Michèle L. Clarke

Effects of storminess, sand supply and the North Atlantic Oscillation on sand invasion and coastal dune accretion in western Portugal

Holocene forested coastal dunes fringe the Atlantic coast of western Portugal. Mapping of dunes in the field and using air photographs shows a range of forms reflecting dominant northwest and westerly onshore wind regimes. Planting of maritime pine forests in the thirteenth and twentieth centuries was initiated because of sand invasion causing problems for human settlement and agriculture. Early Holocene dunes have a well-developed podsol and date to 9.7 and 8.2 ka, suggesting at least some of these sands may have been emplaced during a global cooling event. Significant transgressive dune accretion at 2.2 and 1.5 ka, implies abundant sand supply and strong onshore winds The most recent dune-building period dates to AD 1770-1905 and coincides with a predominantly negative winter North Atlantic Oscillation index (NAOi). Accretion of dunes along the Portuguese coast appears out of phase with dune development in southwest France, which may reflect different Atlantic storm tracks driven by changes in the dominance and state of the NAOi.

The impact of the farming practice of remodelling hillslope topography on badland morphology and soil erosion processes

Abstract Badland landforms, created in Plio–Pleistocene clay landscapes of the Basilicata region of southern Italy, form marginal features in a semi-arid landscape dominated by the widespread cultivation of subsidised cereals. These badland features are high relative relief forms exhibiting a high drainage density and steep slopes, with slope angles typically in excess of 35°. Economic incentives to increase agricultural productivity have resulted in the practice of remodelling these marginal areas using heavy earth-moving equipment. Remodelling the badland features creates longer slopes at lower angles, which can be cultivated using conventional farm machinery. These changes in hillslope morphology have altered the degree to which soil erosion processes operating in these areas are spatially coupled. In the badland areas, erosion and deposition are strongly localised with minimal sediment delivery to ephemeral or perennial channel systems. The economically-driven change in land use from visually striking badland areas to newly remodelled fields for agricultural use results in an increase in the coupling of drainage networks and a net increase in soil erosion.

Long-term survival despite low genetic diversity in the critically endangered Madagascar fish-eagle

The critically endangered Madagascar fish‐eagle (Haliaeetus vociferoides) is considered to be one of the rarest birds of prey globally and at significant risk of extinction. In the most recent census, only 222 adult individuals were recorded with an estimated total breeding population of no more than 100–120 pairs. Here, levels of Madagascar fish‐eagle population genetic diversity based on 47 microsatellite loci were compared with its sister species, the African fish‐eagle (Haliaeetus vocifer), and 16 of these loci were also characterized in the white‐tailed eagle (Haliaeetus albicilla) and the bald eagle (Haliaeetus leucocephalus). Overall, extremely low genetic diversity was observed in the Madagascar fish‐eagle compared to other surveyed Haliaeetus species. Determining whether this low diversity is the result of a recent bottleneck or a more historic event has important implications for their conservation. Using a Bayesian coalescent‐based method, we show that Madagascar fish‐eagles have maintained a small effective population size for hundreds to thousands of years and that its low level of neutral genetic diversity is not the result of a recent bottleneck. Therefore, efforts made to prevent Madagascar fish‐eagle extinction should place high priority on maintenance of habitat requirements and reducing direct and indirect human persecution. Given the current rate of deforestation in Madagascar, we further recommend that the population be expanded to occupy a larger geographical distribution. This will help the population persist when exposed to stochastic factors (e.g. climate and disease) that may threaten a species consisting of only 200 adult individuals while inhabiting a rapidly changing landscape.

IRSL dating of sands : bleaching characteristics at deposition inferred from the use of single aliquots

Single aliquot infra-red stimulated luminescence measurements have been carried out on feldspar from sands from a range of aeolian, colluvial and alluvial environments in an attempt to determine their degree of bleaching at burial. Two coupled parameters have been devised to indicate degree of bleaching and to discriminate samples which would be a problem in IRSL dating. These are used to show that aeolian, as well as colluvial and alluvial sands, can show poorly bleached behaviour.

Quality assurance in luminescence dating

Recent advances in luminescence dating have led to increasing application of the technique to sediments from a wide range of depositional environments, many of which are characterised by rapid transport and deposition under turbid conditions. The complete zeroing of the luminescence signal, by exposure to light, is critical to obtaining an accurate age for deposition of the sediment, before any geomorphological or palaeoenvironmental reconstructions can be undertaken based on this chronology. Comparison of luminescence dates with independent age control is rare, due to a lack of suitable material in natural contexts and thus the accuracy of the luminescence dating technique has yet to be proven in many depositional environments. There is therefore a clear need for both quality assurance in luminescence dating and a method for assessing the accuracy of each date. This paper describes and demonstrates a quality assurance method that can be used as part of a routine dating programme and which can discriminate against samples which would give inaccurate dates.

