applications of organic geochemistry to paleolimnological reconstructions a summary of examples from the laurentian great lakes the organic matter content of lake sediments contains information that helps to reconstruct past environmental conditions evaluate histories of climate change and assess impacts of humans on local ecosystems the elemental isotopic and molecular compositions of organic matter buried in sediment provide evidence of the biota that have lived in a lake and its catchment area and they serve as proxies of organic matter delivery and accumulation sedimentary records from the north american great lakes provide examples of applications of organic geochemistry to paleolimnological reconstructions the records of these lakes date from retreat of the laurentide ice sheet around 12 ka include the mid holocene hypsithermal and show consequences of recent human changes low corg ntotal ratios indicate that most of the sediment organic matter in the great lakes is from algal production yet changes in biomarker molecule compositions also show that varying amounts of land plant organic matter have been delivered to the lakes elevated algal productivity that accompanies nutrient enrichments of lake waters is recorded as excursions to less negative 13c values in the organic matter of sediments that were deposited in the 1960s and 1970s increased organic carbon mass accumulation rates mirror the isotopic excursions in most parts of the great lakes accumulations of petroleum residues and pyrogenic polycyclic aromatic hydrocarbons in sediments identify fluvial and eolian delivery of organic matter components to different parts of the great lakes emerging applications of multiple and novel organic geochemical proxies to paleolimnological reconstructions are promising yet some potentially important measurements remain underutilized

arid and humid phases in southern spain during the last 4000 years the zonar lake record cordoba a multiproxy study of sediment cores from zonar lake 37 29 00 n 4 41 22 w 300 m a s l supported by 11 14c ams dates provides the first high resolution centennial scale reconstruction of past h

aeolian sediment reconstructions from the scottish outer hebrides late holocene storminess and the role of the north atlantic oscillation northern europe can be strongly influenced by winter storms driven by the north atlantic oscillation nao with a positive nao index associated with greater storminess in northern europe however palaeoclimate reconstructions have suggested that the nao storminess relationship observed during the instrumental period is not consistent with the relationship over the last millennium especially during the little ice age lia when it has been suggested that enhanced storminess occurred during a phase of persistent negative nao to assess this relationship over a longer time period a storminess reconstruction from an nao sensitive area the outer hebrides is compared with late holocene nao reconstructions the patterns of storminess are inferred from aeolian sand deposits within two ombrotrophic peat bogs with multiple cores and two locations used to distinguish the storminess signal from intra site variability and local factors the results suggest storminess increased after 1000 cal yrs bp with higher storminess during the medieval climate anomaly mca than the lia supporting the hypothesis that the nao storminess relationship was consistent with the instrumental period however the shift from a predominantly negative to positive nao at c 2000 cal yrs bp preceded the increased storminess by 1000 years we suggest that the long term trends in storminess were caused by insolation changes while oceanic forcing may have influenced millennial variability

tephrological implications of beam size sample size effects in electron microprobe analysis of glass shards this paper concerns the potential consequences of varying procedures for the determination of tephra geochemistry by electron microprobe application of electron probe microanalysis to tephrostratigraphical methods has increasingly facilitated the resolution and refinement of quaternary chronology associated with records of proxy environmental or proxy climatic change the geographical range over which tephras are recovered has expanded significantly with the identification and analysis of crypto or hidden tephras in areas far removed from tephra sources these tephras are dominated by glass shards which in many distal environments may be either small in size m or may be highly pumiceous with low glass void ratios and thin 10 m shard walls we demonstrate that reducing the size of the electron beam used to analyse shard geochemistry cannot be used reliably to permit analysis of thin glass walls this approach distorts the geochemical data creating analytical differences that may generate inappropriate tephrogeochemical fingerprints additional distortion of the geochemical fingerprint in the form of hybrid analyses may be encountered in glass fragments containing micron sized crystalline phases such as feldspar copyright 2001 john wiley sons ltd

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

late holocene hydroclimatic variation in central asia and its response to mid latitude westerlies and solar irradiance abstract hydroclimatic variations in central asia are widely recognized to be tele connected with the north atlantic climate via the mid latitude westerlies however the long term hydroclimatic variation and its response to north atlantic climate changes are not fully understood here we report multi decadal and centennial hydroclimatic variations in central asia during the late holocene using high resolution oxygen and carbon isotopes of bulk carbonate from a remote hydrologically closed alpine lake in tianshan mountains northwestern china the hydroclimatic variations inferred from the covariance between 18ocarb and 13ccarb and published multi proxy in the lake sayram together agree well with the records from the central asia showing enhanced effective moisture at the intervals of 4000 3780 3590 3210 2800 2160 1700 1370 and 890 280 cal yr bp during the negative phase of north atlantic oscillation nao comparison with european hydroclimatic changes on centennial scale our records not only provide valuable insight into the hydroclimatic variations in central asia but also indicate all broadly contemporaneous changes with that of southern europe whereas antiphase with that of northern europe we then conclude that the observed antiphase hydroclimatic pattern between northern europe and southern europe central asia on centennial time scale during the late holocene were persistently controlled by the north south migration of mid latitude westerlies and changes of nao phase in response to solar irradiance forcing however further research is necessary to fully disentangle the natural and anthropogenic forcing mechanisms for enhanced effect moisture and or precipitation in central asia

