A 13‐year record of erosion on badland sites in the Karoo,South Africa

Land degradation in South Africa has been of concern for more than 100 years with both climate change and inappropriate land management (overgrazing) being proposed as primary drivers. However, there are few quantitative studies of degradation and, in particular, few of erosion by water. Badlands, taken here to be the landform which results from extreme erosion, have been notably neglected. We report on 13 consecutive years of erosion pin measurements of badland erosion on 10 study sites in the Sneeuberg uplands of the eastern Karoo in South Africa. The study sites are on Holocene colluvium which mantles footslopes. They have been subject to overgrazing for at least 100 years, c. 1850–1950. Currently they are lightly grazed by sheep. The area receives about 500 mm rainfall per year. The sites are remote, with only informal, farmer‐operated, daily raingauges nearby. The nearest sub‐daily raingauge is c. 55 km distant. Also we report on an analysis of the erosion pin data which focuses on establishing the origins and context of the badlands, including the relationship between study sites and adjacent valley‐bottom gully systems; compare erosion rates on our study sites with rates determined by erosion pins on other badland sites; and discuss the implications of these erosion rates for landscape development and off‐site impacts. Net erosion rates on the study sites are relatively high compared with global badland rates and range from 3.1 to 8.5 mm yr^‐1 which may be extrapolated to 53 to 145 t ha yr^‐1 (using a measured bulk density of 1.7 g cm^‐3). However, comparisons with badland sites elsewhere are difficult because of different measuring methodologies, lithologies, climate and dominant processes. Erosion rates on the study sites are strongly influenced by rainfall amounts and, in particular, by daily rainfall events which exceed ~10 mm: this is the threshold intensity at which runoff has been observed to commence on badlands. Of significance, but of lesser influence, is weathering, mainly by wetting and drying: this prepares bare surfaces for erosion. However, questions remain regarding the role of site characteristics, and of processes at each site, in determining between‐site differences in erosion rate. Crude extrapolation of current rates of erosion, in conjunction with depths of incision into the badlands, suggests that badland development started around 200 years ago, probably as a response to the introduction of European‐style stock farming which resulted in overgrazing. We assume, but cannot quantify, the additional influence of periods of drought and burning in the erosional history of the area. Intermittent connection of these badlands to valley‐bottom gullies and therefore to small farm dams and ultimately to large water storage reservoirs increases their impact on local water resources. Copyright © 2015 John Wiley & Sons, Ltd.     

Cadmium,lead, copper,and zinc in Elminius modestus Darwin (Crustacea,Cirripedia) from Waitemata and Manukau Harbours,Auckland, New Zealand

Abstract

Concentrations of cadmium (Cd), lead (Pb), copper (Cu), and zinc (Zn) were measured in adult barnacles (Elminius modestus Darwin) from Waitemata and Manukau Harbours in the Auckland area, New Zealand. As in studies on sediments reported in the literature, it was possible to identify areas of likely anthropogenic influence, e.g., around the Auckland Harbour Bridge for Pb, Cu, and Zn. Groups of individuals with highest concentrations for these metals showed 19.8–23.8 mg Pb kg^?1, 198–266 mg Cu kg^?1, and 4460–6530 mg Zn kg^?1 (95% confidence limits, dry weight basis). Cd concentrations found for all barnacles from the Auckland area ranged from 0.8 to 3.1 mg kg^?1. Two samples from Omaha Beach, 60 km north of Auckland, were used as a reference. Accordingly, groups of individuals with lowest concentrations for Pb, Cu, and Zn could be allocated to this site using the Student‐Newman‐Keuls Multiple Range Test (0.5–1.3 mg Pb kg^?1, 8–10 mg Cu kg^?1, and 144–214 mg Zn kg^?1 ; 95% confidence limits). Only Cd concentrations were highest at Omaha Beach (8.6–12.1 mg Cd kg^?1 ). This result may have arisen from “naturally” increased bio‐availabilities of certain metals in mangrove systems which are reported in the literature. Generally, metal concentrations in barnacles from the Auckland Harbour area and from Omaha Beach were within the wider range for E. modestus as well as other barnacle species reported in the international literature.     

