rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

340 000 year centennial scale marine record of southern hemisphere climatic oscillation in order to investigate rapid climatic changes at mid southern latitudes we have developed centennial scale paleoceanographic records from the southwest pacific that enable detailed comparison with antarctic ice core records these records suggest close coupling of mid southern latitudes with antarctic climate during deglacial and interglacial periods glacial sections display higher variability than is seen in antarctic ice cores which implies climatic decoupling between mid and high southern latitudes due to enhanced circum antarctic circulation structural and temporal similarity with the greenland ice core record is evident in glacial sections and suggests a degree of interhemispheric synchroneity not predicted from bipolar ice core correlations

coupling of the hemispheres in observations and simulations of glacial climate change abstract we combine reconstructions climate model simulations and a conceptual model of glacial climate change on millennial time scales to examine the relation between the high latitudes of both hemispheres a lead lag analysis of synchronised proxy records indicates that temperature changes in greenland preceded changes of the opposite sign in antarctica by 400 500 yr a composite record of the dansgaard oeschger events shows that rapid warming cooling in greenland was followed by a slow cooling warming phase in antarctica the amplitudes rates of change and time lag of the interhemispheric temperature changes found in the reconstructions are in excellent agreement with climate model simulations in which the formation of north atlantic deep water is perturbed the simulated time lag between high northern and southern latitudes is mainly determined by the slow meridional propagation of the signal in the southern ocean our climate model simulations also show that increased deep water formation in the north atlantic leads to a reduction of the antarctic circumpolar current through diminishing meridional density gradients in the southern ocean we construct a simple conceptual model of interhemispheric dansgaard oeschger oscillations this model explains major features of the recorded temperature changes in antarctica as well as the general shape of the north south phase relation found in the observations including a broad peak of positive correlations for a lead of antarctica over greenland by 1000 2000 yr the existence of this peak is due to the regularity of the oscillations and does not imply a southern hemisphere trigger mechanism contrary to previous suggestions our findings thus further emphasise the role of the thermohaline circulation in millennial scale climate variability

arctic sea ice variability on a timescale of weeks and its relation to atmospheric forcing abstract northern hemisphere sea ice concentration 500 hpa height sea level pressure and 1000 500 hpa thickness at 7 day intervals are examined for the period 1972 1989 with emphasis on the winter season the temporal variability of sea ice concentration is largest along the climatological mean ice edge where its frequency distribution is strongly bimodal with ice free and ice covered conditions being observed much more frequently than partial ice cover these results confirm impressions based on visual inspection of satellite imagery that most of the variability in these regions is associated with the advance and retreat of the ice edge relationships between large scale patterns of atmospheric variability and sea ice variability are investigated making use of singular value decomposition of the temporal covariance matrix the analysis is conducted separately for the atlantic and pacific sectors in agreement with earlier studies based upon monthly mean data on sea ice concentration the strongest

timescales for dust variability in the greenland ice core project grip ice core in the last 100 000 years the calcium representing dust concentration record of the last 100 000 years from the greenland ice core project grip ice core shows a huge dynamic range factor 100 the relationship between dust concentrations and temperature represented by the oxygen isotope ratio is not a simple one as has often been assumed a rapid alternation factor of 5 10 between low concentrations during the dansgaard oeschger interstadial periods and high levels in colder periods is superimposed on a long term trend encompassing a further factor of 5 10 within climate periods there is only a very weak relationship between ca concentration and temperature previous authors biscaye et al 1997 have suggested that the most likely source for the increased dust is eastern asia for the first time we consider each possible cause of both rapid and slow increases from source to deposition we suggest that to account for the size and rapidity of the fast changes significantly higher wind speed in the source area is required although changes in atmospheric residence time could also play a role for the slower long term variability changes in transport speed or possibly route are probably also involved changes in the size of the source area could give some change on longer time periods the probable importance of changes in source area wind speed almost simultaneous with greenland temperature changes confirms that climatic parameters in high and low latitudes were strongly coupled through the atmosphere during glacial climatic changes this adds to evidence that the atmospheric circulation system underwent almost instantaneous large scale changes during the last glacial period

