the sediments of lake logurinn a unique proxy record of holocene glacial meltwater variability in eastern iceland the full holocene development of the large 8100 km 2 vatnajokull ice cap in iceland with its many outlet glaciers is poorly known the idea of an early deglaciation leading to a glacier free period in mid holocene followed by the neoglaciation is still the main concept for the glacial history in the north atlantic region including iceland we have examined a continuous sediment record from the glacier fed lake logurinn in eastern iceland to infer holocene meltwater variability of eyjabakkajokull which is a surge type outlet glacier of the vatnajokull ice cap we focus on the early and mid holocene and our data show that eyjabakkajokull receded rapidly during the final phase of the last deglaciation and did not deliver glacial meltwater to lake logurinn by 9000 years bp suggesting that eyjabakkajokull was significantly smaller than today at that time the return of glacial meltwater transport to lake logurinn and thus a return of eyjabakkajokull is dated to ca 4400 years bp suggesting an almost 5000 years long glacier free period during early and mid holocene during this time period we infer that the 8 2 ka cold event did not cause a significant expansion of eyjabakkajokull however we note a marked decrease in the aquatic productivity in lake logurinn which is suggested to be the result of shorter ice free seasons of lake logurinn the holocene thermal maximum is inferred by a period of maximum holocene aquatic productivity and dated to ca 7900 7000 years bp following the re formation of eyjabakkajokull ca 4400 years bp we suggest that the glacier reached stable conditions ca 1700 years bp and remained fairly stable until the later part of the little ice age when eyjabakkajokull reached its maximum holocene extent c 2012 elsevier ltd all rights reserved less

controls of basement faults on the geometry and evolution of compressional basement involved structures foreland basement involved structures commonly occur in front of major fold thrust belts as irregular chains of uplifts the variation in the map and three dimensional 3 d geometries of individual structures and their mutual spatial and angular relationships are strongly suggestive of the influence of preexisting basement discontinuities three dimensional experimental models were constructed to determine the function of preexisting frontal and lateral discontinuities in determining the geometry of the structures the models consisted of two layers with stiff clay representing the basement and soft clay representing the sedimentary cover laser scanning and 3 d surface modeling were used to determine the map geometry to compare the models with examples of natural structures from the rocky mountain foreland in wyoming planar discontinuities result in doubly plunging structures terminating against a frontal fault with a linear trace sheep mountain anticline whereas intersecting fault sets with sharp or curved intersections result in trapdoor geometries grass creek anticline opposite dipping faults result in uplifted blocks with varying relief and orientations of the structures depending on the relative orientations of the two fault trends significant strike slip faulting along the frontal fault only occurs when the angle between the two fault trends exceeds 30 chains of structures show offsets or relay patterns commonly accompanied by changes in the orientations of adjacent structures the sage creek steamboat butte chains the hamilton dome wagonhound anticline chain the maverick springs circle ridge chain and the grass creek walker dome trend in the rocky mountain foreland are examples of these configurations these patterns suggest the influence of preexisting frontal or lateral discontinuities on the trends and locations of structures lateral discontinuities are either preexisting controlling the lateral extent of structures or form during deformation to accommodate the formation of structures formed along offset frontal faults preexisting lateral faults that penetrate the entire basement commonly result in surface lateral faults bounding the surface structures however reverse faults in the basement connected by buried lateral faults result in surface structures with overlaps between them a relay of reverse faults in the basement not connected by lateral faults may develop parallel or oblique structures with transfer zones between them the mapped geometries of the experimental models and the orientations of secondary faults provide predictive analogs that can be used in the interpretation of surface and subsurface structures

