fingerprinting the 8 2 ka bp event climate response in a coupled climate model using results from coupled climate model simulations of the 8 2 ka climate event that produced a cold period over greenland in agreement with the reconstructed cooling from ice cores we investigate the typical pattern of climate anomalies fingerprint to provide a framework for the interpretation of global proxy data for the 8 2 ka climate event for this purpose we developed an analysis method that isolates the forced temperature response and provides information on spatial variations in magnitude timing and duration that characterise the detectable climate event in proxy archives our analysis shows that delays in the temperature response to the freshwater forcing are present mostly in the order of decades 30 a over central greenland the north atlantic ocean initially cools in response to the freshwater perturbation followed in certain parts by a warm response this delay occurring more than 200 a after the freshwater pulse hints at an overshoot in the recovery from the freshwater perturbation the south atlantic and the southern ocean show a warm response reflecting the bipolar seesaw effect the duration of the simulated event varies for different areas and the highest probability of recording the event in proxy archives is in the north atlantic ocean area north of 40 n our results may facilitate the interpretation of proxy archives recording the 8 2 ka event as they show that timing and duration cannot be assumed to correspond with the timing and duration of the event as recorded in greenland ice cores copyright 2011 john wiley sons ltd

a whiff of oxygen before the great oxidation event high resolution chemostratigraphy reveals an episode of enrichment of the redox sensitive transition metals molybdenum and rhenium in the late archean mount mcrae shale in western australia correlations with organic carbon indicate that these metals were derived from contemporaneous seawater rhenium osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 8 million years ago ma molybdenum and rhenium were probably supplied to archean oceans by oxidative weathering of crustal sulfide minerals these findings point to the presence of small amounts of o 2 in the environment more than 50 million years before the start of the great oxidation event

oxygen and carbon isotopes in modern fresh water ostracod valves assessing vital offsets and autecological effects of interest for palaeoclimate studies abstract the isotopic composition of modern benthic ostracod and mollusc carbonate the oxygen isotope composition of lake water 18ow the carbon isotope composition of dissolved inorganic carbon 13cdic and water temperatures were monitored during a year at different water depths in two lakes ammersee and starnberger see southern germany the aim was to test whether the valves are built in isotopic equilibrium and how seasonal water temperatures are recorded in the shells we find systematic offsets between 18o of valve carbonate compared to 18o expected for a theoretical calcite formed in isotopic equilibrium offsets are higher than 2 for different species of the candona group 1 5 for cytherissa lacustris and around 0 8 for limnocythere inopinata and darwinula stevensoni they seem to be constant for all instars of a species and temperature independent mean water temperatures of the calcification period are readily reflected in the valve 18o if corrected for the offsets 13c of valve carbonate is mainly controlled by 13c of the dissolved carbonate in ambient lake water but seems to be biased by micro habitat effects and vital offsets the study provides information on the seasonality of valve formation thus potentially allowing the quantification of temporal and spatial water temperature gradients from specific 18o records

the 8 2 ka event from greenland ice cores we present a collection of high resolution chemistry and stable isotope records from the plateau of the greenland ice cap during the cold event 8200 yr ago using a composite of four records the cold event is observed as a 160 5 yr period during which decadal mean isotopic values were below average within which there is a central event of 69 yr during which values were consistently more than one standard deviation below the average for the preceding period four cores in north south and central greenland show differences at decadal and shorter timescales it is not yet clear if this represents significant spatial differences in response the results show clear evidence for colder temperatures and a decrease in snow accumulation rate however the changes in chemical concentrations for the ions looked at here are small suggesting only minor changes in atmospheric circulation for this event apart from the decrease in methane concentration greenland ice cores give only weak evidence for effects outside the north atlantic region

molybdenum behavior during early diagenesis insights from mo isotopes 1 we present molybdenum isotope and concentration data from 14 sites in the eastern pacific from the central california to the peru margin the environments studied have been chosen to represent a broad range in oxidation reduction redox potential which provide a framework for the behavior of this redox sensitive element manganese rich hemipelagic sediments from the eastern tropical pacific have a mean characteristic mo isotope signature 98 95mo 0 49 0 04 two times the standard deviation of the mean 2 sdom with n 14 that reflects fractionation between ocean water and authigenic mo associated with mn oxides authigenic fe mo s deposits from reducing continental margin settings also have a characteristic mo isotopic signature 98 95mo 1 64 0 04 2 sdom with n 136 both of these values are in contrast to highly sulfidic 11 m h2saq restricted basin environments which contain mo isotope values analytically indistinguishable from seawater in terms of the mo isotope composition the modern oceanic mo sink is dominated by continental margin type environments where 98 95mo 1 6 and mn rich sediments where 98 95mo approximately 0 5 with a minor contribution from euxinic settings where h2saq 11 m

early holocene climate oscillations recorded in three greenland ice cores a new ice core chronology for the greenland dye 3 grip and ngrip ice cores has been constructed making it possible to compare the 18 o and accumulation signals recorded in the three cores on an almost annual scale throughout the holocene we here introduce the new time scale and investigate 18 o and accumulation anoma lies that are common to the three cores in the early holocene 7 9 11 7 ka before present three time periods with significant and synchronous anomalies in the 18 o and accumulation signals stand out the well known 8 2 ka event an event of shorter duration but of almost similar amplitude around 9 3 ka before present and the preboreal oscillation during the first centuries of the holocene for each of these sections we present a 18 o anomaly curve and a common accumulation signal that represents regional changes in the accumulation rate over the greenland ice cap

climatic forcing of carbon oxygen isotopic covariance in temperate region marl lakes carbon and oxygen stable isotopic compositions of lacustrine carbonate from a southeastern michigan marl lake display linear covariance over a range of 4 0 peedee belemnite pdb in oxygen and 3 9 pdb in carbon mechanisms of delta 13 c delta 18 o coupling conventionally attributed to lake closure in arid region basins are inapplicable to hydrologically open lake systems thus an alternative explanation of isotopic covariance in temperate region dimictic marl lakes is required we propose that isotopic covariance is a direct record of change in regional climate in short residence time temperate region lake basins summer meteoric precipitation is enriched in 18o relative to winter values and summer organic productivity enriches epilimnic dissolved inorganic carbon in 13c thus climate change toward longer summers and or shorter winters could result in greater proportions of warm month meteoric precipitation longer durations of warm month productivity and net long term enrichment in carbonate 18o and 13c isotopic covariance observed in the michigan marl lake cores is interpreted to reflect postglacial warming from 10 to 3 ka followed by cooler mean annual temperature a shift toward greater proportions of seasonal summer precipitation a shortening of the winter season or some combination of these three factors

holocene climate variability in sicily from a discontinuous stalagmite record and the mesolithic to neolithic transition abstract fabric and stable isotopic composition of a holocene stalagmite cr1 from a cave in northern sicily record changes in paleorainfall in the early holocene high 13c stable isotope values in the calcite deposited from ca 8500 to ca 7500 yr ago are interpreted as reflecting periods of high rainfall the wet phase was interrupted by two periods of multi century duration characterized by relatively cool and dry winters centered at ca 8200 and ca 7500 yr ago highlighted by low 13c and 18o values a high variability of 13c values is recorded from ca 7500 to ca 6500 yr ago and indicates that the transition from a pluvial early holocene to the present day climate conditions was punctuated by decadal scale periods of relatively dry winters in northern sicily the traditional elements of the neolithic appear at ca 7700 yr ago it is possible that changes in rainfall influenced the passage from hunter gathering to farming and sheep herding economies

the 8 2 ka event abrupt transition of the subpolar gyre toward a modern north atlantic circulation 1 climate model simulations of the 8 2 ka event show an abrupt strengthening of the atlantic subpolar gyre that allows us to connect two major but apparently contradictory climate events of the early holocene the freshwater outburst from proglacial lakes and the onset of labrador sea water formation the 8 2 ka event is the largest climatic signal of our present interglacial with a widespread cooling in the north atlantic region about 8200 years before present it coincides with a meltwater outburst from north american proglacial lakes that is believed to have weakened the atlantic meridional overturning circulation and northward heat transport followed by a recovery of the deep ocean circulation and rising temperatures after a few centuries marine proxy data however date the onset of deep water formation in labrador sea to the same time the subsequent strengthening of the slope current system created a regional signal recorded as an abrupt and persistent surface temperature decrease although similarities in timing are compelling a mechanism to reconcile these apparently contradictory events was missing our simulations show that an abrupt and persistent strengthening of the atlantic subpolar gyre provides a plausible explanation the intense freshwater pulse triggered a transition of the gyre circulation into a different mode of operation stabilized by internal feedbacks and persistent after the cessation of the perturbation as a direct consequence deep water formation around its center intensifies this corresponds to the modern flow regime and stabilizes the meridional overturning circulation possibly contributing to the holocene s climatic stability

molybdenum and uranium geochemistry in continental margin sediments paleoproxy potential abstract we measured solid phase mo and u concentrations in marine sediments from the california mexico peru and chile margins to ascertain the behavior of mo and u during early diagenesis in continental margin settings at sites along the california mexico and peru margins where there are estimates of mass accumulation rates authigenic u accumulation rates range from 0 50 nmol m 2 day 1 at the california and mexico margin sites mo accumulation rates range from 0 to 134 nmol m 2 day 1 whereas at the peru margin site rates may be as high as 550 nmol m 2 day 1 we observe relationships between metal accumulation rates and the delivery and burial rates of organic carbon corg in the case of mo there appears to be at least two relationships between metal accumulation rate and organic carbon burial for most of the data presented in this manuscript continental margin sediments have a mo corg accumulation rate ratio of 20 mol mol 1 this value is significantly lower however than those reported for anoxic basins 100 mol mol 1 but is consistent with reported mo corg rain ratios from mexico margin sediment traps in contrast to mo there appears to be a single u corg burial ratio of 5 mol mol 1 which includes a range of environments from anoxic basins and open ocean sites we interpret the relationships between metal accumulation and organic carbon to indicate that the reactions that govern authigenic metal accumulation are primarily sensitive to the delivery and burial of organic carbon in these particular settings however we note that any relationship between metal accumulation and organic carbon could be indirect in the particular case of mo based on what is known about mo geochemistry from the literature it is possible that mo authigenesis is coupled to sulfur cycling in many of the environments covered by this study and that the observed association between mo and organic carbon burial is a consequence of the coupling between carbon and sulfur burial using the observed relationships between mo and corg burial as well as constraints from the mo isotope budget we estimate that continental margin sediments are an important sink for mo the magnitude of this sink 0 4 108 mol y 1 or larger may be as much as one quarter of the oceanic removal term and is likely to be larger than the modern anoxic basin sink

