Geological Hypotheses Research Articles

Detrital zircon tales between the Rodinia and Pangea supercontinents – Exploring connections between Avalonia, Cadomia and Central Asia

The Rodinia to Pangea supercontinent cycle represents the most recent complete cycle in the geological record and thus informs models on the processes involved in supercontinent dispersal and assembly. The terranes and/or micro-continents of the Avalonian, Cadomian and Central Asian Orogenic Belts are witnesses to this complete cycle but their relationships to Rodinia, each other, and an intermediate stage in this supercontinent cycle represented by the formation of a large (but not supercontinent) landmass, Gondwana, are at time ambiguous. One method for testing these relationships is detrital zircon geochronology; zircons are a highly resistant and easily dateable mineral and thanks to rapid instrumental advances a huge number of detrital zircon datasets are now available. These data can provide a fingerprint whereby suspect terranes and micro-continental fragments can be linked to each other and their parental continent. Nonetheless, in order to develop coherent and testable geological hypotheses from these data it is necessary to integrate the data into a consistent framework. The presented work explores the possibilities and limitations of using detrital zircon data to better understand the position of the Avalonian, Cadomian, and Central Asian terranes in Rodinia and their journey from there to Pangea.

Deep Electrical Resistivity Tomography for Detecting Gravitational Morpho-Structures in the Becca France Area (Aosta Valley, NW Italy)

Deep-seated gravitational slope deformations (DSGSDs) consist of gravity-induced, large-scale, gradual rock mass movements. In the Aosta Valley region (Valle d’Aosta NW Italy), DSGDs affect wide valley slopes and produce several interconnected morpho-structures that involve bedrock and Quaternary cover. Some DSGSD effects are not visible at the surface because of subglacial abrasion or burial by sediments and, therefore, are difficult to map with standard geomorphological surveys. This is the case for the Pointe Leysser DSGSD in the Aosta Valley, which is heavily influenced by the historical movements of the Verrogne-Clusellaz Glacier and its tributaries. We conducted a new geological investigation, integrated with deep electrical resistivity tomography geophysical surveys (ERTs). The ERT results were initially compared with geological/geomorphological evidence at the surface to define the correlation between the values and spatial distributions of electrical resistivity and the sediments, rocks, or morpho-structures. The resistivity values at various depths were subsequently analysed, interpreted, and discussed in conjunction with geological hypotheses. The geological and geophysical survey revealed three wide buried glacial valleys filled with glacial sediments and mapped the locations of gravitational morpho-structures at depth. These new data allowed us to draw a relationship between glacialism and gravitational evolution, distinguishing between pre-singlacial movements and postglacial movements.

Global geologic expert system and database for predicting carbonate reservoirs from seismic: Application to the Upper Jurassic

Carbonate systems are influenced by a great variety of physical and biological controlling factors that operate from global to local scales. The resulting intrinsic complexity of carbonate platforms makes them difficult to predict, especially when data are limited. Predicting geologic geometries and properties based on limited sampling or uncalibrated seismic data generally relies on a priori knowledge and equivocal interpretations that are marked by geologist perception and personal experience. To overcome these uncertain interpretations of such a complex natural system, which can become critical in frontier exploration, we developed an expert system that relies on a process-based method and a standardized data set using normalized information and parameters. The main innovation relies on the realization of knowledge- and process-based synthetic carbonate stratigraphic architectures that support seismo-stratigraphic interpretations. The workflow consists of four steps: (1) bibliographic compilation of a geologic database for each case study supported by quantitative parameters (e.g., sedimentation duration and thickness) and qualitative parameters (geodynamic context, seismic architecture, and facies model); (2) statistical analyses to establish consistent geologic classes and spatiotemporal trends; (3) process-based modeling to simulate stratigraphic architectures associated with carbonate sedimentation processes in a physically constrained numerical environment and testing different geologic hypotheses; and (4) realization of a predictive palaeogeographic map representing the global distribution of carbonate stratigraphic architectures, and estimation of controlling parameters for unconstrained case studies. The expert system is based on 77 case studies of Upper Jurassic carbonate platforms, which reveal the resemblance of these carbonate systems, in response to uniform global palaeoclimatic conditions and sea level. Significant local differences in stratigraphic architectures are related to specific geodynamic contexts and subsidence trends. The thickest carbonate platforms are developed in extensive/passive geodynamic settings such as the Central Atlantic Ocean margins, while thinner platforms form in intra- and peri-cratonic settings such as those of the Arabian region.