high spatial resolution electron probe microanalysis of tephras and melt inclusions without beam induced chemical modification wavelength dispersive analysis of tephra grains and melt inclusions by epma has been carried out using a focused beam of 3 m diameter without detected loss of sodium or potassium in standard glasses including anhydrous basalts a slightly hydrated rhyolite and a sodium rich intermediate composition the ability to make analyses without chemical modification is strongly dependent upon current density at the analysis site analysis with narrow beams requires extremely low beam currents that are normally associated with energy dispersive analysis experiments indicate that a value of 0 1 na m2 must not be exceeded at least for moderately hydrated samples if sodium loss is to be avoided high resolution analysis without beam induced analytical artefacts enables fully quantitative analysis of very distal and or highly vesicular tephras and very small melt inclusions without the need to use post analysis corrections this development has enabled high quality analyses from crypto tephra layers that were pre

late holocene temperature and precipitation changes in vindelfjallen mid western swedish lapland inferred from chironomid and geochemical data in this article we present results from a palaeolimnological study from lake vuoksjavratje in the mountain tundra region in the vindelfjallen mountains northwest sweden we suggest that the influence of precipitation may be one of the factors causing discrepancies between chironomid based late holocene july temperature julyt reconstructions from fennoscandia we combine quantitative temperature reconstruction using chironomids for the last 5100 years with qualitative analysis of chironomid composition and geochemical analyses such as x ray fluorescence xrf total organic carbon toc and c n analysis the studied sequence is dated by 210pb 137cs and 11 14c datings from terrestrial macrofossils the aim of the study was to use chironomids to reconstruct late holocene summer temperature variation on a multi centennial to centennial timescale and to use geochemical data to identify periods during which the changes in chironomid composition might have been forced by environmental variables other than

biogenic silica deposition in lake malawi east africa over the past 150 000years abstract we report a 150 000 year record of the abundance of biogenic silica bsi in drill cores from the northern and central basins of lake malawi the periods of highest diatom burial at both sites occurred around 65 69 and 51 60 thousand years ago ka after the termination of prolonged intense drought in the region these peaks are believed to reflect elevated delivery of dissolved silica to the lake due to acceleration of chemical weathering in the drainage basin the droughts that preceded these bsi depositional episodes occurred at precessional frequency corresponding to times of minimum austral spring or summer insolation in the region prior to 60 ka subsequent arid spells have not been as severe due to the reduced amplitude of precessional forcing as a result of the superimposed effect of eccentricity however biogenic silica records do not show significant sensitivity to precessional forcing the bsi burial flux in the north basin displays strong millennial scale variability since 50 ka with peak values occurring during cold times in the northern hemisphere as reflected in the greenland ice core records we observe no relationship between bsi and deposition of volcanic tephras in this system the central basin core also displays millennial scale variability in bsi abundance during the past 50 ky of comparable magnitude to that in the north basin but without systematic correlation to the north basin bsi or greenland ice core records in general we conclude that bsi profiles from the north and probably south basins are more readily interpreted in terms of regional climate dynamics than are bsi profiles from more central locations in long narrow tropical rift lakes diatom burial rates are suspected to be too influenced by silica limitations imposed by diatom production and burial in the upwelling systems at either end of these low latitude lacustrine systems

temperature dependence of the oxygen isotopic fractionation between diatom silica and water abstract we present a new paleotemperature scale based on the oxygen isotopic ratio of the non exchangeable fraction of the oxygen from diatom silica the equation t 17 2 2 4 18osilica 18owater 40 0 2 18osilica 18owater 40 2 was derived using recent sediment samples from different oceanic areas the temperature and isotopic composition of the local surface water comparison of our results with other relationships established for quartz water or amorphous silica water at higher temperature suggests no difference in isotopic fractionation between quartz water and biogenic silica water couples

ash layers from iceland in the greenland grip ice core correlated with oceanic and land sediments four previously known ash layers ash zones i and ii saksunarvatn and the settlement layer all originating in iceland have been identified in the central greenland ice core grip this correlation of the ash between the different environments is achieved by comparison of the chemical composition of glass shards from the ash this establishes and confirms detailed correlations between the different types of depositional records and the absolute dating of the younger part of the ice core by counting annual layers dates the eruptions accurately a precise connection with dates obtained by14c beyond the range of dendrochronology is established which provides an excellent confirmation of230th 234u dates from corals four additional icelandic ash layers have also been identified in the core but not yet correlated with known ash deposits

