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1.
Deeply weathered soils cover most of the Piedmont physiographic province of the south-eastern United States of America (USA). These soils have traditionally been inferred to derive from weathered bedrock, but recent work (e.g. Ferguson et al., 2019) suggests that deposited sediments are more prevalent than recognized. Distinguishing sediment from weathered bedrock is integral to understanding critical-zone processes and overall Quaternary landscape evolution, yet the well-developed, red, clay-dominated Ultisols of this temperate and humid region mask differences between transported from non-transported material. Our goal is to determine if optically stimulated luminescence (OSL) methods can distinguish quartz sand from allochthonous (e.g. transported sediment) versus autochthonous (e.g. in situ weathered bedrock) material in soil-profile and core samples from the Redlair Observatory in southwestern North Carolina, USA. Here, we turn to OSL sensitivity and linear-modulated OSL (LM-OSL) to observe the intensity or lack thereof of the fast-decay luminescence component (most light-sensitive signal) in quartz grains from soil horizons and crystalline bedrock-derived saprolite. We find that quartz grains sampled from in situ weathered bedrock as well as from saprolotized clasts of rock have weak luminescence properties and are not dominated by a fast-decay luminescence component. In contrast, quartz grains from transported sediment (e.g. mobile regolith; colluvium; alluvium) contain sensitive grains with more dominant fast components. These results suggest that quartz luminescence sensitivity can be a tool to differentiate between in situ weathered bedrock and similar looking mobile regolith and colluvium over-printed by soil development.  相似文献   

2.
Catastrophic drainage of ice-dammed lakes in the Altai Mountains has been inferred from geomorphological evidence in the Katun Valley (Russia), and is presumed to have occurred during the Pleistocene. The sedimentary features have been difficult to date directly, due to the absence of organic carbon, and the improbability that luminescence signals in sand grains would be reset during transport. However, the development of rock-surface luminescence dating provides a new opportunity to date the features: clasts have a different transport history to sand grains, and their luminescence depth profiles can be inspected for evidence of bleaching before burial. Here we investigate two sites in the Altai Mountains, and use rock-surface luminescence burial dating to constrain the age of the megaflood deposits. In the Katun Valley, we sampled granite cobbles from a frozen sediment clast emplaced as a dropstone within a massive megaflood gravel terrace. Burial ages for the clasts range from 16.7 to 21.4 ka, with a mean age of 19.8 ± 1.5 ka. This represents the depositional age of the fluvial sediments that preceded the lake outburst flood, (and hence places a maximum age on the catastrophic flood). Clasts sampled from mega-ripples in the Kurai Basin are shown to have a mid-to-late Holocene burial age, which is not consistent with the possible origin of these features during a catastrophic drainage of a glacier-dammed lake. Instead, the burial age of the Kurai Basin sediments may reflect local-scale periglacial or seismic processes along the Kurai Fault Zone.  相似文献   

3.
Using the concept of bleaching in optical dating, a new index of sediment sample bleaching percentage (BLP‐2) was developed and applied to evaluate sand grain transport from riverine to deep‐marine environments. As bleached grains in modern sediments have no optically stimulated luminescence (OSL)/infrared stimulated luminescence (IRSL) signal, bleached and unbleached feldspar grains are distinguished by IRSL intensity. The BLP‐2 distribution of present deposits around the Kumano area, on the Pacific coast of central Japan, suggests that sand grains in surface turbidites obtained from the bottom of the Kumano Trough are of flood/storm origin rather than seismogenic origin. The distribution of BLP‐2 tentatively suggests sand grain erosion–transport–depositional processes; for example, origin and transport agencies of shelf sand, and influence of coastal erosion on the beach deposit. Although the present BLP analysis is not yet supported by a rigorous statistical test, it is useful to distinguish recent deposition and remobilization of sand grains. Furthermore, if the depositional age and the luminescence age of sand grains are accurately estimated, sand grain transport processes of old (late Quaternary) sediments may be estimated by the methodology similar to that of the present study.  相似文献   

