Around 56 million years ago, a large release of greenhouse gases, most likely generated by volcanic activity, resulted in a period of intense global warming known as the Paleocene-Eocene Thermal Maximum (PETM).
A smaller episode of warming and ocean acidification triggered by a shorter burst of carbon emissions preceded the PETM, according to a new study.
According to the latest findings, which were published in Science Advances on March 16, the amount of carbon released into the atmosphere during this precursor event was nearly the same as current total carbon emissions from fossil fuel combustion and other human activities.
As a result, the short-lived precursor event indicates what might happen if current emissions are quickly reduced, but the PETM shows the effects of continuing to emit carbon into the atmosphere at the current rate.
“It was a short-lived burp of carbon equivalent to what we’ve already released from anthropogenic emissions,” said coauthor James Zachos, professor of Earth and planetary sciences, and Ida Benson Lynn Chair of Ocean Health at UC Santa Cruz. “If we turned off emissions today, that carbon would eventually get mixed into the deep sea and its signal would disappear because the deep-sea reservoir is so huge.”
By human standards, this process would take hundreds of years, but it would be a breeze compared to the tens of thousands of years it took for Earth’s climate system to recover from the more intense PETM. The new findings are based on an examination of marine sediments deposited in shallow waters along the Atlantic coast of the United States and now forming part of the Atlantic Coastal Plain.
Sea levels were greater at the time of the PETM, and much of Maryland, Delaware, and New Jersey was underwater. The researchers used sediment cores drilled by the United States Geological Survey (USGS) from this location in their investigation.
It was a short-lived burp of carbon equivalent to what we’ve already released from anthropogenic emissions. If we turned off emissions today, that carbon would eventually get mixed into the deep sea and its signal would disappear because the deep-sea reservoir is so huge.
James Zachos
A huge shift in carbon isotope composition and other signs of dramatic changes in ocean chemistry as a result of the ocean absorbing large volumes of carbon dioxide from the atmosphere mark the PETM in marine sediments.
The minuscule shells of tiny sea animals called foraminifera that lived in the ocean’s surface waters can be found in marine sediments. The chemical composition of these shells offers evidence of rising surface water temperatures and ocean acidification, as well as the environmental conditions in which they evolved.
Tali Babila, the study’s first author, began working on it as a postdoctoral fellow with Zachos at UC Santa Cruz and is currently at the University of Southampton in the United Kingdom. The researchers were able to rebuild a precise record of ocean acidification by analyzing the boron isotope composition of each foraminifera using novel analytical methods developed at Southampton.
This was part of a set of geochemical investigations they used to piece together environmental changes between the precursor event and the major PETM.
“Previously, thousands of foraminifera fossil shells were needed for boron isotope measurement. Now we are able to analyze a single shell that’s only the size of a grain of sand,” Babila said.
Sediments from the continental section at Big Horn Basin in Wyoming and a few other sites had previously revealed evidence of a preceding warming episode. However, because it was missing from deep-sea sediment cores, it was unclear whether it was a global signal.
This makes sense, according to Zachos, because deep-sea sedimentation rates are slow, and the signal from a short-lived event would be lost due to sediment mixing by bottom-dwelling marine life.
“The best hope for seeing the signal would be in shallow marine basins where sedimentation rates are higher,” he said. “The problem there is that deposition is episodic and erosion is more likely. So there’s not a high likelihood of capturing it.”
Along the Atlantic Coastal Plain, the USGS and others have drilled several sediment cores (or sections). The PETM is present in all of those portions, and several of them also catch the precursor event, according to the researchers. The latest analysis focuses on two Maryland parts (South Dover Bridge and Cambridge-Dover Airport).
“Here we have the full signal, and a couple of other locations capture part of it. We believe it’s the same event they found in the Bighorn Basin,” Zachos said.
Based on their findings, the researchers determined that the precursor signal in the Maryland parts was part of a global event that lasted several centuries, if not millennia.
The two carbon pulses that drove the PETM, the short-lived precursor and the much larger and longer-lasting carbon emissions, resulted in vastly different mechanisms and time scales for the Earth’s carbon cycle and climate system recovery. Within a thousand years or so, the carbon absorbed by surface waters during the antecedent event was mixed with the deep ocean.
However, carbon emissions during the PETM surpassed the ocean’s buffering capacity, requiring considerably longer processes such as tens of thousands of years of weathering of silicate rocks to remove the surplus carbon.
There are significant distinctions between Earth’s climate system today and during the Paleocene, according to Zachos, including the presence of polar ice sheets today, which increases the climate’s sensitivity to greenhouse warming.
In addition to Babila and Zachos, the coauthors of the paper include Gavin Foster and Christopher Standish at the University of Southampton; Donald Penman at Utah State University; Monika Doubrawa, Robert Speijer, and Peter Stassen at KU Leuven, Belgium; Timothy Bralower at Pennsylvania State University; and Marci Robinson and Jean Self-Trail at the USGS. This work was funded in part by the National Science Foundation.
