East Asia winter monsoon variations on a millennial time‐scale before the last glacial–interglacial cycle

Grain size and magnetic susceptibility measurements on samples from a typical loess–palaeosol sequence on the central Chinese Loess Plateau are used to reconstruct the Pleistocene East Asian monsoon climate. The coarse‐grained fraction, i.e. the weight percentage > 30 μm of the bulk grain‐size distribution, is used as a sensitive proxy index of the East Asia winter monsoon strength. On the basis of an absolute time‐scale, time‐series variations of this proxy show that winter monsoon strengths varied on millennial time‐scales during the periods 145–165, 240–280, 320–350, 390–440, 600–640, 860–890, 900–930 and 1330–1400 kyr BP. The wavelength of these climatic oscillations varied between 1.89 and 4.0 kyr, as is shown by spectral analysis using the multitaper method. Although numerical simulation experiments show that high frequencies also can arise from measurement errors in the grain‐size analysis, the frequencies prove to be sufficiently stable when the spectral analysis is repeated with a different number of tapers. For the time being, we do not correlate these climatic oscillations with palaeoclimatic records in the North Atlantic deep‐sea sediments because both time‐scales need to be further improved. Our data, however, certainly demonstrate that millennial‐scale East Asian winter monsoon variations in the last 1.4 million years can be detected from terrestrial loess records. Copyright © 1999 John Wiley & Sons, Ltd.     

The last Welsh Ice Cap: Part 1 – Modelling its evolution,sensitivity and associated climate

A high‐resolution, three‐dimensional, thermomechanical ice‐flow model is used to investigate the glaciodynamics of the Last Glacial Maximum Welsh Ice Cap – a large, independent ice centre of the British–Irish Ice Sheet. The model uses higher‐order physics to solve longitudinal stresses, and is coupled to climate via a distributed, positive degree‐day mass‐balance scheme. A suite of model experiments driven by the GISP2 δ¹⁸O curve was initiated from a climatic optimum at 38.3 ka BP through to the Devensian/Holocene boundary to identify an icecap configuration compatible with available empirical evidence. An enhanced cooling from present of 11.85°C and strong precipitation suppression are required between 27.4 and 23.5 ka BP for the modelled icecap to attain well‐established empirical limits, a scenario probably associated with Heinrich Event‐2 and the potential collapse of thermohaline circulation in the North Atlantic. The experiments indicate ice‐dispersal centres located in North and Mid Wales, the latter being essential for forcing ice southwards of the Brecon Beacons during the Last Glacial Maximum. Deglaciation of the Welsh Ice Cap was relatively rapid, occurring within one millennium. Dynamic stability is governed largely by the dominance and vigour with which fast‐flowing outlet glaciers drain the icecap interior, which in turn are linked to variations in the climatic forcing. The distribution of permanently cold‐based ice across the uplands and summits indicates the probable preservation of relict landscapes in these areas throughout the full glacial cycle.     

The last deglaciation of the Norwegian Channel – geomorphology,stratigraphy and radiocarbon dating

Based on high‐resolution TOPAS acoustic data, bathymetric data sets and sediment cores from the Norwegian Channel, the last retreat of the Norwegian Channel Ice Stream has been investigated. Mapping of ice‐marginal features such as grounding‐zone wedges and terminal moraines off western Norway suggest that the retreat of the grounding line in this part of the channel was interrupted by frequent stillstands, whereas the channel south of the threshold at Jæren does not have crossing ice‐marginal landforms. Three main seismic units have been identified, and, based on their seismic characteristics, in addition to study of sediment cores, these units are interpreted as till (U1), glacial marine sediment (U2) and Holocene hemipelagic sediment (U3). Based on new and published radiocarbon dates of the lower part of U2, combined with dates from the adjacent areas, it is concluded that the grounding line started to retreat from the shelf edge at about 19 ka and that the inner part of Skagerrak was ice free at 17.6 ka. This gives an average retreat rate of 450 m a⁻¹, which is generally higher than mean retreat rates estimated for other palaeo‐ice streams (15–310 m a⁻¹).     

