Pannotia: A supercontinent on trial
In separate articles published recently by The Geological Society of London, Yale geologists David Evans, R. Damian Nance, and their colleagues debate the existence of Pannotia, a supercontinent that may — or may not — have formed near the South Pole about 600 million years ago, around the time when animals were first evolving.
The articles put Pannotia on trial. Evans, the head of Berkeley College, professor of Earth and planetary sciences, and director of the Yale Paleomagnetism Laboratory, took the role of prosecutor; Nance, professor emeritus of geology at Ohio University and a visiting research fellow at Yale, was part of Pannotia’s defense.
Evans and Nance spoke with YaleNews about the case to be made for — and against — Pannotia.
Why are ancient supercontinents important to understand — and how do they relate to today’s world?
Evans: Most people are familiar with the notion of a Pangea supercontinent, from the early age of dinosaurs about 300 million years ago, when nearly all of our present-day continents were joined together into a single landmass. Less well known are earlier aggregations of supercontinents, which remain murky in our understanding: Did they exist, and if so when did they exist? And furthermore, which of today's continents were joined with which others? Understanding that our present geological environment is merely a single frame within the greater movie of Earth’s history — with many millions of years both past and future — will help us realize the context of our own relatively brief existence on this planet.
Aside from such existential considerations, the great global bounty of our natural resources for the developed world have accumulated throughout that long history, so in practical terms the piecing together of ancient continental puzzles will assist our exploration for such resources to sustain the needs and comfort of billions of people on the planet.
How long has there been speculation about the existence of Pannotia?
Nance: The idea of the existence of a supercontinent about 600 million years ago was first proposed (but not named) on the basis of the fossil record in 1970, and for the following 35 years or so the geologic evidence in support of such a supercontinent was stronger than for any other supercontinent excepting Pangea. Only with the advent of direct data on the positions of the continents stemming from the paleomagnetic record, coupled with increasingly precise age dates, has its existence come into doubt. But that data is still far from definitive.
Evans: It wasn't until the 1980s when my friend and colleague Damian Nance, among others, developed a theory of supercontinental cycles, in which the great landmasses assemble and break apart roughly every 300 to 400 million years. That rhythm would be linked to the steadily overturning motions of rocks deep within the Earth, influencing everything on the surface including major changes in climate and biological evolution, as well as natural resources. For the past 25 years, there has been a big question mark around Pannotia's existence, relative to a greater scientific consensus affirming its predecessors Rodinia (900 million years ago) and Nuna (1,600 million years ago).
What are the dominant arguments in its defense?
Nance: The evidence for Pannotia comes from the geologic record. Many phenomena have been linked to the formation, tenure, and breakup of supercontinents. For example, worldwide mountain building with their assembly; continental rifting with their breakup; low global sea level because supercontinents tend to be thermally uplifted, followed by rapid sea level rise and passive margin development as the dispersing continental fragments cool and subside; cold climates because large land areas draw down atmospheric CO2 and hence reduce the greenhouse effect, followed by climatic warming as the supercontinent breaks up; extinctions due to loss of habitat as the continental shelves become emergent, followed by biotic radiation as they reflood on breakup. The list goes on and on with ocean chemistry, the stable isotope record, biogeochemical cycles, zircon age peaks, et cetera. All of these “proxy” records are consistent with the assembly and breakup of a supercontinent some 600 million years ago.
However, a much more important question is whether or not the supercontinent cycle “turned over” 600 million years ago. We believe Earth’s history has been punctuated by the assembly and breakup of supercontinents for billions of years with profound consequences to its geology, sea level, climate, atmospheric composition, ocean chemistry, and biology. We also believe that the cycle occurs because the gathering of continents affects the dynamics of the underlying mantle in such a way as to cause the assembled continental fragments to disperse. Supercontinents appear to have this effect on the mantle, but we do not know whether we must have a supercontinent to cause this to occur. Perhaps a close approach of continents is sufficient and, if so, Pannotia does not have to have been a supercontinent in order to turn the cycle over. We have an arbitrary man-made definition of a supercontinent — 75% of available continental crust is often used — but the mantle has its own definition, and if it is the supercontinent cycle we are interested in then this is the definition we should be using, even if we are not sure what it is!
What are the arguments against Pannotia?
Evans: The direct lines of evidence are increasingly being called into question. A full-fledged Pannotia cycle requires distinct phases of continental collisions followed by a breakup. Recent dating of the rocks associated with the putative supercontinent-forming collisions (in South America, Africa, Australia, and Antarctica) shows that they are significantly younger than 600 million years old, so the delineation of distinct phases of global collision and breakup is no longer clear. As for the indirect evidence — such as sea level, ocean chemistry, and climate — the hallmarks of collisional assembly are straightforward. But those of an alleged global-scale 500-million-year-old breakup tend to be highly ambiguous in their interpretation. Thus, in my opinion, we have an emerging vision of the 600-to-500 million-year-old collisions as a mere “stepping stone” toward the younger, complete gathering of Pangea, rather than a complete Pannotia assembly/breakup cycle.
If we eliminate Pannotia from the discussion, how does it change our interpretation of Earth’s geological history?
Evans: If Pannotia really existed, then the supercontinent cycle would seem to be speeding up rapidly, from Nuna (1,600 million years ago) to Rodinia (900 million) to Pannotia (600 million) to Pangea (300 million). We would need to find a deep-Earth explanation for the dramatic quickening of this global rhythm, and we might expect that the next supercontinent should be occurring right around the corner, as it were. If Pannotia didn't exist, and is thus removed from that timeline, then Earth's heartbeat is more constant at about 600 million years' duration, and the next supercontinent — rejoining the Americas with Asia, or “Amasia” — wouldn't be expected until far into the future. Whichever is true, the planetary contexts of biological evolutionary history, ancient climatic change, and natural resource endowment all gain different perspectives depending on our assessment of that supercontinental pulse.
Beyond geology, what does the debate over Pannotia tell us about the pursuit of science?
Evans: Uncertainty is an essential aspect of any healthy science. We should all be prepared to rethink our beliefs if new data require us to do so. Also, science isn’t necessarily adversarial. Yes, many scientific fields are competitive; yet the greatest joy of my own career is the camaraderie developed in my community. At various times my colleagues and I have engaged in heated debates about how the Earth works, but at the end of the day we can compartmentalize our differences and continue our lifelong friendships over a warm dinner together.
The current Pannotia debate is one where we all accept that the rocks have been around for hundreds of millions of years, and they can probably wait a little longer for us to sort out their history — so there’s no need to get too distressed about a few remaining unresolved issues. Instead, if we work together we might discover some truly amazing things about our planet.