the active part of charlie gibbs fracture zone a study using sonar and other geophysical techniques a short survey with gloria side scan sonar and other geophysical instruments has revealed new information about charlie gibbs fracture zone between 29 and 36 w the traces of two transform faults have been clearly delineated they fit small circles about the pole of rotation with an rms error of only about 1 km but they do not always follow the deepest parts of the transform valleys the transforms are joined by a short spreading center at 31 45 w the median transverse ridge appears to have been produced by normal seafloor spreading at this center and bears identifiable vine matthews magnetic anomalies a transverse ridge along the eastern inactive part of the northern transform may be an intrusive feature considerable thicknesses of sediment appear to have been deposited in the northern transform valley from norwegian sea overflow water passing through the fracture zone but transverse ridges have prevented the sediment reaching the southern valley

iceland scotland overflow water transport variability through the charlie gibbs fracture zone and the impact of the north atlantic current the charlie gibbs fracture zone cgfz a deep and wide gap in the mid atlantic ridge near 52 n is a gateway between the eastern and western subpolar regions for the atlantic meridional overturning circulation amoc in 2010 2012 an eight mooring array of current meters and temperature salinity sensors was installed across the cgfz between 500 m and the sea floor to measure the mean transport of westward flowing iceland scotland overflow water isow and investigate the impact of the eastward flowing north atlantic current nac on isow transport variability the 22 month record mean isow transport through the cgfz 1 7 0 5 sv 95 confidence interval is 30 lower than the previously published estimate based on 13 months of current only measurements 2 4 sv 1 2 sv saunders 1994 the latter mean estimate may have been biased high due to the lack of continuous salinity measurements although the two estimates are not statistically different due to strong mesoscale variability in both data sets empirical orthogonal function analysis and maps of satellite derived absolute dynamic topography show that weak westward isow transport events and eastward reversals are caused by northward meanders of the nac with its deep reaching eastward velocities these results add to growing evidence that a significant fraction of isow exits the iceland basin by routes other than the cgfz

flow paths and variability of the north atlantic current a comparison of observations and a high resolution model the north atlantic current nac is subject to variability on multiannual to decadal time scales influencing the transport of volume heat and freshwater from the subtropical to the eastern subpolar north atlantic na current observational time series are either too short or too episodic to study the processes involved here we compare the observed continuous nac transport time series at the western flank of the mid atlantic ridge mar and repeat hydrographic measurements at the ovide line in the eastern atlantic with the nac transport and circulation in the high resolution 1 20 ocean model configuration viking20 1960 2008 the modeled baroclinic nac transport relative to 3400 m 24 5 7 1 sv at the mar is only slightly lower than the observed baroclinic mean of 27 4 4 7 sv from 1993 to 2008 and extends further north by about 0 5 in the eastern atlantic the western nac wnac carries the bulk of the transport in the model while transport estimates based on hydrographic measurements from five repeated sections point to a preference for the eastern nac enac the model is able to simulate the main features of the subpolar na providing confidence to use the model output to analyze the influence of the north atlantic oscillation nao model based velocity composites reveal an enhanced nac transport across the mar of up to 6 7 sv during positive nao phases most of that signal 5 4 sv is added to the enac transport while the transport of the wnac was independent of the nao

direct observations of the upper layer circulation in the southern gulf of mexico abstract the upper layer circulation in the bay of campeche is analyzed with three years of data recorded by surface drifters current meter moorings and satellite altimetry the measurements show that the mean cyclonic circulation observed by previous authors extends below 1000 m and that its size and location are delimited by the particular topography of the region a deep basin to the west and a shallower and gentle sloping submarine fan to the east an empirical orthogonal function analysis and large correlations of the surface flow with the deeper currents suggest that the topographic constraint is the result of potential vorticity conservation for an equivalent barotropic flow the variability of the surface currents in the western basin is mostly due to changes in the size form position and intensity of the cyclonic gyre due to its interaction with northern gulf of mexico eddies particularly loop current eddies traveling the southern route towards the western boundary by contrast the eastern basin is characterized by a weak northward drift with the occasional generation of anticyclones in the southeastern boundary the genesis of which remains to be understood this suggests that the variability in the eastern basin is mostly driven by locally generated disturbances rather than by an influx of northern gulf of mexico eddies strong northward flows in the central and eastern basins result from the flow convergence between locally generated anticyclones and the cyclonic gyre

shifting surface currents in the northern north atlantic ocean 1 analysis of surface drifter tracks in the north atlantic ocean from the time period 1990 to 2007 provides evidence that warm subtropical waters have recently increased their penetration toward the nordic seas prior to 2000 the warm water branches of the north atlantic current fed by the gulf stream turned southeastward in the eastern north atlantic since 2001 these paths have shifted toward the rockall trough through which the most saline north atlantic waters pass to the nordic seas these surface drifters are able to overcome the ekman drift which would force them southward under the westerly winds dominating the subpolar atlantic yet the changes in path cannot be accounted for by changes in ekman drift eddy kinetic energy from satellite altimetry shows increased energy along the shifted drifter pathways across the mid atlantic ridge since 2001 these near surface changes have occurred during continual weakening of the north atlantic subpolar gyre as seen by altimetry they are also consistent with the observed increase in temperature and salinity of the waters flowing northward into the nordic seas these findings suggest the changes in the vertical structure of the northern north atlantic ocean its dynamics and exchanges with the higher latitudes wind stress and its curl changes are discussed as a possible forcing of the changes in the pathways of the subtropical waters

temporal variability of the subarctic front near the charlie gibbs fracture zone we document temporal variability of the northernmost branch of the north atlantic current known as the north subarctic front nsaf for the period 1976 1985 the front s location and temperature salinity t s parameters were determined from 120 repeat transects along three standard sections occupied by russian ships 1 ocean weather station ows c 52 75 n 35 5 w to st john s newfoundland 2 along 35 w from 52 n to 36 5 n and 3 ows c to cape st vincent portugal the nsaf exhibits significant interannual variability of its location shifting 200 km to 300 km in a few months along the ows c st john s section the front reached its southernmost location in 1976 1977 then experienced two 250 to 300 km northward shifts in 1978 1979 and 1980 1981 all of the three shifts peaked in wintertime these events nearly coincide with events of the opposite sign along the ows c cape st vincent section the northernmost location in 1976 1977 and two 200 km southward shifts in 1978 1979 and 1980 1981 the nsaf s behavior is thus coherent near the charlie gibbs fracture zone at the 500 km scale frontal time series associated with the moving front of t and s at the 200 m level for the cold side of the nsaf reveal two events an s200 minimum in the winter of 1976 1977 which might be a manifestation of the great salinity anomaly of the 1970s and a sharp drop of s200 in early 1984 which might signal the arrival of another large salinity anomaly the great salinity anomaly of the 1980s

environmental changes in northern iceland since the younger dryas inferred from periglacial slope deposits the slopes in northern iceland show the widespread occurrence of solifluction features indicative of an active periglacial environment due to annual mean temperatures around 3 c at sea level and s

the space time relationship of taxonomic diversity and morphological disparity in the middle jurassic ammonite radiation the middle jurassic ammonite radiation from the late aalenian to the end of the mid bathonian is traced using combined analyses of morphological disparity and taxonomic diversity the global signals of disparity and diversity are compared these signals are then broken down by paleogeographical provinces to detect any heterogeneity in the radiation an examination of the global signals reveals three biodiversity crises discordances between signals where morphological disparity grows while taxonomic diversity declines the subdivision of the signals indicates the radiation was heterogeneous between provinces the global signal is an aggregate of signals from each province the three biological crises have different paleogeographical signatures the first is visible in a few provinces only while the other two are visible in most provinces first order crises can be distinguished from second order ones by studying the number of paleogeographical provinces affected

