Geoscientists discover new consequences of "the collision that changed the world"

When the landmass that is now part of the Indian subcontinent has infiltrated Asia about 50 million years ago, the collision has changed the configuration of the continents, the landscape, the global climate and well Moreover. At present, a team of scientists from Princeton University has identified another effect: oxygen in the world's oceans has increased, thus altering living conditions.

"These results are different from anything people have seen before," said Emma Kast, a graduate student in geoscience and senior author of an article in Science on April 26. "The magnitude of the rebuilt change took us by surprise."

Kast used microscopic shells to create an ocean nitrogen record over a period of 70 million years – shortly before the dinosaurs' extinction – up to 30 million years ago. years. This disc represents a huge contribution to the field of global climate studies, said John Higgins, associate professor of geoscience at Princeton and co-author of the paper.

"In our field, there are documents that you consider to be fundamental, which must be explained by any sort of hypothesis wishing to establish biogeochemical connections," said Higgins. "These are rare, in part because it's very difficult to create records that go back far into the past." Rocks 50 million years old do not readily disclose their secrets. Emma's disk as one of them.From now, people wanting to understand the evolution of the Earth over the last 70 million years will need to be interested in Emma's data. "

In addition to being the most abundant gas in the atmosphere, nitrogen is the key to all life on Earth. "I study nitrogen to study the global environment," said Daniel Sigman, professor of geological and geophysical science at Dusenbury, Princeton, and lead author of the paper. Sigman launched this project in collaboration with Higgins and Daniel Stolper, a Princeton postdoctoral researcher, currently assistant professor of Earth and Planetary Sciences at the University of California at Berkeley.

All organisms on Earth need "fixed" nitrogen, sometimes called "biologically available nitrogen". Nitrogen accounts for 78% of our planet's atmosphere, but few organisms can "fix it" by converting gas into a biologically useful form. In the oceans, cyanobacteria present in surface waters fix nitrogen for all other forms of ocean life. When cyanobacteria and other creatures die and collapse, they decompose.

Nitrogen has two stable isotopes, ¹⁵N and ¹⁴N. In oxygen-poor water, decomposition uses "fixed" nitrogen. This occurs with a slight preference for the lighter nitrogen isotope, ¹⁴N, so the ocean ¹⁵N-to-¹⁴The nitrogen ratio reflects its oxygen levels.

This ratio is incorporated into tiny marine creatures called foraminifera during their lifetime, then preserved in their shells when they die. By analyzing their fossils – collected by the Ocean Drilling Program in the North Atlantic, the North Pacific and South Atlantic – Kast and his colleagues were able to reconstruct the ¹⁵N-to-¹⁴N ratio of the old ocean, and therefore to identify past changes in oxygen levels.

Oxygen controls the distribution of marine organisms, with oxygen-poor waters being poor for most oceans. Numerous global warming events have led to a decrease in ocean oxygen, which limits the habitat of marine creatures, from microscopic plankton to the fish and whales that feed on them. Scientists trying to predict the impact of current and future global warming have warned that low levels of oxygen in the oceans could decimate marine ecosystems, including large fish populations.

When researchers gathered their unprecedented geological data on the ocean's nitrogen, they found that 10 million years after the dinosaurs' disappearance, the 15N-14N ratio was high, suggesting that levels of oxygen in the oceans were low. They first thought that the warm climate of the time was the cause because oxygen is less soluble in warmer waters. But the timing has changed: the transition to a higher oxygen level in the ocean took place about 55 million years ago, while the climate was becoming more and more hot.

"Contrary to our early expectations, the global climate was not the main cause of this change in the cycle of oxygen and nitrogen in the oceans," Kast said. The most likely culprit? Tectonic plates. The collision between India and Asia – dubbed "the collision that changed the world" by legendary geoscientist Wally Broecker, founder of modern climate research – has closed an ancient sea called Tethys, disrupting continental shelves and their connections to the great ocean.

"Over millions of years, tectonic changes could have dramatic effects on ocean circulation," Sigman said. But that does not mean that climate change can be neglected, he added. "On echelons ranging from several years to several millennia, the climate has the upper hand."

"Isotope Nitrogen Evidence for the Expansion of Early Cenozoic Oceanic Suboxia", by Emma R. Kast, Daniel A. Stolper, Alexandra Auderset, John Higgins, Haojia Ren, Xingchen T. Wang and Alfredo Martínez-García, Gerald H. Haug and Daniel M. Sigman, appear in the April 26 issue of Science and was published online on April 25.