the last termination in the central south atlantic lake sediments and peat deposits from two basins on nightingale island 37 degrees s in the tristan da cunha archipelago south atlantic have been analyzed the studies were focused on the time period 16 2 10 0 cal ka bp determined by 36 c 14 dates from the two sequences a wide variety of proxies were used including pollen and diatom analyzes biogenic silica content c and n analyzes stable isotopes c 13 and n 15 elemental concentrations and magnetic susceptibility measurements to detect environmental changes that can be related to shifts of the circulation belts of the southern ocean in addition climate model simulations were carried out we find that the sediments are underlain by a 2 cal ka bp long hiatus possibly representing a dried out lake bed the climate simulations corroborate that the area might have been exposed to arid conditions as a consequence of the heinrich i event in the north and a southward displacement of the itcz the development on the island after 16 2 cal ka bp is determined by the position of the subtropical front stf and the southern hemisphere westerlies shw the period 16 2 14 75 cal ka bp was characterized by varying influence from shw and with stf situated south of tristan da cunha ending with a humidity peak and cooler conditions the stable conditions 14 7 14 1 cal ka bp with cool and fairly arid conditions imply that stf and shw were both north of the islands during the first part of the antarctic cold reversal the most unstable period 14 1 12 7 cal ka bp indicates incessant latitudinal shifts of the zonal circulation perhaps related to climate variability in the northern hemisphere and bipolar seesaw mechanisms as the strength of the atlantic meridional overturning circulation amoc varied at 12 7 cal ka bp the holocene warming began with a gradually drier and warmer climate as a result of a dampened amoc during the younger dryas cooling in the north with itcz stf and shw being displaced southwards peak warming seems to have occurred in the earliest part of the holocene but this period was also characterized by humidity shifts possibly an effect of retraction and expansion phases of shw during amoc variations in the north c 2015 elsevier ltd all rights reserved less

bottom characterization of lagoa das furnas on sao miguel azores archipelago abstract lagoa das furnas is a crater lake located in an area exposed to geohazards from earthquakes and volcanic activity on the island of sao miguel in the azores archipelago geophysical mapping of lagoa das furnas reveals a previously undiscovered volcanic dome this dome is comprised largely of subaquatic pyroclastic debris of trachytic composition sedimentological petrological geochemical and geochronological studies of pyroclastic deposits from the dome link it to the historically documented furnas 1630 eruption the chemistry of glass and crystal fragments sampled from the dome suggests that it is comprised of more evolved magma than that of the main furnas 1630 dome located 1400 m away this suggests that the dome was formed during a final phase of the 1630 eruption in the lagoa das furnas area

environmental response of a fragile semiarid landscape bardenas reales natural park ne spain to early holocene climate variability a paleo and environmental magnetic approach abstract the bardenas reales natural park western ebro basin ne spain constitutes a fragile landscape where the prevailing semiarid climate and erodability of the bedrock have resulted in high sensitivity to erosion sedimentation oscillations during the holocene in this paper we present new chronologic sedimentologic paleomagnetic and environmental magnetic results from the oldest unit qah1 of a system of nested cut and fill alluvial sequences which is 18 m thick and crops out extensively in the central part of the natural park at the bardena blanca depression radiocarbon dating indicates that early holocene unit qah1 is younger than 9 38 8 89 cal kyr bp and gives ages of 8 92 8 42 and 7 59 7 37 cal kyr bp in the middle and uppermost parts of the unit respectively sedimentologic data indicate that distal alluvial sediments of unit qah1 accumulated in sandy mud flats and ephemeral playa lakes under arid conditions the coincidence of enhanced magnetite content with moderate to intense root bioturbation in the lowermost and likely also uppermost sediments of unit qah1 suggests that magnetite formed authigenically in response to relatively wetter conditions at the beginning and end of this dry period these results indicate that onset and cessation of alluvial aggradation mark the passing of a geomorphologic threshold most likely plant cover in the watershed during a progressive drying wetting event the radiocarbon based age model of unit qah1 indicates that alluvial aggradation under arid conditions began at 9 13 cal kyr bp and ended at 7 16 cal kyr bp this period is strikingly centered at around the 8 2 kyr cold event but spans a longer time period supporting the notion of a global climatic deterioration as the underlying cause for this event paleomagnetic directions do not clearly display the characteristic patterns recognized in paleosecular variation curves derived from other european lacustrine sediments and cannot be used to refine the radiocarbon based age model for unit qah1 our results demonstrate a quick and strong response of alluvial systems to early holocene climate variability in fragile mediterranean landscapes such as those at the semiarid bardenas reales natural park