tracking the sea level signature of the 8 2 ka cooling event new constraints from the mississippi delta 1 the ever increasing need for accurate predictions of global environmental change under greenhouse conditions has sparked immense interest in an abrupt century scale cooling around 8200 years ago with a focal point in the north atlantic and with hemispheric teleconnections despite considerable progress in the unraveling of this striking feature including a conceivable driving mechanism rapid drainage of proglacial lake agassiz ojibway and a resulting reduced strength of north atlantic thermohaline circulation several key questions remain unanswered one salient aspect concerns the total amount of freshwater released during this catastrophic event likely echoed by a near instantaneous eustatic sea level rise so far no attempts have been made to perform high resolution sea level studies that explicitly focus on this critical time interval here we present new data from the mississippi delta suggestive of abrupt sea level rise associated with the 8 2 ka event however the amount of sea level rise was likely less than 1 2 m corresponding to a meltwater volume of less than 4 3 1014 m3 values lower than estimates used by several recent studies

pollen based quantitative reconstructions of holocene climate variability in nw romania abstract a modern analogue technique is applied to two high resolution pollen sequences from nw romania to provide the first quantitative evidence for winter summer and annual temperatures and for precipitation across the holocene in this region the pollen based climate reconstructions allow the identification of four main intervals i an early less stable period between 11 700 and 11 200 cal yr bp ii generally stable conditions between 11 200 and 8300 cal yr bp with winter and annual temperatures and precipitation higher than at present and summer temperatures about the same iii lower winter and annual temperatures and higher summer temperatures and precipitation between 8000 and 2400 cal yr bp iv warmer winter and annual temperatures and lower precipitation for the last 2400 years whereas summer temperatures became cooler at steregoiu and remained stable at preluca tiganului the pollen based climate reconstructions at the two sites show similar patterns in annual and winter temperatures and precipitation changes during the holocene but the trends appear to be less consistent for summer temperatures our pollen based reconstructions revealed several short term climatic oscillations during the holocene the strongest of which occur between 10 300 10 100 8300 8000 6800 6400 5100 4900 4000 3600 and 3200 3000 cal yr bp

a mid european decadal isotope climate record from 15 500 to 5000 years b p oxygen isotope ratios of precipitation delta18op inferred from deep lake ostracods from the ammersee southern germany provide a climate record with decadal resolution the record in detail shows many of the rapid climate shifts seen in central greenland ice cores between 15 000 and 5000 years before the present b p negative excursions in the estimated delta18op from both of these records likely reflect short weakenings of the thermohaline circulation caused by episodic discharges of continental freshwater into the north atlantic deviating millennial scale trends however indicate that climate gradients between europe and greenland changed systematically reflecting a gradual rearrangement of north atlantic circulation during deglaciation

geochemical evidence for variations in delivery and deposition of sediment in pleistocene light dark color cycles under the benguela current upwelling system abstract distinctive light dark color cycles in sediment beneath the benguela current upwelling system indicate repetitive alternations in sediment delivery and deposition geochemical proxies for paleoproductivity and for depositional conditions were employed to investigate the paleoceanographic processes involved in creating these cycles in three mid pleistocene intervals from odp sites 1082 and 1084 concentrations of total organic carbon toc vary between 3 5 and 17 1 concentrations of caco3 vary inversely to toc and al which suggests that both carbonate dissolution and terrigenous dilution contribute to the light dark cycles opal concentrations are independent of both toc and caco3 therefore eliminating diatom production and lateral transport of shelf material as causes of the light dark cycles 13corg and 15ntot values do not vary across light dark sediment intervals implying that the extent of relative nutrient utilization did not change the stable 15ntot values represent a balanced change in nitrate supply and export production and therefore indicate that productivity was elevated during deposition of the toc rich layers parallel changes in concentrations of indicator trace elements and toc imply that changes in organic matter delivery influenced geochemical processes on the seafloor by controlling consumption of pore water oxygen cu ni and zn are enriched in the darker sediment as a consequence of greater organic matter delivery redox sensitive metals vary due to loss mn and ba or enrichment mo under reducing conditions created by toc oxidation organic matter delivery impacts subsequent geochemical changes such as carbonate dissolution sulfate reduction and the concentration of metals thus export production is considered ultimately responsible for the generation of the color cycles

cold event at 8200 yr b p recorded in annually laminated lake sediments in eastern europe a quantitative annual mean temperature reconstruction from an annually laminated lake sediment sequence in estonia eastern europe shows a distinct cold period at 8400 8080 yr b p before a d 2000 the timing is consistent with that seen in the greenland ice core data and various high resolution records from western europe during maximal cooling at 8250 8150 yr b p the annual mean temperature in estonia was 2 0 c colder than prior to and 3 0 c colder than after the cooling the pollen stratigraphic and sedimentological data suggest especially cold and snowy winter conditions the duration and amplitude of the cold event agree with the modeled impact of a sudden freshening of the north atlantic surface water and subsequent perturbation of the thermohaline circulation provided that the cold event was caused by a pulse of freshwater from the melting laurentide ice sheet to the north atlantic the results indicate a strong teleconnection between the north atlantic oceanic forcing and the east european climate at least up to long 26 e mediated probably by the changing intensity of the zonal atmospheric circulation

model data comparison for the 8 2 ka bp event confirmation of a forcing mechanism by catastrophic drainage of laurentide lakes abstract to improve our understanding of the mechanism behind the 8 2 ka bp cooling event we compare proxy evidence with climate model simulations in which the thermohaline ocean circulation is perturbed by a freshwater pulse into the labrador sea both the proxy data and model results show a cooling that is mainly concentrated in the north atlantic region ranging from more than 5 c cooling in the nordic seas to about 0 5 1 c over europe and less than 0 5 c over the subtropical north atlantic data and model also indicate a weakening of the summer monsoon and generally a drier circum north atlantic over the south atlantic ocean the model simulates a slight warming mostly less than 0 5 c which falls within the uncertainty of proxy data and thus could not be confirmed to examine in detail the structure of the 8 2 ka bp event we also compare the modeled climatological evolution at two locations with high quality records revealing a generally consistent picture the good model data agreement confirms the hypothesis that the 8 2 ka bp event was forced by a freshwater induced weakening of the thermohaline circulation other forcings are unlikely since they would result in an alternative geographical distribution and expression of the climate response

natural mass dependent variations in the isotopic composition of molybdenum we present the first observations of natural mass dependent fractionation of the isotopic composition of molybdenum mo using multi collector inductively coupled plasma mass spectrometry variations in the isotopic composition of mo are reported as 97 95mo 97mo 95mo sample 97mo 95mo standard 1 1000 external analytical precision of 97 95mo is 1 between sediments deposited under anoxic conditions 97 95mo 1 02 to 1 52 relative to our in house standard and ferromanganese nodules 97 95mo 0 63 to 0 42 97 95mo of pacific ocean seawater 97 95mo 1 48 lies within the range of values for anoxic sediments closest to modern black sea anoxic sediments molybdenites from continental ore deposits have intermediate 97 95mo ranging from 0 26 to 0 09 variations in the abundances of 92mo 95mo 96mo 97mo and 98mo are consistent with mass dependent fractionation a sporadic unidentified interference occurs at mass 94 and 100mo is not measured we hypothesize that the 97 95mo offset between anoxic sediments and ferromanganese nodules results from mo isotope fractionation during inefficient scavenging of mo from seawater by mn oxides under oxic conditions the similarity in 97 95mo of anoxic sediments and seawater is consistent with the very efficient removal of mo from seawater under anoxic conditions in the presence of h2s the data can be interpreted in terms of a steady state mass balance between the mo flux into the oceans from the continents and the mo flux out of the oceans into oxic and anoxic sediments such an interpretation is quantitatively consistent with existing estimates of the removal fluxes of mo to anoxic and oxic sediments these findings suggest that 97 95mo in seawater may co vary with changes in the relative proportions of anoxic and oxic sedimentation in the oceans and that this variation may be recorded in 97 95mo of anoxic sediments hence the mo isotope system may be useful in paleoredox investigations

a 14 ka high resolution δ18o lake record reveals a paradigm shift for the process based reconstruction of hydroclimate on the northern tibetan plateau abstract the influence of the mid latitude westerlies mlw competing with the asian summer monsoons asm over the tibetan plateau tp remains a matter of discussion on how and to which extent both atmospheric systems have been controlling hydro climate during the holocene depleted oxygen isotopes in lake deposits were commonly interpreted in terms of enhanced summer monsoon moisture supply implying a migration of the asm deep into the interior of the plateau during holocene periods in order to test this relationship we used a high resolution oxygen isotope record mean 20 yr resolution in combination with carbonates and mineral phases titanium flux grain size and ostracod abundances derived from a 6 84 m long sediment core in the endorheic kuhai lake basin north eastern tp the results confirm 1 continuous positive co variance between enriched 18ocarb and total carbonates during the last 14 ka indicative of dominant seasonal influence on multi decadal to centennial scale isotopic signatures in lake water and respective carbonate precipitation 2 negative co variance between allochthonous sediment flux and 18ocarb and carbonates attributed to relative increase of flux rates during non summer seasons 3 correspondence of lake level variations with carbonate mineral phases and the occurrence disappearance of ostracod assemblages and 4 inverse relationships between isotopic signatures in asm dominated and mlw controlled lake records across the tp enriched 18ocarb in kuhai lake sediments was primarily a result of high evaporation during the summer seasons while asm related rainfall amount did not play an important role likely counterbalanced by isotopic signatures from different water sources conversely depleted 18ocarb was mainly attributed to water supply during non summer seasons of colder temperatures and generally light isotopic signatures from mlw derived sources this finding may lead to a paradigm shift in such way that depleted 18o in carbonates is primarily not the result from asm related rainfall as previously assumed the reconstructed hydro climatic history of kuhai lake indicates the dominance of westerly derived climate during the younger dryas interval 12 8 11 5 ka under very shallow pond like conditions despite climate amelioration during the early holocene 11 5 7 5 ka hydrological conditions remained unstable with frequent alternations between dominance of summer and winter seasons during the middle holocene 7 5 5 5 ka the lake experienced highest lake levels dominated by summer monsoon related water supply assigned to the holocene hydro climatic optimum frequent high amplitude fluctuations afterwards 5 1 2 9 ka refer to cooling drying events under enhanced mlw influence accompanied by a strong lake level decline the late holocene 2 9 ka present period experienced moderate isotopic variations and fluctuating lake levels in response to variable influence of summer or winter related hydro climatic conditions this seesaw like pattern with amplitudes of 10 in 18ocarb resembles fluctuations in cave records and variations between air and seawater dole effect high correspondence with cooling events derived from north atlantic drift ice and meltwater discharge indicate close ties to northern hemispheric climate transmitted by the mlw across the tp