Botany and geogenomics: Constraining geological hypotheses in the neotropics with large-scale genetic data derived from plants.

Decades of empirical research have revealed how the geological history of our planet shaped plant evolution by establishing well-known patterns (e.g., how mountain uplift resulted in high rates of diversification and replicate radiations in montane plant taxa). This follows a traditional approach where botanical data are interpreted in light of geological events. In this synthesis, I instead describe how by integrating natural history, phylogenetics, and population genetics, botanical research can be applied alongside geology and paleontology to inform our understanding of past geological and climatic processes. This conceptual shift aligns with the goals of the emerging field of geogenomics. In the neotropics, plant geogenomics is a powerful tool for the reciprocal exploration of two long standing questions in biology and geology: how the dynamic landscape of the region came to be and how it shaped the evolution of the richest flora. Current challenges that are specific to analytical approaches for plant geogenomics are discussed. I describe the scale at which various geological questions can be addressed from biological data and what makes some groups of plants excellent model systems for geogenomics research. Although plant geogenomics is discussed with reference to the neotropics, the recommendations given here for approaches to plant geogenomics can and should be expanded to exploring long-standing questions on how the earth evolved with the use of plant DNA.

The fault in our TARs: A critical review of research paradigms for intermediate‐scale bedforms on Mars

AbstractWe provide a critical review of research paradigms for classifying intermediate‐scale aeolian bedforms on Mars and the new terminology that has emerged. The systematic classification of bedforms has always been challenging and debated, and no paradigmatic knowledge organization system exists beyond general agreement on the importance of a distinction between ripples and dunes. The diverse aeolian landscapes of Mars have introduced further topics and challenges to these debates. We argue that Martian aeolian geomorphology's knowledge organization system for intermediate‐scale bedforms is preparadigmatic and that consensus over terminology and their definitions has not been established in the literature. A preparadigmatic science can be functional only if scientists operating in the discipline provide precise, falsifiable definitions or use abductive logic. Drawing on evidence and examples from the literature, we argue that the replacement of the conventional abductive paradigm used in bedform classification with inductive logic has created an emerging disciplinary paradigm based on scientific hesitancy and a dependence on complex inductive research structures, epitomized by the concepts of ‘transverse aeolian ridges’ (TARs) and ‘large Martian ripples’ (LMRs). We show that TAR and, increasingly, LMR are inductive constructs that have been popularized despite them causing significant confusion. Notably, we highlight how the terms are irreconcilably used as both a class of bedform and as a non‐genetic placeholder term for bedforms. Suggestions for moving beyond the need for TAR and LMR are provided, focusing on a return to more direct and local hypothesis‐driven research inspired by W.M. Davis's notion of outrageous geological hypotheses. Recent debate surrounding bedforms in Gale crater is presented as an example of the productivity of such an approach, and it is recommended that TAR and LMR no longer be used.

Bayesian fusion of MT and AEM probabilistic models with geological data: examples from the eastern Gawler Craton, South Australia

When building 3D models of the subsurface, reconciling several geological and geophysical data of diverse nature, resolutions, coverage, or sensitivity, is challenging, both numerically and petrophysically. In this work, we propose a workflow for mapping selected geological features and characterise their uncertainty using a Bayesian Estimate Fusion algorithm. Different datasets such as 1D probabilistic models derived from geophysical data, drillholes and geological data are combined to produce probabilistic maps of selected geological boundaries, relying on petrophysical and geological assumptions. Leveraging large, high-quality geophysical datasets acquired in the eastern Gawler Craton in South Australia, we demonstrate the applicability of our approach with two examples: (1) we map in 3D the top of a stratigraphic unit in the cover, the Tregolana Shale, using 1D magnetotelluric (MT) and 1D Airborne Electromagnetic (AEM) probabilistic models, drill holes and surface geology; (2) we map the depth to basement using 1D probabilistic MT models, drill holes and interpreted structural information. Our results show that the different resolution, data sampling, depth of investigation and reliability of the utilised datasets can be combined in a complementary fashion, overcoming their respective limitations, to find solutions/models that satisfy all the datasets. We show that probabilistic workflows permit characterisation and reduce uncertainty when mapping the location of features of interest, but also permit the testing of geological hypotheses against other geophysical and geological data. These types of models are valuable to better characterise, interpret, and conceptualise the subsurface, enabling better exploration targeting and supporting efforts to discover new mineral deposits.

