Geology

How plate tectonics changed ocean chemistry

Print edition : May 24, 2019

Tectonic changes responsible for changes in ocean oxygen levels. Photo: Emma Kast/Deep Time maps

A team of scientists from Princeton University has found that when the land mass now known as the Indian subcontinent and the Eurasian plate collided about 50 million years ago (mya), resulting in the formation of the Himalayan range, it not only changed the configuration of continents and the global geological landscape but also greatly altered ocean chemistry.

By creating an unprecedented geological record of ocean nitrogen and oxygen levels, the researchers have found that a huge shift in ocean oxygen levels after the India-Asia collision. The findings have been reported in a recent issue of “Science”.

“These results are different from anything people have previously seen,” the press release from Princeton University quotes Emma Kast, a geoscience graduate student and the lead author of the paper. “The magnitude of the reconstructed change took us by surprise.”

Emma Kast used microscopic seashells to create a record of ocean nitrogen over a period from 70 mya, shortly before the extinction of the dinosaurs, until 30 mya. Besides constituting 78 per cent of the atmosphere, nitrogen is key to all life on earth. All organisms require nitrogen in a biologically usable form, but only a few organisms can convert it into this form, in a process known as nitrogen “fixing”. Cyanobacteria in surface ocean waters “fix” nitrogen for all other forms of ocean life. As the cyanobacteria and other creatures die and sink downwards, they decompose.

Nitrogen has two stable isotopes, N15 and N14. In oxygen-deficient waters, decomposition uses up the “fixed” nitrogen, and the process has a slight preference for the lighter nitrogen isotope, N14. So, the ocean’s N15-to-N14 ratio reflects its oxygen levels.

That ratio gets incorporated into tiny sea creatures called foraminifera and is preserved in their shells when they die. By analysing their fossils—which are abundant in oceans over the last 500 million years and were collected by the Ocean Drilling Programme from the North Atlantic, the North Pacific and the South Atlantic—Emma Kast and her colleagues were able to reconstruct the ratio of the ancient ocean and therefore identify past changes in oxygen levels.

Oxygen-poor waters are, of course, bad for most ocean life. Past climate warming events have caused decreases in ocean oxygen (because oxygen is less soluble in warmer water), which affected the habitats of the sea creatures ranging from microscopic plankton to fish and whales. Scientists have warned of similar devastating effects owing to the current and future global warming.

The geological record of ocean nitrogen assembled by the Princeton researchers revealed that in the 10 million years after dinosaurs went extinct, the N15-to-N14 ratio was high, suggesting low ocean oxygen during that period. They first thought that the warm climate of the time was responsible. But the timing of the period of the switch to higher ocean oxygen occurred around 55 mya, around the time the India-Asia collision occurred and which was also a period of continuously warm climate.

“Over millions of years, tectonic changes have the potential to have massive effects on ocean circulation,” said Daniel Sigman, who initiated this project. “But that doesn’t mean climate change can be discounted. On timescales of years to millennia, climate has the upper hand,” he added.

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