itrax description and evaluation of a new multi function x ray core scanner a new automated multi function core scanning instrument named itrax has been developed that records optical radiographic and elemental variations from sediment half cores up to 1 8 m long at a resolution as fine as 200 m an intense micro x ray beam focused through a flat capillary waveguide is used to irradiate samples to enable both x radiography and x ray fluorescence xrf analysis data are acquired incrementally by advancing a split core via a programmable stepped motor drive through the flat rectangular section x ray beam traditional xrf determination of element composition in sediments provides high quality data but it takes a considerable time and normally consumes gram quantities of material that is often only available in limited quantities the itrax core scanner non destructively collects optical and x radiographic images and provides high resolution elemental profiles that are invaluable for guiding sample selection for further destructive detailed sampling this paper presents a description of the construction characteristics and capabilities of the itrax system high resolution itrax data obtained from sediment cores are also presented and compared with results from traditional wavelength dispersive xrf analysis at lower resolution finally some recent technical developments linked to the second generation itrax are presented

a chironomid based reconstruction of summer temperatures in nw iceland since ad 1650 few studies currently exist that aim to validate a proxy chironomid temperature reconstruction with instrumental temperature measurements we used a reconstruction from a chironomid percentage abundance data set to produce quantitative summer temperature estimates since ad 1650 for nw iceland through a transfer function approach and validated the record against instrumental temperature measurements from stykkisholmur in western iceland the core was dated through pb 210 cs 137 and tephra analyses hekla 1693 which produced a well constrained dating model across the whole study period little catchment disturbance as shown through geochemical itrax and loss on ignition data throughout the period further reinforce the premise that the chironomids were responding to temperature and not other catchment or within lake variables particularly cold phases were identified between ad 1683 1710 ad 1765 1780 and ad 1890 1917 with relative drops in summer temperatures in the order of 1 5 2 c the timing of these cold phases agree well with other evidence of cooler temperatures notably increased extent of little ice age lia glaciers our evidence suggests that the magnitude of summer temperature cooling 1 5 2 c was enough to force lia icelandic glaciers into their maximum holocene extent which is in accordance with previous modelling experiments for an icelandic ice cap langjokull

seven tephra isochrones in scotland this paper reports the presence and geochemical characteristics of seven separate tephras in scotland that fell in ad 1510 and about 450 bp 2100 bp 3600 bp 3830 bp 5600 bp and around 6000 bp the results come from 14 peat bogs spread throughout the highlands and islands where 26 tephra deposits have been characterized using over 400 grain discrete electron probe micro analysis firstly tephras are identified and defined by major element characteristics at a reference site secondly tephras are linked between sites on the basis of geochemistry and dated informal names based on british sites are suggested for tephras not yet linked to precise sources in iceland despite some apparent variation in colour all analysed grains in these seven deposits are silicic or intermediate in composition sio2 57 this phenomenon does not appear to be an artifact of preservation for two reasons first small 40 m basic glass shards in peat bogs in northern iceland are found to have undergone minimal alte

palaeoclimate interpretation of stable isotope data from lake sediment archives the isotope composition of authigenic and biogenic carbonates and diatom silica are commonly used as palaeoclimate proxies from lake sediments this article reviews the controls on the isotope composition of lacustrine skeletal and non skeletal deposits and illustrates how stable isotope studies contribute to an understanding of changes in temperature precipitation patterns evaporation and the carbon cycle it highlights the differences in the palaeoclimate potential of a wide range of lakes ranging from open to closed lake basins a large number of the case histories but not all are drawn from studies of temperate lakes from europe large closed lake systems in the tropics and elsewhere lose water predominantly through evaporation and contain sediments with variable and generally high 18o values fluctuations in the isotope composition of authigenic or biogenic minerals are mainly a function of long term changes in the precipitation evaporation ratio in contrast small open lakes which have a degree of through flow typically contain sediments with 18o values that vary by no more than a few these variations are generally ascribed to variations in temperature or the isotope composition of precipitation p from which either an annual or seasonally specific signal can be gained these types of lakes are common in northern europe and at high altitudes the interpretation of isotope data from a lacustrine succession requires a knowledge of the local processes that might control and modify the signal their effects need to be quantified and a robust calibration using the modern lake system is necessary to establish the relationship between the measured signal the isotopic composition of the host waters and climate

