Carbon Dioxide, Oxygen Depletion, and the Mass Extinction in the Permian Era.

[NNadir]

The paper I'll discuss in this post is this one: Temperature-dependent hypoxia explains biogeography and severity of end-Permian marine mass extinction (Penn et al, Science, (2018) Vol. 362, Issue 6419, eaat1327).

This paper is the source material for a news article which came to my attention by a post here: Stanford Study: We Will Be 20% Of The Way To Permian Extinction 2.0 By 2100 With Business As Usual

From the introduction:

Volcanic greenhouse gas release is widely hypothesized to have been the geological trigger for the largestmass extinction event in Earth’s history at the end of the Permian Period [~252 million years (Ma) ago] (1, 2). At least two-thirds of marine animal genera and a comparable proportion of their terrestrial counterparts were eliminated, but the mechanisms connecting environmental change to biodiversity collapse remain strongly debated. Geological and geochemical evidence points to high temperatures in the shallow tropical ocean (3, 4), an expansion of anoxic waters (5–8), ocean acidification (9–12), changes in primary productivity (13, 14), and metal (15) or sulfide (16, 17) poisoning as potential culprits. However, a quantitative, mechanistic framework connecting climate stressors to biological tolerance is needed to assess and differentiate among proposed proximal causes.

In this study, we tested whether rapid greenhouse warming and the accompanying loss of ocean O2—the two best-supported aspects of end- Permian environmental change—can together account for the magnitude and biogeographic selectivity of end-Permianmass extinction in the oceans. Specifically, we simulated global warming across the Permian/Triassic (P/Tr) transition using a model of Earth’s climate and coupled biogeochemical cycles, validated with geochemical data.

This is an in silico evaluation, since the experimental loading of the entire atmosphere with excess carbon dioxide, while well underway, has not been completed, although some preliminary intermediate results are currently being observed. The experimental portion of the work described herein - other than burning all of the world's fossil fuels and dumping the waste in the atmosphere just described - is limited to viewing the metabolic effects of oxygen depletion on extant species. (Trilobites were not available for testing.) The in silico data is also compared with the fossil record, including oxygen isotope ratios in fossil conodonts, eel like animals that lived in those time, generally known from fossils of their teeth.

The following graphic from the paper touches on that point:

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The caption:

• Fig. 1 Permian/Triassic ocean temperature and O2.
(A) Map of near-surface (0 to 70 m) ocean warming across the Permian/Triassic (P/Tr) transition simulated in the Community Earth System Model. The region in gray represents the supercontinent Pangaea. (B) Simulated near-surface ocean temperatures (red circles) in the eastern Paleo-Tethys (5°S to 20°N) and reconstructed from conodont ?18O apatite measurements (black circles) (4). The time scale of the ?18O apatite data (circles) has been shifted by 700,000 years to align it with ?18Oapatite calibrated by U-Pb zircon dates (open triangles) (1), which also define the extinction interval (gray band). Error bars are 1°C. (C) Simulated zonal mean ocean warming (°C) across the P/Tr transition. (D) Map of seafloor oxygen levels in the Triassic simulation. Hatching indicates anoxic regions (O2 < 5 mmol/m^3). (E) Simulated seafloor anoxic fraction ƒanox (red circles). Simulated values are used to drive a published one-box ocean model of the ocean’s uranium cycle (8) and are compared to ?238U isotope measurements of marine carbonates formed in the Paleo-Tethys (black circles). Error bars are 0.1‰. (F) Same as in (C) but for simulated changes in O2 concentrations (mmol/m^3).

The test animal used to perhaps model metabolism is the common crab found along the East Coast of North America Cancer irroratus. Crustaceans, like the trilobites, which inhabited the oceans for 280 million years before their extinction in this event, are members of the phylum Euarthropoda (Arthropods) and like the trilobites, feature an exoskeleton that probably was fairly acid sensitive. It is not clear that the extinction of the trilobites was a function of increased acidity owing to the carbon dioxide content of the oceans, or whether it derived from oxygen depletion or perhaps both. The authors discuss this briefly in the discussion, but in a rather general and somewhat speculative way.

