Churn under sea: Can increase in seafloor spreading speed up global warming

Churn under sea: Can increase in seafloor spreading speed up global warming

Subject: Geography

Introduction–

• During Miocene Climatic Optimum (MCO) period (14-17 Million years ago), 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.
• At a time when human activities are already spewing greenhouse gases into the atmosphere, pushing the planet towards a climate tipping point, it is crucial to understand the factors that triggered those changes.

Sea Floor Spreading and its correlation with Global climate change–

• Mid-ocean ridge system, spewing molten magma from the Earth’s interiors, which gradually moves away from the ridge and cools down to form rocks.
• Since a 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 end 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.

Major Findings–

• Using magnetic records of the seafloor, available in their complete forms for the last 19 million years, Scientists have mapped the spread rates of 18 major mid-ocean ridges.
• 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. So, spreading has slowed by 35 per cent.
• But not all ridges moved alike; while some sped up, others slowed down.
• 15 of the 18 ridges slowed down.
• 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 have 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.
• The reason for this slowdown is unknown, 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.

Definite link to CO2 levels–

• 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.
• 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.
• Faster seafloor spreading is linked with higher CO2 levels during the MCO period.
• During the MCO period, magnetic records show that the 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.
• 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.
• The idea is that the amount of CO2 released from underwater volcanoes should be approximately proportional to the amount of seafloor generated at the mid-ocean ridge.
• Researchers from Australia find that during the Cretaceous Period 145-66 million years ago, when dinosaurs dominated the land, atmospheric CO2 levels shot above 1,000 PPM, taking the mean annual temperatures up to 10°C higher than today.
• The study said that 66 million years ago when Earth entered the Cenozoic era,CO2 levels dropped to 300 PPM and the seafloor spreading slowed down.

Conclusion–

• Seafloor spreading, caused by an upwelling of magma, has led to episodes of global warming in the geologic past; the spread rate has slowed down in the last 19 million years but could gather momentum.