The world looked very different 250 million years ago during the Permian era. The supercontinent Pangaea clung to the Equator and stretched its limbs all the way to the Poles. The climate was warming as the glaciers receded. Trilobites, the iconic animals of the period resembling the progeny of a bizarre crab-centipede mating (see above), had already dominated the oceans for a few hundred million years. Pelycosaurs roamed the shorelines, a hint of the dinosaurs to come.
Despite the differences, scientists believe we can better understand Earth’s future by peering into this particular past. The reason for comparison lies in the Permian’s dramatic end. Two hundred fifty two million years ago, a catastrophic event rocked the entire globe, leading to the extinction of 96% of marine species and 70% of vertebrates on land. Trilobites vanished, as well as most dominant land animals, opening the door for the kingdom of dinosaurs to rise.
But what actually caused the largest extinction event in known history? This is the question puzzling scientists, but a new study in Scientific Reports (1) suggests that ocean acidification played a major role. Since the oceans and atmosphere are always in contact, CO2 constantly exchanges between the two. As CO2 increases in the atmosphere, more and more bleeds into the ocean where it reacts with carbonate ions to form carbonic acid. Less carbonate then resides in the water and marine life cannot produce the calcium carbonate necessary to build shells, leading to massive die-offs. If acidification is found to be the cause, the Permian extinction may provide an important warning of our possible future as global warming and carbon dioxide continue to acidify the oceans around us.
Ocean acidification has already been postulated as a culprit based on model data that suggests a sharp rise in CO2 from volcanic activity. But other causes are possible, such as average global temperature increase and loss of available oxygen. To move beyond model predictions and see what actual experimental data tells us, Clarkson et al measured ancient ocean pH levels using a proxy – the level of boron isotopes in marine carbonates during the same time period. To understand why this works, we need to take a brief detour into carbon chemistry (2).
Boron naturally exists in the ocean, usually as borate – B(OH)3 or B(OH)4-. As clay particles enter the ocean, lighter isotopes of boron prefer to adsorb onto the clay, leaving the ocean with isotopically heavier boron. However, the exact ratio of light-to-heavy isotopes from this reaction depends on the pH levels of the surrounding oceanic environment. Specifically, the amount of heavier boron isotope (B11) decreases with increased acidity. Though knowledge of pH is not directly preserved, scientists can find evidence of boron isotope levels trapped in carbonate samples, providing an indirect glimpse into the ancient pH levels.
With this tool in hand, Clarkson et al explored carbonate samples in the United Arab Emirates to look at isotope trends in the geologic record around 252 millions years ago. They found acidification did not play a role during a first mass extinction at the end of the Permian era, however a second mass extinction tens of millions years later did correlate with an abrupt and massive CO2 injection into the ocean. This second event lasted roughly 10,000 years and removed most of the marine life in the oceans that relied on calcified shells.
So how does this help us understand what we can expect today? The key is understanding the magnitude of the CO2 perturbation in the Permian that caused this mass extinction and to compare it to current anthropogenic CO2 emissions. Based on the boron isotope data that the authors then fed into climate change models, the second Permian mass extinction occurred during a sharp increase in CO2 that is about five times more (24,000 petagrams of carbon (PgC)e ) than what we have could add with conventional fossil fuels (5000 PgC). However, if unconventional sources of CO2 are considered, such as methane hydrates in the Arctic permafrosts, there is a possibility of reaching a perturbation on the order of magnitude of the volcanic-induced Permian disruption.
This should serve as a strong warning – we do not have many opportunities to explore possible futures with such specificity. We are already seeing evidence of ocean acidification, and these new data from the Permian era provide a quantitative sense of how much more CO2 could be a tipping point to cause extinctions of similar extent.
Clarkson MO, Kasemann SA, Wood RA, Lenton TM, Daines SJ, Richoz S, Ohnemueller F, Meixner A, Poulton SW, & Tipper ET (2015). Ocean acidification and the Permo-Triassic mass extinction. Science (New York, N.Y.), 348 (6231), 229-32 PMID: 25859043
Schwarcz HP, Agyei, EK, McMullen CC. “Boron isotopic fractionation during clay adsorption from sea-water.” Earth and Planetary Science Letters, 6(1), 1-5 (1969)