north atlantic nordic seas exchanges abstract the northeastern part of the north atlantic is unique in the sense that it is much warmer in the surface than other ocean areas at similar latitudes the main reason for this is the large northward transport of heat that extends to high latitudes and crosses the greenland scotland ridge to enter the nordic seas and the arctic there the warm atlantic water is converted to colder water masses that return southwards over the ridge partly as surface outflows and partly as overflows through the deep passages across the ridge in this paper the state of knowledge on the exchanges especially across the eastern part of the greenland scotland ridge is reviewed based on results from the ices nansen north atlantic norwegian sea exchanges project from the nordic woce project and from other sources the accumulated evidence allows us to describe the exchanges in fair detail the origins of the waters the patterns of their flow towards and over the ridge and their ultimate fate there is also increasing information on temporal variations of the exchanges although dynamical changes are still not well understood quantitative estimates for the volume transport of most of the overflow branches seem reasonably well established and transport measurements of the atlantic inflows to the nordic seas are approaching acceptable levels of confidence which allows preliminary budgets to be presented the deep overflows are driven by pressure gradients set up by the formation of deep and intermediate water the dominance of deep overflows over surface outflows in the water budget argues that this thermohaline forcing also dominates over direct wind stress and estuarine forcing in driving the atlantic water inflow across the greenland scotland ridge while wind stress seems to influence the characteristics and distribution of the atlantic water north of the ridge

the 8 2 ka cooling event caused by laurentide ice saddle collapse abstract the 8 2 ka event was a period of abrupt cooling of 1 3 c across large parts of the northern hemisphere which lasted for about 160 yr the original hypothesis for the cause of this event has been the outburst of the proglacial lakes agassiz and ojibway these drained into the labrador sea in 0 5 5 yr and slowed the atlantic meridional overturning circulation thus cooling the north atlantic region however climate models have not been able to reproduce the duration and magnitude of the cooling with this forcing without including additional centennial length freshwater forcings such as rerouting of continental runoff and ice sheet melt in combination with the lake release here we show that instead of being caused by the lake outburst the event could have been caused by accelerated melt from the collapsing ice saddle that linked domes over hudson bay in north america we forced a general circulation model with time varying meltwater pulses 100 300 yr that match observed sea level change designed to represent the hudson bay ice saddle collapse a 100 yr long pulse with a peak of 0 6 sv produces a cooling in central greenland that matches the 160 yr duration and 3 c amplitude of the event recorded in ice cores the simulation also reproduces the cooling pattern amplitude and duration recorded in european lake and north atlantic sediment records such abrupt acceleration in ice melt would have been caused by surface melt feedbacks and marine ice sheet instability these new realistic forcing scenarios provide a means to reconcile longstanding mismatches between proxy data and models allowing for a better understanding of both the sensitivity of the climate models and processes and feedbacks in motion during the disintegration of continental ice sheets

mid to late holocene flood frequency changes in the northeastern alps as recorded in varved sediments of lake mondsee upper austria abstract annually laminated varved lake sediments with intercalated detrital layers resulting from sedimentary input by runoff events are ideal archives to establish precisely dated records of past extreme runoff events in this study the mid to late holocene varved sediments of lake mondsee upper austria were analysed by combining sedimentological geophysical and geochemical methods this approach allows to distinguish two types of detrital layers related to different types of extreme runoff events floods and debris flows and to detect changes in flood activity during the last 7100 years in total 271 flood and 47 debris flow layers deposited during spring and summer were identified which cluster in 18 main flood episodes fe 1 18 with durations of 30 50 years each these main flood periods occurred during the neolithic 7100 7050 vyr bp and 6470 4450 vyr bp the late bronze age and the early iron age 3300 3250 and 2800 2750 vyr bp the late iron age 2050 2000 vyr bp throughout the dark ages cold period 1500 1200 vyr bp and at the end of the medieval warm period and the little ice age 810 430 vyr bp summer flood episodes in lake mondsee are generally more abundant during the last 1500 years often coinciding with major advances of alpine glaciers prior to 1500 vyr bp spring summer floods and debris flows are generally less frequent indicating a lower number of intense rainfall events that triggered erosion in comparison with the increase of late holocene flood activity in western and northwestern nw europe commencing already as early as 2800 yr bp the hydro meteorological shift in the lake mondsee region occurred much later these time lags in the onset of increased hydrological activity might be either due to regional differences in atmospheric circulation pattern or to the sensitivity of the individual flood archives the lake mondsee sediments represent the first precisely dated and several millennia long summer flood record for the northeastern ne alps a key region at the climatic boundary of atlantic mediterranean and east european air masses aiding a better understanding of regional and seasonal peculiarities of flood occurrence under changing climate conditions

ensemble water isotope enabled coupled general circulation modeling insights into the 8 2 ka event 1 freshwater forcing has long been postulated as a catalyst for abrupt climate change because of its potential to interfere with thermohaline circulation thc the most recent example may have occurred about 8 2 ka ago with the sudden drainage of glacial lakes agassiz and ojibway into the hudson bay we perform an ensemble of simulations for this freshwater release using the fully coupled atmosphere ocean general circulation model goddard institute for space studies modele r in all cases simulated effects include reduced ocean heat transport and enhanced atmospheric heat transport in the atlantic increased surface albedo through greater low cloud and sea ice cover and local cooling of up to 3 c our suite of ensemble experiments allows us to examine the importance of the initial ocean state in particular the presence or absence of labrador sea water in controlling the magnitude and length of the climate response water isotope tracers included in this model provide an improved means for direct comparisons of the modeled tracer response to water isotope based climate proxy data comparison of model simulations to data implies that there was an abrupt approximate halving of atlantic thc hence providing strong support for the hypothesis that the 8 2 ka event was caused by an abrupt release of fresh water into the north atlantic

molybdenum evidence for expansive sulfidic water masses in 750 ma oceans abstract the ediacaran appearance of large animals including motile bilaterians is commonly hypothesized to reflect a physiologically enabling increase in atmospheric and oceanic oxygen abundances po 2 to date direct evidence for low oxygen in pre ediacaran oceans has focused on chemical signatures in the rock record that reflect conditions in local basins but this approach is both biased to constrain only shallower basins and statistically limited when we seek to follow the evolution of mean ocean chemical state through time because the abundance and isotopic composition of molybdenum mo in organic rich euxinic sediments can vary in response to changes in global redox conditions mo geochemistry provides independent constraints on the global evolution of well oxygenated environments here we establish a theoretical framework to access global marine mo cycle in the past from the abundance and isotope composition of ancient seawater further we investigate the 750 ma walcott member of the chuar group grand canyon which accumulated in a rift basin with open connection to the ocean iron speciation data from upper walcott shales indicate that local bottom waters were anoxic and sulfidic consistent with their high organic content up to 20 wt similar facies in phanerozoic successions contain high concentrations of redox sensitive metals but in the walcott member abundances of mo and u as well as mo toc 0 5 ppm wt are low 98 mo values also fall well below modern equivalents 0 99 0 13 versus 2 35 today these signatures are consistent with model predictions where sulfidic waters cover 1 4 of the global seafloor corresponding to a 20 80 fold increase compared to the modern ocean therefore our results suggest globally expansive sulfidic water masses in mid neoproterozoic oceans bridging a nearly 700 million year gap in previous mo data we propose that anoxic and sulfidic euxinic conditions governed mo cycling in the oceans even as ferruginous subsurface waters re appeared 800 750 ma and we interpret this anoxic ocean state to reflect a markedly lower atmospheric and oceanic o 2 level consistent with the hypothesis that po 2 acted as an evolutionary barrier to the emergence of large motile bilaterian animals prior to the ediacaran period

the behavior of molybdenum and its isotopes across the chemocline and in the sediments of sulfidic lake cadagno switzerland abstract molybdenum mo isotope studies in black shales can provide information about the redox evolution of the earth s oceans provided the isotopic consequences of mo burial into its major sinks are well understood previous applications of the mo isotope paleo ocean redox proxy assumed quantitative scavenging of mo when buried into sulfidic sediments this paper contains the first complete suite of mo isotope fractionation observations in a sulfidic water column and sediment system the meromictic lake cadagno switzerland a small alpine lake with a pronounced oxygen sulfide transition reaching up to h 2 s 200 m in the bottom waters or about 300 m total sulfide s 2 h 2 s hs s 2 we find that mo behaves conservatively in the oxic zone and non conservatively in the sulfidic zone where dissolved mo concentrations decrease from 14 nm to 2 8 nm across this transition dissolved mo in the upper oxic waters has a 98 mo oxic 0 9 0 1 which matches that of the riverine input 98 mo river 0 9 0 1 in the deeper sulfidic waters a subaquatic source delivers mo at 1 55 0 1 but the dissolved mo is even heavier at 98 mo sulfidic 1 8 sediment traps in the sulfidic zone of the lake collect particles increasingly enriched in mo with depth with 98 mo values significantly fractionated at 0 8 to 1 2 both near the chemocline and in the deepest trap suspended particulates in the sulfidic waters carry lighter mo than the ambient dissolved mo pool by 0 3 1 5 sedimentary mo concentrations correlate with total organic carbon and yield mo levels which are two orders of magnitude higher than typical crustal values found in rocks from the catchment area solid phase mo in the sediment shows a slightly positive 98 mo trend with depth from 98 mo 1 2 to 1 4 while the pore waters show dramatic enrichments of mo 2000 nm with a relatively light isotope signature of 98 mo 0 9 1 0 these data are explained if mo is converted to particle reactive oxythiomolybdates in the sulfidic waters and is fractionated during removal from solution onto particles isotope fractionation is expressed in the water column despite the high sulfide concentrations because the rate of mo removal is fast compared to the slow reaction kinetics of thiomolybdate formation however elemental and isotopic mass balances show that mo is indeed quantitatively removed to the lake sediments and thus the isotopic composition of the sediments reflects sources to the sulfidic water this efficient mo drawdown is expected to occur in settings where h 2 s is very much in excess over mo or in a restricted setting where the water renewal rate is slow compared to the mo burial rate we present a model for the mo isotope fractionation in sulfidic systems associated with the slow reaction kinetics and conclude that quantitative removal will occur in highly sulfidic and restricted marine systems

