James R. Wheeley

Rapid marine recovery after the end-Permian mass-extinction event in the absence of marine anoxia

A new Early Triassic marine fauna is described from the Central Oman Mountains. The fauna is Griesbachian in age, on the basis of abundant conodonts and ammonoids, and was deposited in an oxygenated seamount setting off the Arabian platform margin. It is the first Griesbachian assemblage from a well-oxygenated marine setting and thus provides a test for the hypothesis that widespread anoxia prevented rapid recovery. The earliest Griesbachian (parvus zone) contains a low-diversity benthic fauna dominated by the bivalves Promyalina and Claraia. A similar level of recovery characterizes the immediate postextinction interval worldwide. However, the middle upper Griesbachian sedimentary rocks (isarcica and carinata zones) contain an incredibly diverse benthic fauna of bivalves, gastropods, articulate brachiopods, a new undescribed crinoid, echinoids, and ostracods. This fauna is more diverse and ecologically complex than the typical middle to late Griesbachian faunas described from oxygen-restricted settings worldwide. The level of postextinction recovery observed in the Oman fauna is not recorded elsewhere until the Spathian. These data support the hypothesis that the apparent delay in recovery after the end-Permian extinction event was due to widespread and prolonged benthic oxygen restriction: in the absence of anoxia, marine recovery is much faster.

Tunneling trilobites : Habitual infaunalism in an Ordovician carbonate seafloor

Asaphus trilobites preserved in tunnel systems of the trace fossil Thalassinoides from the mid-Ordovician (ca. 465 Ma) Holen Limestone, Sweden, are interpreted as the trace makers, enabled by shallow carbonate firm grounds to construct open tunnel networks and develop habitual infaunal behavior. Their in situ preservation confirms an infaunal ethology inferred for some trilobite taxa from functional morphology. We suggest that predation pressure from large omnivorous nautiloid cephalopods (“Orthoceras” Limestone facies) may have triggered ecologic opportunism. In trilobites well adapted for predatory-scavenging behavior as well as excavation, the tunnel networks functioned primarily for protection, possibly assisting in feeding, breathing, and breeding strategies. Previously, “trilobite burrows” have referred to seafloor traces of locomotion, feeding, and resting ( Cruziana , Rusophycus ). Infaunal, tunneling trilobites provide new evidence of mid-Ordovician partitioning of the skeletal benthos, adding to an ecologic and trophic tier hitherto interpreted as occupied by soft-bodied organisms. Such trilobites also provide an identity for Thalassinoides tracemakers prior to Devonian evolution of decapod crustaceans.

Palaeoecological significance of a new Griesbachian (Early Triassic) gastropod assemblage from Oman

A new Early Triassic (Griesbachian) gastropod fauna from the Al Jil Formation of Oman is described. Early Triassic faunas from elsewhere (e.g. the Italian Dolomites and the western USA) are typically of low diversity and high dominance, usually attributed to environmental stress in the immediate aftermath of the end-Permian mass extinction event. The new Oman fauna has, by contrast, relatively high diversity, low dominance and a more even spread of individuals between taxa. It is the most diverse Griesbachian fauna known to date. This is attributed to the favourable (i.e. well-oxygenated) conditions under which the fauna lived. This uncharacteristic Griesbachian gastropod fauna demonstrates that, in the absence of oceanic anoxia, biotic recovery after the end-Permian extinction event may occur surprisingly quickly (within one conodont zone). The fauna is also partially silicified, which has increased its preservation potential relative to other Griesbachian gastropod assemblages. However, only one reappearing Lazarus genus is present in the Oman fauna. This suggests that there was some other control on the abundance of Lazarus genera at this time, other than the absence of silicified faunas as previously suggested.

Isotopic evidence for long term warmth in the Mesozoic.

Atmospheric CO2 concentrations appear to have been considerably higher than modern levels during much of the Phanerozoic and it has hence been proposed that surface temperatures were also higher. Some studies have, however, suggested that Earths temperature (estimated from the isotopic composition of fossil shells) may have been independent of variations in atmospheric CO2 (e.g. in the Jurassic and Cretaceous). If large changes in atmospheric CO2 did not produce the expected climate responses in the past, predictions of future climate and the case for reducing current fossil-fuel emissions are potentially undermined. Here we evaluate the dataset upon which the Jurassic and Cretaceous assertions are based and present new temperature data, derived from the isotopic composition of fossil brachiopods. Our results are consistent with a warm climate mode for the Jurassic and Cretaceous and hence support the view that changes in atmospheric CO2 concentrations are linked with changes in global temperatures.

