zircon shrimp dating confirms a palaeoarchaean supracrustal terrain in the southeastern kaapvaal craton southern africa abstract we report shrimp zircon ages for felsic volcanic rocks of the early archaean nondweni greenstone belt ngb located in northern kwazulu natal province south africa the ngb is part of a chain of greenstone belt remnants that occurs south of the well known barberton greenstone belt bgb two samples of felsic volcanic rocks of the basal toggekry formation yielded zircon ages of 3 53 ga whereas significantly younger ages of 3 41 ga were obtained for two samples of felsic rocks from near the top of the witkop formation the latter date now constrains the age of stromatolite like structures in cherts associated with the felsic rocks which probably represent some of the oldest preserved evidence of life on earth the zircon ages also indicate that the ngb correlates well chronologically with the bgb and is one of the oldest volcano sedimentary successions in the southeastern kaapvaal craton geochemical data for the felsic ngb rocks suggest formation by partial melting or crystal fractionation of a mafic source that was contaminated by older crust possibly in a back arc environment initial e nd values for ngb rocks range from 1 2 to 0 3 and suggest involvement of pre greenstone continental crust that was possibly extensive in the southeastern part of the kaapvaal craton during palaeoarchaean times

the late devensian periglaciation of upland scotland abstract a wide variety of late devensian periglacial landforms developed on scottish mountains both before ca 13 000 bp and during the loch lomond stadial of ca 11 000 10 000 bp nearly all such features are now inactive late devensian periglacial weathering produced three types of regolith mantle openwork block deposits sandy diamicts and silt rich frost susceptible diamicts each of which supports a characteristic assemblage of relict landforms on upper slopes these include large scale sorted circles and stripes earth hummocks and nonsorted relief stripes sorted and nonsorted solifluction features massive boulder sheets and lobes and nivation benches talus protalus ramparts rock glaciers and alluvial fans also developed at the base of mountain slopes the distribution of late devensian periglacial features on scottish mountains is locally controlled by topography the response of underlying rocks to periglacial weathering and the limits of former glaciers regional variations in the altitude of certain forms of loch lomond stadial age particularly protalus ramparts and rock glaciers indicate a decrease in former snowfall eastwards across the scottish highlands and northwards from the highland boundary fault several upland periglacial features are also diagnostic of former permafrost and complement palaeotemperature reconstructions based on ice wedge casts and the equilibrium firn line altitudes of stadial glaciers these suggest that under stadial conditions mean january temperatures at 600 m and 1000 m on mountains in the western grampians must have been no higher than 20 c and 23 c respectively and possibly several degrees lower

reconstructing the environmental history of the last glacial interglacial transition evidence from the isle of skye inner hebrides scotland abstract a new interpretation is presented of the late devensian late weichselian wisconsinan lateglacial and early flandrian holocene environmental history of the isle of skye inner hebrides scotland this reconstruction based on pollen percentage pollen concentration pollen preservation and chemical data differs in a number of respects from previous explanations of the lateglacial environmental history of the island the evidence suggests that throughout the lateglacial and early flandrian variation in degree of exposure to prevailing westerly winds was the dominant factor influencing the development of regional vegetation patterns the data provide a proxy climatic record for western scotland during the last glacial interglacial transition and this is discussed in the context of both onshore and offshore evidence the results from skye demonstrate the importance of pollen preservation studies in the interpretation of pollen taphonomy and in the evaluation of lateglacial and early flandrian radiocarbon dates

origin of the northern atlantic s heinrich events as first noted by heinrich 1988 glacial age sediments in the eastern part of the northern atlantic contain layers with unusually high ratios of ice rafted lithic fragments to foraminifera shells he estimated that these layers are spaced at intervals of roughly 10 000 years in this paper we present detailed information documenting the existence of the upper five of these layers in odp core 609 from 50 n and 24 w their ages are respectively 15 000 radiocarbon years 20 000 radiocarbon years 27 000 radiocarbon years about 40 000 years and about 50 000 years we also note that the high lithic fragment to foram ratio is the result of a near absence of shells in these layers although we are not of one mind regarding the origin of these layers we lean toward an explanation that the heinrich layers are debris released during the melting of massive influxes of icebergs into the northern atlantic these sudden inputs may be the result of surges along the eastern margin of the laurentide ice sheet

a comparative study of olivine and clinopyroxene spinifex flows from alexo abitibi greenstone belt ontario canada a petrological and geochemical study of an olivine and of a clinipyroxene spinifex textured flow from alexo indicates that the initial liquid in both flows probably came from the same mantle melting event and that the source was incompatible element depleted the starting liquid of the clinopyroxene flow had experienced more olivine fractionation 10 prior to its emplacement at alexo than the initial liquid of the olivine spinifex flow

mineralogy of mafic xenoliths and their reaction zones in the olivine lamproite from prairie creek arkansas and the paragenesis of haggertyite ba fe6ti5mg o19 the recently discovered mineral haggertyite which belongs to the magnetoplumbite family occurs in the prairie creek lamproite exclusively in a reaction zone at the contact between the olivine lamproite and serpentized xenoliths compositional characteristics of the lamproite are an extreme enrichment in elements such as k ba and ti associated with very low al2o3 based on textural evidence it appears that the xenoliths represent a quenched very mafic magma possibly komatiitic an absence of perovskite in the reaction zone is the consequence of si diffusion from the xenolith iron available in the xenolith may be the controlling factor for the crystallization of haggertyite and other fe and ti oxides involved in the reaction other elements k ti and ba diffused from the lamproite toward the xenolith zone but only the outermost edge of the xenoliths reacted the crystallization of haggertyite appears to have resulted from a local chemical environment created by element diffusion between the two significantly contrasting lithologies

the petrography and geochemistry of komatiite flows from the abitibi greenstone belt and a model for their formation abstract the petrology and geochemistry of komatiites at four localities in the abitibi greenstone belt are presented the effects of alteration crystal fractionation and partial melting on their geochemistry are considered it is concluded that na k ca sr and rb have been disturbed during seafloor alteration but the other major elements and the trace elements ni cr co sc ree y zr and hf have not been mobilized variations in the geochemistry within the komatiite flows are the result of olivine and sulphide fractionation the komatiites consisting of branching olivines skeletal textured komatiites and coarse parallel plate olivines a 3 spinifex textured zones may represent crescumulates rather than frozen liquids mass balance calculations indicate that the flows were open systems and could represent lava tubes 26 34 partial melting of a primitive earth mantle 1 4 c 1 chondrite will produce melts similar to the komatiites at each deposit in terms of major elements ni cr co sc hree and y however on average the concentrations of highly incompatible elements lree zr and hf in the komatiites only are 60 70 of the modelled concentrations thus indicating that the komatiites were derived from a mantle depleted in highly incompatible elements

quartz cemented breccias from the mid atlantic ridge samples of a high salinity hydrothermal upflow zone textural mineralogical chemical and fluid inclusion studies on a suite of quartz rich sulfide bearing greenstone breccias indicate that high salinity high fluid flow systems were recently active on the mid atlantic ridge south of the kane fracture zone these systems involved fluids with temperatures close to 300 c and salinities almost 3 times that of seawater rounded pebblelike fragments enclosed within a matrix of euhedral quartz crystals appear to have been cemented by quartz deposition from a moving fluid which attained velocities in excess of 1 m s homogenization temperatures and freezing point depression effects in fluid inclusions from texturally early and late quartz crystals indicate temporal evolution of fluid temperatures and nacl equivalent salinities from 290 c and 10 wt to 200 c and 4 7 wt during alteration and cementation of the breccias highly variable cobalt concentrations in the evolving hydrothermal solutions are indicated by fine scale oscillatory zoning in euhedral pyrite within the breccias a continuous supply of the high salinity fluids may be generated either by supercritical two phase separation at depths well below the level of inclusion entrapment or by retrograde dissolution of hydroxy chloride phases in the deeper portions of the hydrothermal system either model requires the initial temperature of the deeply circulating fluids to have been distinctly higher than 415 c and the fluid to have been cooled by conduction or by mixing with lower temperature seawater prior to entrapment within the inclusions

comment on a vestige of earth s oldest ophiolite the claim by furnes et al reports 23 march 2007 p 1704 that greenland metavolcanic rocks require paleoarchean sea floor spreading is incompatible with their own data the purported sheeted dikes have the composition of pyroxenitic komatiite and could not have fed the adjacent ferroandesitic pillow lavas neither type has ophiolitic analogs and both are likely ensialic