decline of subpolar north atlantic circulation during the 1990s observations of sea surface height reveal that substantial changes have occurred over the past decade in the mid to high latitude north atlantic ocean topex poseidon altimeter data show that subpolar sea surface height increased during the 1990s and the geostrophic velocity derived from altimeter data exhibits declining subpolar gyre circulation combining the data from earlier satellites we find that subpolar circulation may have been weaker in the late 1990s than in the late 1970s and 1980s direct current meter observations in the boundary current of the labrador sea support the weakening circulation trend of the 1990s and together with hydrographic data show that the mid to late 1990s decline extends deep in the water column analysis of the local surface forcing suggests that the 1990s buoyancy forcing has a dynamic effect consistent with altimetric and hydrographic observations a weak thermohaline forcing allows the decay of the domed structure of subpolar isopycnals and weakening of circulation

directly measured mid depth circulation in the northeastern north atlantic ocean the circulation of water masses in the northeastern north atlantic ocean has a strong influence on global climate owing to the northward transport of warm subtropical water to high latitudes but the ocean circulation at depths below the reach of satellite observations is difficult to measure and only recently have comprehensive direct observations of whole ocean basins been possible here we present quantitative maps of the absolute velocities at two levels in the northeastern north atlantic as obtained from acoustically tracked floats we find that most of the mean flow transported northward by the gulf stream system at the thermocline level about 600 m depth remains within the subpolar region and only relatively little enters the rockall trough or the nordic seas contrary to previous work our data indicate that warm saline water from the mediterranean sea reaches the high latitudes through a combination of narrow slope currents and mixing processes at both depths under investigation currents cross the mid atlantic ridge preferentially over deep gaps in the ridge demonstrating that sea floor topography can constrain even upper ocean circulation patterns

island arc carbonates characterization and recognition in the ancient geologic record abstract carbonates of island arc origin that are preserved in paleozoic mesozoic terranes of the north american cordillera exhibit a distinctive suite of paleontologic and lithologic features and share a fundamental similarity with limestones forming in modern volcanic arcs this study provides the first detailed synthesis of carbonate depositional systems in island arcs and documents primary sedimentary constituents based on facies relationships and faunal communities models are developed that show patterns in the long term evolution of shallow marine organisms and the construction evolution and demise of carbonate platforms in island arcs a suite of criteria is identified that may be used to differentiate island arc carbonates from limestones that accumulated in other platform settings biogeographic isolation prolonged subsidence steep submarine slopes and tectonic instability of volcanic edifices contribute to the development of relatively high levels of species endemism impoverished normal marine faunas complex provincial affinities and relict biotas in limestones that are characterized by exceptionally thick platform and periplatform sequences fringing and barrier reefs at the shelf margin extensive lagoonal deposits and rapid lateral and vertical facies changes although destructive tectonic and geologic processes in island arcs may hinder determining the original size and extent of the carbonate platform and particular facies types may not be represented e g fringing and barrier reefs may be replaced by sand shoals at the platform margin many characteristics have potential value for identifying carbonates of island arc origin in the ancient rock record apart from being associated with calc alkaline volcanic and volcaniclastic assemblages the most valuable suite of features for recognizing island arc carbonates is marine biotas that exhibit elevated levels of endemism and mixed paleobiogeographic affinities extraordinary thicknesses of platform shallow marine and periplatform carbonates and rapid facies changes between volcanic and carbonate rocks and between shallow and deep water limestones recognizing that an ensemble of features is distinctive within island arc carbonates considerably enhances identification of volcanic arcs in the ancient geologic record and thus improves the likelihood of successfully unraveling the complex geologic history of ocean basins

a statistical description of the mean circulation and eddy variability in the northwestern atlantic using sofar floats abstract velocity data obtained from trajectories of sofar floats launched as part of several experiments carried out in the northwest atlantic during the last two decades are used to give a statistical description of the mean gulf stream and its recirculation the distribution of eddy variability in the region is also presented a mean gulf stream which bifurcates between 55 and 45 w to feed the north atlantic current and a southern recirculation can be clearly identified a weak interior sverdrup flow is also suggested the spatial distribution of eddy variability is consistent with previous descriptions showing a maximum near the gulf stream

mid depth mixing linked to north atlantic current variability 1 repeated observations at the subpolar front west of the mid atlantic ridge show a link between the position of the north atlantic current nac and the spatial distribution of fine scale variance diapycnal diffusivity and integrated energy dissipation observational data were collected during three cruises a single branch of the nac was found in 2008 approximately at 50 30 n and two branches were observed in 2010 and 2011 with alternating intensities shear variance was elevated below the core of the nac in all cases resulting in an average diapycnal diffusivity that is higher by a factor of 3 compared to the averages north and south of the nac the integrated energy dissipation has maxima at or south of the fronts background dissipation is highest during 2008 also the year with the highest surface eddy kinetic energy

the ecosystem of the mid atlantic ridge at the sub polar front and charlie gibbs fracture zone eco mar project strategy and description of the sampling programme 2007 2010 the work was supported by the uk natural environment research council consortium grant no ne c512961 1 and the census of marine life

marine biodiversity through the late cenomanian early turonian palaeoceanographic controls and sequence stratigraphic biases changes in the marine macro and microfauna sedimentary geochemistry and surface water palaeoproductivity through the last 500 000 years of the cenomanian and first 300 000 years of the turonian are documented these are based on the succession at eastbourne the thickest and most complete section through the late cenomanian and early turonian in the anglo paris basin two levels of rapid faunal and geochemical change are identified one coincident with a significant increase in siliciclastic input at the base of the plenus marls member and the other with a marked drop in surface water productivity near the top of the same unit faunal change is demonstrated to be largely a pattern of immigration emigration rather than true extinction and our sequence stratigraphical analysis shows that it was coincident with major sea level changes no evidence is found to support the hypothesis that reduced bottom water oxygenation developed and was responsible for extinctions amongst the benthos in mid shelf environments the onset of pure chalk facies is interpreted to mark the breakdown of shelf break fronts and the spread of oligotrophic oceanic waters over much of the continental shelf initiated by rising sea level the cenomanian turonian event far from recording a mass extinction of shelf fauna is most probably an artifact caused by a significant switch in the nature of the surviving sedimentary record as a result of a major but perfectly ordinary oceanographic change

interannual thermohaline changes in the northern north atlantic 1991 1996 abstract in the period 1991 1996 the woce hydrographic section a1e ar7e between greenland and ireland was repeated five times the observed thermohaline changes altered the baroclinic structure along the eastern margin of the subpolar gyre significantly between june 1995 and august 1996 an overall increase of the temperature and thickness and a decrease of the density of the subpolar mode water spmw layer were observed accompanied by an increase of its salinity east of the reykjanes ridge and a decrease of its salinity in the irminger sea the changes were most pronounced in the iceland basin where the subarctic front retreated westwards coinciding with a strong weakening of the westerlies as determined by the north atlantic oscillation they are related to a local reduction of the ekman upwelling and the ocean to atmosphere heat flux on the one hand and to the advection of anomalies from the subtropics on the other hand the eastward spreading of the different labrador sea water lsw vintages led to a corresponding cooling of the lsw in the irminger sea and in the iceland basin in the period 1991 1996 the renewal of the lsw in the rockall trough occurred more sporadically indicating that the north atlantic current nac impedes the southward spreading of lsw in the eastern atlantic the changes in 1996 seem to have also counteracted this spreading