bioclimatic analysis of rodent palaeofaunas reveals severe climatic changes in southwestern europe during the plio pleistocene abstract a recently developed method in palaeoclimatology bioclimatic analysis which makes it possible to extract a maximum of information from the available mammal proxy data is applied to a succession of fossil rodent assemblages recorded from the pliocene and pleistocene in the iberian peninsula it consists of both qualitative and quantitative models with the capacity to produce a detailed palaeoclimate reconstruction which is consistent with palaeobotanical and isotopic proxy data and the physics of atmospheric models using this method the palaeoclimate of the iberian plio pleistocene is reconstructed with high resolution from 44 fossil rodent assemblages comprising more than a hundred identified species analysis of these assemblages shows that the climatic evolution of the iberian peninsula during the plio pleistocene can be divided into four distinct climatic phases first a long phase of subtropical climate with summer rainfall and winter drought occurred during the ruscinian second between 3 4 and 2 5 my b p there was a cooling phase coincident with the development of the northern hemisphere ice sheets following this cooling a temperate interlude occurred between 2 5 and 1 8 my b p during this phase the mediterranean climate was established permanently in southwestern europe finally during the pleistocene temperate and colder stages were succeeding in relationship with the northern hemisphere glaciations this sequence of climatic changes is linked with the tectonic rising of the central asian and east african plateaus variations in the atmospheric circulation pattern and changes in the mediterranean sea surface temperatures all these factors led to associated changes in the precipitation regime and a latitudinal displacement of the zonal climatic belts

north atlantic oscillation dynamics recorded in greenland ice cores carefully selected ice core data from greenland can be used to reconstruct an annual proxy north atlantic oscillation nao index this index for the past 350 years indicates that the nao is an intermittent climate oscillation with temporally active coherent and passive incoherent phases no indication for a single persistent multiannual nao frequency is found in active phases most of the energy is located in the frequency band with periods less than about 15 years in addition variability on time scales of 80 to 90 years has been observed since the mid 19th century

the little ice age and medieval warm period in the sargasso sea sea surface temperature sst salinity and flux of terrigenous material oscillated on millennial time scales in the pleistocene north atlantic but there are few records of holocene variability because of high rates of sediment accumulation holocene oscillations are well documented in the northern sargasso sea results from a radiocarbondated box core show that sst was approximately 1 degree c cooler than today approximately 400 years ago the little ice age and 1700 years ago and approximately 1 degree c warmer than today 1000 years ago the medieval warm period thus at least some of the warming since the little ice age appears to be part of a natural oscillation 39 refs 4 figs 1 tab

vegetation and landscape dynamics under natural and anthropogenic forcing on the azores islands a 700 year pollen record from the sao miguel island abstract the azores archipelago has provided significant clues to the ecological biogeographic and evolutionary knowledge of oceanic islands palaeoecological records are comparatively scarce but they can provide relevant information on these subjects we report the palynological reconstruction of the vegetation and landscape dynamics of the sao miguel island before and after human settlement using the sediments of lake azul the landscape was dominated by dense laurisilvas of juniperus brevifolia and morella faya from ca 1280 ce to the official european establishment 1449 ce after this date the original forests were replaced by a complex of erica azorica myrsine africana forests shrublands and grassy meadows which remained until ca 1800 ce extractive forestry cereal cultivation rye maize wheat and animal husbandry progressed until another extensive deforestation ca 1774 ce followed by the large scale introduction 1845 ce of the exotic forest species cryptomeria japonica and pinus pinaster which shaped the present day landscape fire was a significant driver in these vegetation changes the lake levels experienced a progressive rise during the time interval studied reaching a maximum by ca 1778 1852 ce followed by a hydrological decline likely due to a combination of climatic and anthropogenic drivers our pollen record suggests that sao miguel were already settled by humans by ca 1287 ce approximately one century and a half prior to the official historically documented occupation of the archipelago the results of this study are compared with the few palynological records available from other azores islands pico and flores

the azores front since the last glacial maximum the spatial distribution of warm surface water in the atlantic ocean reflects the state of the thermohaline circulation the azores current front which is a recirculation of the gulf stream marks the northeastern boundary of the north atlantic subtropical gyre its position is therefore diagnostic of the width of the atlantic warm water sphere here we report high resolution stable isotope and faunal abundance records of planktonic foraminifera in a sediment core from the gulf of cadiz southwest spain which reflects shifting of the azores front since the last glacial maximum lgm today the azores front does not penetrate into the gulf of cadiz even though the front resides at the same latitude as the gulf of cadiz in the atlantic our results indicate that the azores front is a robust feature of the atlantic surface circulation and that is present both in interglacial times and during the lgm at roughly the same latitude however during the lgm prior to 16 ka bp and during the younger dryas the azores front did penetrate eastward into the gulf of cadiz

mt fuji holocene eruption history reconstructed from proximal lake sediments and high density radiocarbon dating abstract an 8000 year lacustrine sediment record from lake motosu fuji five lakes records several eruptions including potentially unreported events of the active mt fuji volcano which receives approximately 47 million annual visitors a high fidelity age model is constructed from tephra ages and high density radiocarbon dating of terrestrial macrofossil and bulk organic matter variability in lake reservoir age is constrained by modern lake water radiocarbon measurement and reverse calibration of tephra calendar ages we present more accurate ages for known eruptions detect a wider distribution of ejecta for several eruptions including the most recent summit eruption and potentially identify previously undetected flank eruptions there are closely spaced scoria fall layers that may be difficult to differentiate as separate events in land based surveys these results demonstrate the utility of lacustrine sediments as powerful tools for understanding characteristics of volcanic eruptions

a holocene lacustrine record in the central north atlantic proxies for volcanic activity short term nao mode variability and long term precipitation changes a holocene lacustrine record in the central north atlantic proxies for volcanic activity short term nao mode variability and long term precipitation changes