Response of faults to climate‐induced changes of ice sheets,glaciers and lakes

Glacial–interglacial cycles are characterized by strong variations in climatic conditions, which affect the size of continental ice sheets, glaciers and lakes. Such climate‐triggered fluctuations in ice and water masses lead to transient stresses in the Earth's crust, which can be large enough to affect the slip behaviour of faults. In particular, postglacial unloading may increase the slip rate of active faults or re‐activate dormant faults. In the past, numerical modelling has helped to better understand the response of faults to mass fluctuations on Earth's surface. This article provides an overview of the mechanisms and controlling parameters of climate‐induced variations in fault slip as derived from the numerical models. Geological records of postglacial faulting from Scandinavia, the European Alps and the Basin‐and‐Range Province (western USA) are presented. Taken together, modelling and case studies provide a basis for evaluating the future seismic potential in regions that are currently experiencing ice loss or lake regression.     

Deglacial dynamics of the Vestfjorden – Trænadjupet palaeo‐ice stream,northern Norway

Few well‐dated records of the deglacial dynamics of the large palaeo‐ice streams of the major Northern Hemisphere ice sheets are presently available, a prerequisite for an improved understanding of the ice‐sheet response to the climate warming of this period. Here we present a transect of gravity‐core samples through Trænadjupet and Vestfjorden, northern Norway, the location of the Trænadjupet – Vestfjorden palaeo‐ice stream of the NW sector of the Fennoscandian Ice Sheet. Initial ice recession from the shelf break to the coastal area (~400 km) occurred at an average rate of about 195 m a⁻¹, followed by two ice re‐advances, at 16.6–16.4 ka BP (the Røst re‐advance) and at 15.8–15.6 ka BP (the Værøy re‐advance), the former at an estimated ice‐advance rate of 216 m a⁻¹. The Røst re‐advance has been interpreted to be part of a climatically induced regional cold spell while the Værøy re‐advance was restricted to the Vestfjorden area and possibly formed as a consequence of internal ice‐sheet dynamics. Younger increases in IRD content have been correlated to the Skarpnes (Bølling – Older Dryas) and Tromsø – Lyngen (Younger Dryas) Events. Overall, the decaying Vestfjorden palaeo‐ice stream responded to the climatic fluctuations of this period but ice response due to internal reorganization is also suggested. Separating the two is important when evaluating the climatic response of the ice stream. As demonstrated here, the latter may be identified using a regional approach involving the study of several palaeo‐ice streams. The retreat rates reported here are of the same order of magnitude as rates reported for ice streams of the southern part of the Fennoscandian Ice Sheet, implying no latitudinal differences in ice response and retreat rate for this ~1000 km² sector of the Fennoscandian Ice Sheet (~60–68°N) during the climate warming of this period.     

The last Welsh Ice Cap: Part 2 – Dynamics of a topographically controlled icecap

During the Last Glacial Maximum, the British–Irish Ice Sheet was dominated by a number of accumulation centres, including a terrestrially based, semi‐independent icecap centred on Wales. The dynamics of this Welsh Ice Cap (WIC) over the last glacial period are still relatively poorly understood, with few studies taking into consideration the dynamic evolution of the icecap as a whole. Here we contrast results from two modelled reconstructions of the WIC in conjunction with the wider glacial geomorphological record to elucidate understanding of its form, extent and dynamics. Model output was analysed to yield zones of high basal motion and the spatial distribution of potential glacial erosion. We conclude that coherent flowsets of streamlined bedforms are linked to fast‐flowing outlets dominated by basal sliding. Large‐scale changes in dynamics are discussed, with a number of possible major advances proposed over the glacial cycle. Maximum ice thicknesses of ～1200 m in Mid Wales indicate that all mountain summits were probably ice‐covered during the Last Glacial Maximum, even if it was with a thin protective mantle of cold‐based ice, leading to landscape preservation of these upland zones. The distribution, dynamism and landscape modification related to the WIC are further discussed at the regional scale. Model predictions of glacier distribution through the Younger Dryas stadial accord well with geologically reconstructed limits at this time.     