4.
Quantifying glacial erosion contributes to our understanding of landscape evolution and topographic relief production in high altitude and high latitude areas. Combining in situ 10Be and 26Al analysis of bedrock, boulder, and river sand samples, geomorphological mapping, and field investigations, we examine glacial erosion patterns of former ice caps in the Shaluli Shan of the southeastern Tibetan Plateau. The general landform pattern shows a zonal pattern of landscape modification produced by ice caps of up to 4000 km2 during pre-LGM (Last Glacial Maximum) glaciations, while the dating results and landforms on the plateau surface imply that the LGM ice cap further modified the scoured terrain into different zones. Modeled glacial erosion depth of 0–0.38 m per 100 ka bedrock sample located close to the western margin of the LGM ice cap, indicates limited erosion prior to LGM and Late Glacial moraine deposition. A strong erosion zone exists proximal to the LGM ice cap marginal zone, indicated by modeled glacial erosion depth >2.23 m per 100 ka from bedrock samples. Modeled glacial erosion depths of 0–1.77 m per 100 ka from samples collected along the edge of a central upland, confirm the presence of a zone of intermediate erosion in-between the central upland and the strong erosion zone. Significant nuclide inheritance in river sand samples from basins on the scoured plateau surface also indicate restricted glacial erosion during the last glaciation. Our study, for the first time, shows clear evidence for preservation of glacial landforms formed during previous glaciations under non-erosive ice on the Tibetan Plateau. As patterns of glacial erosion intensity are largely driven by the basal thermal regime, our results confirm earlier inferences from geomorphology for a concentric basal thermal pattern for the Haizishan ice cap during the LGM. © 2018 John Wiley & Sons, Ltd.  相似文献   

5.
Sediments of river deltas provide valuable records of past coastal environments. Optically-stimulated luminescence (OSL) dating has become an alternative to radiocarbon dating for constraining the sediment chronology in large deltas that allow for sufficient sunlight bleaching of sediments during the fluvial transport. However, its applicability to smaller deltas with mountainous riverine systems has not been confirmed yet. To check this, we examine multiple signals from two Holocene sediment cores in the wave-dominated Thu Bon River delta in central Vietnam. Two cores were collected, respectively, 3.9 km and 1.2 km inland from the present shoreline and both show a >-25-m thick succession of coarsening-upward mud to sand deposits. Coarse grains (180–250 μm in diameter) of quartz and K-feldspar, and fine grains (4–11 μm in diameter) of quartz and polymineral were extracted from the upper and lower parts of the cores for multi-grain measurements of quartz OSL, and of feldspar infrared-stimulated luminescence (IRSL) at 50 °C (IR50) and post-IR IRSL at 175 °C (pIRIR175) to determine burial ages. In addition, facies analysis and radiocarbon dating were conducted. The landward core consists of transgressive to early regressive estuarine and prodelta facies, which is overlain by a sandy beach-shoreface facies. The seaward core consists of a relatively simple shallowing-upward succession from muddy prodelta facies to sandy beach-shoreface facies. All luminescence ages except for pIRIR175 of fine grains are mostly consistent with the radiocarbon ages. Instead, pIRIR175 ages of fine grains are significantly overestimated with variable offsets. OSL and IR50 of fine grains provide reasonable age estimates, as these grains were likely well bleached during the transport even along a short and steep mountainous river. Consistent age estimates of all signals from sand indicate that sand was well-bleached in the beach and shoreface owing to the frequent sediment reworking by waves and currents. These results support the hypothesis that luminescence dating is applicable to Holocene wave-dominated deltas and reiterate that comparing different luminescence signals is an effective way to check reliability of the age estimates in environments where the sunlight bleaching is not ensured.  相似文献   