Mineral magnetic signatures in a long core from Lake Qarun, Middle Egypt

The analysis and interpretation of changes in mineral magnetic signatures from a long (ca. 8.2 m) sedimentary sequence recovered from Lake Qarun, Middle Egypt in 2003 spanning a timescale of approximately the last 2,000 years is reported. A suite of mass specific susceptibility and magnetic remanence measurements were made at irregular intervals downcore on 39 samples. These samples were selected on the basis of trends and abrupt changes in whole-core magnetic susceptibility measured using a Bartington^® MS2E sensor and were analysed for low and high temperature loss on ignition and their particle size distribution. Trends in all mineral magnetic concentration parameters are remarkably similar and were initially used to divide the core into three magnetically distinct zones. The upper and lower sections of the core (0–119 cm and 445–822 cm depth) are characterised by low values for all magnetic concentration parameters. Between 153 and 380 cm depth, concentration parameters are considerably higher, although somewhat variable. The S ratio and percentage loss of remanence after 24 h (IRM_loss) follow a different trend and are inversely related to each other. A low S ratio (<0.7) is associated with a loss of remanence of >4%. On the basis of these parameters, the core can be divided into four zones, and differences in magnetic mineralogy between these four zones were confirmed by measurement of IRM acquisition curves. The major difference between concentration parameters and ratios or percentage loss of IRM lies in the identification of an additional zone below 619 cm depth where the S ratio is high and IRM_loss is low. There is little evidence to suggest that the magnetic signatures are controlled by particle size or by trends in organic matter and/or carbonate content. The signatures appear to be predominantly detrital and show little evidence of post-depositional alteration through dissolution or authigenic addition of bacterial magnetite or greigite. Analysis of Saharan dust deposition rates in Northern Egypt suggests that atmospheric fallout is likely to make only a very minor contribution (<1%) to sedimentation rates in Lake Qarun. The downcore trends therefore appear to reflect major changes in fluvial sediment sources over the ca. 2,000 year time period spanned by this ～8 m core. Preliminary mineral magnetic characterisations of potential local sources suggest that these cannot account for the range of signatures recorded in the Qarun sediments and it is hypothesised that these sediments are derived from Nile river floods.     

Late Neoproterozoic to Holocene thermal history of the Precambrian Georgetown Inlier,northeast Australia

Carboniferous‐Permian volcanic complexes and isolated patches of Upper Jurassic — Lower Cretaceous sedimentary units provide a means to qualitatively assess the exhumation history of the Georgetown Inlier since ca 350 Ma. However, it is difficult to quantify its exhumation and tectonic history for earlier times. Thermochronological methods provide a means for assessing this problem. Biotite and alkali feldspar ⁴⁰Ar/³⁹Ar and apatite fission track data from the inlier record a protracted and non‐linear cooling history since ca 750 Ma. ⁴⁰Ar/³⁹Ar ages vary from 380 to 735 Ma, apatite fission track ages vary between 132 and 258 Ma and mean track lengths vary between 10.89 and 13.11 μm. These results record up to four periods of localised accelerated cooling within the temperature range of ～320–60°C and up to ～14 km of crustal exhumation in parts of the inlier since the Neoproterozoic, depending on how the geotherm varied with time. Accelerated cooling and exhumation rates (0.19–0.05 km/10⁶ years) are observed to have occurred during the Devonian, late Carboniferous‐Permian and mid‐Cretaceous — Holocene periods. A more poorly defined Neoproterozoic cooling event was possibly a response to the separation of Laurentia and Gondwana. The inlier may also have been reactivated in response to Delamerian‐age orogenesis. The Late Palaeozoic events were associated with tectonic accretion of terranes east of the Proterozoic basement. Post mid‐Cretaceous exhumation may be a far‐field response to extensional tectonism at the southern and eastern margins of the Australian plate. The spatial variation in data from the present‐day erosion surface suggests small‐scale fault‐bounded blocks experienced variable cooling histories. This is attributed to vertical displacement of up to ～2 km on faults, including sections of the Delaney Fault, during Late Palaeozoic and mid‐Cretaceous times.     

Crust restoration for the western Lachlan Orogen using the strain-reversal,area-balancing technique: implications for crustal components and original thicknesses

Strain reversal of structural/stratigraphic profiles at different scales in the western Lachlan Orogen provides a perspective on original crustal thickness estimates, the former depositional basin width of the proto-western Lachlan Orogen, the original sedimentary-fan thickness, and the possible length extent of lower crust lost by subduction. Retrodeformation using strain-reversal techniques allows basin reconstruction giving an original width of the western Lachlan Orogen basin receptor of between 800 km (minimum) and ～1150 km (maximum), depending on the amount of stratal duplication allowed in the turbidites. Crude area balancing of the regional cross-section, adding in sectional volume lost by erosion and assuming strain compatibility between the upper and lower crust, suggests that the predeformation crustal thickness ranges between 15 km and ～21 km, with a lower crustal thickness (oceanic lithosphere) of ～9 km and a turbidite fan thickness of ～6 km (minimum) and ～12 km (maximum allowable), respectively. Disparity between the calculated fan thickness and that derived from measured stratigraphic sections adjusted for strain (～6 km) indicates that some form of crustal stacking must be important in structural thickening of the turbidite crustal component. By varying shortening due to fault stacking, mass balance dictates the mismatch of the upper crustal (uc) and lower crustal (lc) retrodeformed lengths, and therefore provides an estimate of lower crustal loss by subduction. End members range from: (i) a 12 km-thick fan without fault duplication, a basin width of ～800 km where uc = lc giving no lower crustal loss by subduction; to (ii) a ～6 km fan, requiring duplication by faulting, a basin of ～1150 km where uc > lc, and ～360 km of lower crust length (～30%) lost by subduction. This suggests that the total thickness of underplated igneous material in the western Lachlan Orogen is low, probably < ～2 km.     

