Tom Bradwell

The Minch palaeo-ice stream, NW sector of the British–Irish Ice Sheet

Geophysical data from the UK continental shelf off NW Scotland reveal strongly parallel subglacial bedforms on the sea bed. Field mapping on the adjacent landmass has identified closely spaced bedrock megagrooves and highly elongate drumlinoid forms. All these mega-scale glacial lineations probably formed beneath the same fast-flowing zone of the last British–Irish Ice Sheet, The Minch palaeo-ice stream. This ice stream drained the NW sector of the British–Irish Ice Sheet during the Late Devensian Glaciation (marine isotope stage 2) and terminated near the edge of the continental shelf. The size of the adjacent trough-mouth Sula Sgeir Fan and the presence of buried mega-scale glacial lineations within the Quaternary stratigraphic record imply that fast ice-sheet flow in The Minch has been a feature of several mid- to late Pleistocene glaciations.

Growth of crustose lichens: a review.

Abstract. Crustose species are the slowest growing of all lichens. Their slow growth and longevity, especially of the yellow‐green Rhizocarpon group, has made them important for surface‐exposure dating (lichenometry). This review considers various aspects of the growth of crustose lichens revealed by direct measurement including: 1) early growth and development; 2) radial growth rates (RGR, mm yr−1); 3) the growth rate–size curve; and 4) the influence of environmental factors. Many crustose species comprise discrete areolae that contain the algal partner growing on the surface of a non‐lichenized fungal hypothallus. Recent data suggest that ‘primary’ areolae may develop from free‐living algal cells on the substratum while ‘secondary’ areolae develop from zoospores produced within the thallus. In more extreme environments, the RGR of crustose species may be exceptionally slow but considerably faster rates of growth have been recorded under more favourable conditions. The growth curves of crustose lichens with a marginal hypothallus may differ from the ‘asymptotic’ type of curve recorded in foliose and placodioid species; the latter are characterized by a phase of increasing RGR to a maximum and may be followed by a phase of decreasing growth. The decline in RGR in larger thalli may be attributable to a reduction in the efficiency of translocation of carbohydrate to the thallus margin or to an increased allocation of carbon to support mature ‘reproductive’ areolae. Crustose species have a low RGR accompanied by a low demand for nutrients and an increased allocation of carbon for stress resistance; therefore enabling colonization of more extreme environments.

A New Lichenometric Dating Curve For Southeast Iceland.

This paper presents a new lichenometric dating curve for southeast Iceland. The temporal framework for the curve is based on reliably dated surfaces covering the last 270 years, making it the best constrained study of this nature conducted in Iceland. The growth of lichen species within Rhizocarpon Section Rhizocarpon is non‐linear over time, with larger (older) thalli apparently growing more slowly. The linear ‘growth’ curves derived previously by former authors working in Iceland represent only part of a curve which has an overall exponential form. Reasons for the non‐linearity of the new dating curve are probably physiological, although climatic change over the last three centuries cannot be ruled out. Use of linear ‘growth’ curves in Iceland is problematic over time‐spans of more than c. 80 years. Pre‐20th century moraines dated using a constant, linear relationship between lichen size and age are probably older than previously believed. Those moraines lichenometrically ‘dated’ to the second half of the 19th century in Iceland may actually pre‐date this time by several decades (30–100 years), thus throwing doubt on the exact timing of maximum glaciation during the ‘Little Ice Age’.

Lichenometric dating in southeast iceland: the size–frequency approach

Abstract The age of recent deposits can be determined using an intrinsic characteristic of the lichen ‘population’ growing on their surface. This paper presents a calibrated dating curve based on the gradient of the size‐frequency distribution of yellow‐green Rhizocarpon lichens. The dating potential of this new curve is tested on surfaces of known age in southeast Iceland. This particular size—frequency technique is also compared with the more traditional largest‐lichen approach. The results are very encouraging and suggest that the gradient can be used as an age indicator, at least on deposits formed within the last c. 150 years – and probably within the last c. 400 years – in the maritime subpolar climate of southeast Iceland. Using both lichenometric techniques, revised dates for moraines on two glacier forelands are presented which shed new light on the exact timing of the Little Ice Age glacier maximum in Iceland.

Buried glacier ice in southern Iceland and its wider significance

Abstract Geo-electrical resistivity surveys have been carried out at recently deglaciated sites in front of three glaciers in southern Iceland: Skeiðarajokull, Hrutarjokull, and Virkisjokull. The results show the presence of old glacier ice beneath debris mantles of various thickness. We conclude that buried glacier ice has survived for at least 50 years at Virkisjokull and Hrutarjokull, and probably for over 200 years at Skeiðarajokull. Additional data from a further site have identified a discontinuous ice core within 18th-century jokulhlaup deposits. Photographic and lichenometric evidence show that the overlying debris has been relatively stable, and hence melting of the ice at all four sites is proceeding slowly due to the heat-shielding properties of the overburden. The geomorphic implications are pertinent when considering the potential longevity of buried ice. The possible implications for dating techniques, such as lichenometry, radiocarbon dating and cosmogenic surface-exposure dating are also important, as long-term readjustments of surface forms may lead to dating inaccuracy. Finally, it is recognised that landscape development in areas of stagnant ice topography may post-date initial deglaciation by a considerable degree.