isotopic analysis c n and species composition of rodent assemblage as a tool for reconstruction of climate and environment evolution during late quaternary a case study from biśnik cave czestochowa upland poland rodent remains are potentially a powerful tool in paleoecological reconstructions rodents are a worldwide group of mammals inhabiting all climatic zones from arctic tundra to tropical forests some of their representatives have narrow ranges of environmental tolerance while others have cosmopolitan distributions additionally rodent remains are well preserved and numerous in cave sediments and are easily identified to the species level the species composition of rodent assemblage from the specific cave gives valuable data on ecosystem status around the site moreover isotopic composition c n of rodent remains provides information on some environmental factors e g type of vegetation soil activity precipitation and temperature the material for this study was collected from the sediments of the bi nik cave poland the cave is known from its sediment sequence deposited during the middle and late pleistocene and holocene containing a rich collection of the pleistocene fauna remains and archeological artifacts the stratigraphy of the sediment sequence previously was based on archeological findings and several radiometric dates in this study we applied u th dating method to bone collagen rodent tooth enamel and cave bear tooth enamel finally we revised the chronology of sediment sequence from the bi nik cave based on these new data we found significantly higher variability of nitrogen and carbon isotopic composition in the lemming than in the common vole however both species show the same trends in isotopic composition of nitrogen in time at the beginning of the record 15n values were relatively high suggesting intensive soil activity and indirectly mild climate after that from a stratigraphic layer dated to the early wurm glaciation lower 15n values point to lower soil activity deterioration of ecosystem productivity and climate conditions were also confirmed by higher 13c this indicates lower canopy effect and suggests conversion of vegetation around the cave the isotopic record suggests high instability of the environmental conditions during the wurm glaciations with significantly warmer periods before and after the last glacial maximum

centennial eolian cyclicity in the great plains usa a dominant climate pattern of wind transport over the past 4000 years proxy evidence at decadal resolution from late holocene sediments from pickerel lake northeastern south dakota shows distinct centennial cycles 400e700 years in magnetic susceptibility contents of carbonate organic carbon and major elements abundance in ostracodes and d 18 o and d 13 c values in calcite proxies indicate cyclic changes in eolian input productivity and temperature maxima in magnetic susceptibility are accompanied by maxima in aluminum and iron mass accumulation rates mars and in abundances of the ostracode fabaeformiscandona rawsoni this indicates variable windy and dry conditions with westerly wind dominance including during the medieval climate anomaly maxima in carbonates organic carbon phosphorous and high d 13 c values of endogenic calcite indicate moister and less windy periods with increased lake productivity including during the little ice age and alternate with maxima of eolian transport times of the maunder sporer and wolf sunspot minima are characterized by maxima in d 18 o values and aluminum mars and minima in d 13 c values and organic carbon content we interpret these lake conditions during sunspot minima to indicate decreases in lake surface water temperatures of up to 4e5 c associated with decreases in epilimnetic productivity during

anthropogenic and climatic impact on holocene sediment dynamics in se spain a review the climatic and anthropogenic control on holocene sediment dynamics was not well constrained for the western mediterranean basin the paucity of high resolution palaeoenvironmental records long hampered a detailed understanding of the human impact on erosion rates this paper analyses holocene sediment dynamics in the context of regional climatic conditions land cover and human occupation based on an extended compilation of recently published high resolution paleoenvironmental proxy data from both terrestrial and marine environments the palaeoenvironmental reconstruction identified four main aridification episodes a at 5600 4700 a4 4100 3400 a3 2800 2500 a2 and 1900 1300 1200 a1 cal bp most of the severe aridification phases were climatically induced not human driven and well correlated with a large dataset of paleoenvironmental records from the western mediterranean basin and north atlantic ocean the phases of enhanced aridity are generally associated with enhanced sediment dynamics independent of the intensity and type of human occupation in contrast wet climatic conditions are found to be associated with both high and low sediment dynamics dependent on the human impact on the environment the mid late phoenician phoenician ii republican rome civilizations and the christian reconquest are all associated with high human impact on the environment and high erosion rates are reported for the vera basin and ne spain despite the improved hydrological conditions human occupation is not necessarily associated with geomorphic instability as shown by the post argaric omeya and nazarene periods that are characterized by low erosion rates and geomorphic stability

late holocene multi proxy records of environmental change on the south atlantic island tristan da cunha sediment stratigraphies from three sites one lake one overgrown lake and an exposed section on the island tristan da cunha in the temperate south atlantic were analysed by means of pollen analysis total carbon nitrogen and sulphur content determination magnetic susceptibility measurements and detailed radiocarbon dating the aim of these studies was to reconstruct the late holocene vegetation and climatic variations the oldest sediment sequence extends back to 2300 cal years bp the vegetation was relatively stable up to the arrival of humans in the 17th century the appearance of the introduced taxon rumex acetosa acetosella at c 300 cal years bp and a subsequent decline in forest cover on the lowland plain provide evidence of substantial human influence on the vegetation well before the establishment of the first permanent settlement in the 19th century before the first anthropogenic influence centennial scale fluctuations in the proxy records are interpreted as reflections of local hydrological changes probably caused by variations in precipitation as inferred mainly from changing proportions of pollen derived from telmatic and terrestrial taxa and corresponding changes in the deposition of mineral matter by fluvial erosion lake levels were low between c 1450 and 1050 cal years bp and high between c 1050 and 300 cal years bp these variations coincide with known climatic changes in southern africa and in the north atlantic suggesting that the inferred hydrological changes on tristan da cunha were related to large scale variations in the general oceanic and atmospheric circulation in the atlantic region

