A prevailing theory for many decades has held that a major cooling event, accompanied by a significant drop in sea levels, likely impacted Earth’s climate around 34 million years ago.
One of the effects of this large-scale climate event should have been a tremendous amount of sandy sediment deposited on the ocean floor, remaining preserved there today and offering clues about what caused such significant climate changes during this ancient chapter of our planet’s history.
However, the deep ocean sediment that scientists expected to find commemorating one of Earth’s most drastic periods of climate change since the dinosaurs became extinct apparently doesn’t exist.
This, according to new findings by Stanford researchers who reviewed data from hundreds of earlier studies, revealing that no significant amount of sediment coinciding with this period has ever been discovered, a mystery that presents new questions about what some view as an apparent gap in our planet’s geologic record.
A Curious Case of Missing Sediments
Stephan Graham, the Welton Joseph and Maud L’Anphere Crook Professor in the Stanford Doerr School of Sustainability and one of the study’s authors, says his team’s findings led them to an inevitable question.
“The results have left us wondering, ‘where did all the sediment go?'” Graham said. Finding the answers to this perplexing question could finally allow researchers like Graham and his colleagues to unravel the dynamics of what happened to our planet during this period, and the impact of climatic changes on marine sedimentary systems.
The apparent “gap” in the geologic record indicated by the absence of this sediment layer points to the variety of factors that can influence dramatic climate changes, observations which may also help researchers better understand the elements that drive modern changes occurring with Earth’s climate.
Zack Burton, Ph.D., an assistant professor of Earth sciences at Montana State University, said that the new research is the first time that a global perspective on the planet’s oceanic sedimentary systems responded to the period of extreme transition that occurred between the Eocene and Oligocene, the second and third epochs of what was once formally referred to as the Tertiary Period.
“For the first time, we’ve taken a global look at an understudied response of the planet’s largest sediment mass-movement systems during the extreme transition of the Eocene-Oligocene,” said Burton, the new study’s lead author, in a statement.
Earth’s Hot and Cold Climate History
Throughout the period in question, significant planetwide cooling occurred, leading to the accumulation of frozen water on previously ice-free Antarctica. Sea levels around the world dropped as water accumulated near the poles, and the onset of significant climate changes ultimately led to widespread die-offs of animals both on land and at sea.
For millions of years prior to this, climate proxy data indicates that Earth had sustained its warmest temperatures and highest sea levels since the demise of the dinosaurs. An abundance of sandy deposits correlates to this period, as identified in previous research published by Burton and the team which they attribute to erosion from land being carried to sea by intense weather systems during this period.
Taking their research further, the team then began to examine the Eocene and Oligocene periods, undertaking a massive survey of scientific literature going back more than a century, which allowed them to look at the extreme cooling that followed one of Earth’s lengthiest warm periods.
“The actual process of reappraising, reinvestigating, and reanalyzing literature that has in some cases been out for decades is challenging,” Burton said, while noting that it can also “lead to exciting and unexpected findings, like we were able to make here.”
What they found was unexpected, he says, since the team had never anticipated that they would find so little evidence of sediments from the later Eocene and Oligocene.
“We didn’t see abundant sand-rich deposition, as in our study of warm climates of the early Eocene. Instead, we saw that prominent, widespread erosional unconformities—in other words, gaps in the rock record—had developed during the extreme climatic cooling and oceanographic change of the Eocene-Oligocene,” Burton said.
Why a ‘Gap’ May Exist in the Geologic Record
As to what processes could have caused so little sediment deposition to have occurred, Burton and the team suggest that there may have been movement due to ocean bottom currents that prevented sediments from settling during this period.
Another possibility is that sediments normally deposited in basins closer to shore may have been carried further out, eventually coming to rest on the deep abyssal plains, which cover more than 50% of the Earth’s surface, yet also remain some of our planet’s least explored areas.
Whatever processes may have occurred, the researchers feel confident that they were present globally, and affected not just the bottom of the oceans, but also virtually all areas on Earth’s landmasses as well.
“Our findings can help inform us of the kinds of radical changes that can happen on the Earth’s surface in the face of rapid climate change,” Graham said of the team’s findings in a statement, adding that when it comes to climate patterns and their interrelationship to geology, studying the past often paves the way toward a deeper understanding of present day phenomena.
“The geologic past informs the present,” Graham said, “and particularly the future.”
The team’s new paper, “Global Eocene-Oligocene unconformity in clastic sedimentary basins,” was published in the November, 2024 edition of Earth-Science Reviews.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.