holocene climatic instability a prominent widespread event 8200 yr ago the most prominent holocene climatic event in greenland ice core proxies with approximately half the amplitude of the younger dryas occurred 8000 to 8400 yr ago this holocene event affected regions well beyond the north atlantic basin as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane widespread proxy records from the tropics to the north polar regions show a short lived cool dry or windy event of similar age the spatial pattern of terrestrial and marine changes is similar to that of the younger dryas event suggesting a role for north atlantic thermohaline circulation possible forcings identified thus far for this holocene event are small consistent with recent model results indicating high sensitivity and strong linkages in the climatic system

large sensitivity to freshwater forcing location in 8 2 ka simulations the 8 2 ka event is a key test case for simulating the coupled climate response to changes in the atlantic meridional overturning circulation amoc recent advances in quantifying freshwater fluxes at 8 2 ka from the proxy record have improved the realism of the forcing magnitude in model simulations yet this forcing is still generally applied in an unrealistic geographic manner across most of the labrador sea rather than just along the labrador coast previous simulations with eddy resolving ocean models have come to conflicting conclusions regarding the ability of such a coastally confined flow to impact the amoc these simulations have also not incorporated full atmosphere models nor have they used the new meltwater forcing values for 8 2 ka we use the community climate system model version 3 with an ocean model resolution only slightly coarser than that used in previous eddy resolving simulations to test the sensitivity to freshwater forcing location when revised freshwater forcing is applied across the labrador sea the amoc is reduced by 40 and climate anomalies compare well with proxy records of the 8 2 ka event in terms of magnitude and duration when the forcing is added just along the labrador coast however most meltwater joins the subtropical gyre and travels to the subtropics with minor impact to the amoc 10 decrease it is likely that model biases in the placement of the north atlantic current remain an important limitation for correctly simulating the 8 2 ka event

predation in the paleozoic gastropod like drillholes in devonian brachiopods middle devonian articulate brachiopods ludlowville and moscow formations hamilton group new york have external tapered holes with a central boss that are indistinguishable from drillholes of naticid gastropods that are known from the triassic and later drillholes are specific to prey ribbed shells were avoided and specific to sites on prey healed drillholes suggest penetration of live prey as many as 44 percent of the preferred prey are drilled indicating a level of predation that has been reported only from post paleozoic strata

the 8 2 ka event calendar dated glacier response in the alps evidence for an 8 2 ka event related advance for an alpine glacier was missing for a long time in the light of dendrochronological analyses for tree remains found in front of the mont mine glacier swiss alps we present evidence for such an advance related to the 8 2 ka event calendar dates established for dozens of tree remains place this glacier advance 8175 yr before a d 2000 therefore this 8 2 ka advance response of the mont mine glacier terminated a nearly millennial long retreat period with a glacier always shorter than today

terrestrial impact of abrupt changes in the north atlantic thermohaline circulation early holocene uk abrupt cooling events are features of holocene climate and may recur in the future we use lake records from hawes water nw england to quantify the impact of two prominent early holocene climatic events subdecadal oxygen isotope records from sedimentary carbonate 18 oc dated using thermal ionization mass spectrometry tims u series analyses provide evidence for abrupt cold events lasting 50 and 150 yr at 9350 and 8380 yr ago which correlate with the 9 3 ka and 8 2 ka events recognized in greenland ice cores at hawes water mean july air temperatures inferred from chirono mid assemblages decreased by 1 6 c during each event calculations show that the isotopic excursions were dominantly caused by decreases in the isotopic composition of meteoric precipitation 18 op by 1 3 this is interpreted as a direct downstream response to cooling and freshening of northeast atlantic surface water by melting ice sheets intermediate in magnitude between events observed in greenland and central europe the effects are consistent with a partial shutdown of the north atlantic thermohaline circulation

eastern mediterranean hydroclimate over the late glacial and holocene reconstructed from the sediments of nar lake central turkey using stable isotopes and carbonate mineralogy there is a lack of high resolution records of hydroclimate variability in the eastern mediterranean from the late glacial and early holocene more knowledge of the speed of climate shifts and the degree to which they were synchronous with changes in the north atlantic or elsewhere is required to understand better the controls on eastern mediterranean climate using endogenic carbonate from a sediment sequence from nar golu a maar lake in central turkey dated by varve counting and uranium thorium methods we present high resolution 25 years oxygen 18o and carbon isotope records supported by carbonate mineralogy data spanning the late glacial and holocene 18ocarbonate at nar golu has been shown previously to be a strong proxy for regional water balance after a dry period i e evaporation far exceeding precipitation in the younger dryas the data show a transition into the relatively wetter early holocene in the early holocene there are two drier periods that appear to peak at 9 3 ka and 8 2 ka coincident with cooling events seen in north atlantic records after this and as seen in other records from the eastern mediterranean there is a millennial scale drying trend through the mid holocene transition the relatively dry late holocene is punctuated by centennial scale drought intervals at the times of 4 2 ka event and late bronze age societal collapse overall we show that central turkey is drier when the north atlantic is cooler throughout this record and at multiple timescales thought to be due to a weakening of the westerly storm track resulting from reduced cyclogenesis in the north atlantic however some features such as the mid holocene transition and the fact the early holocene dry episodes at nar golu are of a longer duration than the more discrete events seen in north atlantic records imply there are additional controls on eastern mediterranean hydroclimate

model support for forcing of the 8 2 ka event by meltwater from the hudson bay ice dome previous model experiments of the 8 2 ka event forced by the drainage of lake agassiz often do not produce climate anomalies as long as those inferred from proxies in addition to the agassiz forcing there is new evidence for significant amounts of freshwater entering the ocean at 8 2 ka from the disintegration of the laurentide ice sheet lis we use the community climate system model version 3 ccsm3 to test the contribution of this additional meltwater flux similar to previous model experiments we find that the estimated freshwater forcing from lake agassiz is capable of sustaining ocean and climate anomalies for only two to three decades much shorter than the event duration of 150 years in proxies using new estimates of the lis freshwater flux 0 13 sv for 100 years from the collapse of the hudson bay ice dome in addition to the agassiz drainage the ccsm3 generates climate anomalies with a magnitude and duration that match within error those from proxies this result is insensitive to the duration of freshwater release a major uncertainty if the total volume remains the same an analysis of the modeled north atlantic freshwater budget indicates that the agassiz drainage is rapidly transported out of the north atlantic while the lis contribution generates longer lasting freshwater anomalies that are also subject to recirculation by the subtropical gyre back into the north atlantic thus the meltwater flux originating from the lis appears to be more important than the agassiz drainage in generating 8 2 ka climate anomalies and is one way to reconcile some model data discrepancies

simulated resumption of the north atlantic meridional overturning circulation slow basin wide advection and abrupt local convection abstract the resumption process of the north atlantic meridional overturning circulation moc is investigated in a series of freshwater hosing experiments using a comprehensive coupled climate model four different freshwater perturbations are applied to the north atlantic between 50 n and 70 n leading to a substantial weakening of moc and an expansion of winter sea ice cover over the greenland iceland norwegian gin seas deactivating the freshwater forcing leads to a two phase recovery of the moc the first phase is a slow basin wide advection process whereas the second phase is an abrupt decadal scale transition located in the north atlantic the slow basin wide advection process to an intermediate state of the moc is characterized by a restart of the deep water production in the area of the subpolar gyre and the irminger basin however the ice cover over the gin seas remains whereas the surface density gradually increases in the gin seas on a century time scale the length of this intermediate state depends on the strength of freshwater perturbation which determines the magnitude of the meridional density gradient in the north atlantic the second transition phase is triggered by a model specific density threshold reached in the gin seas when passed the moc abruptly intensifies by 60 within 60 80 years the analysis shows that this abrupt intensification only depends on surface salinity in the gin seas the model results reveal that the simulated sea ice retreat in the gin seas and a rapid increase of the greenland air temperature of the second phase of the resumption of the moc are similar in amplitude to temperature proxies observed during dansgaard oeschger events of the last ice age

middle holocene dry climate caused by change in atmospheric circulation patterns evidence from lake levels and stable isotopes sediment hiatuses detritus layers and inwashed terrestrial moss layers in five cores at crawford lake ontario canada show that the lake level was low between ca 4 8 and 2 ka 1 ka 1000 14 c yr b p this low level is attributed to a dry and warm climate which has also been documented at other sites in southern ontario southern michigan and southern wisconsin oxygen isotope 18 o values from authigenic marl show a negative shift of 2 4 from 9 to 11 4 vienna peedee belemnite between ca 5 and 2 ka enhanced evaporation under dry and likely closed basin conditions would lead to more enrichment in 18 o so we suggest that the trend to depleted 18 o indicates a significant change in the 18 o value of the source meteoric water the major moisture source for the great lakes region is the gulf of mexico from which the amount and seasonality of precipitation are affected by the interplay of air masses from the gulf of mexico and north pacific probably controlled by jet stream positions and storm tracks in the late middle holocene the isotopically heavy moisture from the gulf might have contributed less precipitation and or a higher proportion in winter months probably caused by more frequent eastward extension of dry pacific air depleted in 18 o this hypothesis implies that the 18 o values of paleoprecipitation in the middle holocene reflected moisture source history more strongly than paleotemperature

the 8k event cause and consequences of a major holocene abrupt climate change abstract a prominent abrupt climate event about 8200 years ago brought generally cold and dry conditions to broad northern hemisphere regions especially in wintertime in response to a very large outburst flood that freshened the north atlantic changes were much larger than typical climate variability before and after the event with anomalies up to many degrees contributing to major displacement of vegetative patterns this 8k event provides a clear case of cause and effect in the paleoclimatic realm and so offers an excellent opportunity for model testing the response to north atlantic freshening has the same general anomaly pattern as observed for older events associated with abrupt climate changes following north atlantic freshening and so greatly strengthens the case that those older events also reflect north atlantic changes the north atlantic involvement in the 8k event helps in estimating limits on climate anomalies that might result in the future if warming caused ice melt and hydrologic cycle intensification at high latitudes lead to major changes in north atlantic circulation few model experiments have directly addressed the 8k event and most studies of proxy records across this event lack the time resolution to fully characterize the anomalies so much work remains to be done

