Supporting evidence from the New York drumlin field that elongate subglacial bedforms indicate fast ice flow

Although drumlins and other subglacial bedforms are well-studied features, controls on their formation and morphometry have remained elusive. Of current interest is the hypothesis that elongate bedforms (length:width ratios ≥ 10) indicate fast ice flow, and perhaps the location of past ice streams. This hypothesis is explored by analysing drumlins from the New York State drumlin field. A subset of 548 drumlins between Oneida Lake and Lake Ontario was digitized using 10-m grid cell digital elevation data. Because bedform elongation is greatest along the axis of a reconstructed lobe and increases down flowline, elongate bedforms are best explained by fast ice flow. The swath of elongate bedforms between lakes Ontario and Oneida, the boundaries of which do not coincide with topography, may signify the location of an ice stream during deglaciation.     

The meltwater hypothesis for subglacial bedforms

This paper presents an historical and in places informal account of the meltwater hypothesis, which invokes enormous outburst floods for the formation of subglacial bedforms. It begins with a brief discussion of the difficulties of determining processes of formation for landforms, which are not seen in formation. Analogy provides a solution to these difficulties. Analogy between erosional marks at the bases of turbidites and drumlins, which were the starting point for this hypothesis, rests on the idea that inverted erosional marks at the ice bed are subsequently infilled to form drumlins. Field tests on the sedimentology, architecture, and landform associations of drumlins in the Livingstone Lake drumlin field are outlined before more extensive work on bedrock erosional forms and flood routes is introduced. Bedrock erosional forms played a central part in establishing the hypothesis since their form and ornamentation are confidently interpreted as fluvial. Their form and genesis are discussed mainly with reference to sites at French River and Wilton Creek, Ontario, though some remarkable bedrock erosional forms in Antarctica support their regional extent. Initially in the meltwater hypothesis, drumlins were thought to be cavity fills and erosional drumlins were recognized later. This development is shown to be central to the realization that drumlin composition may be inferred from drumlin form. The scale of drumlin fields, measured at about 10³ km², and the magnitude of the inferred floods require that the flood events were regional. Regional-scale flood tracts in Ontario, Quebec, Alberta and the Northwest Territories extending over 1000 km in length and several hundred kilometers in width, support this suggestion. Floods, had they occurred, would have caused rapid rates of sea level rise and may have changed climate through their effects on ocean stratification and sea surface temperatures. The meltwater hypothesis covers a range of bedforms besides drumlins and bedrock erosional marks—fluting, Rogen moraine, hummocky terrain, and transverse ridges. Recent work shows how these forms are best explained by the meltwater hypothesis. The roles of water storage and release, which underpin the theory of the meltwater hypothesis, remain poorly understood.     

Size and shape characteristics of drumlins,derived from a large sample,and associated scaling laws

Ice sheets flowing across a sedimentary bed usually produce a landscape of blister-like landforms streamlined in the direction of the ice flow and with each bump of the order of 10² to 10³ m in length and 10¹ m in relief. Such landforms, known as drumlins, have mystified investigators for over a hundred years. A satisfactory explanation for their formation, and thus an appreciation of their glaciological significance, has remained elusive. A recent advance has been in numerical modelling of the land-forming process. In anticipation of future modelling endeavours, this paper is motivated by the requirement for robust data on drumlin size and shape for model testing.From a systematic programme of drumlin mapping from digital elevation models and satellite images of Britain and Ireland, we used a geographic information system to compile a range of statistics on length L, width W, and elongation ratio E (where E = L/W) for a large sample. Mean L, is found to be 629 m (n = 58,983), mean W is 209 m and mean E is 2.9 (n = 37,043). Most drumlins are between 250 and 1000 metres in length; between 120 and 300 metres in width; and between 1.7 and 4.1 times as long as they are wide. Analysis of such data and plots of drumlin width against length reveals some new insights. All frequency distributions are unimodal from which we infer that the geomorphological label of ‘drumlin’ is fair in that this is a true single population of landforms, rather than an amalgam of different landform types. Drumlin size shows a clear minimum bound of around 100 m (horizontal). Maybe drumlins are generated at many scales and this is the minimum, or this value may be an indication of the fundamental scale of bump generation (‘proto-drumlins’) prior to them growing and elongating. A relationship between drumlin width and length is found (with r² = 0.48) and that is approximately W = 7 L ^1/2 when measured in metres. A surprising and sharply-defined line bounds the data cloud plotted in E–W–L space, and records a scale-dependent maximum elongation limit (approximated by E_max = L^1/3, when L measured in metres). For a given length, for some reason as yet unknown, drumlins do not exceed the elongation ratio defined by this scaling law. We also report and compare our statistics to an amalgamated sample (25,907 drumlins) of measures derived from around 50 published investigations. Any theory must be able to explain the drumlin statistics and fundamental scaling properties reported herein and they thus provide powerful tests for drumlin modelling.     