With this editor and the type of text used by Science I cannot produce the equation for the "metabolic index" used here, but for those with a modicum of a science back ground, this index is proportional to the partial pressure of oxygen divided by a term that looks very much like an Arrhenius term, an exponential operator on the negative value of energy (here measured in electron-volts), divided by the Boltzman constant (R/No) times the difference between reciprocal temperatures. The proportionality constant has units of inverse pressure and therefore the metabolic index, ?, is dimensionless. This metabolic index (which differs from what your fitbit might put out or what you can see on a "lose your fat and look good" website) is described here: Climate change tightens a metabolic constraint on marine habitats, which seems to be along the same lines as the paper under discussion.

A graphic about the metabolic index:

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The caption:

• Fig. 2 Physiological and ecological traits of the Metabolic Index (? ) and its end-Permian distribution.
(A) The critical O2 pressure (pO2crit) needed to sustain resting metabolic rates in laboratory experiments (red circles, Cancer irroratus) vary with temperature with a slope proportional to Eo from a value of 1/Ao at a reference temperature (Tref), as estimated by linear regression when ? = 1 (19). Energetic demands for ecological activity increase hypoxic thresholds by a factor ?crit above the resting state, a value estimated from the Metabolic Index at a species’ observed habitat range limit. (B) Zonal mean distribution of ? in the Permian simulation for ecophysiotypes with average 1/Ao and Eo (~4.5 kPa and 0.4 eV, respectively). (C and D) Variations in ? for an ecophysiotype with weak (C) and strong (D) temperature sensitivities (Eo = 0 eV and 1.0 eV, respectively), both with 1/Ao ~ 4.5 kPa. Example values of ?crit (black lines) outline different distributions of available aerobic habitat for a given combination of 1/Ao and Eo.

Text touching on the metabolic index is this paper:

pO2 and T are the O2 partial pressure and temperature of ambient water, respectively; kB is Boltzmann’s constant; and the parameters Ao (kPa^(?1)) and Eo (eV) represent fundamental physiological traits of a species. The inverse of Ao (i.e., 1/Ao, in kPa) is the minimum pO2 that can sustain the resting metabolic rate (i.e., the “hypoxic threshold”) at a reference temperature (Tref), and Eo is the temperature sensitivity of that threshold (Fig. 2A). The Metabolic Index measures the capacity of an environment to support aerobic activity by a factor of F above an organism’sminimumrequirement in a complete resting state (F = 1). For both marine and terrestrial animals, the energy required for sustained activity (e.g., feeding, reproduction, defense) is elevated by a factor of ~1.5 to 7 above resting metabolic demand (18, 25) and represents an ecological trait, termedFcrit. If climate warming and O2 loss reduce the Metabolic Index for an organism below its species-specific Fcrit, the environment would no longer have the capacity to support active aerobic metabolism and, by extension, long-term population persistence.

The graphic immediately following the one above:

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The caption:

• Fig. 3 Aerobic habitat during the end-Permian and its change under warming and O2 loss.
(A) Percentage of ocean volume in the upper 1000 m that is viable aerobic habitat (? ? ?crit) in the Permian for ecophysiotypes with different hypoxic threshold parameters 1/Ao and temperature sensitivities Eo. (B) Relative (percent) change in Permian aerobic habitat volume (?Vi, where i is an index of ecophysiotype) under Triassic warming and O2 loss. Colored contours are for ecophysiotypes with ?crit = 3. Measured values of 1/Ao and Eo in modern species are shown as black symbols, but in (B) these are colored according to habitat changes at a species’ specific ?crit where an estimate of this parameter is available. The gray region at upper left indicates trait combinations for which no habitat is available in the Permian simulation.

Some information about the distribution of oxygen depletion in the oceans:

