Patterns and mechanisms of early Pliocene warmth

About five to four million years ago, in the early Pliocene epoch, Earth had a warm, temperate climate. The gradual cooling that followed led to the establishment of modern temperature patterns, possibly in response to a decrease in atmospheric CO2 concentration, of the order of 100 parts per million, towards preindustrial values. Here we synthesize the available geochemical proxy records of sea surface temperature and show that, compared with that of today, the early Pliocene climate had substantially lower meridional and zonal temperature gradients but similar maximum ocean temperatures. Using an Earth system model, we show that none of the mechanisms currently proposed to explain Pliocene warmth can simultaneously reproduce all three crucial features. We suggest that a combination of several dynamical feedbacks underestimated in the models at present, such as those related to ocean mixing and cloud albedo, may have been responsible for these climate conditions

The Sail-Backed Reptile Ctenosauriscus from the Latest Early Triassic of Germany and the Timing and Biogeography of the Early Archosaur Radiation

Background Archosaurs (birds, crocodilians and their extinct relatives including dinosaurs) dominated Mesozoic continental ecosystems from the Late Triassic onwards, and still form a major component of modern ecosystems (>10,000 species). The earliest diverse archosaur faunal assemblages are known from the Middle Triassic (c. 244 Ma), implying that the archosaur radiation began in the Early Triassic (252.3–247.2 Ma). Understanding of this radiation is currently limited by the poor early fossil record of the group in terms of skeletal remains. Methodology/Principal Findings We redescribe the anatomy and stratigraphic position of the type specimen of Ctenosauriscus koeneni (Huene), a sail-backed reptile from the Early Triassic (late Olenekian) Solling Formation of northern Germany that potentially represents the oldest known archosaur. We critically discuss previous biomechanical work on the ‘sail’ of Ctenosauriscus, which is formed by a series of elongated neural spines. In addition, we describe Ctenosauriscus-like postcranial material from the earliest Middle Triassic (early Anisian) Röt Formation of Waldhaus, southwestern Germany. Finally, we review the spatial and temporal distribution of the earliest archosaur fossils and their implications for understanding the dynamics of the archosaur radiation. Conclusions/Significance Comprehensive numerical phylogenetic analyses demonstrate that both Ctenosauriscus and the Waldhaus taxon are members of a monophyletic grouping of poposauroid archosaurs, Ctenosauriscidae, characterised by greatly elongated neural spines in the posterior cervical to anterior caudal vertebrae. The earliest archosaurs, including Ctenosauriscus, appear in the body fossil record just prior to the Olenekian/Anisian boundary (c. 248 Ma), less than 5 million years after the Permian–Triassic mass extinction. These earliest archosaur assemblages are dominated by ctenosauriscids, which were broadly distributed across northern Pangea and which appear to have been the first global radiation of archosaurs

Multiple phases of carbon cycle disturbance from large igneous province formation at the Triassic-Jurassic transition

The end-Triassic mass extinction (ca. 201.4 Ma) coincided with a major carbon cycle perturbation, based on an ~5%‰-6%‰ negative excursion in δ13CTOC (total organic carbon) records. Both events coincided directly with the onset of massive flood basalt volcanism in the Central Atlantic Magmatic Province (CAMP). Organic carbon isotope data from the western Tethys Ocean (Austria) and the Germanic basin (UK and Germany), however, demonstrate earlier disruption of the global carbon cycle, preceding CAMP eruptions. A 2%‰-3%‰ late Rhaetian precursor negative excursion in marine and continental δ13CTOC records is matched by a negative perturbation in δ13Cleaf data, suggesting multiple events of Rhaetian atmospheric 13C depletion. Intruding dike and sill systems, preceding CAMP eruptive volcanic activity, may have released ~3000-7000 Gt of isotopically light carbon as thermogenic methane from subsurface organic-rich strata. This possibly caused an end-Triassic atmospheric pCO2 increase and reduced ecosystem stability before the actual onset of eruptive volcanic activity in the CAMP region. We present a model that identifies three phases of disturbances in global biogeochemical cycles related to the formation of this large igneous province. © 2011 Geological Society of America

