According to highly cited conventional models, cooling and a major drop in sea levels about 34 million years ago should have led to widespread continental erosion and deposited gargantuan amounts of sandy material onto the ocean floor. This was, after all, one of the most drastic climate transitions on Earth since the demise of the dinosaurs.
Yet a new Stanford review of hundreds of studies going back decades contrastingly reports that across the margins of all seven continents, little to no sediment has ever been found dating back to this transition. The discovery of this extensive gap in the geologic record was published this week in Earth-Science Reviews.
"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 offers fresh insights into sediment deposition and erosion processes, as well as the broader environmental signals from dramatic climate change, which could help researchers better grasp the scale of today's changing climate.
"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, PhD '20, who is now an assistant professor of Earth sciences at Montana State University.
Tim McHargue, an adjunct professor of Earth and planetary sciences at Stanford, is also a co-author on the study.
From hothouse to icehouse
During the Eocene-Oligocene period, Earth underwent profound planetary cooling. Giant ice sheets appeared in Antarctica, which was previously ice-free, global sea level plunged, and land and marine life suffered severe die-offs.
Prior to that, in the early part of the Eocene that lasted from about 56 million to 34 million years ago, Earth had the warmest temperatures and highest sea levels since dinosaurs walked the Earth more than 66 million years ago, according to climate-proxy records.
Burton and colleagues initially focused on exploring the effects of early Eocene conditions on deep-sea depositional systems. The resulting study - published in Nature Scientific Reports in 2023 - found abundant sand-rich deposits in the ocean basins along Earth's continental margins. The research team attributed this deposition increase mainly to intensified climatic and weather conditions boosting erosion from land. Their curiosity piqued, Burton and colleagues then extended the investigation to the late Eocene and early Oligocene, when Earth suddenly went from "hothouse" and "greenhouse" climates to the opposite, an "icehouse" climate.
For the new study, the researchers painstakingly pored over scientific and technical literature documenting ancient sediment up to several kilometers beneath the sea floor going back more than a century. The literature included offshore oil and gas drilling studies, onshore rock outcrop studies, and even interpretations of seismic data to infer Eocene-Oligocene sediment characteristics. In total, just over a hundred geographical sites worldwide were included, outlining 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."
The researchers offer a few theories about why this lack of deposition occurred. Vigorous ocean bottom currents, driven by temperature and salinity of the waters, may have been triggered or magnified by the major climate shift, potentially eroding the ocean floor and sweeping away sediment that flowed off the continents. Meanwhile, mechanisms from continental shelves exposed by sea-level fall could have allowed sediments to entirely bypass the closer-in sedimentary basins, sending deposits much farther out onto the abyssal plain of the ocean floor. More regionally restricted processes, like glacial erosion around Antarctica, likely played a part, too.
Whatever mechanisms may have been in play, they collectively created similar scenes of erosion in oceanic basins around every continent. That ubiquity points to what the researchers referred to as global controls - meaning that profound climatic change was felt everywhere, from the tallest landmasses down into the deepest waters.
In this way, the abrupt climatic event at the Eocene-Oligocene boundary and its newly observed, substantial effects along continental margins could help researchers better grasp the global enormity of today's unfolding climate change. Although the human-caused climate change of the past couple centuries is currently much smaller in overall magnitude compared to the Eocene-Oligocene transition, it is happening at an alarmingly faster pace, the Stanford researchers said.
"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."