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Fig. 4. Global and regional extinction at the end of the Permian. (A) Global extinction versus latitude, as predicted for model ecophysiotypes and observed in marine genera from end-Permian fossil occurrences in the Paleobiology Database (PBDB). Model extinction is calculated from the simulated changes in Permian global aerobic habitat volume (DVi) under Triassic warming and O2 loss (19). The maximum depth of initial habitat and fractional loss of habitat resulting in extinction (Vcrit) are varied from 500 to 4000 m (colors) and from 40 to 95% (right-axis labels), respectively.The observed extinction of genera combines occurrences from all phyla in the PBDB (points). Error bars are the range of genera extinction across two taxonomic groupings: phyla multiply sampled in the modern physiology data (arthropods, chordates, and mollusks) and all other phyla. Latitude bands with fewer than five Permian fossil collections are excluded. The average range is used for latitude bands missing extinction estimates from both taxonomic groupings (i.e., 80°S, 30°S, and 40°N). The main latitudinal trend—increased extinction away from the tropics—is found when including all data together and when restricting to the best-sampled latitude bands (fig. S14). In all panels, model values are averaged across longitude and above 500 m. (B) Average hypoxic threshold and Fcrit across ecophysiotypes versus latitude in the Permian. In (B) to (D), shading represents the 1s standard deviation at each latitude. (C) Regional extinction (i.e., extirpation) versus latitude for model ecophysiotypes, with individual contributions from warming and the loss of seawater O2 concentration. Extirpation occurs in locations where the Metabolic Index meets the active demand of an ecophysiotype in the Permian (F ? Fcrit) but falls below this threshold in the Triassic (F < Fcrit). (D) Same as (C) but including globally extinct ecophysiotypes (using a maximum habitat depth of 1000 m and Vcrit = 80%), and as observed in marine genera from end-Permian and early Triassic fossil occurrences of all phyla in the PBDB. Observed extirpation magnitudes are averaged across tropical and extratropical latitude bands (red points and horizontal lines). Regional 1s standard deviations are shown as vertical lines.

The authors conclude with somewhat obvious remarks on the relevance of this study to the present times:

The end-Permian mass extinction resulted in the largest loss of animal diversity in Earth’s history, and its proposed geologic trigger—volcanic greenhouse gas release—is analogous to anthropogenic climate forcing. Predicted patterns of future ocean O2 loss under climate change (30, 31) are broadly similar to those simulated here for the P/Tr boundary. Moreover, greenhouse gas emission scenarios projected for the coming centuries (32) predict a magnitude of upper ocean warming by 2300 CE that is ~35 to 50% of that required to account for most of the end-Permian extinction intensity. Given the fundamental nature of metabolic constraints from temperature-dependent hypoxia in marine biota, these projections highlight the potential for a future mass extinction arising from depletion of the ocean’s aerobic capacity that is already under way.

But you already knew that, didn't you?

To be clear, this paper refers to oxygen in the oceans, and not the atmosphere. Almost all of the oxygen now on earth originates in the oceans, but it's not clear how it partitions between the oceans and the air. In general, gases are less soluble in hot water than in cold water, as is clear to anyone who's messed around with carbonated beverages, but I'm not aware in any quantitative sense of how these solubility relations relate to oxygen as compared with carbon dioxide. (The latter is controlled, in water, by the equilibrium between solvated CO2, its water adduct, carbonic acid, bicarbonate and carbonate, all of which are present.) It is quite possible that the warm surface layers, rich with algae or other photosynthetic species, cranked out lots of oxygen after the Permian extinction, but that it all went into the air and did not remain in the ocean.

(From the text of the paper, one factor seems to have been the circulation patterns of oceanic water, which were arrested by the heating.)

I didn't mean to divert your attention from all the hoopla surrounding the orange fool, but frankly, he doesn't matter and has never mattered, and his ultimate significance will prove to be that of Caligula, so much as Caligula matters today - he doesn't - except for the amusing historical fact that Caligula put a horse in the Senate and the orange idiot has a turtle in the Senate.

Same difference.

Have a nice day tomorrow.

[Crutchez_CuiBono]

1. Good LORD man(?). Thatll take some reading. Thanks!!

[NNadir]

2. You're welcome.

Speaking only for myself, I find reading is good for me.

[littlemissmartypants]

3. I love your discussions!

Thanks for sharing them. I often search and read scholarly articles. Unfortunately, many are now behind paywalls, which is stifling to the scientific research community and troubling in the pursuit of a broadly informed medical community. Sometimes I have to settle for the abstracts. These are great!

[NNadir]

4. Thank you!

I appreciate having readers, but if I have none, the writing these teaches me stuff, and forces me to learn things.

I understand that my strategy may not work in many places, but in New Jersey, there are several academic libraries that offer open access if you are physically in the library.

Rutgers will give you two hours a day on "guest computers" of which most of their libraries have several.

I pay to use Princeton's libraries, an annual fee, but some of their libraries are open for free at least for portions of the day.

These two libraries cover most of the world's scientific literature.

In other areas, MIT has a few open computers available for two hours a day.

I have a son in an out of town university with a very good library. If I can't get something, which is rare, I ask him and often he can fill in the blanks if he's not too busy.

The American Chemical Society website also allows you to search for open access papers directly.

Thanks much for caring about science.