mo total organic carbon covariation in modern anoxic marine environments implications for analysis of paleoredox and paleohydrographic conditions 1 sedimentary molybdenum mo s has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic rich marine facies deposited under oxygen depleted conditions a detailed analysis of mo s total organic carbon toc covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that 1 mo s while useful in distinguishing oxic from anoxic facies is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and 2 patterns of mo s toc covariation can provide information about paleohydrographic conditions especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal these inferences are based on data from four anoxic silled basins the black sea framvaren fjord cariaco basin and saanich inlet and one upwelling zone the namibian shelf representing a spectrum of aqueous chemical conditions related to water mass restriction in the silled basin environments increasing restriction is correlated with a systematic decrease in mo s toc ratios from 45 5 for saanich inlet to 4 5 1 for the black sea this variation reflects control of mo s by mo aq which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal the basin reservoir effect the temporal dynamics of this process are revealed by high resolution chemostratigraphic data from framvaren fjord and cariaco basin sediment cores which exhibit long term trends toward lower mo s toc ratios following development of water column stratification and deepwater anoxia mo burial fluxes peak in weakly sulfidic environments such as saanich inlet owing to a combination of greater mo aq availability and enhanced mo transport to the sediment water interface via fe mn redox cycling and are lower in strongly sulfidic environments such as the black sea and framvaren fjord these observations demonstrate that at timescales associated with deepwater renewal in anoxic silled basins decreased sedimentary mo concentrations and burial fluxes are associated with lower benthic redox potentials i e more sulfidic conditions these conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction e g silled basins and are not valid for low oxygen facies in open marine settings such as continent margin upwelling systems

surface and deep ocean interactions during the cold climate event 8200 years ago evidence from a north atlantic deep sea sediment core reveals that the largest climatic perturbation in our present interglacial the 8200 year event is marked by two distinct cooling events in the subpolar north atlantic at 8490 and 8290 years ago an associated reduction in deep flow speed provides evidence of a significant change to a major downwelling limb of the atlantic meridional overturning circulation the existence of a distinct surface freshening signal during these events strongly suggests that the sequenced surface and deep ocean changes were forced by pulsed meltwater outbursts from a multistep final drainage of the proglacial lakes associated with the decaying laurentide ice sheet margin

freshwater outbursts to the oceans from glacial lake agassiz and their role in climate change during the last deglaciation abstract lake agassiz was the largest lake in north america during the last deglaciation as the laurentide ice sheet lis retreated large volumes of water stored in this proglacial lake were episodically released into the oceans these waters were variably routed to the gulf of mexico arctic ocean north atlantic ocean and hudson bay during this period the three largest cooling events in the northern hemisphere closely followed 4 of the 5 largest outbursts from lake agassiz 1 the younger dryas which was preceded by a release of 9500 km 3 2 the preboreal oscillation preceded by releases of 9300 km 3 and 5900 km 3 and 3 the 8 2 ka cold event preceded by a 163 000 km 3 outburst these are respectively fluxes of 0 30 sv 0 29 sv 0 19 sv and 5 2 sv if released in 1 year because the influx of freshwater reaching the north atlantic ocean can inhibit thermohaline circulation partly depending on whether the ocean was in a glacial interglacial or transitional mode of circulation we believe that at least these large outbursts from lake agassiz may have provided the triggers for changes in ocean circulation and in turn for widespread climate change

biotic turnover driven by eutrophication before the sturtian low latitude glaciation a dramatic oceanic biotic shift from eukaryotic phytoplankton to bacteria occurred about 740 million years ago microfossil and geochemical data from the chuar group in the southwestern united states link this biotic turnover to widespread eutrophication of surface waters reconstructions of the diversity of precambrian microorganisms suggest a pronounced biotic turnover coinciding with the onset of neoproterozoic low latitude glaciation in which diverse assemblages of organic walled microfossils known as acritarchs were replaced by assemblages of simple smooth walled forms called leiosphaerids and the remnants of bacterial blooms1 2 3 4 this turnover has been interpreted as the mass extinction of eukaryotic phytoplankton1 2 3 4 and the subsequent proliferation of bacteria5 however the causes of this mass extinction and its exact temporal relationship to the glaciations remain unclear1 2 3 4 here we present palaeontological data from the 742 6 myr old chuar group from arizona which indicate that the biotic turnover occurred before the first low latitude sturtian glaciation constrained to be between 726 and 660 myr in age6 in our record the turnover is associated with the appearance of abundant and diverse protozoan fossils and a shift to rising total organic carbon suggestive of increased primary productivity spurred by the influx of nutrients this is followed by an increase in the ratio of highly reactive iron to total iron which we interpret as persistent water column anoxia we therefore conclude that the biotic turnover recorded in the chuar group was driven by widespread eutrophication of surface waters rather than low latitude glaciation

palaeoecology of late glacial and holocene profundal ostracoda of pre alpine lake mondsee austria a base for further palaeo biological research succession of profundal ostracod palaeoassemblages in response to environmental changes during the late glacial and holocene was studied in a similar to 15 m long sediment sequence from pre alpine lake mondsee austria first local ostracod assemblage zone loaz 1 prior to 15 700 a bp with low abundances of leucocythere mirabilis and limnocytherina sanctipatricii followed by cytherissa lacustris corresponds to the pleniglacial phase of clastic detrital sedimentation at relatively high rates most of the key species of loaz 2 15 590 to 13 940 a bp including the pleniglacial late glacial transition i e limnocytherids fabaeformiscandona cf harmsworthi f tricicatricosa c lacustris and candona candida reveal the significant association with high al contents and low sedimentation rates and are classified as preferring low productivity conditions in contrast candona neglecta dominant in loaz 3 13 820 to 9960 a bp palynologically defined as balling early holocene and in the last loaz 4 9780 a bp to present as well as cypria ophtalmica second key species in loaz 4 show relationship with higher productivity elevated sedimentation rates and decreasing al contents furthermore valve biodegradation and shape disparity at different time periods and in relation to sedimentation rate changes were investigated in a 3 juveniles of candona neglecta the highest percentage of valves biodegraded by actinobacteria was recorded during the allerod younger dryas and early holocene when the lowest sedimentation rates occurred whereas significantly lower frequencies of biodegraded valves were recorded during the mid holocene and late holocene when sediment accumulation was much higher it is also hypothesised that the degree of the valve shape variation was related to the lake productivity during the allerod younger dryas and early holocene a valve shape variation significantly lower than that recorded during late holocene intervals characterised by intensified lake productivity was observed additionally an agenda of potential questions and approaches that should be considered and form the core of further palaeo biological research projects is offered c 2014 elsevier b v all rights reserved

amplitude and timing of temperature and salinity variability in the subpolar north atlantic over the past 10 k y paired planktic foraminiferal 18 o and mg ca data reveal trends of increasing temperatures 3 c and salinities in the subpolar north atlantic over the course of the holocene which were punctuated by abrupt events the trends likely refl ect an insolation forced northward retreat of the boundary between polar and north atlantic subsurface waters the superimposed variability does not appear to be periodic but tends to recur within a broad millennial band the records provide convincing evidence of open ocean cooling nearly 2 c and freshening during the 8 2 ka event and suggest similar conditions at 9 3 ka however the two largest temperature oscillations in our record 2 c occurred during the past 4 k y suggesting a recent increase in temperature variability relative to the mid holocene perhaps in response to neoglaciation which began at about this time

lake isotope records of the 8200 year cooling event in western ireland comparison with model simulations the early holocene cooling which occurred around 8200 calendar years before present was a prominent abrupt event around the north atlantic region here we investigate the timing duration magnitude and regional coherence of the event as expressed in carbonate oxygen isotope records from three lakes on northwest europe s atlantic margin in western ireland namely loch avolla loch gealain and lough corrib an abrupt negative oxygen isotope excursion lasted about 200 years comparison of records from three sites suggests that the excursion was primarily the result of a reduction of the oxygen isotope values of precipitation which was likely caused by lowered air temperatures possibly coupled with a change in atmospheric circulation comparison of records from two of the lakes loch avolla and loch gealain which have differing bathymetries further suggests a reduction in evaporative loss of lake water during the cooling episode comparison of climate model experiments with lake sediment isotope data indicates that effective moisture may have increased along this part of the northeast atlantic seaboard during the 8200 year climatic event as lower evaporation compensated for reduced precipitation

paleohydraulics of the last outburst flood from glacial lake agassiz and the 8200 bp cold event abstract during the last deglaciation of north america huge proglacial lakes formed along the southern margin of the laurentide ice sheet the largest of these was glacial lake agassiz which formed about 11 7 14 c kyr and drained into hudson bay about 7 7 14 c kyr 8 45 cal kyr overflow from these lakes was variably directed to the mississippi st lawrence and mackenzie drainage systems and it is thought that switches in routing were accompanied by a response in ocean circulation that produced abrupt climate events when the ice dam across hudson bay finally was breached a massive flood drained lake agassiz which was routed through hudson strait to the labrador sea in terms of stored water volume the largest reservoir was associated with the kinojevis level of lake agassiz for this maximum filling the impounded water volume available to produce floods is estimated as 40 000 151 000 km 3 depending on the location of the ice margin and route used the timing of this rapid release of stored freshwater just precedes the early holocene cooling event at 8 2 cal kyr bp we use the spring hutter theory to simulate flood hydrographs for floods that originate in subglacial drainage conduits and find that flood magnitude and duration are 5 sv and 0 5 yr multiple fillings and floods are possible as are single floods having a complex multipulse structure modelling results suggest that the outburst flood from lake agassiz may have terminated before the lake surface elevation dropped to sea level and that the flood ended when a stable drainage channel was established connecting lake agassiz to the tyrrell sea

the cold event 8200 years ago documented in oxygen isotope records of precipitation in europe and greenland stable oxygen isotope ratios of ostracod valves in late glacial and holocene sediments of core as 92 5 from deep lake ammersee southern germany reflect variations of mean oxygen isotope ratios in past atmospheric precipitation the record reconfirms the strong similarity of climate evolution in europe and greenland during the last deglaciation for the first time in europe we find a 200 year long negative 18o excursion which is contemporaneous with the strongest negative 18o excursion in the greenland ice around 8 2 ky before present the 8 2 ky isotopic event on both sides of the north atlantic ocean is interpreted as a cold period most probably induced by a perturbation of the north atlantic thermohaline circulation we discuss two possible triggering mechanisms 1 weak forcing as proposed by alley et al and 2 forcing by a strong and sudden freshwater pulse from the collapse of the hudson ice dome

