Ice marginal dynamics during surge activity,Kuannersuit Glacier,Disko Island,West Greenland

The Kuannersuit Glacier surged 11 km between 1995 and 1998. The surge resulted in the formation of an ice cored thrust moraine complex constructed by subglacial and proglacial glaciotectonic processes. Four main thrust zones are evident in the glacier snout area with phases of compressional folding and thrusting followed by hydrofracture in response to the build-up of compressional stresses and the aquicludal nature of submarginal permafrost and naled. Various types of stratified debris-rich ice facies occur within the marginal zone: The first (Facies I) comprises laterally continuous strata of ice with sorted sediment accumulations, and is reworked and thrust naled ice. The second is laterally discontinuous stratified debris-rich ice with distinct tectonic structures, and is derived through subglacial extensional deformation and localised regelation (Facies II), whilst the third type is characterised by reworked and brecciated ice associated with the reworking and entrainment of meteoric ice (Facies III). Hydrofracture dykes and sills (Facies IV) cross-cut the marginal ice cored thrust moraines, with their sub-vertically frozen internal contact boundaries and sedimentary structures, suggesting supercooling operated as high-pressure evacuation of water occurred during thrusting, but this is not related to the formation of basal stratified debris-rich ice. Linear distributions of sorted fines transverse to ice flow, and small stratified sediment ridges that vertically cross-cut the ice surface up-ice of the thrust zone relate to sediment migration along crevasse traces and fluvial infilling of crevasses. From a palaeoglaciological viewpoint, marginal glacier tectonics, ice sediment content and sediment delivery mechanisms combine to control the development of this polythermal surge valley landsystem. The bulldozing of proglacial sediments and the folding and thrusting of naled leads to the initial development of the outer zone of the moraine complex. This becomes buried in bulldozed outwash sediment and well-sorted fines through surface ablation of naled. Up-ice of this, the heavily thrust margin becomes buried in sediment melted out from basal debris-rich ice and subglacial diamicts routed along thrusts. These mechanisms combine to deliver sediment to supraglacial localities, and promote the initial preservation of structurally controlled moraines through insulation, and the later development of kettled dead ice terrain.     

The cause for the north-south orientation of the crustal dichotomy and the equatorial location of Tharsis on Mars

The crustal dichotomy and the Tharsis rise are the most prominent topographic features on Mars. The dichotomy is largely an expression of different crustal thicknesses in the northern and southern hemispheres, while Tharsis is centered near the equator at the dichotomy boundary. However, the cause for the orientation of the dichotomy and the equatorial location of Tharsis remains poorly understood. Here we show that the crustal thickness variations associated with the dichotomy may have driven true polar wander, establishing the north-south orientation of the dichotomy very early in martian history. Such a reorientation that placed the dichotomy boundary near the equator would also have constrained the Tharsis region on the dichotomy boundary to have originated near the equator. We present a scenario for the early generation and subsequent reorientation of the hemispheric dichotomy, although the reorientation is independent of the formation mechanism. Our results also have implications for the sharply different remanent magnetizations between the two hemispheres.     

Timing and pace of ice-sheet withdrawal across the marine–terrestrial transition west of Ireland during the last glaciation

Understanding the pace and drivers of marine-based ice-sheet retreat relies upon the integration of numerical ice-sheet models with observations from contemporary polar ice sheets and well-constrained palaeo-glaciological reconstructions. This paper provides a reconstruction of the retreat of the last British–Irish Ice Sheet (BIIS) from the Atlantic shelf west of Ireland during and following the Last Glacial Maximum (LGM). It uses marine-geophysical data and sediment cores dated by radiocarbon, combined with terrestrial cosmogenic nuclide and optically stimulated luminescence dating of onshore ice-marginal landforms, to reconstruct the timing and rate of ice-sheet retreat from the continental shelf and across the adjoining coastline of Ireland, thus including the switch from a marine- to a terrestrially-based ice-sheet margin. Seafloor bathymetric data in the form of moraines and grounding-zone wedges on the continental shelf record an extensive ice sheet west of Ireland during the LGM which advanced to the outer shelf. This interpretation is supported by the presence of dated subglacial tills and overridden glacimarine sediments from across the Porcupine Bank, a westwards extension of the Irish continental shelf. The ice sheet was grounded on the outer shelf at ~26.8 ka cal bp with initial retreat underway by 25.9 ka cal bp. Retreat was not a continuous process but was punctuated by marginal oscillations until ~24.3 ka cal bp. The ice sheet thereafter retreated to the mid-shelf where it formed a large grounding-zone complex at ~23.7 ka cal bp. This retreat occurred in a glacimarine environment. The Aran Islands on the inner continental shelf were ice-free by ~19.5 ka bp and the ice sheet had become largely terrestrially based by 17.3 ka bp. This suggests that the Aran Islands acted to stabilize and slow overall ice-sheet retreat once the BIIS margin had reached the inner shelf. Our results constrain the timing of initial retreat of the BIIS from the outer shelf west of Ireland to the period of minimum global eustatic sea level. Initial retreat was driven, at least in part, by glacio-isostatically induced, high relative sea level. Net rates of ice-sheet retreat across the shelf were slow (62–19 m a⁻¹) and reduced (8 m a⁻¹) as the ice sheet vacated the inner shelf and moved onshore. A picture therefore emerges of an extensive BIIS on the Atlantic shelf west of Ireland, in which early, oscillatory retreat was followed by slow episodic retreat which decelerated further as the ice margin became terrestrially based. More broadly, this demonstrates the importance of localized controls, in particular bed topography, on modulating the retreat of marine-based sectors of ice sheets.     

