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1.
The Little Ice Age glacier maximum in Iceland and the North Atlantic Oscillation: evidence from Lambatungnajökull, southeast Iceland 总被引:3,自引:0,他引:3
TOM BRADWELL REW J. DUGMORE DAVID E. SUGDEN 《Boreas: An International Journal of Quaternary Research》2006,35(1):61-80
This article examines the link between late Holocene fluctuations of Lambatungnajökull, an outlet glacier of the Vatnajökull ice cap in Iceland, and variations in climate. Geomorphological evidence is used to reconstruct the pattern of glacier fluctuations, while lichenometry and tephrostratigraphy are used to date glacial landforms deposited over the past ˜400 years. Moraines dated using two different lichenometric techniques indicate that the most extensive period of glacier expansion occurred shortly before c . AD 1795, probably during the 1780s. Recession over the last 200 years was punctuated by re-advances in the 1810s, 1850s, 1870s, 1890s and c . 1920, 1930 and 1965. Lambatungnajökull receded more rapidly in the 1930s and 1940s than at any other time during the last 200 years. The rate and style of glacier retreat since 1930 compare well with other similar-sized, non-surging, glaciers in southeast Iceland, suggesting that the terminus fluctuations are climatically driven. Furthermore, the pattern of glacier fluctuations over the 20th century broadly reflects the temperature oscillations recorded at nearby meteorological stations. Much of the climatic variation experienced in southern Iceland, and the glacier fluctuations that result, can be explained by secular changes in the North Atlantic Oscillation (NAO) Advances of Lambatungnajökull generally occur during prolonged periods of negative NAO index. The main implication of this work relates to the exact timing of the Little Ice Age in the Northeast Atlantic. Mounting evidence now suggests that the period between AD 1750 and 1800, rather than the late 19th century, represented the culmination of the Little Ice Age in Iceland. 相似文献
2.
CLIFFORD TOM N.; BARTON ERIKA S.; STERN RICHARD A.; DUCHESNE JEAN-CLAIR 《Journal of Petrology》2004,45(4):669-691
The O'okiep Copper District is underlain by voluminous 10351210Ma granite gneiss and granite with remnants of metamorphosedsupracrustal rocks. This assemblage was intruded by the 1030Ma copper-bearing Koperberg Suite that includes jotunite, anorthosite,biotite diorite and hypersthene-bearing rocks ranging from leuconoriteto hypersthenite. New sensitive high-resolution ion microprobeage data demonstrate the presence of 17002000 Ma zirconas xenocrysts in all of the intrusive rocks, and as detritalzircon in the metasediments of the Khurisberg Subgroup. Thesedata are consistent with published SmNd model ages ofc. 1700 Ma (TCHUR) and c. 2000 Ma (TDM) of many of the intrusivesthat support a major crust-forming event in Eburnian (Hudsonian)times. In addition, UThPb analyses of zirconsfrom all major rock units define two tectono-magmatic episodesof the Namaquan Orogeny: (1) the O'okiepian Episode (11801210Ma), represented by regional granite plutonism, notably theNababeep and Modderfontein Granite Gneisses and the Concordiaand Kweekfontein Granites that accompanied and outlasted (e.g.Kweekfontein Granite) regional tectonism [F2(D2)] and granulite-faciesmetamorphism (M2); (2) the Klondikean Episode (10201040Ma), which includes the intrusion of the porphyritic RietbergGranite and of the Koperberg Suite that are devoid of regionalplanar or linear fabrics. Klondikean tectonism (D3) is reflectedby major eastwest-trending open folds [F3(D3a)], andby localized eastwest-trending near-vertical ductilefolds [steep structures; F4(D3b)] whose formationwas broadly coeval with the intrusion of the Koperberg Suite.A regional, largely thermal, amphibolite- to granulite-faciesmetamorphism (M3) accompanied D3. This study demonstrates, interalia, that the complete spectrum of rock-types of the KoperbergSuite, together with the Rietberg Granite, was intruded in ashort time-interval (<10 Myr) at c. 1030 Ma, and that therewere lengthy periods of about 150 Myr of tectonic quiescencewithin the Namaquan Orogeny: (1) between the O'okiepian andKlondikean Episodes; (2) from the end of the latter to the formalend of Namaquan Orogenesis 800850 Ma ago. KEY WORDS: UPb, zircon; O'okiep, Namaqualand; granite plutonism; granulite facies; Koperberg Suite; Namaquan (Grenville) Orogeny 相似文献
3.
Quaternary geology and deglaciation of the continental shelf off Troms, north Norway 总被引:1,自引:0,他引:1
The study is based mainly on 4700 km shallow seismic profiling, soil mechanical, and micropaleontological analyses from forty localities, and seven radiocarbon datings. Six foraminiferal assemblages are recognized. The thickness of Quaternary deposits ranges from sparse to more than 200 m. They consist of top sand, soft sensitive clay, and glacial drift divided into four seismic stratigraphic units. The oldest unit, which is the thickest and most extensive, is built up of various sediments and is partly eroded, especially in the southern part. The remaining units occur as three complex linear belts running broadly parallel to the present coast and were deposited during deglaciation. Dating of the outer unit, the Nordvestsnaget Drift, suggests a maximum age of 13,300 years B.P., and datings from the top of the middle unit, the Mulegga Drift, have given a minimum age of about 12,200 years B.P. Seismic stratigraphy shows that the inner unit, the Havbrobakken Drift, is younger than the Mulegga Drift, but no datings have been obtained so far. 相似文献
4.