Macro‐ and micro‐sedimentology of a modern melt‐out till – Matanuska Glacier,Alaska, USA

The macro‐ and micro‐sedimentology of a supraglacial melt‐out till forming at the Matanuska Glacier was examined in relationship to the properties of the stratified basal zone ice and debris from which it is originating. In situ melting of the basal ice has produced a laminated to bedded diamicton consisting mainly of silt. Macroscopic properties include: discontinuous laminae and beds; lenses of sand, silt aggregates and open‐work gravel; deformed and elongate clasts of clay; widely dispersed pebbles and cobbles, those that are prolate usually with their long axes subparallel to parallel to the bedding. Evidence for deformation is absent except for localized bending of beds over or under rock clasts. Microscopic properties are a unique element of this work and include: discontinuous lineations; silt to granule size laminae; prolate coarse sand and rock fragments commonly with their long axis subparallel to bedding; subangular to subrounded irregular shaped clay clasts often appearing as bands; sorted and unsorted silt to granule size horizons, sometimes disrupted by pore‐water pathways. Limited deformation occurs around rock clasts and thicker parts of lamina. This study shows that in situ melting of debris‐rich basal ice can produce a laminated and bedded diamicton that inherits and thereby preserves stratified basal ice properties. Production and preservation of supraglacial melt‐out till require in situ melting of a stagnant, debris‐rich basal ice source with a low relief surface that becomes buried by a thick, stable, insulating cover of ice‐marginal sediment. Also required are a slow melt rate and adequate drainage to minimize pore‐water pressures in the till and overlying sediment cover to maintain stability and uninterrupted deposition. Many modern and ancient hummocky moraines down glacier of subglacial overdeepenings probably meet these process criteria and their common occurrence suggests that both modern and pre‐modern supraglacial melt‐out tills may be more common than previously thought.     

The role of megatides and relative sea level in controlling the deglaciation of the British–Irish and Fennoscandian ice sheets

Key external forcing factors have been proposed to explain the collapse of ice sheets, including atmospheric and ocean temperatures, subglacial topography, relative sea level and tidal amplitudes. For past ice sheets it has not hitherto been possible to separate relative sea level and tidal amplitudes from the other controls to analyse their influence on deglaciation style and rate. Here we isolate the relative sea level and tidal amplitude controls on key ice stream sectors of the last British–Irish and Fennoscandian ice sheets using published glacial isostatic adjustment models, combined with a new and previously published palaeotidal models for the NE Atlantic since the Last Glacial Maximum (22 ka BP). Relative sea level and tidal amplitude data are combined into a sea surface elevation index for each ice stream sector demonstrating that these controls were potentially important drivers of deglaciation in the western British Irish Ice Sheet ice stream sectors. In contrast, the Norwegian Channel Ice Stream was characterized by falling relative sea level and small tidal amplitudes during most of the deglaciation. As these simulations provide a basis for observational field testing we propose a means of identifying the significance of sea level and tidal amplitudes in ice sheet collapse.     

Diatom‐based reconstruction of summer sea‐surface salinity in the South China Sea over the last 15 000 years

We present a new reconstruction of summer sea‐surface salinity (SSS) over the past 15 000 years based on a diatom record from piston core 17940, located on the northern slope of the South China Sea (SCS). The reconstructed diatom‐based summer SSS values for the modern period are in accord with instrumental observations of summer SSS in the area. Here, the modern summer SSS is primarily controlled by river runoff, in particular from the Pearl River. The reconstruction presented in this study shows that the summer SSS varied between 33.3 and 34.2 psu over the past 15 000 years. The long‐term summer SSS trend closely followed the trend of the orbitally controlled solar insolation at 20°N, suggesting that orbital forcing was the dominant driver of changes in summer SSS in this area. Comparisons to speleothem δ¹⁸O data and studies of surface hydrography in the region suggest that changes in solar insolation affected the summer SSS through changes in the East Asian Monsoon and sea‐level changes associated with the last deglaciation. Univariate spectral analyses indicate that centennial‐scale oscillatory variations in summer SSS were superimposed on the long‐term trend. During the deglacial period (c. 12 000–9000 cal. a BP), the dominant periodicity was centred around 230–250 years, whereas a ～350‐year oscillation dominated in the period 2200–4500 cal. a BP. The balance of evidence suggests that these centennial‐scale changes in summer SSS may have been driven by solar‐induced changes in the East Asian Monsoon, but further evidence is needed to firmly establish this relationship.     