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

the holocene and upper pleistocene pollen record from padul granada spain a new study two borings were made at the site of padul granada pollen analysis of 200 spectra from two successive sequences 14 8 and 8 m deep enables a description of the vegetational and climatic history of this region the most southern one in europe from early wurmian times the chronology is supported by twenty one 14c dates original equivalents are found for the great european prewurm interstadials a markedly arid though thermically not extreme episode exactly equivalent to stage 4 of the marine isotopic stratigraphy closes this relatively temperate period the middle part of the last glacial shows climatic fluctuations that are poorly characterized as is often the case in europe for this period a long section very probably corresponding to the complete last pleniglacial does not show any climatic amelioration towards 15 000 yr b p a change in the diagram is noted that can only be interpreted as reflecting a larger expansion of a regional steppe cover this event also reported on three occasions in southeast france marks the beginning of the oldest dryas the climatic amelioration of ca 13 000 yr b p is far more pronounced here than anywhere else in europe whereas that at 10 000 yr b p is not so clearly marked this unexpected result may be accounted for by the fact that padul is the first continental site so far south and so near pleniglacial refuges to be known on the other hand this result is consistent with what is known from oceanic isotopic stratigraphy a quite new late glacial and holocene vegetational history is revealed characterized by the early appearance and dominance of quercus ilex oak forests of a southern type and the early occurrence of quercus suber and olea it is the first account of a complete history of the postglacial reafforestation in a region with a modern semi arid mediterranean climate from the palaeoclimatic point of view it shows that the hot and humid holocene optimum was attained slightly before 8000 yr b p and that the holocene climatic fluctuations were of but small amplitude

implications of 36cl exposure ages from skye northwest scotland for the timing of ice stream deglaciation and deglacial ice dynamics geochronological constraints on the deglaciation of former marine based ice streams provide information on the rates and modes by which marine based ice sheets have responded to external forcing factors such as climate change this paper presents new 36cl cosmic ray exposure dating from boulders located on two moraines glen brittle and loch scavaig in southern skye northwest scotland ages from the glen brittle moraines constrain deglaciation of a major marine terminating ice stream the barra donegal ice stream that drained the former british irish ice sheet depending on choice of production method and scaling model this occurred 19 9 1 5 17 6 1 3 ka ago we compare this timing of deglaciation to existing geochronological data and changes in a variety of potential forcing factors constrained through proxy records and numerical models to determine what deglaciation age is most consistent with existing evidence another small section of moraine the scavaig moraine is traced offshore through multibeam swath bathymetry and interpreted as delimiting a later stillstand readvance stage following ice stream deglaciation additional cosmic ray exposure dating from the onshore portion of this moraine indicate that it was deposited 16 3 1 3 15 2 0 9 ka ago when calculated using the most up to date scaling scheme this time of deposition is within uncertainty the same as the timing of a widely identified readvance the wester ross readvance observed elsewhere in northwest scotland this extends the area over which this readvance has potentially occurred reinforcing the view that it was climatically forced

metabasalts axial hot springs and the structure of hydrothermal systems at mid ocean ridges knowledge of the chemical transfer and mineralogical transformations that occur when sea water reacts with basalt at elevated temperatures and pressures can be used along with geological and geophysical data to deduce the typical structure and evolutionary sequence for hydrothermal systems within the oceanic crust along the axis of a mid ocean ridge studies of metabasalt and metadiabase dredged from fault scarps along the axial valley of the mid atlantic ridge reveal consistent relationships among the bulk chemistry of the altered rocks and their secondary mineralogy mineral abundances and mineral compositions especially for chlorite these relationships can be interpreted in terms of the distribution of alteration with respect to time temperature and water rock ratio in young crust assemblages of chl ab ep act chl ab ep act qtz chl ab qtz and chl qtz are produced at successively higher effective sea water to rock ratios within the temperature range of the greenschist facies 250 to 450 c toward higher ratios chlorites tend to become more mg rich pillow basalts of layer 2 are commonly altered under these conditions by sea water on the descending rather than the ascending limb of a convection system this type of alteration which reflects moderately high water rock ratios may characteristically occur above a still partially molten magma chamber as the magma chamber solidifies and is then penetrated more pervasive alteration of the deeper part of layer 2 and of layer 3 occurs at lower water rock ratios producing solutions such as those recovered from springs on the galapagos rift and the east pacific rise at 21 n localized upwelling limbs of the convection system can produce veins filled with quartz sulfides and fe rich chlorite

the east pacific rise near 21 n 13 n and 20 s inferences for along strike variability of axial processes of the mid ocean ridge abstract data obtained along various segments of the mid ocean ridge mor are used to construct an idealized model for crustal accretion the model seeks to predict the topographic volcanic tectonic and hydrothermal characteristics of any given spreading segment of the mor as a function of distance away from the bounding transform faults this model is based on a series of detailed mapping efforts carried out on segments of the mor having a broad range of spreading rates between 2 and 16 cm yr this paper includes the results of the french searise program carried out during the summer of 1980 aboard the n o jean charcot and two american cruises conducted in 1981 aboard the r v melville

distal volcanic ash layers in the lateglacial interstadial gi 1 problems of stratigraphic discrimination abstract a new icelandic ash layer has been detected in mid interstadial sediments in a number of scottish lateglacial sequences and has been named the penifiler tephra it is rhyolitic in composition and possesses a chemistry which is similar to the borrobol tephra of early lateglacial interstadial age which also occurs in a number of these same sequences where the borrobol tephra has been identified in these sequences it consistently exhibits a diffuse distribution accompanied in some cases by stratigraphic bimodality a number of sedimentological and taphonomic factors are considered in order to account for this distribution one possibility is that these distributions are produced by taphonomic factors another possibility is that the borrobol tephra may not be the product of a single icelandic eruption but of two events closely spaced in time in at least two of the sequences investigated in this study basaltic shards were found in association with the penifiler and borrobol tephras suggesting either a basaltic phase associated with these eruptions or coincident eruptions from a separate basaltic volcanic centre the discovery of the new penifiler tephra makes a contribution to the regional tephrostratigraphic framework and provides an additional isochron for assessing the synchroneity of palaeoenvironmental changes during the interstadial the true stratigraphic nature and age of the borrobol tephra however remains unresolved and therefore its use as an isochron is more problematic the possible occurrence of basaltic populations may strengthen correlations with basaltic tephras recently detected in the ngrip ice core