palaeobiogeography and the ordovician and mesozoic cenozoic biotic radiations curves of taxonomic diversity through geological time consistently show major evolutionary radiations during the ordovician period and from the mid mesozoic to the present day both intervals were characterized by marked biotic provincialism reflecting episodes of major continental break up and global tectonism and their later histories featured steep global climatic gradients the ordovician radiation can be recognized at a wide spectrum of taxonomic levels from species to class and the biogeographical patterns associated with the radiation of individual clades reflect a complex combination of plate distribution tectonic activity sedimentary environment sea level rise and ultimately glaciation the true scale of the mid mesozoic cenozoic biotic radiation is currently a topic of intense debate but there is no doubt that it affected plants and animals in both the marine and terrestrial realms the role of land bridges and ocean gateways in controlling the for mation of biodiversity patterns has been a persistent theme in mesozoic cenozoic bio geography and a complex set of neogene tectonic events probably aided the development of both latitudinal and longitudinal provinces during the cenozoic the present volume highlights some of the successes across a spectrum of approaches to unravelling the ordovician and mesozoic cenozoic radiations within the context of palaeobiogeography

a high resolution salinity time series 1993 2012 in the north atlantic from argo and altimeter data the study of salinity changes has been hampered by the lack of temporal and spatial resolution of the observations in order to improve the spatial and temporal distribution of salinity observations we used the gravest empirical mode gem technique to calculate high resolution salinity distributions as a function of dynamic height for the period 1993 2012 this technique combined argo and altimeter data to exploit the relationship between t s profiles and dynamic height in the north atlantic the method was valid in the upper 700 m mainly at and near the pathways of the north atlantic current nac but failed in regions with weak stratification or with ambiguities in the t s relationships coherent multiannual large scale variability was observed with many features present in all regions albeit with weaker amplitudes in the eastern basins some of the interannual features in the northeastern atlantic basins were unrelated to the variability further south and west pointing to an occasional advection of subtropical water in the eastern atlantic origin and advection of salinity anomalies with the nac from the north american basin into the western subpolar north atlantic are correlated with the state of the north atlantic oscillation nao and dampened by the surface freshwater fluxes other mechanisms influencing the salinity pattern are the changing location of the subpolar front also related to the nao the large multiyear variability in the 20 year time series obscured any potential trends caused by global warming only the rockall trough showed a salinity increase of 0 03 per decade

late holocene land surface change in a coupled social ecological system southern iceland a cross scale tephrochronology approach this work is supported by a uk natural environment research council nerc phd studentship ne f00799x 1

long term observations of north atlantic current transport at the gateway between western and eastern atlantic in the western north atlantic warm and saline water is brought by the north atlantic current nac from the subtropics into the subpolar gyre four inverted echo sounders with high precision pressure sensors pies were moored between 47 40 n and 52 30 n to study the main pathways of the nac from the western into the eastern basin the array configuration that forms three segments northern central and southern allows partitioning of the nac and some assessment of nac flow paths through the different mid atlantic ridge fracture zones we exploit the correlation between the nac transport measured between 2006 and 2010 and the geostrophic velocity from altimeter data to extend the time series of nac transports to the period from 1992 to 2013 the mean nac transport over the entire 21 years is 27 5 sv consisting of 60 warm water of subtropical origin and 40 subpolar water we did not find a significant trend in the total transport time series but individual segments had opposing trends leading to a more focused nac in the central subsection and decreasing transports in the southern and northern segments the spectral analysis exhibits several significant peaks the two most prominent are around 120 days identified as the time scale of meanders and eddies and at 4 9 years most likely related to the nao transport composites for the years of highest and lowest nao indices showed a significantly higher transport 2 9 sv during strong nao years mainly in the southern segment

interaction of the north atlantic current with the deep charlie gibbs fracture zone throughflow the charlie gibbs fracture zone cgfz a passage of 3600 m sill depth through the mid atlantic ridge near 52 n is a known gateway for the passage of deep waters from the northeast atlantic into the western basin during a shipboard survey of august 1997 deep current profiling yielded eastward deep flow through the passage while geostrophy calculated against an intermediate reference level resulted in westward relative deep transport the reason was an unusual and deep reaching northward excursion of the north atlantic current nac inspection of historical data showed that such interference of the nac with the cgfz regime occured occasionally in the past relocation of surface circulation patterns by decadal ocean climate anomalies may thus be of significance also for the deep circulation

geochemistry of historical age silicic tephras in iceland the major element chemistry of nine silicic tephras of historical age from iceland is assessed as a key step in the development of the recent tephrochronology of the north atlantic region the tephras include the largest such layers produced by each of the five central volcanoes hekla o raefajokull eyjafjallajokull torfajokull and askja since the ninth century ad h 1104 o1362 e 1821 landnam tephra c 870 a 1875 and four other tephras h 1158 h 1510 h 1947 o1727 the determination of grain discrete major element chemistry of the glass fraction is a fundamental stage in the identification and correlation of tephra and allows links to be made between icelandic source areas with precise dating evidence and distal deposits elsewhere in the north atlantic region although major element data can be used to discriminate between tephra layers produced by the different central volcanoes on its own it cannot be used to identify all the holocene layers produc

topographic and bioclimatic controls on soil paleosol morphogenesis in the norra storfjallet mountains sweden abstract retreat of the stabre glacier in the norra storfjallet mountains of northern sweden led to the emplacement of a broad expanse of glaciofluvial sediment a virtual blanket of debris interspersed with recessional and push moraines episodic mass wasting of the glaciofluvial moraine and loessic deposits yielded a pedostratigraphic succession of 14 c dated sub boreal and sub atlantic cryosols in soliflucted sediment that provide weathering data related to neoglacial perturbations whereas weathering within polygonal complexes yielded single profile one event cryosolic gleysols of similar age both provide a database of paleosol extremes controlled by topography and bioclimate together the paleosols provide a database of weathering of mixed amphibolitic gneiss trachytic lavas and granite weathering under two variable redox conditions one soliflucted sediment fully or partially aerated and free draining under a subaerial atmosphere with high redox potential the other polygonal soil partly reduced under the influence of an active layer of sporadic permafrost paleosols dated to the early neoglacial 3600 4200 cal 14 c yr bp have variable horizon development dependent upon landform associations and provide maximum ages for the second oldest moraine stillstand of ice retreat into the highlands on the arctic circle of sweden the cryosolic gleysol yields surprisingly high extractable fe and al suggestive of considerable redox fluctuations over the time of morphogenesis

the north atlantic current and surrounding waters at the crossroads the north atlantic current is a well defined western boundary current that flows north along the east side of the grand banks from 40 to 51 n where it turns sharply to the east and begins its journey across the ocean the current is unique in transporting warm tropical waters to much higher latitudes than any other western boundary current and thus plays a crucial role in ameliorating the climate of the european subcontinent the north atlantic current originates in the gulf stream when the latter curves north around the southeast newfoundland rise a major submarine ridge that stretches se from the grand banks a well defined front delineates the path of the current as long as it flows north as a western boundary current after the current turns east in the north it broadens into a widening band of eastward drift without a sharp or permanent front in the sense of the eastward flowing gulf stream after it separates from cape hatteras the north atlantic current transports more than 40 sv 1 sv 106 m s 1 in the south and about 20 sv by the time it flows east across the mid atlantic ridge the currents along the northward flowing front are quite swift with typical maximum average speeds in the upper 300 m near 1 m s 1 2 knots the current meanders almost as wildly as a snaking river but unlike steep meanders in the gulf stream these meanders appear to be stable and with one exception have not been observed to break off to form pools of warm and or cold waters as frequently occurs in the gulf stream the meanders appear to be induced by major topographic features along the path of the current namely the southeast newfoundland rise the newfoundland seamounts and flemish cap strong recirculations develop on the concave side of the meanders one of these the mann eddy at the first meander crest of the north atlantic current should be regarded as a permanent feature of the north atlantic circulation other meanders also contain recirculations that can persist for months under certain conditions these can merge together to form an extended sw flow recirculation just east of the north atlantic current