The Barents Sea Ice Sheet — A model of its growth and decay during the last ice maximum

On the basis of geomorphological and sedimentological data, we believe that the entire Barents Sea was covered by grounded ice during the last glacial maximum. ¹⁴C dates on shells embedded in tills suggest marine conditions in the Barents Sea as late as 22 ka BP; and models of the deglaciation history based on uplift data from the northern Norwegian coast suggest that significant parts of the Barents Sea Ice Sheet calved off as early as 15 ka BP. The growth of the ice sheet is related to glacioeustatic fall and the exposure of shallow banks in the central Barents Sea, where ice caps may develop and expand to finally coalesce with the expanding ice masses from Svalbard and Fennoscandia.The outlined model for growth and decay of the Barents Sea Ice Sheet suggests a system which developed and existed under periods of maximum climatic deterioration, and where its growth and decay were strongly related to the fall and rise of sea level.     

BRITICE Glacial Map,version 2: a map and GIS database of glacial landforms of the last British–Irish Ice Sheet

During the last glaciation, most of the British Isles and the surrounding continental shelf were covered by the British–Irish Ice Sheet (BIIS). An earlier compilation from the existing literature (BRITICE version 1) assembled the relevant glacial geomorphological evidence into a freely available GIS geodatabase and map (Clark et al. 2004: Boreas 33, 359). New high‐resolution digital elevation models, of the land and seabed, have become available casting the glacial landform record of the British Isles in a new light and highlighting the shortcomings of the V.1 BRITICE compilation. Here we present a wholesale revision of the evidence, onshore and offshore, to produce BRITICE version 2, which now also includes Ireland. All published geomorphological evidence pertinent to the behaviour of the ice sheet is included, up to the census date of December 2015. The revised GIS database contains over 170 000 geospatially referenced and attributed elements – an eightfold increase in information from the previous version. The compiled data include: drumlins, ribbed moraine, crag‐and‐tails, mega‐scale glacial lineations, glacially streamlined bedrock (grooves, roches moutonnées, whalebacks), glacial erratics, eskers, meltwater channels (subglacial, lateral, proglacial and tunnel valleys), moraines, trimlines, cirques, trough‐mouth fans and evidence defining ice‐dammed lakes. The increased volume of features necessitates different map/database products with varying levels of data generalization, namely: (i) an unfiltered GIS database containing all mapping; (ii) a filtered GIS database, resolving data conflicts and with edits to improve geo‐locational accuracy (available as GIS data and PDF maps); and (iii) a cartographically generalized map to provide an overview of the distribution and types of features at the ice‐sheet scale that can be printed at A0 paper size at a 1:1 250 000 scale. All GIS data, the maps (as PDFs) and a bibliography of all published sources are available for download from: https://www.sheffield.ac.uk/geography/staff/clark_chris/britice .     