Late Quaternary hillslope evolution recorded in eastern South African colluvial badlands

Abstract Infrared stimulated luminescence (IRSL) ages have been obtained from a sequence of sandy colluvial deposits in northern KwaZulu–Natal. Comparison of the IRSL ages on sand-sized, potassium-rich feldspars, with radiocarbon dates on A-horizon organic matter from buried palaeosols within the colluvial succession shows good agreement between the dating techniques. When compared with palaeoenvironmental records, the data suggest that the colluvium accumulated during arid stages of the Late Quaternary, whilst pedogenesis repeatedly occurred at intervals of hillslope stability which may reflect periods of greater humidity. Temperature does not seem to have been a forcing factor in the landscape development. Extensive colluviation took place during the arid Last Glacial Maximum, but the occurrence of several palaeosols indicates that it was a time of fluctuating climatic conditions.

Infra-red stimulated luminescence ages from aeolian sand and alluvial fan deposits from the eastern Mojave Desert, California

Abstract Infra-red stimulated luminescence (IRSL) from coarse-grained (180–212 μm) potassium-rich feldspars has been used to date deposits associated with the western piedmont of the Providence Mountains in the eastern Mojave Desert. These deposits consist of alluvial fans, aeolian sand, which exists in the form of lenses within the fan matrices, and sand from the Kelso dune field which abuts the distal end of the fan sequence. Deposition and subsequent stabilisation of these sand units appears to have occurred in both aird and intermittent climates associated with pluvial phases during the Holocene and Late Pleistocene. Ages of 6600 ± 870 and 7840 ± 1790 years were obtained from dune sand bracketing the Mazama ash at Skull Creek Dunes, Oregon, giving an independent age control on the single aliquot IRSL method which was used on the Mojave samples.

A revised chronology for aeolian activity in subarctic Fennoscandia during the Holocene

Detailed sedimentological studies and parallel sampling for 14C and infra-red stimulated luminescence age assays were undertaken at six sites lying beyond and below the tree-line with the objective of establishing the historical development of aeolian landforms in Lapland. The main issues were: (a) the timing of dune activity and processes of stabilization; (b) the specific processes responsible for the past and contemporary activity. New data indicate that many of the primary dunes were active for several millennia after deglaciation. Diachronous stabilization at both local and regional scales appears likely. Initial stabilization occurred in local interdune areas and where regional dune fields were colonized by pine (generally before 7 ka). Local large dunes and some regional dunefields only became stable around 4.3 ka, when the regional groundwater table rose and late pine colonization was possible. The latest episode of aeolian activity dates from ad 1100–1650 across the area. A unique deflation triggering factor was not identifiable, suggesting that several agents acted in combination. Climate-vegetational parameters, especially the‘Little Ice Age’ event, together with reindeer trampling, appear responsible for the continuing aeolian activity at the tundra sites. At the margin of the pine forest zone, forest fires may be the cause of aeolian reactivation.

Infra-red stimulated luminescence spectra of alkali feldspars

IRSL emission spectra from alkali feldspars show a UV emission peak, centred on 290 nm, which is associated with a sodium aluminosilicate lattice structure, either as the dominant phase in sodium feldspar (albite) or as exsolved features in perthitic potassium feldspars (orthoclases and microclines). The trapped charge population associated with recombination at the 290 nm centre during IRSL is thermally unstable. After laboratory irradiation, luminescence intensity in this emission band decays and in sodium feldspars, this decay results in charge transfer into other recombination centres. Use of a sustained preheating regime removes the unstable trapped charge population associated with the 290 nm emission and is essential in an IRSL dating routine.

Luminescence dating of recent dunes on Inch Spit, Dingle Bay, southwest Ireland

Fifteen samples of dune sand, and three samples from a core taken in the beach face, were collected from a sand spit which protrudes into Dingle Bay in Southwest Ireland. The potassium-rich feldspar grains from the samples were dated using a single aliquot luminescence protocol for infrared stimulated luminescence (IRSL) signals. No ages over 600 years were obtained, which demonstrates the youth of the dune forms currently observed. The youngest ages were about 150 years old, but for these samples the scatter in the equivalent dose obtained as the mean of 18 measurements was higher than for the older samples.