Ancient climate analysis reveals unknown global processes

According to widely cited conventional models, cooling and a significant drop in sea level around 34 million years ago should have led to widespread continental erosion and the deposition of gargantuan quantities of sandy material on the ocean floor. After all, this is one of the most drastic climate transitions on Earth since the disappearance of the dinosaurs.

Yet a new Stanford review of hundreds of studies going back decades contrastingly reports that on the margins of all seven continents, little or no sediment has ever been found dating back to this transition. The discovery of this globally extensive gap in the geological record was published this week in Earth Science Exams.

“The results made us wonder ‘where did all the sediment go?'” said study lead author Stephan Graham, the Welton Joseph and Maud L’Anphere Crook Professor at the Stanford Doerr School of Sustainability . “Answering this question will help us better understand how sedimentary systems work and how climate change is imprinted on the deep marine sedimentary record.”

The geological divide offers new insights into processes of sediment deposition and erosion, as well as broader environmental signals due to dramatic climate change, which could help researchers better understand the global enormity of climate change current.

“For the first time, we have taken a global look at a little-studied response of the planet’s largest sediment mass movement systems during the extreme Eocene-Oligocene transition,” said lead author of the paper. study, Zack Burton, Ph.D. ’20, who is now an assistant professor of Earth sciences at Montana State University.

Tim McHargue, assistant professor of earth and planetary sciences at Stanford, is also a co-author of the study.

From greenhouse to cooler

During the Eocene-Oligocene period, Earth experienced profound global cooling. Giant ice sheets appeared in Antarctica, which was previously ice-free, sea levels plunged, and land and marine life suffered severe disappearances.

Before that, during the early Eocene, which lasted about 56 to 34 million years ago, Earth had the hottest temperatures and highest sea levels since dinosaurs walked the Earth more than 66 million years ago, according to climate records. .

Burton and colleagues initially focused on exploring the effects of early Eocene conditions on deep-water depositional systems. The resulting study, published in Scientific reports in 2023, they discovered abundant sand-rich deposits in ocean basins along Earth’s continental margins.

The research team attributed this increase in deposition mainly to intensifying climatic and weather conditions favoring land erosion. Their curiosity piqued, Burton and his colleagues then extended the investigation to the late Eocene and early Oligocene, when Earth suddenly shifted from a “hothouse” and “hothouse” climate to one “ice greenhouse” climate.

For this new study, the researchers carefully studied scientific and engineering literature documenting ancient sediments up to several kilometers below the seafloor, analyzing studies published in the last decade or more than a century ago. The literature included offshore oil and gas drilling studies, onshore rock outcrop studies, and even interpretations of seismic data to infer characteristics of Eocene-Oligocene sediments. In total, just over a hundred geographic sites around the world were included, describing each landmass.

Although the method of literature analysis used in the study is not new in itself, the scale of such an approach made possible by large online databases could prove very illuminating, said Graham. “There might be other similar events in the geologic past that merit further investigation,” Graham said, “and the way to start is to do exactly what we did: a very thorough study of the geologic literature worldwide for certain suspicious periods in time.

“The current process of re-evaluating, re-investigating and re-analyzing literature that in some cases has been available for decades is challenging, but can be very fruitful,” Burton said. “The method can lead to exciting and unexpected discoveries, as we were able to do here.”

Totally unexpected

As Burton and his colleagues looked through the inventory of compiled data, they became increasingly perplexed by the apparent sedimentary non-presentation.

“We did not observe sand-rich deposits, as in our study of warm early Eocene climates,” Burton said. “Instead, we found that large and widespread erosional unconformities – in other words, gaps in the rock record – had developed during extreme climatic cooling and oceanographic change in the Eocene -Oligocene.”

Researchers offer some theories on the reasons for this lack of deposition. Vigorous ocean bottom currents, driven by water temperature and salinity, may have been triggered or amplified by major climate change, potentially eroding the ocean floor and sweeping away sediment that flowed from the continents.

Meanwhile, the mechanisms of continental shelves exposed by falling sea levels could have allowed sediments to bypass the nearest sedimentary basins entirely, sending the deposits much further into the abyssal plain of the ocean floor. More regionally restricted processes, such as glacial erosion around Antarctica, likely also played a role.

Whatever mechanisms are at play, they have collectively created similar erosion scenes in the ocean basins of all continents. This pervasiveness reflects what researchers call global controls, that is, profound climate changes were felt everywhere, from the highest lands to the deepest waters.

In this way, the abrupt climate event at the Eocene-Oligocene boundary and its newly observed substantial effects along continental margins could help researchers better understand the global enormity of ongoing climate change. Although human-caused climate change over the past two centuries is currently much smaller in overall magnitude than during the Eocene-Oligocene transition, it is occurring at an alarming and faster rate, the researchers said. Stanford.

“Our findings can help inform us about the types of radical changes that can occur on Earth’s surface in the face of rapid climate change,” Graham said. “The geological past sheds light on the present, and particularly the future.”