calibration of xrf core scanners for quantitative geochemical logging of sediment cores theory and application abstract on line analysis of split sediment cores by xrf core scanners has become increasingly popular in the past decade because it allows nondestructive extraction of near continuous records of element intensities from sediment cores with a minimum of analytical effort a disadvantage of xrf core scanning relative to conventional geochemical analysis is the problematic conversion of core scanner output to element concentrations the main reason for this long standing problem is the poorly constrained measurement geometry attributable to inhomogeneity of the specimens e g variable water content and grain size distribution irregularities of the split core surface and in some setups spatial variations in thickness of an adhesive pore water film which forms directly below a protective foil covering the core surface we propose a log ratio calibration model for xrf core scanners derived from a combination of xrf spectrometry theory principles of compositional data analysis and empirical evidence the log ratio calibration model provides accurate and precise predictions of sediment composition element concentrations from xrf core scanner output with a limited number of parameters namely 2 d 1 where d equals the number of chemical elements whose concentrations are to be estimated the model can accommodate the inherent non linearity of the relation between relative intensities and concentrations which is apparent from the fact that it provides unbiased estimates an immediate corollary of our results is that log ratios of element intensities which are related to log ratios of element abundances by a simple linear transformation provide the most easily interpretable signals of relative changes in chemical composition consistent use of log ratios of element intensities or concentrations should minimise the risk of drawing erroneous conclusions from geochemical proxies the relative standard deviation precision of predicted element concentrations in core geob7920 is less than 2 stochastic simulations indicate that this level of precision can be attained with 40 randomly selected calibration specimens improved control over input errors and development of robust goodness of fit statistics allows xrf core scanning to be developed into a rigorous quantitative measurement technology the log ratio calibration equation derived in this study may be adapted to inter laboratory and inter instrument calibration as well

determination of molybdenum isotope fractionation by double spike multicollector inductively coupled plasma mass spectrometry 1 molybdenum isotopic compositions are precisely determined by mc icp ms measurements using a mo double spike the double spike is added prior to chemical purification so that laboratory and instrumental mass fractionations are separated from natural mass dependent fractionation fractionation is determined on four mo mass ratios providing an internal consistency check the external standard reproducibility is at 0 06 per mil on the 98mo 95mo ratio 2 standard deviation using a normal microconcentric nebuliser with a cyclonic spray chamber the minimum quantity of mo is 1 g for high precision results a hydrothermal molybdenite shows fractionation of 0 3 per mil on the 98mo 95mo ratio relative to our standard johnson matthey 1000 g ml 0 3 icp standard solution lot 602332b fine grained sediments show fractionation of 0 1 and 0 3 per mil on the 98mo 95mo ratio the observed mo isotope fractionation is small but resolvable with the presented high resolution technique

the molybdenum isotopic composition in river water constraints from small catchments we report molybdenum isotope compositions and concentrations in water samples from a variety of river catchment profiles in order to investigate the influence of anthropogenic contamination catchment geology within river precipitation and seasonal river flow variations on riverine molybdenum our results show that the observed variations in 98 95mo from 0 to 1 9 are primarily controlled by catchment lithology particularly by weathering of sulfates and sulfides erosion in catchments dominated by wet based glaciers leads to very high dissolved molybdenum concentrations in contrast anthropogenic inputs affect neither the concentration nor the isotopic composition of dissolved molybdenum in the rivers studied here seasonal variations are also quite muted the finding that catchment geology exerts the primary control on the delivery of molybdenum to seawater indicates that the flux and isotope composition of molybdenum to seawater has likely varied in the geologic past

consistently dated records from the greenland grip gisp2 and ngrip ice cores for the past 104 ka reveal regional millennial scale δ18o gradients with possible heinrich event imprint abstract we present a synchronization of the ngrip grip and gisp2 ice cores onto a master chronology extending back to 104 ka before present providing a consistent chronological framework for these three greenland records the synchronization aligns distinct peaks in volcanic proxy records and other impurity records chemo stratigraphic matching and assumes that these layers of elevated impurity content represent the same instantaneous event in the past at all three sites more than 900 marker horizons between the three cores have been identified and our matching is independently confirmed by 24 new and previously identified volcanic ash tephra tie points using the reference horizons we transfer the widely used greenland ice core chronology gicc05modelext to the two summit cores grip and gisp2 furthermore we provide gas chronologies for the summit cores that are consistent with the gicc05modelext timescale by utilizing both existing and new gas data ch4 concentration and 15n of n2 we infer that the accumulation contrast between the stadial and interstadial phases of the glacial period was 10 greater at summit compared to at ngrip the 18o temperature proxy records from ngrip grip and gisp2 are generally very similar and display synchronous behaviour at climate transitions the 18o differences between summit and ngrip however changed slowly over the last glacial interglacial cycle and also underwent abrupt millennial to centennial scale variations we suggest that this observed latitudinal 18o gradient in greenland during the glacial period is the result of 1 relatively higher degree of precipitation with a pacific signature at ngrip 2 increased summer bias in precipitation at summit and 3 enhanced rayleigh distillation due to an increased source to site distance and a potentially larger source to site temperature gradient we propose that these processes are governed by changes in the north american ice sheet nais volume and north atlantic sea ice extent and or sea surface temperatures sst on orbital timescales and that changing sea ice extent and ssts are the driving mechanisms on shorter timescales finally we observe that maxima in the summit ngrip 18o difference are roughly coincident with prominent heinrich events this suggests that the climatic reorganization that takes place during stadials with heinrich events possibly driven by a southward expansion of sea ice and low ssts in the north atlantic are recorded in the ice core records

holocene climate variability although the dramatic climate disruptions of the last glacial period have received considerable attention relatively little has been directed toward climate variability in the holocene 11 500 cal yr b p to the present examination of similar to50 globally distributed paleoclimate records reveals as many as six periods of significant rapid climate change during the time periods 9000 8000 6000 5000 4200 3800 3500 2500 1200 1000 and 600 150 cal yr b p most of the climate change events in these globally distributed records are characterized by polar cooling tropical aridity and major atmospheric circulation changes although in the most recent interval 600 150 cal yr b p polar cooling was accompanied by increased moisture in some parts of the tropics several intervals coincide with major disruptions of civilization illustrating the human significance of holocene climate variability c 2004 university of washington all rights reserved

centennial scale climate cooling with a sudden cold event around 8 200 years ago the extent of climate variability during the current interglacial period the holocene is still debated temperature records derived from central greenland ice cores show one significant temperature anomaly between 8 200 and 8 100 years ago which is often attributed to a meltwater outflow into the north atlantic ocean and a slowdown of north atlantic deep water formation this anomaly provides an opportunity to study such processes with relevance to present day freshening of the north atlantic anomalies in climate proxy records from locations around the globe are often correlated with this sharp event in greenland but the anomalies in many of these records span 400 to 600 years start from about 8 600 years ago and form part of a repeating pattern within the holocene more sudden climate changes around 8 200 years ago appear superimposed on this longer term cooling the compounded nature of the signals implies that far field climate anomalies around 8 200 years ago cannot be used in a straightforward manner to assess the impact of a slowdown of north atlantic deep water formation and the geographical extent of the rapid cooling event 8 200 years ago remains to be determined

a 200 year mid european air temperature record preserved in lake sediments an extension of the δ18op air temperature relation into the past abstract a detailed history of the water oxygen isotope ratios of lake ammersee southern germany was established for the last two centuries by analyzing oxygen isotope ratios of ostracod shells from a sediment core in a three to five year resolution empirical analysis and modelling of the isotope hydrology of the lake show that the oxygen isotopic composition of the lake water is mainly controlled by the isotopic composition of local precipitation for the last 200 years the lake isotope record is strongly correlated to the air temperature record from hohenpeisenberg situated within the lake s catchment area thus the mid european relation between 18 o in precipitation and air temperature documented for the last 20 years can be extended over a longer time and a larger temperature range the results provide further support for the validity of ostracod 18 o from ammersee sediments as a quantitative proxy for mean annual air temperature probably extending back to the last glaciation

morphometric analysis of predatory drillholes abstract quantitative studies of post jurassic predatory drillholes assume that drill holes were made by either naticid or muricid gastropods however other potential predators can leave similar traces and the traditional method of discrimination based on differences in vertical shape of drillholes cylindrical vs countersunk is not always applicable poor preservation or thin shells may prevent identification a multivariate morphometric approach including principal component analysis pca canonical variate analysis cva and size free discriminant analysis is suggested here as a supplementary method i test the approach by analyzing 98 drillholes made in the valves of the thin shelled bivalve mulinia coloradoensis collected from subfossil deposits in northeastern baja california using traditional criteria 10 10 2 of 98 drillholes were not correctly identified morphometric analyses indicate that in planar view muricid and naticid drillholes do not differ in shape however they perfectly discriminate in size the cylindrical drillholes oichnus simplex attributed to muricid predators are significantly smaller than countersunk ones oichnus paraboloides made most likely by naticid gastropods the results do not necessarily undermine the validity of the traditional ichnotaxonomy muricids tend to produce smaller drillholes than naticids nevertheless the vertical shape of a drillhole may be a function of both the type of predator and the size of hole drilled more than half of the unidentified drillholes 6 out of 10 from an independent group characterized by a more oval shape octopod drilling the traditional method supported by the morphometric methods employed here is a powerful tool for analysis of drillholes it may be valuable not only as a tool for analysis of muricid and naticid drillholes but also as a method for comparative analysis of drillholes of unknown source found throughout the phanerozoic fossil record

simulation of the cold climate event 8200 years ago by meltwater outburst from lake agassiz meltwater pulse is assumed to have a volume of 1 6 10 14 m 3 and a period of discharge of 2 years on the basis of glaciological modeling of the decay of the laurentide ice sheet lis we present a possible mechanism which can explain the centennial duration of the 8 2 ka cold event the mechanism is related to the existence of an additional equilibrium climate state with reduced north atlantic deep water nadw formation and a southward shift of the nadw formation area hints at the additional climate state were obtained from the largely varying duration of the pulse induced cold episode in response to overlaid random freshwater fluctuations in monte carlo simulations the model equilibrium state was attained by releasing a weak multicentury freshwater flux through the st lawrence pathway completed by the meltwater pulse the existence of such a climate mode appears essential for reproducing climate anomalies in close agreement with paleoclimatic reconstructions of the 8 2 ka event the results furthermore suggest that the temporal evolution of the cold event was partly a matter of chance index terms 3344 meteorology and atmospheric dynamics paleoclimatology 4255 oceanography general numerical modeling 4532 oceanography physical general circulation 4215 oceanography general climate and interannual variability 3309 9325 information related to geographic region atlantic ocean

