Pattern,style and timing of British–Irish Ice Sheet advance and retreat over the last 45 000 years: evidence from NW Scotland and the adjacent continental shelf     

Tom Bradwell  Derek Fabel  Chris D. Clark  Richard C. Chiverrell  David Small  Rachel K. Smedley  Margot H. Saher  Steven G. Moreton  Dayton Dove  S. Louise Callard  Geoff A. T. Duller  Alicia Medialdea  Mark D. Bateman  Matthew J. Burke  Neil McDonald  Sean Gilgannon  Sally Morgan  David H. Roberts  Colm ó Cofaigh 《第四纪科学杂志》2021,36(5):871-933

Predicting the future response of ice sheets to climate warming and rising global sea level is important but difficult. This is especially so when fast-flowing glaciers or ice streams, buffered by ice shelves, are grounded on beds below sea level. What happens when these ice shelves are removed? And how do the ice stream and the surrounding ice sheet respond to the abruptly altered boundary conditions? To address these questions and others we present new geological, geomorphological, geophysical and geochronological data from the ice-stream-dominated NW sector of the last British–Irish Ice Sheet (BIIS). The study area covers around 45 000 km² of NW Scotland and the surrounding continental shelf. Alongside seabed geomorphological mapping and Quaternary sediment analysis, we use a suite of over 100 new absolute ages (including cosmogenic-nuclide exposure ages, optically stimulated luminescence ages and radiocarbon dates) collected from onshore and offshore, to build a sector-wide ice-sheet reconstruction combining all available evidence with Bayesian chronosequence modelling. Using this information we present a detailed assessment of ice-sheet advance/retreat history, and the glaciological connections between different areas of the NW BIIS sector, at different times during the last glacial cycle. The results show a highly dynamic, partly marine, partly terrestrial, ice-sheet sector undergoing large size variations in response to sub-millennial-scale climatic (Dansgaard–Oeschger) cycles over the last 45 000 years. Superimposed on these trends we identify internally driven instabilities, operating at higher frequency, conditioned by local topographic factors, tidewater dynamics and glaciological feedbacks during deglaciation. Specifically, our new evidence indicates extensive marine-terminating ice-sheet glaciation of the NW BIIS sector during Greenland Stadials 12 to 9 – prior to the main ‘Late Weichselian’ ice-sheet glaciation. After a period of restricted glaciation, in Greenland Interstadials 8 to 6, we find good evidence for rapid renewed ice-sheet build-up in NW Scotland, with the Minch ice-stream terminus reaching the continental shelf edge in Greenland Stadial 5, perhaps only briefly. Deglaciation of the NW sector took place in numerous stages. Several grounding-zone wedges and moraines on the mid- and inner continental shelf attest to significant stabilizations of the ice-sheet grounding line, or ice margin, during overall retreat in Greenland Stadials 3 and 2, and to the development of ice shelves. NW Lewis was the first substantial present-day land area to deglaciate, in the first half of Greenland Stadial 3 at a time of globally reduced sea-level c. 26 kabp , followed by Cape Wrath at c. 24 kabp. The topographic confinement of the Minch straits probably promoted ice-shelf development in early Greenland Stadial 2, providing the ice stream with additional support and buffering it somewhat from external drivers. However, c. 20–19 kabp , as the grounding-line migrated into shoreward deepening water, coinciding with a marked change in marine geology and bed strength, the ice stream became unstable. We find that, once underway, grounding-line retreat proceeded in an uninterrupted fashion with the rapid loss of fronting ice shelves – first in the west, then the east troughs – before eventual glacier stabilization at fjord mouths in NW Scotland by ~17 kabp. Around the same time, ~19–17 kabp , ice-sheet lobes readvanced into the East Minch – possibly a glaciological response to the marine-instability-triggered loss of adjacent ice stream (and/or ice shelf) support in the Minch trough. An independent ice cap on Lewis also experienced margin oscillations during mid-Greenland Stadial 2, with an ice-accumulation centre in West Lewis existing into the latter part of Heinrich Stadial 1. Final ice-sheet deglaciation of NW mainland Scotland was punctuated by at least one other coherent readvance at c. 15.5 kabp , before significant ice-mass losses thereafter. At the glacial termination, c. 14.5 kabp , glaciers fed outwash sediment to now-abandoned coastal deltas in NW mainland Scotland around the time of global Meltwater Pulse 1A. Overall, this work on the BIIS NW sector reconstructs a highly dynamic ice-sheet oscillating in extent and volume for much of the last 45 000 years. Periods of expansive ice-sheet glaciation dominated by ice-streaming were interspersed with periods of much more restricted ice-cap or tidewater/fjordic glaciation. Finally, this work indicates that the role of ice streams in ice-sheet evolution is complex but mechanistically important throughout the lifetime of an ice sheet – with ice streams contributing to the regulation of ice-sheet health but also to the acceleration of ice-sheet demise via marine ice-sheet instabilities.  相似文献   