Bedform topography and basal conditions beneath a fast-flowing West Antarctic ice stream

A grid of seismic reflection lines has been used to image basal topography and infer basal conditions and flow processes beneath ～140 km² of Rutford Ice Stream, West Antarctica. The subglacial topography in this region consists of two troughs flanking a central high and the bed is composed of water-saturated sediments. The two troughs are filled with deforming sediment, whereas the bed in the central region appears to undergo a transition from largely deforming conditions upstream to basal sliding downstream. The deforming bed is very flat along flow, but undulates across flow. Sliding areas show rougher bed topography. Cross-stream bed topography is characterised by streamlined mounds of deforming sediment aligned in the ice flow direction. These bedforms occur superimposed on the bed in regions of both basal sliding and sediment deformation. In places, they form finger-like mounds of material, which extend into the sliding region further downstream. Mean bedform height is 22 m, mean width is 267 m, and many of them extend for at least 1–2 km along flow. We interpret most of these bedforms as drumlins and one as a mega-scale glacial lineation. The juxtaposition of different basal conditions is consistent with models proposed from terrestrial studies in which the glacier bed is a mosaic of stable and deforming bed areas, variable both spatially and temporally. Any theory of subglacial sediment rheology must also be able to account for our conclusion that, at any given time, pervasive deformation extends at least a few metres into the bed and can persist over a considerable area (many km²). Bedform geometry and basal conditions concur with interpretations of former ice streams, with evidence for increasing elongation ratio with distance downstream. However, those studies also identified bedrock cropping out at the ice-bed interface, for which there is no evidence on Rutford Ice Stream.     

Subglacial bedforms and conditions associated with the 1991 surge of Skeiðarárjökull, Iceland

Much previous research at surge-type glaciers has sought to identify features diagnostic of surge-type behaviour. However, in comparatively little work have subglacial landform–sediment characteristics been used to reconstruct changing basal processes and conditions during surge events. Subglacial bedforms described in this article are associated with the 1991 surge of Skeiðarárjökull, Iceland, and include a series of drumlins with superimposed flutes and basal crevasse-fill ridges. The drumlins were formed by the subglacial erosion of ice-contact fans. Sedimentary evidence indicates a shift from rigid-bed to soft-bed conditions during the surge. The presence of eroded but undeformed fan sediments suggests that they acted as a rigid bed when initially overridden. Subsequent deposition of a layer of deformation till resulted in a change to soft-bed conditions and the generation of flutes and subglacial crevasse-fill ridges. The lack of mixing between this till and the underlying stratified sediments indicates that subglacial sediment deformation was restricted to a thin layer and that its deposition resulted in a cessation of subglacial erosion. The drumlin is therefore a composite of both rigid-bed and soft-bed processes that illustrates changes in basal conditions and processes during the course of the event. The limited time frame in which the drumlin formed and the presence of kettleholes across its surface are distinctive features that may warrant further investigation in the search for features diagnostic of past surge events.     