Uncertainty Quantification in Reservoir Simulation Using Modern Data Assimilation Algorithm

Production forecasting using numerical simulation has become a standard in the oil and gas industry. The model construction process requires an explicit definition of multiple uncertain parameters; thus, the outcome of the modelling is also uncertain. For the reservoirs with production data, the uncertainty can be reduced by history-matching. However, the manual matching procedure is time-consuming and usually generates one deterministic realization. Due to the ill-posed nature of the calibration process, the uncertainty cannot be captured sufficiently with only one simulation model. In this paper, the uncertainty quantification process carried out for a gas-condensate reservoir is described. The ensemble-based uncertainty approach was used with the ES-MDA algorithm, conditioning the models to the observed data. Along with the results, the author described the solutions proposed to improve the algorithm’s efficiency and to analyze the factors controlling modelling uncertainty. As a part of the calibration process, various geological hypotheses regarding the presence of an active aquifer were verified, leading to important observations about the drive mechanism of the analyzed reservoir.

Field And Well Evidence For Major Unconformities In North Sarawak, Compared To Southwest Sabah, Malaysia

A review of biostratigraphic and lithofacies data is used to show that there is no major unconformity in the stratigraphic record of north Sarawak or southwest Sabah near the end of the Early Miocene (approximately 17-15 Ma). The existence of such an unconformity has been cited in many papers in the past decade and used as a data point in the construction of regional geological hypotheses. Exploration well and outcrop data identifies two major unconformities in SW Sabah (the base and top of Stage III; BMU and DRU; roughly 24 and 13-12 Ma), and in offshore west Sarawak a third unconformity (MMU c. 16 Ma) which fades in effect towards onshore Sarawak. In recent years the names of these distinct unconformities have become conflated as workers had overlooked the origins and definitions of these features. This history is reviewed here in order to clarify future work.

Alteration history of Séítah formation rocks inferred by PIXL x-ray fluorescence, x-ray diffraction, and multispectral imaging on Mars.

Collocated crystal sizes and mineral identities are critical for interpreting textural relationships in rocks and testing geological hypotheses, but it has been previously impossible to unambiguously constrain these properties using in situ instruments on Mars rovers. Here, we demonstrate that diffracted and fluoresced x-rays detected by the PIXL instrument (an x-ray fluorescence microscope on the Perseverance rover) provide information about the presence or absence of coherent crystalline domains in various minerals. X-ray analysis and multispectral imaging of rocks from the Séítah formation on the floor of Jezero crater shows that they were emplaced as coarsely crystalline igneous phases. Olivine grains were then partially dissolved and filled by finely crystalline or amorphous secondary silicate, carbonate, sulfate, and chloride/oxychlorine minerals. These results support the hypothesis that Séítah formation rocks represent olivine cumulates altered by fluids far from chemical equilibrium at low water-rock ratios.

Thermal history modeling techniques and interpretation strategies: Applications using HeFTy

Abstract Advances in low-temperature thermochronology, and the wide range of geologic problems that it is used to investigate, have prompted the routine use of thermal history (time-temperature, tT) models to quantitatively explore and evaluate rock cooling ages. As a result, studies that investigate topics ranging from Proterozoic tectonics to Pleistocene erosion now commonly require a substantial numerical modeling effort that combines the empirical understanding of chronometer thermochemical behavior (kinetics) with independent knowledge or hypotheses about a study area's geologic history (geologic constraints). Although relatively user-friendly programs, such as HeFTy and QTQt, are available to facilitate thermal history modeling, there is a critical need to provide the geoscience community with more accessible entry points for using these tools. This contribution addresses this need by offering an explicit discussion of modeling strategies in the program HeFTy. Using both synthetic data and real examples, we illustrate the opportunities and limitations of thermal history modeling. We highlight the importance of testing the sensitivity of model results to model design choices and describe a strategy for classifying model results that we call the Path Family Approach. More broadly, we demonstrate how HeFTy can be used to build an intuitive understanding of the thermochronologic data types and model design strategies that are capable of discriminating among geologic hypotheses.

