The L3–6 chondritic regolith breccia Northwest Africa (NWA) 869: (II) Noble gases and cosmogenic radionuclides

Abstract– We measured cosmogenic radionuclides and noble gases in the L3–6 chondrite breccia Northwest Africa (NWA) 869, one of the largest meteorite finds from the Sahara. Concentrations of ¹⁰Be, ²⁶Al, and ³⁶Cl in stone and metal fractions of six fragments of NWA 869 indicate a preatmospheric radius of 2.0–2.5 m. The ¹⁴C and ¹⁰Be concentrations in three fragments yield a terrestrial age of 4.4 ± 0.7 kyr, whereas two fragments show evidence for a recent change in shielding, most likely due to a recent impact on the NWA meteoroid, approximately 10⁵ yr ago, that excavated material up to approximately 80 cm deep and exposed previously shielded material to higher cosmic‐ray fluxes. This scenario is supported by the low cosmogenic ³He/²¹Ne ratios in these two samples, indicating recent loss of cosmogenic ³He. Most NWA samples, except for clasts of petrologic type 4–6, contain significant amounts of solar Ne and Ar, but are virtually free of solar helium, judging from the trapped ⁴He/²⁰Ne ratio of approximately 7. Trapped planetary‐type Kr and Xe are most clearly present in the bulk and matrix samples, where abundances of ¹²⁹Xe from decay of now extinct ¹²⁹I are highest. Cosmogenic ²¹Ne varies between 0.55 and 1.92 × 10^?8 cm³ STP g^?1, with no apparent relationship between cosmogenic and solar Ne contents. Low cosmogenic (²²Ne/²¹Ne)_c ratios in solar gas free specimens are consistent with irradiation in a large body. Combined ¹⁰Be and ²¹Ne concentrations indicate that NWA 869 had a 4π cosmic‐ray exposure (CRE) age of 5 ± 1 Myr, whereas elevated ²¹Ne concentrations in several clasts and bulk samples indicate a previous CRE of 10–30 Myr on the parent body, most probably as individual components in a regolith. Unlike many other large chondrites, NWA 869 does not show clear evidence of CRE as a large boulder near the surface of its parent body. Radiogenic ⁴He concentrations in most NWA 869 samples indicate a major outgassing event approximately 2.8 Gyr ago that may have also resulted in loss of solar helium.     

From thick to thin regional identities?

Regions and regional identity have become more important over the last decades. At the same time regions have become less discernable as distinct historically rooted spatial entities. Globalisation and the decline of collective identities through individualisation transform both this regional reality and how regions are conceptualised. This article analyses the shifts in types of regional identities used by regional administrations in an increasingly competitive environment. It uses the contrast between ‘thick’ traditional and historical rooted well-established regional identities, and ‘thin’ regional identities which are more transitory and focus more on economic competitiveness. These concepts are used to analyse the regional identity of regional administrations in Northwest Germany and the Netherlands. Hybrid regional identities combining a locally specific mix of thick and thin elements of regional identity, and which link up with regional identities at other relevant scales, appear to be the most effective regional identities for regional administrations facing the challenges of both globalisation and the decline in collective identities.     

Periglacial microjointing and faulting in Weichselian fluvio-aeolian deposits

A Weichselian Late Pleniglacial fluvio-aeolian deposit has been investigated in the southern Netherlands. Three main structural lineaments have been distinguished: (1) very small, vertical platy structures (microjoints), in a parallel and a columnar configuration; (2) large joints and normal faults with minor displacement (‘Grubbenvorst type’), arranged in a conjugate fault-system; (3) large joints and normal faults (‘wedge-type’), located adjacent to ice-wedge casts. Since clay is absent, the occurrence of the vertical platy structures cannot be attributed to desiccation cracking. The vertical platy structures are interpreted as the result of thermal contraction cracking of a relatively thin layer, due to a sudden temperature drop. The large joints and normal faults of the Grubbenvorst type are the result of failure of the sediment due to the melting of the permafrost in the Late Pleniglacial, just before the formation of the Beuningen Gravel Bed. In other areas large periglacial convolutions have been formed during the same period. The normal faults and joints of the wedge type are more generally known. They are the result of failure of the sediment adjacent to a melting ice wedge.     