Regional reconstruction of subglacial hydrology and glaciodynamic behaviour along the southern margin of the Cordilleran Ice Sheet in British Columbia,Canada and northern Washington State,USA

Subglacial landsystems in and around Okanagan Valley, British Columbia, Canada are investigated in order to evaluate landscape development, subglacial hydrology and Cordilleran Ice Sheet dynamics along its southern margin. Major landscape elements include drumlin swarms and tunnel valleys. Drumlins are composed of bedrock, diamicton and glaciofluvial sediments; their form truncates the substrate. Tunnel valleys of various scales (km to 100s km length), incised into bedrock and sediment, exhibit convex longitudinal profiles, and truncate drumlin swarms. Okanagan Valley is the largest tunnel valley in the area and is eroded >300 m below sea level. Over 600 m of Late Wisconsin-age sediments, consisting of a fining-up sequence of cobble gravel, sand and silt fill Okanagan Valley. Landform–substrate relationships, landform associations, and sedimentary sequences are incompatible with prevailing explanations of landsystem development centred mainly on deforming beds. They are best explained by meltwater erosion and deposition during ice sheet underbursts.During the Late-Wisconsin glaciation, Okanagan Valley functioned as part of a subglacial lake spanning multiple connected valleys (few 100s km) of southern British Columbia. Subglacial lake development started either as glaciers advanced over a pre-existing sub-aerial lake (catch lake) or by incremental production and storage of basal meltwater. High geothermal heat flux, geothermal springs and/or subglacial volcanic eruptions contributed to ice melt, and may have triggered, along with priming from supraglacial lakes, subglacial lake drainage. During the underburst(s), sheetflows eroded drumlins in corridors and channelized flows eroded tunnel valleys. Progressive flow channelization focused flows toward major bedrock valleys. In Okanagan Valley, most of the pre-glacial and early-glacial sediment fill was removed. A fining-up sequence of boulder gravel and sand was deposited during waning stages of the underburst(s) and bedrock drumlins in Okanagan Valley were enhanced or wholly formed by this underburst(s).Subglacial lake development and drainage had an impact on ice sheet geometry and ice volumes. The prevailing conceptual model for growth and decay of the CIS suggests significantly thicker ice in valleys compared to plateaus. Subglacial lake development created a reversal of this ice sheet geometry where grounded ice on plateaus thickened while floating valley ice remained thinner (due to melting and enhanced sliding, with significant transfer of ice toward the ice sheet margin). Subglacial lake drainage may have hastened deglaciation by melting ice, lowering ice-surface elevations, and causing lid fracture. This paper highlights the importance of ice sheet hydrology: its control on ice flow dynamics, distribution and volume in continental ice masses.     

Quantitative assessment of the hydraulic role of subglaciofluvial interbeds in promoting deposition of deformation till (Northern Till,Ontario)

The Northern Till is a thick (>65 m) deformation till underlying some 7500 km² of Southern Ontario, Canada including the Peterborough Drumlin Field. It was deposited below the Lake Ontario ice stream of the Laurentide Ice Sheet. The till rests on glaciotectonized aquifer sediments and consists of multiple beds of till up to 6 m thick. These are separated by boulder lags, sometimes in the form of striated pavements, with thin (<30 cm) interbeds of poorly sorted waterlaid sand. The composite till stratigraphy indicates ‘punctuated aggradation’ where the subglacial bed was built up incrementally by the repeated ‘immobilization’ of deforming overpressured till layers. Boulders and sands indicate pauses in subglacial aggradation marked by sluggish sheet flows of water that reworked the top of the underlying till. Interbeds are laterally extensive and correlated using downhole electrical conductivity, core recovery and natural gamma data. A 3-D finite element model (FEFLOW) using data from 200 cored and geophysically logged boreholes, and a large digital water well dataset of 3400 individual records shows that the till functions as a ‘leaky aquitard’ as a consequence of water flow through interbeds. It is proposed that interbeds played a similar role in the subglacial hydraulic system below the Laurentide Ice Sheet by allowing drainage of excess porewater pressures in deforming sediment and promoting deposition of till. This is in agreement with theoretical studies of deforming bed dynamics and observations at modern glaciers where porewater in the deforming layer is discharged into underlying aquifers. In this way, the presence of interbeds may be fundamental in retarding downglacier transport of deforming bed material thereby promoting the build-up of thick subglacial till successions.     