cherts of the barberton greenstone belt interpreted as products of submarine exhalative activity cherts studied from the southern barberton greenstone belt are identified as replacement products of various pillow lavas pyroclastic deposits and sedimentary deposits silica filled fracture systems within the cherts have been found to be associated with this silicification process these systems include 1 silica filled fractures 2 silica vein stockworks 3 silica cemented breccias and 4 pseudoconglomerate bodies some of the silica filled fracture systems can be shown to have formed soon after sedimentation silica vein stockworks can be seen to develop laterally into silica cemented breccias the fragments of which have sometimes become rounded within the fracture systems to form pseudoconglomerate the cherts are interpreted as silicified hemi pelagic sediments that overlie pillow lavas some of which are now also cherts cells of convective sea water interacted with oceanic crust to produce silica rich fluids these fluids silicified lava and sediments on re entering the near surface env

considerations on the chronological and causal relationships between talus flatirons and palaeoclimatic changes in central and northeastern spain abstract talus flatirons are relatively common landforms in arid and semiarid areas they are frequent in the duero tajo and ebro tertiary basins in spain the generation of a flatiron requires the alternation of periods with a prevalence of accumulation processes followed by periods with a preponderance of erosion processes in the three tertiary basins considered here talus flatirons have been found in numerous places and up to five stages of slope evolution have been recognised in some locations the second youngest stage s 2 has been dated in several places in the ebro and duero basins through 14 c the age of this stage ranges from 2529 52 to 3590 40 14 c ybp the third youngest stage s 3 has yielded dates of 27 862 444 radiocarbon years bp in the ebro basin and 28 550 130 14 c ybp in the duero basin s 4 has been dated in the ebro basin to 35 570 490 14 c ybp the s 2 slope facets correspond to the iron and bronze age cold stages the s 3 and s 4 flatirons may be correlated with the heinrich events h 3 and h 4 these dates may indicate that the accumulation periods on the slopes correspond to cold global events the dates obtained for the stages s 3 and s 4 in the central and northeastern sectors of spain and their good correlation with heinrich events suggest that flatirons could be related to climatic sequences in the upper pleistocene and holocene

ore deposits associated with mafic magmas in the kaapvaal craton mafic and ultramafic magmatism played an important role in the 3 5 ga long history of the kaapvaal craton the oldest 3 5 ga greenstone belts contain mafic and ultramafic volcanics that erupted in an oceanic environment probably in oceanic plateaus then followed a series of continental flood basalts from the 3 4 ga old commondale and nondweni sequences to the 180 ma karoo basalts the history was dominated however by the emplacement 2 1 ga ago of the bushveld complex an enormous layered ultramafic mafic felsic intrusion three types of ore deposits might be found in such a sequence ni cu fe sulfides in komatiites of the greenstone belts noril sk type ni cu pge deposits in the karoo and other flood basalts and deposits of cr platinum group elements pge and v in the bushveld and other layered intrusions only the latter are present it is tempting to attribute the absence of komatiite hosted deposits to the specific character of the ultramafic rocks in kaapvaal greenstone belts which are older that the 2 7 ga komatiites that host deposits in australia canada and zimbabwe and are of the less common al depleted type however a review of mantle melting processes found no obvious connection between the character of the mantle melts and their capacity to form ore deposits the lack of this type of deposit may be due to differences in the volcanic environment or it may be fortuitous the barberton and other belts are small and could fit into deposit free parts of the much larger australian or canadian belts still more puzzling is the absence of noril sk type deposits the karoo and older flood basalt sequences appear to contain all the important elements of the volcanic sequences that host the siberian deposits it is now recognised that these deposits formed through the segregation of sulfide from magma flowing rapidly through conduits en route from deeper magma chambers to the surface an exploration approach aimed at understanding the fluid dynamics of such systems seems warranted although the bushveld intrusion has been studied for decades and its deposits are taken as type examples of magmatic mineralisation the origin of its pge deposits remains unclear opinion is divided on the relative importance of sulfide segregation from magma filling a large chamber at the time of emplacement and the scavanging of pge from fluids circulating through cumulates at a late magmatic stage answers to these questions may come from studies designed to gain a better understanding of the mechanisms through which the magma chamber filled and solidified

the proximal volcanic environment of the scotia nickel deposits detailed exploration of the stratigraphy associated with the scotia nickel sulfide deposit has led to the identification of a succession of ultramafic flows displaying decreasing relative magnesium content both up the pile and along strike from the deposit in addition vent facies ultramafic pyroclastics and massive olivine peridotite flows dominate the locally thickened pile immediately overlying the ore interflow sediments are absent from this vent facies material but are present both higher in the sequence and along its flanks the relatively undeformed and weakly recrystallized stratigraphy at scotia affords an opportunity to reconstruct the development of an ultramafic succession proximal to a volcanic vent the presence of ultramafic pyroclastics amygdaloidal units and a crosscutting ultramafic intrusive characterize the vent environment and distinguish the overlying pile at scotia from that at other western australian nickel sulfide deposits

the genesis and significance of hummocky moraine evidence from the isle of skye scotland abstract the range of genetic and climatic interpretations of scottish hummocky moraine is reviewed and new data are presented from the isle of skye western scotland which are used as the basis of a genetic classification hummocky moraine on skye is shown to consist of three principal sediment landform associations 1 recessional moraines 2 chaotic ice stagnation moraines and 3 drumlins and fluted moraines the recessional moraines consist of transverse moraine ridges and chains of mounds and were formed by a combination of glaciotectonics and debris accumulation at active ice margins second chaotic moraines consist of randomly distributed hummocks mounds and rim ridges and record deposition in contact with inactive ice finally drumlins and fluted moraines are longitudinally oriented subglacial bedforms formed by a combination of lodgement and sediment deformation individual occurrences of hummocky moraine may comprise one two or all of these associations the detailed study and differentiation of scottish hummocky moraine provides a valuable source of information on former glacier dynamics and landscape change

rapid environmental changes in southern europe during the last glacial period oxygen isotope records from greenland ice cores1 2 indicate numerous rapid climate fluctuations during the last glacial period north atlantic marine sediment cores show comparable variability in sea surface temperature and the deposition of ice rafted debris3 4 5 in contrast very few continental records of this time period provide the temporal resolution and environmental sensitivity necessary to reveal the extent and effects of these environmental fluctuations on the continents here we present high resolution geochemical physical and pollen data from lake sediments in italy and from a mediterranean sediment core linked by a common tephrochronology our lacustrine sequence extends to the past 102 000 years many of its features correlate well with the greenland ice core records demonstrating that the closely coupled ocean atmosphere system of the northern hemisphere during the last glacial4 extended its influence at least as far as the central mediterranean region numerous vegetation changes were rapid frequently occurring in less than 200 years showing that the terrestrial biosphere participated fully in last glacial climate variability earlier than 65 000 years ago our record shows more climate fluctuations than are apparent in the greenland ice cores together the multi proxy data from the continental and marine records reveal differences in the seasonal character of climate during successive interstadials and provide a step towards determining the underlying mechanisms of the centennial millennial scale variability

catastrophic arid episodes in the eastern mediterranean linked with the north atlantic heinrich events the response of continental climate to the well documented climate oscillations during the last glacial period has been a subject of intense interest yet much less is known about the influence on regional continental climates than in the marine or polar realms of earth the detailed lake level history of the closed lake lisan paleo dead sea in the middle east has been reconstructed from shoreline indications and high resolution u th and 1 4 c chronologies thus providing data on the response of the lake s catchment area to the climate changes during the corresponding period we present a correlation between the newly developed lake lisan level curve for the past 55 k y and the north atlantic heinrich events the correlation indicates a closely connected climate response between these north atlantic events and the hydrologic conditions that prevailed in the eastern mediterranean our findings show that although the generally cooler conditions that prevailed during the last glaciation favored high levels of the lake catastrophic events in the north atlantic which are associated with maximum cooling have been responsible for droughts in the eastern mediterranean we infer that cold water input to the mediterranean originating in the collapse of the north atlantic deep water circulation caused the reduction of evaporation and less precipitation in the eastern mediterranean