persistent positive north atlantic oscillation mode dominated the medieval climate anomaly the medieval climate anomaly mca was the most recent pre industrial era warm interval of european climate yet its driving mechanisms remain uncertain we present here a 947 year long multidecadal north atlantic oscillation nao reconstruction and find a persistent positive nao during the mca supplementary reconstructions based on climate model results and proxy data indicate a clear shift to weaker nao conditions into the little ice age lia globally distributed proxy data suggest that this nao shift is one aspect of a global mca lia climate transition that probably was coupled to prevailing la nina like conditions amplified by an intensified atlantic meridional overturning circulation during the mca

midges as quantitative temperature indicator species lessons for palaeoecology calibration data sets give a unique opportunity to establish patterns of biological existence and their statistical associations with environmental variables by use of calibration data sets environmental variables can be inferred quantitatively the resulting long time series may assist in distinguishing natural environmental variability from human induced variability both in terms of climate change and biotic turnover however the validity of the palaeoenvironmental reconstructions depends on their accuracy precision and sensibility before performing palaeoenvironmental inferences key mechanisms controlling contemporary species distribution abundances and dynamics should be identified and understood an inference model is developed to produce reconstructions a major challenge lies in validating and interpreting the reconstructions calibration data sets involving midges diptera chironomidae suggest that climate has a broad scale regional control over midge existence and abundance often ove

pressure leaching of boron from argillaceous sediments for facies analysis abstract for the extraction of b and al from argillaceous sediments a pressure leaching procedure has been developed which involves treatment of samples with 6 n hcl at 150 c in autoclaves compared with total digestion methods the leaching procedure is simpler and less time consuming elemental concentrations were measured using icp aes the b al ratio has proved to be a valuable geochemical indicator for the distinction of marine sediments from non marine sediments

reconstructions of surface ocean conditions from the northeast atlantic and nordic seas during the last millennium we undertake the first comprehensive effort to integrate north atlantic marine climate records for the last millennium highlighting some key components common within this system at a range of temporal and spatial scales in such an approach careful consideration needs to be given to the complexities inherent to the marine system composites therefore need to be hydrographically constrained and sensitive to both surface water mass variability and three dimensional ocean dynamics this study focuses on the northeast ne north atlantic ocean particularly sites influenced by the north atlantic current a composite plus regression approach is used to create an inter regional ne north atlantic reconstruction of sea surface temperature sst for the last 1000 years we highlight the loss of spatial information associated with large scale composite reconstructions of the marine environment regional reconstructions of ssts off the norwegian and icelandic margins are presented along with a larger scale reconstruction spanning the ne north atlantic the latter indicates that the medieval climate anomaly warming was most pronounced before ad 1200 with a long term cooling trend apparent after ad 1250 this trend persisted until the early 20th century while in recent decades temperatures have been similar to those inferred for the medieval climate anomaly the reconstructions are consistent with other independent records of sea surface and surface air temperatures from the region indicating that they are adequately capturing the climate dynamics of the last millennium consequently this method could potentially be used to develop large scale reconstructions of ssts for other hydrographically constrained regions