The Pleistocene glaciations of the North Sea Basin

We review the Quaternary geology of the North Sea area, summarising evidence for extents, configurations and timing of former glacial activity, focusing attention on key sites across the basin, and for the first time, integrating the stratigraphy with up-to-date information on the geomorphic (morphological) framework of the Pleistocene glacial sequence.

Cosmogenic 10Be age constraints for the wester ross readvance moraine : Insights into british ice-sheet behaviour

Abstract This study presents the first absoluteage constraints from a palaeo‐ice‐sheet margin in western Scotland. Cosmogenic 10Be from four Lewisian gneiss boulders on the Gairloch Moraine in NW Scotland have yielded reliable exposure ages. Three of these dates, taken from a single moraine ridge, cluster around c. 15.5–18 ka BP, with a weighted mean of 16.3 ± 1.6 ka BP. These findings indicate that the last British Ice Sheet had retreated to the present‐day coastline in NW Scotland by this time. It is suggested that the Wester Ross Readvance represents an ice‐sheet oscillation during, or in the immediate aftermath of, Heinrich Event 1 (c. 17–18 ka BP).

High Latitude Dust in the Earth System

Natural dust is often associated with hot, subtropical deserts, but significant dust events have been reported from cold, high latitudes. This review synthesizes current understanding of high-latitude (≥50°N and ≥40°S) dust source geography and dynamics and provides a prospectus for future research on the topic. Although the fundamental processes controlling aeolian dust emissions in high latitudes are essentially the same as in temperate regions, there are additional processes specific to or enhanced in cold regions. These include low temperatures, humidity, strong winds, permafrost and niveo-aeolian processes all of which can affect the efficiency of dust emission and distribution of sediments. Dust deposition at high latitudes can provide nutrients to the marine system, specifically by contributing iron to high-nutrient, low-chlorophyll oceans; it also affects ice albedo and melt rates. There have been no attempts to quantify systematically the expanse, characteristics, or dynamics of high-latitude dust sources. To address this, we identify and compare the main sources and drivers of dust emissions in the Northern (Alaska, Canada, Greenland, and Iceland) and Southern (Antarctica, New Zealand, and Patagonia) Hemispheres. The scarcity of year-round observations and limitations of satellite remote sensing data at high latitudes are discussed. It is estimated that under contemporary conditions high-latitude sources cover >500,000 km2 and contribute at least 80–100 Tg yr−1 of dust to the Earth system (~5% of the global dust budget); both are projected to increase under future climate change scenarios.

Lichenometric dating: a commentary, in the light of some recent statistical studies

Abstract. This commentary article discusses the relative merits of new mathematical approaches to lichenometry. It highlights their strong reliance on complex statistics; their user unfriendliness; and their occasional mistreatment of existing lichenometric techniques. The article proposes that the success of lichenometric dating over the past 50 years has stemmed from its relative simplicity, transparency, and general field applicability. It concludes that any new techniques which ignore these principles are likely to be unjustified, unsuitable to the user community and inappropriate for the subject matter. Furthermore, the article raises a more general philosophical question: can statistical complexity and high precision in a ‘geobotanical’ dating technique, fraught with high degrees of environmental variability and inbuilt uncertainty, ever be scientifically valid?

Re-Dating the Moraines at Skálafellsjökull and Heinabergsjökull using different Lichenometric Methods: Implications for the Timing of the Icelandic Little Ice Age Maximum

Abstract Little Ice Age (LIA) moraines along the margins of Skálafellsjökull and Heinabergsjökull, two neighbouring outlet glaciers flowing from the Vatnajökull ice‐cap, have been re‐dated to test the reliability of different lichenometric approaches. During 2003, 12 000 lichens were measured on 40 moraine fragments at Skálafellsjökull and Heinabergsjökull to provide surface age proxies. The results are revealing. Depending on the chosen method of analysis, Skálafellsjökull either reached its LIA maximum in the early 19th century (population gradient) or the late 19th century (average of five largest lichens), whereas the LIA maximum of Heinabergsjökull occurred by the mid‐19th century (population gradient) or late‐19th century (average of 5 largest lichens). Discrepancies (c. 80 years for Skálafellsjökull and c. 40 years for Heinabergsjökull) suggest that the previously cited AD 1887 LIA maxima for both glaciers should be reassessed. Dates predicted by the lichen population gradient method appear to be the most appropriate, as mounting evidence from other geochronological reconstructions and sea‐ice records throughout Iceland tends to support an earlier LIA glacier maximum (late 18th to mid‐19th century) and probably reflects changes in the North Atlantic Oscillation. These revised chronologies shed further light on the precise timing of the Icelandic LIA glacier maximum, whilst improving our understanding of glacier‐climate interactions in the North Atlantic.