Hyper extended rifted margins: A computational procedure for a stretching-thinning factor estimation

Crustal extension mechanisms control rifted margin formation. During these extensional processes, the crust stretches and thins following both non-uniform and non-symmetric patterns. The modeling of main features of these processes is a relevant issue, as well as its quantification by the calculation of the stretching-thinning factors along the margins. This work presents a computational procedure for reproducing the crustal extension process due to Andersonian faulting and estimating the evolution of its extension-thinning factors. This procedure uses an algorithm based on geological hypotheses, geodynamic analysis results and kinematic relationships. The algorithm applies pure-shear extension, where stretching and thinning are based on Andersonian faulting formation. It is supposed to be applied in two-layer (upper/lower) crust configurations in two ways: a) including a new active fault; b) reactivating a previously activated fault. Kinematic relationships consider area conservation for the upper crust. The first step is the application of a crustal extension increment. This increment activates the faulting by simple shear in the upper crust and induces plastic distributed pure-shear deformation in the lower crust. This double effect generates a local thinning of both upper and lower crusts. Slip-in-fault also occurs as part of the thinning deformation. Additionally, a geodynamic analysis was carried out with an in-house developed system, based on the Finite Element Method. Coupled with advanced constitutive models, this approach is useful to reproduce stress-strain-temperature evolution during the crustal extension. Geodynamic analysis guided the definition of some improved kinematic relationships for the proposed procedure: a variable thinning zone, nonlinear thinning configurations, a variable slip/extension ratio, a shoulder uplift and out-of-section fault pivot. The procedure was applied to reproduce part of a crustal stretch of a hyper extended margin due to Andersonian faulting, estimating its thinning and extension factors. The results show that this procedure is able to qualitatively reproduce crustal extensional configurations.

Probabilistic Evaluation of Geoscientific Hypotheses With Geophysical Data: Application to Electrical Resistivity Imaging of a Fractured Bedrock Zone

AbstractAs climate changes and populations grow, groundwater sustainability is becoming increasingly important. Hydrogeologic models, which are based on a conceptual understanding of the subsurface, are crucial tools for informing decisions. Conceptual models of the subsurface incorporate knowledge of geological processes, and, frequently, observations from geophysical data into a common subsurface parameterization where the parameters may still be uncertain. Many methods exist to test how different conceptual subsurface parameterizations compare to geophysical data, but a frequent problem in hydrogeologic model development occurs when multiple geological phenomena could explain a single subsurface parameterization. In this work, we present a framework for testing geological hypotheses in conditions where a geological feature is observed in geophysical data, but its physical characteristics are uncertain. The framework uses Popper‐Bayes methods developed in previous studies, and is applied to study a fractured bedrock zone in a mountainous watershed in southwest Colorado. First, we propose six hypotheses based on the geological history of the watershed. Then, using the proposed Popper‐Bayes approach, we demonstrate that two of the hypotheses are inconsistent with the electrical resistivity tomography data. Finally, we discuss the importance of the prior model, and in what other scenarios the framework can be applied to.

Herpetological phylogeographic analyses support a Miocene focal point of Himalayan uplift and biological diversification.

The Himalaya are among the youngest and highest mountains in the world, but the exact timing of their uplift and origins of their biodiversity are still in debate. The Himalayan region is a relatively small area but with exceptional diversity and endemism. One common hypothesis to explain the rich montane diversity is uplift-driven diversification—that orogeny creates conditions favoring rapid in situ speciation of resident lineages. We test this hypothesis in the Himalayan region using amphibians and reptiles, two environmentally sensitive vertebrate groups. In addition, analysis of diversification of the herpetofauna provides an independent source of information to test competing geological hypotheses of Himalayan orogenesis. We conclude that the origins of the Himalayan herpetofauna date to the early Paleocene, but that diversification of most groups was concentrated in the Miocene. There was an increase in both rates and modes of diversification during the early to middle Miocene, together with regional interchange (dispersal) between the Himalaya and adjacent regions. Our analyses support a recently proposed stepwise geological model of Himalayan uplift beginning in the Paleocene, with a subsequent rapid increase of uplifting during the Miocene, finally giving rise to the intensification of the modern South Asian Monsoon.