The origin of chloritoid – 3‐mica pseudomorph growth in staurolite–muscovite schist,Bangriposi (Eastern India)

At Bangriposi, variable stages in replacement of staurolite by chloritoid – Na–K–Ca mica shimmer aggregates in muscovite schists provides insight into the complex interplay between fluid flow, mass transfer, and dissolution–precipitation during pseudomorph growth. Idioblastic chloritoid growing into mica caps without causing visible deformation, and monomineralic chloritoid veins (up to 300 μm wide) within shimmer aggregates replacing staurolite attest to chloritoid nucleation in fluid‐filled conduits along staurolite grain boundaries and crystallographic planes. The growth of shimmer aggregates initiated along staurolite margins, and advanced inwards into decomposing staurolite along networks of crystallographically controlled fluid‐filled conduits. Coalescence among alteration zones adjacent to channel fills led to dismemberment and the eventual demise of staurolite. Mass balance calculation within a volume‐fixed, silica‐conserved reference frame indicate the shimmer aggregates grew via precipitation from fluids in response to mass transport that led to the addition of H₂O, K₂O, Na₂O and CaO in the reaction zone, and Al₂O₃ was transported outward from the inward‐retreating margin of decomposing staurolite. This aided precipitation of chloritoid in veins and in the outer collars, and as disseminated grains in the shimmer aggregates at mid‐crustal condition (~520 ± 20 °C, 5.5 ± 2.0 kbar). Computation using one‐dimensional transport equation suggests that staurolite decomposition involved advection dominating over diffusive transport; the permeation of externally derived H₂O caused flattening of chemical potential gradients in H₂O and aqueous species, for example, and , computed using the Gibbs method. This suggests that staurolite decomposition was promoted by the infiltration of a large volume of H₂O that flattened existing chemical potential gradients. In the initial stages of replacement, chloritoid super‐saturation in fluid caused preferential nucleation and growth of chloritoid at staurolite grain boundaries and in crystallographic planes. As reaction progressed, further chloritoid nucleation was halted, but chloritoid continued to grow as the 3‐mica aggregates continued to replace the remaining staurolite in situ, while the chloritoid‐compatible elements were transported in the water‐rich phase facilitating continued growth of the existing chloritoid grains.     

The time–deflection behaviour of a rigid under-reamed anchor in a deep clay layer

The behaviour of an under-reamed anchor in a consolidating soil is examined by approximating the anchor as an impermeable circular plate embedded in a deep soil layer. Hankel and Laplace transforms are applied to the equations governing the consolidation process, and this greatly simplifies the equations, allowing a solution to be obtained for the transformed variables. Numerical inversion of both the Hankel and Laplace transforms is used to obtain the solution at any time. A particular feature of the solution method is that the unknown contact stress between the anchor and the soil and the unknown flows in the plane of the anchor are approximated as a series of simple functions with unknown coefficients. By determining the coefficients of the terms in the series, the complete solution may be found. Computations have been carried out using the method proposed, and results are presented for the time–settlement behaviour of an impermeable anchor. These results are compared with some published and some recomputed finite element data, and this highlights some of the difficulties encountered in using such numerical techniques.     

A major ice drainage pathway of the last British–Irish Ice Sheet: the Tyne Gap,northern England

The Tyne Gap is a wide pass, situated between the Scottish Southern Uplands and the English Pennines that connects western and eastern England. It was a major ice flow drainage pathway of the last British–Irish Ice Sheet. This study presents new glacial geomorphological and sedimentological data from the Tyne Gap region that has allowed detailed reconstructions of palaeo‐ice flow dynamics during the Late Devensian (Marine Isotope Stage 2). Mapped lineations reveal a complex palimpsest pattern which shows that ice flow was subject to multiple switches in direction. These are summarised into three major ice flow phases. Stage I was characterised by convergent Lake District and Scottish ice that flowed east through the Tyne Gap, as a topographically controlled ice stream. This ice stream was identified from glacial geomorphological evidence in the form of convergent bedforms, streamlined subglacial bedforms and evidence for deformable bed conditions; stage II involved northerly migration of the Solway Firth ice divide back into the Southern Uplands, causing the easterly flow of ice to be weakened, and resulting in southeasterly flow of ice down the North Tyne Valley; and stage III was characterised by strong drawdown of ice into the Irish Sea Ice Basin, thus starving the Tyne Gap of ice and causing progressive ice sheet retreat westwards back across the watershed, prior to ice stagnation. Copyright © 2009 John Wiley & Sons, Ltd.