the medieval warm period the little ice age and simulated climatic variability the csiro mark 2 coupled global climatic model has been used to generate a 10 000 year simulation for present climatic conditions the model output has been analysed to identify sustained climatic fluctuations such as those attributed to the medieval warm period mwp and the little ice age lia since no external forcing was permitted during the model run all such fluctuations are attributed to naturally occurring climatic variability associated with the nonlinear processes inherent in the climatic system comparison of simulated climatic time series for different geographical locations highlighted the lack of synchronicity between these series the model was found to be able to simulate climatic extremes for selected observations for century timescales as well as identifying the associated spatial characteristics other examples of time series simulated by the model for the usa and eastern russia had similar characteristics to those attributed to the mwp and the lia but smaller amplitudes and clearly defined spatial patterns a search for the frequency of occurrence of specified surface temperature anomalies defined via duration and mean value revealed that these were primarily confined to polar regions and northern latitudes of europe asia and north america over the majority of the oceans and southern hemisphere such climatic fluctuations could not be sustained for reasons explained in the paper similarly sustained sea ice anomalies were mainly confined to the northern hemisphere an examination of mechanisms associated with the sustained climatic fluctuations failed to identify a role for the north atlantic oscillation the el nino southern oscillation or the pacific decadal oscillation it was therefore concluded that these fluctuations were generated by stochastic processes intrinsic to the nonlinear climatic system while a number of characteristics of the mwp and the lia could have been partially caused by natural processes within the climatic system the inability of the model to reproduce the observed hemispheric mean temperature anomalies associated with these events indicates that external forcing must have been involved essentially the unforced climatic system is unable to sustain the generation of long term climatic anomalies

relaxation oscillators in concert a framework for climate change at millennial timescales during the late pleistocene 1 using a box model of the north atlantic ocean and a parameterization of heinrich events we suggest that self sustained oscillations of the large scale oceanic circulation provide a framework to accommodate crucial elements of late pleistocene climate variability i dansgaard oeschger style oscillations with varying interstadial length ii synchronization between dansgaard oeschger stadials and heinrich events and iii younger dryas type events the latter result from the restart of the oscillations after a glacial maximum and can be regarded as dansgaard oeschger stadials overprinted by rapidly changing boundary conditions

a study of cleaning procedures used for foraminiferal mg ca paleothermometry the various cleaning steps required for preparation of foraminiferal samples for mg ca and sr ca analysis are evaluated for their relative importance and effects on measured elemental ratios it is shown that the removal of silicate contamination is the most important step for the measurement of mg ca ratios in an example bulk sample mg ca decreases from 10 5 to 2 5 mmol mol 1 during clay removal oxidation of organic material causes a lowering of sample mg ca in the order of 10 or approximately 1 c when converted to temperature use of dilute acid leaching to remove adsorbed contaminants causes partial dissolution of the sample carbonate and a corresponding decrease in mg ca reductive treatment also causes dissolution of the sample and a decrease in the mg ca ratio of up to 10 15 sample preparation for sr ca analysis does not require the same degree of rigor as is necessary for mg ca work the within run reproducibility of the method described here for g ruber in a core top sample from the arabian sea was 1 8 mean sample ratio was 4 72 mmol mol 1 when converted to temperature this becomes 28 0 2 c the equivalent result for sr ca was 0 5 mean ratio 1 44 mmol mol 1

millennial scale variability in south east australian hydroclimate between 30 000 and 10 000 years ago abstract global climate variability during the late quaternary is commonly investigated within the framework of the bipolar seesaw pattern of asynchronous temperature variations in the northern and southern polar latitudes the terrestrial hydrological response to this pattern in south eastern australia is not fully understood as continuous high resolution well dated proxy records for the hydrological cycle in the region are sparse here we present a well dated highly resolved record of moisture balance spanning 30000 10000 calendar years before present 30 10 ka bp based on x ray fluorescence and organic carbon isotope 13com measurements of a sedimentary sequence from lake surprise in south eastern australia the data provide a locally coherent record of the hydrological cycle elevated si reflecting windblown quartz and clays and relatively high 13com indicate an extended period of relative aridity between 28 and 18 5 ka bp interrupted by millennial scale episodes of decreased si and 13com suggesting increased moisture balance this was followed by a rapid deglacial shift to low si and 13com at 18 5 ka bp indicative of wetter conditions we find that these changes are coeval with other records from south eastern australia and new zealand and use a monte carlo empirical orthogonal function approach to extract a common trend from three high resolution records our analyses suggest that drivers of the regional hydrological cycle have varied on multi millennial time scales in response to major shifts in global atmosphere ocean dynamics during the last glacial interglacial transition southern ocean processes were the dominant control on hydroclimate during glacial times via a strong influence of cold sea surface temperatures on moisture uptake and delivery onshore following the last deglaciation around 18 ka bp the southward migration of cold southern ocean fronts likely resulted in the establishment of conditions more like those of the present day millennial scale variability in records from the region is dominated by a persistent ca 2300 year periodicity consistent with other records across the southern hemisphere mid latitudes however this pervasive periodicity is not obviously linked to the bipolar seesaw and the mechanism remains equivocal