Oxygen isotope variability in conodonts: implications for reconstructing Palaeozoic palaeoclimates and palaeoceanography

Conodonts have the potential to elucidate the intricacies of Palaeozoic climates, especially if δ18O values of single apatitic tooth-like ‘elements’ can be used to map evolving sea surface temperatures and differentiate oceanic water masses. Their ecological distribution as pelagic and nektobenthic organisms, high-resolution biostratigraphy, and abundance in Cambrian–Triassic rocks qualifies them as potentially robust climate archives. Previous ion microprobe conodont δ18O studies have proceeded directly to palaeotemperature interpretation without appreciation of inter- and intra-element variability or post-mortem artefacts. Here, ion microprobe analyses of Ordovician and Silurian conodonts establishes that: intra-element crown tissue δ18O typically varies by ≤1‰ (53% of conodonts analysed), is normally ≤2‰ (92% of analyses), and rarely varies by 2–4‰; δ18O can vary across elements, suggesting a microstructural and/or diagenetic control; δ18O can vary between species representatives by c. 3‰; δ18O of pelagic and nektobenthic taxa can be offset by 2–3‰; elements processed with formic acid have highly variable δ18O; and thermal alteration does affect δ18O. Conodont ion microprobe δ18O values are comparable with those of bulk methods, but utilization of material with no consideration of geological context or processing history may introduce significant artefacts. A protocol for future conodont oxygen isotope ion microprobe studies is proposed. Supplementary material: Full results of oxygen isotope analyses reported in this paper are available at www.geolsoc.org.uk/SUP18516.

Provenance of microcrystalline carbonate cement in limestone-marl alternations (LMA): aragonite mud or molluscs?

Aragonite derived from marine molluscs is evaluated as the source for microcrystalline carbonate cements of limestone–marl alternations (LMA). Calculations demonstrate that extremely low levels of mollusc-derived aragonite, well below the production rates of molluscs in modern marine settings, could have provided sufficient carbonate to cement examples of Ordovician, Silurian and Jurassic LMA in non-tropical or tropical settings. It is likely that even in the Palaeozoic molluscs provided sufficient carbonate entirely to source microcrystalline cements of LMA. Autochthonous molluscan aragonite is the only viable aragonite precursor for LMA microcrystalline cements of cool-water settings where temperatures precluded calcified algae and abiotic carbonate precipitation. In ‘calcite seas’ where Mg:Ca ratios inhibited both abiogenic aragonite precipitation and aragonite generation by calcified algae, molluscan aragonite was again the most likely main contributor. In some epeiric seas where brackish wedges switched off the shallow-water carbonate mud factories molluscan aragonite is the parsimonious source of carbonate for LMA microcrystalline cements.

Taphonomic Bias in Shelly Faunas Through Time: Early Aragonitic Dissolution and Its Implications for the Fossil Record

Early diagenetic dissolution of skeletal carbonate in environments from seafloor to shallow burial has the potential to skew the marine fossil record of aragonitic shells, particularly molluscs. Taphonomic windows leading to the preservation of labile skeletal components include relatively rare occurrences of early mineral replacement by silica (skeletal lagerstatten). Another, much more frequent process is event deposition where dissolution is halted by rapid burial of shells. Shell plasters form in basinal mud or low energy lagoonal environments during temporary dysoxic episodes, such as are caused by algal blooms. Preservation potential for aragonitic fossils may be enhanced by early cementation during shallow burial (hardgrounds) that protects the delicate dissolution moulds from destruction by bioturbation, or in high energy shoal environments where the drive for microbial dissolution is reduced. A data-based environmental model summarizes the main taphonomic zones, and illustrates significant taphonomic bias against aragonitic shells in lower energy settings of platform interiors and mid-outer ramps/shelves. The temporal distribution of various taphonomic windows shows the limited occurrence of silicified faunas, while the nature and extent of shell beds also change, but there is no obvious correlation with periods of ‘calcite’ and ‘aragonite seas’.