Reply to Comment on “Post‐impact event bed (tsunamite) at the Cretaceous–Palaeogene boundary deposited on a distal carbonate platform interior”          下载免费PDF全文

Tvrtko Korbar  Iain McDonald  Vlasta Premec Fućek  Ladislav Fuček  Hrvoje Posilović 《地学学报》2017,29(5):332-334

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Titanite dates crystallization: Slow Pb diffusion during super‐solidus re‐equilibration     

Michael I. H. Hartnady  Christopher L. Kirkland  Chris Clark  Catherine V. Spaggiari  R. Hugh Smithies  Noreen J. Evans  Brad J. McDonald 《Journal of Metamorphic Geology》2019,37(6):823-838

Titanite can be found in rocks of wide compositional range, is reactive, growing or regrowing during metamorphic and hydrothermal events, and is generally amenable to U–Pb geochronology. Experimental evidence suggest that titanite has a closure temperature for Pb ranging from 550 to 650°C, and thus titanite dates are commonly interpreted as cooling ages. However, this view has been challenged in recent years by evidence from natural titanite which suggests the closure temperature may be significantly higher (up to 800°C). Here, we investigate titanite in an enclave of migmatitic gneiss included within a granite intrusion. The titanite crystals exhibit textural features characteristic of fluid‐mediated mass transfer processes on length scales of <100 µm. These textural features are associated with variation in both Pb concentrations and distinct U–Pb isotopic compositions. Zr‐in‐titanite thermometry indicates that modification of the titanite occurred at temperatures in excess of 840°C, in the presence of a high‐T silicate melt. The Pb concentration gradients preserved in these titanite crystals are used to determine the diffusivity of Pb in titanite under high‐T conditions. We estimate diffusivities ranging from 2 × 10^?22 to 5 × 10^?25 m²/s. These results are significantly lower than experimental data predict yet are consistent with other empirical data on natural titanites, suggesting that Pb diffusivity is similar to that of Sr. Thus our data challenge the wide‐held assumption that U–Pb titanite dates only reflect cooling ages.  相似文献   

Transition from arid to hyper-arid environment in the southern Levant deserts as recorded by early Pleistocene cummulic Aridisols     

Rivka Amit  Ori Simhai  Avner Ayalon  Yehouda Enzel  Ari Matmon  Onn Crouvi  Naomi Porat  Eric McDonald 《Quaternary Science Reviews》2011,30(3-4):312-323

The time at which deserts established their current arid or hyper-arid conditions remains a fundamental question regarding the history of Earth. Cosmogenic isotope exposure ages of desert pavement and welded, calcic–gypsic–salic Reg soils that developed on relatively flat alluvial surfaces ～2 Ma ago in the Negev Desert indicate long geomorphic stability under extremely dry conditions. Over a short interval during their initial stage of development between 1–2 Ma, these cumulative soils are characterized by calcic soils reaching maximum stage III of carbonate morphology. This interval is the only period when calcic soil horizons formed on stable abandoned alluvial surfaces in the southern Negev Desert. Since ～1 Ma pedogenesis changed toward more arid soil environment and the formation of gypsic–salic soil horizons that were later followed by dust accumulation. The dichotomy of only moderately-developed calcic soil (stages II–III) during a relatively long time interval (10⁵–10⁶ years) indicates an arid environment that does not support continuous development but only occasional calcic soil formation. The very low δ¹⁸O and relatively high δ¹³C values of these early pedogenic carbonates support soil formation under arid climatic conditions. Such an environment was probably characterized by rare and relatively longer duration rainstorms which occasionally allowed deeper infiltration of rainwater and longer retention of soil moisture. This, in turn enabled the growth of sparse vegetation that enhanced deposition of pedogenic carbonate. At ～1 Ma these rare events of slightly wetter conditions ceased and less atmospheric moisture reached the southern Negev Desert leading to deposition of soluble salts and dust deposited in the soils. The combination of long-term hyperaridity, scarcity of vegetation and lack of bioturbation, salts cementation, dust accumulation and tight desert pavement cover, has protected the surfaces from erosion forming one of the most remarkably stable landscapes on Earth, a landscape that essentially has not eroded, but accumulated salt and dust for more than 10⁶ yr.  相似文献   

A comprehensive evaluation of tourism climate suitability in Qinghai Province, China     