widespread evidence of 1500 yr climate variability in north america during the past 14 000 yr there is debate concerning the spatial extent and magnitude of the recently identified 1500 yr climate oscillation existing evidence is largely restricted to the north atlantic and adjacent landmasses the spatial extent magnitude and effects of these climate variations within the terrestrial environment during the holocene have not been established we show that millennial scale climate variability caused changes in vegetation communities across all of north america with a periodicity of 1650 500 yr during the past 14 000 calendar years cal yr times of major transitions identified in pollen records occurred at 600 1650 2850 4030 6700 8100 10 190 12 900 and 13 800 cal yr b p consistent with ice and marine records we suggest that north atlantic millennial scale climate variability is associated with rearrangements of the atmospheric circulation with far reaching influences on the climate

reevaluation of the oxygen isotopic composition of planktonic foraminifera experimental results and revised paleotemperature equations cultured planktonic foraminifera orbulina universa symbiotic and globigerina bulloides nonsymbiotic are used to reexamine temperature 18o relationships at 15 25 c relationships for both species can be described by linear equations equations for o universa grown under low light ll and high light hl share a slope of 4 80 0 21 c 1 with a hl ll offset of 0 33 due to symbiont photosynthetic activity the effect of co32 on o universa is 0 002 mol 1 kg 1 and is insensitive to temperature for g bulloides ontogenetic effects produce size related trends in temperature 18o whereby larger shells are enriched in 18o relative to smaller specimens the o universa temperature 18o relationships are more accurate than previously published equations for describing plankton tow data our equations do not explain planktonic core top data with the same precision but provide a good fit to benthic cibicidoides data below 10 c temperature 18o relationships for g bulloides provide good agreement with field data for this species from the northeast pacific

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

old radiocarbon ages in the southwest pacific ocean during the last glacial period and deglaciation marine radiocarbon 14c dates are widely used for dating oceanic events and as tracers of ocean circulation essential components for understanding ocean climate interactions past ocean ventilation rates have been determined by the difference between radiocarbon ages of deep water and surface water reservoirs but the apparent age of surface waters currently approximately 400 years in the tropics and approximately 1 200 years in antarctic waters might not be constant through time as has been assumed in radiocarbon chronologies and palaeoclimate studies here we present independent estimates of surface water and deep water reservoir ages in the new zealand region since the last glacial period using volcanic ejecta tephras deposited in both marine and terrestrial sediments as stratigraphic markers compared to present day values surface reservoir ages from 11 900 14c years ago were twice as large 800 years and during glacial times were five times as large 2 000 years contradicting the assumption of constant surface age furthermore the ages of glacial deep water reservoirs were much older 3 000 5 000 years the increase in surface to deep water age differences in the glacial southern ocean suggests that there was decreased ocean ventilation during this period

salinity bias on the foraminifera mg ca thermometry correction procedure and implications for past ocean hydrographic reconstructions 1 mg ca in foraminiferal calcite has recently been extensively used to estimate past oceanic temperatures here we show however that the mg ca temperature relationship of the planktonic species globigerinoides ruber is significantly affected by seawater salinity with a 1 psu change in salinity resulting in a 1 6 c bias in mg ca temperature calculations if not accounted for such a bias could lead for instance to systematic overestimations of mg ca temperatures during glacial periods when global ocean salinity had significantly increased compared to today we present here a correction procedure to derive unbiased sea surface temperatures sst and 18osw from g ruber tmg ca and 18of measurements this correction procedure was applied to a sedimentary record to reconstruct hydrographic changes since the last glacial maximum lgm in the western pacific warm pool while uncorrected tmg ca data indicate a 3 c warming of the western pacific warm pool since the lgm the salinity corrected sst result in a stronger warming of 4 c

