Evidence for climatic variability and its impact on human development during the Neolithic from Loughmeenaghan,County Sligo,Ireland

High‐resolution pollen and geochemical analyses conducted on a sediment profile from a small lake in County Sligo, Ireland, revealed that human development during the Neolithic was influenced by pronounced climatic oscillations. The primeval woodland around the lake experienced a considerable transformation coinciding with the elm decline at 3810 BC. The subsequent increase in summer temperatures and decrease in precipitation favoured wheat cultivation in the lake's catchment area, which was practised for approximately 140 years. A shift towards pastoral farming took place with the establishment of exceptionally dry conditions between 3650 and 3560 BC, when lake level and influx of allochthonous material were notably low. The onset of cool and wet conditions at the transition from the Early to Middle Neolithic possibly caused the initial decline of human activity in the area. Periods of particularly high precipitation during the Middle and early Late Neolithic contributed to the abandonment of the area by the first farmers. Comparison of the proxy record from the study site with other palynological and archaeological records from Ireland suggests that climatic variability on the decadal to centennial scale represented a primary control on the nature and duration of farming practices during the Neolithic. Copyright © 2012 John Wiley & Sons, Ltd.     

A high-resolution sedimentary archive from landslide-dammed Lake Mengda,north-eastern Tibetan Plateau

Lacustrine sediments have been widely used to investigate past climatic and environmental changes on millennial to seasonal time scales. Sedimentary archives of lakes in mountainous regions may also record non-climatic events such as earthquakes. We argue herein that a set of 64 annual laminae couplets reconciles a stratigraphically inconsistent accelerator mass spectrometry (AMS) ¹⁴C chronology in a ~4-m-long sediment core from Lake Mengda, in the north-eastern Tibetan Plateau. The laminations suggest the lake was formed by a large landslide, triggered by the 1927 Gulang earthquake (M = 8.0). The lake sediment sequence can be separated into three units based on lithologic, sedimentary, and isotopic characteristics. Starting from the bottom of the sequence, these are: (1) unweathered, coarse, sandy valley-floor deposits or landslide debris that pre-date the lake, (2) landslide-induced, fine-grained soil or reworked landslide debris with a high organic content, and (3) lacustrine sediments with low organic content and laminations. These annual laminations provide a high-resolution record of anthropogenic and environmental changes during the twentieth century, recording enhanced sediment input associated with two phases of construction activities. The high mean sedimentation rates of up to 4.8 mm year^?1 underscore the potential for reconstructing such distinct sediment pulses in remote, forested, and seemingly undisturbed mountain catchments.     

High-T, low-P metamorphism in the Palaeoproterozoic Halls Creek Orogen, northern Australia: the middle crustal response to a mantle-related transient thermal pulse

High‐T, low‐P metamorphic rocks of the Palaeoproterozoic central Halls Creek Orogen in northern Australia are characterised by low radiogenic heat production, high upper crustal thermal gradients (locally exceeding 40 °C km^?1) sustained for over 30 Myr, and a large number of layered mafic‐ultramafic intrusions with mantle‐related geochemical signatures. In order to account for this combination of geological and thermal characteristics, we model the middle crustal response to a transient mantle‐related heat pulse resulting from a temporary reduction in the thickness of the mantle lithosphere. This mechanism has the potential to raise mid‐crustal temperatures by 150–400 °C within 10–20 Myr following initiation of the mantle temperature anomaly, via conductive dissipation through the crust. The magnitude and timing of maximum temperatures attained depend strongly on the proximity, duration and lateral extent of the thermal anomaly in the mantle lithosphere, and decrease sharply in response to anomalies that are seated deeper than 50–60 km, maintained for <5 Myr in duration and/or have half‐widths <100 km. Maximum temperatures are also intimately linked to the thermal properties of the model crust, primarily due to their influence on the steady‐state (background) thermal gradient. The amplitudes of temperature increases in the crust are principally a function of depth, and are broadly independent of crustal thermal parameters. Mid‐crustal felsic and mafic plutonism is a predictable consequence of perturbed thermal regimes in the mantle and the lowermost crust, and the advection of voluminous magmas has the potential to raise temperatures in the middle crust very quickly. Although pluton‐related thermal signatures significantly dissipate within <10 Myr (even for very large, high‐temperature intrusive bodies), the interaction of pluton‐ and mantle‐related thermal effects has the potential to maintain host rock temperatures in excess of 400–450 °C for up to 30 Myr in some parts of the mid‐crust. The numerical models presented here support the notion that transient mantle‐related heat sources have the capacity to contribute significantly to the thermal budget of metamorphism in high‐T, low‐P metamorphic belts, especially in those characterised by low surface heat flow, very high peak metamorphic geothermal gradients and abundant mafic intrusions.     

