direct correlation of millennial scale changes in western north american vegetation and climate with changes in the california current system over the past 60 kyr terrestrial pollen and marine climate proxies from seven sediment cores taken on the continental margin of western north america between 32 and 43 n show systematically related short term changes superposed on milankovitch scale oscillations of the past 60 kyr during oxygen isotope stage 3 brief warming events in california and oregon appear correlative with interstadial events in waters offshore as do the rapid climate oscillations of the last glacial transition 14 10 ka estimated temperature depression of the coastal lowlands ranges from 10 c in southern california to 5 c in oregon during the last glacial the absence of long distance migration of pacific coastal plant assemblages concurs with marine evidence that southward movement of the polar front during the last glacial maximum was limited correlative changes in terrestrial and marine ecosystems imply that broad reorganizations of circulation in the northeast pacific ocean were synchronous with reorganizations of atmospheric circulation over western north america

enso and solar driven sub milankovitch cyclicity in the palaeogene greenhouse world high resolution pollen records from eocene lake messel germany the annually laminated oil shale from the eocene maar lake at messel germany provides unique palaeoenvironmental and palaeoclimatological information for the palaeogene greenhouse phase two palynological analyses of 6 3 and 70 kyr long records with a temporal resolution of 70 and 700 years respectively confirm vegetation and climate variability in the sub milankovitch range this variability clearly corresponds to cyclic climate fluctuations indicating the influence of solar activity and a millennial scale variability of the el nino southern oscillation also seen during the quaternary icehouse system this provides strong evidence of cyclic fluctuations in the sub milankovitch frequency band that represent a pervasive and persistent system not only of the quaternary icehouse but also throughout the entire neogene and the palaeogene greenhouse this is proof for the long term stability of the driving forces of natural climate change on earth fluctuations in the frequency of palynomorphs indicate cyclic short term quantitative changes in the composition of the vegetation these changes were related to precipitation and corresponding fluctuations of the lake level as well as to changes in pollen production transport and deposition supplementary material a complete list of variables and palynomorphs used for pollen diagrams and statistical analyses as well as additional time series analyses are available at https doi org 10 6084 m9 figshare c 3458778

extreme climatic conditions recorded in santa barbara basin laminated sediments the 1835 1840macoma event abstract the santa barbara basin sbb record of laminated sediment well known for its excellent time resolution shows the presence of an unusual once in a millenium event in the period 1835 1840 at a depth of 55 57 cm below the surface of the seafloor this macoma event is the brief flourishing of a macrofaunal community in the deep center of the sbb from which large organisms are normally excluded by the lack of oxygen shells of the pelecypodmacoma leptonoidea dall and associated fossils which congregate in a thin layer dated at 1840 provide evidence for a sudden local extinction of macrobenthos by suffocation various stratigraphic evidence suggests thatm leptonoidea colonized the basin floor for a few years prior to 1840 as a result of temporarily decreased productivity and increased oxygen content in the water spilling over the sill into the basin historical eyewitness reports indicate unusually severe winter storms from southeasterly directions during the 1830s in the sbb we suggest that coastal upwelling was severely reduced during 1835 1839 with the effect of reducing the intensity of the oxygen minimum layer themacoma event apparently is one manifestation of large scale climatic anomalies felt throughout north america and which were possibly linked to the major volcanic eruption of cosiguina nicaragua in 1835

laurentide ice sheet instability during the last deglaciation the factors leading to the full retreat of ice sheets during deglaciation are debated numerical modelling suggests that the laurentide ice sheet retreated only after a threshold for warming and radiative forcing was passed in the holocene

evidence of 550 year and 1000 year cyclicities in north atlantic circulation patterns during the holocene sedimentological colour data obtained from north atlantic deep sea sediment core neap15k pro vide a continuous climate record for the last 11500 yr with a mean sampling interval of 15 yr variations in sediment lightness a proxy for changes in north atlantic deep water nadw circulation are characterized by high frequency fluctuations with periodicities equivalent to 550 and 1000 yr as well as a 1600 yr cyclicity comparable to that reported in other studies of holocene climate cross spectral analysis suggests that the 550 yr and 1000 yr cyclical variations in nadw circulation were coherent with fluctuations in atmospheric con ditions over greenland where both these periodicities have been identified previously in the holocene gisp2 d18o record stuiver et al 1995 a similar statistical comparison provides evidence that high frequency 550 yr variability in nadw circulation patterns may have been linked to short term variations in atmospheric 14c values during the holocene in general nadw pro

simulations of northern hemisphere ice sheet retreat sensitivity to physical mechanisms involved during the last deglaciation a 3 dimensional thermomechanical ice sheet model is used to simulate the evolution of the geometry of northern hemisphere ice sheets through the last deglaciation the ice sheet model is forced by a time evolving climatology provided by the linear interpolation through time of climate snapshots simulated by the lmd5 3 atmospheric general circulation model agcm at different periods of the last deglaciation 21 15 9 6 and 0 kyr bp the agcm is driven by insolation atmospheric co2 content ice sheet configuration and sea surface temperatures although our approach is able to produce the complete continental ice retreat our simulated deglaciation presents a phase lag with reconstructions based on observational evidences this suggests that physical mechanisms related to climate forcing and or ice sheet internal dynamics are not properly represented the influence of millennial scale forcing feedback mechanisms between ice sheet elevation and surface mass balance and parameterization of the ice flow is also tested through a set of sensitivity experiments the rapid variability has a strong impact on the evolution of the ice volume because of nonlinear effects in temperature mass balance relationships fennoscandia appears to be strongly sensitive to the small scale ice sheet instability both ice sheets are to some extent sensitive to an increased basal sliding

greenland glacial history and local geodynamic consequences summary space time reconstructions of the continental ice sheets that existed at last glacial maximum lgm have previously been produced using two entirely independent methodologies one based upon the use of theoretical models of ice sheet accumulation and flow and one based upon the geophysical inversion of relative sea level rsl histories from previously ice covered regions the analyses described in this paper demonstrate the significant advantages that derive from the simultaneous application of both methods to the particular case of greenland we thereby show that the ice 4g reconstruction of the glaciation history of this region from lgm to present which was based upon the geophysical inversion of rsl data alone was reasonably accurate in the peripheral regions where rsl data were available but inaccurate in the interior of the ice sheet which was unconstrained by such information we test the new model of greenland glacial history determined by the simultaneous application of the constraints that derive from ice sheet modelling and the geophysical inversion of rsl data by employing recently published geodetic inferences of mass balance over the entire interior region of the ice sheet and of gps measurements of vertical crustal motion these observations which were not employed to constrain the ice sheet reconstruction provide significant support for the new glacial history for greenland that our analyses have led us to infer

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

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

late pliocene greenland glaciation controlled by a decline in atmospheric co2 levels it is thought that the northern hemisphere experienced only ephemeral glaciations from the late eocene to the early pliocene epochs about 38 to 4 million years ago and that the onset of extensive glaciations did not occur until about 3 million years ago several hypotheses have been proposed to explain this increase in northern hemisphere glaciation during the late pliocene here we use a fully coupled atmosphere ocean general circulation model and an ice sheet model to assess the impact of the proposed driving mechanisms for glaciation and the influence of orbital variations on the development of the greenland ice sheet in particular we find that greenland glaciation is mainly controlled by a decrease in atmospheric carbon dioxide during the late pliocene by contrast our model results suggest that climatic shifts associated with the tectonically driven closure of the panama seaway with the termination of a permanent el nino state or with tectonic uplift are not large enough to contribute significantly to the growth of the greenland ice sheet moreover we find that none of these processes acted as a priming mechanism for glacial inception triggered by variations in the earth s orbit

ice age paleotopography a gravitationally self consistent theory of postglacial relative sea level change is used to infer the variation of surface ice and water cover since the last glacial maximum lgm the results show that lgm ice volume was approximately 35 percent lower than suggested by the climap reconstruction and the maximum heights of the main laurentian and fennoscandian ice complexes are inferred to have been commensurately lower with respect to sea level use of these ice age boundary conditions in atmospheric general circulation models will yield climates that differ significantly from those previously inferred on the basis of the climap data set

a high resolution holocene record on the southern brazilian shelf paleoenvironmental implications abstract a high resolution multi proxy record has been used to determine the environmental changes during theholocene on the southern brazilian shelf present oceanographic conditions reveal wind and freshwaterinput as the determinants of short term productivity changes in the study area magnetic susceptibilityand grain size variations together with proxies of productivity organic carbon carbon accumulationrate ba sr and ca content ba al ba ti and al ti ratios were analyzed and compared with proxies ofredox condition v ti ratio terrigenous input fe ca and ti ca ratios as well as other element ti ratios to evaluate the paleoceanographic and paleoclimatic changes over the period the core covers a time interval of about 7650 years with sedimentation rates varying from 0 025 to0 250 cma 1 which represent time intervals of between 8 and 80 a per sample there is a clear change inthe sedimentation rate at about 2800 b p all grain size and elemental results indicate the occurrence of conspicuous changes between 5200 and5000 cal b p as well as between 3000 and 2800 cal b p a comparison of the results with the palyno logical information available from the adjacent continental areas suggests that the sedimentary changesin this last interval may be correlated with the onset of modern climatic conditions in south america andespecially with the onset of the plata plume water a water mass that carries cold less saline waterstowards the north however minor changes are observed at ca 1500 b p and are correlated with anincrease in the atmospheric humidity furthermore a time series analysis undertaken using severalproxies indicated the occurrence of sub milankovitch cycles which may be compared with thosereported worldwide 2008 elsevier ltd and inqua all rights reserved