flame foliation evidence for a schistosity formed normal to the extension direction foliations are normally thought to develop approximately parallel to the xy plane of the finite strain ellipsoid i e perpendicular to the main shortening direction we present a new type of schistosity named flame foliation that forms orthogonal to the main extension direction approximately parallel to the yz plane of finite strain flame foliation consists of anastomosing biotite rich selvedges overprinting s1 in pelitic layers of metaturbitites in nw namibia the biotite crystals in the selvedges are peculiar because they are oriented oblique or orthogonal to the flame foliation itself and parallel to the previous s1 cleavage a feature no other foliation shows in some cases biotite flames flank minor quartz veinlets implying that the flame foliation developed by infiltration of fluids and biotite growth along extensional fractures perpendicular to the main d1 extension direction where overprinting ductile d2 deformation is strong the flame foliation is transformed into a normal crenulation cleavage by rotation of biotite in the flames flame foliation is found preferentially close to syntectonic syenite granite plutons and their formation in metapelitic rocks may be enhanced by fluid overpressure due to devolatilisation reactions 2006 elsevier ltd all rights reserved

was a change in thermohaline circulation responsible for the little ice age long term 800 year and short term 20 year estimates of the rate of deep water formation in the southern ocean are in serious disagreement this disagreement suggests to me that the rate of deep water formation may have been larger during the little ice age lia than it is now

holocene volcanic history as recorded in the sulfate stratigraphy of the european project for ice coring in antarctica dome c edc96 ice core 1 a detailed history of holocene volcanism was reconstructed using the sulfate record of the european project for ice coring in antarctica dome c edc96 ice core this first complete holocene volcanic record from an antarctic core provides a reliable database to compare with long records from antarctic and greenland ice cores a threshold method based on statistical treatment of the lognormal sulfate flux distribution was used to differentiate volcanic sulfate spikes from sulfate background concentrations ninety six eruptions were identified in the edc96 ice core during the holocene with a mean of 7 9 events per millennium the frequency distribution events per millennium showed that the last 2000 years were a period of enhanced volcanic activity edc96 volcanic signatures for the last millennium are in good agreement with those recorded in other antarctic ice cores for older periods comparison is in some cases less reliable mainly because of dating uncertainties sulfate depositional fluxes of individual volcanic events vary greatly among the different cores a volcanic flux normalization volcanic flux tambora flux ratio was used to evaluate the relative intensity of the same event recorded at different sites in the last millennium normalized flux variability for the same event showed the highest value in the 1100 1500 ad period this pattern could mirror changes in regional transport linked to climatic variations such as slight warming stages in the southern hemisphere southern hemisphere medieval warming like period

wobbly ocean conveyor circulation during the holocene abstract compared to the last glacial period the climate of the holocene has been quite stable however the record does suggest that at high latitudes in the north temperatures have cooled at least since the middle holocene and that this cooling has been modulated by millennial duration fluctuations that culminated in the medieval warm little ice age oscillation in this paper we explore the possibility that these variations were associated with concurrent changes in the strength of the atlantic s conveyor circulation three lines of evidence are considered one involves records that relate directly to ocean operation a second involves the tie between sea ice extent in the northern atlantic and the extent of mountain glaciers in the european alps and in the cordillera of western north america and a third involves evidence for a bipolarity in climate changes in the northern and southern polar regions in addition we look into clues provided by the atlantic multidecadal oscillation amo although at this point no firm conclusion can be drawn the case for ocean involvement is sufficiently strong to merit further attention