significance of snowblow in the generation of loch lomond stadial younger dryas glaciers in the western pennines northern england geomorphological mapping has identified the former existence of five cirque glaciers that formed during the loch lomond stadial in the western pennines northern england landforms in mallerstang which previously have been interpreted as moraine ridges are shown to be better explained as large complex landslides reconstruction of these former glaciers has allowed the calculation of the former equilibrium line altitudes ela by a number of different methods values for the ela show a range across the area from 311 m to 608 m od but with low values 311 m and 428 m for the two western glaciers these are explained by the existence of large plateau areas adjacent to the former glaciers which contributed additional mass to the glaciers by snowblow on to the glacier surfaces delimitation of the potential snowblow area for each glacier shows that it has a distinct orientation with the western sector 225 315 being statistically significant with the ela indicating the importance of winds from this direction in determining the existence of these palaeoglaciers the significance of snowblow for the generation of small glaciers in marginal areas means that such local factors must be investigated before regional firn line trends are determined in former glaciated areas

the behaviour of so called immobile elements in hydrothermally altered rocks associated with volcanogenic submarine exhalative ore deposits evidence is available that some elements notably zr tio2 y sc ce and nb are largely immobile during the alteration of volcanic rocks owing to metamorphism hydrothermal events and weathering e g floyd and winchester 1978 however it is shown by reference to analyses of rocks from the environment of five volcanogenic massive sulphide bodies that while zr tio2 and ce are mostly immobile even during intense hydrothermal alteration y and particularly sc and nb may be extremely mobile when elements are removed by solution in a hydrothermal fluid it seems that reaction rates are such that these elements are almost totally removed from the rock therefore of the so called immobile trace elements only zr and tio2 may be used with any reliability to identify the degree of magmatic differentiation in an hydrothermally altered rock however if an element has been mobile it is usually readily identified as having moved

the last ice sheet in north west scotland reconstruction and implications recent models of the last scottish ice sheet suggest that nunataks remained above the ice surface in areas peripheral to the main centres of accumulation this proposition has been investigated on 140 mountains over an area of 10 000 km2 in nw scotland outside the limits of the later loch lomond readvance in this area there is evidence for a single high level weathering limit that separates glacially eroded terrain from higher areas of in situ frost debris this limit occurs at altitudes ranging from 425 to 450 m in the outer hebrides to 950 m on the mainland and is best developed on lithologies that resisted breakdown after ice sheet downwastage interpretation of this weathering limit as a periglacial trimline cut by the last ice sheet at its maximum thickness is supported by 1 joint depth and schmidt hammer measurements that indicate significantly more advanced rock breakdown above the weathering limit 2 a much greater representation of gibbsite a pre late devensian weathering product in the clay fraction of soils above the limit 3 cosmogenic isotope dating of the exposure ages of rock outcrops above and below the limit 4 the sharpness of the limit at some sites and its regular decline along former ice flowlines and 5 shear stress calculations based on the inferred altitude and gradient of the former ice surface reconstruction of the ice surface based on trimline evidence indicates that the mainland ice shed lay near or slightly east of the present watershed and descended northwards from 900 m to ca 550 m at the north coast independent dispersion centres fed broad ice streams that occupied major troughs on skye an ice dome 800 m deflected the northwestwards movement of mainland ice but the mountains of rum were over ridden by mainland ice up to an altitude of ca 700 m the outer hebrides supported an independent ice cap that was confluent with mainland ice in the minches extrapolation of the trimline evidence indicates that most reconstructions of ice extent are too conservative and suggests that low gradient ice streams extended across the hebridean shelf offshore wider implications of this research are 1 that blockfields and other periglacial weathering covers are not all of the same age or significance depending on the resistance of different lithologies to frost weathering 2 that the contrasting degree of glacial modification in the western and eastern highlands of scotland may reflect a former cover of predominantly warm based ice in the former and predominantly cold based ice in the latter and 3 that the approach and techniques developed in this study have potential application for constraining ice sheet models not only in areas peripheral to the main centres of ice accumulation in britain and ireland but also in other mountain areas where nunataks protruded through warm based late pleistocene ice masses

a high resolution seismic stratigraphy from a scottish sea loch and its implications for loch lomond stadial deglaciation a detailed high resolution seismic stratigraphy calibrated by core data and terrestrial geomorphological mapping has been constructed for loch ainort isle of skye this study has provided a palaeoenvironmental history of the area as well as important corroborative evidence for the stepped deglaciation of the loch lomond stadial ice field on skye the ainort glacier reworked pre loch lomond glacial deposits terminating in a grounded tidewater ice front potentially 800 m beyond the previously extrapolated limit the first stage of deglaciation was characterised by the formation of de geer moraines indicative of a period of interrupted retreat the second phase by contrast produced hummocky relief with sporadic linear moraines suggesting periods of uninterrupted retreat with occasional stillstands readvances paraglacial reworking of terrestrial slopes resulted in the deposition of thick subaqueous debris flows which graded into fluvioglacial dominated sediments and ultimately modern fjordic deposits the identification of an initial period of active retreat punctuated by numerous readvances correlates directly with the terrestrial record however the offshore stratigraphy suggests that although the second phase was dominated by uninterrupted retreat occasional stillstands readvances did occur

why was flood volcanism on submerged continental platforms so common in the precambrian abstract many archean and proterozoic greenstone belts contain volcanic rocks that erupted subaqueously but interacted with continental crust on their way to the surface this points to emplacement in a tectonic setting that was rare to absent throughout the phanerozoic the eruption of flood basalts onto submerged continental platforms to explain the situation requires that sea levels were high during periods of flood volcanism perhaps because the ocean basins were partially filled by extensive active and high standing oceanic ridges or by oceanic plateaus the common preservation of crust contaminated submarine basalts even in the oldest supracrustal sequences may indicate that continental crust was voluminous even at an early stage in earth history

a smnd and pb isotope study of archaean greenstone belts in the southern kaapvaal craton south africa abstract a sm nd and pb study on a wide variety of lithologies in archaean greenstone belt fragments in the southern kaapvaal craton reveals a complex petrogenetic history the fragments are important because they represent a 350 km transect through the craton south of barberton to its southern margin the commondale greenstone belt yields a precise sm nd age of 3334 18ma on an exceptionally well preserved peridotite suite of komatiitic affinity the wide range of sm nd from 0 6 to 1 0 is attributed to the unusual occurrence of orthopyroxene in the spinifex bearing rocks a considerably younger age of about 3 2 ga is suggested for the nondweni greenstone belt close to the southern margin of the craton on the basis of separate sm nd isochrons on individual lithologies ranging from komatiite through komatiitic basalt and basalt to felsic volcanic rocks the most precise sm nd age for the greenstone belt of 3208 87ma is based on a combined mineral whole rock isochron for komatiitic basalts but the possibility cannot be excluded that this represents a later metamorphic age on the basis of the present study the greenstone belts appear to have been emplaced at progressively younger ages towards the southern margin of the craton of greater significance is that irrespective of age the individual lithologies of the nondweni sequence have isotopically distinct initial ratios combining isotopic data from these lithologies invariably yields apparent older ages the isotopic characteristics are consistent with a model of progressive contamination by older sialic crust thought to be the basement to the nondweni sequence