responses of mountain ice caps in central iceland to holocene climate change abstract a chronology of holocene fluctuations of small outlet glaciers from the regnabu ajokull ice cap on hrutfell central iceland allows comparison of their sensitivity with the margin of the nearby langjokull icefield to ascertain which frequencies of climatic variability are recorded by adjacent glaciers of different size dating utilised tephra layers in aeolian soils lying on between and beneath moraine ridges key marker isochrones dating from the hekla 4 tephra c 3 8 ka bp to katla a d 1918 provide unequivocal bracketing ages the stratigraphy and geochemical fingerprinting of tephras on younger moraines allows subdivision of little ice age moraines five groups of moraines are identified at c 4 5 5 0 c 3 0 3 5 ka bp c 2 0 2 5 ka bp and from the little ice age at c a d 1700 and in the late 19th early 20th century these represent a neoglacial sequence in which steep small glaciers readvanced to similar positions during what are here termed little ice age type periods liatps in contrast the nearby margins of langjokull show evidence of a late nineteenth century advance only suggesting that this was the holocene maximum for this glacier the contrasting responses of local glaciers and the large icefield are explained by their different sizes and response times so that the preserved moraine record is largely pre conditioned by the glacier type in general the forefields of steep fast responding glaciers contain more complete archives of holocene climatic changes than do the margins of the large icefields

palaeoenvironmental trends in the history of trace fossils tracks trails and burrows made by organisms are commonly preserved in phanerozoic sedimentary rocks these biogenic sedimentary structures or trace fossils have been most intensively studied from rocks deposited in marine environments and have proven to be extremely useful in palaeoenvironmental reconstruc tions our systematic analysis of occurrences in the stratigraphic record of two of the most common trace fossils zoophycos and ophiomorpha indicates that occurrences for each show an environmentally linked directional pattern through time zoophycos is commonly reported from palaeozoic strata deposited from nearshore to deep sea environments whereas in post palaeozoic stratait is generally reported only from rocks deposited in deeper water environments ophiomorpha first appeared in permian shallowwater environments and was common in deep sea fan environments by the cretaceous and has been distributed from nearshore to deep sea environments since then these onshore offshore trends which are similar to those previously reported for phanerozoic benthic macroinvertebrate assemblages and individual higher taxa require integration into future palaeoenvironmental reconstructions that use trace fossils

the north atlantic current based on trajectories of satellite tracked buoys and hydrographic sections both dietrich s and worthington s schemes of the north atlantic current are reconsidered ninety satellite tracked buoys have been analyzed in order to distinguish between these schemes neither could be confirmed the north atlantic current appears as a superposition of a broad west wind drift and a frontal jet in the western part of the north atlantic north of 40 n which becomes independent from the subtropical gyre neither a permanent branching suggested by dietrich nor a confinement of the subpolar gyre to the east of newfoundland per worthington can be found the north atlantic current extends over the entire north atlantic carrying more than 20 sverdrups sv toward the north this northward transport is mainly due to the northward flow around flemish cap and a sudden turn of the north atlantic current east of the mid atlantic ridge the buoy tracks have been used to determine speed and direction in the upper layers 100 m we combine these results with hydrographic data in order to obtain an estimate of the mass transport transport across the mid atlantic ridge is based on recent sections fahrbach et al 1985 while transport in the northeastern north atlantic has been derived from the erika dan sections at 53 5 n and 59 5 n according to these two sections the return flow occurs mainly below 1000 m after water mass conversion in the northern areas the results imply that about 7 sv of atlantic water are transformed into deep water in the area between 53 5 n and 60 n

ocean rafted pumice constrains postglacial relative sea level and supports holocene ice cap survival abstract distally deposited tephra from explosive volcanic eruptions can be a powerful tool for precise dating and correlation of sedimentary archives and landforms however the morphostratigraphic and chronological potential of ocean rafted pumice has been under utilized considering its long observational history and widespread distribution on modern and palaeo shorelines around the world here we analyze the geochemical composition and elevation data of 60 samples of ocean rafted pumice collected since 1958 from raised beaches on svalbard comparison of pumice data with postglacial relative sea level history suggests eight distinct pumice rafting events throughout the north atlantic during the middle and late holocene analyzed ocean rafted pumice exhibit consistent silicic composition characteristic of deposits from iceland s volcanic system katla eruption triggered jokulhlaups are key drivers of the transport of pumice from the katla caldera to beyond the coast of iceland and into the surface currents of the north atlantic ocean thus the correlation of distinct high concentration pumice horizons from katla deposited along raised middle holocene beach ridges in svalbard further advocates for the persistence of the m rdalsjokull ice cap through the holocene thermal maximum

holocene land degradation and climatic change in northeastern iceland the spatial and temporal pattern of land degradation in northeastern iceland during the holocene is analysed in relation to climatic fluctuations tephrochronology is used to date the infer red terrestrial changes and remote sensing and geographical information systems to assess the dynamic relationships between topographic relief geomorphic processes and soil structure changes in vegetation and soil cover during the holocene are found to he more dynamic than previously reported highlighted by three substantial degradation phases two of which occurred prior to the recorded viking settlement in the ninth century ad c 5000 bp and c 2500 bp the results demonstrate the role of climate in modifying land cover hence triggering land degradation without anthropogenic influence however anthropogenic activity probably had a significant role in the acceleration of the third degradation phase in the sixteenth century ad when the system was possibly forced beyond its threshold of recovery resulting in land degradation on a catastrophic scale less

on the movements of the north atlantic subpolar front in the preinstrumental past abstractthree sediment records of sea surface temperature sst are analyzed that originate from distant locations in the north atlantic have centennial to multicentennial resolution are based on the same reconstruction method and chronological assumptions and span the past 15 000 yr using recursive least squares techniques an estimate of the time dependent north atlantic sst field over the last 15 kyr is sought that is consistent with both the sst records and a surface ocean circulation model given estimates of their respective error co variances under the authors assumptions about data and model errors it is found that the 10 c mixed layer isotherm which approximately traces the modern subpolar front would have moved by 15 of latitude southward northward in the eastern north atlantic at the onset termination of the younger dryas cold interval yd a result significant at the level of two standard deviations in the isotherm position in contrast meridional movements of the isotherm in

reconstructing late holocene environmental change in iceland using high resolution tephrochronology high resolution records of sediment accumulation are necessary to evaluate subtle temporal and spatial variations in sediment flux especially in the context of decadal scale human environment inte

vertically averaged velocities in the north atlantic current from field trials of a lagrangian electric field float abstract a subsurface lagrangian float that utilizes motional induction to calculate vertically averaged velocities was tested in the north atlantic current nac taking advantage of existing cruises and infrastructure the electric field float eff is a rafos float with horizontal electrodes that measures its own velocity by rafos tracking and calculates vertically averaged velocities when merged with the electrode system the observations showed depth averaged velocities that were fast in the core of the nac 0 6 0 9 m s 1 and moderate in adjacent recirculations and eddies 0 3 0 4 m s 1 a float at 850 dbar moved at close to the depth averaged velocity while shallower floats followed surface intensified flow on top of the depth averaged motion integral time scales of depth averaged velocity 1 3 1 6 0 4 d are slightly shorter than time scales of float velocity 1 6 2 0 0 3 d while integral length scales of depth averaged water velocity 35 10 km for u 18 6 km for v are slightly shorter than length scales of float motion 53 12 km for u 28 6 km for v velocity spectra of depth averaged velocity show significant variance at inertial periods quantitative and qualitative validation with multiple independent data sets confirms the accuracy of the instrument and sampling strategy in the nac advancing the limited observational knowledge of depth averaged circulation in subpolar regions