implementation and initial evaluation of the glimmer community ice sheet model in the community earth system model abstractthe glimmer community ice sheet model glimmer cism has been implemented in the community earth system model cesm glimmer cism is forced by a surface mass balance smb computed in multiple elevation classes in the cesm land model and downscaled to the ice sheet grid ice sheet evolution is governed by the shallow ice approximation with thermomechanical coupling and basal sliding this paper describes and evaluates the initial model implementation for the greenland ice sheet gis the ice sheet model was spun up using the smb from a coupled cesm simulation with preindustrial forcing the model s sensitivity to three key ice sheet parameters was explored by running an ensemble of 100 gis simulations to quasi equilibrium and ranking each simulation based on multiple diagnostics with reasonable parameter choices the steady state gis geometry is broadly consistent with observations the simulated ice sheet is too thick and extensive however in some marginal regions where the smb is anomalousl

a holocene record of ocean productivity and upwelling from the northern california continental slope abstract the holocene upwelling history of the northern california continental slope is examined using the high resolution record of tn062 o550 40 9 n 124 6 w 550 m water depth this 7 m long marine sediment core spans the last 7500 years and we use it to test the hypothesis that marine productivity in the california current system ccs driven by coastal upwelling has co varied with holocene millennial scale warm intervals a combination of biogenic sediment concentrations opal total organic c and total n stable isotopes organic matter 13c and bulk sedimentary 15n and key microfossil indicators of upwelling were used to test this hypothesis the record of biogenic accumulation in tn062 o550 shows considerable holocene variability despite being located within 50 km of the mouth of the eel river which is one of the largest sources of terrigenous sediment to the northeast pacific ocean margin a key time interval beginning at 2900 calibrated years before present cal yr bp indicates the onset of modern upwelling in the ccs and this period also corresponds to the most intense period of upwelling in the last 7500 years when these results are placed into a regional ccs context during the holocene it was found that the timing of upwelling intensification at tn062 o550 corresponds closely to that seen at nearby odp site 1019 as well as in the santa barbara basin of southern california other ccs records with less refined age control show similar results which suggest late holocene upwelling intensification may be synchronous throughout the ccs based on the strong correspondence between the alkenone sea surface temperature record at odp site 1019 and the onset of late holocene upwelling in northern california we suggest that ccs warming may be conducive to upwelling intensification though future changes are unclear as the mechanisms forcing sst variability may differ

biogenic sedimentation beneath the california current system for the past 30 kyr and its paleoceanographic significance a north south transect of 17 cores was constructed along the eastern boundary of the california current system from 33 to 42 n to investigate the changes in biogenic sedimentation over the past 30 kyr percentages and mass accumulation rates of caco3 corg and biogenic opal were assembled at 500 to 1000 years sample to provide relatively high resolution time space maps reveal a complex pattern of changes that do not follow a simple glacial interglacial two mode model biogenic sedimentation shows responses that are sometimes time transgressive and sometimes coeval and most of the responses show more consistency within a limited geographic area than any temporal consistency reconstructed conditions during late oxygen isotope stage 3 were more like early holocene conditions than any other time during the last 30 kyr coastal upwelling and productivity during oxygen isotope stage 3 were relatively strong along the central california margin but were weak along the northern california margin precipitation increased during the last glacial interval in the central california region and the waters of the southern california margin had relatively low productivity productivity on the southern oregon margin was relatively low at the beginning of the last glacial interval but by about 20 ka productivity in this area significantly increased this change suggests that the center of the divergence of the west wind drift shifted south at this time the end of the last glacial interval was characterized by increased productivity in the southern california margin and increased upwelling along the central california margin but upwelling remained weak along the northern california margin a sudden 300 years decrease in caco3 corg and biogenic opal occurred at 13 ka the changes suggest a major reorientation of the atmospheric circulation in the north pacific and western north america and the establishment of a strong seasonality in the central california region a carbonate preservation event occurred at 10 ka that appears to reflect the uptake of co2 by the terrestrial biosphere as the northern latitudes were reforested following retreat of the glaciers the holocene has been a period of relatively high productivity in the southern california margin relatively strong coastal upwelling along the central california margin relatively weak upwelling along the northern california margin and the northward migration of the divergence zone of the west wind drift

strong coherence between solar variability and the monsoon in oman between 9 and 6 kyr ago variations in the amount of solar radiation reaching the earth are thought to influence climate but the extent of this influence on timescales of millennia to decades is unclear a number of climate records show correlations between solar cycles and climate1 but the absolute changes in solar intensity over the range of decades to millennia are small2 and the influence of solar flux on climate is not well established the formation of stalagmites in northern oman has recorded past northward shifts of the intertropical convergence zone3 whose northward migration stops near the southern shoreline of arabia in the present climate4 here we present a high resolution record of oxygen isotope variations for the period from 9 6 to 6 1 kyr before present in a th u dated stalagmite from oman the 18o record from the stalagmite which serves as a proxy for variations in the tropical circulation and monsoon rainfall allows us to make a direct comparison of the 18o record with the 14c record from tree rings5 which largely reflects changes in solar activity6 7 the excellent correlation between the two records suggests that one of the primary controls on centennial to decadal scale changes in tropical rainfall and monsoon intensity during this time are variations in solar radiation

deglaciation of a soft bedded laurentide ice sheet we present a series of numerical reconstructions of the laurentide ice sheet during the last deglaciation 18 7 14c ka that evaluates the sensitivity of ice sheet geometry to subglacial sediment deformation these reconstructions assume that the laurentide ice sheet flowed over extensive areas of water saturated deforming sediment soft beds corresponding to the st lawrence lowland the great lakes region the western prairies of the u s and canada and the hudson bay and hudson strait regions sediment rheology is based on a constitutive law that incorporates experimental results from late wisconsin till deposited by the laurentide ice sheet which suggest only mildly nonlinear viscoplastic behavior by varying the effective viscosity of till we produced four reconstructions for the ice sheet during the last glacial maximum 18 14c ka and two reconstructions each of the ice sheet at 14 13 12 11 and 10 14c ka we also produced one reconstruction for 9 8 4 8 and 7 14c ka reconstructions that assume a low effective viscosity for all areas of deforming sediment show a multidomed ice sheet with a large bowl shaped depression over hudson bay and thin ice 1000 m above modern sea level over the western and southern margins those reconstructions that assume a higher effective viscosity of till in the hudson bay region than for the western and southern margins also show a multidomed ice sheet but with considerably thicker ice over hudson bay and a more northerly position of the central ice divide these two different geometries may represent ice sheet orographic changes associated with a heinrich event further increases in effective viscosity of till approaching the effective viscosity of ice would result in a high monolithic ice dome centered over hudson bay reinforcing the notion that a multidomed ice sheet reflects the distribution of substrate geology modeled ice surface geometry at the last glacial maximum shows many of the same general features as previous reconstructions that incorporate deformable beds our reconstructions with higher effective till viscosities in hudson bay also agree with the ice 4g reconstructions peltier 1994 which are based on inversion of relative sea level data for the early part of the last deglaciation 18 13 14c ka but then depart significantly from ice 4g beginning at about 12 14c ka due to differing assumptions of the history of deglaciation modeled ice volume for the last glacial maximum suggests a glacioeustatic change of 50 55 m by a soft bedded laurentide ice sheet this would increase as the effective viscosity of till increases subsequent ice volume changes through the last deglaciation generally parallel the trend of eustatic rise recorded at barbados new guinea and tahiti but suggest that the laurentide ice sheet was not the source of meltwater pulse 1a

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

a lacustrine sedimentary record of holocene periglacial activity from the uinta mountains utah u s a abstract a lake sediment core from the uinta mountains of northern utah was analyzed to constrain the timing of late holocene periglacial activity records of numerous physical properties were converted to time series spanning the past 5300 years using a depth age model based on four ams 14 c dates long term decreases in organic content and increases in bulk density attest to increasing inputs of clastic sediment abundance of mineral p signaling physical bedrock weathering reaches maximum values ca 2900 2150 and 1400 cal yr bp coincident with finer median grain size and a shift toward darker red sediment these peaks interpreted as signals of periglacial activity align with pulses of rock glacier activity in colorado determined from lichenometry the youngest peak coincides with lichenometric ages previously determined for periglacial deposits upstream from the lake a pulse of renewed periglacial activity ca 400 cal yr bp represents the little ice age the late 20th century witnessed extremely high values of organic matter and biogenic silica and unprecedented low values of c n reflecting greatly enhanced in lake productivity likely due to disturbance in the watershed