6.
Till deposition by glacier submarginal,incremental thickening   总被引：1，自引：0，他引：1  
Macro‐ and micro‐scale sedimentological analyses of recently deposited tills and complex push/squeeze moraines on the forelands of Icelandic glaciers and in a stacked till sequence at the former Younger Dryas margin of the Loch Lomond glacier lobe in Scotland are used to assess the depositional processes involved in glacier submarginal emplacement of sediment. Where subglacial meltwater is unable to flush out subglacial sediment or construct thick debris‐rich basal ice by cumulative freeze‐on processes, glacier submarginal processes are dictated by seasonal cycles of refreezing and melt‐out of tills advected from up‐ice by a combination of lodgement, deformation and ice keel and clast ploughing. Although individual till layers may display typical A and B horizon deformation characteristics, the spatially and temporally variable mosaic of subglacial processes will overprint sedimentary and structural signatures on till sequences to the extent that they would be almost impossible to classify genetically in the ancient sediment record. At the macro‐scale, Icelandic tills display moderately strong clast fabrics that conform to the ice flow directions documented by surface flutings; very strong fabrics typify unequivocally lodged clasts. Despite previous interpretations of these tills as subglacial deforming layers, micro‐morphological analysis reveals that shearing played only a partial role in the emplacement of till matrixes, and water escape and sediment flowage features are widespread. A model of submarginal incremental thickening is presented as an explanation of these data, involving till slab emplacement over several seasonal cycles. Each cycle involves: (1) late summer subglacial lodgement, bedrock and sediment plucking, subglacial deformation and ice keel ploughing; (2) early winter freeze‐on of subglacial sediment to the thin outer snout; (3) late winter readvance and failure along a decollement plane within the till, resulting in the carriage of till onto the proximal side of the previous year's push moraine; (4) early summer melt‐out of the till slab, initiating porewater migration, water escape and sediment flow and extrusion. Repeated reworking of the thin end of submarginal till wedges produces overprinted strain signatures and clast pavements. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

7.
In the Arctic Ocean, direct dating methods are needed as an alternative to the radiocarbon (14C) method and to various indirect approaches for a longer stratigraphy. In past attempts to develop a luminescence sediment dating, the use of fine-silt (4–11 μm) mixture of quartz and feldspar grains from core tops has often produced large age overestimates by several ka. A recent application of micro-focused laser (‘micro-hole’) photon-stimulated luminescence (PSL) to medium-silt to fine-sand quartz grains (11–105 μm) from the core tops at the Alaska margin has been usefully accurate. To extend this approach to the central Arctic Ocean and to a larger grain size range, we applied micro-hole PSL dating to >11 μm quartz grains from core tops (0.5–2 cm horizon) from two sites on the central Lomonosov Ridge. We obtain a burial age estimate of ca. 2 ka for 11–62 μm grains at a multicore site 18 MC within a perched intra-ridge basin, in accord with 14C ages obtained on foraminifers. At nearby site 19 MC on the erosive ridge top, the micro-hole PSL dating of >90 μm quartz grains produces a burial age estimate of ∼ca. 25 ka, in accord with a foraminiferal 14C age of ca. 26 ka. However, the 11–90 μm grains from the same sample produce a much younger burial age estimate of ca. 9 ka. Thus, these two size fractions of quartz grains record different burial times and different deposition agents (icebergs vs. sea ice), providing insight into past sedimentary processes. Overall, our results confirm an earlier conclusion from micro-hole PSL dating study at the Alaska margin that medium to coarse silt fractions of quartz grains (11–90 μm or at least 62 μm) is the preferred material for direct dating of the last daylight exposure of detrital sediment in the Arctic Ocean.  相似文献   

