Research Highlight: History's Warning

With frequently slick volcanic mud at its topmost layer and 2,000 feet of crumbly red limestone between the rim and the valley below, the centerpiece amphitheater at Cedar Breaks National Monument can seem like a confection made in hell to hikers.

But despite the quality of palpable erosion and instability that has rendered Cedar Breaks a repository of wind- and rain-sculpted rock, the formation is robust in one aspect. It contains what may be the clearest record anywhere on Earth of a rapid climate change event 55 million years ago that could give us the best hint at what climate has in store for us in the next century.      

Cedar Breaks is the smaller neighbor to Utah’s Bryce Canyon National Park, both made famous by their brilliant red sedimentary cliffs and fanciful stone pillars known as hoodoos. More importantly to the Scripps researchers who visited here in September, Cedar Breaks is older, enough so to bear sedimentary rocks formed during the Paleocene-Eocene Thermal Maximum, known as the PETM to geologists.

The PETM was but one of many rapid-warming episodes to have taken place throughout history, but one of the biggest. Existing knowledge suggests that some event, possibly a large-scale melting of seafloor methane deposits, released between 1,200 and 2,500 gigatons of carbon dioxide into the atmosphere. The incident took place in less than 1,000 years. Whether it happened in one year or 50 or 500 is unknown but by the end of the millennium, global temperatures had risen between 4° and 7° C (7.2° and 12.6° F).

Many climate scientists have speculated about what such a circumstance would look like today but PETM already gives a rough account — little of it encouraging for a planet that now has 6.5 billion human inhabitants. The warming extended to the deep oceans and was enough to reverse global circulation patterns. Chemical evidence shows that the heat created oxygen-starved acidic waters intolerable to many life forms. The presence of acidic water suggests that the outgassing event and subsequent dissolution of CO₂ in the water must have happened quickly, too fast for natural processes to absorb the excess carbon.

Mass die-offs ensued. The acid may have prevented many marine invertebrates from being able to form shells. Fossils of some of the ocean’s most plentiful shelled organisms, tiny creatures called foraminifera, reveal that some combination of the low pH and greatly reduced oxygen levels led to the extinction of more than 50 percent of all deep-sea foraminifera species.

On land, the warming trend made palm trees grow in present-day Wyoming and Montana and the ancestors of today’s crocodiles migrate as far north as Canada. There is evidence that the Arctic Ocean had no ice during this time and that the first primates and horses took advantage of the warmth to expand from their place of origin in central Asia to other continents on their way to replacing the position of dominance ceded by dinosaurs 10 million years earlier. All this from one 1,000-year burp.

Scripps geologist Richard Norris, the principal investigator on the Cedar Breaks trip, considers two other components of the PETM climate. One is that, cataclysmic as it was, the PETM might not have unfolded rapidly enough to serve as an adequate analogy to what’s happening today. The second is that once the atmosphere was loaded with CO₂, it may have taken as much as 200,000 years for the skies and oceans and all the organisms within them to return to their pre-PETM states. (Video here.)

“If we can figure out how rapid the input was, then we can make a much better assessment of the extent to which this is comparable to modern times,” he said.

 The hiking area around to the northwest of the Cedar Breaks ranger station and visitors center is called Twisted Forest for its wind-sculpted trees. Somebody in the research group called it “Twisted Sister” as a joke and the name stuck. After a day of scouting, Norris and Lisa Tauxe, a Scripps researcher and expert in paleomagnetism, decided that Twisted Sister bears what Norris calls the best “low-hanging fruit,” accessible samples of 55-million-year-old rock.

The rocks from Twisted Sister promise to contribute to reconstructing the events of the PETM. The case for the PETM being the trigger that transitioned the Earth from one geologic epoch to another has only developed in the last decade. Most of what is inferred about it comes from ocean floor and terrestrial sediments, two sources that come with limitations.  Sediment accumulates steadily on the seafloor at a rate of roughly one centimeter per millennium, leaving researchers interested in time scales smaller than 1,000 years with little detail with which to work.

Norris and graduate student Johnnie Lyman, a co-leader of this trip, have been part of efforts in recent years to recover records of PETM both from the seafloor and from terrestrial sources in Wyoming. In these locations, 1,000 years might be represented by a band of rock one meter (3.28 feet) thick, but because of the sporadic way sediment accumulates on land, those results need confirmation from another source.

Cedar Breaks is an ancient lakebed, where deposits collected in steady fashion year after year. Its sediments could help validate land records and provide more detailed samples than the ocean can yield.  Lyman is hoping for millennial slices as much as 10 centimeters (3.93 inches) thick but that is only one obstacle to useful data.  Somewhere within the sedimentary record of Cedar Breaks is the record of what happened during the PETM. But how does one find it?

Tauxe came with the group to help them read what she calls the “bar code” of the rocks by dating a cross-section of a cliff face below Twisted Sister and from another promising dig site. Throughout Earth’s history, the planet’s magnetic field has shifted from pole to pole with the compass last pointing south 840,000 years ago. The PETM happens to have taken place during a relatively stable period between two reversals.

The team took rock samples at two-meter (6.56-foot) intervals from a 200-meter (656-foot) section of cliff. Norris collected another batch of rocks at even narrower intervals within that same section. Lyman’s first analytical task was to look for relative quantities of carbon and oxygen isotopes, the echo left behind by the ancient lake’s algae and other rudimentary life forms. An “excursion,” or sudden deviation in carbon isotopes, could be the calling card of the PETM. If she finds the excursion, Lyman will conduct a second round of analysis in which she examines the magnetic properties of the rocks to look for reversals of the Earth’s magnetic field. The patterns they form will be plotted against the timeline of known magnetic reversals to confirm whether the team’s rocks are in the right historical ballpark.

Norris and Lyman note that there are known key differences between the PETM and today, the most significant being that the world 55 million years ago was hotter to begin with before the event started. But if the rate and amount of the PETM’s infusion of carbon into the atmosphere turns out to be comparable to today’s, then history’s lesson is sobering regardless. Researchers have estimated that 368 gigatons of carbon dioxide were emitted into the atmosphere between 1850 and 1995, with slightly more than half of that being reabsorbed naturally through processes like plant photosynthesis. A United Nations science advisory panel projected that we will be adding another 20 gigatons per year by the year 2100 if trends continue for a total input of 2,368 gigatons since the start of the industrial revolution. 

With such comparable numbers, Norris believes the ancient event could well serve as an instructive precedent of climatic upheaval. But even in its aftermath, there could be another parallel, he said. There is evidence that foraminifera were able to come back from even such a destabilizing event in as few as 10,000 years because of a variety of factors that made them adaptable, an apparent affirmation of one of Darwin’s most basic observations about survival. 

Indeed, a possibly encouraging aspect of the PETM is that, with the exception of the deep sea, there were few extinctions on land or in the surface ocean despite the intense global warming. Evidently, the combination of ocean acidification and warming was not enough to tip most species over the brink of extinction.  On the other hand, Norris notes, it is possible that humans will end up burning the roughly 5,000 gigatons of carbon locked in all the coal and oil reserves known and create a dire set of conditions.

“If we don't get our fossil fuel diet under control, the PETM will not be much comfort as a predictor of the effects on the Earth's climate,” Norris said. “We will have stepped well into the unknown with a rapid carbon release more the twice the size of any event currently known from Earth's history.”

- Robert Monroe

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