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

glacial ventilation rates for the deep pacific ocean 1 a key constraint in attempts to reconstruct the patterns and rates of the ocean s thermohaline circulation during the last glacial period is the difference between the 14c to c ratio in surface and deep water while imperfect it is our best index of past deep sea ventilation rates in this paper we review published ventilation rate estimates based on the measured radiocarbon age difference between coexisting benthic and planktic foraminifera from glacial age pacific sediments we also present new results from a series of eastern equatorial pacific sediment cores the conclusion is that the scatter in these results is so large that the apparent 14c age of glacial deep pacific water could lie anywhere between double and half today s further it is not clear what is responsible for the wide scatter in the radiocarbon results

the gisp2 δ18o climate record of the past 16 500 years and the role of the sun ocean and volcanoes measured 18o 16o ratios from the greenland ice sheet project 2 gisp2 ice core extending back to 16 500 cal yr b p provide a continuous record of climate change since the last glaciation high resolution annual 18o 16o results were obtained for most of the current millennium a d 818 1985 and record the medieval warm period the little ice age and a distinct 11 yr 18o 16o cycle volcanic aerosols depress central greenland annual temperature 1 5 c maximally and annual 18o 16o for about 4 yr after each major eruptive event on a bidecadal to millennial time scale the contribution of solar variability to holocene greenlandic temperature change is 0 4 c the role of thermohaline circulation change on climate problematic during the holocene is more distinct for the 16 500 10 000 cal yr b p interval analogous to 14c age calibration terminology we express time in calibrated cal yr b p a d 1950 0 cal yr b p the oldest dryas bolling older dryas allerod younger dryas sequence appears in great detail bidecadal variance in 18o 16o but not necessarily in temperature is enhanced during the last phase of lateglacial time and the younger dryas interval suggesting switches of air mass transport between jet stream branches the branched system is nearly instantaneously replaced at the beginning of the bolling and holocene at 14 670 and 11 650 cal yr b p respectively by an atmospheric circulation system in which 18o 16o and annual accumulation initially track each other closely thermodynamic considerations of the accumulation rate temperature relationship can be used to evaluate the 18 o 16o temperature relationship the gisp2 ice layer count years of major gisp2 climate transitions also support the use of coral 14c ages for age calibration

quantified intermediate water oxygenation history of the ne pacific a new benthic foraminiferal record from santa barbara basin the oxygen minimum zone omz of the late quaternary california margin experienced abrupt and dramatic changes in strength and depth in response to changes in intermediate water ventilation ocean productivity and climate at orbital through millennial time scales expansion and contraction of the omz is exhibited at high temporal resolution 107 126 year by quantitative benthic foraminiferal assemblage changes in two piston cores forming a vertical profile in santa barbara basin 569 m basin floor 481 m near sill depth to 34 and 24 ka respectively variation in the omz is quantified by new benthic foraminiferal groupings and new dissolved oxygen index based on documented relations between species and water mass oxygen concentrations foraminiferal based paleoenvironmental assessments are integrated with principal component analysis bioturbation grain size caco3 total organic carbon and c 13 to reconstruct basin oxygenation history fauna responded similarly between the two sites although with somewhat different magnitude and taxonomic expression during cool episodes younger dryas and stadials the water column was well oxygenated most strongly near the end of the glacial episode 17 16 ka heinrich 1 in contrast the omz was strong during warm episodes bolling allerod interstadials and pre boreal during the bolling allerod the omz shoaled to 360 m of contemporaneous sea level its greatest vertical expansion of the last glacial cycle assemblages were then dominated by bolivina tumida reflecting high concentrations of dissolved methane in bottom waters short decadal intervals were so severely oxygen depleted that no benthic foraminifera were present the middle to late holocene 6 0 ka was less dysoxic than the early holocene

climatic solar oceanic and geomagnetic influences on late glacial and holocene atmospheric 14c 12c change late glacial and holocene 14c12c ratios of atmospheric co2 vary in magnitude from a few per mil for annual decadal pertubations to more than 10 for events lasting millennia a data set illuminating 10 to 104 yr variability refines our understanding of oceanic climatic versus geomagnetic or solar forcing of atmospheric 14c12c ratios most of the variance in the holocene atmospheric 14c12c record can be attributed to the geomagnetic millennia time scale and solar century time scale influence on the flux of primary cosmic rays entering the atmosphere attributing the observed atmospheric 14c12c changes to climate alone leads to ocean circulation and or global wind speed changes incompatible with proxy records climate ocean related 14c redistribution between carbon reservoirs while evidently playing a minor role during the holocene may have perturbed atmospheric 14c12c ratios measurably during the late glacial younger dryas event first order corrections to the radiocarbon time scale 12 000 30 000 14c yr b p are calculated from adjusted lake sediment and tree ring records and from geomagnetically defined model 14c histories paleosunspot numbers 100 9700 cal yr b p are derived from the relationship of model 14c production rates to sunspot observations the spectral interpretation of the 14c12c atmospheric record favors higher than average solar activity levels for the next century minimal evidence was found for a sun weather relationship

deglacial rapid sea level rises caused by ice sheet saddle collapses a simulation of ice sheet retreat reveals two meltwater pulses coinciding with mwp 1a and the 8 200 year event caused by saddle collapses between ice domes during the last deglaciation the retreating ice sheets occasionally released massive amounts of fresh water these meltwater pulses are often implicated as important influences on abrupt climate change but the underlying triggering mechanisms are not clear here lauren gregoire et al show that saddle zones that connect adjacent high domes on ice sheets can be lowered by ongoing warming to an elevation at which they rapidly degrade leading to separate domes and a meltwater pulse that is consistent with geological observations although the mechanism itself is not novel the authors establish that two important sudden cooling events meltwater pulse 1a and the 8 200 year event were probably caused by saddle separation the last deglaciation 21 to 7 thousand years ago was punctuated by several abrupt meltwater pulses which sometimes caused noticeable climate change1 2 around 14 thousand years ago meltwater pulse 1a mwp 1a the largest of these events produced a sea level rise of 14 18 metres over 350 years3 although this enormous surge of water certainly originated from retreating ice sheets there is no consensus on the geographical source or underlying physical mechanisms governing the rapid sea level rise4 5 6 here we present an ice sheet modelling simulation in which the separation of the laurentide and cordilleran ice sheets in north america produces a meltwater pulse corresponding to mwp 1a another meltwater pulse is produced when the labrador and baffin ice domes around hudson bay separate which could be associated with the 8 200 year event the most pronounced abrupt climate event of the past nine thousand years7 for both modelled pulses the saddle between the two ice domes becomes subject to surface melting because of a general surface lowering caused by climate warming the melting then rapidly accelerates as the saddle between the two domes gets lower producing nine metres of sea level rise over 500 years this mechanism of an ice saddle collapse probably explains mwp 1a and the 8 200 year event and sheds light on the consequences of these events on climate

paleoproductivity ventilation and organic carbon burial in the santa barbara basin odp site 893 off california since the last glacial biogenic burial rates and coccolith assemblage data were generated for the past 16 ka at odp site 893 in the santa barbara basin off california to determine if dysoxic events in the basin were related to changes in marine productivity coccolith abundance data show that changes in surface water conditions did indeed change in concert with oxygen levels at the sea floor however organic carbon burial rates varied independently indicating that oxygenation of the water column is related mainly to intermediate water ventilation a strong correlation of organic carbon burial rates with lithogenic and to a lesser extent with carbonate and opal accumulation rates confirms recent conclusions that the export of organic carbon from the sea surface is largely controlled by the presence of ballast minerals

brief interstadial events in the santa barbara basin ne pacific during the past 60 kyr the instability of the northern hemisphere glacial climate over the past 100 kyr has been revealed by at least 20 brief warm interstadial episodes called dansgaard oeschger events recorded in greenland ice cores1 3 and in north atlantic sedimentary records4 5 a few of these events have been recognized elsewhere6 8 here we describe a record of ocean oxygenation and circulation from the santa barbara basin in the northeast pacific ocean which correlates well with the greenland ice core records we see 19 of the 20 dansgaard oeschger events in the form of laminated sediments deposited under anoxic conditions and we can correlate at least 16 of these with the 17 ice core interstadials of the past 60 kyr thus these short term events were not restricted to the north atlantic region the events had substantial ecological and oceanographic effects in the santa barbara basin including changes in benthic faunal populations and in the age and composition of bottom waters similar ventilation changes have been seen in the gulf of california9 10 suggesting that these changes may have been widespread and synchronous along the northeast pacific margin these results suggest sensitivity of broad areas of the ocean atmosphere cryosphere system to short term climate change

last glacial maximum and holocene climate in ccsm3 abstract the climate sensitivity of the community climate system model version 3 ccsm3 is studied for two past climate forcings the last glacial maximum lgm and the mid holocene the lgm approximately 21 000 yr ago is a glacial period with large changes in the greenhouse gases sea level and ice sheets the mid holocene approximately 6000 yr ago occurred during the current interglacial with primary changes in the seasonal solar irradiance the lgm ccsm3 simulation has a global cooling of 4 5 c compared to preindustrial pi conditions with amplification of this cooling at high latitudes and over the continental ice sheets present at lgm tropical sea surface temperature sst cools by 1 7 c and tropical land temperature cools by 2 6 c on average simulations with the ccsm3 slab ocean model suggest that about half of the global cooling is explained by the reduced lgm concentration of atmospheric co2 50 of present day concentrations there is an increase in the antarctic circumpolar current an