a bi polar signal recorded in the western tropical pacific northern and southern hemisphere climate records from the pacific warm pool during the last ice age abstract western tropical pacific sea surface temperatures and pacific deep water temperatures during marine isotope stage 3 have been reconstructed from the 18o and mg ca of planktonic and benthic foraminifera from marion dufresne core md98 2181 this 36 m marine core was collected at 6 3 n from a water depth of 2114 m with sediment accumulation rates of up to 80 cm ky it provides a decadally resolved history of ocean variability during the last glacial period surface temperatures and salinities at this site varied in close association with millennial scale atmospheric temperature swings at high northern latitudes as reflected in the gisp2 ice core at times of colder atmospheric temperatures over greenland the western pacific was more saline and summer season ssts were 2 c colder these millennial scale changes within the tropics are attributed to a southward displacement of the summer season itcz in response to steeper meridional temperature gradients within the pacific the benthic 18o record from md98 2181 documents upper pacific deep water temperature and salinity variability benthic 18o variations of 0 3 0 5 during mis 3 indicate deep waters within the pacific were varying by 1 1 5 c with the possibility that some of the variability was due to changing salinity and minor glacial eustatic changes the observed deep water variability correlates to changes in antarctic surface temperatures and thus reflects changes in southern ocean temperatures at the site of pacific deep water formation the combined planktonic and benthic records from md98 2181 thus provide a northern and southern hemispheric climate record of anti phased variability during mis 3 as has been inferred previously from ice core records furthermore the deep sea temperature excursions appear to have led millennial variations in atmospheric co2 as recorded in the edml ice core by 1 kyr

on the fidelity of shell derived δ18oseawater estimates article i nfo 18 oseawater a robustproxyforsurfacesalinity we assessthe fidelity of shell derived 18 oseawater estimates for the surface dwelling foraminifer globigerinoides ruber white using an atlantic meridional coretop transect spanning basin scale temperature and salinity gradients shell derived and observed 18 oseawater values are well correlated r 2 0 77 but a large systematic bias is observed shell mg ca ratios are significantly elevated above values expected from observed and isotopic calcification temperatures in the saline subtropical gyres of both hemispheres this temperature independent shell mg ca ratio variability termed excess mg ca i s highly correlated with surface salinity r 2 0 77 and the observed salinity dependence 27 4 is much higher than indicated by culture studies 6 2 our coretop data are used to develop new atlantic basin temperature and salinity calibration equations that are accurate 1 1 c and 0 20 respectively precise r 2 0 82 and 0 81 respectively and verifiable using previously published data these results are valid for the relatively high salinities of the subtropical atlantic 35 5 37 3 we discover that inclusion of other published data from lower salinity regions b35 indicates little or no excess mg ca taken together these results point to a strongly non linear positive salinity effect on shell mg ca ratios that significantly affects the accuracy of sst and 18

coherent resonant millennial scale climate oscillations triggered by massive meltwater pulses the role of mean and stochastic freshwater forcing on the generation of millennial scale climate variability in the north atlantic is studied using a low order coupled atmosphere ocean sea ice model it is shown that millennial scale oscillations can be excited stochastically when the north atlantic ocean is fresh enough this finding is used in order to interpret the aftermath of massive iceberg surges heinrich events in the glacial north atlantic which are characterized by an excitation of dansgaard oeschger events based on model results it is hypothesized that heinrich events trigger dansgaard oeschger cycles and that furthermore the occurrence of heinrich events is dependent on the accumulated climatic effect of a series of dansgaard oeschger events this scenario leads to a coupled ocean ice sheet oscillation that shares many similarities with the bond cycle further sensitivity experiments reveal that the timescale of the oscillations can be decomposed into stochastic linear and nonlinear deterministic components a schematic bifurcation diagram is used to compare theoretical results with paleoclimatic data

core top calibration of mg ca in tropical foraminifera refining paleotemperature estimation 1 optimal use of mg ca as a paleotemperature proxy requires establishing calibrations for different species of foraminifera and quantifying the influence of dissolution to achieve this goal we have measured mg ca and 18o in a series of tropical and subtropical core tops including four depth transects the ceara rise the sierra leone rise and the rio grande plateau in the atlantic and the ontong java plateau in the pacific focusing on spinose mixed layer dwelling species globigerinoides ruber and globigerinoides sacculifer and nonspinose thermocline dwelling neogloboquadrina dutertrei shell mg ca in g sacculifer is 5 15 lower than in g ruber while n dutertrei mg ca is 49 55 lower than in g ruber this statistically significant offset has allowed us to establish different calibrations for each species multilinear regression analysis was used to develop calibration equations that include a correction term for the dissolution effect on mg ca in foraminiferal calcite presented in this paper are two sets of calibrations one set using core depth as a dissolution correction and another using co32 as a dissolution parameter the calibrations suggest that g ruber is the most accurate recorder of surface temperature while g sacculifer records temperatures below the surface at 20 30 m the depth habitat of n dutertrei is more uncertain owing to the wide range in habitat depths depending on hydrographic conditions but on average mg ca and 18o data suggest it is at 50 m of the three species n dutertrei is the most sensitive to dissolution up to 23 decrease in shell mg ca per km while g sacculifer is the most resistant