8.
The ongoing debate over the effects of global environmental change on Earth's cryosphere calls for detailed knowledge about process rates and their variability in cold environments. In this context, appraisals of the coupling between glacier dynamics and para‐glacial erosion rates in tectonically active mountains remain rare. We contribute to filling this knowledge gap and present an unprecedented regional‐scale inventory of supra‐glacial sediment flux and hillslope erosion rates inferred from an analysis of 123 large (> 0·1 km2) catastrophic bedrock landslides that fell onto glaciers in the Chugach Mountains, Alaska, as documented by satellite images obtained between 1972 to 2008. Assuming these supra‐glacial landslide deposits to be passive strain markers we infer minimum decadal‐scale sediment yields of 190 to 7400 t km–2 yr–1 for a given glacier‐surface cross‐section impacted by episodic rock–slope failure. These rates compare to reported fluvial sediment yields in many mountain rivers, but are an order of magnitude below the extreme sediment yields measured at the snouts of Alaskan glaciers, indicating that the bulk of debris discharged derives from en‐glacial, sub‐glacial or ice‐proximal sources. We estimate an average minimum para‐glacial erosion rate by large, episodic rock–slope failures at 0·5–0·7 mm yr–1 in the Chugach Mountains over a 50‐yr period, with earthquakes likely being responsible for up to 73% of this rate. Though ranking amongst the highest decadal landslide erosion rates for this size of study area worldwide, our inferred rates of hillslope erosion in the Chugach Mountains remain an order of magnitude below the pace of extremely rapid glacial sediment export and glacio‐isostatic surface uplift previously reported from the region. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

9.
Single grain optical dating of glacigenic deposits   总被引：4，自引：0，他引：4  
Determining the age of glacigenic sediments is difficult for many geochronological methods because of the lack of suitable materials for analysis. Luminescence dating can be applied to the mineral grains making up the glacigenic sediments. However a major source of uncertainty in previous studies has been whether the mineral grains were exposed to sufficient daylight prior to deposition for the luminescence signal to be reset. Measurements of the optically stimulated luminescence signal from single sand-sized quartz grains offers the potential for explicitly identifying if a sediment contains grains that were not exposed to sufficient daylight to reset their signal. Statistical analysis of the resulting data can then reject those grains to allow the age of the sample to be determined. This study is the first to apply single grain optical dating to glacigenic sediments, and demonstrates the issues involved by analysis of samples from Chile and Scotland. Ages from 2.4±0.5 to 17.3±1.5 ka are produced. Comparison of the results with independent age control suggests that the ages are reliable. The results also show that the extent of bleaching at deposition varies considerably from one sample to another. For the most incompletely bleached sample, luminescence measurements based on the average of many hundreds or thousands of grains would have overestimated the age of the sample by 60 ka, but the single grain method proposed here was able to reliably date it.  相似文献   

10.
Grounding-zone wedges (GZWs) mark the grounding terminus of flowing marine-based ice streams and, in the presence of an ice shelf, the transition from grounded ice to floating ice. The morphology and stratigraphy of GZWs is predominantly constrained by seafloor bathymetry, seismic data, and sediment cores from deglaciated continental shelves; however, due to minimal constraints on GZW sedimentation processes, there remains a general lack of knowledge concerning the production of these landforms. Herein, outcrop observations are provided of GZWs from Whidbey Island in the Puget Lowlands (Washington State, USA). These features are characterized by prograded diamictons bounded by glacial unconformities, whereby the lower unconformity indicates glacial advance of the southern Cordilleran Ice Sheet and the upper unconformity indicates locally restricted ice advance during GZW growth; the consistent presence of an upper unconformity supports the hypothesis that GZWs facilitate ice advance during landform construction. Based on outcrop stratigraphy, GZW construction is dominated by sediment transport of deformation till and melt-out of entrained basal debris at the grounding line. This material may be subsequently remobilized by debris flows. Additionally, there is evidence for subglacial meltwater discharge at the grounding line, as well as rhythmically bedded silt and sand, indicating possible tidal pumping at the grounding line. A series of GZWs on Whidbey Island provides evidence of punctuated ice sheet movement during retreat, rather than a rapid ice sheet lift-off. The distance between adjacent GZWs of 102–103 m and the consistency in their size relative to modern ice stream grounding lines suggests that individual wedges formed over decades to centuries. © 2018 John Wiley & Sons, Ltd.  相似文献   

