The Little Ice Age glacier maximum in Iceland and the North Atlantic Oscillation: evidence from Lambatungnajökull, southeast Iceland

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.     

U-Pb Zircon Calendar for Namaquan (Grenville) Crustal Events in the Granulite-facies Terrane of the O'okiep Copper District of South Africa

The O'okiep Copper District is underlain by voluminous 1035–1210Ma 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 1700–2000 Ma zirconas xenocrysts in all of the intrusive rocks, and as detritalzircon in the metasediments of the Khurisberg Subgroup. Thesedata are consistent with published Sm–Nd model ages ofc. 1700 Ma (T_CHUR) and c. 2000 Ma (T_DM) of many of the intrusivesthat support a major crust-forming event in Eburnian (Hudsonian)times. In addition, U–Th–Pb analyses of zirconsfrom all major rock units define two tectono-magmatic episodesof the Namaquan Orogeny: (1) the O'okiepian Episode (1180–1210Ma), 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 (1020–1040Ma), 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 east–west-trending open folds [F₃(D_3a)], andby localized east–west-trending near-vertical ductilefolds [‘steep structures’; F₄(D_3b)] 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 800–850 Ma ago. KEY WORDS: U–Pb, zircon; O'okiep, Namaqualand; granite plutonism; granulite facies; Koperberg Suite; Namaquan (Grenville) Orogeny     

Quaternary geology and deglaciation of the continental shelf off Troms, north Norway

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.     

Controlling factors of volumetrically important marine carbonate cementation in deep slope settings

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.     

Geology in the Falkland Islands

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.     

Multi-Stage Modification of the Northern Slave Mantle Lithosphere: Evidence from Zircon- and Diamond-Bearing Eclogite Xenoliths Entrained in Jericho Kimberlite, Canada

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 Rb–Sr phlogopite megacrysts, 172·8 ±0·7 Ma U–Pb eclogite rutile, 178 ± 5 MaU–Pb eclogite zircon lower intercept) and JD-3 (173 ±2 Ma Rb–Sr phlogopite megacryst; 176·6 ±3·2 Ma U–Pb perovskite), and all yielded identicalresults within analytical uncertainty. As there is no discernibledifference in the radiometric ages obtained for these two pipes,the composite Rb–Sr 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 (0·7043–0·7084)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 (2–3%) 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 (22·6–27·5wt %) nature of garnet and the high-Na₂O (8·47–9·44wt %) and high-Al₂O₃ (13·12–14·33 wt %)character of the clinopyroxene. These eclogite whole-rock compositionsare highly enriched in high field strength elements (HFSE) suchas Nb (133–1134 ppm), Ta (5–28 ppm), Zr (1779–4934ppm) and Hf (23–64 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 (19·6–21·3 wt %) garnet andultralow-Na₂O (0·44–1·50 wt %) clinopyroxene.Paleotemperature estimates indicate that both the zircon- anddiamond-bearing eclogites have similar equilibration temperaturesof 950–1020°C and 990–1030°C, respectively,corresponding to mantle depths of 150–180 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 1·0–1·3 Ga and isrecorded by relatively consistent whole-rock eclogite modelNd ages and secondary U–Pb 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 1·88–1·84Ga 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 150–180 km. KEY WORDS: zircon- and diamond-bearing eclogites; Jericho kimberlite, geochronology; Precambrian metasomatism, northern Slave Craton     

Assessment of dispersive pressure as a beach placer mechanism

The concept of sediment sorting extending from the Bagnold constitutive relationship for sediment under shear is examined in the context of beach placer formation. The traditional interpretation of shear sorting is found to be inconsistent with data from new granular flow experiments. A binary mixture satisfying the shear equivalence relationship displayed pronounced segregation during shear. This observation is contrary to shear sorting as applied in the literature where no such segregation for this pairing of mineral species is expected. Further experiments minimizing the potential of grain percolation of smaller grains or kinetic sieving also demonstrated sorting patterns inconsistent with the existing shear sorting interpretation. Consequently, the shear sorting process, as currently formulated, may not be an appropriate means to describe beach placer formation. Kinetic sieving appears to be a more likely mechanism in this context, enhanced by the shearing and dilation of the granular‐fluid flow.     

Lateglacial environmental changes recorded in calcareous gyttja deposits at Gulickshof, southern Netherlands

Environmental changes were reconstructed from Lateglacial calcareous gyttja deposits at Gulickshof, southern Netherlands. Analyses of pollen, fresh-water mollusca, stable isotopes and geo-chemistry were performed and the combined evidence was put into a chronostratigraphic framework based on regional biostratigraphy and accelerator mass spectrometry radiocarbon dating. The pollen diagram showed vegetation development from the Bφlling interstadial into the Late Dryas stadial within 2.9 m of lacustrine deposits. Early in the Allerφd, around 11 900 BP, the composition of aquatic taxa and stable isotopes of calcium carbonate changed significantly. It appears that the final melting of relic ground-ice and subsequent re-establishment of the hydrological cell early in the Allerφd were responsible for these changes. Later in the Allerφd, around 11 250 BP, the disappearance of molluscs, changes in vegetation and isotope composition all point to colder climate conditions attributable to the Gerzensee oscillation. The combination of different lines of evidence in these types of deposits proves to be an excellent tool for unravelling climate and environmental signals.