strong hemispheric coupling of glacial climate through freshwater discharge and ocean circulation the climate of the last glacial period was extremely variable characterized by abrupt warming events in the northern hemisphere accompanied by slower temperature changes in antarctica and variations of global sea level it is generally accepted that this millennial scale climate variability was caused by abrupt changes in the ocean thermohaline circulation here we use a coupled ocean atmosphere sea ice model to show that freshwater discharge into the north atlantic ocean in addition to a reduction of the thermohaline circulation has a direct effect on southern ocean temperature the related anomalous oceanic southward heat transport arises from a zonal density gradient in the subtropical north atlantic caused by a fast wave adjustment process we present an extended and quantitative bipolar seesaw concept that explains the timing and amplitude of greenland and antarctic temperature changes the slow changes in antarctic temperature and its similarity to sea level as well as a possible time lag of sea level with respect to antarctic temperature during marine isotope stage 3

paleoclimate history of galapagos surface waters over the last 135 000 yr abstract records of planktonic and benthic foraminiferal 18 o and planktonic mg ca from core tr163 22 just northwest of the galapagos islands reveal a detailed 250 450 year resolution climate history of the region over the last 135 thousand years kyr sea surface temperatures ssts reconstructed from g ruber mg ca averaged 24 3 0 4 c during the holocene 22 6 0 6 c during marine isotope stage mis 2 3 and 4 and 26 0 0 9 c during mis 5e changes in sst lead changes in both planktonic and benthic 18 o by an average 3 kyr suggesting that sst changes in this region predated continental ice volume changes changes in sst display clear millennial scale variability especially in marine isotope stage 3 with behavior somewhat similar to antarctic proxy air temperature and south pacific sst records removal of the temperature component from the planktonic 18 o record demonstrates that glacial interglacial 18 o water changes at this site were 1 0 0 2 similar to estimates for mean ocean shifts implying that salinity changes due to regional hydrological variation between the last glacial maximum and holocene in the galapagos region were minimal comparison between the tr163 22 sst record and an sst record from core tr163 19 north of the equatorial front reveals a largely similar broad scale climate history suggesting that changes in the galapagos region were caused by large scale forcing rather than by local or regional dynamical changes changes in atmospheric greenhouse forcing are the most plausible explanation for the observed large scale climate changes in the eastern equatorial pacific

response of deep groundwater to land use change in desert basins of the trans pecos region texas usa effects on infiltration recharge and nitrogen fluxes quantifying the effects of anthropogenic processes on groundwater in arid regions can be complicated by thick unsaturated zones with long transit times human activities can alter water and nutrient fluxes but their impact on groundwater is not always clear this study of basins in the trans pecos region of texas links anthropogenic land use and vegetation change with alterations to unsaturated zone fluxes and regional increases in basin groundwater no3 concentrations median increases in groundwater no3 by 0 7 0 9 mg n l over periods ranging from 10 to 50 years occurred despite low precipitation 220 360 mm year high potential evapotranspiration 1570 mm year and thick unsaturated zones 10 150 m recent model simulations indicate net infiltration and groundwater recharge can occur beneath trans pecos basin floors and may have increased due to irrigation and vegetation change these processes were investigated further with chemical and isotopic data from groundwater and unsaturated zone cores some unsaturated zone solute profiles indicate flushing of natural salt accumulations has occurred results are consistent with human influenced flushing of naturally accumulated unsaturated zone nitrogen as an important source of no3 to the groundwater regional mass balance calculations indicate the mass of natural unsaturated zone no3 122 910 kg n ha was sufficient to cause the observed groundwater no3 increases especially if augmented locally with the addition of fertilizer n groundwater no3 trends can be explained by small volumes of high no3 modern recharge mixed with larger volumes of older groundwater in wells this study illustrates the importance of combining long term monitoring and targeted process studies to improve understanding of human impacts on recharge and nutrient cycling in arid regions which are vulnerable to the effects of climate change and increasing human reliance on dryland ecosystems