tephra geochemistry a discussion of some persistent analytical problems distal tephrochronological studies are reliant upon precise and accurate geochemical quantitation this quantitation can involve difficulties not often encountered in more conventional petrology solving these problems is of particular importance in the holocene and late quaternary of northwest europe where numerous tephra layers of icelandic origin have been detected in peat bogs and lake and marine sediments electron probe microanalysis epma has proved the most applicable analytical technique when insufficient quantities of tephra are available for bulk analysis standard epma techniques require fine tuning to the tephra problem as the geochemical instability of glass is a common feature in many previous studies geochemical data of avoidably low quality have been produced and little attention appears to have been directed to modification of epma methods it is suggested that the black box approach to geochemical analysis should be avoided by the consideration of several factors these include

recent climatic and anthropogenic imprints on lacustrine systems in the pyrenean mountains inferred from minerogenic and organic clastic supply vicdessos valley pyrenees france high resolution seismic profiling has been combined with geochemical analyses of both watershed samples and five lacustrine cores retrieved from two natural lacustrine basins of glacial origin lake majeur and lake sigriou 1630 m a s l and 1995 m a s l respectively eastern french pyrenees identifying specific minerogenic and organic markers of autochthonous and allochthonous supply data allow documenting past climatic and anthropogenic pressures over the past century the lacustrine sediment of lake majeur has been essentially composed of algae drastically contrasting with the natural sedimentary infill of the basin mainly resulting from soil erosion from the mid late holocene since ad 1907 the lake majeur has been used for hydroelectricity production human induced lake level regulations affecting up to 37 of the lacustrine surface have increased by fourfold the accumulation rate of the lake and favoured water enrichment rubidium abundance within the lacustrine sediments of the two lakes reflects the mid late holocene palaeohydrology after dam construction in ad 1907 greater quantities of rubidium found in lake majeur sedimentary infills indicate drier climatic periods such as from ad 1975 to ad 1982 during which water reservoirs were particularly in demand inversely before the dam was built rubidium fluctuations were correlated with wetter conditions and hydrological events were recorded as sandy layers deposited by canyon reactivation synchronous with european climatic deterioration phases we notably document that the mediaeval climate anomaly was interrupted by some humid periods dated c ad 940 ad 1080 ad 1100 and ad 1250 we also date the onset of the little ice age c ad 1360 and identify that this period was wetter after c ad 1500

late holocene climatic variability south of the alps as recorded by lake level fluctuations at lake ledro trentino italy a lake level record for the late holocene at lake ledro trentino northeastern italy is presented it is based on the sediment and pollen analysis of a 1 75 m high stratigraphic section observed on the southern shore site ledro i and a 3 2 m long sediment core taken from a littoral mire on the southeastern shore site ledro ii the chronology is derived from 15 radiocarbon dates and pollen stratigraphy the late holocene composite record established from these two sediment sequences gives evidence of centennial scale fluctuations with highstands at c 3400 2600 1700 1200 and 400 cal bp in agreement with various palaeohydro logical records established in central and northern italy as well as north of the alps in addition high lake level conditions at c 2000 cal bp may be the equivalent of stronger river discharge observed at the same time in central italy s rivers in agreement with the lake level record of accesa tuscany the ledro record also suggests a relatively complex palaeohydrological pattern for the period around 4000 cal bp on a millennial scale sediment hiatuses observed in the lower part of the ledro i sediment sequence indicate that except for a high stand occurring just after 7500 cal bp lower lake levels generally prevailed rather before c 4000 cal bp than afterwards finally the lake level data obtained at lake ledro indicate that the relative continuity of settlements in humid areas of northern italy during the bronze age in contrast to their general abandonment north of the alps between c 3450 and 3150 cal bp does not reflect different regional patterns of climatic and palaeohy drological conditions in contrast the rise in lake level dated to c 3400 cal bp at ledro appears to coincide with a worldwide climate reversal observed in both the hemispheres while palaeoenvironmental and archaeological data collected at lake ledro may suggest as a working hypothesis a relative emancipation of proto historic societies from climatic conditions