the loch lomond readvance on the isle of mull scotland glacier reconstruction and palaeoclimatic implications geomorphological mapping of mull provides evidence for an icefield 143 km2 in area flanked by six corrie glaciers with a total area of ca 13 km2 the absence of lateglacial periglacial features shorelines and pollen sites from the area occupied by this readvance together with radiocarbon dating of shell fragments confirm that it occurred during the loch lomond younger dryas stade the thickness of glacigenic deposits within the area of the readvance is attributed to reworking of paraglacial sediments up valley continuity of recessional moraines indicates that the ice remained active and near to equilibrium during retreat consistent with slow warming following the coldest part of the stade reconstructed equilibrium line altitudes elas imply vigorous snow blowing by westerly winds and are consistent with a general southwestwards decline in elas across the scottish highlands an ela of 250 m was calculated for the mull icefield using an ablation accumulation balance ratio abr approach palaeotemperature and palaeoprecipitation estimates were derived by calculating a theoretical regional ela from meteorological data and assuming that the combination of temperature and precipitation implied by the theoretical ela approximates conditions at 250 m on mull during the loch lomond stade the result indicates a mean july sea level temperature of 5 7 0 5 c and a mean annual precipitation at 250 m of ca 2700 3800 mm best estimate 3200 mm indicating higher precipitation totals than at present owing to more vigorous atmospheric circulation copyright 2002 john wiley sons ltd

deep sea records from the southeast labrador sea ocean circulation changes and ice rafting events during the last 160 000 years 1 results from two deep sea cores from northeast of newfoundland at 1251 and 2527 m water depth respectively indicate that during the time period from 160 000 to 10 000 years bp ice rafting events in the labrador sea were accompanied by rapid variations in deep and surface water circulation twelve ice rafting events occurred each coinciding with high concentrations of detrital carbonate and oxygen isotopic depletion of both surface and bottom waters eleven of these can be correlated with the north atlantic heinrich events h1 h11 the remaining very conspicuous ice rafting event took place early in mis substage 5e at a time when the planktic faunal assemblage suggests marked warming of the sea surface in the shallower core benthic delta 13 c values rise from a minimum during the deglaciation to peak substage 5e values following the last ice rafting event indicating that the ventilation of intermediate depths was renewed after the deglaciation was complete and continued throughout substage 5e the benthic foraminifera suggest that this well ventilated water mass was comparable to the modern labrador sea water lsw the benthic faunas suggest that a relatively warm intermediate water mass entered the se labrador sea during heinrich events generally low benthic delta 13 c values indicate that this water mass was poorly ventilated and rich in inorganic nutrients isotope data and benthic faunal distributions indicate that north atlantic deep water nadw formed in the norwegian greenland sea reached the se labrador sea between the heinrich events

reconstruction of a lateglacial younger dryas mountain ice field in sutherland northwestern scotland and its palaeoclimatic implications quaternary geomorphological and geological mapping of a large area 1800 km2 in the northwestern scottish highlands has led to the reconstruction of a coherent mountain ice field approximately 45 km long and 30 km wide at its maximum extent this west sutherland ice field covered an area of 350 km2 and had a mean equilibrium line altitude of 334 m assuming that the outlet glaciers all reached their maxima simultaneously numerical dating strongly suggests a younger dryas 12 9 11 7 ka age of the mapped limits of this mountain ice field this study also shows that the area of northwestern scotland covered by younger dryas glaciers was five times larger than previously thought palaeo temperature estimates obtained from chironomid and beetle assemblages elsewhere have been used to reconstruct palaeo precipitation during the younger dryas for west sutherland using a previously published global empirical dataset annual precipitation of 2344 248 mm a 1 at sea level is inferred from our new palaeo glacier reconstruction suggesting slightly wetter conditions compared to the present day these figures are in good agreement with those reconstructed for other parts of the scottish highlands and islands and this may indicate a marked w e palaeo precipitation gradient during the younger dryas this paper goes on to discuss the wider validity of palaeo climatic reconstructions from inferred palaeo glacier geometries and presents a number of possible scottish lateglacial climate scenarios which warrant further testing copyright 2010 john wiley sons ltd

late devensian and holocene relative sea level changes on the isle of skye scotland uk detailed litho and biostratigraphical analyses from three coastal sites in contrasting coastal settings on the isle of skye scotland uk reveal evidence for several changes in relative sea level during the late devensian and holocene at the start of the record relative sea level in the area was high at ca 12 500 14c ca 14 800 cal yr bp but then fell reaching a low point during the younger dryas at ca 11 000 10 000 14c ca 13 000 11 600 cal yr bp when a rock platform correlated with the main rock platform was formed in the early middle holocene relative sea level was rising by ca 8000 14c ca 8800 cal yr bp and in northeast skye a lagoonal surface correlated with the main postglacial shoreline was formed at ca 6600 14c ca 7500 cal yr bp by the late holocene relative sea level was again falling but a rise registered at at least two sites began probably before ca 4000 14c ca 4500 cal yr bp and a second lagoonal surface in northeast skye correlated with the blairdrummond shoreline was formed although by ca 3000 14c ca 3200 cal yr bp relative sea level in the area had resumed its downward trend the pattern of relative sea level changes disclosed is compared with evidence elsewhere in scotland

allochthonous and autochthonous mire deposits slope instability and palaeoenvironmental investigations in the borve valley barra outer hebrides scotland ashmore p brayshay b a edwards k j gilbertson d grattan j kent m pratt k weaver r allochthonous and autochthonous mire deposits slope instability and palaeoenvironmental investigations in the borve valley barra outer hebrides scotland the holocene 2000 10 1 pp 97 108

cosmogenic cl 36 dating of postglacial landsliding at the storr isle of skye scotland major postglacial rock slope failures are a common feature of the scottish highlands and other mountainous areas that were deglaciated at the end of the pleistocene but evaluation of the causes and triggers of failure has been hindered by a lack of reliable dating evidence we report the result of a pilot study designed to establish the absolute age of a large postglacial rotational rockslide at the storr on the isle of skye scotland using 36cl surface exposure dating exposure ages of 6 3 0 7 cal ka bp and 6 6 0 8 cal ka bp were obtained for rock samples from two separate landslide blocks giving an overall age estimate of 6 5 0 5 cal ka bp for rock slope failure at this site this date is consistent with ams radiocarbon dating of windblown sand derived from the failure scarp and with previous inferences based on relative dating evidence concerning an early holocene age for most rock slope failures in the scottish highlands the long time lag 7 ka between deglaciation and failure sugge

geochemistry of the mafic ultramafic volcanic associations of the nondweni greenstone belt south africa and constraints on their petrogenesis abstract the 3 4 ga nondweni greenstone belt is situated near the southern margin of the kaapvaal craton and consists of predominantly pillowed and spinifex textured volcanic rocks primary volcanic structures and compositional characteristics of this succession are unique in currently known archaean geology none of the lavas exceeds 23 mgo anhydrous and pyroxene rather than olivine dominates the crystallisation assemblages lower mgo higher sio 2 contents and different incompatible trace element ratios e g lower ti zr of the mafic ultramafic lithotypes distinguish the nondweni greenstone belt from the barberton and other archaean belts the geochemistry permits subdivision into five categories komatiites komatiitic basalts komatiitic andesites basalts and basaltic andesites progressive fractionation or partial melting of a discrete mantle source although supported by co linearity on some variation diagrams cannot account for a group of incompatible depleted basaltic rocks or for the high silica content of some komatiitic andesites contamination of mafic ultramafic melts by felsic material is supported by isotopic data but cannot alone account for sio 2 contents of up to 58 in some of the komatiitic rocks ree chemistry also argues against consaguinuity of the various rock types the komatiites have gd yb n 1 and are either lree depleted or lree enriched komatiitic basalts and komatiitic andesites are invariably lree enriched but the latter show overall depletion in ree relative to the komatiitic basalts a similar relation characterises the basalts and basaltic andesites polybaric melting of a mantle source and contamination by sialic crust followed by fractional crystallisation may explain the compositional characteristics of the nondweni lavas