assessing the effect of dissolution on planktonic foraminiferal mg ca ratios evidence from caribbean core tops in order to assess the dissolution effect on foraminiferal mg ca ratios we analyzed mg ca of seven planktonic foraminiferal species and four of their varieties from caribbean core tops from 900 4700 m water depth depending on the foraminiferal species and variety mg ca start to decline linearly below co32 levels of 18 26 mol kg by 0 04 0 11 mmol mol per 1 mol kg co32 similar to decreases of 0 5 0 8 mmol mol per kilometer below 2500 3000 m water depth above these species specific critical levels mg ca remains stable with higher intraspecific mg ca variability than below we developed routines to correct mg ca from below these critical thresholds for dissolution effects which reduce the overall intraspecific variability by 24 64 and provide dissolution corrected mg ca appropriate to calculate holocene paleotemperatures when taking into account only dissolution unaffected mg ca from 2000 m the systematic succession of foraminiferal species according to their mg ca reflects expected calcification depths

trace element proxies for surface ocean conditions a synthesis of culture calibrations with planktic foraminifera abstract the trace element composition of planktic foraminiferal calcite provides a useful means of determining past surface ocean conditions we have assembled the results of culture experiments for three species of symbiont bearing planktic foraminifera globigerinoides ruber globigerinoides sacculifer and orbulina universa and one symbiont barren species globigerina bulloides to evaluate their responses to temperature salinity ph carbonate ion and dissolved inorganic carbon dic growth conditions trace element ratios li ca b ca mg ca sr ca mn ca cd ca ba ca na ca and u ca were measured simultaneously on samples grown with the same culture techniques which provides robust relatable calibrations that may be used together in multi proxy paleoceanographic studies our data confirm that temperature is the dominant control on foraminiferal mg ca under the ranges of conditions studied and that the potential effects of salinity and co32 on mg ca of these tropical species across late pleistocene glacial cycles are relatively small carbonate system experiments suggest that sr ca may be useful for reconstructing large dic changes na ca increases with salinity in g ruber pink but not in g sacculifer as these emerging proxy relationships become more firmly established the synthesis of multiple trace element ratios may help paleoceanographers isolate the effects of different environmental parameters in paleo records calcification rates g day vary among species and do not respond consistently to any experimental parameter comparison of our calcification rates with those observed in inorganic calcite precipitation experiments suggest that foraminifera calcify 100 more slowly than inorganic calcites grown in similar solutions we suggest that calcification rate does not typically exert a dominant control on trace element partitioning in planktic foraminiferal calcite though it may play a role for some elements under certain circumstances differences in average growth rate cannot explain composition differences among species pointing to alternative controls that may be biological in origin

assessing millennial scale variability during the holocene a perspective from the western tropical pacific we investigate the relationship between tropical pacific and southern ocean variability during the holocene using the stable oxygen isotope and magnesium calcium records of cooccurring planktonic and benthic foraminifera from a marine sediment core collected in the western equatorial pacific the planktonic record exhibits millennial scale sea surface temperature sst oscillations over the holocene of 0 5 c while the benthic 18oc document 0 10 millennial scale changes of upper circumpolar deep water ucdw a water mass which outcrops in the southern ocean solar forcing as an explanation for millennial scale sst variability requires 1 a large climate sensitivity and 2 a long 400 year delayed response suggesting that if solar forcing is the cause of the variability it would need to be considerably amplified by processes within the climate system at least at the core location we also explore the possibility that sst variability arose from volcanic forcing using a simple red noise model our best estimates of volcanic forcing falls short of reproducing the amplitude of observed sst variations although it produces power at low frequency similar to that observed in the md81 record although we cannot totally discount the volcanic and solar forcing hypotheses we are left to consider that the most plausible source for holocene millennial scale variability lies within the climate system itself in particular ucdw variability coincided with deep north atlantic changes indicating a role for the deep ocean in holocene millennial scale variability