11.
The provenance of loess in Chinese Loess Plateau, including origin, transport pathways and source areas, has long been one of the most important questions. In this study, the vertical variations of the luminescence sensitivity of quartz grains from the central Chinese Loess Plateau were investigated by using optically stimulated luminescence (OSL) techniques. Our results indicate that the luminescence sensitivity of quartz grains of paleosols can be much higher than that of loess beds. In addition, the quartz grains from the loess-paleosol sequence exhibit a temporal trend in the strength of luminescence sensitivity, characterized by higher values in soils and lower values in loess beds. The OSL sensitivity of quartz grains of the loess-paleosol sequence also shows very similar trend to the magnetic susceptibility and particle size fluctuations, implying that the luminescence sensitivity might be climatic dependent. The possible factors affecting the variations of luminescence sensitivity were discussed including particle size, natural radioactivity, and the provenance of eolian deposits. We suggest that the temporal variations of luminescence sensitivity can be attributed to the retreat-advance of deserts, the different contributions of glacial origin quartz particles associated with mountain processes, and wind patterns during glacial/interglacial cycles. Therefore, the secular variations of luminescence sensitivity of quartz grains are ultimately influenced by past climatic change through its controlling on sediment provenance changes.  相似文献   

12.
The electron spin resonance (ESR) dating of tills using germanium-doped (Ge) paramagnetic centers in quartz has advantages over other dating techniques, as quartz is common, processing is easy, and the technique has the potential for dating features several hundreds of thousands years old. ESR dating of moraines is based on the supposition that either subglacial comminution or exposure to sunlight resets the signal. However, actual dating suggests that a signal that is initially present cannot be bleached to zero by grinding alone. We found that grinding coarse samples (0.5–1 mm in diameter) to the mean grain size of fine sand (0.125–0.193 mm) reduced the signal intensity to 53–69% of its original value. From the value of the signal difference, one can devise a correction factor for ESR ages of subglacial sediment. Polymineralic grains are commonly present in till. Exposure of them to sunlight for several days can reduce the signal intensity to 7–8% of its original value within 1–2 mm thick of the sediment surface. However, within 5–8 mm of the sediment surface, exposure to sunlight for over one week only reduced the signal intensity to mean plateau values of 42–50% of the initial value. Mixing upper and lower layers of the samples during exposure to sunlight changed the signal intensity. This suggests that the amount of bleaching varies spatially. Sediments initially deposited at the margins of ice caps or ice sheets and subsequently overridden may have been sufficiently exposed to sunlight to allow ESR dating of moraines. The purity of the quartz and the grain size have significant impacts on signal intensity; intensive purification and the use of a uniform fine sand fraction are thus recommended.  相似文献   

13.
We evaluate the paraglacial activity in Nexpayantla, a subtropical mountainous gorge in Popocatépetl volcano (Central Mexico), fully deglaciated in the 20th century. Glacial advances are evidenced by the presence of moraines. Fluvio-glacial terraces and an alluvial megafan resulted from the gorge deglaciation. Current reworking of the glacigenic material is done by landslides and debris flows produced on the moraines and terraces. To study the different phases of mobilization of glacigenic sediment, we used an approach based on the study of the optically stimulated luminescence (OSL) signals obtained from a portable OSL (POSL) reader in samples extracted from both glacigenic and paraglacial deposits. The luminescence (POSL) results obtained at moraines increase as altitude decreases, which is expected for deglaciated valleys where the oldest moraines are located at lower elevations. We evaluate the grade of luminescence signal reset of the glacigenic sediments during the proglacial stage, and the subsequent deglaciation phases. Our results indicate that there is a marked transition between glacial and fluvially dominated processes at Nexpayantla Gorge. We find that the grade of luminescence signal resetting in the paraglacial deposits is a good indicator to trace paraglacial stages and the beginning of exhaustion of the paraglacial activity in mountain areas. OSL ages confirm that the oldest fluvio-glacial terraces found at the middle sector of Nexpayantla Gorge are ~2 ka, which is also supported by an AMS 14C age. OSL dating was found challenging, since quartz grains have low sensitivity because of their volcanic origin; POSL signals, however, are in good agreement with the location and distribution of geomorphic markers. We propose that luminescence data obtained from the POSL unit can be useful to provide information about sediment mobilization in paraglacial environments during different climatic pulses – even for the case where mineral grains have low sensitivity, such as in volcanic sediments. © 2020 John Wiley & Sons, Ltd.  相似文献   