holocene variability of the north atlantic irminger current on the south and northwest shelf of iceland abstract the irminger current ic transports heat and nutrients to the icelandic shelf and its fluctuations are strongly linked to variations in the strength of the north atlantic surface flow from the south by using benthic foraminiferal assemblages and their stable isotopic composition in two marine sediment cores located in the pathway of the ic on the west iceland shelf we were able to derive bottom water temperature estimates at a 100 year resolution and define the temperature difference between the core sites throughout the holocene these are the first transfer function bottom water temperatures presented from the icelandic shelf based on benthic foraminiferal assemblages our findings demonstrate a delayed holocene thermal maximum in the marine current system around iceland compared to the northern hemisphere summer insolation receipts the maximum heat transport in the ic occurred between 8 and 4 ka years later than some previous records report a reconstruction of the ambient seawater 18o based on paired samples of c2 transfer function temperature estimates and oxygen isotopic composition indicates repeated freshwater input to the n atlantic inflow in the early holocene impacting the current system before 8 ka two fundamental regime shifts are evident in the paleoceanography and foraminiferal properties during the holocene occurring around 8 ka towards more stable conditions and again around 4 ka towards more unstable conditions increasing fluctuation with similarities to the great salinity anomaly in the 1970s occur after 4 ka

non linear holocene climate evolution in the north atlantic a high resolution multi proxy record of glacier activity and environmental change from hvitarvatn central iceland abstract iceland is well situated to monitor north atlantic holocene climate variability terrestrial sites there offer the potential for well dated high resolution continuous records of environmental change and or glacier activity laminated sediments from the proglacial lake hvitarvatn provide a continuous record of environmental change and the development of the adjacent langjokull ice cap for the past 10 2 ka replicate lake sediment cores collected from multiple locations in the basin are placed in a secure geochronology by splicing a varve chronology for the past 3 ka with a tephra constrained paleomagnetic secular variation derived chronology for older sediments multiple proxies including sedimentation rate bulk density ice rafted debris sediment organic matter biogenic silica and diatom abundance allow annual to multi decadal resolution and reveal a dynamic holocene terrestrial climate following regional deglaciation of the main iceland ice sheet summer temperatures were high enough that mountain ice caps had already melted or were contributing insignificant sediment to the lake pronounced increases in sedimentation rate sediment density and the influx of terrestrial organic matter between 8 7 and 7 9 ka suggest early holocene warmth was interrupted by two distinct pulses of cold summers leading to widespread landscape destabilization and possibly glacier growth the holocene thermal maximum htm 7 9 to 5 5 ka was characterized by high within lake productivity and ice free conditions in the watershed neoglaciation is recorded as a non linear transition toward cooler summers landscape destabilization and the inception and expansion of langjokull beginning ca 5 5 ka with notable increases in ice cap size and landscape instability at 4 2 and 3 0 ka the past two millennia are characterized by the abrupt onset of sustained cold periods at ca 550 and 1250 ad separated by an interval of relative warmth from ca 950 to 1150 ad the greatest holocene extent of langjokull occurred in the nineteenth century and is coincident with peak landscape instability followed by ice recession throughout the twentieth century

the boron isotope geochemistry of smectites from sodium magnesium and calcium bentonite deposits abstract the mineralogical chemical and isotopic analyses of smectites with variable interlayer cation occupancies from bentonite deposits in various depositional environments reveal new insights into the boron sources and the fluids involved in bentonitization in marine and non marine environments smectites from bentonites have non exchangeable structural boron concentrations of 0 2 to 196 g g b smectites from sodium bentonites have higher boron concentrations 30 g g than those from magnesium or calcium bentonites most smectites have a small interstratified illitic component that has a major influence on boron concentrations requiring the use of modified fluid mineral boron partitioning coefficients which indicate that bentonites formed from fluids of highly variable boron concentrations of 100 mg l b and a chlorine content of 76 4 mg l to 59 076 mg l the sodium bentonites formed from boron rich saline fluids or brines whereas the fluids involved in calcium and magnesium bentonite formation have a more variable boron composition and salinity the 11b values of the structural boron in tetrahedral sites of smectites range from 30 1 to 12 2 the smectites from terrestrial depositional settings have 11b values of 30 1 to about 0 whereas smectites from marine depositional settings have negative as well as positive 11b values the boron isotope values indicate that all examined bentonites from terrestrial depositional settings as well as many bentonites from marine depositional settings formed from basinal fluids or brines the boron isotope geochemistry of smectites is demonstrated to be a tool for elucidating the fluids involved in the formation of clay mineral deposits it also has great potential for tracing fluids in other settings involving authigenic clay minerals such as sedimentary basins and surficial crystalline rocks as well as man made applications such as in disposal sites for highly active nuclear waste