a fast and easy method to derive highest resolution time series datasets from drillcores and rock samples abstract we present a fast method to obtain highest resolution greyvalue datasets from digital core images these datasets are processed using a software package developed by us artificially introduced trends in the datasets are corrected disturbances i e cracks voids are eliminated to reduce the high level noise in data we have developed a vector filter that effectively removes such noise application of the method to odp drillcores from the santa barbara basin allowed exact correlation of two drillcores from the same basin the influence of the termination of the last glacial cold period on the flux rates in the basin are reflected in the greyvalue datasets we found a transitional period of about 3000 yr between the end of the last glacial cold period and the onset of post glacial climatic oscillation in the basin fourier analyses show that the high resolution greyvalue time series record climatic cycles with frequencies as high as 1 cycle yr varves

on solar forcing of holocene climate evidence from scandinavia the focus of this study is the possible correlation between changes in the climate of scandinavian and changes in solar irradiation reliable information about holocene climatic change in sweden and norway is currently available from two main sources the 14c dating of pine wood retrieved from above the present pine tree limit and studies of glacier variations based on proglacial lacustrine sediments and on moraines the reconstructed alpine tree limit reveals that summer temperature in general was warmer during the early holo cene than it was during the late holocene superimposed on this general trend are several fluctuations of a few hundred years duration relatively cold periods with a duration of the order of 300 600 years occurred frequently during the holocene in this paper dates of the major climatic events are compared with an index of solar activity the so called 14c anomalies for most of the last 9000 years a good correspondence is demonstrated between the timing of cold events in scandi

the antarctica component of postglacial rebound model ice 6g_c vm5a based on gps positioning exposure age dating of ice thicknesses and relative sea level histories

space geodesy constrains ice age terminal deglaciation the global ice 6g_c vm5a model a new model of the last deglaciation event of the late quaternary ice age is here described and denoted as ice 6g c vm5a it differs from previously published models in this sequence in that it has been explicitly refined by applying all of the available global positioning system gps measurements of vertical motion of the crust that may be brought to bear to constrain the thickness of local ice cover as well as the timing of its removal additional space geodetic constraints have also been applied to specify the reference frame within which the gps data are described the focus of the paper is upon the three main regions of last glacial maximum ice cover namely north america northwestern europe eurasia and antarctica although greenland and the british isles will also be included if peripherally in the discussion in each of the three major regions the model predictions of the time rate of change of the gravitational field are also compared to that being measured by the gravity recovery and climate experiment satellites as an independent means of verifying the improvement of the model achieved by applying the gps constraints several aspects of the global characteristics of this new model are also discussed including the nature of relative sea level history predictions at far field locations in particular the caribbean island of barbados from which especially high quality records of postglacial sea level change are available but which records were not employed in the development of the model although ice 6g c vm5a is a significant improvement insofar as the most recently available gps observations are concerned comparison of model predictions with such far field relative sea level histories enables us to identify a series of additional improvements that should follow from a further stage of model iteration

a data calibrated distribution of deglacial chronologies for the north american ice complex from glaciological modeling article i nfo past deglacial ice sheet reconstructions have generally relied upon discipline specific constraints with no at tention given to the determination of objective confidence intervals reconstructions based on geophysical inversion of relative sea level rsl data have the advantage of large sets of proxy data but lack ice mechanical constraints conversely reconstructions based on dynamical ice sheet models are glaciologically self consistent but depend on poorly constrained climate forcings and sub glacial processes as an example of a much better constrained methodology that computes explicit error bars we present a dis tribution of high resolution glaciologically self consistent deglacial histories for the north american ice com plex calibrated against a large set of rsl marine limit and geodetic data the history is derived from ensemble based analyses using the 3d mun glacial systems model and a high resolution ice margin chronol ogy derived from geological and geomorphological observations isostatic response is computed with the vm5a viscosity structure bayesian calibration of the model is carried out using markov chain monte carlo methods in combination with artificial neural networks trained to the model results the calibration provides a posterior distribution for model parameters and thereby modeled glacial histories given the observational data sets that takes data uncertainty into account final ensemble results also account for fits between com puted and observed strandlines and marine limits given the model including choice of calibration parameters input and constraint data sets and vm5a earth rhe ology we find the north american contribution to mwp1a was likely between 9 4 and 13 2 m eustatic over a 500 year interval this is more than half of the total 16 to 26 m meltwater pulse over 500 to 700 years with lower values being more probable indicated by the barbados coral record fairbanks 1989 peltier and fairbanks 2006 if one assumes a 5 meter living range for the acropora palmata coral 20 ka ice volume for north america was likely 70 1 2 0 m eustatic or about 60 of the total contribution to eustatic sea level change we suspect that the potentially most critical unquantified uncertainties in our analyses are those related to model structure especially climate forcing deglacial ice margin chronology and earth rheology

wildfire and abrupt ecosystem disruption on california s northern channel islands at the allerod younger dryas boundary 13 0 12 9 ka abstract sedimentary records from california s northern channel islands and the adjacent santa barbara basin sbb indicate intense regional biomass burning wildfire at the allerod younger dryas boundary 13 0 12 9 ka all age ranges in this paper are expressed in thousands of calendar years before present ka radiocarbon ages will be identified and clearly marked 14c years multiproxy records in sbb ocean drilling project odp site 893 indicate that these wildfires coincided with the onset of regional cooling and an abrupt vegetational shift from closed montane forest to more open habitats abrupt ecosystem disruption is evident on the northern channel islands at the allerod younger dryas boundary with the onset of biomass burning and resulting mass sediment wasting of the landscape these wildfires coincide with the extinction of mammuthus exilis pygmy mammoth the earliest evidence for human presence on these islands at 13 1 12 9 ka 11 000 10 900 14c years is followed by an apparent 600 800 year gap in the archaeological record which is followed by indications of a larger scale colonization after 12 2 ka although a number of processes could have contributed to a post 18 ka decline in m exilis populations e g reduction of habitat due to sea level rise and human exploitation of limited insular populations we argue that the ultimate demise of m exilis was more likely a result of continental scale ecosystem disruption that registered across north america at the onset of the younger dryas cooling episode contemporaneous with the extinction of other megafaunal taxa evidence for ecosystem disruption at 13 12 9 ka on these offshore islands is consistent with the younger dryas boundary cosmic impact hypothesis firestone r b west a kennett j p becker l bunch t e revay z s schultz p h belgya t kennett d j erlandson j m dickenson o j goodyear a a harris r s howard g a kloosterman j b lechler p mayewski p a montgomery j poreda r darrah t que hee s s smith a r stich a topping w wittke j h wolbach w s 2007 evidence for an extraterrestrial impact 12 900 years ago that contributed to the megafaunal extinctions and younger dryas cooling proceedings of the national academy of sciences 104 16016 16021

high resolution sea surface reconstructions off cape hatteras over the last 10 ka 1 this study presents high resolution foraminiferal based sea surface temperature sea surface salinity and upper water column stratification reconstructions off cape hatteras a region sensitive to atmospheric and thermohaline circulation changes associated with the gulf stream we focus on the last 10 000 years 10 ka to study the surface hydrology changes under our current climate conditions and discuss the centennial to millennial time scale variability we observed opposite evolutions between the conditions off cape hatteras and those south of iceland known today for the north atlantic oscillation pattern we interpret the temperature and salinity changes in both regions as co variation of activities of the subtropical and subpolar gyres around 8 3 ka and 5 2 3 5 ka positive salinity anomalies are reconstructed off cape hatteras we demonstrate for the 5 2 3 5 ka period that the salinity increase was caused by the cessation of the low salinity surface flow coming from the north a northward displacement of the gulf stream blocking the southbound low salinity flow concomitant to a reduced meridional overturning circulation is the most likely scenario finally wavelet transform analysis revealed a 1000 year period pacing the 18o signal over the early holocene this 1000 year frequency band is significantly coherent with the 1000 year frequency band of total solar irradiance tsi between 9 5 ka and 7 ka and both signals are in phase over the rest of the studied period

ventilation changes in the northeast pacific during the last deglaciation under present climate conditions convection at high latitudes of the north pacific is restricted to shallower depths than in the north atlantic to what extent this asymmetry between the two ocean basins was maintained over the past 20 kyr is poorly known because there are few unambiguous proxy records of ventilation from the north pacific we present new data for two sediment cores from the california margin at 800 and 1600 m depth to argue that the depth of ventilation shifted repeatedly in the northeast pacific over the course of deglaciation the evidence includes benthic foraminiferal cd ca 18o 16o and 13c 12c data as well as radiocarbon age differences between benthic and planktonic foraminifera a number of features in the shallower of the two cores including an interval of laminated sediments are consistent with changes in ventilation over the past 20 kyr suggested by alternations between laminated and bioturbated sediments in the santa barbara basin and the gulf of california keigwin and jones 1990 kennett and ingram 1995 behl and kennett 1996 data from the deeper of the two california margin cores suggest that during times of reduced ventilation at 800 m ventilation was enhanced at 1600 m depth and vice versa this pronounced depth dependence of ventilation needs to be taken into account when exploring potential teleconnections between the north pacific and the north atlantic