magma derived co2 emissions recorded in and content of plants growing in furnas caldera azores abstract the environmental impact of fumarolic and soil emanations of magma derived carbon dioxide across furnas caldera has been investigated by measuring the 14 c and 13 c content of 40 specimens of different c3 plants leaves growing within and outside the degassing areas the results demonstrate a significant to large 14 c depletion in many of the plants due to assimilation of 14 c free endogenous co2 during photosynthesis and leading to artificial radiocarbon ageing of up to 4400 years the extent of 14 c ageing broadly correlates with the intensity of gas manifestations at the sampling sites as inferred from field observations and measurements of excess co2 concentrations in the volcanic ground it also provides a time integrated measure of the amount of volcanic co2 locally admixed to the ambient air at several sites this accounts for 15 to 40 of total co2 420 to 600 ppm in enriched air in some of the plant species azalea camellia and fern 14 c depletion is correlated with an enrichment of 13 c due to assimilation of magma derived co2 with a 4 higher 13 c than normal atmosphere the rate of 13 c enrichment averages ca 0 18 by percent of volcanic carbon fixed in the plant and includes enhanced 13 c discrimination during photosynthesis as a consequence of increased ambient pco2 inferred at 0 0306 per added ppm of volcanic co2 furnas is one of the few volcanoes where clear 13 c enrichment in plants due to endogenous degassing has been evidenced our results can be used to estimate the local intensity of volcanic soil gas fluxes in the emanating areas of furnas caldera they also have implications for radiocarbon dating of past eruptive events in the caldera since plants artificially aged by previous degassing could be trapped in volcanic deposits

new major element analyses of proximal tephras from the azores and suggested correlations with cryptotephras in north west europe the azores archipelago is one of the most active volcanic areas in the north atlantic region approximately 30 eruptions have been reported over the last 600 years with some major vei 5 volcanic e

a well verified multiproxy reconstruction of the winter north atlantic oscillation index since a d 1400 abstract a new well verified multiproxy reconstruction of the winter north atlantic oscillation nao index is described that can be used to examine the variability of the nao prior to twentieth century greenhouse forcing it covers the period a d 1400 1979 and successfully verifies against independent estimates of the winter nao index from european instrumental and noninstrumental data as far back as 1500 the best validation occurs at interannual timescales and the weakest at multidecadal periods this result is a significant improvement over previous proxy based estimates which often failed to verify prior to 1850 and is related to the use of an extended reconstruction model calibration period that reduced an apparent bias in selected proxies associated with the impact of anomalous twentieth century winter nao variability on climate teleconnections over north atlantic sector land areas although twentieth century nao variability is somewhat unusual comparable periods of persistent positive phase

late holocene morphodynamics in the littoral zone of the iwik peninsula area banc d arguin mauritania abstract in the littoral zone of the banc d arguin both littoral sedimentary units and man made deposits shell middens abound they were formed during the late holocene after the final onset of the post glacial transgression here a geo archaeological approach is used to study them the geomorphological and sedimentological characteristics of the coastal features define two distinct coastal sedimentary units in the iwik aouatil zone between an ancient shoreline which formed on the tafaritian substratum ca 6 7 5 7 cal ka bp and the present one i very extensive sand flats and ii linear relief features both covered by a thin veneer of anthropogenic anadara senilis shell middens they developed under a stable sea level regime the sand flats expanded at variable rates throughout the period the linear relief features represent beach barriers and coastal dunes which can be dated archaeologically thanks to the superimposed shell middens supplemented by 14 c dating two series of linear sedimentary units are identified first a complex north south system of dunes and hook ended sand barriers developed from 4 8 to 4 1 cal ka bp and secondly two regular and linear beach barriers developed between 3 7 and 3 3 cal ka bp their morphological characteristics suggest that they were generated through littoral drift processes directional changes observed between the first and the second groups demonstrate that the wave refraction pattern was dramatically modified after the closure of the strait between the former iwik island and the land a comparison with the nearby jerf el oustani coastal area in the late holocene shows that both sites shared similarities in depositional patterns but also differences in the chronology of the sedimentary units processes involved in the development of the linear units are indicative of the enhancement of both sand influx and longshore drift due to climatic change

holocene climate and vegetation dynamics on nightingale island south atlantic an apparent interglacial bipolar seesaw in action abstract high resolution analyses of a sediment sequence from an in filled lake on nightingale island at 37 s in the south atlantic have revealed hydrological variations during the last 10 700 cal yr bp which are linked to shifts in the southern hemisphere westerlies and the meridional overturning circulation moc in the atlantic the lowermost part of the sequence 10 700 8600 cal yr bp consists of peat the peat is overlaid by gyttja which continues up to the final overgrowing of the basin around 500 years ago the transition from peat to gyttja at 8600 cal yr bp implies a significant increase in effective humidity probably caused by a northward shift of the southern hemisphere westerlies after the end of the antarctic thermal optimum recurring phases of increased erosion with high deposition of minerogenic particles terrestrial organic matter high c n ratios and pollen grains from plant taxa growing in more distal parts of the catchment indicate periods with higher precipitation these high precipitation periods correspond to periods of southward shifted southern hemisphere westerlies inferred from south america and with periods of weaker moc in the north atlantic therefore we conclude that these periods also characterized by decreased influence from marine aerosols were not caused by the stronger westerlies but by weaker moc and less heat transport from the south to the north atlantic causing warmer sea surface temperatures ssts and increased precipitation in the south atlantic this is the expression of the bipolar seesaw mechanism and implies that it may have been operational also during interglacial conditions