Late Pleistocene glaciolacustrine sedimentation and paleogeography of southeastern Michigan,USA

The geomorphic, stratigraphic and sedimentological characteristics of glaciolacustrine sediments in the metropolitan Detroit, Michigan area were studied to determine environments of deposition and make paleogeographic reconstructions. Nine lithofacies were identified and paleoenvironments interpreted based on their morphostratigraphic relationships with relict landforms. The sediments studied are found southeast of the Defiance and Birmingham moraines lying beneath a lowland characterized by a low morainal swell (Detroit moraine) and a series of lacustrine terraces that descend progressively in elevation southeastward. The glaciolacustrine sediments were deposited approximately 14.3–12.4 kA BP during the Port Bruce and Port Huron glacial phases of late Wisconsinan time, and are related to proglacial paleolakes Maumee, Arkona, Whittlesey, Warren, Wayne, Grassmere, Lundy and Rouge. The glaciolacustrine section is typically 2–4 m thick and consists of a basal unit of wavy-bedded clayey diamicton overlain by a surficial deposit of stratified and cross-stratified sand and gravel. The basal unit is comprised of subaqueous debris flow deposits that accumulated as subaqueous moraine in paleolake Maumee along the retreating front of the Huron lobe. The surficial deposits of sand and gravel were formed by traction, resulting from lacustrine wave activity and fluvial processes, in lakebed plain, beach ridge and deltaic depositional settings. Much of the lake-margin sand and gravel was derived from clayey diamicton by lacustrine wave action and winnowing, and that associated with paleolakes of the Port Huron phase is largely reworked Port Bruce sediment. Paleogeographic reconstructions show that the Defiance, Birmingham and Detroit moraines, Defiance and Rochester channels, and the Rochester delta, were deposited penecontemporaneously as paleolake Maumee expanded northward across the map area. A unique type of wavy bedform is characteristic of clayey diamicton deposited by subaqueous mass flow in the study area that is useful for differentiating sediment: 1) deposited by mass flow in subaqueous vs. subaerial settings, and 2) deposited by subaqueous mass flow vs. basal till. These bedforms are a useful tool for identifying subglacial meltwater deposits, and facilitate the mapping and correlation of glacial sediments based on till sheets. The map area provides a continental record of ice sheet dynamics along the southern margin of the Laurentide ice sheet during Heinrich event H-1. The record reveals rapid glacial retreat (～ 0.8 km/yr) contemporaneous with the discharge of a large volume of meltwater. Evidence in the study area for subglacial meltwater is problematic, but indications that periglacial conditions persisted in the map area until ～ 12.7 kA BP, and extended for 200 km or more south of the ice front suggest that a frozen substrate may have contributed to instability of the LIS.     

Subglacial and ice‐marginal landforms in south‐central Ontario: implications for ice‐sheet reconfiguration during deglaciation

Regional‐scale, high‐resolution terrain data permit the study of landforms across south‐central Ontario, where the bed of the former Laurentide Ice Sheet is well exposed and passes downflow from irregular topography on Precambrian Shield highlands to flat‐lying Palaeozoic carbonate bedrock, and thick (50 to >200 m) unconsolidated sediment substrates. Rock drumlins and megagrooves are eroded into bedrock and mega‐scale glacial lineations (MSGL) occur on patchy streamlined till residuals in the Algonquin Highlands. Downflow, MSGL pass into juxtaposed rock and drift drumlins on Palaeozoic bedrock and predominantly till‐cored drumlins in areas of thick drift. The Lake Simcoe Moraines, now traceable for more than 80 km across the Peterborough drumlin field (PDF), form a distinct morphological boundary: downflow of the moraine system, drumlins are larger, broader and show no indication of subsequent reworking by the ice, whereas upflow of the moraines, a higher degree of complexity in bedform pattern and morphology is distinguished. Discrete radial and/or cross‐cutting flowset terminate at subtle till‐cored moraine ridges downflow of local topographic lows, indicating multiple phases of late‐stage ice flow with strong local topographic steering. More regional‐scale flow switching is evident as NW‐orientated bedforms modify drumlins south of the Oak Ridges Moraine, and radial flowset emanate from areas within the St. Lawrence and Ottawa River valleys. Most of the drumlins in the PDF formed during an early, regional drumlinization phase of NE–SW flow that followed the deposition of a thick regional till sheet. These were subsequently modified by local‐scale, topographically controlled flows that terminate at till‐cored moraines, providing evidence that the superimposed bedforms record dynamic ice (re)advances throughout the deglaciation of south‐central Ontario. The patterns and relationships of glacial landform distribution and characteristics in south‐central Ontario hold significance for many modern and palaeo‐ice sheets, where similar downflow changes in bed topography and substrate lithology are observed.     