14.
Linking the timing of glacial episodes and behaviour to climatic shifts that are documented in ice and marine sedimentary archives is key to understanding ocean-land interactions. In the NW Scottish Highlands a large number of closely spaced (‘hummocky’) moraines formed at retreating glacier margins. Independent age control on one palaeo-glacier limit is consistent with the timing of Younger Dryas (YD) glaciation in the area, but adjacent glacier lobes have remained undated due to the lack of sites and material for 14C dating. Direct dating of ice-marginal moraines using optically stimulated luminescence (OSL) techniques has never been attempted before in Scotland, but if successful, they may be the most appropriate methods for constraining the age of sediment deposition in the absence of organic material. Coarse-grained quartz and K-feldspar minerals from supraglacial sheet flow deposits and glacilacustrine sediments within ice-marginal moraines were analysed using the single-aliquot regenerative-dose (SAR) protocol. Independent age control and clear geomorphological relationships indicate that all samples should yield YD or post-Last Glacial Maximum (LGM) ages. Quartz OSL shine down curves showed low luminescence sensitivity, significant medium-to-slow components, a weak fast component, and scattered SAR data; Linearly Modulated-OSL (LM-OSL) measurements confirmed that the fast component was weak or absent. In contrast, feldspar infrared stimulated luminescence (IRSL) was highly sensitive with excellent SAR data. However, SAR data from 3 mm diameter aliquots of feldspar (200 grains) give higher than expected equivalent doses (De) by an order of magnitude. SAR measurements of small clusters of feldspar grains (ranging from 1–8) considerably broaden the apparent De distribution, but even the lowest value is about 2–3 times the expected De. Two possibilities arise to explain the quartz and feldspar data: (1) that glacial sequences in the NW Highlands re-work inherited (older) glacial deposits and that some of the pre-Devensian existing glacial landforms have only been modified during Devensian glaciation; or (2) that the sedimentary processes operating in these ice-marginal environments are not conducive to adequate bleaching of quartz and feldspar grains. Our study implies that ice-proximal supraglacial sediments from this region in NW Scotland reflect older ages of deposition, but dating YD sediments has not been possible.  相似文献   

15.
Optically stimulated luminescence (OSL) dating is becoming a useful technique to yield absolute age of organic-poor sandy deposits. The buried tidal sand body (BTSB) in the coastal zone of northern Jiangsu Province, China, has been suggested to have the same origin as the offshore radial sand ridge in the Yellow Sea. However, chronological constrain of the BSTB is still quite limited. In this study, OSL measurements were conducted using silt-sized multi-grain and coarse-grained single-grain quartz to constrain the depositional history of a 25.6 m core from the BTSB. A low luminescence sensitivity of quartz was observed, and only ∼1.04% of the grains passed the standard rejection criterion for single-grain measurement. Analysis of paired OSL ages from two grain-size fractions using different protocols showed that silt-sized quartz ages were underestimated of 0.14–1.35 ka in comparison to coarse-grained quartz in the depth interval of 5.8–22.4 m. We interpret such an age discrepancy as the effects of lateral infiltration of fine-grained sediment into the sand body due to dynamic feature of channel-ridge system on the shelf. As far as we know, it is the first time that such infiltration is demonstrated through OSL dating. Our OSL data indicated that there is a significant hiatus between the Late Pleistocene stiff clay layer (50–18 ka) and the Holocene sequence. Holocene deposits only occurred in the last 2 ka, with rapid accumulation of ∼17 m-thick sediments at ∼2–1 ka, a slower accumulation between ∼1 and 0.1 ka and rapid land emergence through an accretion of ∼4 m-thick sediment over the past ∼0.1 ka. This study highlights the complexity of OSL dating in highly dynamic sedimentary environments. Therefore, examining different grain size fractions and comparing different measurement protocols are highly deserved in carrying out OSL dating in such environments.  相似文献   