early holocene climate variability and the timing and extent of the holocene thermal maximum htm in northern iceland the magnitude and timing of holocene maximum warmth in the arctic and sub arctic has been the subject of considerable recent interest particularly in the context of future climate change although lying at a crucial location in the north atlantic close to significant atmospheric and oceanic boundaries terrestrial holocene climatic data from iceland are few and predominantly derive from glacial and palaeoecological evidence here we present new datasets from trollaskagi based on chironomid inferred temperatures ci t using sub fossil chironomids from the same lake sediments supplemented by pollen data july air temperatures have been derived using an icelandic training set and the data suggest optimal temperatures at sea level up to 1 5 c above current levels around 8 k cal yr bp a time when birch woodland was well developed in trollaskagi but when woodland had still not fully developed in the more isolated nw peninsula our data thus suggest that optimal summer warmth did not occur in iceland until 8 kcal yr bp at the earliest possibly lasting until 6 7 kcal yr bp the amount of warming for july was therefore at least 1 5 c but possibly up to 2 3 c higher than the 1961 1990 average on the basis of the tree line data comparison with data from elsewhere in adjacent arctic regions greenland and eastern arctic canada show peak warmth to be later in iceland and less pronounced it also appears that there were enhanced temperature gradients during the first half of the holocene between the two study areas trollaskagi and the nw peninsula and that they influenced patterns of vegetation colonisation with current spatial temperature patterns only developing as holocene climate deteriorated after around 6 kcal yr bp

recommendations for using xrf core scanning as a tool in tephrochronology x ray fluorescence xrf core scanning is a relatively new arrangement of a classic analytical technique which allows for non destructive in situ xrf analysis of sediment cores from submillimetre

holocene and latest pleistocene climate and glacier fluctuations in iceland abstract multiproxy climate records from iceland document complex changes in terrestrial climate and glacier fluctuations through the holocene revealing some coherent patterns of change as well as significant spatial variability most studies on the last glacial maximum and subsequent deglaciation reveal a dynamic iceland ice sheet iis that responded abruptly to changes in ocean currents and sea level the iis broke up catastrophically around 15 ka as the polar front migrated northward and sea level rose indications of regional advance or halt of the glaciers are seen in late allerod early younger dryas time and again in preboreal time due to the apparent rise of relative sea level in iceland during this time most sites contain evidence for fluctuating tidewater glacier termini occupying paleo fjords and bays the time between the end of the younger dryas and the preboreal was characterized by repeated jokulhlaups that eroded glacial deposits by 10 3 ka the main ice sheet was in rapid retreat across the highlands of iceland the holocene thermal maximum htm was reached after 8 ka with land temperatures estimated to be 3 c higher than the 1961 1990 reference and net precipitation similar to modern such temperatures imply largely ice free conditions across iceland in the early to mid holocene several marine and lacustrine sediment climate proxies record substantial summer temperature depression between 8 5 and 8 ka but no moraines have been detected from that time termination of the htm and onset of neoglacial cooling took place sometime after 6 ka with increased glacier activity between 4 5 and 4 0 ka intensifying between 3 0 and 2 5 ka although a distinct warming during the medieval warm period is not dramatically apparent in icelandic records the interval from ca ad 0 to 1200 is commonly characterized by relative stability with slow rates of change the literature most commonly describes little ice age moraines ca ad 1250 1900 as representing the most extensive ice margins since early holocene deglaciation with temperature depressions of 1 2 c compared to the ad 1961 1990 average steep north south and west east temperature gradients are reconstructed in the holocene records of iceland suggesting a strong maritime influence on the terrestrial climate of iceland