Are long subglacial bedforms indicative of fast ice flow?

It has been suggested that extremely long subglacial bedforms (e.g. attenuated drumlins and mega-scale glacial lineations) record former areas of fast-flowing ice and that bedform elongation ratio is a useful proxy for ice velocity. Despite the availability of much data pertaining to the measurement and analysis of subglacial bedforms, these assumptions have rarely been explicitly addressed in detail. In this paper, we demonstrate that long subglacial bedforms (length:width ratios ≥10:1) are indicative of fast ice flow. Using satellite imagery, we mapped over 8000 lineaments associated with a highly convergent flow pattern near Dubawnt Lake, District of Keewatin, Canada. This flow pattern is unusual in that it displays a large zone of convergence feeding into a main 'trunk' and then diverging towards the inferred ice margin. The 'bottleneck' pattern is taken to record an increase and subsequent decrease in ice velocity and we analysed transverse and longitudinal variations in bedform morphometry. The main trunk of the flow pattern (down-ice of the convergent zone) is characterized by mega-scale glacial lineations of great length (up to 13 km) and high elongation ratios (up to 43:1). The down-ice variations in elongation ratio reflect exactly what we would expect from a terrestrial ice stream whose velocity increases in the onset zone passes through a maximum in the main trunk and slows down as the ice diverges at the terminus. It is suggested that any unifying theory of drumlin formation must be able to account for the association between long subglacial bedforms and fast ice flow, although it is not assumed that fast ice flow always produces attenuated bedforms. A further implication of this work is that many more ice streams may be identified on the basis of attenuated subglacial bedforms, radically altering our views on the flow dynamics of former ice sheets.     

Fades of subglacial channel sedimentation in late-Pleistocene drumlins, Northern Ireland

The sedimentology of three sand-cored drumlins in central Ulster. Northern Ireland is described. The sand-cores of the drumlins consist of complex sequences of cohesive sediment gravity forms, grain-flow deposits, turbiditic sands and deltaic sands and gravels, showing complex fades relationships. The bulk of the sediments formed in water-filled cavities associated with a major subglacial meltwater escape route excavated in bedrock. The sand-cores are overlain by a streamlined carapace of basal melt-out till. A model is presented to account for spatial variations in faeies arrangement within the sand-cores. Three specific meltwater depositional subenvironments were recognised: (1) main channel axis, (2) main channel flank, and (3) minor tributary channel. The sand-cores formed prior to the main phase of basal melt-out till deposition and before drumlin streamlining.     

The palaeoglaciology of the central sector of the British and Irish Ice Sheet: reconciling glacial geomorphology and preliminary ice sheet modelling

Digital elevation models of the area around the Solway Lowlands reveal complex subglacial bedform imprints relating the central sector of the LGM British and Irish Ice Sheet. Drumlin and lineation mapping in four case studies show that glacier flow directions switched significantly through time. These are summarised in four major flow phases in the region: Phase I flow was from a dominant Scottish dispersal centre, which transported Criffel granite erratics to the Eden Valley and forced Lake District ice eastwards over the Pennines at Stainmore; Phase II involved easterly flow of Lake District and Scottish ice through the Tyne Gap and Stainmore Gap with an ice divide located over the Solway Firth; Phase III was a dominant westerly flow from upland dispersal centres into the Solway lowlands and along the Solway Firth due to draw down of ice into the Irish Sea basin; Phase IV was characterised by unconstrained advance of Scottish ice across the Solway Firth. Forcing of a numerical model of ice sheet inception and decay by the Greenland ice core record facilitates an assessment of the potential for rapid ice flow directional switching during one glacial cycle. The model indicates that, after fluctuations of smaller radially flowing ice caps prior to 30 ka BP, the ice sheet grows to produce an elongate, triangular-shaped dome over NW England and SW Scotland at the LGM at 19.5 ka BP. Recession after 18.5 ka BP displays a complex pattern of significant ice flow directional switches over relatively short timescales, complementing the geomorphologically-based assessments of palaeo-ice dynamics. The palaeoglaciological implications of this combined geomorphic and modelling approach are that: (a) the central sector of the BIIS was as a major dispersal centre for only ca 2.5 ka after the LGM; (b) the ice sheet had no real steady state and comprised constantly migrating dispersal centres and ice divides; (c) subglacial streamlining of flow sets was completed over short phases of fast flow activity, with some flow reversals taking place in less than 300 years.     