changes in the thermocline structure of the indonesian outflow during terminations i and ii we present centennial records of sea surface and upper thermocline temperatures in core md01 2378 from the timor sea which provide new insights into the variability of the indonesian outflow across the last two glacial terminations mg ca in globigerinoides ruber white s s indicates an overall increase of 3 2 degrees c in sea surface temperature sst over termination i following an early holocene plateau at 11 3 6 4 ka ssts cooled by 0 6 degrees c during the middle to late holocene 6 4 0 7 ka the early holocene warming occurred in phase with increasing northern hemisphere summer insolation coinciding with northward displacement of the intertropical convergence zone enhanced boreal summer monsoon and expansion of the indo pacific warm pool thermocline temperatures pulleniatina obliquiloculata mg ca gradually decreased from 24 5 to 21 5 degrees c since 10 3 ka reflecting intensification of a cool thermocline throughflow the vertical structure of the upper ocean in the timor sea evolved in similar fashion during the holocene and mis5e although the duration of sst plateaux differed 11 3 to 6 4 ka in termination i and from 129 to 119 ka in termination 11 which was probably due to the more intense northern hemisphere summer insolation during mis 5e during both terminations sst increased simultaneously in the southern high latitudes and the tropical eastern indian ocean suggesting virtually instantaneous atmospheric climate feedbacks between the high and low latitudes c 2008 elsevier b v all rights reserved

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

coccolith evidence for instabilities in surface circulation south of iceland during holocene times abstract the coccolith based micropaleontological investigation of an exceptionally thick deep sea holocene sediment core was conducted in order to document the pattern and timing of surface circulation changes south of iceland over the gardar contour drift during the last 12 000 years fluctuations in bulk carbonate content at the core location are primarily driven by the coccolith fraction the observed overall correlation between bottom flow speed as given by the sortable silt mean size index and the bulk coccolith concentration abundance g dry sediment suggests that the accumulation of this fine carbonate fraction is as a first order controlled by processes of sediment redistribution by bottom current the down core variations in coccolith assemblage structure species indicate that changes in properties of surface waters south of iceland occurred as two distinct steps at ca 11 2 and 6 ka the onset of north atlantic drift water influence over the gardar drift after 11 2 ka was associated with excess export flux of coccolith carbonate in the vicinity of an active frontal system the period from 10 to 6 ka saw the progressive warming of the study area which culminated between 6 and 7 ka cooling of the surface waters after 6 ka as indicated changes in the relative abundances of the dominant coccolith species took place in two phases the present hydrological regime being only established after a last cooling step between 3 5 and 2 8 ka these long term reorganisations of the surface hydrology are interpreted as the response of the north atlantic to the combined force of the solar insolation and the waning laurentide ice sheet millennial scale perturbations of the surface hydrology are documented by changes in accumulation of the species emiliania huxleyi these successive decreases in the export fluxes of e huxleyi exhibit a distinct millennial pacing in phase with previously recorded holocene advection of cool ice bearing waters from the greenland iceland seas to the british islands from a clear analogy with the most recent extreme variation in global ocean climate we argue that mechanisms responsible for these observed millennial scale perturbations might be found in long term modulations of the atmospheric processes associated with the north atlantic oscillation nao

controls on shell mg ca and sr ca in cultured planktonic foraminiferan globigerinoides ruber white mg ca and sr ca ratios were determined on a single species of planktonic foraminiferan globigerinoides ruber white collected from the gulf of eilat and cultured in seawater at five different salinities 32 to 44 five temperatures 18 to 30 c and four ph values 7 9 to 8 4 the mg ca temperature calibration of cultured g ruber with an exponential slope of 8 3 c agrees well with previously published calibrations from core tops and sediment traps however the dependence of mg ca on salinity with an exponential slope of 5 3 psu is also significant and should be included in the calibration equation with this purpose we calculated a calibration equation for g ruber dependent on both temperature and salinity within the 95 confidence limits mg ca mmol mol exp 0 06 0 02 s psu 0 08 0 02 t c 2 8 1 0 r2 0 95 the influence of ph on mg ca ratios is negligible at ambient seawater ph 8 1 to 8 3 however we observe a dominating ph control on shell mg ca when the ph of seawater is lower than 8 0 sr ca in g ruber shows a significant positive correlation with average growth rate presumably part of the variability in shell sr ca in the geological record is linked to changes in growth rates of foraminifera as a response to changing environmental conditions

