So, What Do We Know Now?

Today’s post is quite wordy, but I’ve put some nice photos in as a reward! Happy reading and I hope it’s kept you interested. 

Over the past few weeks, we’ve been looking at a series of concepts and processes that have helped us to understand the PETM and its significance. Today, I’m going to review the evidence, put all the pieces together and hopefully draw some conclusions over what exactly caused the PETM.

So, what do we know? We know, from a variety of sources, that 55.5 Ma, there was a large isotopically light excursion of C, of around 2-3‰ into various biomes over the world, resulting in a δ¹³C shift of 2-3‰. Estimates vary quite considerably from 4000-7000Pg of C - thats a window of 75% of the lower limit. In short, its very imprecise! In any case, an excursion of even 4000Pg of C to give a isotopic shift δ¹³C of 2-3‰ points towards new sources of isotopically depleted C. That is to say, C sources that contains more ¹²C than their standard, the Pee Dee Belemnite. After solving the mass balance problem, this cannot be explained by methane hydrate stores alone, and so we’ve come across other potential sources including wildfires, increased volcanic activity and even meteorite impacts.

A fossilised Belemnite

A reconstructed Belemnite

Methane hydrates were first proposed, based on the mass balance arguments, to explain the carbon release across the PETM. Within a decade, this excursion had been linked to various seismic and sedimentological data from the western margins of the Atlantic Ocean, which is thought to have resulted in the release of methane hydrates stored in the sediments. 

As studies probed further into the evidence of the PETM, focus shifted to the early PETM (ePETM), as knowledge of our own modern climate system developed with the advent of concepts such as tipping points and positive feedback. It was here that many researchers have intended to find a localised, intense source of the carbon isotope excursion (CIE). Rather sound data have come from the Vøring and Møre basins of the North Atlantic in support of a link between seismic disturbances and CIE. It is here that large hydrothermal vent complexes were found along seismic reflection profiles, in association with a variety of sill complexes. As has already been explained before, the carbon rich sediments, once in contact with the sills, were metamorphosed into the contact aureoles that we see today, at the expense of releasing at least 7 times as much C as from ordinary basaltic melt. 

Prior to such discoveries, proposals (Katz et al., 2003) from terrestrial sources, notably the burning of peatlands and coal. Such mechanisms too closely resemble the anthropogenic factors that cause modern climate change; ie the burning of fossil fuels and positive feedback mechanisms associated with them. Yet again, the mass balance issue plagued these theories, as it is calculated that with a -26‰ signature of biospheric carbon, for 7000Pg C to be released with a δ¹³C of -2‰ to -3‰, more organic matter would have to be burnt that is present in the total organic carbon reservoir. Basically, there isn’t enough C there to explain it! Whilst the volcanic origins of the ePETM warming solve these problems, there is insufficient resolution on the data to accurately date them. 

Subsequent warming from the ePETM resulted in a positive feedback effect, where it just kept getting warmer and warmer. The largest effect on the biota is seen in benthic foram species, where 35-50% of species became extinct. From evidence from Barite accumulation in sediments, there have been some support of a negative feedback mechanism with planktonic forams. As waters become warmer and richer, the forams are able to fix more CO₂ from the oceans, and thus remove it from the atmosphere, returning the Earth to its original equilibrium. In my personal opinion, this theory is still quite unfounded and is based upon a proposed model by the same people of Barium cycling in the oceans; not to mention that the concentrations involved are far lower than most concentrations of other solutes in the oceans, making the data highly susceptible to tiny perturbations in chemical composition, sometimes not necessarily related to CO₂. Below are examples of forams, exemplifying their morphological diversity, which is extremely useful for dating ancient sedimentary basins. 

Belemnites are extremely diverse

In another proposal, we came across was the extraterrestrial impact theory based upon an iridium anomaly at Zumaya, Spain. The iridium “spike” at Zumaya was dated to be of similar time to the magnetization of minerals within Kaolinite minerals in the sedimentary basins off the eastern coast of North America. Similar observations were made over the KT boundary at the extinction of the dinosaurs. - cue highly inaccurate, but awesome looking KT impact picture: 

The KT Extinction resulted in the demise of the dinosaurs and
wiped out 75% of all species on the Earth

However, these magnetized particle within the rocks were very constrained in their grain sizes; not something you’d expect to see from an event that produced 100 teratonnes of TNT (which is over a billion times the energy that was dropped on Nagasaki and Hiroshima)! Instead, the authors concluded that the particles may have been biogenic is origin, as this is known to occur in nature. But how to explain the iridium anomaly.....

The anomaly itself is small. It is measured in 133-143ppt, which is small, so we’re not looking at another KT event. Other evidence for a KT-style impact is either localised, absent or misleading. The key give-away for the KT extinction was the Yantucan peninsula where the chixuclub crater was the “smoking gun” for the extinction of the dinosaurs. Such signs are absent for the 55.5Ma extinction. Now it is believed that the PETM occurred at a time where there was potentially a large meteor shower. Fossil evidence certainly points towards there being no major terrestrial extinction events; which is an unlikely scenario for an impact theory. 

So, there we have it. I conclude that the PETM was initiated by increased volcanic in the North Atlantic, and after crossing a tipping point, resulted in a series of positive feedback mechanisms, that ultimately led to a runaway greenhouse gas excursion, the most obvious measure of which was the release of methane from methane hydrates due to changes in the  climatic systems, resulting in warmer conditions globally. Primarily, by the feedback mechanisms of the flora of both the oceans and land that CO₂ levels could then again be lowered and returned to a more stable condition. 

Next time I intend to look at life across the PETM and extinctions. Today we are thought to be entering a 6th mass extinction through a combination of climatic change and other disturbances by how we live and develop and grow. We weren’t around then, so were there species shifts across the PETM? Did taxa go extinct? What changed?

Ciao for now.