During the Cretaceous Period around 100 million years ago, Earth’s oceans were nearly unrecognizable. Below the waves swam marine reptiles: lizard-like mosasaurs, long-necked plesiosaurs and gargantuan sea turtles. These behemoths lived alongside squid-like ammonites encased in tightly-coiled shells and a slew of bizarre fish.
94 million years ago, these strange seas became nearly uninhabitable. Oxygen levels plummeted, and the ocean acidified during an episode known as the Oceanic Anoxic Event 2 (OAE2) that sent ripples through marine ecosystems worldwide. “As geologists, we’re drawn to times when things went wrong during the past,” said Matt Jones, a former research fellow at the National Museum of Natural History who now works with the United States Geological Survey. “We’re trying to understand why the oceans lost so much oxygen content in the mid Cretaceous.”
To determine what sparked this dramatic bout of prehistoric climate change, Jones and his colleagues boarded a research vessel with the International Ocean Discovery Program (IODP) to plumb the depths of the Indian Ocean in 2017. They were interested in collecting deep-sea cores — sediment samples collected by drilling down into the seafloor with a hollow tube — from the Mentelle Basin, a stretch of deep seafloor off the coast of Western Australia. “When you go deep sea drilling, it's kind of like fishing,” Jones said. “You don't know what you're actually going to recover.”
The researchers aimed to collect samples from below the seafloor, including sediments deposited during the OAE2. During the Cretaceous, Australia was much closer to the South Pole and slowly splintering apart from Antarctica, a landmass it had been welded to as part of the supercontinent Gondwana. According to Brian Huber, a curator in the museum’s Department of Paleobiology and Co-Chief Scientist of the expedition, Australia’s former position near the South Pole made it a good spot to look for signs of ancient climate change. “The high latitudes are really our bellwether of global climate change,” Huber said. “The most dramatic swings are going to be at the polar latitudes.”
As they retrieved samples from the seafloor, the team noticed the pale-colored cores were punctuated by a green and black band of sediment several centimeters thick — a sign that something dramatic had happened to the oceanic environment. Based on its position in the core, they estimated that this band of dark sediment was deposited during the OAE2.
To determine what conditions caused this dark interval in the rock record, Jones and Huber collected samples from the core and ran them through a battery of tests with collaborators when they were back on land. The green and black mudstones lacked the shells of microscopic marine organisms that are deposited in the layers above and below the event. The disappearance of these miniscule shells revealed that the ancient sea experienced ocean acidification. As the water acidifies, carbonate shells in the plankton and on the seafloor of the ocean’s continental slope dissolve, and it may become difficult for animals living in the surface waters to craft new shells. The researchers also documented that the ratio of isotopes of the dense element osmium drastically shifted in the mudstone sediments of the Mentelle Basin right at the beginning of the OAE2. According to Jones, this often happens when massive tracts of underwater volcanoes known as large igneous provinces erupt.
In a paper published earlier this month in the journal Nature Geoscience, Jones, Huber and their team of co-authors–including scientists from Northwestern University, Durham University, and shipboard scientists from the IODP expedition–reported on this phenomenon of ocean acidification during OAE2 and determined it was sparked by large igneous province eruptions spewing massive amounts of carbon dioxide into the ocean and atmosphere. This excess carbon dioxide acidified the water and likely increased the local water’s temperature.
In addition to belching out carbon dioxide, the volcanoes also released massive amounts of nutrients. According to Huber, who specializes in examining the fossilized shells of microscopic protists called foraminifera for clues to ancient water chemistry, plankton gobbled up the nutrients, like iron and nitrogen, emitted by the erupting volcanoes. This caused the plankton populations to explode, which siphoned immense amounts of oxygen from the water. Eventually, the levels of dissolved oxygen levels plummeted in many parts of the ocean and remained low for more than half a million years.
The disastrous convergence of acidification, warming and anoxic conditions during the OAE2 is eerily reminiscent of how oceans today are responding to climate change. Similar to the conditions of Cretaceous seas, modern bouts of acidification and warming are sparked by excess amounts of carbon dioxide entering the ocean. But instead of erupting marine volcanoes, humans are now the main culprit behind this surplus of carbon.
Although the causes are different, Jones thinks the Cretaceous sediment below the seafloor can help place modern climate change in context. “Today, people are very concerned about how climate change will play out in the future,” he said. “As geologists, we can look at these events in the past and find an actual example of what transpired during a massive geologic release of co2.”
Huber concurs, noting that Cretaceous oceans were not able to rebound overnight after the OAE2. “That’s another lesson for the future: once the genie is out of the bottle, it's hard for the earth system to recapture that carbon.”
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