holocene alluvial morphopedosedimentary record and environmental changes in the bardenas reales natural park ne spain abstract a holocene alluvial morphostratigraphy based on geomorphological mapping sedimentological analysis soil development and radiocarbon chronology is presented for the semiarid blanca basin in the bardenas reales natural park ne spain four morphopedosedimentary alluvial units mu1 mu2 mu3 and mu4 were differentiated which may correspond to two palaeogeographical stages the first one includes the morphopedosedimentary unit mu1 and is the most extensive in the area it is made up of a complex aggradation fill up sequence comprising a braided channel system a sand mud flat 9200 7900 cal bp and a playa lake environment related to a nearly closed depression an intensive fluvial entrenchment period is recognized leading the opening of the blanca basin before the second stage that includes the units mu2 mu3 and mu4 this set of units forms a sequence of cut and fill and nested fill terraces recording the alluvial activity in the blanca basin during the last 6 ka the mu2 unit 5900 3300 cal bp in age represents a period of high geomorphic instability with rapid alluvial sedimentation channel incision oscillating changes on the other hand the mu3 unit has an age of 1050 500 cal bp and appears as a thin extensive alluvial cover overlying previous units finally the mu4 unit comprises superposition of numerous flood depositional sequences centimetric in scale dated from 450 10 cal bp the established soil chronosequence supports the numerical chronology of the four units these accumulation periods mu1 mu2 mu3 mu4 are associated with dry phases and or high hydrological variability floods and droughts at a regional scale changes on the north atlantic atmospheric circulation mode over southern europe may provide the key to explain the nature of alluvial activity during holocene in bardenas reales

arctic north atlantic oscillation signature in holocene sea surface temperature trends as obtained from alkenone data 1 the variability in alkenone derived sea surface temperatures ssts in the north atlantic realm shows that a continuous sst decrease in the northeast atlantic from the early to the late holocene was accompanied by a persistent warming over the western subtropical atlantic the eastern mediterranean sea and the northern red sea based on the analysis of the instrumental data and of atmospheric general circulation model experiments we show that this variation in ssts during the holocene can be attributed to a continuous weakening of a northern hemisphere atmospheric circulation pattern similar to that of the arctic north atlantic oscillation

coherent high and low latitude climate variability during the holocene warm period a faunal record of sea surface temperature sst variations off west africa documents a series of abrupt millennial scale cooling events which punctuated the holocene warm period these events evidently resulted from increased southward advection of cooler temperate or subpolar waters to this subtropical location or from enhanced regional upwelling the most recent of these events was the little ice age which occurred between 1300 to 1850 a d when subtropical ssts were reduced by 3 to 4 c these events were synchronous with holocene changes in subpolar north atlantic ssts documenting a strong in phase link between millennial scale variations in high and low latitude climate during the holocene

long term coordinated changes in the convective activity of the north atlantic the north atlantic is a peculiarly convective ocean the convective renewal of intermediate and deep waters in the labrador sea and greenland iceland sea both contribute significantly to the production and export of north atlantic deep water thus helping to drive the global thermohaline circulation while the formation and spreading of 18 degree water at shallow to intermediate depths off the us eastern seaboard is a major element in the circulation and hydrographic character of the west atlantic for as long as time series of adequate precision have been available to us it has been apparent that the intensity of convection at each of these sites and the hydrographic character of their products have been subject to major interannual change as shown by aagaard 1968 clarke et al 1990 and meincke et al 1992 for the greenland sea in the ows bravo record from the labrador sea eg lazier 1980 et seq and at the panulirus hydrostation s site in the northern sargasso off bermuda eg jenkins 1982 talley and raymer 1982 this paper reviews the recent history of these changes showing that the major convective centres of the greenland and labrador seas are currently at opposite convective extrema in our postwar record with vertical exchange at the former site limited to 1000 m or so but with labrador sea convection reaching deeper than previously observed to over 2300 m as a result greenland sea deep water has become progressively warmer and more saline since the early 70 s due to increased horizontal exchange with the arctic ocean through fram strait while the labrador sea water has become progressively colder and fresher over the same period through increased vertical exchange most recently convection has become deep enough there to reach into the more saline nadw which underlies it so that cooler but now saltier and denser lsw has resulted

the great salinity anomaly in the northern north atlantic 1968 1982 the widespread freshening of the upper 500 800m layer of the northern north atlantic which this paper describes represents one of the most persistent and extreme variations in global ocean climate yet observed in this century though a range of explanations have been advanced to explain this event including in situ changes in the surface moisture flux this paper describes the great salinity anomaly as largely though not entirely an advective event traceable around the atlantic subpolar gyre for over 14 years from its origins north of iceland in the mid to late 1960s until its return to the greenland sea in 1981 1982 the overall propagation speed around this subpolar gyre is estimated at about 3cm s 1 of the total salt deficit associated with the anomaly as it passed south along the labrador coast in 1971 1973 about 72 109 tonnes a deficit equivalent to about two thirds of this figure 47 109 tonnes ultimately passed through the faroe shetland channel to the barents sea arctic ocean and greenland sea during the mid 1970s possible effects on deep water formation and on the representativeness of historical section data are discussed