a data constrained large ensemble analysis of antarctic evolution since the eemian abstract reconstructing the historical behaviour of the antarctic ice sheet is essential for understanding and predicting climatic interactions ice sheet sensitivities and sea level contributions furthermore to interpret such reconstructions with any degree of confidence meaningful uncertainty estimates are required working toward this goal this article presents results from a large ensemble data constrained study of antarctic evolution over the last glacial cycle ice sheet chronologies have been generated using a 3 d glacial systems model that includes sheet stream shelf flow a parametrized basal drag coefficient accounting for sub grid topographic roughness sediment likelihood and systematic model to observation thickness misfits a sub grid grounding line flux parametrization a visco elastic bedrock response component parametrized climate forcing separate shelf and tidewater calving treatments and a physically motivated empirically derived sub shelf melt component there are 31 ensemble parameters to capture uncertainties in the glacial cycle climate mass balance processes and ice dynamics once generated the ensemble of model runs is constrained using a suite of observational data including constraints on relative sea level past ice thickness and grounding line retreat data points as well as present day ice volumes and configuration and an evaluation methodology that produces a misfit to observations score for each run assuming variants of a gaussian error model the scores are used to generate probability distributions for the past evolution of the ice sheet in our ensemble based analysis the last glacial maximum lgm of antarctic ice volume occurs at 25 24 ka the lgm ice volume excess relative to present day is likely between 5 6 and 14 3 m equivalent sea level mesl and with less confidence 10 mesl there is little change in the grounded ice volume from 24 ka until 17 16 ka at which time widespread deglaciation commences the ross and weddell sea sectors are the largest sources of ice mass loss the major period of grounding line retreat in the weddell sea sector resulting in the present day ronne filchner shelf system occurs after 10 ka the grounding line in the ross sea sector start its major retreat phase after 12 ka there is no significant contribution during the eemian the modelled antarctic ice sheet has a minimal ice volume at 114 ka contributing 2 mesl relative to present day the mass loss is principally due to a major retreat of the grounding line in the siple coast sector of the ross sea we present standard deviation plots of the ensemble results for present day and at the lgm these plots highlight geographic areas that would most benefit from additional constraint and therefore offer priorities and guidance for future field campaigns

geographic coherence of millennial scale climate cycles during the holocene varve thickness measurements and variation in sediment texture have been compiled for an intermittently varved sequence of holocene sediments at odp site 893 in the santa barbara basin sbb off california a varve based age model is generated from the varve counts using interpolation across homogeneous intervals with this age model the varve thickness and sediment texture time series are compared to published gisp2 stable oxygen isotope and aerosol data as well as to the atmospheric 14c record variation in sediment texture at site 893 shows the presence of a 1000 year cycle which is attributed to fluctuations in the degree of ventilation of the basin cross spectral analysis shows that this cycle is coherent with the gisp2 18o record and with atmospheric 14c throughout the holocene the correlation with the 14c record indicates that this 1000 year cycle is related to solar cycles which appear to have a noticeable impact especially on circum north atlantic climate the presence of the 1000 year cycle in the sbb is attributed to a tele connection with the northern high latitudes most likely via intermediate water formation in the north pacific in contrast the gisp2 aerosol record and variation in varve thickness at odp site 893 show a coherent 2750 year cycle which does not correlate with the 1000 year cycle varve thickness in sbb has been shown to correlate with annual rainfall which occurs mainly during winter when prevailing winds slacken both the gisp2 aerosol and sbb varve thickness records are therefore interpreted as a measure for a combination of wind stress and aridity the correlation is negative in that thick varves wet climate in sbb occur during times that the aerosol load is high at gisp2 dry the presence of two distinct patterns of millennial scale variation points to regional trends in climate change during the holocene however the fact that both patterns occur at two distant localities suggest that they represent differential behaviour of supra regional climate systems of which the effects can be traced over large parts of the globe

strong evidence for the influence of solar cycles on a late miocene lake system revealed by biotic and abiotic proxies the late miocene paleogeography of central europe and its climatic history are well studied with a resolution of c 106 years small scale climatic variations are yet unresolved observing past climatic change of short periods however would encourage the understanding of the modern climatic system therefore past climate archives require a resolution on a decadal to millennial scale to detect such a short term evolution a continuous 6 m core of the paleo lake pannon was analyzed in 1 cm sample distance to provide information as precise and regular as possible measurements of the natural gamma radiation and magnetic susceptibility combined with the total abundance of ostracod shells were used as proxies to estimate millennial to centennial scale environmental changes during the mid tortonian warm period patterns emerged but no indisputable age model can be provided for the core due to the lack of paleomagnetic reversals and the lack of minerals suitable for absolute dating therefore herein we propose another method to determine a hypothetic time frame for these deposits based on statistical processes including lomb scargle and redfit periodograms along with wavelet spectra several distinct cyclicities could be detected calculations considering established off shore sedimentation rates of the tortonian vienna basin revealed patterns resembling holocene solar cycle records well the comparison of filtered data of miocene and holocene records displays highly similar patterns and comparable modulations a best fit adjustment of sedimentation rate results in signals which fit to the lower and upper gleissberg cycle the de vries cycle the unnamed 500 year and 1000 year cycles as well as the hallstatt cycle each of these cycles has a distinct and unique expression in the investigated environmental proxies reflecting a complex forcing system hence a single proxy analysis as often performed on holocene records should be considered cautiously as it might fail to capture the full range of solar cycles

holocene periodicity in north atlantic climate and deep ocean flow south of iceland climate fluctuations during the past millennium are relatively well documented1 on a longer timescale there is growing evidence of millennial scale variability of holocene climate at periodicities of 2 500 and 950 years possibly caused by changes in solar flux 2 3 and 1 500 years maybe related to an internal oscillation of the climate system 4 5 6 but the involvement of deep water masses in these holocene climate changes has yet to be established here we use sediment grain size data from the iceland basin to reconstruct past changes in the speed of deep water flow the study site is under the influence of iceland scotland overflow water isow the flow of which is an important component of the thermohaline circulation that modulates european climate flow changes coincide with some known climate events the little ice age and the mediaeval warm period and extend over the entire holocene epoch with aquasi periodicity of 1 500 years the grain size data indicate afaster isow flow when the climate of northern europe is warmer however a second mode of operation is observed in the early holocene when warm climate intervals are associated with slower isow flow at that time the melting remnant of land based glacial age ice may have provided a sufficient source of fresh water to the ocean to reduce isow flow south of iceland

cyclic variation and solar forcing of holocene climate in the alaskan subarctic high resolution analyses of lake sediment from southwestern alaska reveal cyclic variations in climate and ecosystems during the holocene these variations occurred with periodicities similar to those of solar activity and appear to be coherent with time series of the cosmogenic nuclides 14c and 10be as well as north atlantic drift ice our results imply that small variations in solar irradiance induced pronounced cyclic changes in northern high latitude environments they also provide evidence that centennial scale shifts in the holocene climate were similar between the subpolar regions of the north atlantic and north pacific possibly because of sun ocean climate linkages

sub milankovitch solar forcing of past climates mid and late holocene perspectives a comparison was performed of solar activity and terrestrial temperature records both derived from tree rings i e without dating uncertainties with identification of detailed and highly quantified time and timescale dependent characteristics of solar forcing on climate through the current interglacial in the context of oceanic variability the tree ring derived temperature record from high latitudes of europe lapland exhibits persistent annual to millennial scale variations with multidecadal to multicentennial periodicities reminiscent of the sun9s periodicities at millennial scales cool temperatures coincided with large scale glacial maxima moreover millennial and bimillennial modes of climate variability were correlative with variations in sunspot numbers on similar scales with near century and near zero lags respectively although they were subtle in amplitude the sub milankovitch scale changes in the reception of the sun9s energy could thus suffice to noticeably modulate interglacial climate variations the relative significance of timescale dependent sun climate linkages has likely varied during the mid and late holocene times respectively thus the warmer and cooler paleotemperatures during the medieval climate anomaly and little ice age were better explained by solar variations on a millennial rather than bimillennial scale the observed variations may have occurred in association with internal climate amplification likely thermohaline circulation and el nino southern oscillation activity the near centennial delay in climate in responding to sunspots indicates that the sun9s influence on climate arising from the current episode of high sunspot numbers may not yet have manifested itself fully in climate trends if neglected in climate models this lag could cause an underestimation of twenty first century warming trends