Early Palaeozoic cooling events: peri-Gondwana and beyond

Abstract The short-lived end-Ordovician Hirnantian glaciation allied to marine mass extinction is variously considered as a short-lived event or as the peak of long-drawn-out climatic cooling through at least late Ordovician–early Silurian times. Evidence from Early Palaeozoic facies, faunas and stable isotope excursions used to interpret climatic cooling events ranges farther, from late Mid-Cambrian to late Silurian times. Glacigenic sediments, structures and geomorphology provide direct evidence of glacial episodes. Cool-water carbonate deposition, which is particularly widespread during the late Ordovician Boda event in high-latitude peri-Gondwana–Gondwana, and beyond into mid–low palaeolatitudes, is interpreted as indicating global cooling, not warming as has been proposed. Such carbonates also characterize mid-latitude continents widely at horizons earlier in the Ordovician, and more locally in the mid-Silurian in high-latitude Gondwana. Cool-water carbonate mounds have distinctive facies-controlled mound faunas across palaeocontinents. Other facies evidence for palaeoclimates includes black shale deposition, including deglacial organic-rich ‘hot shales’, which indicate transgression in epeiric seas, and sea-level curves interpreted from facies and faunal successions. Correlation is shown between facies evidence and positive C isotope excursions, from which cyclicities are apparent. The possible interface of orbitally controlled rhythms is considered against evolving palaeobiogeography, and changes in global sea level and in pCO2. Facies and faunal evidence from peri-Gondwanan terranes (Armorica, Central Europe, Alborz) is assessed with that from Gondwana (mostly North Africa, South America) and correlatives in Avalonia, Baltica and Laurentia to establish a wider picture of early Palaeozoic cooling events.

Long-period orbital climate forcing in the early Palaeozoic?

Facies indicators and geochemical proxies of early Palaeozoic global climate cooling suggest episodes of fluctuating glacio-eustasy and severe cold or glaciation from the Mid–Late Cambrian to Silurian (c. 85 myr), with a mean frequency of 2.6 myr. Long-period orbital time series predicted through the Phanerozoic are used to generate sine waves to test against these data; the null hypothesis of no orbital influence is rejected with a high confidence level. Cooling episodes appear most frequent through the Late Ordovician leading up to the Hirnantian glacial maximum, but even ‘greenhouse’ intervals of the Early–Mid-Ordovician and early Silurian provide evidence for periodic cooling episodes.

Abrupt global-ocean anoxia during the Late Ordovician–early Silurian detected using uranium isotopes of marine carbonates

Significance The Late Ordovician mass extinction (LOME) terminated one of the greatest biodiversity radiations in Earth history eliminating ∼85% of marine animals, and it is coincident with the first major glaciation of the Phanerozoic. To evaluate LOME origins, we use uranium isotopes from marine limestones as a proxy for global-ocean redox conditions. Our results provide evidence of an abrupt global-ocean anoxic event coincident with the LOME onset and its continuation after the biologic recovery, through peak glaciation, and the following early Silurian deglaciation. These results also provide evidence for widespread ocean anoxia initiating and continuing during icehouse conditions. Widespread marine anoxia is hypothesized as the trigger for the second pulse of the Late Ordovician (Hirnantian) mass extinction based on lithologic and geochemical proxies that record local bottom waters or porewaters. We test the anoxia hypothesis using δ238U values of marine limestones as a global seawater redox proxy. The δ238U trends at Anticosti Island, Canada, document an abrupt late Hirnantian ∼0.3‰ negative shift continuing through the early Silurian indicating more reducing seawater conditions. The lack of observed anoxic facies and no covariance among δ238U values and other local redox proxies suggests that the δ238U trends represent a global-ocean redox record. The Hirnantian ocean anoxic event (HOAE) onset is coincident with the extinction pulse indicating its importance in triggering it. Anoxia initiated during high sea levels before peak Hirnantian glaciation, and continued into the subsequent lowstand and early Silurian deglacial eustatic rise, implying that major climatic and eustatic changes had little effect on global-ocean redox conditions. The HOAE occurred during a global δ13C positive excursion, but lasted longer indicating that controls on the C budget were partially decoupled from global-ocean redox trends. U cycle modeling suggests that there was a ∼15% increase in anoxic seafloor area and ∼80% of seawater U was sequestered into anoxic sediments during the HOAE. Unlike other ocean anoxic events (OAE), the HOAE occurred during peak and waning icehouse conditions rather than during greenhouse climates. We interpret that anoxia was driven by global cooling, which reorganized thermohaline circulation, decreased deep-ocean ventilation, enhanced nutrient fluxes, stimulated productivity, which lead to expanded oxygen minimum zones.