ALBERT

Description: A new conodont biostratigraphic study is presented from a key section on the flanks of the Permian-Triassic Great Bank of Guizhou, an isolated carbonate platform from South China, that has recently provided much key data for understanding the nature of this mass extinction interval. Detailed investigation at Bianyang (Guizhou Province) has revealed ten conodont zones, in ascending order: Clarkina yini Zone; Hindeodus changxingensis Zone; Hindeodus parvus Zone; Sweetospathodus kummeli Zone; Neospathodus dieneri Zone; Neospathodus cristagalli Zone; Discretella discreta Zone; Pachycladina-Parachirognathus assemblage zone; Icriospathodus collinsoni Zone; Triassospathodus homeri Zone. This allowed the Permian-Triassic boundary (PTB) to be defined in the section based on the first occurrence of Hindeodus parvus . Furthermore, it is proposed that the first occurrence of Discretella discreta can be used as an auxiliary reference for defining the Induan-Olenekian boundary when Novispathodus waageni is absent both at Bianyang and elsewhere.

Description: :  Progradational fluvio-deltaic systems tend towards but cannot reach equilibrium, a state in which the longitudinal profile does not change shape and all sediment is bypassed beyond the shoreline. They cannot reach equilibrium because progradation of the shoreline requires aggradation along the longitudinal profile. Therefore progradation provides a negative feedback, unless relative sea level falls at a sufficient rate to cause non-aggradational extension of the longitudinal profile. How closely fluvio-deltaic systems approach equilibrium is dependent on their progradation rate, which is controlled by water depth and downstream allogenic controls, and governs sediment partitioning between the fluvial, deltaic, and marine domains. Here, six analogue models of coastal fluvio-deltaic systems and small prograding shelf margins are examined to better understand the effect of water depth, subsidence, and relative sea-level variations upon longitudinal patterns of sediment partitioning and grain-size distribution that eventually determine large-scale stratigraphic architecture. Fluvio-deltaic systems prograding in relatively deep-water environments are characterized by relatively low progradation rates compared to shallow-water systems. This allows these deeper water systems to approach equilibrium more closely, enabling them to construct less concave and steeper longitudinal profiles that provide low accommodation to fluvial systems. Glacio-eustatic sea-level variations and subsidence modulate the effects of water depth on the longitudinal profile. Systems are closest to equilibrium during falling relative sea level and early lowstand, resulting in efficient sediment transport towards the shoreline at those times. Additionally, the strength of the response to relative sea-level fall differs depending on water depth. In systems prograding into deep water, relative sea-level fall causes higher sediment bypass rates and generates significantly stronger erosion than in shallow-water systems, which increases the probability of incised-valley formation. Water depth in the receiving basin thus forms a first-order control on the sediment partitioning along the longitudinal profile of fluvio-deltaic systems and the shelf clinoform style. It also forms a control on the availability of sand-grade sediment at the shoreline that can potentially be remobilized and redistributed into deeper marine environments. Key findings are subsequently applied to the literature of selected shelf clinoform successions.

Description: : The frequency and origin of thickening-upward packages in the sediments of deep-sea environments has been a topic of much recent debate. Excellent bed-scale exposures in the Carboniferous Ross Formation, western Ireland, allow single surfaces to be traced laterally and the detailed architecture of whole packages to be evaluated using multiple vertical logs. The deposits comprise architectural elements including bed-sets, lobe-elements and composite lobes, with the lobe-elements being arranged in thickening-upward depositional packages. Results show that these packages are the result of the progradation of each lobe-element, whereby each package records a vertical trend from distal to proximal deposits accompanied by an increasing frequency of megaflutes and ultimately the development of broad erosional surfaces. We propose a six-stage model for lobe-element evolution that documents successive phases of deposition, sediment bypass, erosion and lobe abandonment. This new model provides a mechanism for lobe-element switching and explains the development of thickening-upward successions. This allows the re-examination of existing process models for these sedimentary packages in deep-sea sediments.

Description: 〈p〉The Permian-Triassic mass extinction was the worst crisis faced by life; it killed 〉90% of marine species in less than 0.1 million years (Ma). However, knowledge of its macroecological impact over prolonged time scales is limited. We show that marine ecosystems dominated by non-motile animals shifted to ones dominated by nektonic groups after the extinction. In Triassic oceans, animals at high trophic levels recovered faster than those at lower levels. The top-down rebuilding of marine ecosystems was still underway in the latest Triassic, ~50 Ma after the extinction, and contrasts with the ~5-Ma recovery required for taxonomic diversity. The decoupling between taxonomic and ecological recoveries suggests that a process of vacant niche filling before reaching the maximum environmental carrying capacity is independent of ecosystem structure building.〈/p〉