SIMULATION MODELLING IN THE STRUCTURAL GRAVITY PROSPECTING

In accordance with the purpose of geophysical exploration, the gravity data interpretation is aimed at prospecting mineral resources which is based on the study of the geological cross-section structure. The task of quantitative interpretation, which uses methods of gravity modeling and gravity inversion, is the modelling of a gravity field (gravity modeling) and of a density structure of geological environments (gravity inversion). The article presents the definition and steps of the gravity data modelling technique. This technique is based on the construction of an informal sequence of equivalent solutions. The technological and geological features of methods for modelling the density structure of complex geological environments are given; among them geological content, consistency with a priori data and the subordination of modelling to geological hypotheses are important. The topicality and methods of simulation modelling are outlined. The purpose of simulation modelling is to study the properties of gravity inversion in the general formulation, as well as to evaluate the degree of detail and reliability of the methods and technologies of gravity modelling, which claim to be an effective solution to geological problems. The example of structural simulation testing of the methods of informal sequence of equivalent solutions and its computer technologies shows that a complex interpretation of seismic and gravity measurements data enables the creation of detailed density models of structural cross-sections. The ways of increasing the veracity of gravity data modelling of structural cross-sections have been studied. It is revealed that the best approximation of the regional background is an inclined plane which approximates the observed field of gravity according to characteristic pickets over the research areas that are better studied. The increase in the veracity of modeling can also be achieved by rebuilding the near side zones in the structural type models in an interactive process of solving structural gravity inversion problems. Substantive modeling depends primarily on the experience of the interpreter since computer technologies for gravity modeling and gravity inversion are merely an interpretation tool.

Imitation modelling technology for gravity inversion cases

Formulation of the problem. A gravity method is aimed at prospecting and exploration of mineral resources which are based on the study of the geological section structure. The task of quantitative interpretation of the gravimetric materials, which uses methods for solving direct and inverse gravity problems, is the modelling of a gravity field (direct problem) and geological media’s density structure (inverse problem). The important features of methods for density structure modelling of complex geological media are geological content, consistency with a priori data and its subordination to geological hypotheses. It is proposed to analyze these properties by a imitation technique. The purpose of the article is to describe the imitation gravimetric modelling method, based on the construction of an informal sequence of equivalent solutions. The purpose of imitation modelling is to study the properties of gravity inversion in general formulation as well as to assess the degree of detail and reliability of the methodology and technologies of gravity modelling, which is claimed to be an effective solution to geological problems. Methods. Imitation modelling technology and methods of solving gravity direct and inverse problems for geodensity model of complex geological environment. Results. The examples of density and structural simulation testing of the informal sequence of equivalent solutions and its computer technologies show that complex interpretation of wells, seismic and gravity data enables to create detailed density models of geological medium. Studies have also been conducted of ways to increase the reliability of gravitational modelling. Scientific novelty and practical significance. It is revealed that the best approximation of the regional background is an inclined plane, which approximates the observed gravity field along characteristic pickets over the research areas that are better studied. Also, an increase in the reliability of modelling can be achieved by rebuilding near side zones in structural type models in an interactive process of solving structural inverse gravity problems. Substantive modelling depends primarily on the experience of the interpreter, since computer technologies for solving direct and inverse gravity problems are only an interpretation tool.

The 2016 UK Space Agency Mars Utah Rover Field Investigation (MURFI)