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

holocene climate variability in the western mediterranean region from a deepwater sediment record 1 the detailed analysis of the international marine past global changes study core md99 2343 recovered from a sediment drift at 2391 m water depth north of the island of minorca illustrates the effects of climate variability on thermohaline circulation in the western mediterranean during the last 12 kyr geochemical ratios associated with terrigenous input resulted in the identification of four phases representing different climatic and deepwater overturning conditions in the western mediterranean basin during the holocene superimposed on the general trend eight centennial to millennial scale abrupt events appear consistently in both grain size and geochemical records which supports the occurrence of episodes of deepwater overturning reinforcement in the western mediterranean basin the observed periodicity for these abrupt events is in agreement with the previously defined holocene cooling events of the north atlantic region thus supporting a strong atlanticmediterranean climatic link at high frequency time intervals during the last 12 kyr the rapid response of the mediterranean thermohaline circulation to climate change in the north atlantic stresses the importance of atmospheric teleconnections in transferring climate variability from high latitudes to midlatitudes

complexity of holocene climate as reconstructed from a greenland ice core glaciochemical time series developed from summit greenland indicate that the chemical composition of the atmosphere was dynamic during the holocene epoch concentrations of sea salt and terrestrial dusts increased in summit snow during the periods 0 to 600 2400 to 3100 5000 to 6100 7800 to 8800 and more than 11 300 years ago the most recent increase and also the most abrupt coincides with the little ice age these changes imply that either the north polar vortex expanded or the meridional air flow intensified during these periods and that temperatures in the mid to high northern latitudes were potentially the coldest since the younger dryas event

insolation driven 100 000 year glacial cycles and hysteresis of ice sheet volume comprehensive climate and ice sheet models show that insolation and internal feedbacks between the climate the ice sheets and the lithosphere asthenosphere system explain the 100 000 year period on which the northern hemisphere ice sheets grow and shrink for the past million years earth s climate has been dominated by a 100 000 year cycle of glaciation involving the advance and retreat of the northern hemisphere ice sheets variations in insolation the solar radiation reaching earth s surface caused by constant changes in earth s orbit and orientation to the sun are closely associated with glaciations but the physical mechanisms driving the cycle have remained unclear now ayako abe ouchi and colleagues demonstrate that insolation variations interact with the size of the ice sheets and the solid earth to control the 100 000 year cycles their model shows that internal feedbacks between insolation climate and the elevation of the land beneath the ice sheet allows ice sheets to grow gradually until a maximum size is reached at that point the lowered ice sheet elevation caused by the delayed rebound of the solid earth means that a small increase in insolation can destroy the ice sheet in just a few thousand years the growth and reduction of northern hemisphere ice sheets over the past million years is dominated by an approximately 100 000 year periodicity and a sawtooth pattern1 2 gradual growth and fast termination milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles3 and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity obliquity and precession cycles4 5 yet insolation alone cannot explain the strong 100 000 year cycle suggesting that internal climatic feedbacks may also be at work4 5 6 7 earlier conceptual models for example showed that glacial terminations are associated with the build up of northern hemisphere excess ice 5 8 9 10 but the physical mechanisms underpinning the 100 000 year cycle remain unclear here we show using comprehensive climate and ice sheet models that insolation and internal feedbacks between the climate the ice sheets and the lithosphere asthenosphere system explain the 100 000 year periodicity the responses of equilibrium states of ice sheets to summer insolation show hysteresis11 12 13 with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles the hysteresis loop of the north american ice sheet is such that after inception of the ice sheet its mass balance remains mostly positive through several precession cycles whose amplitudes decrease towards an eccentricity minimum the larger the ice sheet grows and extends towards lower latitudes the smaller is the insolation required to make the mass balance negative therefore once a large ice sheet is established a moderate increase in insolation is sufficient to trigger a negative mass balance leading to an almost complete retreat of the ice sheet within several thousand years this fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound14 15 16 which is the lithosphere asthenosphere response carbon dioxide is involved but is not determinative in the evolution of the 100 000 year glacial cycles

millennial scale variability in atlantic water advection to the nordic seas derived from holocene coccolith concentration records abstract abundance patterns of coccolith species in two holocene marine cores retrieved off norway and northern iceland are indicative of millennial scale modulations in the flow of the main norwegian atlantic current and secondary north iceland irminger current branches of the north atlantic drift to the nordic seas long term trends in coccolith abundance changes reflect major holocene steps in atlantic water transfer to the nordic seas at orbital scale with important constraints on the convective activity of the nordic seas that leads to the formation of the precursor water mass of north atlantic deep water millennial scale holocene episodes of increased advection of atlantic waters off norway are associated with enhanced winter precipitation over scandinavia increased sea salt fluxes over greenland and strengthened wind over iceland thereby suggesting a common atmospheric forcing the location and intensity of the westerlies and the associated changes in mid to high latitude pressure gradients our biotic data indicate an opposite pattern of atlantic water inflow at suborbital scale between the western denmark and eastern iceland scotland straits of the northern atlantic throughout the holocene this as supported by present observational and simulated data further highlights the role of atmospheric oscillations in the recent history of the north atlantic nordic seas water mass exchanges across the greenland scotland ridge such atmospheric processes are thought to explain the observed coupling between periods of excess export of arctic sea ice to the nordic seas and intervals of maximum inflow of atlantic water to the norwegian sea throughout the last 11 000 years

north american ice sheet reconstructions at the last glacial maximum abstract the areal extent of the last glacial maximum lgm ice sheets is well known in north america but there is no direct geological proxy for ice sheet thickness or volume uncertainties associated with glaciological and geophysical reconstructions give widely varying estimates of north american ice sheet nais volume at lgm in an effort to quantify the uncertainty associated with glaciological reconstructions we conducted a suite of 190 numerical simulations of the last glacial cycle in north america prescribing different climatic mass balance glaciologic and isostatic treatments for the least constrained model variables lgm ice sheet reconstructions were evaluated using the well established geologic record of ice sheet area and southern extent at lgm dyke and prest 1987 these constraints give a subset of 33 simulations that produce reasonable lgm ice cover in north america ice sheet dispositions and the associated parameter settings in this subset of tests provide insight into the plausible range of nais thickness form and mass balance regime at lgm ice volume in this subset of tests spans a range of 28 5 38 9 10 15 m 3 at lgm with a predominant cluster at 32 36 10 15 m 3 taking floating ice and displaced continental water into account this corresponds to 69 94 m eustatic sea level msl more than 75 of the accepted tests fall in the range 78 88 msl we argue that this is a plausible estimate of the volume of water locked up in the nais at lgm

climatically related millennial scale fluctuations in strength of california margin oxygen minimum zone during the past 60 k y a strong oxygen minimum zone omz currently exists along the california margin because of a combination of high surface water productivity and poor intermediate water ventilation however the strength of this omz may have been sensitive to late quaternary ocean circulation and productivity changes along the margin although sediment lamination strength has been used to trace ocean oxygenation changes in the past oxygen levels on the open margin are not sufficiently low for laminations to form in these regions benthic foraminifera are highly sensitive monitors of omz strength and their fossil assemblages can be used to reconstruct past fluctuations benthic foraminiferal assemblages from ocean drilling program site 1017 off point conception exhibit major and rapid faunal oscillations in response to late quaternary millennial scale climate change dansgaard oeschger cycles on the open central california margin these faunal oscillations can be correlated to and are apparently synchronous with those reported from santa barbara basin together they represent major fluctuations in the strength of the omz which were intimately associated with global climate change weakening perhaps disappearing during cool periods and strengthening during warm periods these rapid major omz strength fluctuations were apparently widespread on the northeast pacific margin and must have influenced the evolution of margin biota and altered biogeochemicalmore cycles with potential feedbacks to global climate change less

effects of mountains and ice sheets on global ocean circulation abstractthe impact of mountains and ice sheets on the large scale circulation of the world s oceans is investigated in a series of simulations with a new coupled ocean atmosphere model oregon state university university of victoria model osuvic in which the height of orography is scaled from 1 5 times the actual height at t42 resolution to 0 no mountains the results suggest that the effects of mountains and ice sheets on the buoyancy and momentum transfer from the atmosphere to the surface ocean determine the present pattern of deep ocean circulation higher mountains reduce water vapor transport from the pacific and indian oceans into the atlantic ocean and contribute to increased decreased salinities and enhanced reduced deep water formation and meridional overturning circulation in the atlantic pacific orographic effects also lead to the observed interhemispheric asymmetry of midlatitude zonal wind stress the presence of the antarctic ice sheet cools winter air temperatures by more th