geological and biological consequences of giant impacts it seems inescapable that large impacts have been an ongoing phenomenon throughout geological time in the phanerozoic the estimated frequency of impact by bodies of 5 km diameter or more is on the order of 1 0 every 1 00 myr the 5 km lower limit is chosen arbitrarily but it is shown in this review that such a body is certainly large enough to cause very large terrestrial effects a small decrease in this size limit would lead to a large increase in the frequency of occurrence it may be claimed that bodies of a size likely to have drastic and sudden effects on niany physical and biological features of the earth have arrived with a frequency comparable to the average duration of a biostratigraphic stage nothing is said of the interval between arrivals which may be random i e stochastic currently there is not enough geological evidence to allow recognition of other than random spacing between a few positively identified events during the phanerozoic since impacts by extraterrestrial bodies have been clearly established it is reasonable to postulate that they must leave a record in the stratigraphic column why then do we currently have so few horizons that have been claimed to contain evidence of a global impact event since the effects we are considering are unquestionably global in this paper we bring together evidence from several disciplines that

first direct estimates of volume and water mass transports across the reykjanes ridge the reykjanes ridge is a major topographic feature located south of iceland in the north atlantic ocean that strongly influences the subpolar gyre spg circulation based on velocity and hydrographic measurements carried out along the crest of the reykjanes ridge from the icelandic continental shelf to 50 n during the rrex cruise in june july 2015 we derived the first direct estimates of volume and water mass transports over the reykjanes ridge north of 53 15 n circulation was mainly westward south of this latitude it was mainly eastward the westward transport was estimated at 21 9 2 5 sv sv 106 m3 s 1 and represents the spg intensity the westward flows followed two main pathways at 57 n near the bight fracture zone and at 59 62 n we argue that those pathways were connected to the northern branch of the north atlantic current and to the sub arctic front respectively which were both intersected by the southern part of the section in addition to this horizontal circulation mixing and bathymetry shaped the water mass distribution water mass transformations in the iceland basin lead to the formation of weakly stratified subpolar mode water spmw we explain why spmw the main water mass contributing to the westward flow was denser at 57 n than at 59 62 n at higher densities both intermediate water and icelandic slope water contributed more to the westward transport across the reykjanes ridge than the sum of labrador sea water and iceland scotland overflow water plain language summary the reykjanes ridge the northern section of the mid atlantic ridge strongly influences the cyclonic circulation of the north atlantic subpolar gyre a major component of the climate system up to now no dedicated dataset was available to describe the circulation across this ridge to fill this gap surface to bottom measurements of flow velocity and water mass properties were carried out along the crest of the ridge from iceland to 50 degree sign n in 2015

interannual variability in the pathways of the north atlantic current over the mid atlantic ridge and the impact of topography abstract recent studies have indicated that the north atlantic ocean subpolar gyre circulation undergoes significant interannual to decadal changes in response to variability in atmospheric forcing there are also observations however suggesting that the southern limb of the subpolar gyre namely the eastward flowing north atlantic current nac may be quasi locked to particular latitudes in the central north atlantic by fracture zones gaps in the mid atlantic ridge this could constrain the current s ability to respond to variability in forcing in the present study subsurface float trajectories at 100 1000 m collected during 1997 99 and satellite derived surface geostrophic velocities from 1992 to 2006 are used to provide an improved description of the detailed pathways of the nac over the ridge and their relationship to bathymetry both the float and satellite observations indicate that in 1997 99 the northern branch of the nac was split into two branches as it crossed the ridge one quasi lock

isow spreading and mixing as revealed by deep argo floats launched in the charlie gibbs fracture zone to improve our understanding of deep circulation we deployed five deep argo floats 0 4 000 m in the charlie gibbs fracture zone cgfz which channels the flow of iceland scotland overflow water isow a dense water mass of the north atlantic ocean the floats were programed to drift at 2 750 dbar in the isow layer the floats mainly moved westward in the cgfz although some of them followed different routes for few cycles depending on northward intrusions of the north atlantic current over the cgfz one float revealed a direct route for isow from cgfz to the deep western boundary current at flemish cap in the cgfz oxygen data acquired by the floats revealed that the isow layer characterized by salinity higher than 34 94 and density greater than 27 8 kg m was mainly composed of the highly oxygenated isow and the less oxygenated north east atlantic deep water neadw a complex water mass from the east atlantic in the isow layer the relative contribution of isow was generally larger in the northern valley than in the southern valley of cgfz northward intrusions of the north atlantic current above the cgfz increased the relative contribution of neadw in the northern valley and favors mixing between isow and neadw the isow neadw signal flowing westward from the cgfz toward the deep western boundary current was progressively diluted by labrador sea water and denmark strait overflow water oxygen measurements from deep argo floats are essential for a better understanding and characterization of the mixing and spreading of deep water masses

eddy kinetic energy of the north atlantic subpolar gyre from satellite altimetry the surface eddy kinetic energy field of the north atlantic subpolar gyre 40 65 n 5 60 w is studied using the altimetric satellites geosat 1987 1988 and ers 1 1992 1993 this is on the whole an area of low eddy activity previous studies of the north atlantic have usually concentrated on the highly variable gulf stream system we review what is known of the surface circulation of the region we confirm that there is a close relationship between the eddy kinetic energy and the mean currents and that it is strongly constrained by the topography the eddy field occurs in deep water and follows the contours particularly around the flemish cap high eddy kinetic energy is seen on the flanks of topography and is higher where the slope is steeper this is particularly clear on both east and west flanks of the rockall plateau interannual changes in the currents are revealed by the differences between the geosat and ers 1 eddy kinetic energy maps in some years 1988 and 1992 1993 the northern core of the north atlantic current nac usually assumed to be tied to the charlie gibbs fracture zone 52 n is considerably weaker and may not exist at all while in 1987 there is a distinct core here in 1988 and 1992 1993 the main eddy region shifts farther to the south to about 49 n in 1988 and 50 n in 1992 1993 in 1987 there is also a current core even farther south centered at about 47 n implying that in that year the nac is arranged into two distinct branches

observed variability of the north atlantic current in the rockall trough from 4 years of mooring measurements the rockall trough is one of the main conduits for warm atlantic water to the nordic seas ocean heat anomalies originating from the eastern subpolar gyre are known to influence arctic sea ice ex

palaeoatmospheres in the phanerozoic except in the recent geological past palaeoatmospheric composition cannot be measured but only inferred from palaeontological or geochemical observations the likely controls on atmospheric composition are reviewed and the role of organisms stressed as monitors recorders motors and moderators of palaeoatmospheric changes whereas there is a good general case for changes in po 2 and pco 2 during phanerozoic time proposals for timing and mechanism in the earlier phanerozoic are speculative at present it seems probable that the late palaeozoic spread of land plants raised o 2 and reduced co 2 levels in the atmosphere the end cretaceous crisis had a transient impact on the atmosphere whose general constancy through geological time is one of the most remarkable features of earth history

