In the past it has been widely reported that high and abruptly changing CO2 concentrations during the Permian led to climate conditions that were “too hot for complex life to survive” on the planet. Today, scientists have determined that the opposite may be true: the Permian mass extinction event occurred during a period of global cooling, expansive ice sheet growth, relatively low CO2 levels, and a marine-habitat-destroying sea level drop of 100 meters.
Image Source: Kani et al., 2018
A year ago, the press release for a paper published in Scientific Reports argued that during the Permian mass extinction event, “the majority of marine species” were killed off by an “extreme cold” period that coincided with widespread glaciation and a dramatic drop in global sea levels.
“Analysis of the newly dated layers showed a significant reduction of seawater levels during the [Permian] extinction event. The only explanation for such a dramatic decrease in water levels is a sudden increase in ice. The ice age lasted just 80,000 years, but the extreme cold was enough to kill off the majority of marine species.”
Within the last few months, at least two more papers have been published that also affirm that the Permian mass extinction event that annihilated up to 90% of marine species and 70% of land-dwelling species coincided with extreme global cooling, ice sheet expansion over land, and dramatically-falling sea levels — 100 meters lower than they were in previously warmer climates.
The lowering of sea levels alone may have been enough to destroy a substantial percentage of marine habitats, and the expansion of ice sheets may have austerely limited the habitat ranges for land-dwelling fauna.
CO2 Concentrations And Mass Extinctions: A Questionable Link
Further analysis reveals that, contrary to commonly popularized claims, neither the Ordovician mass extinction event nor the Permian mass extinction event had a clear causal link to atmospheric CO2 concentrations. Indeed, it has long been documented that CO2 concentrations may have fluctuated between about 280 ppm and 2800 ppm during the Permian, with the low CO2 values coinciding with cool periods and the high values coinciding with warm periods (Saunders and Reichow, 2009).
While both extinction events occurred during global cooling periods accompanied by significantly lowered sea levels, the CO2 concentrations were relatively high (“over 2000 ppm”) during the Ordovician but relatively low (~300 ppm) during the Permian extinction event. The latter CO2 values would appear to undermine the contention that CO2-driven ocean “acidification” and too-high CO2 concentration levels were causally connected to the extinction of marine species during the Permian. And the relatively high CO2 values during the Ordovician are not compatible with the accompanying global cooling, glaciation, and plummeting sea levels of that period.
In sum, a growing body of evidence suggests that commonly-held assumptions about a direct causal link between CO2 concentration flux and mass extinction events may not be as clear as previously thought.
The Hirnantian (Late Ordovician) and end-Guadalupian
(Middle Permian) mass-extinction events compared
“Besides the similarity in extinction patterns with the preferential elimination of particular clades, we can recognize a major resemblance but of non-biotic nature between the [Ordovician] Hirnantian and [Permian] Capitanian [mass extinction] events, that is the significant cooling coupled with global sea-level drop.”
“A sea-level drop of nearly 100 m can be achieved solely by transferring vast seawater onto land in the form of ice. It is noteworthy that sea level was much higher in the Late Ordovician (ca. 60 m above the present-day level) than in the Middle Permian (ca. 80 m below the present-day level), even after the sea-level drop in the same magnitude.”
“The lines of evidence for cooling and relevant sealevel drop during the Hirnantian [Ordovician] are listed as follows: (1) sequence stratigraphy (Haq & Schutter 2008; Fig. 2); (2) regional occurrence of glacial sediments, including tillite/dropstone, mostly in Gondwana (e.g. Brenchley et al. 1994; Ghienne 2003); and (3) isotope signature in seawater (Trotter et al. 2008). On the basis of these, there was a solid consensus among researchers for the link between the global cooling and the first Hirnantian extinction; however, recent studies indicate that the cooling may have started not necessarily at the beginning of the Hirnantian but much earlier, probably already in the Middle Ordovician (Vandenbroucke et al. 2010; Finnegan et al. 2011; Nardin et al. 2011; Rasmussen et al. 2016). Nevertheless, sea level dropped nearly 100 m in the Hirnantian (e.g. Haq & Schutter 2008). Traditionally, the existence of a continental block (Gondwana) over the South Pole was required for the development of ice sheets.”