Assessment of hydrodynamic simulation results for eco‐hydraulic and eco‐hydrological applications: a spatial semivariance approach

Output from a three‐dimensional numerical flow model (SSIIM) is used in conjunction with high‐resolution topographic and velocity data to assess such models for eco‐hydraulic applications in river channel design and habitat appraisal. A new methodology for the comparison between field measurement and model output is detailed. This involves a comparison between conventional goodness‐of‐fit approaches applied to a spatially structured (riffle and pool) sample of model and field data, and a ‘relaxation’ method based upon the spatial semivariance of model/field departures. Conventional assessment indicates that the model predicts point‐by‐point velocity characteristics on a 0·45 m mesh to within ±0·1 m s⁻¹ over 80% of the channel area at low flow, and 50% of the area at high in‐bank flow. When a relative criterion of model fit is used, however, the model appears to perform less well: 60–70% of channel area has predicted velocities that depart from observed velocities by more than 10%. Regression analysis of observed and predicted velocities gives more cause for optimism, but all of these conventional indicators of goodness of fit neglect important spatial characteristics of model performance. Spatial semivariance is a means of supplementing model appraisal in this respect. In particular, using the relaxation approach, results are greatly improved: at a high in‐bank flow, the model results match field measurements to within 0·1 m s⁻¹ for more than 95% of the total channel area, provided that model and field comparisons are allowed within a radius of approximately 1 m from the original point of measurement. It is suggested that this revised form of model assessment is of particular relevance to eco‐hydraulic applications, where some degree of spatial and temporal dynamism (or uncertainty) is a characteristic. The approach may also be generalized to other environmental science modelling applications where the spatial attributes of model fits are of interest. Copyright © 2005 John Wiley & Sons, Ltd.     

Global change revealed by palaeolimnological records from remote lakes: a review

Over recent decades, palaeolimnological records from remote sites have provided convincing evidence for the onset and development of several facets of global environmental change. Remote lakes, defined here as those occurring in high latitude or high altitude regions, have the advantage of not being overprinted by local anthropogenic processes. As such, many of these sites record broad-scale environmental changes, frequently driven by regime shifts in the Earth system. Here, we review a selection of studies from North America and Europe and discuss their broader implications. The history of investigation has evolved synchronously with the scope and awareness of environmental problems. An initial focus on acid deposition switched to metal and other types of pollutants, then climate change and eventually to atmospheric deposition-fertilising effects. However, none of these topics is independent of the other, and all of them affect ecosystem function and biodiversity in profound ways. Currently, remote lake palaeolimnology is developing unique datasets for each region investigated that benchmark current trends with respect to past, purely natural variability in lake systems. Fostering conceptual and methodological bridges with other environmental disciplines will upturn contribution of remote lake palaeolimnology in solving existing and emerging questions in global change science and planetary stewardship.     

Climate change during the Triassic and Jurassic

The transition from the Triassic to Jurassic is associated with dramatic changes in Earth's climate. Pangaea was breaking up as North America rifted away from Africa, the Central Atlantic Magmatic Province erupted, and the concentration of atmospheric carbon dioxide increased dramatically. This article summarises the changes in Earth's climate associated with this transition, including a discussion of the various impacts of the increased carbon dioxide on the Earth system, the question of whether the wet episode in the Carnian was a global or regional event, the formation of bauxite deposits, and how dinosaur distributions changed over time. Palaeoclimate model simulations reveal the spatial changes in climate between the Triassic and Jurassic, illustrating the subtropics becoming slightly cooler and wetter despite the warming trend for the Earth's average temperature.     

Fault‐controlled fluid flow inferred from hydrothermal vents imaged in 3D seismic reflection data,offshore NW Australia

Fluid migration pathways in the subsurface are heavily influenced by pre‐existing faults. Although studies of active fluid‐escape structures can provide insights into the relationships between faults and fluid flow, they cannot fully constrain the geometry of and controls on the contemporaneous subsurface fluid flow pathways. We use 3D seismic reflection data from offshore NW Australia to map 121 ancient hydrothermal vents, likely related to magmatic activity, and a normal fault array considered to form fluid pathways. The buried vents consist of craters up to 264 m deep, which host a mound of disaggregated sedimentary material up to 518 m thick. There is a correlation between vent alignment and underlying fault traces. Seismic‐stratigraphic observations and fault kinematic analyses reveal that the vents were emplaced on an intra‐Tithonian seabed in response to the explosive release of fluids hosted within the fault array. We speculate that during the Late Jurassic the convex‐upwards morphology of the upper tip‐lines of individual faults acted to channelize ascending fluids and control where fluid expulsion and vent formation occurred. This contribution highlights the usefulness of 3D seismic reflection data to constraining normal fault‐controlled subsurface fluid flow.