high resolution holocene record in the southeastern bay of biscay global versus regional climate signals abstract core ks10b north east atlantic 43 50 n 2 03 w water depth 550 m was recovered at a location of high sediment deposition and hence includes a high resolution palaeoclimatic record for the southeastern bay of biscay over the last 9000 cal yr bp the effect of global and regional climatic forcing factors e g orbital forcing north atlantic oscillation nao and their subsequent control on water temperature sea level hydrology and continental influence were investigated by studying planktic and benthic foraminiferal faunas 150 m sedimentology and stable isotopes results indicate probable episodic long term incursions of the warm and salty iberian poleward current ipc into the bay of biscay during the holocene by the intermittent presence of subtropical globigerinoides ruber in the otherwise temperate fauna these incursions seem to be triggered by negative nao like conditions our data show five main climatic periods which are in general agreement with the literature data on the climatic variability in the north atlantic and the iberian peninsula the early holocene 9000 7400 cal yr bp is characterized by low sea level significant downslope transport eutrophic benthic settings and high surface water productivity these environmental characteristics are probably due to the general wet and warm climate under a prevailing negative nao increasing precipitation and river runoff and favouring the incursion of the ipc into the bay of biscay the climatic optimum appears between 7400 and 6000 cal yr bp under a generally positive nao index in the following 6000 3500 cal yr bp surface water cooling is matching a steep decline of the northern hemisphere summer insolation meanwhile benthic settings are changing progressively towards present day trophic conditions the time interval from 3500 to 1800 cal yr bp encompasses the iberian roman humid period characterized by warm and humid conditions and increasing primary production under a prevailing negative nao index the last 1800 years were characterized by stable cool conditions reflecting a change towards a positive nao state

seismicity source mechanisms and tectonics of the azores gibraltar plate boundary abstract a study and partial revision of the seismicity and source mechanisms of earthquakes are presented for the oceanic plate boundary of eurasia and africa from the azores islands to the strait of gibraltar focal mechanisms of 36 earthquakes wave form modelling of 4 and seismic moments and dimensions of 28 have been calculated these results indicate that the boundary is divided into three sections with different characteristics the azores the central section and the east section the azores section has high seismicity of moderate magnitude and a low slip rate 0 76 cm yr the predominant mechanism is normal or transform faulting with horizontal tensions on average in a n25 e direction in the central section large earthquakes occur with strike slip right lateral motion along faults in an e w direction there is a high slip rate 3 39 cm yr the east section is of more complex nature large earthquakes also occur but the predominant mechanism is reverse faulting with a horizontal pressure axis in the n30 w direction resulting in underthrusting of africa from the slip vectors the pole of rotation for the african plate is located at 28 2 n 21 1 w nearer to the boundary than previous locations close to the continent there is a continuation in the ocean crust of some structural features of the iberian peninsula

influence of cycling pore water pressures and principal stress ratios on drained deformations in clay seasonal changes in groundwater levels affect the rate of downhill creep movements in slopes this process has been studied in triaxial tests on undisturbed specimens of a natural clay from bluefish lake 50 km north of yellowknife n w t specimens were first anisotropically consolidated to low stresses that correspond to conditions at shallow depths in creeping slopes pore water pressures back pressures in the specimens were then cycled systematically over periods lasting 4 48 h with the drainage leads open resulting axial and volumetric strains were measured and shear and lateral strains deduced from them strain rates decreased with increasing total times of testing they increased with increasing values of the ratio u uf with increasing values of deviator stress q and with decreasing values of systematically increasing the pore water pressures in the specimens produced clear estimates of failure at low stresses key words slope clay creep cyclic loading ground water triaxial

north pacific and north atlantic sea surface temperature variability during the holocene holocene climate variability is investigated in the north pacific and north atlantic realms using alkenone derived sea surface temperature sst records as well as a millennial scale simulation with a coupled atmosphere ocean general circulation model aogcm the alkenone sst data indicate a temperature increase over almost the entire north pacific from 7 cal kyr bp to the present a dipole pattern with a continuous cooling in the northeastern atlantic and a warming in the eastern mediterranean sea and the northern red sea is detected in the north atlantic realm similarly sst variations are opposite in sign between the northeastern pacific and the northeastern atlantic a 2300 year long aogcm climate simulation reveals a similar sst seesaw between the northeastern pacific and the northeastern atlantic on centennial time scales our analysis of the alkenone sst data and the model results suggests fundamental inter oceanic teleconnections during the holocene

holocene climate variability as derived from alkenone sea surface temperature and coupled ocean atmosphere model experiments holocene climate modes are identified by the statistical analysis of reconstructed sea surface temperatures ssts from the tropical and north atlantic regions the leading mode of holocene sst variability in the tropical region indicates a rapid warming from the early to mid holocene followed by a relatively weak warming during the late holocene the dominant mode of the north atlantic region sst captures the transition from relatively warm cold conditions in the eastern north atlantic and the western mediterranean sea the northern red sea to relatively cold warm conditions in these regions from the early to late holocene this pattern of holocene sst variability resembles the signature of the arctic oscillation north atlantic oscillation ao nao the second mode of both tropical and north atlantic regions captures a warming towards the mid holocene and a subsequent cooling the dominant modes of holocene sst variability emphasize enhanced variability around 2300 and 1000 years the leading mode of the coupled tropical north atlantic holocene sst variability shows that an increase of tropical sst is accompanied by a decrease of sst in the eastern north atlantic an analogy with the instrumental period as well as the analysis of a long term integration of a coupled ocean atmosphere general circulation model suggest that the ao nao is one dominant mode of climate variability at millennial time scales