the early archaean nondweni greenstone belt southern kaapvaal craton south africa part ii characteristics of the volcanic rocks and constraints on magma genesis the early archaean nondweni greenstone belt is dominated by mafic volcanic rocks which have a wide range in compositions primary volcanic structures and compositional characteristics point to this succession being unique in archaean geology and an important indicator of controls of magma formation and environment of deposition compositionally the nondweni lavas are broadly similar to those of the barberton sequence high values for cao al2o3 ti y and low al2o3 tio2 they are also distinctly different in that sio2 content is markedly higher and incompatible trace element ratios are also different the high silica content has promoted the subdivision of komatiitic and basaltic rocks into the following categories komatiite komatiitic andesite komatiitic basalt basalt and basaltic andesite the recognition of the distinct suite of komatiitic andesites is aided by the lower zr ti ratios than those lithologies of equivalent mgo content reported from other greenstone belts and sio2 contents as high as 58 these characteristics suggest similarity to recent high mg andesites none of the lava compositions exceed 23 mgo anhydrous and pyroxene rather than olivine dominates the crystallization assemblage olivine spinifex textures are entirely absent in the nondweni greenstone belt and instead pyroxene or amphibole after pyroxene spinifex is abundantly developed and is characterised by spectacular radial conical structures which have their apices towards the tops of the flow units some of the spinifex cone structures attain heights of up to 6 m branching pyroxene crystals and feathery textured plagioclase typify the spinifex textures of the komatiitic andesites the common occurrence of coned and branching spinifex textures in a wide range of magma compositions indicates that these were controlled by thermal characteristics and depositional environment for the flow units rather than solely by composition this may have been facilitated by emplacement in shallow water rather than in deep water as envisaged for the onverwacht group of the barberton sequence shallow water depositional environments in the nondweni group are supported by sedimentary lithofacies well preserved pillow structures also show a wide range in textural forms sizes of pillows degree of welding of pillow margins development of hyaloclastite and formation of shelf structures formed by repeated filling of lava tubes are related to both compositional type as well as physical controls such as extrusion rate and nature of the substrate komatiites seldom form pillows and very large pillow formation is restricted to the komatiitic andesite lava type komatiite flow successions are typified by spinifex textured units as well as by massive and highly brecciated aphyric flow units unaltered pyroxene and plagioclase characterise some of the massive flow units and pillowed basalts of the witkop formation the present paper is the second part of a series on the nondweni greenstone belt part i wilson and versfeld 1994 describes the stratigraphy sedimentology mineralization and depositional environment there are strong of kaapvaal craton may have been different to that of the barberton greenstone belt a present day analogue to the nondweni tectonic setting may be an ensialic back arc basin adjacent to a continental margin

scarp retreat rates in semiarid environments from talus flatirons ebro basin ne spain abstract triangular slope facets or talus flatirons are presented as a useful tool for the calculation of scarp retreat rates in arid regions in the central sector of the ebro basin sequences of talus flatirons have been developed around mesas capped by miocene limestone these talus flatirons are grouped in four stages of slope evolution s2 s3 s4 s5 by the extrapolation of the relict talus profiles with the adjusted log functions the position of the scarp for each one of the stages is located the mean values obtained indicate a scarp retreat of 23 m between stages s3 and s2 8 m between s4 and s3 and 26 m between s5 and s4 the ages obtained by radiocarbon dating are 2529 52 b p and 2930 60 b p for stage s2 27 864 444 b p for s3 and 35 570 490 b p for s4 consequently the mean scarp retreat rates are 0 9 mm yr for the s2 s3 interval and 1 mm yr for s3 s4 the scarp retreat rate is related to the lithological and structural features of both the caprock and the underlying clay sediments and to the palaeoclimatic evolution of the region that controls the intensity of weathering and water erosion processes that affect the slope system

emplacement crystallization and alteration of spinifex textured komatiitic basalt flows in the archaean nondweni greenstone belt southern kaapvaal craton south africa a well exposed succession of spinifex textured komatiite flows is reported from the archaean nondweni greenstone belt located near the southern margin of the kaapvaal craton the flows are relatively thin 1 5 m compared to similar occurrences in other greenstone belts they are characterised by well developed cone structures of highly elongate amphibole crystals after augite which fan downwards from the tops of the flows extreme development of coned spinifex has not been reported from other greenstone belts and points to specific thermal conditions prevailing in the nondweni environment the zones of bladed spinifex are contained between layers of random spinifex and overlie a lower cumulus layer originally of augite orthopyroxene and minor olivine the observed major and trace element distributions through a 1 7 m thick spinifex textured flow are consistent with a model involving concentration of phenocryst phases resulting in significant fractionation upwards in the flow approximately 40 of the spinifex textured phenocrysts grew in situ after the lithological units were established collapse and displacement of the coned crystal networks originally attached to the top of the flow are shown to have influenced the distribution of liquid within the flow and accentuated the fractionation associated with the spinifex textured units are massive aphyric and brecciated flows which show distinct chemical cycles through the succession the brecciated zones have compositions with 18 mgo and are characterised by ovoid bodies that are not pillows and may represent magmatic reworking and movement of a partly congealed flow post solidus alteration is considered to have caused early hydration of the original mineralogy and also introduced sio2 and na2o into the upper part of the flow by way of microfractures the observed alteration is different to that of mid ocean ridge basalts and a subaerial shallow water environment is suggested

vedde ash layer discovered in a small lake basin on the scottish mainland a thin layer of micro tephra particles has been discovered in a lake sediment sequence from sutherland scotland the tephra is attributed to the vedde ash on the basis of ash geochemistry the vedde ash which has been dated to c 10 300 radiocarbon years bp and which was thus deposited during the younger dryas chronozone has previously been reported from lake sequences in norway from sediment cores from the ne north atlantic and from greenland ice cores but this is the first reported record from the british mainland it is likely that this ash layer also occurs in other lake sediment sequences in northern britain thus providing the potential for more precise correlations of younger dryas sequences than has hitherto been the case the way in which correlation by tephrochronology may revolutionize approaches to reconstructing the sequence of events in thene atlantic region during the last glacial interglacial transition is briefly considered

emplacement and cooling of komatiite lavas we propose that komatiite lavas were emplaced as turbulent flows accompanied by vigorous forced convection with cooling rates often in excess of hundreds of c h 1 they melted and assimilated up to 10 of the ground over which they flowed forming deep channels nickel sulphide mineralization may have resulted from incorporation of sulphur rich sediment after emplacement high cooling rates persisted resulting in spinifex textures due to compositional convection

three new distal tephras in sediments spanning the last glacial interglacial transition in scotland three new microtephras are reported from a number of lake sites from the inner hebrides and scottish mainland one occurs stratigrapically in the middle of greenland interstadial 1 gi 1 and has been named the penifiler tephra it is rhyolitic and possesses a geochemical signature that is very similar to that of the borrobol tephra which also occurs in three of the sequences reported here but which lies close to the lower boundary of gi 1 the second occurs stratigraphically in the early holocene below the saksunarvatn ash and is named the ashik tephra this tephra is geochemically bimodal with a rhyolitic component comparable to the an druim tephra that occurs later in the holocene and a basaltic component which is similar to the saksunarvatn ash a third tephra occurs stratigraphically above the saksunarvatn ash and is provisionally named the breakish tephra the consistent inter site correlation demonstrated for these new tephras at several sites enhances the regional tephrostratigraphic framework and increases the potential for correlating palaeoenvironmental events during gi 1 and the early holocene however the occurrence of multiple tephras with similar geochemistry in close stratigraphic and temporal proximity has implications for the rigour with which tephrostratigraphic investigations must be performed copyright 2006 john wiley sons ltd