evidence for higher than average air temperatures after the 8 2 ka event provided by a central european δ 18 o record the so called 8 2 ka event represents one of the most prominent cold climate anomalies during the holocene warm period accordingly several studies have addressed its trigger mechanisms absolute dating and regional characteristics so far however knowledge about subsequent climate recovery is still limited although this might be essential for the understanding of rapid climatic changes here we present a new sub decadally resolved and precisely dated oxygen isotope o record for the interval between 7 7 and 8 7 ka bp 10 calendar years before ad 1950 derived from the calcareous valves of benthic ostracods preserved in the varved lake sediments of pre alpine mondsee austria besides a clear reflection of the 8 2 ka event showing a good agreement in timing duration and magnitude with other regional stable isotope records the high resolution mondsee lake sediment record provides evidence for a 75 year long interval of higher than average o values directly after the 8 2 ka event possibly reflecting increased air temperatures in central europe this observation is consistent with evidence from other proxy records in the north atlantic realm thus most probably reflecting a hemispheric scale climate signal rather than a local phenomenon as a possible trigger we suggest an enhanced resumption of the atlantic meridional overturning circulation amoc supporting assumptions from climate model simulations

molybdenum isotope fractionation during adsorption by manganese oxides the isotopic composition of mo 97 95mo in seawater is 2 heavier than mo in marine ferromanganese crusts and nodules barling et al earth planet sci lett 193 2001 447 457 siebert et al earth planet sci lett 211 2003 159 171 to explore this phenomenon we have conducted an experimental investigation into the mass dependent fractionation of mo isotopes during adsorption onto mn oxyhydroxide two series of experiments were carried out a time series in which adsorption proceeded for 2 96 h and a ph series in which ph varied from 6 5 to 8 5 the extent of mo adsorption by mn oxyhydroxides decreases with increasing ph a trend typical of anion adsorption and takes 48 h to reach steady state lighter mo isotopes are preferentially adsorbed experimentally determined fractionation factors soln mnox exhibit no systematic variation with either time or experimental ph the mean soln mnox for all experiments is 1 0018 0 0005 2 s d comparison of the mo isotopic data for experimental solutions and mo adsorbed to mn oxyhydroxide with predictions for closed system equilibrium and rayleigh fractionation models indicates that isotope fractionation occurs as a result of closed system equilibrium exchange between dissolved and adsorbed mo the isotopic offset between dissolved and adsorbed mo is comparable to that observed between mo in seawater and mo in ferromanganese nodules and crusts it is therefore likely that adsorption of mo to mn oxyhydroxides is a significant factor in the fractionation of mo isotopes in the oceans

high resolution isotope records of early holocene rapid climate change from two coeval stalagmites of katerloch cave austria abstract two coeval stalagmites from katerloch cave show pronounced intervals of low 18o values around 8 2 9 1 and 10 0 kyr all ages are reported before the year 2000 ad and represent an inorganic u th dated climate archive from the southeast of the european alps a region where only very few well dated climate records exist the o isotope curves providing near annual resolution allow a direct comparison to the greenland ice core records as temperature was the primary factor controlling the o isotopic composition of katerloch speleothems the 8 2 kyr climate anomaly lasted about one century from 8196 to 8100 yr with a maximum amplitude of 1 1 at 8175 yr the event is characterized by a rapid onset and a more gradual demise and u th data as well as annual lamina counting support a rapid climate change towards cooler conditions within 10 20 yr there is no strong evidence that the 8 2 kyr anomaly was superimposed on a pronounced longer term cooling episode nor do the new data support two separate cooling events within the 8 2 kyr event as reported by other studies our record also shows a distinct climate anomaly around 9 1 kyr which lasted 70 110 yr and showed a maximum amplitude of 1 0 i e it had a similar duration and amplitude as the central 8 2 kyr event compared to the 8 2 kyr event the 9 1 kyr anomaly shows a more symmetrical structure but onset and demise still occurred within a few decades only the different progression of the 8 2 asymmetrical and 9 1 kyr anomaly symmetrical suggests a fundamental difference in the trigger and or the response of the climate system moreover both stalagmites show evidence of a climate anomaly around 10 0 kyr which was of comparable magnitude to the two subsequent events using a well constrained modern calibration between air temperature and 18o of precipitation for the study area and cave monitoring data confirming speleothem deposition in katerloch reflecting cave air temperature a maximum cooling by ca 3 c can be inferred at 8 2 and 9 1 kyr which is similar to other estimates e g from lake ammersee north of the alps the o isotopic composition of meteoric precipitation however is a complex tracer of the hydrological cycle and these temperature estimates do not take into account additional effects such as changes in the source area or synoptic shifts apart from that the relative thickness of the seasonally controlled lamina types in the katerloch stalagmites remains rather constant across the intervals comprising the isotopic anomalies i e the stalagmite petrography argues against major shifts in seasonality during the early holocene climate excursions

a 1600 yr seasonally resolved record of decadal scale flood variability from the austrian pre alps we present a record of extreme spring summer runoff events for the past 1600 yr preserved in the varved sediments of lake mondsee austrian pre alps combined sediment microfacies analyses and high resolution micro x ray fluorescence element scanning allow us to identify 157 detrital event layers deposited in spring summer and to discriminate between regional flood and local debris flow deposits higher spring summer flood activity with a mean event recurrence of 3 5 yr occurred in several well confined multidecadal episodes during the dark ages cold period and medieval time a d 450 480 590 640 700 750 and 1140 1170 as well as during the early little ice age lia a d 1300 1330 and 1480 1520 in contrast lowest spring summer flood activity with an event recurrence of only 30 100 yr is observed during the medieval climate anomaly a d 1180 1300 and the coldest interval of the lia a d 1600 1700 these findings indicate a complex relationship between temperature conditions and extreme hydro meteorological events and suggest that enhanced summer mediterranean cyclogenesis triggers large scale floods in the northeast alps during climatic transitions the lake mondsee data demonstrate the climatic sensitivity of spring summer floods and prove the potential of varved sediment records to investigate the impact of changing climate boundary conditions on seasonal flood activity for pre instrumental time

environmental responses to lateglacial climatic fluctuations recorded in the sediments of pre alpine lake mondsee northeastern alps investigation of the sedimentary record of pre alpine lake mondsee upper austria focused on the environmental reaction to rapid lateglacial climatic changes results of this study reveal complex proxy responses that are variable in time and influenced by the long term evolution of the lake and its catchment a new field sampling approach facilitated continuous and precisely controlled parallel sampling at decadal to sub annual resolution for mu xrf element scanning carbon geochemistry stable isotope measurements on ostracods pollen analyses and large scale thin sections for microfacies analysis the holocene chronology is established through microscopic varve counting and supported by accelerator mass spectrometric 14 c dating of terrestrial plant macrofossils whereas the lateglacial age model is based on delta 18 o wiggle matching with the greenland ngrip record using the gicc05 chronology microfacies analysis enables the detection of subtle salimentological changes proving that depositional processes even in rather large lake systems are highly sensitive to climate forcing comparing periods of major warming at the onset of the lateglacial and holocene and of major cooling at the onset of the younger dryas reveals differences in proxy responses reflecting threshold effects and ecosystem inertia temperature increase vegetation recovery decrease of detrital flux and intensification of biochemical calcite precipitation at the onset of the holocene took place with only decadal leads and lags over a ca 100 a period whereas the spread of woodlands and the reduction of detrital flux lagged the warming at the onset of the lateglacial interstadial by ca 500 750a cooling at the onset of the younger dryas is reflected by the simultaneous reaction of delta 18 o and vegetation but sedimentological changes reduction of endogenic calcite content increase in detrital flux were delayed by about 150 300 a three short term lateglacial cold intervals corresponding to greenland isotope substages gl 1d gl 1c2 and gl 1b also show complex proxy responses that vary in time copyright c 2011 john wiley sons ltd

glacial dynamics at the beginning of termination i in the eastern alps and their stratigraphic implications abstract results of a geological re investigation of the type localities of the buhl stadial the supposed first halt during ice decay of the inn glacier in the basin of hopfgarten and at the southern flank of the mountain range wilder kaiser northern tyrol austria are presented it is shown that in this region no stationary or advancing inn glacier tongue existed following the last glacial maximum lgm ice extent all sedimentary and morphological evidence indicates that large parts of the inn glacier and all other eastern alpine valley glaciers became stagnant followed by massive down wasting hence this period after the lgm at the beginning of termination i wurm lateglacial is best defined as phase of early lateglacial ice decay which can be tentatively correlated to greenland stadial 2c 21 2 19 5 kyr bp only smaller glaciers advanced towards their lateglacial maximum position lmp this happened mostly due to short lasting climatic oscillations as indicated in the windau valley hopfgarten and at steinach am brenner a mechanically induced glacier advance is reconstructed for the cirque glaciers of the wilder kaiser at an advanced stage of down melting they lost their blockade established by the inn glacier during lgm and advanced downvalley it is proposed to define the processes of this phase by lithostratigraphic type sections

trace element and ree composition of a low temperature ridge flank hydrothermal spring abstract warm 25 c hydrothermal springs have been sampled on baby bare a basaltic outcrop on 3 5 ma old crust 100 km east of the endeavor segment of the juan de fuca ridge the source for these springs is a 62 to 64 c formation water that has cooled conductively as it ascends to feed the springs this water originated as bottom seawater that probably descended into basement 52 km to the southwest at another much larger outcrop called grizzly bare as this seawater flows towards baby bare it is heated and altered by reactions within basaltic basement and by diffusive fluxes to and from the overlying sediment concentrations of mn co ni zn cd and mo in the spring waters are greater than in bottom seawater indicating that the oceanic crust is a source for these elements to the oceans at least a portion of this increase probably results from the redox cycling of mn in sedimentary sources near the basement interface that produces a diffusive flux to basement formation waters additional removal of mo and inputs of the other five elements to two of the three springs are observed locally near sites of venting where density gradients can form shallow circulation cells within the sediment and diffusive exchange occurs concentrations of cu u v y and the rare earth elements rees excluding ce in these samples are less than in bottom seawater indicating that the oceanic crust is a net sink for these elements in this environment copper is probably removed into newly formed carbonate and or sulfide phases removal of the oxyanions u and v is consistent with a net removal of phosphate demonstrated previously for ridge flank hydrothermal systems similarly removal of y and the rees is associated with carbonate phosphate rich and oxide phases calculated maximum global chemical fluxes from warm ridge flank hydrothermal systems such as baby bare are insignificant relative to riverine fluxes for these elements except possibly for mn and mo the impact on global geochemical budgets for these elements from lower temperature