Late Pleistocene and Holocene palaeogeography of the Lower Tagus Valley (Portugal): effects of relative sea level, valley morphology and sediment supply

Because of the deep glacial incision, the Lower Tagus Valley hosts a sedimentary record since 20 000 cal BP, making this a unique site along the European Atlantic margin with respect to palaeogeographic and sea level changes. Based on nine cross-sections and 55 radiocarbon dates together with a newly created relative sea level curve, we constructed five palaeogeographic maps of the infill of the Lower Tagus Valley since 20 000 cal BP. We illustrate that relative sea level rise and fluvial sediment supply were the prime forcing factors determining the depositional history and palaeogeographic changes. Around 20 000 cal BP a deeply incised braided river existed, which was directly connected to the ocean across the narrow continental shelf. After that (12 000 cal BP) the gradually moister and warmer climate caused a change to a single-channel river. During the following period (12 000–7000 cal BP) relative sea level rise resulted in a transgression in the Lower Tagus Valley and the establishment of extensive tidal environments. After relative sea level rise had ended (7000 cal BP) the valley was progressively filled by a fluvial wedge and tidally influenced bayhead delta. Since 1000 cal BP the valley-fill history was dominated by increased sediment input due to human-induced degradation of catchment slopes. Generally, climate was of subordinate importance during the entire studied period, merely causing a single-channel river resulting from the change from the cold Heinrich event 1 to the temperate Bölling–Allerød interstadial. Despite the tectonic activity in the region, neotectonic uplift or subsidence was limited, as supported by the horizontal relative sea level curve since 7000 cal BP. Neotectonics played a minor role due to the large distance from the Fennoscandian ice sheet and the narrow continental shelf, which prohibited strong glacio- and hydro-isostatic movements.     

Identifying Environmental and Human Factors Associated With Tick Bites using Volunteered Reports and Frequent Pattern Mining

Tick populations and tick‐borne diseases like Lyme borreliosis have been steadily increasing since the mid‐1990s. Realizing the threat that ticks pose to public health, two Dutch citizen science projects have collected tick bite reports since 2006. This unique volunteered geographical dataset, which currently has nearly 35,000 reports, was used to identify environmental and other circumstantial factors associated with tick bites. For this, we first enriched the tick bite reports with temperature, precipitation, vegetation and volunteered data associated with the location of the tick bite. Using this enriched dataset, we then derived a series of features to characterize the environmental and volunteer‐related conditions in which each tick bite occurred. Next, we discretized these features using the Jenks Natural Breaks algorithm and, after that, we mined frequent environmental patterns associated with tick bites using the AprioriClose algorithm. Finally, we checked that these patterns are specifically associated with the tick bites by comparing them with the frequent patterns mined from pseudo‐random locations. The frequent patterns were visualized using heat maps and ring maps and two representative patterns associated with tick bites were projected into geographic space to study their spatio‐temporal distribution. Our results show that factors linked to human activity are more relevant to model tick bites than seasonal accumulations of temperature, vegetation or precipitation. In particular, the number of warm and dry days per season are present in a significant number of patterns and the majority of tick bites are produced within a distance of half a kilometer of a forest, recreational or built‐up area. The study of patterns in the time‐series revealed that there are several persistent patterns consistently occurring each year and the validation process showed that the volunteer tick bites collection is capturing environmental conditions associated with tick bites, suggesting that these reports have a high scientific value. These results support the creation of a Dutch tick bite risk map that, in turn, will open the door to the design of public health interventions to reduce the incidence of Lyme disease.     