komatiites i eruption and flow because of their high eruption temperatures and ultrabasic composition komatiite lavas had low viscosities which typically ranged from 0 1 to 10 pa s a major consequence of this low viscosity is that most lavas erupted as turbulent flows an analysis of their ascent through the lithosphere suggests ascent velocities in the range of 1 to over 10ms 1 and reynolds numbers much greater than the critical value of 2000 the lavas would have remained turbulent for most or all of their subsequent flow and emplacement typical horizontal flow rates are estimated to range from 0 5 to 100 m2 s per unit width of flow such turbulent lava flows would have lost their heat by convection to the surroundings at rates which are orders of magnitude greater than the rates for laminar flows which cool by conduction a quantitative analysis of the cooling of komatiites indicates cooling rates from over 1000 c hr to a few c hr 1 while the flows remained turbulent these rates are in an appropriate range to cause phenomena such as high nucleation rates strong supersaturation of the lava delayed nucleation of olivine and skeletal or dendritic crystal morphologies komatiites often flowed over ground composed of rocks with lower melting temperatures it is proposed that the turbulent lavas melted the ground to form deep thermal erosion channels melting rates at the lava source are calculated at several metres per day and deep troughs with depths of several metres to hundreds of metres and lengths of several kilometres probably formed laboratory experiments performed to simulate thermal erosion show qualitative agreement with the theory with channel depth decreasing downstream the experiments also revealed that the channel margins become undercut during thermal erosion to form overhanging sides of the channel some sinuous rilles observed in the mare regions of the moon are thought to have formed by thermal erosion hulme 1973 they provide analogues of the channels postulated to form in komatiite eruptions where conditions were in fact more favourable for thermal erosion an assessment of the role of olivine crystals precipitated in the cooling turbulent flows indicates that they will remain in suspension until the lava has come to rest contamination of komatiite lava by underlying rock can be as much as 10 per cent some illustrative calculations show how the major element and trace element compositions of residual melts can be significantly modified by combined assimilation and fractional crystallization in a moving flow assimilation of tholeiitic basalt into a komatiite can cause incompatible trace element ratios such as ti zr and y zr and the rare earth patterns of derivative lavas to vary substantially some of the variations in such geochemical parameters which are often ascribed to mantle heterogeneity also could have resulted from assimilation of the ground assimilation could have modified the isotope geochemistry of lava suites and led to apparent ages which differ from the true eruption age the thermal erosion model also provides an explanation of the formation of some nickel sulphide ores found at the bottom of thick komatiite flows it is proposed that ores can form by assimilation of sulphur rich sediment which combines with ni from the komatiite to form an immiscible liquid

extent and deglacial chronology of the last british irish ice sheet implications of exposure dating using cosmogenic isotopes over 160 10be and 36cl exposure ages pertaining to the extent and chronology of the last british irish ice sheet biis are critically reviewed despite uncertainties inherent in the use of exposure ages this approach has demonstrated that at the last glacial maximum lgm ca 26 21 ka the biis extended over all low ground in scotland and all or almost all of ireland consistent with recent models depicting extension of the last biis to the atlantic shelf edge all exposure ages obtained for high level sites above trimlines on the mountains of northwestern scotland and ireland pre date the lgm this finding confirms that high plateaux in these areas escaped significant glacial erosion probably under a protective cover of cold based glacier ice that remained frozen to the underlying substrate in terms of deglaciation chronology the southern irish sea basin was probably deglaciated by 19 18 ka with extensive deglaciation of low ground in ireland and northeastern scotland prior to ca 14 5 ka ice cover on low ground in northwestern scotland apparently persisted after 14 0 ka suggesting survival of ice in favourable locations during the lateglacial interstade ca 14 5 12 9 ka attempts to date readvances have had mixed success the wester ross readvance in northwestern scotland was initially dated to ca 16 3 ka but more recent research implies a much younger age ca 14 0 13 5 ka exposure ages indicating a prolonged stillstand during ice sheet retreat in the cairngorms at ca 14 5 14 0 ka conflict with ages obtained on postglacial rockslides and basal radiocarbon dates which indicate deglaciation before ca 15 5 15 0 ka exposure dating has been successfully employed to confirm a loch lomond stadial ca 12 9 11 5 ka age for moraines in several mountain areas and to constrain the minimum altitude of the ice cap that extended over the western grampians at that time copyright 2009 john wiley sons ltd

the impact of cold north atlantic sea surface temperatures on climate implications for the younger dryas cooling 11 10 k the sensitivity of global climate to colder north atlantic sea surface temperatures is in vestigated with the use of the giss general circulation model north atlantic ocean temperatures 18 000 b p resembling those prevalent during the younger dryas were incorporated into the model of the present climate and also into an experiment using orbital parameters and land ice characteristic of 11 000 b p the results show that with both 11 000 b p and present conditions the colder ocean temperatures produce cooling over western and central europe in good agreement with younger dryas paleoclimatic evidence cooling also occurs over extreme eastern north america although the precise magnitude and location depends upon the specification of ocean temperature change in the western atlantic despite the presence of increased land ice and colder ocean temperatures the younger dryas summer air temperatures at northern hemisphere midlatitudes in the model are warmer than those of today due to changes in the orbital parameters chiefly precession and atmospheric subsidence at the perimeter of the ice sheets

millennial scale iceberg discharges in the irminger basin during the last glacial period relationship with the heinrich events and environmental settings high resolution records of coarse lithic content and oxygen isotope have been obtained in a piston core from the irminger basin the last glacial period is characterized by numerous periods of increased iceberg discharges originating partly from iceland and corresponding to millennial scale instabilities of the coastal ice sheets and ice shelves in the nordic area a comparison with midlatitude sediment cores shows that ice rafted material corresponding to the heinrich events was deposited synchronously from 40 to 60 n there are thus two oscillating systems every 5 10 kyr massive iceberg armadas are released from large continental ice caps whereas more frequent instabilities of the coastal ice sheets in the high latitude regions occur every 1 2 3 8 kyr at the time of the heinrich events the synchroneity of the response from all the northern hemisphere ice sheets attests the existence of strong interactions between the two systems

multiple talus flatirons variations of scarp retreat rates and the evolution of slopes in almazan basin semi arid central spain abstract we studied a group of talus flatirons situated at the foot of structural platforms capped by miocene limestones in the almazan basin a southeastern extension of the tertiary duero basin in northern spain this group of talus flatirons exhibits five different stages of slope evolution s 1 s 2 s 3 s 4 and s 5 they are concave slopes separated from the limestone scarp by a distance that increases with the age of the slope we measured their profiles and fitted them to mathematical functions by extrapolating these functions we obtained the intersection point with the scarp at every stage this gives different distances retreated by the scarp during each interval between consecutive stages of slope evolution remains of charcoal found in the debris slopes corresponding to stages s 1 s 3 and s 4 have been dated by means of 14 c to 1090 40 3590 40 and 28 550 130 bp respectively these values allowed us to calculate the following rates of scarp retreat 0 5 mm year during the time interval between stages s 4 and s 3 and 10 5 mm year for the interval between s 3 and s 1 there is a good correlation between the ages of the slope accumulations and cold periods s 3 is correlated with the middle bronze age cold epoch and s 4 may correspond to the heinrich event h 3 these correlations seem to confirm that talus flatirons in arid environments are good climatic indicators