a high resolution study of holocene paleoclimatic and paleoceanographic changes in the nordic seas 1 high resolution records from images core md95 2011 in the eastern norwegian sea provide evidence for relatively large and small scale high latitude climate variability throughout the holocene during the early and mid holocene a situation possibly driven by consistent stronger westerlies increased the eastward influence of arctic intermediate and near surface waters for the late holocene a relaxation of the atmospheric forcing resulted in increased influence of atlantic water the main changes in holocene climate show no obvious connection to changing solar irradiance and spectral analysis reveals no consistent signature for any periodic behavior of holocene climate at millennial or centennial timescales there are however indications of consistent multidecadal variability

spatial variability in oceanic redox structure 1 8 billion years ago the deposition of iron formations ceased about 1 84 billion years ago reconstructions of ocean chemistry suggest that the advent of euxinic conditions along ocean margins preferentially removed dissolved iron from the water column in the form of the mineral pyrite inhibiting widespread iron oxide mineral deposition

simulation of holocene cooling events in a coupled climate model three potential mechanisms behind centennial scale holocene cooling events are studied in simulations performed with the coupled climate model ecbilt clio 1 internal variability 2 solar forcing and 3 freshwater forcing in experiments with constant preindustrial forcings three centennial scale cooling events occur spontaneously in 15 000 years these rare events represent an unstable internal mode of variability that is characterised by a weaker thermohaline circulation a more southward location of the main site of deep water formation expanded sea ice cover and cooling of 101c over the nordic seas this mode is visited more frequently when the climate is cooled by abruptly reducing the solar constant by 5 or 3 wm 2 prescribing a solar forcing of the same magnitude but following a sinusoidal function with a period of 100 or 1000 years does not result in any centennial scale cooling events the latter forcing does however result in more frequent individual cold years in the north atlantic region that are related to local weakening of the deep convection and sea ice expansion adding realistic freshwater pulses to the labrador sea is also able to trigger centennial scale cooling events with temperature anomalies resembling proxy evidence for the cooling event at 8 2 kyr bp suggesting that freshwater forcing is a valid explanation for early holocene cooling events r 2007 elsevier ltd all rights reserved

links between early holocene ice sheet decay sea level rise and abrupt climate change the beginning of the holocene interglacial was marked by ice sheet melting and sea level rise a review of sea level and climate records identifies two sea level jumps associated with the final drainage of glacial lake agassiz and links them to an abrupt cooling event

lgm and late glacial fluctuations in the eastern alps abstract studies of glacier development in the eastern alps around the last glacial maximum lgm for more than a hundred years have created a reasonably well based knowledge of glacier extent sedimentation and palaeoclimate as well as chronology this was obtained from broad and detailed mapping palynological investigations and radiocarbon dating despite a very cold climate between 31 and 21 ka bp topographical constraints meant that glacier extent was limited to tributary valleys of the main trenches until about 24 ka bp then rapid ice build up in the main longitudinal valleys led to extended ice streams and to piedmont glaciers in the foreland this build up virtually ended at ca 21 ka bp according to radiocarbon dates obtained from organic matter embedded in the outwash and periglacial gravels niederterrasse the lgm possibly lasted 3000 4000 years and shows a short maximalstand and a more prolonged hochstand both connected with outwash deposits the ice decay was apparently very fast and the glaciers vanished from the longitudinal valleys probably within some hundreds of years this retreat was interrupted by two short oscillations of active glaciers in contact with huge masses of inactive ice occupying the main valley floors before these became totally ice free these events at the beginning of the oldest dryas ca 16 ka bp were followed by a distinct glacier readvance gschnitz marked by end moraines and outwash fans according to some radiocarbon dates this occurred around 14 ka bp during the bolling interstade the valleys became almost ice free the next two glacier readvances daun and egesen were limited to cirques and the higher parts of the valleys and are dated to the older and younger dryas stades respectively

interactions between environmental change and shallow water carbonate buildup along the northern tethyan margin and their impact on the early cretaceous carbon isotope record 1 the evolution of the early cretaceous northern tethyan carbonate platform was not only influenced by changes in sea level detrital influx and surface water temperature but also by changes in trophic levels we distinguish between phases of carbonate production dominated by oligotrophic photozoan communities and by mesotrophic and eventually colder water heterozoan communities superimposed on this bimodal trend in platform evolution were phases of platform demise for which we provide improved age control based on ammonite biostratigraphy the initial phase of these episodes of platform demise corresponds in time to episodes of oceanic anoxic events and environmental change in general on the basis of a comparison between the temporal changes in an early cretaceous ammonite calibrated d 13 c record from southeastern france and coeval changes in the platform record we suggest that the history of carbon fractionation along the northern tethyan margin was not only influenced by changes in the oceanic carbon cycle such as in the rate of production and preservation of organic and carbonate carbon and in the size of the oceanic dissolved inorganic carbon reservoir but it was also influenced by the above mentioned changes in the ecology and geometry of the adjacent carbonate platform phases of photozoan carbonate production induced positive trends in the hemipelagic carbonate d 13 c record phases of heterozoan carbonate production pushed the d 13 c system toward more negative values platform drowning episodes implied an initial increase in d 13 c values followed by a longerterm decrease in d 13 c values

anthropogenic and climate controls on vegetation changes between 1500 bce and 500 ce reconstructed from a high resolution pollen record from varved sediments of lake mondsee austria abstract this study reports a precisely dated pollen record with a 20 year resolution from the varved sediments of lake mondsee in the north eastern european alps 47 49 n 13 24 e 481 m above sea level the analysed part of core spans the interval between 1500 bce and 500 ce and allows changes in vegetation composition in relation to climatic changes and human activities in the catchment to be inferred intervals of distinct but modest human impact are identified at ca 1450 1220 740 490 and 340 190 bce and from 80 bce to 180 ce while the first two intervals are synchronous with prominent salt mining phases during the bronze age and early iron age at the nearby unesco world heritage site of hallstatt the last two intervals fall within the late iron age and roman imperial era respectively comparison with published records of extreme runoff events obtained from the same sediment core shows that human activities including agriculture and logging around lake mondsee were low during intervals of high flood frequency as indicated by a higher number of intercalated detrital event layers but intensified during hydrologically stable intervals comparison of the pollen percentages of arboreal taxa with the stable oxygen isotope and potassium ion records of the ngrip and gisp2 ice cores from greenland reveals significant positive correlations for fagus and negative correlations for betula and alnus this underlines the sensitivity of vegetation around lake mondsee to temperature fluctuations in the north atlantic as well as to moisture fluctuations controlled by changes in the intensity of the siberian high and the north atlantic oscillation nao regime

processes governing the recovery of a perturbed thermohaline circulation in hadcm3 abstract in an experiment with the latest version of the hadley centre climate model the model response has been analyzed after the thermohaline circulation thc in the atlantic ocean has been suppressed the suppression is induced by a strong initial perturbation to the salinity distribution in the upper layer of the northern north atlantic the model is then allowed to adjust freely salinity gradually increases and deep water formation in the greenland and norwegian seas restarts later also in the labrador sea the meridional overturning recovers after about 120 yr in the first few decades when the overturning is very weak surface air temperature is dominated by cooling of much of the northern hemisphere and weak warming of the southern hemisphere leading to maximum global cooling of 0 9 c the disruption to the atmosphere s radiation balance results in a downward flux anomaly at the top of the atmosphere maximally 0 55 w m 2 in the first decade then decreasing with the thc recovery the processes

basinal restriction black shales re os dating and the early toarcian jurassic oceanic anoxic event 1 profiles of mo total organic carbon toc through the lower toarcian black shales of the cleveland basin yorkshire united kingdom and the posidonia shale of germany and switzerland reveal water mass restriction during the interval from late tenuicostatum zone times to early bifrons zone times times which include that of the putative early toarcian oceanic anoxic event the degree of restriction is revealed by crossplots of mo and toc concentrations for the cleveland basin which define two linear arrays with regression slopes ppm of 0 5 and 17 the slope of 0 5 applies to sediment from the upper semicelatum and exaratum subzones this value which is one tenth of that for modern sediments from the black sea mo toc regression slope 4 5 reveals that water mass restriction during this interval was around 10 times more severe than in the modern black sea the renewal frequency of the water mass was between 4 and 40 ka the mo toc regression slope of 17 applies to the overlying falciferum and commune subzones the value shows that restriction in this interval was less severe and that the renewal frequency of the water mass was between 10 and 130 years the more restricted of the two intervals has been termed the early toarcian oceanic anoxic event but is shown to be an event caused by basin restriction local to nw europe crossplots of re os and mo against toc show similar trends of increasing element concentration with increase in toc but with differing slopes together with modeling of 187 os 188 os and d 98 mo the element toc trends show that drawdown of re os and mo was essentially complete during upper semicelatum and exaratum subzone times mo toc regression slope of 0 5 drawdown sensitized the restricted water mass to isotopic change forced by freshwater mixing so that continental inputs of re os and mo via a low salinity surface layer created isotopic excursions of up to 1 3 in d 98 mo and up to 0 6 for 187 os 188 os restriction thereby compromises attempts to date toarcian black shales and possibly all black shales using re os chronology and introduces a confounding influence in the attempts to use d 98 mo and initial 187 os 188 os for palaeo oceanographic interpretation

molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the early jurassic relatively brief periods of severe paleoenvironmental change during the jurassic and cretaceous were associated with the widespread accumulation of organic rich marine deposits termed oceanic anoxic events oaes these intervals involved abrupt global warming of 5 10 c higher rates of continental weathering elevated extinction rates and large scale perturbations to the global carbon cycle the major oaes also overlapped temporally the emplacement of large igneous provinces however despite being known as oaes the extent of seawater anoxia at those times is undefined and the causative processes remain unclear here we show how changes in seawater molybdenum isotope ratios a proxy for seawater anoxia during the toarcian early jurassic oae define the onset and expansion of oxygen deficient conditions our data also place constraints on the areal extent of marine anoxia during the event and demonstrate that anoxia expanded and contracted periodically broadly in line with precession driven changes in 13 corg despite their intermittent occurrence over geological history oaes have an important contemporary relevance because the magnitude and high rates of environmental change then were broadly similar to those occurring at the present day