seasonal and interannual changes in the north atlantic subpolar gyre from geosat and topex poseidon altimetry sea surface slopes from the altimetric satellites geosat and topex poseidon are used to calculate eddy kinetic energy of the north atlantic subpolar gyre 40 65 n 60 5 w using two years of data from each satellite december 1986 to december 1988 and october 1992 to september 1994 respectively interannual differences in the north atlantic current nac are revealed in regions of strong currents the eddies are driven by baroclinic instability of the mean flow are not seasonally varying and may therefore be used as a surrogate for the mean flow itself wind stress curl fields for the same periods show that the northward and southward shifts in the current branches across the mid atlantic ridge are related to interannual differences in the winter wind stress curl pattern when the zero in wind stress curl is well defined and wind stress is at a maximum outside the nac the eddies are driven primarily by wind stress indicated by a significant seasonality time series of eddy kinetic energy and wind stress are generated for the 4 years and the magnitude phase and significance of annual and semiannual signals are determined interannual changes in the timing of the maximum eddy kinetic energy are associated with that of the maximum wind stress with each being shifted by a month or two later or earlier throughout much of the northern north atlantic a significant annual signal in eddy kinetic energy is observed lagging the wind stress by on average 6 weeks a negligible lag is found in the region of the east greenland current and irminger sea whereas the greatest lag about 2 months is found in the labrador sea

thermal front variability along the north atlantic current observed using microwave and infrared satellite data thermal fronts detected using multiple satellite sensors have been integrated to provide new information on the spatial and seasonal distribution of oceanic fronts in the north atlantic the branching of the north atlantic current nac as it encounters the mid atlantic ridge mar is reflected in surface thermal fronts which preferentially occur at the charlie gibbs fracture zone cgfz and several smaller fracture zones north of the cgfz there are few thermal fronts contrasting with the region to the south where there are frequent surface thermal fronts that are persistent seasonally and interannually the alignment of the fronts confirms that the shallower reykjanes ridge north of the cgfz is more of a barrier to water movements than the ridge to the south comparison of front distributions with satellite altimetry data indicates that the mar influence on deep ocean currents is also frequently exhibited in surface temperature the improved spatial and temporal resolution of the front analysis has revealed consistent seasonality in the branching patterns these results contribute to our understanding of the variability of the nac and the techniques for visualising oceanic fronts can be applied in other regions to reveal details of surface currents that cannot be resolved using satellite altimetry or in situ measurements

temperature flux carried by individual eddies across 47 n in the atlantic ocean surface geostrophic velocity fields derived from satellite altimetry between january 1993 and april 2014 are used to detect and investigate eddies in the north atlantic between 40 55 n and 60 10 w focus is on a zonal section along 47 n roughly at the boundary between the subpolar and the subtropical gyres sea surface temperature data are used to quantify the temperature anomalies associated with eddies and the respective surface temperature fluxes related to these eddies identified eddy pathways across 47 n are related to the mean background velocity from full depth ship observations carried out on 11 cruises between 2003 and 2014 the analysis is repeated in two model simulations with 1 4 and 1 12 horizontal resolution respectively for the period 2002 2013 the analysis reveals almost 37000 altimeter derived eddies with a lifetime longer than one week in the area the highest number of eddies is found along the pathway of the north atlantic current roughly following the 4000 m isobath and on the grand banks of newfoundland time series of temperature fluxes by eddies crossing 47 n reveal that single isolated eddies with large sst signatures contribute 25 to the surface temperature flux relating the observed eddies to the observed top to bottom velocity distribution at 47 n points to the existence of eddy pathways across 47 the highest temperature fluxes are linked to the fastest and most pronounced current branches in the western newfoundland basin while there are fewer eddies in both model simulations the key findings are consistent between the observations and the two model simulations this article is protected by copyright all rights reserved

the northern north atlantic ocean mean circulation in the early 21st century the decadal mean circulation in the northern north atlantic was assessed for the early 21st century from repeated ship based measurements along the greenland portugal ovide line from satellite altimetry and from earlier reported transports across 59 5 n and at the greenland scotland sills the remarkable quantitative agreement between all data sets allowed us to draw circulation pathways with a high level of confidence the north atlantic current nac system is composed of three main branches referred to as the northern central and southern branches which were traced from the mid atlantic ridge mar to the irminger sea the greenland scotland ridge and the subtropical gyre at ovide the northern and central branches of the nac fill the whole water column and their top to bottom integrated transports were estimated at 11 0 3 sv and 14 2 6 4 sv 1 sv 106 m3 s 1 respectively those two branches feed the cyclonic circulation in the iceland basin and the flow over the reykjanes ridge into the irminger sea this cross ridge flow was estimated at 11 3 4 2 sv westward north of 58 5 n the southern nac branch is strongly surface intensified and most of its top to bottom integrated transport estimated at 16 6 2 sv is found in the upper layer it is composed of two parts the northern part contributes to the flow over the rockall plateau and through the rockall trough towards the iceland scotland ridge the southern part feeds the anticyclonic circulation towards the subtropical gyre summing over the three nac branches the top to bottom transport of the nac across ovide was estimated at 41 8 3 7 sv because of the surface intensification of the southern nac branch the intermediate water is transported to the northeast atlantic mostly by the northern and central branches of the nac 11 9 1 8 sv eastward this water circulates cyclonically in the iceland basin and anticyclonically in the west european basin with similar transport intensities

the north atlantic oscillation surface current velocities and sst changes in the subpolar north atlantic changes in surface circulation in the subpolar north atlantic are documented for the recent interannual switch in the north atlantic oscillation nao index from positive values in the early 1990s to negative values in 1995 96 data from lagrangian drifters which were deployed in the north atlantic from 1992 to 1998 were used to compute the mean and varying surface currents ncep winds were used to calculate the ekman component allowing isolation of the geostrophic currents the mean ekman velocities are considerably smaller than the mean total velocities that resemble historical analyses the northeastward flow of the north atlantic current is organized into three strong cores associated with topography along the eastern boundary in rockall trough in the iceland basin the subpolar front and on the western flank of the reykjanes ridge irminger current the last is isolated in this eulerian mean from the rest of the north atlantic current by a region of weak velocities on the east side of the reykjanes ridge the drifter results during the two different nao periods are compared with geostrophic flow changes calculated from the nasa pathfinder monthly gridded sea surface height ssh variability products and the advanced very high resolution radiometer avhrr sst data during the positive nao years the northeastward flow in the north atlantic current appeared stronger and the circulation in the cyclonic gyre in the irminger basin became more intense this was consistent with the geostrophic velocities calculated from altimetry data and surface temperature changes from avhrr sst data which show that during the positive nao years with stronger westerlies the subpolar front was sharper and located farther east sst gradients intensified in the north atlantic current irminger basin and east of the shetland islands during the positive nao phase associated with stronger currents sst differences between positive and negative nao years were consistent with changes in air sea heat flux and the eastward shift of the subpolar front sst advection as diagnosed from the drifters likely acted to reduce the sst differences

small scale lobate hillslope features on mars a comparative 3d morphological study with terrestrial solifluction lobes and zebra stripe lobes abstract small scale lobes ssl observed at the surface of mars are thought to be the result of solifluction hence their existence has important implications for our understanding of recent climate history the distribution of thaw liquids and its geomorphic effects as water is believed not to have been recently stable at the martian surface these small scale lobes are limited to sloping terrains and often occur alongside polygonal patterned ground attesting to the presence of ice in the environment in this study we perform a comparative 3d morphometric analysis of the lobes on mars with terrestrial analogues we use data from high resolution imaging science experiment hirise camera in order to create digital terrain models dtms on mars at 1 m pix for the terrestrial analogues we use dtms and orthophotos from five periglacial environments with solifluction lobes and from one site with lobate features found in the atacama desert the lobate features in the atacama desert are found within features called zebra stripes which are apparently unique to the atacama and are now generally accepted to be the result of seismic shaking we show that ssl on mars overlap morphometrically with terrestrial solifluction lobes and are less similar to zebra stripe lobes the majority of martian ssl occur below the angle of repose which demonstrates that a dry fall mechanism cannot be responsible for their formation and that lubrication of the subsurface is required we found that martian ssl only occur on slopes greater than 10 a constraint not observed for terrestrial solifluction lobes nor zebra stripe lobes in atacama we hypothesise that this apparent constraint does not result from the lowered gravity on mars but instead from differing soil properties and or thermal regime between mars and earth which affects the slope at which creep can initiate we cannot rule out a role of co2 sublimation in lobe formation based on our terrestrial measurements