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

an outline tephrochronology for the holocene of the north of ireland geochemically typed thin layers of icelandic holocene tephras form a tephrochronology for the north of ireland tephrochronology construction is based on geochemical linkage with dated historic icelandic eruptions and high precision multisample radiocarbon dating comparison with geochemically typed holocene layers at sites throughout scotland and northern england shows that some layers are widespread throughout much of the northern british isles other layers of known historic date have so far been detected only in ireland

the tephrochronology of iceland and the north atlantic region during the middle and late quaternary a review the tephrochronology of iceland and the north atlantic region is reviewed in order to construct a unified framework for the last 400 kyr bp nearly all of the tephra layers described are also characterised geochemically a number of new tephra layers are analysed for the first time for their geochemical signature and a number of pre holocene tephra layers have been given an informal denotation the tephrostratigraphy of ash zone ii is highlighted where possible the rhyolitic tephra layers found outside iceland have been correlated to known icelandic tephra layers or to the volcanic source area the application of tephra fallout in various depositional environments is described and discussed copyright 2000 john wiley sons ltd

a 3000 year varved record of glacier activity and climate change from the proglacial lake hvitarvatn iceland a suite of environmental proxies in annually laminated sediments from hvitarvatn a proglacial lake in the central highlands of iceland are used to reconstruct regional climate variability and glacial activity for the past 3000 years sedimentological analysis is supported by tephrostratigraphy to confirm the continuous annual nature of the laminae and a master varve chronology places proxies from multiple lake cores in a secure geochronology varve thickness is controlled by the rate of glacial erosion and efficiency of subglacial discharge from the adjacent langjokull ice cap the continuous presence of glacially derived clastic varves in the sediment fill confirms that the ice cap has occupied the lake catchment for the duration of the record varve thickness varve thickness variance ice rafted debris total organic carbon mass flux and bulk concentration and c n of sedimentary organic matter reveal a dynamic late holocene climate with abrupt and large scale changes in ice cap size and landscape stability a first order trend toward cooler summers and ice cap expansion is punctuated by notable periods of rapid ice cap growth and or landscape instability at ca 1000 bc 600 bc 550 ad and 1250 ad the largest perturbation began ca 1250 ad signaling the onset of the little ice age and the termination of three centuries of relative warmth during medieval times consistent deposition of ice rafted debris in hvitarvatn is restricted to the last 250 years demonstrating that langjokull only advanced into hvitarvatn during the coldest centuries of the little ice age beginning in the mid eighteenth century this advance represents the glacial maximum for at least the last 3 ka and likely since regional deglaciation 10 ka the multi centennial response of biological proxies to the hekla 3 tephra deposition illustrates the significant impact of large explosive eruptions on local environments and catchment sensitivity to perturbations

climatic variability during the last millennium in western iceland from lake sediment records the aim of this research was to create a decadal scale terrestrial quantitative palaeoclimate record for nw iceland from lake sediments for the last millennium geochemical stable isotope and chironomid reconstructions were obtained from a lake sequence constrained by tephra deposits on the snaefellsnes peninsula western iceland obtaining a quantitative record proved problematic but the qualitative chironomid record showed clear trends associated with past summer temperatures and the sedimentological records provided evidence for past changes in precipitation mediated through catchment soil in wash when the full range of chronological uncertainty is considered four clear phases of climatic conditions were identified 1 a relatively warm phase between ad 1020 and 1310 2 a relatively stable period between ad 1310 and 1510 cooler than the preceding period but still notably warmer than the second half of the millennium 3 a consistent reduction of temperatures between ad 1560 and 1810 with the coolest period between ad 1680 and 1810 and 4 ad 1840 2000 has temperatures mainly warmer than in the preceding two centuries with a rising trend and increased variability from c ad 1900 onwards the reconstructions show clearly that the first half of the millennium experienced warmer climatic conditions than the second half with a return to the warmer climate only occurring in the last c 100 years much of the variability of the chironomid record can be linked to changes in the north atlantic oscillation nao the reconstructions presented can track low frequency and long term trends effectively and consistently but high resolution and calibrated quantitative records remain more of a challenge not just in finding optimal sedimentary deposits but also in finding the most reliable proxy it is this that presents the real challenge for holocene climate reconstruction from this key area of the north atlantic