The 2016 Mars Utah Rover Field Investigation (MURFI) was a Mars rover field trial run by the UK Space Agency in association with the Canadian Space Agency's 2015/2016 Mars Sample Return Analogue Deployment mission. MURFI had over 50 participants from 15 different institutions around the UK and abroad. The objectives of MURFI were to develop experience and leadership within the UK in running future rover field trials; to prepare the UK planetary community for involvement in the European Space Agency/Roscosmos ExoMars 2020 rover mission; and to assess how ExoMars operations may differ from previous rover missions. Hence, the wider MURFI trial included a ten-day (or ten-‘sol’) ExoMars rover-like simulation. This comprised an operations team and control centre in the UK, and a rover platform in Utah, equipped with instruments to emulate the ExoMars rovers remote sensing and analytical suite. The operations team operated in ‘blind mode’, where the only available data came from the rover instruments, and daily tactical planning was performed under strict time constraints to simulate real communications windows. The designated science goal of the MURFI ExoMars rover-like simulation was to locate in-situ bedrock, at a site suitable for sub-surface core-sampling, in order to detect signs of ancient life. Prior to “landing”, the only information available to the operations team was Mars-equivalent satellite remote sensing data, which were used for both geologic and hazard (e.g., slopes, loose soil) characterisation of the area. During each sol of the mission, the operations team sent driving instructions and imaging/analysis targeting commands, which were then enacted by the field team and rover-controllers in Utah. During the ten-sol mission, the rover drove over 100 m and obtained hundreds of images and supporting observations, allowing the operations team to build up geologic hypotheses for the local area and select possible drilling locations. On sol 9, the team obtained a subsurface core sample that was then analyzed by the Raman spectrometer. Following the conclusion of the ExoMars-like component of MURFI, the operations and field team came together to evaluate the successes and failures of the mission, and discuss lessons learnt for ExoMars rover and future field trials. Key outcomes relevant to ExoMars rover included a key recognition of the importance of field trials for (i) understanding how to operate the ExoMars rover instruments as a suite, (ii) building an operations planning team that can work well together under strict time-limited pressure, (iii) developing new processes and workflows relevant to the ExoMars rover, (iv) understanding the limits and benefits of satellite mapping and (v) practicing efficient geological interpretation of outcrops and landscapes from rover-based data, by comparing the outcomes of the simulated mission with post-trial, in-situ field observations. In addition, MURFI was perceived by all who participated as a vital learning experience, especially for early and mid-career members of the team, and also demonstrated the UK capability of implementing a large rover field trial. The lessons learnt from MURFI are therefore relevant both to ExoMars rover, and to future rover field trials.

Enhancing Model Consistency in Ensemble-Based History Matching

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 186049, “Enhancing the Geological Models Consistency in Ensemble-Based History Matching: An Integrated Approach,” by A. Perrone, F. Pennadoro, A. Tiani, and E. Della Rossa, Eni, and J. Saetrom, Resoptima, prepared for the 2017 SPE Reservoir Characterization and Simulation Conference and Exhibition, Abu Dhabi, 8–10 May. The paper has not been peer reviewed. The aim of this work is to present the effectiveness of a fully integrated approach for ensemble-based history matching on a complex real-field application. The predictive ability of the ensemble of models is greatly enhanced through an integrated work flow promoting collaboration between all subsurface disciplines. One key feature of the ensemble-based method that is especially important for complex reservoirs is that it overcomes the typical limitation of the traditional approaches where the number of uncertainty parameters resulting from practical or algorithm constraints often has to be reduced. Methodology The distinguishing feature of ensemble-based methodologies is their capability of integrating multiple sources of data while quantifying and propagating the uncertainty in reservoir-model parameters. In order to achieve a proper uncertainty assessment, one must couple the ensemble-based data-assimilation technique with an integrated work flow covering the entire reservoir-modeling process. The core of this methodology is an iterative data-assimilation process, which incorporates the reservoir-modeling steps in order to condition the ensemble of model realizations to the available data. The integrated reservoir-modeling process is used to generate an initial guess for the reservoir models, representing prior knowledge of the reservoir uncertainties. Throughout this process, the relevant model uncertainties must be properly identified and quantified. It is worth noting that significant differences among models may arise from different geological hypotheses or scenarios and from the random component typically introduced by geostatistical methods. In the complete paper, the authors use the ensemble-smoother-with-multiple-data-assimilation (ES-MDA) algorithm to condition the ensemble of reservoir models to the historical observations. According to this methodology, the ensemble predictions are used to compute a statistical approximation of the sensitivity to the input (uncertain) variables, which are modified in a predefined number of iterations to reduce the mismatch between the simulated and measured dynamic data. The methodology requires the definition of an error associated with each observed source (e.g., field, wells) and data type (e.g., rates, pressures). The ensemble approach allows for a grid-based parameterization, which, driven by the ensemble correlations, provides the ability to modify the value of a variable at some specific location without inappropriately forcing a change to other variables or locations. Consequently, all uncertain model parameters are retained because they may still have a relevant effect on the production forecasts.