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

in situ cosmogenic exposure ages from the isle of skye northwest scotland implications for the timing of deglaciation and readvance from 15 to 11 ka we present 10 in situ cosmogenic exposure ages from two moraines on the isle of skye the strollamus medial moraine was deposited during deglaciation of the devensian ice sheet and yields a mean exposure age from five samples of 14 3 0 9 ka the moraine age indicates that a significant ice mass existed on skye at the time of a regional readvance recorded in wester ross northwest scotland taken at face value the ages suggest that deglaciation did not occur until well into greenland interstade 1 the slapin moraine represents the local limit of the loch lomond readvance llr and yields a mean exposure age from five samples of 11 5 0 7 ka which is consistent with deposition relating to the llr these ages suggest that the maximum extent may have been reached late in the stadial and that some glaciers may have remained active until after the climatic amelioration that marks its end this scenario is considered unlikely given the nature of the climate during this period which leads us to call for a locally calibrated production rate copyright 2011 john wiley sons ltd

palaeoclimatic reconstruction from loch lomond readvance glaciers in the west drumochter hills scotland geomorphological mapping in the west drumochter hills provides evidence of a readvance of locally nourished glaciers during the loch lomond younger dryas stade in the form of an icefield 67 7 km2 in area drained by outlet glaciers the icefield limits accord broadly with those proposed by sissons 1980 but all geomorphic stratigraphic and sedimentological evidence conflicts with a recent proposal that the landforms in the area reflect southwestwards retreat of the last ice sheet up valley continuity of recessional moraines indicates that the ice remained active and close to climatic equilibrium during the earlier stages of glacier retreat consistent with slow warming following the coldest part of the stade the pattern of equilibrium line altitudes elas across the icefield is consistent with transfer of snow by westerly and southerly winds the ela of the reconstructed icefield as a whole is 622 629 m although this figure is likely to be lower than the regional climatic ela because the icefield probably received additional snow blown from adjacent plateau surfaces and slopes inclusion of potential snow blow areas in the ela calculation yields a value of 648 656 m the climatic ela is therefore likely to have lain between 622 and 656 m mean june to august temperature at the ela based on chironomid assemblages at two sites falls within the range 4 0 0 7 c empirical relationships between temperature and precipitation at modern glacier elas indicate that mean annual precipitation map at the ela was 1977 464 mm statistically indistinguishable from modern values comparison with precipitation values calculated for the isle of mull on the west coast suggest that the precipitation gradient across the central highlands of scotland was steeper during the loch lomond stade than at present probably as the result of efficient scavenging of precipitation from westerly airflows by the west highland icefield copyright 2005 john wiley sons ltd

retreat velocity of the north atlantic polar front during the last deglaciation determined by 14 c accelerator mass spectrometry the glacial oscillations which dominate the quaternary global climate are closely related to changes in incoming solar radiation but the details of palaeoclimatic spectra suggest that other parameters usually referred to as feedback have significantly increased the climatic response to extraterrestrial forcing1 the last deglaciation appears to be one of the more characteristic examples of nonlinearity because its abruptness strongly contrasts with the smooth shape of the associated solar radiation maximum dated at 11 000 yr bp in addition the presence of the younger dryas cold event concomitant with the solar radiation maximum could be interpreted as a transitory phenomenon caused by the rapid meltwater influx linked to the first phase of the ice sheet s disintegration2 3 here using accelerator mass spectrometry we date precisely the movements of the polar front during the whole period of glacial retreat 15 000 8 000 yr bp our results show that the polar front has moved between 35 n and 55 n in 1 000 yr for the earliest retreat 12 500 12 000 bp and moved almost instantaneously that is 400 yr during the two abrupt climatic changes associated with the younger dry as cold event

a reinterpretation of the lateglacial environmental history of the isle of skye inner hebrides scotland this paper presents a major revision of the late devensian lateglacial environmental history of the isle of skye scotland based upon a combination of geomorphological biostratigraphical and radiocarbon evidence the distribution of glacial and periglacial landforms and of raised shorelines suggests that there was only one extensive readvance of local glaciers in southern skye following the wastage of the late devensian ice sheet pollen stratigraphic evidence from 10 sites inside and 4 sites outside the mapped ice limits indicates that this readvance occurred during the loch lomond stadial at that time over 180km2 of the uplands of south central skye were covered by glacier ice a much more extensive glaciation than previously envisaged palynological evidence from four lateglacial profiles implies that degree of exposure to strong westerly winds was the principal factor determing vegetational contrasts on the island and that regional differences in vegetational type were less pronounced than has hitherto been suggested the glacial and palaeobotanical reconstructions reported here are more compatible with lateglacial data from the scottish mainland and hebridean islands than were the previously published accounts for the isle of skye

changes in sea surface hydrology associated with heinrich event 4 in the north atlantic ocean between 40 and 60 n abstract the changes in distribution of sea surface temperature and salinity in the north atlantic between 40 and 60 n were reconstructed for the time interval between 40 and 30 kyr bp which includes the large iceberg discharge event associated with the deposition of heinrich layer 4 we found that the meltwater input during deposition of heinrich layer 4 resulted in a 1 2 kyr temperature decrease of about 2 c and a salinity decrease in the range of 1 5 3 5 between 40 and 50 n sites above 50 n did not experience significant salinity variations a much larger area was affected by the reduction in sea surface temperature the amplitude of the sea surface temperature shift was however much smaller than the atmospheric temperature changes over greenland at gisp and grip sites

oldest reliable 40ar 39ar ages for terrestrial rocks barberton mountain komatiites we report here the first 40ar 39ar ages for komatiites and komatiitic basalts from the barberton mountain greenstone belt on the transvaal swaziland border in southern africa1 both rock types display remarkable argon retentivity while some variation is found among the samples the best argon age estimate for the time of metamorphism is in the 3 450 3 490 myr range this age is only slightly less than that found in komatiites from the same area by sm nd dating 3 540 15 myr 2 within the uncertainties of the decay constants involved the two age estimates are not different the results show that the principal pervasive greenschist metamorphism1 in the area must have occurred within 100 myr of the eruption of the komatiite analytically precise ages were obtained despite potassium contents as low as 80 p p m in the presence of high calcium concentrations 3 5 ca to our knowledge the results represent by far the oldest reliable ages obtained for terrestrial rocks using the k ar system

did large ice caps persist on low ground in north west scotland during the lateglacial interstade recent research based primarily on exposure ages of boulders on moraines has suggested that extensive ice masses persisted in fjords and across low ground in north west scotland throughout the lateglacial interstade greenland interstade 1 ca 14 7 12 9 ka and that glacier ice was much more extensive in this area during the older dryas chronozone ca 14 0 ka than during the younger dryas stade ca 12 9 11 7 ka we have recalibrated the same exposure age data using locally derived 10be production rates this increases the original mean ages by 6 5 12 implying moraine deposition between ca 14 3 and ca 15 1 ka and we infer a most probable age of ca 14 7 ka based on palaeoclimatic considerations the internal consistency of the ages implies that the dated moraines represent a single readvance of the ice margin the wester ross readvance pollen stratigraphic evidence from a lateglacial site at loch droma on the present drainage divide demonstrates deglaciation before ca 14 0 ka and therefore implies extensive deglaciation of all low ground and fjords in this area during the first half of the interstade ca 14 7 14 0 ka this inference appears consistent with lateglacial radiocarbon dates for shells recovered from glacimarine sediments and a dated tephra layer our revised chronology conflicts with earlier proposals that substantial dynamic ice caps persisted in scotland between 14 and 13 ka that large active glaciers probably survived throughout the lateglacial interstade and that ice extent was greater during the older dryas period than during the younger dryas stade copyright 2011 john wiley sons ltd