According to recent research led by Oregon State University, the Ontong Java Plateau, an underwater volcanic plateau in the Pacific Ocean north of the Solomon Islands, is younger and its eruption lasted longer than previously believed.
The results, which were just published in Science, also call into question long-held beliefs that the formation of the plateau, which is roughly the size of Alaska, was the reason for an oceanic black shale deposit around the world.
“This type of shale is formed when there is very limited oxygen in the ocean. This layer was formed about 120 million years ago and can be found preserved everywhere around the world in geological formations,” said Anthony Koppers, a professor of marine geology in OSU’s College of Earth, Ocean, and Atmospheric Sciences, and a co-author of the study.
“A massive volcanic eruption like the one that formed Ontong Java Plateau could deplete the ocean of oxygen, and it was believed that this massive volcanic activity and the shale deposits were connected. But our findings suggest that is not the case.”
Ontong Java Plateau covers roughly 1% of the Earth’s surface. The plateau is a fragment of the Ontong Java Nui, a superplateau that broke apart shortly after its formation, creating Ontong Java, Manihiki Plateau and Hikurangi Plateau.
The thickest and oldest rock that makes up the Pacific marine crust is composed of sediment and an underlying basalt basement. Between 1973 and 2000, researchers drilled cores into these layers to collect samples for study.
The National Science Foundation-funded Deep Sea Drilling Project and Ocean Drilling Program used the scientific ocean drilling vessels Glomar Challenger and JOIDES Resolution to acquire the cores.
All of the tests showed a high level of internal consistency in the ages measured and a remarkable reproducibility between multiple samples from the same volcanic eruptive units, providing us with a tremendous degree of confidence. These results mean we can’t connect Ontong Java anymore to the anoxic event that led to the shale deposits. The dates no longer line up.
Peter Davidson
Ontong Java was thought to have been created in a single, relatively recent volcanic eruption approximately 120 million years ago, around the time of the shale deposit, according to a previous study employing these cores.
However, according to Koppers, an international authority on large-scale geodynamic processes and associate vice president for research advancement and strategy in OSU’s research office, there have been questions raised regarding the veracity of the data used to make that judgment.
“Understanding the timing of these volcanic eruptions is essential to establishing a link between the eruptions themselves and the formation of the black shales,” Koppers said. “Establishing this causality is important to understanding large changes in ocean chemistry, similar to what is happening today through climate change due to human activity.”
Koppers and the study’s lead author, Peter Davidson, who worked on the project as a doctoral student and just completed his degree at Oregon State, replicated the original 1993 dating studies to see if they could achieve the same results while also taking advantage of major improvements in scientific techniques and in mass spectrometry equipment used in chemical analysis.
In an effort to confirm the earlier findings, Davidson conducted 40 trials using a number of identical core samples from the initial drilling initiatives. Of the 40, a surprisingly large number 38 failed when applying modern-day data quality standards.
“The new results showed that the original samples were greatly affected by an unwanted process during the irradiation of the samples, which causes the ages to appear to be too old,” Davidson said. “This irradiation issue, called recoil, is a problem that could not easily be seen with the older equipment used decades ago, but our new, highly sensitive instrumentation can easily identify this problem.”
Plagioclase, a type of feldspar or rock-forming mineral, was the second mineral phase of the original samples that Davidson used in a new set of dating experiments.
“The data from the plagioclase is much higher resolution than past data and more importantly, it is entirely devoid of the recoil issue that skewed the original ages for the Ontong Java Plateau that were based on the basalt,” Koppers said.
The updated testing revealed that Ontong Java was up to 10 million years younger than anticipated and that its formation probably took millions of years.
“All of the tests showed a high level of internal consistency in the ages measured and a remarkable reproducibility between multiple samples from the same volcanic eruptive units, providing us with a tremendous degree of confidence,” Davidson said. “These results mean we can’t connect Ontong Java anymore to the anoxic event that led to the shale deposits. The dates no longer line up.”
The evidence also casts doubt on beliefs that the Manihiki Plateau and Ontong Java were previously connected before they split away, indicating that they are millions of years older than one other. The evidence instead points to the possibility that volcanic activity may have originated beneath the Manihiki Plateau and gradually moved hundreds of kilometers across the Pacific.
Koppers observed that the discovery is likely to leave scientists scratching their heads because there is no longer an explanation for the significant oxygen depletion event that left shale deposits all over the world.
“There is also a possibility that lower portions of Ontong Java are older,” he said. “The available samples from Ontong Java were drilled only into the top couple hundred meters of the plateau, whereas the plateau is a geologic megastructure up to 35 kilometers thick, with the top 8 to 9 kilometers considered the eruptive portion.”
“In that sense, we have only scratched the surface of Ontong Java,” Koppers said.
In order to continue the study, Davidson plans to track down samples from the plateau’s older regions and conduct fresh tests using these cutting-edge methods. Up to four kilometers of the plateau’s top are exposed in some locations in the Solomon Islands and may be reachable.
“We hope that by targeting further samples from deeper in the volcanic stratigraphy, we can uncover potentially even older portions of the Ontong Java Plateau,” Davidson said. “It might be possible that these older portions of the plateau did cause the black shale deposition, but they might not.”
“These future studies should further help us understand Ontong Java, which is the largest volcanic feature on Earth’s surface, and how volcanic eruptions of this magnitude can potentially cause global environmental disruptions.”
Additional co-authors are Takashi Sano of the National Museum of Nature and Science in Tsukuba, Japan and Takeshi Hanyu of the Japan Agency for Marine-Earth Science and Technology in Yokosuka, Japan.