Milankovitch-scale palynological turnover across the Triassic-Jurassic transition at St. Audrie's Bay, SW UK

A high-resolution palynological study of the Triassic-Jurassic boundary in the St. Audrie's Bay section revealed a palynofloral transition interval with four pronounced spore peaks in the Lilstock Formation. Regular cyclic increases in palynomorph concentrations can be linked with periods of increased runoff, and correspond to the orbital eccentricity cycle. Spore peaks can be related to precession-induced variations in monsoon strength. An implication is that the initial carbon isotope excursion lasted for at least 20 ka. Emergence during deposition of the Cotham Member had an influence on one of the peaks, which is dominated by spore-producing pioneer plants (e.g. horsetails and liverworts). There is no compelling evidence of a global end-Triassic spore spike that, by analogy with the K-T boundary fern spike, could be related to a catastrophic mass extinction event. Climate change is a more plausible mechanism to explain the increased amount of spores. © 2010 Geological Society of London

Triassic-Jurassic organic carbon isotope stratigraphy of key sections in the western Tethys realm (Austria)

The late Triassic period is recognized as one of the five major mass extinctions in the fossil record. All these important intervals in earth history are associated with excursions in C-isotope records thought to have been caused by perturbations in the global carbon cycle. The nature and causes of C-isotopic events across the Triassic-Jurassic (T-J) transition however, are poorly understood. We present several new high resolution organic C-isotope records from the Eiberg Basin, Austria, including the proposed Global boundary Stratotype Section and Point (GSSP) for the base of the Jurassic. The Triassic-Jurassic boundary interval in these records is characterized by the initial and main negative organic carbon isotope excursions (CIE) of up to 8‰. The initial and main CIEs are biostratigraphically constrained by first and last occurrences of boundary defining macro- and microfossils (e.g. ammonites). High resolution C-isotope records appear to be an excellent correlation proxy for this period in the Eiberg Basin. Pyrolysis analysis demonstrates increased Hydrogen Index (HI) values for organic matter coinciding with the initial CIE. Terrestrial organic matter influx and mass occurrences of green algae remains may have influenced the C-isotope composition of the sedimentary organic matter. This may have contributed to the extreme amplitude of the initial CIE in the Eiberg Basin. © 2009 Elsevier B.V. All rights reserved

Climate change driven black shale deposition during the end-Triassic in the western Tethys

Several new Triassic-Jurassic boundary sections from the Eiberg Basin (Northern Calcareous Alps, Austria) have been studied at high resolution. We present integrated geochemical and biological proxy data from this western Tethys shelf basin. High-resolution correlation of Kuhjoch, the Global boundary Stratotype Section and Point (GSSP) for the base of the Jurassic, Hochalplgraben and Tiefengraben shows that the initial and main Carbon Isotope Excursions (CIE) are contemporaneous with first and last occurrences of boundary defining macro- and microfossils. We focus on the end-Triassic initial CIE at the transition from the limestones of the Kössen Formation to the marls of the Kendlbach Formation. This change coincides with a dramatically increased influx of conifer (Cheirolepidiaceae) pollen and increased total organic carbon (TOC) values, succeeded by an acme of green algae (Cymatiosphaera). We present a model in which increased terrestrial organic matter influx is related to enhanced seasonality and increased erosion of the hinterland. Reduced salinity of the surface waters led to the mass occurrence of green algae. Stratification of the water column may have caused anoxic bottom water conditions and black shale deposition during the initial CIE at the base of the Kendlbach Fm. © 2009 Elsevier B.V. All rights reserved

Sedimentary organic matter characterization of the Triassic-Jurassic boundary GSSP at Kuhjoch (Austria)