Exploring former subglacial Hodgson Lake,Antarctica. Paper II: palaeolimnology

Direct exploration of subglacial lakes buried deep under the Antarctic Ice Sheet has yet to be achieved. However, at retreating margins of the ice sheet, there are a number of locations where former subglacial lakes are emerging from under the ice but remain perennially ice covered. One of these lakes, Hodgson Lake (72°00.549′S, 068°27.708′W) has emerged from under more than 297–465 m of glacial ice during the last few thousand years. This paper presents data from a multidisciplinary investigation of the palaeolimnology of this lake through a study of a 3.8 m sediment core extracted at a depth of 93.4 m below the ice surface. The core was dated using a combination of radiocarbon, optically stimulated luminescence, and relative palaeomagnetic intensity dating incorporated into a chronological model. Stratigraphic analyses included magnetic susceptibility, clast provenance, organic content, carbonate composition, siliceous microfossils, isotope and biogeochemical markers. Based on the chronological model we provisionally assign a well-defined magnetic polarity reversal event at ca 165 cm in the lake sediments to the Mono Lake excursion (ca 30–34 ka), whilst OSL measurements suggest that material incorporated into the basal sediments might date to 93 ± 9 ka. Four stratigraphic zones (A–D) were identified in the sedimentological data. The chronological model suggests that zones A–C were deposited between Marine Isotope Stages 5–2 and zone A during Stage 1, the Holocene. The palaeolimnological record tracks changes in the subglacial depositional environment linked principally to changing glacier dynamics and mass transport and indirectly to climate change. The sediment composition in zones A–C consists of fine-grained sediments together with sands, gravels and small clasts. There is no evidence of overriding glaciers being in contact with the bed reworking the stratigraphy or removing this sediment. This suggests that the lake existed in a subglacial cavity beneath overriding LGM ice. In zone D there is a transition to finer grained sediments characteristic of lower energy delivery coupled with a minor increase in the organic content attributed either to increases in allochthonous organic material being delivered from the deglaciating catchment, a minor increase in within-lake production or to an analytical artefact associated with an increase in the clay fraction. Evidence of biological activity is sparse. Total organic carbon varies from 0.2 to 0.6%, and cannot be unequivocally linked to in situ biological activity as comparisons of δ¹³C and C/N values with local reference data suggest that much of it is derived from the incorporation of carbon in catchment soils and gravels and possibly old CO₂ in meteoric ice. We use the data from this study to provide guidelines for the study of deep continental subglacial lakes including establishing sediment geochronologies, determining the extent to which subglacial sediments might provide a record of glaciological and environmental change and a brief review of methods to use in the search for life.     

Boron content of Lake Ulubat sediment: A key to interpret the morphological history of NW Anatolia,Turkey

Freshwater Lake Ulubat (c. 1.5 m deep and c. 138 km²) receives sediment from a 10.414 km² area in the seismically active Susurluk Drainage Basin (SDB) of NW Turkey. The B and trace element contents of the lake infill seem to be a link between the fresh landforms of the SDB and the lacustrine sediment. Deposition in Lake Ulubat has been 1.60 cm a⁻¹ for the last 50 a according to radionucleides; however the sedimentation rate over the last millennium was 0.37 cm a⁻¹ based on ¹⁴C dating. The B content of the lacustrine infill displays a slight increase at 0.50 m and a drastic increase at 4 m depth occurring c. 31 a and c. 1070 a ago, respectively. Probably the topmost change corresponds to the start of open mining in the SDB and the second one to the natural trenching of borate ore-deposits. These dates also show indirectly a 1.4 cm a⁻¹ erosion rate during the last millennium as the borate beds were trenched up to 15 m. By extrapolation, it is possible to establish that the formation of some of the present morphological features of the southern Marmara region, especially river incision, began in the late Pleistocene, and developed especially over the last 75 ka.     