Atmospheric dust variability from Arabia and China over the last 500,000 years

Atmospheric mineral dust aerosols affect Earth’s radiative balance and are an important climate forcing and feedback mechanism. Dust is argued to have played an important role in past natural climate changes through glacial cycles, yet temporal and spatial dust variability remain poorly constrained, with scientific understanding of uncertainties associated with radiative perturbations due to mineral dust classified as “very low”. To advance understanding of the dust cycle, we present a high-resolution dust record from the Red Sea, sourced principally from Arabia, with a precise chronology relative to global sea level/ice volume variability. Our record correlates well with a high-resolution Asian dust record from the Chinese Loess Plateau. Importing our age model from the Red Sea to the Chinese Loess Plateau provides a first detailed millennial-scale age model for the Chinese loess, which has been notoriously difficult to date at this resolution and provides a basis for inter-regional correlation of Chinese dust records. We observe a high baseline of dust emissions from Arabia and China, even through interglacials, with strong superimposed millennial-scale variability. Conversely, the distal EPICA Dome C Antarctic ice core record, which is widely used to calculate the radiative impact of dust variations, appears biased to sharply delineated glacial/interglacial contrasts. Calculations based on this Antarctic dust record will therefore overestimate the radiative contrast of atmospheric dust loadings on glacial/interglacial timescales. Additional differences between Arabian/Asian and circum-Saharan records reveal that climate models could be improved by avoiding ‘global mean’ dust considerations and instead including large-scale regions with different dust source variability.     

Internal structure of a trough blowout, determined from migrated ground-penetrating radar profiles

Ground‐penetrating radar (GPR) was used to investigate the relationship between the geomorphological development of a large aeolian trough blowout and the stratigraphy and internal sedimentary structure of its associated deposits. Although analogous, many of the data‐processing techniques routinely applied in seismic reflection are very rarely applied in GPR studies. In this study, a simple migration program was used that significantly enhanced the quality of GPR images from a large trough blowout at Raven Meols on the Sefton coast, northwest England. These improvements aided subsequent data interpretation, which was achieved through application of the principles of radar stratigraphy. GPR shows the pre‐blowout dunes to have a complex internal structure that suggests they were formed in the presence of at least a partial vegetation cover. Subsequent to stabilization of these dunes a thin soil developed. This dune soil forms an important radar sequence boundary and delineates a complex topography beneath the depositional lobe of the blowout. The internal structure of the depositional lobe of the blowout does not conform to a model of simple radial foreset deposition, as derived from contemporary process studies reported in the literature. Instead, the pattern of deposition has been extensively modified by the antecedent dune topography and by varying spatial and temporal exposure to important sand‐transporting winds that is partly controlled by interactions between the regional wind pattern and local dune morphology. Trough blowout deposits in coastal aeolian sedimentary sequences are likely to be recognized by the presence of laterally continuous packets of relatively high‐angle cross‐strata, which often display a spatially‐variable radial dip pattern that is only very poorly or partially developed. In addition, a soil, or other surface representing a significant hiatus in dune deposition, is likely to underlie the blowout deposits, the topography of which will show a clear relationship to the dip and orientation of the overlying cross‐strata.     

Constraining slip rates and spacings for active normal faults

Numerous observations of extensional provinces indicate that neighbouring faults commonly slip at different rates and, moreover, may be active over different time intervals. These published observations include variations in slip rate measured along-strike of a fault array or fault zone, as well as significant across-strike differences in the timing and rates of movement on faults that have a similar orientation with respect to the regional stress field. Here we review published examples from the western USA, the North Sea, and central Greece, and present new data from the Italian Apennines that support the idea that such variations are systematic and thus to some extent predictable. The basis for the prediction is that: (1) the way in which a fault grows is fundamentally controlled by the ratio of maximum displacement to length, and (2) the regional strain rate must remain approximately constant through time. We show how data on fault lengths and displacements can be used to model the observed patterns of long-term slip rate where measured values are sparse. Specifically, we estimate the magnitude of spatial variation in slip rate along-strike and relate it to the across-strike spacing between active faults.     

Impedance spectroscopy of single and polycrystalline olivine: Evidence for grain boundary transport

The impedance spectra of single and polycrystalline olivine display two and three impedance arcs, respectively. Impedance spectra of single crystal olivine, polycrystalline olivine compacts, and natural dunite are compared to deduce the causes of the different impedance arcs. Variation of sample dimensions and use of three- and four-electrode configurations aid in the interpretation. The resistance of the two highest frequency mechanisms varies directly with the length to area ratio (l/A) of the sample. Experiments using the four-electrode configuration confirm that the lowest frequency impedance arc is caused by processes at the sample-electrode interface. In both single and polycrystalline samples the highest frequency mechanism is interpreted as bulk (grain interior) conduction, and the lowest frequency mechanism is attributed to sample-electrode interface effects. In the polycrystalline samples, the intermediate frequency mechanism is interpreted as the grain boundary conduction mechanism. The resistances of the grain interior and grain boundary mechanisms add in a series manner.