Controlling factors of volumetrically important marine carbonate cementation in deep slope settings 总被引:1,自引:0,他引:1
BRAM VAN DER KOOIJ ADRIAN IMMENHAUSER THOMAS STEUBER JUAN RAMON BAHAMONDE RIONDA OSCAR MERÍNO TOMÉ 《Sedimentology》2010,57(6):1491-1525
The controlling parameters of early marine carbonate cementation in shoal water and hemipelagic to pelagic domains are well‐studied. In contrast, the mechanisms driving the precipitation of early marine carbonate cements at deeper slope settings have received less attention, despite the fact that considerable volumes of early marine cement are present at recent and fossil carbonate slopes in water depths of several hundreds of metres. In order to better understand the controlling factors of pervasive early marine cementation at greater water depths, marine carbonate cements observed along time‐parallel platform to basin transects of two intact Pennsylvanian carbonate slopes are compared with those present in the slope deposits of the Permian Capitan Reef and Neogene Mururoa Atoll. In all four settings, significant amounts of marine cements occlude primary pore spaces downslope into thermoclinal water depths, i.e. in a bathymetric range between some tens and several hundreds of metres. Radial, radiaxial and fascicular optic fibrous calcites, and radiaxial prismatic calcites are associated with re‐deposited facies, boundstones and rudstones. Botryoidal (formerly) aragonitic precipitates are common in microbially induced limestones. From these case studies, it is tentatively concluded that sea water circulation in an extensive, near‐sea floor pore system is a first‐order control on carbonate ion supply and marine cementation. Coastal upwelling and internal or tidal currents are the most probable mechanisms driving pore water circulation at these depths. Carbonate cements precipitated under conditions of normal to elevated alkalinity, locally elevated nutrient levels and variable sea water temperatures. The implications of these findings and suggestions for future work are discussed. 相似文献
5.
Geology in the Falkland Islands 总被引:2,自引:0,他引:2
RICHARD CLARK EMMA EDWARDS STEPHEN LUXTON TOM SHIPP & PETER WILSON 《Geology Today》1995,11(6):217-223
In the next few years we are likely to hear and learn much about the offshore geology of the Falkland Islands as exploratory drilling for hydrocarbons begins. The offshore geology may become better known than the onshore, of which there has been little detailed investigation in the 200+ years since settlements were established. Here we outline the history of geological investigations and present information gathered during recent fieldwork. 相似文献
6.
HEAMAN LARRY M.; CREASER ROBERT A.; COOKENBOO HARRISON O.; CHACKO TOM 《Journal of Petrology》2006,47(4):821-858
The Jericho kimberlites are part of a small Jurassic kimberlitecluster in the northern Slave craton, Canada. A variety of datingtechniques were applied to constrain the nature and age of twoJericho kimberlites, JD-1 (170·2 ± 4·3Ma RbSr phlogopite megacrysts, 172·8 ±0·7 Ma UPb eclogite rutile, 178 ± 5 MaUPb eclogite zircon lower intercept) and JD-3 (173 ±2 Ma RbSr phlogopite megacryst; 176·6 ±3·2 Ma UPb perovskite), and all yielded identicalresults within analytical uncertainty. As there is no discernibledifference in the radiometric ages obtained for these two pipes,the composite RbSr phlogopite megacryst date of 173·1± 1·3 Ma is interpreted as the best estimate forthe emplacement age of both Jericho pipes. The initial Sr isotopecomposition of 0·7053 ± 0·0003 derivedfrom phlogopite megacrysts overlaps the range (0·70430·7084)previously reported for Jericho whole-rocks. These strontiumisotope data, combined with the radiogenic initial 206Pb/204Pbratio of 18·99 ± 0·33 obtained in thisstudy, indicate that the Jericho kimberlites are isotopicallysimilar to Group 1 kimberlites as defined in southern Africa.The Jericho kimberlites are an important new source of mantlexenoliths that hold clues to the nature of the Slave cratonsubcontinental mantle. A high proportion (30%) of the Jerichomantle xenolith population consists of various eclogite typesincluding a small number (23%) of apatite-, diamond-,kyanite- and zircon-bearing eclogites. The most striking aspectof the Jericho zircon-bearing eclogite xenoliths is their peculiargeochemistry. Reconstructed whole-rock compositions indicatethat they were derived from protoliths with high FeO, Al2O3and Na2O contents, reflected in the high-FeO (22·627·5wt %) nature of garnet and the high-Na2O (8·479·44wt %) and high-Al2O3 (13·1214·33 wt %)character of the clinopyroxene. These eclogite whole-rock compositionsare highly enriched in high field strength elements (HFSE) suchas Nb (1331134 ppm), Ta (528 ppm), Zr (17794934ppm) and Hf (2364 ppm). This HFSE enrichment is linkedto growth of large (up to 