Structure of the Mid Atlantic Ridge province between 12° and 18°N

An analysis is given of air-gun profiler and magnetic data obtained in the central North Atlantic between 12° and 18°N. Eight fracture zones were crossed, one of which (the 15°20N fracture zone) was traced over a distance of 1500 km. The mode of adjustment of fracture zones to a change in direction of spreading is discussed. It is shown that also if this new direction would lead to an opening of the fracture zone, and adjustment fracture can originate and actually does so in several instances.The about E-W fracture zones dominate the structure of the Ridge province entirely, both with regard to the topography and to the magnetics. A magnetic model is proposed accounting for the different types of anomalies found over fracture zones. No intrusive bodies are needed to explain these anomalies.The origin of fracture zones is related to thermal contraction of a cooling lithosphere while moving from the ridge. Thermal contraction may also explain how the American and the African plates are freed from the grip they are caught in by the fanning of the fracture zones in the central North Atlantic. The fanning of fracture zones has consequences for the determination of the pole of spreading. This pole can only be found as a best fit from a synthesis of the total plate boundary, i.e. from the Azores to Bouvet Island. Local poles have only restricted value, since deviations up to 5 deg occur from a small circle pattern based on existing data.Several huge structures, viz. Researcher Ridge and Royal Trough, are found in the area which seem to parallel the flow lines of the fracture zone system. No adequate explanation exists for these structures.     

Analytic modeling of groundwater dynamics with an approximate impulse response function for areal recharge

An analytic approach is presented for the simulation of variations in the groundwater level due to temporal variations of recharge in surficial aquifers. Such variations, called groundwater dynamics, are computed through convolution of the response function due to an impulse of recharge with a measured time series of recharge. It is proposed to approximate the impulse response function with an exponential function of time which has two parameters that are functions of space only. These parameters are computed by setting the zeroth and first temporal moments of the approximate impulse response function equal to the corresponding moments of the true impulse response function. The zeroth and first moments are modeled with the analytic element method. The zeroth moment may be modeled with existing analytic elements, while new analytic elements are derived for the modeling of the first moment. Moment matching may be applied in the same fashion with other approximate impulse response functions. It is shown that the proposed approach gives accurate results for a circular island through comparison with an exact solution; both a step recharge function and a measured series of 10 years of recharge were used. The presented approach is specifically useful for modeling groundwater dynamics in aquifers with shallow groundwater tables as is demonstrated in a practical application. The analytic element method is a gridless method that allows for the precise placement of ditches and streams that regulate groundwater levels in such aquifers; heads may be computed analytically at any point and at any time. The presented approach may be extended to simulate the effect of other transient stresses (such as fluctuating surface water levels or pumping rates), and to simulate transient effects in multi-aquifer systems.     

Estimating Travel Time in Bank Filtration Systems from a Numerical Model Based on DTS Measurements

An approach is presented to determine the seasonal variations in travel time in a bank filtration system using a passive heat tracer test. The temperature in the aquifer varies seasonally because of temperature variations of the infiltrating surface water and at the soil surface. Temperature was measured with distributed temperature sensing along fiber optic cables that were inserted vertically into the aquifer with direct push equipment. The approach was applied to a bank filtration system consisting of a sequence of alternating, elongated recharge basins and rows of recovery wells. A SEAWAT model was developed to simulate coupled flow and heat transport. The model of a two‐dimensional vertical cross section is able to simulate the temperature of the water at the well and the measured vertical temperature profiles reasonably well. MODPATH was used to compute flowpaths and the travel time distribution. At the study site, temporal variation of the pumping discharge was the dominant factor influencing the travel time distribution. For an equivalent system with a constant pumping rate, variations in the travel time distribution are caused by variations in the temperature‐dependent viscosity. As a result, travel times increase in the winter, when a larger fraction of the water travels through the warmer, lower part of the aquifer, and decrease in the summer, when the upper part of the aquifer is warmer.