trilobite diversity patterns in the middle cambrian of southwestern europe a comparative study abstract this paper describes the pattern of trilobite diversity throughout the middle cambrian in two fossiliferous basins of the western gondwana margin the iberian chains northeastern spain and the montagne noire southern france the documented fluctuations of species diversity allow us to recognize 1 a substantial extinction event recorded in latest early cambrian times named valdemiedes event which separates the culmination of a widespread decline of trilobites and the stepwise immigration of cosmopolitan invaders 2 a major trilobite radiation occurring in the earliest leonian and culminating across the caesaraugustian languedocian transition in which a peak in diversity of trilobites and carpoids was close to a major flooding surface 3 a major reduction of trilobite taxa across the early middle languedocian transition related to a well documented regressional trend and 4 a second immigration of trilobite fauna in the late languedocian which coincided with trangressional pulses and the establishment of suitable shaly substrates in addition we summarize the known trilobite occurrences from the middle cambrian of southwestern europe maps of the distribution of some relevant taxa genera and species over the western gondwana margin are documented

the flux of overflow water through the charlie gibbs fracture zone we report the results of a set of measurements made within the charlie gibbs fracture zone in 1988 1989 the principal data sources are year long current meter records 16 in number and 23 conductivity temperature depth stations the aim was to determine the flux of overflow water passing westward out of the iceland basin in the eastern north atlantic within the fracture zone overflow water is defined either as having salinity greater than 34 94 at the present time or more generally as having a potential density 0 27 8 kg m3 in 1988 1989 the core salinity had a value equal to that reported in 1981 and 1983 but was fresher by about 0 02 than was found in 1964 the flux was estimated twice per day and was found to fluctuate strongly with energy spread in a broad band of periods ranging from 10 to 400 days no annual signal could be detected the mean value was 2 4 0 5 sv less than all previous estimates based on hydrography alone it is suggested that hydrography may systematically overestimate transport because diffusion is neglected flux estimates from current meter arrays are rarely used in initializing numerical ocean models consequently measurements like those reported here are an independent and important means for assessing the performance of such models at the deepest levels water enters the two transform valleys of the fracture zone from the west in the northern valley the flow is periodically flushed out by energetic bursts of overflow water a critical value of an inverse froude number is shown to define the flushing threshold

deep water formation the subpolar gyre and the meridional overturning circulation in the subpolar north atlantic on interannual to decadal times scales model simulations suggest a strong relationship between anomalies in the deep water formation rate the strength of the subpolar gyre and the meridional overturning circulation in the north atlantic whether this is valid can only be confirmed by continuous long observational time series several measurement components are already in place but crucial arrays to obtain time series of the meridional volume and heat transport in the subpolar north atlantic are still missing here we summarize the recent developments of the deep water formation rates and the subpolar gyre transports we discuss how existing observational components in the subpolar north atlantic could be supplemented to provide long term monitoring of the meridional heat and volume transport through a combined analysis of observations and model results the temporal and spatial scales that had to be covered with instruments are discussed together with the key regions with the highest variability in the velocity and temperature fields

circulation and transports in the newfoundland basin western subpolar north atlantic the southwestern part of the subpolar north atlantic east of the grand banks of newfoundland and flemish cap is a crucial area for the atlantic meridional overturning circulation here the exchange between subpolar and subtropical gyre takes place southward flowing cold and fresh water is replaced by northward flowing warm and salty water within the north atlantic current nac as part of a long term experiment the circulation east of flemish cap has been studied by seven repeat hydrographic sections along 47 degrees n 2003 2011 a 2 year time series of current velocities at the continental slope 2009 2011 19 years of sea surface height and 47 years of output from an eddy resolving ocean circulation model the structure of the flow field in the measurements and the model shows a deep reaching nac with adjacent recirculation and two distinct cores of southward flow in the deep western boundary current dwbc one core above the continental slope with maximum velocities at mid depth and the second farther east with bottom intensified velocities the western core of the dwbc is rather stable while the offshore core shows high temporal variability that in the model is correlated with the nac strength about 30 sv of deep water flow southward below a density of sigma 27 68 kg m 3 in the dwbc the nac transports about 110 sv northward approximately 15 sv originating from the dwbc and 75 sv recirculating locally east of the nac leaving 20 sv to be supplied by the nac from the south

a review of the holocene environmental history of iceland abstract the purpose of this paper is to review the present knowledge of the holocene environmental history of iceland focusing on glacier fluctuations the ideas of the holocene being a relatively flat epoch in terms of environmental changes have been superseded by new studies reflecting active fluctuations the present paper promotes a revised model of the holocene glacier fluctuation history of iceland and recognises a long period of neoglaciation present data suggests an onset of glacier expansion in iceland around 5 ka bp which can be roughly correlated between different parts of the country and coincides with wider environmental changes specific glacier advances have been bracketed to 5 4 5 ka bp ca 4 2 ka bp ca 3 ka bp 2 ka bp and 1 5 1 2 ka bp during the little ice age lia glaciers in iceland begun to advance in the late 18th century and were biggest in the late 19th century

timing and significance of mid holocene glacier advances in northern and central iceland tephrochronology is used to date mid holocene moraines in iceland in the granavatn cirque trollaskagi peninsula the h 4 tephra ca 3830 yr bp lies within sediment accumulated against the distal slope of a lateral moraine providing a minimum age for glaciation and the absence of h 5 ca 5950 yr bp provides a maximum age by implication the cirque was glaciated in the early neoglacial period a large rock glacier the development of which post dates the moraine probably formed within the little ice age lia of the sixteenth to nineteenth centuries ad at eystri lo mundarjokull in central iceland frozen colluvium containing the h 3 ca 2900 yr bp and h 4 tephras provides a minimum age of ca 3830 yr bp for two terminal moraines and a maximum age of ca 2900 yr bp for a third the older moraines are correlated with the vatnsdalur i drangagil advances ca 4700 yr bp and 4500 yr bp respectively and the begisardalur i advance ca 4200 yr bp they imply that early neoglacial advances throughout iceland were locally more extensive than those of the lia the apparent lia holocene glacial maximum may reflect i selective sampling of large ice cap outlet glaciers where the preservation potential of pre lia deposits is low and ii reliance on lichenometric dating which has a restriced temporal range copyright 2001 john wiley sons ltd

a comparison of the lichenometric and schmidt hammer dating techniques based on data from the proglacial areas of some icelandic glaciers abstract measurements of rhizocarpon section and schmidt hammer r values are reported from the proglacial geomorphic features on the forelands of the icelandic glaciers of kviarjokull holarjokull and heinabergsjokull oraefi and south vatnajokull sandfellsjokull and oldufellsjokull east m rdalsjokull and bruarjokull eyjabakkajokull and west snaefell north vatnajokull these data are used in reconstructions of patterns of glacier recession since the little ice age maximum and the geomorphic signals of climatic versus non climatic events are discussed age control was obtained from various dated substrates by utilizing historical accounts aerial photographs and grave stones three lichen growth rates are calculated a 0 51 mm a 1 corrected to 0 50 mm a 1 with a colonization lag time of