16.
Outburst floods from glacier‐dammed lakes are major events associated with glacier thinning and volume reduction. This paper investigates jökulhlaups emanating from the glacier‐dammed lake Øvre Messingmalmvatn at Rundvassbreen, an outlet glacier of the Blåmannsisen ice cap in northern Norway. Since 2001, the lake has several times been observed to drain suddenly, causing jökulhlaup outbursts into the pro‐glacial lake Rundvatnet. Varve analysis and lead‐210 (210Pb) dating were used to date sediment cores taken from Rundvatnet. It was found that sedimentation from jökulhlaups is recognizable in the lake as distinct sand layers embedded in the varved silt‐clay sequence which represents the normal lake sedimentation. Sand fractions were carried in suspension because of the extreme hydraulic conditions of jökulhlaups. The thickest sand layer was deposited during the 2001 jökulhlaup which lasted three days and had a total volume of 40 ×106 m3. Jökulhlaups were also recorded in 2005, 2007, 2009, and 2010; they each resulted in a sand layer. Annual sediment accumulation in Rundvatnet increased up to 10‐fold during the years with jökulhlaup outburst floods, from a normal value of 1–2 mm yr?1 to 8–10 mm yr?1. Five other jökulhlaups were identified from the 1910–1930 sedimentation interval, in addition to those observed in 2001–2010; there appear to have been none for 70 years during 1931–2000. Each jökulhlaup was preceded by a period when the glacier thinned to a critical volume and could no longer withstand the hydrostatic pressure of Øvre Messingmalmvatn; consequently a tunnel developed beneath the glacier, leading to a jökulhlaup. Statistical analyses of the correlations between the pro‐glacial sedimentation rate and temperature and precipitation suggested that although climate conditions are expected to influence sedimentation in the pro‐glacial catchment, a host of other interacting factors moderate the availability and delivery of sediment to the pro‐glacial system, making the processes responsible for changes in pro‐glacial sedimentation to remain uncertain. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

17.
Glacial Lake Benson formed in west-central Minnesota as the Des Moines lobe of the Laurentide ice sheet retreated north of a small moraine in the Minnesota River lowland. Although previous research has constrained the timing of glacial Lake Agassiz immediately to the north, little age control is available for the formation of glacial Lake Benson and ice-marginal positions to the south. In order to constrain the age of glacial Lake Benson and test the application of single-grain optically stimulated luminescence (OSL) dating to ice-marginal deposits, seven OSL samples were collected from a variety of depositional settings. These included deltaic deposits linked to specific lake levels, pro-glacial fluvial, ice-contact and supra-glacial deposits. Single-grain OSL results indicate evidence for incomplete resetting (partial bleaching) of the luminescence signal, as expected for glacial environments, and therefore ages were calculated using a minimum age model. OSL results constrain the timing of ice-margin retreat and lake formation to 14.4–14.8 ka. Analysis of single-grain equivalent dose distributions indicates that deposits created by glacial-dominated processes typically had higher over-dispersion (>50%) and greater positive skew (>0.9) than deposits originating from fluvial processes. These results suggest that water-lain deposits should be targeted for OSL sampling over those created by glacial processes when dating ice-proximal settings.  相似文献   