Lake ecosystem dynamics and links to climate change inferred from a stable isotope and organic palaeorecord from a mountain lake in southwestern China (ca. 22.6–10.5 cal ka BP)

A detailed understanding of long-term climatic and environmental change in southwestern China is hampered by a lack of long-term regional palaeorecords. Organic analysis (%TOC, %TN, C/N ratios and δ¹³C values) of a sediment sequence from Lake Shudu, Yunnan Province (ca. 22.6–10.5 cal ka BP) indicates generally low aquatic palaeoproductivity rates over millennial timescales in response to cold, dry climatic conditions. However, the record is punctuated by two marked phases of increased aquatic productivity from ca. 17.7 to 17.1 cal ka BP and from ca. 11.9 to 10.5 cal ka BP. We hypothesise that these shifts reflect a marked, stepwise lacustrine response to Asian summer monsoon strengthening during the last deglaciation.     

Variability of Quaternary glacial erosion rates – A global perspective with special reference to the Eastern Pyrenees

Glaciers erode bedrock but are also efficient conveyors of debris supplied during a cycle of glaciation by processes other than basal erosion. In this dual capacity as both an eroding and a transporting agent lies the ambiguity of ‘glacial erosion’ as a geomorphic process, with implications for methods of measuring the removal of rock mass by glaciers in the geological past, and for interpreting what exactly the consequences have been on topography and elevation change. A global review of ～400 Quaternary glacial denudation rates estimated from five different measurement techniques provides values ranging between 10^?4 and 10 mm yr^?1. We investigate the causes of such wide variability by examining the respective influences of environmental setting and methodological bias. A reference frame chosen for assessing these issues is the Massif du Carlit (Pyrenees, France), where a quantified mass balance of the well preserved glacial, periglacial and paraglacial deposits was made possible by detailed geomorphological mapping and terrestrial cosmogenic nuclide dating of extant erosional and depositional landform sequences. Resulting age brackets helped to define three main episodes of ice-cap growth and decline, each characterized by a volume of debris and a mappable source area. Erosion rates were expressed in two ways: (i) as spatially averaged denudation rates (D) during the successive stages of glacial advance to the line of maximum ice extent (MIE), post-MIE ice recession, and Lateglacial cirque readvance, respectively; and (ii) as cirque-wall recession rates (R) where moraine facies criteria indicated a supraglacial provenance of debris. Results indicate low erosion (D ≈ 0.05 mm yr^?1) during the ice advance phase, probably because of thin or passive ice covering the low-gradient subglacial topography that occurs just above the late Pleistocene equilibrium line altitude (2.2–2.4 km). Erosion rates peaked (D ≈ 0.6 mm yr^?1 and R ≈ 2.4–4.5 mm yr^?1) during the main transition to ice-free conditions, when deglacial debuttressing promoted the rapid response of freshly exposed slope systems to new equilibrium conditions in the steep crest zone. Lateglacial D- and R-values declined to 0.2–0.3 mm yr^?1, with indications of spatially variable R controlled by lithology. In this environment glaciers overall behaved more as conveyors of debris supplied by supraglacial rock exposures in the mountain crest zone than as powerful modifiers of subglacial topography. This explains the widespread preservation of deep, in situ preglacial weathering profiles on relict Cenozoic land surfaces in the deglacierized part of the Eastern Pyrenees. When plotted on the global data set analyzed and discussed in the review, the East Pyrenean erosion rates stand out as being amongst the lowest on record.     

Forms of phosphorus in sediments from the Goczałkowice Reservoir

The occurrence of Total P (P-Tot) and the mineral forms of P (P-Min) in sediments from Goczałkowice Reservoir were investigated using inductively coupled plasma-optical emission spectrometer (ICP-OES). The method is based on sequential extractions of the sediment samples, each releasing four forms of inorganic P: loosely bound P, P bound to Al (P-Al), P bound to Fe (P-Fe) and P bound to Ca (P-Ca). Sediment samples for studies presented in this work were collected in April, August and November of 2009 from four sites of the Goczałkowice Reservoir. Significant variations in the P forms with sediment grain size were observed. The most abundant form of inorganic P in the bottom sediment was P-Fe. Other P forms were as follows: P-Ca > P-Al > loosely bound-P. Moreover, P compounds were studied using ³¹P NMR. It was found that the predominant form was inorganic orthophosphate.     

