Eddie Gonzales Jr. – MessageToEagle.com – Conventional models suggest that 34 million years ago, cooling and a major drop in sea levels should have caused significant continental erosion and deposited large amounts of sand on the ocean floor.
This was one of Earth’s most drastic climate transitions since the dinosaurs’ extinction.
A new Stanford review of hundreds of studies reports that little to no sediment from this transition has been found across all seven continents.
“The results have left us wondering, ‘where did all the sediment go?'” said study senior author Stephan Graham, the Welton Joseph and Maud L’Anphere Crook Professor in the Stanford Doerr School of Sustainability. “Answering that question will help us get a better fundamental understanding about the functioning of sedimentary systems and how climatic changes imprint on the deep marine sedimentary record.”
The geological gap provides new insights into sediment deposition, erosion, and environmental signals from drastic climate changes, aiding researchers in understanding today’s global climate changes.
“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 study lead author Zack Burton, Ph.D. ’20, who is now an assistant professor of Earth sciences at Montana State University.
From hothouse to icehouse
During the Eocene-Oligocene period, Earth experienced significant cooling. Ice sheets formed in Antarctica, sea levels dropped, and life on land and sea faced severe die-offs. Earlier in the Eocene (56-34 million years ago), Earth had its warmest temperatures and highest sea levels since dinosaurs roamed over 66 million years ago.
Burton and colleagues studied early Eocene effects on deep-sea depositional systems. Their 2023 Scientific Reports study found abundant sand-rich deposits in ocean basins along continental margins.
The research team linked the increase in deposition to intensified climate and weather-boosting land erosion. Intrigued, Burton and colleagues extended their study to the late Eocene and early Oligocene, when Earth’s climate shifted from a “hothouse” and “greenhouse” to an “icehouse.”
Researchers reviewed scientific and technical literature on ancient sediment several kilometers beneath the sea floor. They surveyed studies from the past decade to over a century ago, including offshore oil and gas drilling, onshore rock outcrops, and seismic data interpretations. Over a hundred global sites were included, covering every continental landmass.
While the study’s method of literature analysis is not new on its own, the scale of such an approach made possible by vast online databases could prove highly illuminating, Graham said.
“There could be other similar events in the geologic past that would bear a closer investigation,” said Graham, “and the way to start that is by doing exactly what we did—a really thorough canvassing of the global geologic literature for certain suspect periods in time.”
“The actual process of reappraising, reinvestigating, and reanalyzing literature that has in some cases been out for decades is challenging, but can be very fruitful,” Burton said. “The method can lead to exciting and unexpected findings, like we were able to make here.”
Wholly unanticipated
As Burton and his colleagues made their way through the compiled data inventory, they grew increasingly perplexed by the apparent sedimentary no-show.
“We didn’t see abundant sand-rich deposition, as in our study of warm climates of the early Eocene,” said Burton. “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.”
Researchers propose theories for the lack of deposition.
Vigorous ocean bottom currents, driven by water temperature and salinity, may have been intensified by a major climate shift, eroding the ocean floor and sweeping away sediment from continents.
Mechanisms from exposed continental shelves due to sea-level falls may have let sediments bypass nearby basins and reach the ocean floor’s abyssal plain. Regionally, glacial erosion around Antarctica likely contributed too.
Various mechanisms together caused similar erosion in oceanic basins worldwide. This ubiquity suggests global controls, meaning profound climate change affected everything from the tallest landmasses to the deepest waters.
The abrupt climatic event at the Eocene-Oligocene boundary and its effects on continental margins could help researchers understand today’s climate change. Although current human-caused climate change is smaller in magnitude than the Eocene-Oligocene transition, it is occurring at a much faster pace, according to Stanford researchers.
“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,” said Graham. “The geologic past informs the present, and particularly the future.”
Paper
Zachary F.M. Burton et al, Global Eocene-Oligocene unconformity in clastic sedimentary basins, Earth-Science Reviews (2024). DOI: 10.1016/j.earscirev.2024.104912
Written by Eddie Gonzales Jr. – MessageToEagle.com Staff Writer