phase relationships between millennial scale events 64 000 24 000 years ago a core recovered on the iberian margin off southern portugal can be correlated with greenland ice cores using oxygen isotope variability in planktonic foraminifera which closely matches the ice core records of temperature over greenland our age model identifies the base of every interstadial between 64 000 and 24 000 years ago and uses the greenland ice core project grip timescale the oxygen isotope signal in benthic foraminifera on this grip based timescale is quite different from the planktonic record and resembles the temperature record over antarctica when this is synchronized with greenland using the record of methane in the atmospheric air in the polar ice cores we interpret the benthic record as indicating significant fluctuations in ice volume during millennial events and we suggest that antarctic temperature changed as a function of ice volume

antarctic climate signature in the greenland ice core record a numerical algorithm is applied to the greenland ice sheet project 2 gisp2 dust record from greenland to remove the abrupt changes in dust flux associated with the dansgaard oeschger d o oscillations of the last glacial period the procedure is based on the assumption that the rapid changes in dust are associated with large scale changes in atmospheric transport and implies that d o oscillations in terms of their atmospheric imprint are more symmetric in form than can be inferred from greenland temperature records after removal of the abrupt shifts the residual dejumped dust record is found to match antarctic climate variability with a temporal lag of several hundred years it is argued that such variability may reflect changes in the source region of greenland dust thought to be the deserts of eastern asia other records from this region and more globally also reveal antarctic style variability and suggest that this signal is globally pervasive this provides the potential basis for suggesting a more important role for gradual changes in triggering more abrupt transitions in the climate system

atmospheric co2 and climate from 65 to 30 ka b p 1 using new and existing ice core co2 data from 65 30 ka a new chronology for co2 is established and synchronized with greenland ice core records to study how high latitude climate change and the carbon cycle were linked during the last glacial period atmospheric co2 rose several thousand years before abrupt warming in greenland associated with dansgaard oeschger events 8 12 14 17 four large warm events that follow heinrich events the co2 rise terminated at the onset of greenland warming for each of these events atmospheric co2 is strongly correlated with the antarctic isotopic temperature proxy with an average time lag of 720 370 yr mean 1 during the time interval studied the new data and chronology should provide a better target for models attempting to explain co2 variability and abrupt climate change

southern ocean origin for the resumption of atlantic thermohaline circulation during deglaciation during the two most recent deglaciations the southern hemisphere warmed before greenland at the same time the northern atlantic ocean was exposed to meltwater discharge which is generally assumed to reduce the formation of north atlantic deep water yet during deglaciation the atlantic thermohaline circulation became more vigorous in the transition from a weak glacial to a strong interglacial mode here we use a three dimensional ocean circulation model to investigate the impact of southern ocean warming and the associated sea ice retreat on the atlantic thermohaline circulation we find that a gradual warming in the southern ocean during deglaciation induces an abrupt resumption of the interglacial mode of the thermohaline circulation triggered by increased mass transport into the atlantic ocean via the warm indian ocean and cold pacific ocean water route this effect prevails over the influence of meltwater discharge which would oppose a strengthening of the thermohaline circulation a southern ocean trigger for the transition into an interglacial mode of circulation provides a consistent picture of southern and northern hemispheric climate change at times of deglaciation in agreement with the available proxy records