Exploring the MIS M2 glaciation occurring during a warm and high atmospheric CO2 Pliocene background climate

Prior to the Northern Hemisphere glaciation around ∼2.7 Ma, a large global glaciation corresponding to a 20 to 60 m sea-level drop occurred during Marine Isotope Stage (MIS) M2 (3.312–3.264 Ma), interrupted the period of global warmth and high CO2 concentration (350–450 ppmv) of the mid Piacenzian. Unlike the late Quaternary glaciations, the M2 glaciation only lasted 50 kyrs and occurred under uncertain CO2 concentration (220–390 ppmv). The mechanisms causing the onset and termination of the M2 glaciation remain enigmatic, but a recent geological hypothesis suggests that the re-opening and closing of the shallow Central American Seaway (CAS) might have played a key role. In this article, thanks to a series of climate simulations carried out using a fully coupled Atmosphere Ocean General Circulation Model (GCM) and a dynamic ice sheet model, we show that re-opening of the shallow CAS helps precondition the low-latitude oceanic circulation and affects the related northward energy transport, but cannot alone explain the onset of the M2 glaciation. The presence of a shallow open CAS, together with favourable orbital parameters, 220 ppmv of CO2 concentration, and the related vegetation and ice sheet feedback, led to a global ice sheet build-up producing a global sea-level drop in the lowest range of proxy-derived estimates. More importantly, our results show that the simulated closure of the CAS has a negligible impact on the NH ice sheet melt and cannot explain the MIS M2 termination.

Historical volcanism and within-island genetic divergence in the Tenerife skink (Chalcides viridanus)

This study examines the genetic diversity of the Tenerife skink (Chalcides viridanus) within the context that Tenerife is now thought to have arisen as a single island as opposed to two/three precursor islands. DNA sequences were obtained from two mitochondrial regions and five nuclear loci. MtDNA divergence was substantial with four geographical clusters being detected. Two of these corresponded to two ancient areas that have undergone little recent eruptive activity (Anaga, Teno), while two further clusters were found within the more volcanically active central region. Nuclear divergence was low and revealed no strong geographical pattern. Estimated divergence of the Anaga group was 0.4-2.1 Ma ago (95% posterior interval), while the divergence time of the Teno cluster was 0.1-1.2 Ma ago. Phylogeographic distributions correspond well with ancient edifices, but divergence times postdate those expected under the previous ‘unification of ancient islands’ geological scenario. There is evidence of a major recent expansion of the central group following a decline in eruptive activity in this region, which also fits well with current geological hypotheses. Previously-described within-island evolutionary patterns in other Tenerife species also need to examined within this new geological context.

Biodiversity signature of the Last Glacial Maximum at South Georgia, Southern Ocean

AbstractAimHigh‐latitude biodiversity distributions can preserve signals of the timing and geography of past glaciations, and as such ground truth ice‐sheet models. Discrete polar archipelagos offer the fewest confounding factors for testing historic ice position records in extant biodiversity. At South Georgia, two competing geological hypotheses suggest that either the Last Glacial Maximum (LGM) ice was extensive, nearly covering the continental shelf (H1Big ice) or restricted to the inner fjords (H2Little ice). We examined the past configuration of the South Georgia ice cap using seabed biodiversity.LocationSouth Georgia, Southern Ocean.MethodsWe used a bespoke camera lander (SUCS) and Agassiz trawl deployments across ‘big ice’ and ‘little ice’ hypothesized positions ofLGMgrounded ice around the South Georgia continental shelf. We investigated faunal assemblage structure and richness, especially of brooders, and modelled low dispersal taxa, for example, those with limited pelagic larvae (bryozoans and sponges).ResultsWe found a striking ‘line’ of major richness discontinuity, with significantly higher richness, especially of brooders and low dispersal model taxa, mainly conforming to the ‘big ice’ hypothesized position. What few bryozoans and sponges occurred inside this line were a subset of those outside.Main conclusionsBenthic biodiversity is consistent with extensiveLGMgrounded ice advancing to near the shelf break in most, but not all locations around South Georgia's shelf, for example, the eastern shelf area. We suggest that most of the shelf is still undergoing recolonization from when grounded ice covered the shelf ~20 kyr ago. Our alternative hypothesis ofLGMice position, H3‘Limited‐Extensive ice’, best fitted our data and is easily further testable, but if verified, shows that shelf recolonization following glaciation is much slower than previously thought. This contrasts with surprisingly rapid colonization of continental shelves after ice‐shelf collapses, but these are not grounded, which may be crucial to polar recolonization rates.