THE PERMIAN-TRIASSIC BOUNDARY CRISIS: CURRENT STATE OF KNOWLEDGE AND FUTURE RESEARCH DIRECTIONS

Major progress has been made in understanding the ~252-Ma end-Permian mass extinction (EPME), the largest biocrisis in Earth history, marked by loss of ~90% of marine invertebrate species. The ultimate trigger is regarded as eruption of the Siberian Traps Large Igneous Province (STLIP), which released large quantities of greenhouse gases (CO₂ and CH₄) and sulfate aerosols, triggering a catastrophic global warming of ~10°C and acidification of both continents and oceans. On land, a massive die-off of vegetation led to a transient episode of rapid soil erosion and a longer-term increase in weathering rates linked to elevated temperatures. In the ocean, widespread anoxia developed concurrently with the EPME, triggered by ocean-surface warming that reduced dissolved oxygen solubility in seawater and that intensified vertical stratification. Expanded anoxia led to massive burial of organic matter and reduced sulfur, although the evidence for this is indirect (C, U and S isotopes); few organic-rich deposits of Early Triassic age have been found, suggesting that organic sedimentation occurred mainly on continental slopes or in the deep ocean. Other aspects of the end-Permian crisis remain under debate. For example, there is no consensus regarding changes in marine productivity levels in the aftermath of the EPME, which were stimulated by enhanced subaerial weathering but depressed by reduced overturning circulation--the evidence to date may favor localized positive and negative changes in productivity. Also under scrutiny is evidence for volcanic eruptions and environmental perturbations during the ~100 kyr prior to the EPME, which are likely to have occurred but remain poorly dated and quantified. The greatest uncertainty, however, may surround the nature of the proximate kill mechanism(s) during the EPME. Many hypotheses have been advanced including mechanisms based on redox, temperature, and pH changes. Determining which environmental factors were most harmful to end-Permian life is inherently difficult as the taphocoenotic clues are challenging to interpret, and many organisms may have succumbed to a combination of environmental effects. Understanding the details of the EPME may provide key insights into the future of the Earth’s biosphere as present-day global warming and ocean acidification intensify.