When the landmass, now the Indian subcontinent, arrived in Asia some 50 million years ago, the collision changed continent configuration, landscape, global climate, and more. Now, a team of Princeton University scientists has identified another effect: Oxygen in the oceans of the world has increased and changed living conditions.
"These results are different from anything people have seen so far," said student Emma Kast in Earth Sciences and lead author on a paper published on April 26, Science . "The extent of the reconstructed change surprised us."
Kast used microscopic shells to produce a record of ocean nitrogen over a period of 70 million years ago, shortly before dinosaur extinction, to 30 million years ago. This record is a tremendous contribution to the field of global climate studies, said John Higgins, a professor of geosciences at Princeton and co-author of the paper.
"In our field there are records that you consider fundamental, that must be explained by any kind of hypothesis that wants to make biogeochemical links," said Higgins. "These are few and far between, in part because it's very hard to make records that go back far to the past, fifty-million-year-old rocks do not give up their secrets voluntarily, I would certainly consider Emma's record as one of them Those who want to deal with Earth's changes over the last 70 million years must now deal with Emma's data. "
Apart from being the richest gas in the atmosphere, nitrogen is the key to all life on earth. "I'm studying nitrogen so I can study the global environment," said Daniel Sigman, Princeton Professor of Geology and Geophysics and Dusenbury Professor. Sigman initiated this project with Higgins and then Princeton postdoctoral fellow Daniel Stolper, who is now Assistant Professor of Earth and Planetary Sciences at the University of California-Berkeley.
Every organism on Earth needs "solid" nitrogen ̵
Nitrogen has two stable isotopes, 15N and 14N. In oxygen-poor waters, the decomposition consumes "solid" nitrogen. This is done with a slight preference for the lighter nitrogen isotope 14N, so that the ratio of 15 N to 14 N reflects the oxygen content of the ocean.
This relationship is incorporated into tiny marine animals called foraminifera during their lifetime and then preserved in their state as shells die. By analyzing their fossils – collected by the Ocean Drilling Program of the North Atlantic, North Pacific, and South Atlantic – Kast and her colleagues were able to reconstruct the 15N-to-14N quotient of the ancient ocean and detect previous changes in oxygen concentrations.
Oxygen controls the distribution of marine organisms, with low-oxygen waters being unfavorable to most marine creatures. Many past climate warming events have led to a decrease in marine oxygen, which has limited marine habitats, from the microscopic plankton to the fish and whales that feed on them. Scientists, who want to predict the impact of present and future global warming, warned that low levels of oxygen in the ocean could decimate marine ecosystems, including key fish stocks.
When the researchers collected their unprecedented geological data on nitrogen from sea level, they found that in water 10 million years after dinosaur extinction, the ratio of 15 N to 14 N was high, indicating a low level of oxygen in the ocean , They first thought that the warm climate of the time was responsible because oxygen is less soluble in warmer water. The timing, however, revealed a different story: the change to a higher oxygen content of the ocean took place about 55 million years ago in a time of continuous warm climate.
"Contrary to our first expectation, the global climate was not the main cause of this change in the ocean oxygen and nitrogen cycle," said Kast. The more likely the offender? Plate tectonics. The collision of India with Asia – "the collision that has changed the world" of legendary geoscientist Wally Broecker, a founder of modern climate research, completed an ancient sea called Tethys and disrupted the continental shelves and their connections to the open ocean.
"For millions of years, tectonic changes have the potential to have a massive impact on ocean currents," Sigman said. However, that does not mean that climate change can not be taken into account, he added. "On the time scale of years to millennia, the climate has the upper hand."