temperature and accumulation of high altitude firn in the alps data on temperature and accumulation of high altitude firn in the alps are compiled and discussed firn temperature varies with incoming radiation slope aspect at a given altitude the altitudinal gradient of temperature in highly permeable firn bodies appears to be about twice as high as the mean lapse rate of air temperature cold infiltration takes place above about 3500 m a s firn temperatures on the highest peaks are around 15 c accumulation net balance also decreases with increasing altitude from about 3m h20 at 3500 m a s to around 0 5 m h20 at wind exposed sites between 4300 and 4800 m a s l probably this is strongly due to wind erosion and topographical effects however temperature and accumulation not only appear to be interrelated but also seem to be positively correlated to the heat applied to the surface assuming the observed altitudinal gradients have remained constant in time it can be estimated that high altitude firn bodies have become considerably warmer since the last century co2 induced atmospheric warming could lead to a drastic change in the mass turnover and flow activity of high glaciers in wind exposed places where wind erosion of the snowpack becomes a controlling factor of accumulation introduction studies on mass and heat exchange at the surface of alpine firn have usually been carried out at altitudes below about 3500 m a s where firn is temperate and accumulation is high eg lang and others 1977 muller 1977 access to firn regions of higher altitudes is difficult however rather unusual construction work eg refuge lenkins on mont blanc and a number of tunnels dug into the firn of the monte rosa region by j e fisher in the 1950s furnished some information which indicated that at high altitudes firn is cold and accumulation does not further increase with altitude interest in temperature and accumulation of high altitude firn has grown recently in connection with i englacial temperatures 2 ice avalanches and 3 alpine core drilling temperature distribution in the tongue of grenzgletscher one of the largest ice bodies in the alps must be caused by advection of cold ice from high altitudes blatter and haeberli 1984 the frequency of ice avalanches ultimately depends on the accumulation of firn alean 1985 and the stability of steep mountain glaciers seems to depend on firn and ice temperatures rothlisberger 198 1 alean 1984 finally core drilling projects in cold high alpine firn and ice provide important information about the evolution of the composition of the atmosphere in heavily industrialized regions over past decades and centuries eg wagenbach and others in press but require knowledge about the distribution of accumulation in space and time the aims of the present study are to compile information available and point to possible effects of recent atmospheric warming firn temperature temperature measurements from deep pits tunnels and shallow bore holes in high altitude firn of the alps have been compiled by haeberli 1976 cf also hooke and others 1983 new information is available from chli titlis jungfraujoch sphinxgrat and monte rosa colle gnifetti all swiss alps at chli titlis 3070 m a s i 15 m temperature in the summit s 22 m thick firn and ice was 0 7 c 1979 80 cf haeberli and others 1979 temperatures in the sphinxgrat crest were measured in a 70 m borehole through firn ice and perennially frozen rocks in connection with recent construction work at the jungfraujoch the ice rock interface 10 m below the surface at 3525 m a s had a temperature close to 6 c 198 1 temperatures around 14 c at 15 m depth were registered in all core drilling boreholes made so far on colle gnifetti 4450 m a s the fim ice transition in this cold firn saddle occurs about 40 m below the surface schotterer and others 1981 when presently known firn temperatures are compiled figure i two groups seem to exist group i incorporates summits crests and probably also very steep slopes 45 which mainly consist of impermeable ice or high density firn with high concentrations of ice layers temperatures in such ice bodies decrease with increasing altitude at a rate about i c ioo m which is slightly higher than the assumed lapse rate of mean annual air temperature 0 65 c ioo m no temperate firn bodies are known in such situations in contrast more gentle slopes and basins group 2 where highly permeable firn accumulates are temperate up to about 3600 m a s above this upper boundary of the warm infiltration zone of the alps percolating meltwater cannot

influence of northwest pacific productivity on north pacific intermediate water oxygen concentrations during the bolling allerod interval 14 7 12 9 ka elevated productivity in the northwest pacific is suggested as a new possible control driving past intervals of low o2 intermediate water along the western continental margin of north america according to this mechanism o2 consumption would occur near the site of formation of north pacific intermediate water npiw due to increased respiration of organic carbon in response to a high productivity event evidence is provided for such a productivity increase during the bolling allerod interval 14 7 12 9 ka a time when laminated sediments were deposited along the northern california margin by this mechanism low o2 events in intermediate waters off the western north american margin could occur without significant changes in the rate of npiw ventilation

spectrophotometer analysis of holocene sediments from an anoxic fjord saanich inlet british columbia canada abstract a borehole core through the holocene sediments of saanich inlet british columbia canada was analyzed using high resolution spectrophotometry to determine whether spectral signal variations can be used as environmental climatic proxies the effects of post drilling oxidation and water loss on the core sample were assessed by comparing spectral measurements taken onboard the research ship immediately after the core was opened with shore based measurements taken one year later for saanich inlet sediments the shore based measurements provided more reliable indications of the degree of basin anoxia our spectral analyses allowed us to identify all the major features of the saanich inlet sediment sequence mazama ash and fraser river drainage stratigraphic markers oxygenation state of the basin by using the first derivative value at 675 nm as a proxy for changes in organic matter content and the colorimetric parameter b as an indicator of diatom content we were able to recognize changes in terrestrially derived organic matter om and diatom concentrations in the sediment we suggest that these variations are due to changes in upwelling intensity and fluctuations in sea level during the holocene our results confirm the potential value of spectrophotometry in identifying sediment composition and in reconstructing paleoenvironments

the california current of the last glacial maximum reconstruction at 42 n based on multiple proxies multiple paleoceanographic proxies in a zonal transect across the california current near 42 n record modern and last glacial maximum lgm thermal and nutrient gradients the offshore thermal gradient derived from foraminiferal species assemblages and oxygen isotope data was similar at the lgm to that at present warmer offshore but average temperatures were 3 3 1 5 c colder observed gradients require that the sites remained under the southward flow of the california current and thus that the polar front remained north of 42 n during the lgm carbon isotopic and foraminiferal flux data suggests enhanced nutrients and productivity of foraminfera in the northern california current up to 650 km offshore in contrast marine organic carbon and coastal diatom burial rates decreased during the lgm these seemingly contradictory results are reconciled by model simulations of the lgm wind field which suggest that wind stress curl at 42 n and thus open ocean upwelling increased while offshore ekman transport and thus coastal upwelling decreased during the last ice age the ecosystem of the northern california current during the lgm approximated that of the modern gulf of alaska cooling and production in this region was thus driven by stronger open ocean upwelling and or southward flow of high latitude water masses rather than by coastal upwelling

a 9000 year flood history for southern california a revised stratigraphy of varved sediments in santa barbara basin abstract the center of santa barbara basin sbb preserves annual laminations through most of the holocene providing an important locality for high resolution late quaternary paleoclimate and paleoceanography reconstructions an accurate chronology is necessary for these sediment based records and enables comparisons with more distal data time series of similar quality here we present an improved high resolution radiocarbon chronology for the last 9000 yrs based on 89 accelerator mass spectrometric ams 14c dates of mixed planktonic foraminiferal carbonate from three sediment cores collected in sbb mv0811 14jc spr0901 06kc and odp hole 893a accurate core to core correlation is demonstrated using prominent gray flood and olive turbidite layers identified in sbb and dated in multiple cores gray layer deposits were found more frequently in wetter intervals as determined by multi proxy hydroclimate records in southern california e g lake deposits and tree rings mass accumulation rates mars calculated for the sbb depocenter using the improved radiocarbon age model indicate high mars are associated with these gray layers that have been associated with floods thus sediment accumulation in sbb is largely controlled by sediment delivery via river runoff following precipitation events although mars may also be influenced by other factors that enhance erosion in river catchments folded sediment is observed within the basal core section of mv0811 14jc which stratigraphically correlates with 14c age reversals in odp hole 893a we associate this sediment slump feature with the large goleta submarine slide complex in santa barbara channel and estimate the submarine slide event occurred at 9000 200 yrs bp furthermore small olive turbidite layers identified and dated in sbb can be temporally linked with large earthquakes along san andreas fault and can therefore potentially reconstruct earthquake history in southern california

origin and palaeoenvironmental significance of c25 and c27 n alk 1 enes in a 25 000 year lake sedimentary record from equatorial east africa we studied the distribution of long chain alkenes n c23 to n c31 in well dated sediments from lake challa a deep crater lake near mt kilimanjaro in equatorial east africa to reveal signatures of palaeo environmental and palaeo climatic changes affecting the production of these compounds during the last 25 kyr the apolar fractions of organic sediment extracts dated to the last 16 kyr showed an unusual dominance of d13c depleted n c25 1 and n c27 1 alk 1 enes these alkenes were not detected in soil and litter from near the shoreline and from the inner rim of the crater pointing to an autochthonous aquatic source analysis of suspended particulate matter indicated that the n alk 1 enes are produced in the well oxygenated upper 30 m of the water column indicating a phytoplanktonic origin sedimenting particles collected monthly from december 2006 to november 2007 showed increased fluxes of n alk 1 enes following the locally prominent short rain season in november december green algae and or cyanobacteria were identified as candidate sources of these alkenes production of the n c25 1 and n c27 1 alkenes in lake challa was much reduced during the last glacial maximum and early late glacial period suggesting a temperature or co2 effect on habitat suitability we explored the potential of n alk 1 ene accumulation rates and of a derived alkene index n c27 1 n c25 1 n c27 1 to record longer term climatic changes the alkene index record of lake challa over the past 25 kyr shows clear periodicity with a dominant frequency of 2 3 kyr potentially indicative of monsoon variability directly or indirectly forced by variation in solar radiation

mid to late holocene climate change an overview the last 6000 years are of particular interest to the understanding of the earth system because the boundary conditions of the climate system did not change dramatically in comparison to larger glacial interglacial changes and because abundant detailed regional palaeoclimatic proxy records cover this period we use selected proxy based reconstructions of different climate variables together with state of the art time series of natural forcings orbital variations solar activity variations large tropical volcanic eruptions land cover and greenhouse gases underpinned by results from general circulation models gcms and earth system models of intermediate complexity emics to establish a comprehensive explanatory framework for climate changes from the mid holocene mh to pre industrial time the redistribution of solar energy due to orbital forcing on a millennial timescale was the cause of a progressive southward shift of the northern hemisphere nh summer position of the intertropical convergence zone itcz this was accompanied by a pronounced weakening of the monsoon systems in africa and asia and increasing dryness and desertification on both continents the associated summertime cooling of the nh combined with changing temperature gradients in the world oceans likely led to an increasing amplitude of the el nin o southern oscillation enso and possibly increasingly negative north atlantic oscillation nao indices up to the beginning of the last millennium on decadal to multi century timescales a worldwide coincidence between solar irradiance minima tropical volcanic eruptions and decadal to multi century scale cooling events was not found however reconstructions show that widespread decadal to multi century scale cooling events accompanied by advances of mountain glaciers occurred in the nh e g in scandinavia and the european alps this occurred namely during the little ice age lia between ad w1350 and 1850 when the lower summer insolation in the nh due to orbital forcing coincided with solar activity minima and several strong tropical volcanic eruptions the role of orbital forcing in the nh cooling the southward itcz shift and the desertifi cation of the sahara are supported by numerous model simulations other simulations have suggested that the fingerprint of solar activity variations should be strongest in the tropics but there is also evidence that changes in the ocean heat transport took place during the lia at high northern latitudes with possible additional implications for climates of the southern hemisphere sh