2 mm) zircon and niobian rutile crystals(up to 3 modal %) near the time of eclogite metamorphism. Thediamond-bearing eclogites on the other hand are characterizedby high-MgO (19·621·3 wt %) garnet andultralow-Na2O (0·441·50 wt %) clinopyroxene.Paleotemperature estimates indicate that both the zircon- anddiamond-bearing eclogites have similar equilibration temperaturesof 9501020°C and 9901030°C, respectively,corresponding to mantle depths of 150180 km. Integrationof petrographic, whole-rock and mineral geochemistry, geochronologyand isotope tracer techniques indicates that the Jericho zircon-bearingeclogite xenoliths have had a complex history involving Paleoproterozoicmetamorphism, thermal perturbations, and two or more episodesof Precambrian mantle metasomatism. The oldest metasomatic event(Type 1) occurred near the time of Paleoproterozoic metamorphism(1·8 Ga) and is responsible for the extreme HFSE enrichmentand growth of zircon and high-niobian rutile. A second thermalperturbation and concomitant carbonatite metasomatism (Type2) is responsible for significant apatite growth in some xenolithsand profound light rare earth element enrichment. Type 2 metasomatismoccurred in the period 1·01·3 Ga and isrecorded by relatively consistent whole-rock eclogite modelNd ages and secondary UPb zircon upper intercept dates.These eclogite xenoliths were derived from a variety of protoliths,some of which could represent metasomatized pieces of oceaniccrust, possibly linked to east-dipping subduction beneath theSlave craton during construction of the 1·881·84Ga Great Bear continental arc. Others, including the diamond-bearingeclogites, could be cumulates from mafic or ultramafic sillcomplexes that intruded the Slave lithospheric mantle at depthsof about 150180 km. KEY WORDS: zircon- and diamond-bearing eclogites; Jericho kimberlite, geochronology; Precambrian metasomatism, northern Slave Craton 相似文献
7.
JORIE CLARK A. MARSHALL McCABE CHRISTOPH SCHNABEL PETER U. CLARK STEWART FREEMAN COLIN MADEN SHENG XU 《Boreas: An International Journal of Quaternary Research》2009,38(1):111-118
A well-preserved moraine on the northern coast of County Donegal, Ireland, has played a critical role in our understanding of the glacial history of this sector of the Irish Ice Sheet (IIS). Because of a lack of numerical dating of the moraine, however, previous interpretations of its age and significance to the glacial history of this region have varied widely. Here we report eight in situ cosmogenic 10 Be ages on boulders sampled from the moraine. Two of these ages are outliers, with the remaining six ranging from 18.8±1.0 10 Be kyr to 20.9±1.3 10 Be kyr, with an uncertainty-weighted mean age of 19.4±0.3 10 Be kyr (19.4±1.2 kyr accounting for production rate uncertainty). Our results confirm one previous 10 Be age obtained from the moraine, with the combined data ( n =7) constraining the age of initial deglaciation of the IIS from its LGM position on the continental shelf to be 19.3±0.3 10 Be kyr (19.3±1.2 kyr accounting for production rate uncertainty). These ages are in excellent agreement with calibrated 14 C ages that constrain retreat of the IIS margin from the continental shelf elsewhere in northwestern and western Ireland and the Irish Sea Basin associated with the start of the Cooley Point Interstadial (≥20–≤18.2 cal. kyr BP), suggesting widespread deglaciation of the IIS ∼19.5–20 kyr ago. 相似文献
8.
9.
BY TIMOTHY F. JOHNSEN HELENA ALEXANDERSON DEREK FABEL STEWART P.H.T. FREEMAN 《Geografiska Annaler: Series A, Physical Geography》2009,91(2):113-120
The overall pattern of deglaciation of the southern part of the Scandinavian Ice Sheet has been considered established, although details of the chronology and ice sheet dynamics are less well known. Even less is known for the south Swedish Upland because the area was deglaciated mostly by stagnation. Within this area lies the conspicuous Vimmerby moraine, for which we have used the terrestrial cosmogenic nuclide (10 Be) exposure dating technique to derive the exposure age of six glacially transported boulders. The six 10 Be cosmogenic ages are internally consistent, ranging from 14.9 ± 1.5 to 12.4 ± 1.3 ka with a mean of 13.6 ±0.9 ka. Adjusting for the effects of surface erosion, snow burial and glacio-isostatic rebound causes the mean age to increase only by c. 6% to c. 14.4± 0.9 ka. The 10 Be derived age for the Vimmerby moraine is in agreement with previous estimates forthe timing of deglaciation based on radiocarbon dating and varve chronology. This result shows promise for further terrestrial cosmogenic nuclide exposure studies in southern Sweden. 相似文献
10.
DEFORMATIONAL BEHAVIOURS OF ALLUVIAL UNITS DETECTED BY ADVANCED RADAR INTERFEROMETRY IN THE VEGA MEDIA OF THE SEGURA RIVER,SOUTHEAST SPAIN
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