changes in the indonesian throughflow during the past 2000 yr the indonesian throughflow itf is the only low latitude conduit for exchange of surface and thermocline waters between the indian and pacific oceans 80 of the itf waters pass through the makassar strait the flux of heat and salt through the strait is in part a function of the density difference between its southern and northern ends here the mg ca and stable oxygen isotope compositions of the planktonic foraminifer globigerinoides ruber sampled from two sediment cores collected at the northern and southern ends of the makassar strait are used to reconstruct surface water temperature sst sea surface salinity sss and seawater density variability over the past 2000 yr maximum sst and sss occurred at both sites between 850 and 700 yr ago coinciding with the medieval solar maximum and medieval warm period ca 1000 700 yr ago sst and sss declined at both locations after 700 yr ago and reached minimum values during the little ice age between 300 and 100 yr ago however the magnitude of sst and sss change over the past 2000 yr was distinctly different between the two sites especially during the period between 2000 and 900 yr ago this led to changes in the south to north density gradient within the makassar strait specifically the time interval from ca 2000 to 800 yr ago was marked predominantly by higher densities at the northern end relative to the southern end of the strait suggesting reduced heat flux from the pacific to the indian ocean this is consistent with previous work showing that this time interval was also marked by frequent el nino conditions conversely the period from ca 800 to 300 yr ago including the lia was marked by frequent periods when surface density was higher at the southern end of the strait indicative of enhanced heat flux to the indian ocean

a pacific centennial oscillation predicted by coupled gcms internal climate variability at the centennial time scale is investigated using long control integrations from three state of the art global coupled general circulation models in the absence of external forcing all three models produce centennial variability in the mean zonal sea surface temperature sst and sea level pressure slp gradients in the equatorial pacific with counterparts in the extratropics the centennial pattern in the tropical pacific is dissimilar to that of the interannual el nino southern oscillation enso in that the most prominent expression in temperature is found beneath the surface of the western pacific warm pool some global repercussions nevertheless are analogous such as a hemispherically symmetric atmospheric wave pattern of alternating highs and lows centennial variability in western equatorial pacific sst is a result of the strong asymmetry of interannual ocean heat content anomalies while the eastern equatorial pacific exhibits a lagged bjerknes like response to temperature and convection in the west the extratropical counterpart is shown to be a flux driven response to the hemispherically symmetric circulation anomalies emanating from the tropical pacific significant centennial length trends in the zonal sst and slp gradients rivaling those estimated from observations and model simulations forced with increasing co 2 appear to be inherent features of the internal climate dynamics simulated by all three models unforced variability and trends on the centennial time scale therefore need to be addressed in estimated uncertainties beyond more traditional signal to noise estimates that do not account for natural variability on the centennial time scale

dynamics of north atlantic deep water masses during the holocene high resolution flow speed reconstructions of two core sites located on gardar drift in the northeast atlantic basin and orphan knoll in the northwest atlantic basin reveal a long term decrease in flow speed of northeast atlantic deep water neadw after 6 500 years benthic foraminiferal oxygen isotopes of sites currently bathed in neadw show a 0 2 depletion after 6 500 years shortly after the start of the development of a carbon isotope gradient between neadw and norwegian sea deep water we consider these changes in near bottom flow vigor and benthic foraminiferal isotope records to mark a significant reorganization of the holocene deep ocean circulation and attribute the changes to a weakening of neadw flow during the mid to late holocene that allowed the shoaling of lower deep water and deeper eastward advection of labrador sea water into the northeast atlantic basin

low latitude hydrological cycle and rapid climate changes during the last deglaciation sea surface temperature and oxygen isotopic records from two well dated indian ocean cores covering the last deglaciation show the occurrence of two periods of increased salinity along the route of warm surface water transport from the indian to the atlantic ocean one between 18 and 14 5 ka and the other during the younger dryas our results imply that during these periods salt accumulated in the tropical atlantic creating favorable conditions for an abrupt resumption of the thermohaline circulation and abrupt northern hemisphere warming furthermore we suggest that the observed pattern of millennial climate variability during the last glacial and deglaciation resulted from the interaction between the relatively slow rhythm of expansion and decay of the northern hemisphere ice sheets and el nino southern oscillation variability through changes in the position of the intertropical convergence zone this interaction generated an oscillator with millennial time response that operated at times of sufficient northern hemisphere ice sheets extent