thermohaline instability in the north atlantic during meltwater events stable isotope and ice rafted detritus records from core so75 26kl portuguese margin a benthic isotope record has been measured for core so75 26kl from the upper portuguese margin 1099 m water depth to monitor the response of thermohaline overturn in the north atlantic during heinrich events evaluating benthic 18o in ts diagrams in conjunction with equilibrium c fractionation implies that advection of mediterranean outflow water mow to the upper portuguese margin was significantly reduced during the last glacial 15 compared to 30 today the benthic isotope record along core so75 26kl therefore primarily monitors variability of glacial north atlantic conveyor circulation the 14c accelerator mass spectrometry ages of 13 54 07 and 20 46 12 ka for two ice rafted detritus ird layers in the upper core section and an interpolated age of 36 1 ka for a third ird layer deeper in the core are in the range of published 14c ages for heinrich events h1 h2 and h4 marked depletion of benthic 13c by 0 7 1 1 during the heinrich events suggests reduced thermohaline overturn in the north atlantic during these events close similarity between meltwater patterns inferred from planktonic 18o at site 609 and ventilation patterns inferred from benthic 13c in core so75 26kl implies coupling between thermohaline overturn and surface forcing as is also suggested by ocean circulation models benthic 13c starts to decrease 1 5 2 5 kyr before heinrich events hl and h4 fully increased values are reached 1 5 3 kyr after the events indicating a successive slowdown of thermohaline circulation well before the events and resumption of the conveyor s full strength well after the events benthic 13c changes in the course of the heinrich events show subtle maxima and minima suggesting oscillatory behavior of thermohaline circulation a distinct feature of thermohaline instability in numerical models inferrred gradual spin up of thermohaline circulation after hl and h4 is in contrast to abrupt wanning in the north atlantic region that is indicated by sudden increases in greenland ice core 18o and in marine faunal records from the northern north atlantic from this we infer that thermohaline circulation can explain only in part the rapid climatic oscillations seen in glacial sections of the greenland ice core record

global geochemical cycles of carbon sulfur and oxygen time resolved data on the carbon isotopic composition of carbonate minerals and the sulfur isotopic composition or sulfate minerals show a strong negative correlation during the cretaceous carbonate minerals are isotopically heavy during this period while sulfate minerals are isotopically light the implication is that carbon is being transferred from the oxidized carbonate reservoir to the reservoir of isotopically light reduced organic carbon in sedimentary rocks while sulfur is being transferred from the reservoir of isotopically light sedimentary sulfide to the oxidized sulfate reservoir these apparently oppositely directed changes in the oxidation state of average sedimentary carbon and sulfur are surprising because of a well established and easy to understand correlation between the concentrations of reduced organic carbon and sulfide minerals in sedimentary rocks rocks rich in reduced carbon are also rich in reduced sulfur the isotopic and concentration data can be reconciled by a model which invokes a significant flux of hydrothermal sulfide to the deep sea at least during the cretaceous

a 70 kyr sea surface temperature record off southern chile ocean drilling program site 1233 1 we present the first high resolution alkenone derived sea surface temperature sst reconstruction in the southeast pacific ocean drilling program site 1233 covering the major part of the last glacial period and the holocene the record shows a clear millennial scale pattern that is very similar to climate fluctuations observed in antarctic ice cores suggesting that the southern hemisphere high latitude climate changes extended into the midlatitudes involving simultaneous changes in air temperatures over antarctica sea ice extent extension of the antarctic circumpolar current and westerly atmospheric circulation a comparison to other midlatitude surface ocean records suggests that this antarctic millennial scale pattern was probably a hemisphere wide phenomenon in addition we performed sst gradient reconstructions over the complete latitudinal range of the pacific eastern boundary current system for different time intervals during the last 70 kyr the main results suggest an equatorward displaced subtropical gyre circulation during marine isotope stages 2 and 4

irregular glacial interstadials recorded in a new greenland ice core the greenland ice sheet offers the most favourable conditions in the northern hemisphere for obtaining high resolution continuous time series of climate related parameters profiles of 18o 16o ratio along three previous deep greenland ice cores1 3 seemed to reveal irregular but well defined episodes of relatively mild climate conditions interstadials during the mid and late parts of the last glaciation but there has been some doubt as to whether the shifts in oxygen isotope ratio were genuine representations of changes in climate rather than artefacts due to disturbed stratification here we present results from a new deep ice core drilled at the summit of the greenland ice sheet where the depositional environ ment and the flow pattern of the ice are close to ideal for core recovery and analysis the results reproduce the previous findings to such a degree that the existence of the interstadial episodes can no longer be in doubt according to a preliminary timescale based on stratigraphic studies the interstadials lasted from 500 to 2 000 years and their irregular occurrence suggests complexity in the behaviour of the north atlantic ocean circulation

