth (DTE).
So what triggered these changes? Understanding these factors is important at a time when human activities are already pumping greenhouse gases into the atmosphere, pushing the planet towards a climate tipping point.
Over the past seven years, Herbert and his fellow researchers in the United States and Hong Kong have attempted to solve this puzzle by examining submarine volcanoes that span 75,000 km along divergent plate boundaries, where the tectonic plates are moving apart.
These volcanoes, known as the mid-ocean ridge system, spew molten magma from inside the Earth, which gradually moves away from the ridge and cools to form rocks.
Since new ocean floor or crust is created during such seafloor spreading, to maintain planetary balance, the Earth returns a similar area in the deep mantle elsewhere by pushing the old seafloor towards subduction zones, where heavier tectonic plate sinks under lighter plate in 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 ends up in the atmosphere.
They also influence the sea level. “When the plates spread rapidly, the entire base of the seabed rises, and so does the sea level. But during slow motion, both the base and the sea level fall. as the crustal material cools,” Herbert said.
Using magnetic seafloor records, available in their complete form for the last 19 million years, Herbert and his team mapped the propagation rates of 18 major mid-ocean ridges.
In a study published in Geophysical Research Letters in March of this year, they write that the spread has slowed by 35%. About 15 million years ago, the rate of seabed expansion was 200 mm per year, while today it averages 140 mm per year.
“Many scientists are convinced that plate behavior is extremely slow. But our results show that it was significantly faster during the MCO. This period may seem too old, but not for geologists,” Herbert said.
But not all ridges moved the same; while some accelerated, others slowed down.
Douglas Wilson, a research geophysicist at the Marine Science Institute, University of California, USA, who is part of the study, said DTE that the fastest known rate of spread of a tectonic plate is 210 to 220 mm per year, roughly the growth rate of human hair.
Slow plates spread less than a tenth as much as their faster counterparts. Ridges along the eastern Pacific show such slabs with spreading rates nearly 100mm per year slower than 19 million years ago, lowering the global average.
The reduced rates in this region could be because the Pacific Ocean is shrinking while the Atlantic and Indian Oceans are expanding, Wilson pointed out. Overall, 15 of the 18 ridges slowed down, wrote Colleen A Dalton, associate professor of geological sciences at Brown University, US, who led the research, in the study.
The reason for the slowdown is unknown, Herbert said, adding that the mantle circulation could be driving it. “It’s similar to how water moves when you boil it on a stove. As circulation slows, it changes the rates of spread,” 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 on ocean ridges, CO2 trapped in lava escapes into the atmosphere, Herbert said.
In subduction zones, gas is removed from the surface when organisms such as corals and plankton die and sink to the bottom of the sea floor. Their shells, made of calcium carbonate, combine with sediment to form limestone which transports carbon trapped in the mantle.
However, even here, some of the carbon escapes into the atmosphere as rocks melt in subduction zones. CO2 in the interiors then makes its way back into the atmosphere at mid-ocean ridges, continuing the cycle.
“We don’t know if CO2 release is more on mid-ocean ridges or in 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 is linked to higher CO2 levels during the MCO period.
“Think of the inside of the Earth (containing CO2) like a bottle of carbonated soda, and the plates like the cap of the bottle. The more the cap is opened – or in other words, the faster the plaque spreads – the more CO2 is released,” Herbert said.
During the MCO period, magnetic records show that the total production rate of new crust was 3.5 km2 per year due to rapidly spreading plates. Since then, the rate of production of new crust has fallen to just over 2.5 km2 per year.
To estimate CO2 levels, the team analyzed the ratio of boron isotopes found in fossils of foraminifera, a single-celled organism that builds complex shells using minerals in seawater.
Once it dies, it sinks to the bottom of the seabed and becomes trapped in layers of sediment. Analysis shows that CO2 levels varied between 500 and 1,000 PPM during the MCO.
“It doesn’t prove that CO2 contributed to the rate of spread, but there is a strong link between the two,” Herbert said. “The idea is that the amount of CO2 released by submarine volcanoes should be approximately proportional to the amount of seabed generated at the mid-ocean ridge,” he added.
Australian researchers have collected similar evidence on the link between CO2 levels and plate tectonics.
Researchers led by Dietmar Müller and Ben Mather from the School of Geosciences at the University of Sydney have modeled the Earth’s carbon cycle over the past 250 million years. “We show a strong correlation between high atmospheric carbon concentrations and plate tectonic cycles,” Mather said.
They found that during the Cretaceous period, 145 to 66 million years ago, when dinosaurs ruled the earth, CO2 levels in the atmosphere exceeded 1,000 PPM, bringing annual temperatures averages up to 10°C warmer than today.
They published their findings in Nature in May 2022. Scientists used a computer model to simulate this greenhouse world and found that tectonic plates moved rapidly during this time, which doubled the release of CO2 from mid-ocean ridges. .
The study says that 66 million years ago, when Earth entered the Cenozoic Era, CO2 levels dropped to 300 PPM and seafloor spreading slowed.
Herbert and his team plan to continue research into what factors are driving this slowing and increasing spread rates, and if there is a trend to be seen. Magnetic records, however, become increasingly incomplete further in time as they are destroyed in subduction zones.
“It’s not hopeless to go back 30 or 40 million years, but we haven’t done it carefully yet,” Wilson said.
This was first published in the September 1-15, 2022 edition of Down to earth
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