3 October 2015: Layers of Meaning
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.
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. Our international research team has discovered that this current paradigm isn’t valid as we have found volcanic ash deposits within close proximity to the vertebrate-defined crisis that indicate a turnover event more than 1.6 million years older than the marine crisis.
- SOUTH AFRICAN
- Dr. Johann Neveling, Council for Geosciences, Pretoria
- Dr. Marion Bamford, Director, Evolutionary Studies Institute, University of Witwatersrand, Johannesburg
- Dr. Sandra Kamo, Jack Satterly Laboratory for Geochronology, Toronto
- U.S. colleagues
- Dr. John Geissman, University of Dallas, Texas
- Dr. Neil Tabor, Southern Methodist University, Dallas, Texas
- Dr. Cindy Looy, University of California-Berkeley, California
- Mr. Timothy Stonesifer, IT, Media, and Drone Pilot, Colby College
- 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:
- the recognition and climate characterization of “ghost” landscapes in the upper Daptocephalus and lower Lystrosaurus Assemblage Zones, using recalcitrant, calcite-cemented soil nodules concentrated in lag deposits at the base of river channel sandstone bodies in the Eastern Cape Province. Stable isotope δ13C and δ18O values of the nodule cements reflect an overall trend in cool and wet climates controlling calcite precipitation towards the vertebrate biozone boundary. Thereafter, stable-isotope trends indicate an increasing seasonality and warming into the base of the Lystrosaurus Assemblage Zone, as defined at the time of publication (Gastaldo et al. 2020c). Hence, we now demonstrate that climate variance characterized the latest Permian in southern Africa, without evidence for any unidirectional increasing warming.
- a high precision, U-Pb CA-ID-TIMS age from a pristine airfall ash deposit in the lower part of the Lystrosaurus Assemblage Zone, traditionally assigned to the Triassic, that is 252.24 +/- 0.11 Ma (2 σ) indicates the biozone is latest Permian. There is no evidence to support the hypothesis that the boundary between the Daptocephalus and Lystrosaurus Assemblage Zones marks the terrestrial expression of the Permian—Triassic Boundary. Hence, the response of terrestrial and marine ecosystems in the latest Permian is decoupled (Gastaldo et al., 2020a).
- paleoclimate estimates of Mean Annual Temperature and Mean Annual Precipitation, based on geochemistry of calcium-rich paleoVertisols in the latest Permian Upper Daptocephalus Assemblage Zone, indicate a cool (MAT 10° C) and humid (900-1100 mm/yr MAP) climate. These estimates are in contrast to the prevailing hypothetical model of increasing temperatures and aridity that precedes the transition to the Lystrosaurus AZ (Gastaldo et al. 2020b).
- a reverse magnetic polarity chron spanning the purported PTB on the Bethel farm, Free State, from where 73% of the vertebrate-fossil record used to construct the end-Permian extinction crisis originates; the end-Permian marine extinction event is reported to occur in a normal polarity chron (Gastaldo et al. 2019).
- a late Permian (Changhsingian) spore-and-pollen assemblage in the lower part of the, reportedly Triassic, Lystrosaurus Assemblage Zone on the Donald 207 (Fairydale) farm in the Free State. The palynological assemblage is found ~50 m above the reported Daptocephalus/Lystrosaurus assemblage-zone boundary as identified by other workers on the Bethel farm (Gastaldo et al. 2019).
- evidence that negates an interpreted change in paleoclimate from wet to arid conditions at the stratigraphic position proposed for a cataclysmic turnover in vertebrate fossils at the “type” section on the Bethel Farm, Free State. This reportedly “unique” horizon, interpreted to represent playa-lake (evaporite) deposits is used by many workers as time-equivalent to the end-Permian event in the oceans. We have shown that there is no physical sedimentological, geochemical, or rock magnetic property evidence to support the evaporite lake model (Gastaldo et al., 2019). Rather, the interval is neither unique for traceable in the area, let alone across the basin marking a single point in time (isochron). .
- a test of the accuracy of the vertebrate biostratigraphy in all localities that have been used to propose an end-Permian mass extinction event wherein we have identified significant problems with the original database on which the crisis is identified. In addition, we document “complacent” wood anatomy of a tree trunk and impressions of Glossopteris leaves, both indicative of a wet landscape, in the Lystrosaurus Assemblage Zone which, according to the currently accepted model, represents arid conditions following the turnover (Gastaldo et al., 2017). .
- identification of the source of color change in fine-grained mudrocks at Old Lootsberg Pass, a feature used by other authors to identify Triassic, post-extinction rocks. A change in siltstone color to reddish-gray is the result of fine-grained hematite coatings on clay particles, likely the result of changes in pore-water chemistries. This feature is associated with sediments deposited in abandoned rivers rather than being an indicator of aridity in the Lystrosaurus Assemblage Zone. (Li et al., 2017) .
- the first Uranium-Lead (U-Pb) isotope dilution–thermal ionization mass spectrometry (ID-TIMS) zircon age of 253.48 ± 0.15 Ma (early Changhsingian) is from a silicified ash layer ~60 m below the current vertebrate-defined boundary at Old Lootsberg Pass, in a new magnetostratigraphic framework. When conservatively interpreted, these, along with new paleontological data, indicate that the turnover between the Daptocephalus (formerly Dicynodon) and Lystrosaurus biozones, currently used to define the end-Permian crisis on land, occurred in the early Changhsingian and was not coeval with the marine mass extinction event (Gastaldo et al., 2015). .
- See The Economist’s 2 October 2015 article: Layers of Meaning
- 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) .
- 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) .
- 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) .
- 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).
- 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) .
- 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) .
- 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) .
- 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) .
- 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, Eastern Cape Province, and Bethulie, Free State Province. 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, 1624302), 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 and Barrett T. Dixon Geology Research and Internship Fund for student research.