PT Boundary

The response of terrestrial ecosystems to the extinction event recorded at the Permian-Triassic Boundary (PTB), identified by Doug Erwin of the Smithsonian as the “Mother” of all mass extinctions, is reported to parallel the biodiversity loss in the marine realm.  Terrestrial ecosystem disruption is thought to be synchronous with that of the oceans in the northern hemisphere, although recent palynological studies in the Southern Barents Sea, Norway, contradict the pattern.  The model for the terrestrial response in the Angaran (Russia) and Cathyasian (China) realms also envisions that both primary producers and vertebrates underwent extinction concurrently with catastrophic oceanic events.  But, the presence of older (Permian) relicts in the earliest Triassic at the Chache (Guizhou) section and documentation by other workers of Triassic elements in the latest Permian make it difficult to pinpoint the terrestrial Permian-Triassic boundary based on fossils, alone.

Carlton Heights, South Africa

Our understanding of the response in southern Africa is based primarily on the vertebrate biostratigraphic record from South Africa.  Here, the latest Permian Dicynodon Assemblage Zone (AZ) is replaced by the Early Triassic Lystrosaurus AZ and linked to changes in Karoo Basin river  architecture that transitions from the Balfour to Katberg Formations.  This change previously was interpreted as a sedimentological response to increasing aridity, although an increasingly wet trend for the same rocks has been reported, and the loss of the Glossopteris flora that vegetated floodplains.  Both events are interpreted to occur essentially coeval with the boundary event, claimed to be associated with a “unique” boundary facies described from the Bethulie area, Free State, South Africa.

Ganora Guest Ranch, Field Crew 2012

The research team lead by Professor Gastaldo, working in collaboration with:

  • SOUTH AFRICAN
  • Dr. Rose Prevec, Albany Museum, Rhodes University, Grahamstown
  • Dr. Johann Neveling, Council for Geosciences, Pretoria
  • CANADIAN
  • Dr. Sandra Kamo, Jack Satterly Laboratory for Geochronology, Toronto

and

  • U.S. colleagues
  • Dr. John Geissman, University of Dallas, Texas
  • Dr. Cindy Looy, University of California-Berkeley, California
  • Dr. Conrad Labandeira, USNM, Smithsonian Institute, Washington, DC
  • Colby College undergraduate research assistants

has challenged the currently accepted model of terrestrial ecosystem response in the rock record of the Karoo Basin, South Africa.For more than a decade, the research team has undertaken high resolution sedimentological, stratigraphic, and paleontological studies of all outcrop exposures reported to contain the transitional rocks from the latest Permian into the earliest Triassic.  Their major findings include:

  1. the first estimates of latest Permian paleoatmospheric pCO2 based on soil carbonate nodules which range from lows of 500 ppm to 1300 ppm to high estimates of 900 ppm to 1900 ppm, depending upon fractionation temperature (Gastaldo et al. 2014) .
  2. the presence of mudclast aggregates, originating from soil horizons associated with more seasonably dry climates, with pedogenic nodular conglomerate, channel-lag deposits, in the Lower Triassic Katberg Formation (Gastaldo et al., 2013) .
  3. the absence of the “unique” interval, dubbed “the dead zone” or “event bed” and used to identify the PT boundary, in the Karoo Basin indicating that previous correlations were in error.  They demonstrate that there is no practical way to identify the PT boundary in this part of the southern hemisphere (Gastaldo et al., 2009) .
  4. the presence of plant-fossil assemblages within a few meters of the purported PT boundary in several localities throughout the basin, negating the contention that evidence for terrestrial ecosystems was solely confined to the vertebrate-fossil record (Gastaldo et al., 2005).
  5. the first identification of a plant-fossil assemblage in the Karoo Basin, other than peat, in which an in situ, forest-floor litter is preserved.  This Glossopteris-dominated assemblage occurs ~70 m below what has been identified as the PT boundary, based on vertebrate biostratigraphy, at Wapadsberg Pass (Prevec et al., 2010) .
  6. the presence of volcaniclastic sediments, contrary to all published claims, near the PT boundary in which datable zircon crystals are recoverable (Prevec et al., 2010) .
  7. the recognition that the trace fossil, Katbergia gen. nov., is not a unique life-form confined to the “dead zone” or “event bed,” but is found in Permian paleosols (ancient soils) below and Triassic paleosols above the boundary.  The animal responsible for making the burrow was similar to a decapod crustacean, and lived under wet soil conditions.  This is in direct contradiction to an increasingly arid climate prior to the boundary event (Gastaldo and Rolerson, 2008)  .
  8. the rivers that developed following the Mass Extinction event represent anabranching systems that formed under seasonally dry climates.  But, contrary to previous interpretations, the Lower Triassic rocks record: intervals of landscape buildup (aggradation) under a seasonably wet climate (Pace et al., 2009); punctuated by the development of dry soils (Vertisols) in which calcite-cemented nodules and mudclast aggregates (Gastaldo et al., 2013) ; which, then, were subjected to landscape erosion with the return of highly seasonal (e.g., monsoonal) climates (Pace et al., 2009) .
  9. In addition, early work by Rob Selover ’04 and the team resulted in the identification of an Early Permian-aged type Ecca deposit in the Middle Permian Escourt Formation, requiring the recognition of younger deep-water turbidite sediments than had been recognized in the Karoo Basin .

Currently, the research team is investigating a Permian-Triassic boundary section in the area near Graaf Reinet.  Here, the same in situ plant-fossil assemblage has been found, in association with several volcanic ash horizons from which zircons are recovered.  These are being placed into a high resolution stratigraphy from which several hundred drill-core samples have been taken to assess the paleomagnetic signature in these rocks.

Professor Gastaldo’s research team has undertaken their work through grant support from the National Science Foundation (NSF EAR 0417317, 0749895, 0934077, 1123570), the Fulbright U.S. Scholar Program, the Council for Geosciences (Pretoria), the Mellon Foundation, and Colby College’s Dean of Faculty and Department of Geology endowments including the Selover Family Endowment.