Description: We thank Davies et al. (2017) for their comments and welcome the opportunity to further discuss the role of early land plants in fluvial environments. Critically, Davies et al. (2017) note that although testable hypotheses exist for the possible role of early land plants they remain untested, and thus there is correlation (between an increase in meandering systems and early plants) without any mechanistic causation. In our paper (Santos et al. 2017) we challenged this causation, offering an alternative mechanism for increasing meandering channels based on a unique set of tectonic and environmental controls. It is important to be clear that Santos et al. (2017) focused on this meandering transition and the possible role of early land plants. Davies et al. (2017) conflate our comments regarding the impacts of early land-plant evolution with the longer-term influence of vegetation; there is little doubt over the longer-term role played by vegetation, particularly in stabilizing deeply rooted soils. Fluvial sedimentary successions are strongly influenced by tectonics, climate, sediment flux and sea level, and we question whether early land plants of limited size and restricted occurrence were able to dominate over such controls during terrestrialization (see Leeder 2007). We maintain our principal argument that a series of abiotic factors conspired to induce a worldwide increase in the proportion of meandering rivers, which in turn favoured development of an environment appropriate for land-plant evolution and colonization of the continents.

Description: A detailed, 10 m.y. redox history of Changhsingian to Anisian (latest Permian to Middle Triassic) oceans in ramp settings is reconstructed based on framboidal pyrite analysis from South China. The result shows that the well-established phenomenon of intense ocean euxinia-anoxia is faithfully recorded in pyrite framboid data. Three major euxinia-anoxia episodes, namely, the end-Changhsingian to end-Smithian, middle to late Spathian, and early to middle Anisian, have been recognized from the ramp successions. The first reducing episode is subdivided into four subepisodes: Permian-Triassic boundary, Griesbachian-Dienerian boundary, earliest Smithian, and end-Smithian. Redox variations broadly track other oceanographic proxies. Euxinia-anoxia episodes coincide with positive excursions of conodont Ce anomalies, negative excursions of 34 S cas (carbonate-associated sulfate), increases in sea-surface temperature, and negative excursions of 13 C in most cases. However, euxinia-anoxia near the Dienerian-Smithian boundary coincided with positive excursions of 13 C and a general cooling period. This exception may be the result of locally developed water-column anoxia. The Permian-Triassic boundary subepisode witnessed two ephemeral euxinia-anoxia events separated by a dysoxic to oxic period. The former, together with a rapid increase in sea-surface temperature (up to 8 °C), may have been responsible for the biodiversity crisis, while the latter anoxic event destroyed ecosystem trophic structures. In addition to the Permian-Triassic boundary euxinia-anoxia event, which spread over habitats in all oceans, the Spathian and Anisian euxinia-anoxia episodes also prevailed in global oceans. Variation of the oxygen minimum zone are suggested as the driving mechanism that facilitated the movement of oxygen-poor water columns in various paleogeographic settings over this critical period.

Description: The Permian-Triassic mass extinction was the worst crisis faced by life; it killed 〉90% of marine species in less than 0.1 million years (Ma). However, knowledge of its macroecological impact over prolonged time scales is limited. We show that marine ecosystems dominated by non-motile animals shifted to ones dominated by nektonic groups after the extinction. In Triassic oceans, animals at high trophic levels recovered faster than those at lower levels. The top-down rebuilding of marine ecosystems was still underway in the latest Triassic, ~50 Ma after the extinction, and contrasts with the ~5-Ma recovery required for taxonomic diversity. The decoupling between taxonomic and ecological recoveries suggests that a process of vacant niche filling before reaching the maximum environmental carrying capacity is independent of ecosystem structure building.

Description: The controversial Capitanian (Middle Permian, 262 Ma) extinction event is only known from equatorial latitudes, and consequently its global extent is poorly resolved. We demonstrate that there were two, severe extinctions amongst brachiopods in northern Boreal latitudes (Spitsbergen) in the Middle to Late Permian, separated by a recovery phase. New age dating of the Spitsbergen strata (belonging to the Kapp Starostin Formation), using strontium isotopes and d 13 C trends and comparison with better-dated sections in Greenland, suggests that the first crisis occurred in the Capitanian. This age assignment indicates that this Middle Permian extinction is manifested at higher latitudes. Redox proxies (pyrite framboids and trace metals) show that the Boreal crisis coincided with an intensification of oxygen depletion, implicating anoxia in the extinction scenario. The widespread and near-total loss of carbonates across the Boreal Realm also suggests a role for acidification in the crisis. The recovery interval saw the appearance of new brachiopod and bivalve taxa alongside survivors, and an increased mollusk dominance, resulting in an assemblage reminiscent of younger Mesozoic assemblages. The subsequent end-Permian mass extinction terminated this Late Permian radiation.