Glaciation and deglaciation of the SW Lake District,England: implications of cosmogenic 36Cl exposure dating

Exposure dating using cosmogenic ³⁶Cl demonstrates that the summit plateau of Scafell Pike (978 m) in the SW Lake District escaped erosion by glacier ice during the last glacial maximum (LGM; c. 26–21 kyr) and probably throughout the Devensian Glacial Stage (MIS 5d-2). Exposure ages obtained for ice-moulded bedrock on an adjacent col at 750–765 m confirm over-riding and erosion of bedrock by warm-based glacier ice during the LGM. The contrast between the two sites is interpreted in terms of preservation of tors, frost-shattered outcrops and blockfields on terrain above 840–870 m under cold-based ice. An exposure age of 17.3 ± 1.1 kyr for the col at 750–765 m suggests that substantial downwastage of the last ice sheet had occurred by c. 17 kyr, consistent with deglacial exposure ages obtained for other high-level sites in the British Isles. An exposure age of 12.5 ± 0.8 kyr obtained for a glacially transported rockfall boulder within the limits of later corrie glaciation confirms that the final episode of local glaciation in the Lake District occurred during the Loch Lomond Stade (c. 12.9–11.7 kyr). This research also demonstrated the difficulties of obtaining reliable exposure ages from rhyolite and andesite bedrock that has proved resistant to glacial abrasion.     

The Allerød–Younger Dryas–Holocene sequence in the west-central Champlain Sea,eastern Ontario: a record of glacial,oceanographic, and climatic changes

The aragonite mineralogy and geochemistry of the mollusc faunas preserved at Navan and Bearbrook, Ontario, serve as proxies of original seawater chemistry. The composite section spanning 12,980–10,980 cal yr BP includes the Younger Dryas (YD) paleoclimatic oscillation. Oxygen isotopes demonstrate the onset of cooling with the YD event, in addition to the lowering of marine values by the influx of isotopically light glacial meltwater from Lake Agassiz. Impact of cooling and dilution is reduced or eliminated with the start of the Holocene, when water temperatures and salinities for Champlain Sea (CS) seawater were 8–16 °C and 27–34 ppt, respectively. Overall, oxygen isotope values deceased to −3.5% during the YD mainly due to freshening by glacial meltwater. Carbon isotopes confirm the rise in atmospheric CO₂ concentration at the YD–Holocene transition. Marine strontium isotope values for the Allerød–YD–earliest Holocene range from 0.709151 (16,210 cal yr BP) to 0.709145 (12,980 cal yr BP) and 0.709142 (10,950 cal yr BP). The oceanographic changes recorded for the CS are in agreement with the evolutionary phases of Lake Agassiz and deglaciation dynamics of the Laurentide Ice Sheet. The volume and direction of meltwater discharge from Lake Agassiz alternated between the Gulf of Mexico during the Allerød, via the Great Lakes through the CS to the North Atlantic during the YD, and back to the Gulf of Mexico during the early Holocene, but with diminished impact.     

Magnetostratigraphy of deep drilling core SG-1 in the western Qaidam Basin (NE Tibetan Plateau) and its tectonic implications

The Qaidam Basin is the largest intermontane basin of the northeastern Tibetan Plateau and contains a continuous Cenozoic sequence of lacustrine sediments. A ~ 1000-m-deep drilling (SG-1) with an average core recovery of ~ 95% was carried out in the depocenter of the Chahansilatu playa (sub-depression) in the western Qaidam Basin, aimed to obtain a high-resolution record of the paleoenvironmental evolution and the erosion history. Stepwise alternating field and thermal demagnetization, together with rock magnetic results, revealed a stable remanent magnetization for most samples, carried by magnetite. The polarity sequence consisted of 16 normal and 15 reverse zones which can be correlated with chrons 1n to 2An of the global geomagnetic polarity time scale. Magnetostratigraphic results date the entire core SG-1 at ~ 2.77 Ma to ~ 0.1 Ma and yielded sediment accumulation rate (SAR) ranging from 26.1 cm/ka to 51.5 cm/ka. Maximum SARs occurred within the intervals of ~ 2.6–2.2 Ma and after ~ 0.8 Ma, indicating two episodes of erosion, which we relate to pulse tectonic uplift of the NE Tibetan Plateau with subsequent global cooling.