KRISTEN McDonald 《山地科学学报》2012,9(3):403-413

Qinghai Province is an important component of the Qinghai-Tibet Plateau in China. Scientific evaluation of the suitability of Qinghai’s climate for tourism can contribute to overcoming obstacles posed by climate on sustainable tourism development in Qinghai Province, including disparities between the low and high seasons, high altitude health concerns, and weather events. A tourism climate suitability evaluation model of the Qinghai-Tibet Plateau is constructed (Tourism Climate Suitability Index, or TCSI), and tourism climate suitability is comprehensively evaluated for Qinghai Province from climate data from 1960 to 2009. Results show that: (I) There is clear distributional characteristics of spatial-temporal variability of TCSI values in Qinghai Province. (II) Tourism climate suitability in Qinghai Province has significant seasonal and regional differences. The year is divided into a very suitable period (July and August), suitable tourism periods (from April and October), less suitable periods (From Nov to Mar). June to August is the most suitable tourism period in Qinghai. Qinghai Province is divided into five levels of tourism climate suitability: most suitable regions, very suitable regions, suitable regions, less suitable regions, and unsuitable region. (III) The key factor which influences regional differences in tourism climatic suitability is atmospheric oxygen. And the key factors which chiefly influence seasonal differences of tourism climate suitability are temperature and humidity, the wind chill factor, and barrier weather.  相似文献   

Zircon geochronology reveals polyphase magmatism and crustal anatexis in the Buchan Block,NE Scotland: Implications for the Grampian Orogeny     

T.E. Johnson  C.L. Kirkland  D.R. Viete  S. Fischer  S.M. Reddy  N.J. Evans  B.J. McDonald 《地学前缘(英文版)》2017,8(6):1469-1478

The type locality for high-temperature,low-pressure regional metamorphism,the Buchan Block in NE Scotland,exhibits profound differences to the rest of the Grampian Terrane.These differences have led some to regard the Buchan Block as an exotic crustal fragment comprising Precambrian basement gneisses and cover rocks thrust into their current position during Grampian orogenesis.Although rocks of the Buchan Block are now generally correlated with Dalradian strata elsewhere,the origin of the gneisses and the cause of the high heat flow and associated magmatism is debated.We report SIMS U-Pb and LA-ICPMS Hf isotopic data in zircon from high-grade rocks from the northeast(Inzie Head Gneiss)and northwest(Portsoy)corners of the Buchan Block.Around Inzie Head,upper amphibolite to granulite facies metasedimentary gneisses coexist with diorite sheets that were emplaced contemporaneously with partial melting of their host rocks,at least locally.U-Pb geochronology indicates a crystallisation age for the diorite of 486±9 Ma.Highly-deformed diorites within the Portsoy Gabbro have a crystallisation age of 493±8 Ma.Ages of ca.490 Ma for magmatism and high-grade metamorphism,which are broadly contemporaneous with ophiolite obduction and the onset of orogenesis,are significantly older than the established peak of Grampian metamorphism(ca.470 Ma).We propose a new model for the Grampian Orogeny involving punctuated tectonothermal activity due to tectonic switching during accretionary orogenesis.Rollback of a NW-dipping subduction zone at ca.490 Ma produced a back-arc environment(the Buchan Block)with associated arc magmatism and high dT/dP metamorphism.Arrival of an outboard arc resulted in shortening(the initial phase of the Grampian Orogeny)at ca.488 Ma.Rollback of a NW-dipping subduction zone to the SE of the ca.488 Ma suture began at 473 Ma and led to lithospheric-scale extension,decompression melting and advective heating of the middle crust,producing the widespread ca.470 Ma Grampian(classic Barrovian and Buchan)regional metamorphism.Resumed hinge advance and the final phase of shortening cut off the heat supply at ca.465 Ma,marking the end of the Grampian Orogeny.  相似文献   

A spectral atlas of post-main-sequence stars in ω Centauri: kinematics, evolution, enrichment and interstellar medium     

Jacco Th. van Loon  Floor van Leeuwen  Barry Smalley  rew W. Smith  Nicola A. Lyons  Iain McDonald  Martha L. Boyer 《Monthly notices of the Royal Astronomical Society》2007,382(3):1353-1374

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Post-peak,fluid-mediated modification of granulite facies zircon and monazite in the Trivandrum Block,southern India     

Richard J. M. Taylor  Chris Clark  Ian C. W. Fitzsimons  M. Santosh  M. Hand  Noreen Evans  Brad McDonald 《Contributions to Mineralogy and Petrology》2014,168(2):1-17