Rheological control on the tectonic evolution of a continental suture zone: the Variscan example from NW Iberia (Spain)

The Variscan continental suture zone exposed in NW Iberia is examined to uncover the long-lived rheological control exerted by the strata deposited over the external parts of Gondwana on its geodynamic evolution. The suture occurs within a set of allochthonous terranes whose limits were taken as domain boundaries to interpret the Variscan stacking of Paleozoic continental domains and retrodeform the resulting nappe pile. The suture zone formed due to closure of ocean basins located between Gondwana and Laurussia during the Late Paleozoic and consists of relics of oceanic and transitional crust. The suture zone exhibits a tabular to lens shape due to repeated tectonic events dominated by non-coaxial deformation (thrusts and low-angle normal faults). Thrusting and normal faulting also involved the margins of the continents bounding the suture. The structure of the continental blocks, however, is dominated by folds, particularly large nappe folds with pronounced superimposed flattening. The upper part of the basal allochthonous units comprises a rheologically incompetent domain below the suture zone. This domain is typified by the carbonaceous-rich strata, which are probably Ordovician–Silurian sediments based on U–Pb detrital zircon populations. The rheology of this layer determined the location of the first accretionary thrust that initiated the Late Devonian subduction of the Gondwana margin below the suture zone. By favoring fault development, the upper sequence of the basal allochthonous units as a whole influenced the exhumation of deep-seated continental crust, the transference of the suture zone over Gondwana, and the re-equilibration of the resulting overthickened crust.     

Inflation along Kilauea's Southwest Rift Zone in 2006

We report on InSAR and GPS results showing the first crustal inflation along the southwest rift zone at Kilauea volcano in over 20 years. Two independent interferograms (May 2–August 2, 2006 and June 22–Nov 7, 2006) from the ALOS PALSAR instrument reveal domal uplift located southwest of the main caldera. The uplift is bounded on the northeast by the caldera and follows the southwest rift zone for about 12 km. It is approximately 8 km wide. We use data derived from permanent GPS stations to calibrate the InSAR displacement data and estimate uplift of 7.7 cm during the first interferogram and 8.9 cm during the second with line-of-sight volumes of 2.8 × 10⁶ m³ and 3.0 × 10⁶ m³ respectively. The earthquake record for the periods before, during, and after inflation shows that a swarm of shallow earthquakes (z < 5 km) signaled the beginning of the uplift and that elevated levels of shallow seismicity along the rift zones occurred throughout the uplift period. GPS data indicate that the inflation occurred steadily over nine months between mid-January and mid-October, 2006 making injection of a sill unlikely. We attribute the inflation to recharge of a shallow ductile area under the SWRZ.     

An integrated lake-catchment approach for determining sediment source changes at Aqualate Mere,Central England

This paper raises fundamental questions about the sole use of paleolimnological techniques to identify sediment sources and develop catchment management plans. The concept of an integrated lake: catchment framework was established 30 years ago, yet paleolimnologists occasionally fail to appreciate the dynamics of the contributing catchment. This is especially critical when the predominant source of sediment accumulating in a lake is allochthonous. In this paper we argue that a detailed appraisal of catchment sources and investigation of historical documentary evidence is needed to identify and evaluate the relative significance of sediment sources. We used such an approach at Aqualate Mere, Shropshire, UK. Mineral magnetic and radionculide signatures of potential catchment sources and accumulating lake sediments were compared in an attempt to match the sources to sediments deposited in the Mere. Dated lake sediments indicate there has been an increase in sedimentation rate and the relative amount of minerogenic material delivered to the Mere over the last 200 years. In contrast to a previous study at the same site, there is no evidence to attribute this increase to an overspill from a nearby canal. Other catchment disturbances include landscaping in parkland surrounding the Mere in the early nineteenth century and drainage systems installed to improve catchment agriculture over the last ca. 150 years. Both activities may explain the change in sedimentation rates and types, independent of the hypothesized canal origin. Although our results exclude the canal as a major sediment source, identifying the contribution of other potential catchment sources remains problematic. ¹³⁷Cs inventories for the lake are similar to those recorded at a local reference site, suggesting little influx of ¹³⁷Cs-bearing topsoil, yet ¹³⁷Cs activities remain high in the upper 20–30 cm of the lake sediment profile, indicating a topsoil origin. Combined radionuclide and mineral magnetic signatures proved to be relatively poor discriminators of potential sources, and the high atmospheric pollution load from the West Midland conurbation has probably altered recent lake sediment signatures. Although further research is required to identify the origins of recent (last ca. 200 years) minerogenic sediment inputs to the Mere, we suggest that the combined lake: catchment approach offers a more rigorous method for understanding the impact of catchment disturbance than analysis of the paleolimnological record alone.