southern ocean control on the extent of denitrification in the southeast pacific over the last 70 ka abstract temporal changes in oceanic denitrification the bacterial reduction of nitrate under suboxic conditions highlight the potential importance of n inventory changes and the production of n 2 o on the climate system at the same time the cause of the globally observed variation in denitrification remains unclear high resolution benthic foraminiferal oxygen isotope and bulk sediment nitrogen isotope records from odp site 1234 on the chile margin record integrated denitrification changes within the peru chile upwelling system over the last 70 ka denitrification changes in the southeast pacific are coherent with antarctic climate changes recorded by the byrd ice core 18 o record and lead northern hemisphere climate events the southern hemisphere character of the chile margin 15 n record suggests that episodes of reduced denitrification in the se pacific represent times when more oxygen was supplied as the result of changes in the ventilation and preformed nutrient content of subantarctic mode water samw which forms in the subantarctic zone of the southern ocean and feeds into the low latitude thermocline

greenland antarctic phase relations and millennial time scale climate fluctuations in the greenland ice cores abstract the greenland grip gisp2 and antarctic byrd ice cores are examined in the frequency domain with the data synchronized using common methane variations using conventional time series analysis a simple picture emerges there is low frequency periods longer than about 10 kyr coherence between the two records consistent with a simple time delay antarctica leading by 1 2 kyr two geographical data points are however inadequate to infer causality from the south to north time lag at higher frequencies in the millennial band there is no measurable average relationship between the records and they appear to represent different processes with a regional character a serious question concerns the extent to which the greenland cores reflect tracer concentration change without corresponding abrupt climate change the large abrupt shifts in ice 18 o can be rationalized as owing to wind trajectory shifts perhaps of rather modest size many different physical phenomena probably do however contribute to the record as a function of time and time scale

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

phase relationships between antarctic and greenland climate records comparison of climate records from antarctic and greenland ice cores shows that the two regions respond asynchronously during millennial scale climate changes the apparent out of phase relationship between the records has been described as a climate seesaw in which cooling in the northern hemisphere is balanced by warming in the southern hemisphere the same relationship has also been attributed to the initiation of climate change events in the southern hemisphere rather than the north atlantic as is conventionally assumed a simple statistical approach band pass filtering combined with lag correlation tests used to examine the phase relationships in more detail shows that neither an anti phase nor a phase lag relationship adequately describes the observations whereas antarctic and greenland climate records do exhibit approximate anti phase behavior about 50 of the time they are generally in phase during cooling a phase lead of southern hemisphere climate of 1000 1600 years is statistically indistinguishable from a lag of 400 800 years whether for dansgaard oeschger heinrich or longer duration events the seesaw or southern lead appearance of the data arises from the fundamentally different characteristics of the climate time series most importantly the absence of rapid warming events in antarctica comparable to those in greenland to be consistent with the observations climate models will need to capture these characteristics in addition to reproducing the correct phase relationships

regional variability of ice core dust composition and provenance in greenland 1 mineralogical and isotopic composition sr and nd of six dust samples obtained from six widely spread ice coring sites in greenland were analyzed in order to investigate the regional geographic variability of dust provenance we show that long range transport from eastern asian deserts provides mineral dust with essentially the same composition to all elevated interior sites dye 3 site a grip and northgrip while most material deposited at sites located closer to the edge and at lower altitude hans tausen and renland derives from proximal source regions no contribution from other sources is apparent at any of the interior sites from the mineralogical and isotopic composition of the dust samples each of which represents several decades of dust deposition during the 17th 18th century these results provide additional evidence that african and north american deserts do not play a significant role in the dust deposited over greenland which has implications for ice core record interpretation and atmospheric dust transport model validation