A NEW ROLE FOR ELEVATED CO₂ IN THE PROLONGED PERMIAN-TRIASSIC BIOTIC CRISIS

Atmospheric CO₂ has risen continuously since the Industrial Era primarily due to anthropogenic input. As CO₂ diffuses into the oceans, the pH and CaCO₃ saturation state of seawater decrease, and experiments and observations indicate that small changes measured in surface ocean chemistry today are causing a physiological and biocalcification crisis for Earth's calcareous marine biota. The degree and rate of calcification among corals, coccolithophores, foraminiferans, and coralline red algae decrease as the CaCO₃ saturation state of seawater decreases, and low-metabolism organisms may experience reduced growth and reproductive potential and even death if acid-base imbalance is not achieved. Atmospheric CO₂ has been elevated at intervals in Earth's history, and it has been hypothesized to have played a role in some ancient biotic crises. Elevated CO₂ is hypothesized to have caused the extinction of metabolically slow organisms and reef organisms; a decrease in nannoplankton size and calcification; and extinction and dissolution of calcareous microorganisms during the end-Triassic mass extinction, the early Toarcian crisis, and the Paleocene-Eocene Thermal Maximum, respectively.

Elevated atmospheric CO₂ has been hypothesized to have played a critical role in causing the end-Permian mass extinction, too, by facilitating development of a vertically stratified, low O₂, H₂S-rich deep ocean. However, biotic crisis continued for 5- to 6- million years after the extinction, and some stressed Early Triassic shallow marine faunas do not exhibit any indications of influence from toxic deep water. The absence of metazoan reefs and the numerical dominance of bivalves and tiny adult gastropods in Early Triassic shallow marine environments may indicate that the direct effects of increased atmospheric CO₂ on ocean surface waters and biota during this time have been underappreciated previously. We propose that elevated atmospheric CO₂ was an important mechanism for sustained ecological degradation in Early Triassic shallow-marine environments. Future research is necessary to test this hypothesis further, but it is likely that the elevated level of atmospheric CO₂ during the latest Permian-Early Triassic had detrimental effects on the Earth's shallow-marine biota.