“Contrary to our first expectations, global climate was not the primary cause of this change in ocean oxygen and nitrogen cycling,” said biogeochemist Emma Kast currently at the University of Cambridge about the planet’s dramatic increase in oxygen 55 million years ago. The more likely culprit? Plate tectonics. The collision of India with Asia — dubbed “the collision that changed the world” by Columbia University geoscientist Wally Broecker, a pioneer in the ocean’s role in climate change.— closed off an ancient sea called the Tethys, disturbing the continental shelves and their connections with the open ocean.
Earth’s Backstory –“Climate has the Upper Hand”
When Princeton researchers assembled their unprecedented geologic record of ocean nitrogen, they found that in the 10 million years after dinosaurs went extinct, the 15N-to-14N nitrogen ratio (nitrogen-15 to nitrogen-14; abundance ratio of the two nitrogen isotopes) was high, suggesting that ocean oxygen levels were low. They first thought that the warm climate of the time was responsible, as oxygen is less soluble in warmer water. But the timing told another story: the change to higher ocean oxygen occurred around 55 million years ago, during a time of continuously warm climate.
“Over millions of years, tectonic changes have the potential to have massive effects on ocean circulation,” said Princeton’s Daniel Sigman, Dusenbury Professor of Geological and Geophysical Sciences. But that doesn’t mean climate change can be discounted, he added. “On timescales of years to millennia, climate has the upper hand.” Sigman is an authority on the cycles of biologically important elements and their interaction with changing environmental conditions through the course of Earth history.
When the landmass that is now the Indian subcontinent slammed into Asia about 50 million years ago, the collision changed the configuration of the continents, the landscape, global climate and more. A team of Princeton University scientists has identified one more effect: the oxygen in the world’s oceans increased, altering the conditions for life.
Results a Surprise
“These results are different from anything people have previously seen,” said Kast, a graduate student in geosciences and the lead author on a paper coming out in Science on April 26. “The magnitude of the reconstructed change took us by surprise.”
Paleomap comparison 65 million years ago vs 50 million years ago
Neither the continents nor the oceans have always looked the way they do now. These “paleomaps” show how the continents and oceans appeared before (top) and during (bottom) “the collision that changed the world,” when the landmass that is now the Indian subcontinent rammed northward into Asia, closing the Tethys Sea and building the Himalayas. Global ocean levels were higher then, creating salty shallow seas (pale blue) that covered much of North Africa and parts of each of the continents.
Unprecedented Record of Ocean Nitrogen and Oxygen Levels
A team of Princeton researchers, using samples gathered at the three starred locations, created an unprecedented record of ocean nitrogen and oxygen levels from 70 million years ago through 30 million years ago that shows a major shift in ocean chemistry after the India-Asia collision. Another shift came 35 million years ago, when Antarctica began accumulating ice and global sea levels fell. Images were created by Emma Kast, using paleogeographic reconstructions from Deep Time Maps, with their permission
Kast used microscopic seashells to create a record of ocean nitrogen over a period from 70 million years ago — shortly before the extinction of the dinosaurs — until 30 million years ago. This record is an enormous contribution to the field of global climate studies, said Princeton’s John Higgins, a co-author on the paper. Higgins is a leading authority on evolution of the carbon cycle and the global climate system over Earth history.
“In our field, there are records that you look at as fundamental, that need to be explained by any sort of hypothesis that wants to make biogeochemical connections,” Higgins said. “Those are few and far between, in part because it’s very hard to create records that go far back in time. Fifty-million-year-old rocks don’t willingly give up their secrets. I would certainly consider Emma’s record to be one of those fundamental records. From now on, people who want to engage with how the Earth has changed over the last 70 million years will have to engage with Emma’s data.”
Nitrogen is Key to All Life on Earth
In addition to being the most abundant gas in the atmosphere, nitrogen is key to all life on Earth. “I study nitrogen so that I can study the global environment,” said Daniel Sigman. Sigman initiated this project with Higgins and then-Princeton postdoctoral researcher Daniel Stolper, who is now an assistant professor of Earth and planetary science at the University of California-Berkeley.
Every organism on Earth requires “fixed” nitrogen — sometimes called “biologically available nitrogen.” Nitrogen makes up 78% of our planet’s atmosphere, but few organisms can “fix” it by converting the gas into a biologically useful form. In the oceans, cyanobacteria in surface waters fix nitrogen for all other ocean life. As the cyanobacteria and other creatures die and sink downward, they decompose.
Nitrogen has two stable isotopes, 15N and 14N. In oxygen-poor waters, decomposition uses up “fixed” nitrogen. This occurs with a slight preference for the lighter nitrogen isotope, 14N, so the ocean’s 15N-to-14N ratio reflects its oxygen levels.
That ratio is incorporated into tiny sea creatures called foraminifera during their lives, and then preserved in their shells when they die. By analyzing their fossils — collected by the Ocean Drilling Program from the North Atlantic, North Pacific, and South Atlantic — Kast and her colleagues were able to reconstruct the 15N-to-14N ratio of the ancient ocean, and therefore identify past changes in oxygen levels.
Oxygen controls the distribution of marine organisms, with oxygen-poor waters being bad for most ocean life. Many past climate warming events caused decreases in ocean oxygen that limited the habitats of sea 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 ocean oxygen could decimate marine ecosystems, including important fish populations.
“Ongoing global warming will have major impacts on ocean oxygenation,” Daniel Sigman, Dusenbury Professor of Geological and Geophysical Sciences at Princeton University, ominously told The Daily Galaxy, “but our work suggests that the longer term changes, once the warming has become established, will be markedly different from the short term changes that have so far been recognized.”
Paleobiologist Richard D. Norris at the Scripps Institution of Oceanography told The Daily Galaxy: “Both observations and models suggest that the modern oceans are becoming more hypoxic as the oxygen minimum zones expand in the midwater oceans. We can anticipate that as deoxygenation continues, the nitrogen isotope ratio will increase, as seen in the geologic past.”