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

reconciling the ice 6g_c reconstruction of glacial chronology with ice sheet dynamics the cases of greenland and antarctica we describe a theoretical and numerical framework that has been developed to investigate the compatibility of the ice 6g c reconstruction of the glaciation histories of the greenland and antarctic ice sheets with the latest understanding of ice physics the ice 6g c reconstruction has been produced solely on the basis of the theory of the glacial isostatic adjustment gia process and it has remained an issue as to whether such reconstructions of the time dependent thickness variations of grounded continental ice sheets were compatible with physics based ice mechanical considerations our analyses focus on the evolution over the last glacial cycle of these extent ice sheet complexes and demonstrate that the gia inferred models are entirely consistent with such considerations when uncertainties in net mass balance history are taken fully into account

ice free glacial northern asia due to dust deposition on snow during the last glacial maximum lgm 21 kyr bp no large ice sheets were present in northern asia while northern europe and north america except alaska were heavily glaciated we use a general circulation model with high regional resolution and a new parameterization of snow albedo to show that the ice free conditions in northern asia during the lgm are favoured by strong glacial dust deposition on the seasonal snow cover our climate model simulations indicate that mineral dust deposition on the snow surface leads to low snow albedo during the melt season this in turn caused enhanced snow melt and therefore favoured snow free peak summer conditions over almost the entire asian continent during the lgm whereas perennial snow cover is simulated over a large part of eastern siberia when glacial dust deposition is not taken into account

effects of solar forcing and north atlantic oscillation on the climate of continental scandinavia during the holocene abstract 10 000 year long varved sediment records from lakes nautajarvi and korttajarvi finland provide evidence of climate and environment oscillations at multi decadal to millennial timescales we used two independent methods to extract periodic features from these time series of clastic laminae and assess their statistical reliability analyses revealed that seasonal sediment fluxes correspond to environmental changes with statistically significant periodicities of 1500 1800 1000 600 800 nearly 300 nearly 200 150 170 nearly 90 and 47 years showing variable coherency with different climate forcing factors and other palaeoproxy records in the northern hemisphere results indicate that the holocene winter climate in continental scandinavia was forced by a combination of several factors at least by solar variability and the north atlantic ocean atmosphere circulation patterns with varying influences through time

forcing of the antarctic circumpolar wave by el nino southern oscillation teleconnections we address several issues regarding the antarctic circumpolar wave acw these include the extent to which the acw is related to el nino southern oscillation enso cycles and the need to reconcile differences between the observed zonal wavenumber 2 pattern and the wavenumber 3 pattern found in coupled ocean atmosphere models in the present study we revisit the sea surface temperature sst signal of the observed acw we find that the associated sst variations may be decomposed into several empirical orthogonal functions eofs that are linearly independent the first four eofs describe 84 of the total variance on timescales of 3 7 years and superposition of sst signals of the four eofs reproduces sst signals of the observed acw given that eof analysis can only describe standing oscillations we then employ complex eof ceof analysis suitable for studying propagating waves the first two ceofs denoted ceofsst1 and 2 account for 70 of the total variance and the sst variability reconstructed from them again reproduces the observed acw the real part of ceofsst1 44 of the total variance is similar to the first real eof eofsst1 both eofsst1 and ceofsst1 may be identified with enso and have a wavenumber 2 zonal pattern in the southern middle to high latitudes ceofsst1 is the major constituent of the acw in the period of 1981 1997 the associated atmospheric circulation displays a pacific south america psa pattern statistical analyses demonstrate that the psa pattern teleconnected to enso is the major mechanism that forces this principal constituent of the acw ceofsst2 26 of the total variance has a zonal wavenumber 3 pattern in the same latitude band similar to that simulated by several coupled models we suggest that if enso is not adequately resolved in coupled models but structures with zonal wavenumber 3 pattern are then the modeled acw would appear to have a zonal wavenumber 3 pattern

sediment fluxes and varve formation in santa barbara basin offshore california results from a year long sediment trap study provide the first direct observations of seasonal variability in sediment fluxes to the sea floor in santa barbara basin offshore california and are used to evaluate varve formation in this basin sediment fluxes throughout the year are dominated by lithogenic material biogenic sediments primarily silica are of secondary importance we conclude that the combination of high lithogenic fluxes and low biogenic fluxes during the fall winter period results in the deposition of dark laminae the onset of upwelling and high surface productivity in the spring results in high biogenic silica fluxes that last into the summer the relative contribution of silica to the total sediment flux is highest during this spring summer period and we suggest that light laminae are formed at that time this is consistent with previous suggestions that sediment input is the primary factor controlling varve formation in this basin

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

sensitive moisture response to holocene millennial scale climate variations in the mid atlantic region usa millennial scale climate variability has been increasingly recognized as one of the most prominent features of the holocene however regional responses especially in terms of moisture conditions are poorly documented and understood here we present lithologic and magnetic evidence from white lake in northern new jersey usa to show that low lake levels occurred at about 1 3 3 0 4 4 and 6 1 ka 1 ka 1000 cal yr bp the low lake levels are indicated by heterogeneous coarse calcareous sediment layers showing strong magnetic intensities these detrital layers were likely formed during low stands when shallow water marls were exposed oxidized transported and redeposited this model is supported by laboratory experiments showing that oxidation of marls can enhance magnetic intensities the dry periods inferred from the low lake levels of white lake appear to occur concurrently with the cold periods recorded in the north atlantic sediments the correlation between millennial scale dry wet cycles inferred from lake level fluctuations of white lake and cold warm cycles in north atlantic sediments suggests sensitive moisture responses to holocene millennial scale climate variability the dry cold or wet warm association is supported by instrumental records of the last century showing that the mid atlantic region was dominated by wet conditions while most parts of the conterminous usa experienced droughts when the north atlantic ocean was warm the consistent moisture responses of the mid atlantic region to temperature changes of the north atlantic ocean may have persisted for the past 6000 years

applying the community ice sheet model to evaluate pmip3 lgm climatologies over the north american ice sheets we apply the community ice sheet model cism2 to determine the extent to which the last glacial maximum lgm temperature and precipitation climatologies from the paleoclimate modelling intercomparison project 3 pmip3 simulations support the large north american ice sheets that were prescribed as a boundary condition we force cism2 with eight pmip3 general circulation models gcms and an additional model genmom seven gcms simulate lgm climatologies that support positive surface mass balances of the laurentide and cordilleran ice sheets lis cis consistent with where ice was prescribed in the gcms two gcms simulate july temperatures that are too warm to support the ice sheets four of the nine cism2 simulations support ice sheets in beringia in absence of prescribed ice in the driving gcms and in disagreement with geologic evidence that indicates the area remained ice free during the lgm we test the sensitivity of our results to a range of snow and ice positive degree day factors daily monthly and climatological temperature and precipitation inputs and we evaluate the role of albedo and snow in the simulations areas with perennial snow in the gcm simulations correspond well to the presence of ice in the cism2 simulation gcms with unrealistically low surface albedos over the lis yield simulations that fail to simulate realistic ice sheets

millennial scale climate changes manifest milankovitch combination tones and hallstatt solar cycles in the devonian greenhouse world sub milankovitch rhythmic features in sedimentary records have been reported from throughout geological time however their origin remains enigmatic in particular during so called greenhouse periods in earth s history to better understand such short term climatic changes we sampled two 3 m thick intervals of early devonian hemipelagic carbonate at 1 cm resolution in the pod barrandovem section czech republic greenhouse conditions prevailed during early devonian times and the chosen resolution enables the detection of millennial scale climate change as recorded by elemental abundances we used a previously published astrochronology for the section to transform the studied series from the stratigraphic into the time domain spectral analysis of the time calibrated log transformed ti records reveal obliquity and precession cycles confirming the applied astrochronology additional spectral peaks with periods of 2 3 2 7 6 8 and 10 12 k y appear in both records furthermore a 1 5 k y periodicity close to the pleistocene dansgaard oeschger oscillation is also identified but only in the record with higher accumulation rate 3 5 cm k y bi coherence spectra reveal that the 6 8 and 10 12 k y periodic components are combination tones of milankovitch cycles we infer the shorter 2 5 k y periodicity to be the result of solar forcing related to the hallstatt cycle these new observations strengthen the case for an external origin of millennial scale features