“The evidence for the late Capitanian global sealevel drop (for up to 100 m) is robust. … The sharp erosion of reef limestone in low-latitude mid-ocean implies a large eustatic sea-level drop, in other words, the appearance of a global cooling. … The biotic responses in the Hirnantian and Capitanian appear compliant with all these lines of evidence for cooling, in particular, the latitudinal contraction of faunal distributions towards tropics together with the preferential elimination of preexisting tropical fauna. Regardless of the second Hirnantian episode, the first decline in biodiversity in both cases occurred during global cooling. In general, a relative drop in seawater temperature, particularly in shallow seas, is critical for the metabolism of almost all contemporary marine organisms. The updated lines of evidence therefore confirmed the classic notion of a putative link between the global cooling and extinction (e.g. Stanley 1988) for both cases [Ordovician and Permian mass extinction events].”
“[T]he atmospheric CO2 decreased signiﬁcantly from over 2000 ppm in the Late Ordovician down to ca. 300 ppm during the Middle Permian, almost close to the present level (Royer et al. 2014).”
Middle Permian (Capitanian) seawater 87 Sr/ 86 Sr minimum
coincided with disappearance of tropical biota and reef
collapse in NE Japan and Primorye (Far East Russia)
“The end-Middle Permian extinction was in fact a prolonged but gradual decrease in diversity from the Wordian to the end of the Capitanian (Clapham et al., 2009). The main phase of the extinction appears to have occurred almost simultaneously during the Capitanian minimum of Sr isotope records. The present study on Sr isotopes eventually confirms that the carbonate deposition declined and consequently ceased during the interval called the “Capitanian minimum” with extremely low 87Sr/86Sr ratios below 0.7070, at least in the northern part of Greater South China.”
“These studies confirmed that unusually the low 87Sr/86Sr ratio in seawater (as low as 0.7068) persisted throughout the Capitanian Stage of the Guadalupian Series (the last one-third of the Guadalupian). This extremely low Sr isotope value naturally reflected a minimum flux from continental crust with respect to that from mid-oceanic ridges. For the cause of this unique phenomenon, a conventional explanation might prefer a high sea level under global warming, which can suppress the global total weathering/erosion as a result of concealing vast continental coastal zones. Nonetheless, the sea level during the Capitanian contradictorily recorded the lowest stand of the Phanerozoic (Haq and Schutter, 2008), suggesting a global cooling instead. Ice coverage and/or the predominance of arid climates under cooling during the Permian likely accelerated the decrease in the seawater Sr ratio.”
“In general, the termination of shallow marine carbonates may occur with a decrease in the seawater temperature. Two possible causes can be inferred for the late Capitanian temperature decrease, i.e., 1) the appearance of global cooling, and 2) the migration of depositional sites to higher latitudes with cooler climates.”
“The Capitanian global cooling was proposed first on the basis of the unique carbon isotope record in low-latitude paleo-atoll carbonates (12°S) (Isozaki et al., 2007), a signature that was reproduced later in other parts of the world, e.g., in Croatia and in South China (Isozaki et al., 2011; Chen and Benton, 2012). In addition, in a global summary of sequence stratigraphy (Haq and Schutter, 2008), the findings of coeval glacial deposits in eastern Australia and in Mongolia (Fielding et al., 2008; Fujimoto et al., 2012), the selective extinction of tropical fauna (Isozaki and Aljinovic, 2009), the migration of mid-latitude fauna to low latitudes (Shen and Shi, 2002), and the Milankovitch tuning (Fang et al., 2017), all support the onset of cooling in the Capitanian.”
“[T]he Capitanian under a cooling trend […] [coincided with the] lowest sea level of the Phanerozoic and […] the preferential elimination of tropical fauna. … [T]he appearance of extensive ice coverage over continental blocks might have occurred, and the weathering/erosion of continental crust could have been suppressed to drive a lower riverine flux with high 87Sr/86Sr ratios. Alternatively, an arid weathering regime might have developed extensively under the cool climate, particularly on the vast supercontinent Pangea, which also contributed to suppressing the riverine fluxes (Korte et al., 2006).”