The Triassic-Jurassic (T-J) boundary interval coincides with enhanced extinction rates in the marine realm and pronounced changes in terrestrial ecosystems on the continents. It is further marked by distinct negative excursions in the δ13Corg and δ13Ccarb signature that may represent strong perturbations of the global carbon cycle. We present integrated geochemical, stable-isotope and palynological data from the Kuhjoch section, the Global boundary Stratotype Section and Point (GSSP) for the base of the Jurassic (Northern Calcareous Alps, Austria). We show that the initial carbon isotope excursion (CIE), coinciding with the marine extinction interval and the formation of black shales in the western Tethys Eiberg Basin, is marked by only minor changes in kerogen type, which is mainly of terrestrial origin. Increased Total Organic Carbon (TOC) concentrations of 9% at the first half of the initial CIE coincide with Hydrogen Index (HI) values of over 600 mg HC/g TOC. The high correlation (with R2 = 0.93) between HI values and terrestrial Cheirolepidiaceaen conifer pollen suggests a terrestrial source for the hydrogen enriched organic compounds. The lack of major changes in source of the sedimentary organic matter suggests that changes in the δ13Corg composition are genuine and represent true disturbances of the global C-cycle. The sudden decrease in total inorganic carbon (TIC) concentrations likely represents the onset of a biocalcification crisis. It coincides with a 4.5‰ negative shift in δ13Corg values and possibly corresponds to the onset of CAMP related volcanic activity. The second half of the initial CIE is marked by the dramatic increase of green algae remains in the sediment. The simultaneous increase of the Corg/Ntot ratio suggests increased marine primary production at the final stage of black shale formation. © 2010 Elsevier B.V

Atmospheric carbon injection linked to end-Triassic mass extinction.

The end-Triassic mass extinction (~201.4 million years ago), marked by terrestrial ecosystem turnover and up to ~50% loss in marine biodiversity, has been attributed to intensified volcanic activity during the break-up of Pangaea. Here, we present compound-specific carbon-isotope data of long-chain n-alkanes derived from waxes of land plants, showing a ~8.5 per mil negative excursion, coincident with the extinction interval. These data indicate strong carbon-13 depletion of the end-Triassic atmosphere, within only 10,000 to 20,000 years. The magnitude and rate of this carbon-cycle disruption can be explained by the injection of at least ~12 × 10(3) gigatons of isotopically depleted carbon as methane into the atmosphere. Concurrent vegetation changes reflect strong warming and an enhanced hydrological cycle. Hence, end-Triassic events are robustly linked to methane-derived massive carbon release and associated climate change

Astronomical constraints on the duration of the early Jurassic Hettangian stage and recovery rates following the end-Triassic mass extinction (St Audrie's Bay/East Quantoxhead, UK)

The end-Triassic environmental crisis with major extinctions in the marine realm is followed by successive recovery in the lower Jurassic Hettangian Stage. Accurate timing of events is however still poorly constrained. In this study, combined field observations and physical and chemical proxy records, covering the uppermost Triassic and lower Jurassic marine successions of St Audrie's Bay and East Quantoxhead (UK), have been used to construct a floating astronomical time-scale of ~2.5Myr in length. This time-scale is based on the recognition of meters thick cycles in limestone and (black) shale predominance and concurrent variability in physical and chemical proxy records. Three to five individual black-shale beds occur within these meter-scale sedimentary bundles and are interpreted to reflect precession-controlled changes in monsoon intensity, while the bundles are interpreted as forced by the ~100-kyr eccentricity cycle. On the basis of these findings, we propose an astronomically constrained duration of the Hettangian stage of 1.8Myr in the UK and unequal duration of Hettangian ammonite zones (Psiloceras planorbis zone: ~250kyr; Alsatites liasicus zone: ~750kyr; Schlotheimia angulata zone: ~800kyr). Within this astronomical framework, the extinction interval and coinciding negative CIE represent 1 to 2 precession cycles (~20-40kyr). The amount of time succeeding the end-Triassic negative carbon isotope excursion (CIE) and preceding the first Jurassic ammonite occurrence (in the UK) is constrained to 6 climatic precession cycles (~120kyr). Cyclostratigraphic correlation to the astronomically-tuned sedimentary record of the continental Newark basin (USA) allows to locate the stratigraphic position of the marine defined Triassic-Jurassic and Hettangian-Sinemurian boundary in the continental realm. Continuous low δ13CTOC values throughout the Hettangian and early Sinemurian, succeeding volcanic activity in the Central Atlantic Magmatic Province (CAMP), may suggest a long-term change in Earth's global biogeochemical cycles, which do not fully recover for several million years. © 2010 Elsevier B.V