ocean gyre circulation changes associated with the north atlantic oscillation abstract observational evidence is presented for interannual to interdecadal variability in the intensity of the north atlantic gyre circulation related to the atmospheric north atlantic oscillation nao patterns a two point baroclinic pressure difference between the subtropical and subpolar gyre centers an oceanic analogue to the much used sea level pressure slp based atmospheric nao indices is constructed from time series of potential energy anomaly pea derived from hydrographic measurements in the labrador basin and at station s near bermuda representing the upper 2000 db eastward baroclinic mass transport between the two centers the transport index indicates a gulf stream and north atlantic current that gradually weakened during the low nao period of the 1960s and then intensified in the subsequent 25 years of persistently high nao to a record peak in the 1990s the peak to peak amplitude difference was 15 20 megatons per second mt s 1 with a 43 yr mean of about 60 mt s 1 a change of 25 33

geochemistry dispersal volumes and chronology of holocene silicic tephra layers from the katla volcanic system iceland at least 12 silicic tephra layers silk tephras erupted between ca 6600 and ca 1675 yr bp from the katla volcanic system have been identified in southern iceland in addition to providing significant new knowledge on the holocene volcanism of the katla system which typically produces basaltic tephra the silk tephras form distinct and precise isochronous marker horizons in a climatically sensitive location close to both the atmospheric and marine polar fronts with one exception the silk tephras have a narrow compositional range with sio2 between 63 and 67 geochemically they are indistinguishable from ocean transported pumice found on beaches in the north atlantic region although they differ significantly from the silicic component of the north atlantic ash zone one naazo volumes of airborne silk tephra range from 0 05 to 0 3 km3 we present new isopach maps of the six largest layers and demonstrate that they originate within the katla caldera the apparently stable magma system conditions that produced the silk tephras may have been established as a consequence of the eruption of the silicic component of naazo ca 10 3 ka and disrupted by another large scale event the tenth century ad eldgja eruption ca 1 ka despite the current long repose silicic activity of this type may occur again in the future presenting hitherto unknown hazards copyright 2001 john wiley sons ltd

climate and material controls on periglacial soil processes toward improving periglacial climate indicators one of the distinguished efforts of a l washburn was to reconstruct mean annual air temperature using periglacial features as climate indicators this paper reviews existing periglacial indicators and proposes a strategy to improve their thermal resolution based on recent periglacial process studies with a focus on solifluction and thermal contraction cracking and associated landforms structures landforms resulting from solifluction reflect both the depth subjected to freeze thaw and the thickness of frost susceptible soils the thickness of a solifluction structure can be used to infer the dominant freeze thaw regime and minimum seasonal frost depth ice wedge pseudomorphs have limited potential as a climate indicator because 1 they mainly reflect extreme winter temperatures 2 their thermal thresholds depend on the host material and 3 they need to be distinguished from frost wedges of other origin produced under different thermal and or material conditions monitoring studies of currently active ice wedges suggest that ice wedge cracking requires a combination of low temperature and large temperature gradients in the frozen active layer further field monitoring of periglacial processes and their controlling factors under various climate conditions and in various materials are needed however to improve the resolution of periglacial paleoclimate indicators

simulated decadal variability of the meridional overturning circulation across the a25 ovide section andhighlatitudes 12sv 1sv 10 6 m 3 s 1 and a cellinternal to the subpolar gyre 4sv the decadal moc variability is associated with synchronized transport changes of the subtropical and subpolar inflow within the north atlantic current nac the varying strength of the moc is further related to changes in the upper horizontal transport distribution when the moc is in a strong phase early 1990s the northern branch of the nac in the iceland basin is strong while the southern branch at the rockall trough entrance is relatively weak the inverse situation holds for a persistent weak moc state 1970s contrary to the conclusions of earlier studies variability in the strength and shape of the subpolar gyre does not stand as the main driver of the changing nac structure which is largely induced by the horizontal variability of the subtropical inflow additionally the recently shown intrusion of subtropical waters into the northeastern atlantic late 1960s early 1980s and 2000s are shown to primarily occur during periods of weak moc circulation at a25 ovide

permian marine paleoecology and its implications for large scale decoupling of brachiopod and bivalve abundance and diversity during the lopingian late permian abstract the transition during the permian triassic interval from brachiopod rich benthic marine assemblages to communities with diverse molluscs has been well constrained by measures of global taxonomic richness but its ecological context has not been studied using quantitative relative abundance data new relative abundance data from early middle and late permian silicified fossil assemblages from offshore shelf carbonate environments indicate that molluscs increased significantly in relative abundance from 0 8 in the middle permian guadalupian to 65 4 in the late permian lopingian these numerical changes are accompanied by increases in the abundance of infaunal bivalves and motile gastropods in the late permian although this ecological change was coincident with the end guadalupian extinction a new global diversity compilation indicates that the overall genus level severity of that biotic crisis was substantially lower than previous estimates the pronounced ecological shift was instead concurrent with the onset of deep marine anoxic or euxinic conditions around the guadalupian lopingian boundary suggesting that increased environmental variability in proximity to the deep water mass may have given eurytopic molluscs a competitive advantage over more stenotopic brachiopods however the lack of global changes in relative genus richness implies dramatic decoupling of global taxonomic and local ecological processes the pronounced increase in molluscan abundance and alpha diversity in the silicified assemblages contrasts with their unchanged global taxonomic richness and may also have resulted from increased environmental stress in offshore shelf environments these results demonstrate that the ecological transition from the brachiopods to molluscs was more complex than inferred from global measures of taxonomic richness and imply that initial stages of permo triassic environmental deterioration may have been responsible for the decoupling of local ecological from global taxonomic processes during the dramatic late permian rise of the modern fauna

the role of vegetation cover and slope angle in tephra layer preservation and implications for quaternary tephrostratigraphy financial support was provided by the national science foundation of america through grant 1202692 comparative island ecodynamics in the north atlantic and grant 1249313 tephra layers and early warning signals for critical transitions and the support of the carnegie trust for the universities of scotland

the production of north atlantic deep water sources rates and pathways updating an earlier account by dickson et al 1990 this paper reviews the initial development phase of north atlantic deep water nadw production from the points where the dense inflows from nordic seas cross the greenland scotland ridge to the point off south greenland where the buildup of new production appears almost complete in particular three long term current meter arrays totaling 91 instruments and set at 160 km intervals south from the denmark strait sill are used to validate earlier short term arrays by others and in combination with these earlier arrays to describe the downstream evolution of mean speed depth and entrainment the variability of the overflow current in space and time and the likely contribution of the other three main constituents of nadw production at densities greater than 27 8 from the points of overflow 5 6 sv the transport within this range increases by entrainment and confluence with other contributory streams to around 13 3 sv at cape farewell while recirculating elements prevent us from determining the net southgoing transport a nadw transport of this order appears consistent with recent estimates of net abyssal flow passing south through the north and south atlantic

variability of the directly observed middepth subpolar north atlantic circulation satellite views of the ocean have suggested a decline of the subpolar north atlantic surface circulation during the 1990s and 2000s this was a period of unprecedented observational capacity in the basin thanks to the presence of many hundreds of profiling floats we use more than 40 000 subsurface displacements of these floats to characterize the circulation at 1000 m depth and its evolution from 1997 2013 we show a statistically significant slowdown in the labrador sea boundary currents of 0 8 cm s 1 per decade 95 confidence interval of 1 4 to 0 15 cm s 1 per decade a conservative estimate of the uncertainty otherwise the middepth circulation field was largely stable our analysis of the location where the north atlantic current crosses the mid atlantic ridge shows that profiling floats can reveal steering by bathymetric features but do not reveal of decadal variability in the position where the current crosses the ridge