18.
Optically stimulated luminescence (OSL) dating technique is a reliable method to determine the ages of sand dune sediments. While it seems logical to assume that for these windblown materials (such as sand dune sediments) grains from different sized fractions are suitable for optical dating and would yield identical ages, this was not previously explicitly demonstrated yet. In this study, six samples were selected from the sand dunes intercalated in loess strata near Lanzhou, western Chinese Loess Plateau, and different grain-size quartz fractions (e.g. 38–63 μm, 90–150 μm, 150–200 μm, 200–250 μm and 250–300 μm) were extracted to compare the OSL ages of different grain-size quartz. The results show that: (1) quartz OSL ages derived from different grain-size fractions produce identical ages within errors, confirming that the ages resulting from both coarse silt-sized (or middle grain of 38–63 μm) and sand-sized (90–300 μm) quartz can represent the periods of sand dune accumulation; (2) the OSL ages of the selected sand dune samples fall into ca. 28–18 ka, suggesting that the sand dune accumulation occurred during the marine isotope stage 2 (MIS 2) in current study area, which might imply regional increased aridity on the western Chinese Loess Plateau.  相似文献   

19.
We use three different approaches of optically stimulated luminescence (OSL) to study young fluvial sediments located at the main channels of one of the largest fluvial systems of North America: the Usumacinta–Grijalva. We use the pulsed photo‐stimulated luminescence (PPSL) system also known as portable OSL reader, full OSL dating and profiling OSL dating in samples extracted from vertical sediment profiles (n = 9) of riverbanks to detect changes in depositional rates of sediments and to obtain the age of the deposits. The results of the PPSL system show that the luminescence signals of vertical sediment profiles highly scattered from the top to the bottom contrast with the luminescence pattern observed on well‐reset sequences of fluvial deposits where luminescence increase from the top to the bottom of the profile. The profiling and full OSL ages yielded large uncertainty values on their ages. Based on the inconsistencies observed in both ages and luminescence patterns of profiles we suggest that these fluvial deposits were not fully reset during their transport. As an explanation, we propose that in the Usumacinta and Grijalva rivers the cyclonic storms during the wet season promote the entrainment of large volumes of sediments due to high‐erosional episodes around the basin resulting from hyper‐concentrated and turbid flows. We conclude that the PPSL, profiling and full OSL dating of sediments are useful tools to quantify and to assess the depositional patterns in fluvial settings during the Holocene. These techniques also can yield information about sites where increases in the sediment load of rivers may produce poorly resetting of grains affecting the results of OSL dating. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

20.
On the high altitude polar plateau of Amundsenisen, western Dronning Maud Land, East Antarctica, a subglacial valley, with a broad horizontal valley floor interpreted as a sediment floodplain or valley delta, was studied by radio echo sounding. In addition, a small, probably glacial, valley was mapped within the same subglacial massif. Basal ice temperatures were calculated using field data on precipitation, air temperature and ice sheet thickness. Discoveries of old landforms which have been preserved more or less intact beneath the former Fennoscandian and Laurentide ice sheets have received increasing attention during the last decade. The aim of this study is to investigate whether preservation of landforms occurs under the East Antarctic Ice Sheet, and to discuss under that climatological and glaciological circumstances preservation may take place. The results show that the ice sheet covering the investigated localities is frozen to bed, and therefore has an insignificant erosional capability. The observations suggest that a large-scale subglacial sediment deposit and a small valley formed by glacial erosion have survived beneath a cold-based ice sheet marginal zone for a long time period. The process of glacial preservation, recognized for bedrock features and tentatively observed for sediment accumulations, should act on similar large-scale landforms under any cold-based ice sheet, present or past. On the basis of existing studies of the age and stability of the East Antarctic Ice Sheet, a Middle Pliocene age is suggested for the preserved landforms. The presence of the presumed sediment-filled valley further indicates that no prolonged periods of basal melting have occurred at the Amundsenisen study area during the ice sheet history, which includes the Quaternary glaciation periods. Finally, calculations of basal temperature for localities at different altitudes within the same subglacial massif were used to demonstrate local altitudinal control of glacial preservation. © 1997 by John Wiley & Sons, Ltd.  相似文献   

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