The quarry at Kottavattom in the Trivandrum Block of southern India contains spectacular examples of fluid-assisted alteration of high-grade metamorphic rocks. Garnet-biotite gneiss has undergone a change in mineral assemblage to form submetre scale orthopyroxene-bearing patches, later retrogressed to form an amphibole-bearing lithology. These patches, often referred to as arrested or incipient charnockite, crosscut the original metamorphic foliation and are typically attributed to passage of a low aH₂O fluid through the rock. Whilst this conversion is recognised as a late stage process, little detailed chronological work exists to link it temporally to metamorphism in the region. Zircon and monazite analysed from Kottavattom not only record metamorphism in the Trivandrum Block but also show internal, lobate textures crosscutting the original zoning, consistent with fluid-aided coupled dissolution-reprecipitation during formation of the orthopyroxene-bearing patches. High-grade metamorphism at the quarry occurred between the formation of metamorphic monazite at ~585 Ma and the growth of metamorphic zircon at ~523 Ma. The fluid-assisted alteration of the garnet-biotite gneiss is poorly recorded by altered zircon with only minimal resetting of the U–Pb system, whereas monazite has in some cases undergone complete U–Pb resetting and records an age for fluid infiltration at ~495 Ma. The fluid event therefore places the formation of the altered patches at least 25 Myr after the zircon crystallisation in the garnet-biotite gneiss. The most likely fluid composition causing the modification and U–Pb resetting of zircon and monazite is locally derived hypersaline brine.  相似文献   

Hydrogeological processes controlling the release of arsenic in parts of 24 Parganas district,West Bengal     

Neha Singh  Ravi Prakash Singh  Saumitra Mukherjee  Kyle McDonald  K. J. Reddy 《Environmental Earth Sciences》2014,72(1):111-118

A study was conducted to understand the hydrogeological processes dominating in the North 24 Parganas and South 24 Parganas based on representative 39 groundwater samples collected from selected area. The abundance of major ions was in the order of Ca²⁺ > Na⁺ > Mg²⁺ > K⁺ > Fe²⁺ for cations and HCO₃ ^? > PO₄ ^3? > Cl^? > SO₄ ^2? > NO₃ ^? for anions. Piper trilinear diagram was plotted to understand the hydrochemical facies. Most of the samples are of Ca-HCO₃ type. Based on conventional graphical plots for (Ca + Mg) vs. (SO₄ + HCO₃) and (Na + K) vs. Cl, it is interpreted that silicate weathering and ion exchange are the dominant processes within the study area. Previous studies have reported quartz, feldspar, illite, and chlorite clay minerals as the major mineral components obtained by the XRD analysis of sediments. Mineralogical investigations by SEM and EDX of aquifer materials have shown the occurrence of arsenic as coating on mineral grains in the silty clay as well as in the sandy layers. Excessive withdrawal of groundwater for irrigation and drinking purposes is responsible for fluctuation of the water table in the West Bengal. Aeration beneath the ground surface caused by fluctuation of the water table may lead to the formation of carbonic acid. Carbonic acid is responsible for the weathering of silicate minerals, and due to the formation of clay as a product of weathering, ion exchange also dominates in the area. These hydrogeological processes may be responsible for the release of arsenic into the groundwater of the study area, which is a part of North 24 Parganas and South 24 Parganas.  相似文献   

The Need for a Common Framework for Collection and Interpretation of Data in Platinum-Group Element Geochemistry     

Iain McDonald 《Geostandards and Geoanalytical Research》1998,22(1):85-91

Platinum-group element (PGE) concentrations and the distribution of the metals in rocks serve as important tracers of mantle processes, as well as extraterrestrial input into crustal environments, but common standards regarding the gathering and presentation of PGE data have never been formalized. Effective modelling assumes that concentration data are within acceptable levels of precision, yet the practices used in some studies to determine precision do not adequately assess precision and, as a result, the uncertainties on PGE concentrations and PGE ratios are sometimes consistently underestimated. This article argues that replicate analyses of unknowns must be adopted more widely in order to overcome this problem. Related to the issue of uncertainties on PGE concentrations, is the issue of uncertainty associated with normalisation. Arguments have recently been put forward as to the significance of small positive or negative anomalies on chondrite nor-malized plots. At least four CI chondrite PGE datasets (of varying age and quality) are currently used for normalisation and significantly different patterns can be derived simply by using one dataset rather than another. This article is intended to open a debate within the PGE research community by asking whether more consistency needs to be applied in PGE analysis and in the subsequent interpretation of data. A rigorous assessment of the real uncertainties on PGE concentrations and the adoption of a standard CI chondrite PGE dataset, in order to eliminate bias from normalisation, are suggested to be central to this.  相似文献