sedimentology of a fine grained aggrading floodplain abstract historical land use changes accelerated both soil erosion and subsequent floodplain alluviation within the galena river basin wisconsin a cross valley transect in a low gradient downstream section of this watershed permitted documentation of the lateral and vertical sedimentological properties occurring during this aggradational phase mechanical and statistical analyses of 97 sediment samples reveal two dominant controls determining the resulting floodplain sedimentology firstly the initial fine grained loessial texture dominated the particle size of the early post settlement alluvium generally yielding fine textured sediments across the valley bottom further landscape de stabilization promoted upland gullying into coarser textured residuum below the loess with a concomitant particle size increase in the foodplain alluvium thus the resulting foodplain stratigraphy exhibits a coarsening upward sequence secondly distance from the stream channel exerts a strong control on overall sedimentological properties distal settings possess massive high clay content sediments usually with less than 1 sand samples become coarser and more stratified near the modern stream channel with channel fill facies containing approximately 20 sand in the upper sections the fine grained loessial sediment source affected numerous sedimentological properties of the deposited alluvium sorting correlates negatively with particle size because all particle sizes were entrained during large overbank floods dominance of silt in this loess mantled basin maintains the maintains the median of the particle size distribution within a fairly narrow range only the tails of the distribution vary and they are primarily controlled by the distance to the stream channel in distal settings the fine tail emerges whereas the coarse tail appears in the channel fill and proximal settings although all sediments were deposited in an overbank floodplain environment passega s cm diagrams erroneously indicate a pelagic depositional environment

calibration of mg ca thermometry in planktonic foraminifera from a sediment trap time series 1 paired mg ca and 18o measurements on planktonic foraminiferal species g ruber white g ruber pink g sacculifer g conglobatus g aequilateralis o universa n dutertrei p obliquiloculata g inflata g truncatulinoides g hirsuta and g crassaformis from a 6 year sediment trap time series in the sargasso sea were used to define the sensitivity of foraminiferal mg ca to calcification temperature habitat depths and calcification temperatures were estimated from comparison of 18o of foraminifera with equilibrium calcite based on historical temperature and salinity data when considered together mg ca mmol mol of all species except two show a significant r 0 93 relationship with temperature t c of the form mg ca 0 38 0 02 exp 0 090 0 003 t equivalent to a 9 0 0 3 change in mg ca for a 1 c change in temperature small differences exist in calibrations between species and between different size fractions of the same species o universa and g aequilateralis have higher mg ca than other species and in general data can be best described with the same temperature sensitivity for all species and pre exponential constants in the sequence o universa g aequilateralis g bulloides g ruber g sacculifer other species this approach gives an accuracy of 1 2 c in the estimation of calcification temperature the 9 sensitivity to temperature is similar to published studies from culture and core top calibrations but differences exist from some literature values of pre exponential constants different cleaning methodologies and artefacts of core top dissolution are probably implicated and perhaps environmental factors yet understood planktonic foraminiferal mg ca temperature estimates can be used for reconstructing surface temperatures and mixed and thermocline temperatures using g ruber pink g ruber white g sacculifer n dutertrei p obliquiloculata etc the existence of a single mg thermometry equation is valuable for extinct species although use of species specific equations will where statistically significant provide more accurate evaluation of mg ca paleotemperature

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

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

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

evidence from wavelet analysis for a mid holocene transition in global climate forcing a strong mid holocene transition has been identified by wavelet analyses in several sea ice cover records from the circum antarctic area ice core records taylor dome byrd and tropical marine records the results are compared with those previously published in a synthesis of north atlantic records and with 4 new records from the norwegian and icelandic seas and from a coastal site in ireland these new records confirm the previous pattern for the north atlantic area extend this pattern nearly to the arctic circle and include a continental record we further tested the possibility of extending this scheme using continental records from south america the holocene pattern proposed here confirms the importance of external forcing during the early holocene solar activity 1000 years cyclicity and 2500 years during the entire holocene even if the signal is disturbed by meltwater fluxes the second part of the holocene is then marked by the gradual appearance of internal forcing thermohaline circulation around 1500 years associated with a stabilisation of the signal coupling between ocean and atmosphere seems to play a fundamental role in the observed frequencies which vary accordingly in the atlantic circum antarctic and pacific areas the north atlantic area seems to be the instigator of thermohaline circulation as shown by its sensitivity to meltwater discharges during the early holocene even though each sector is independent with regards to its frequency content around 1600 years for atlantic area around 1250 years for antarctica the holocene methane pattern still under debate ruddiman w f 2003a orbital insolation ice volume and greenhouse gases quaternary science review 22 1597 1629 ruddiman w f 2003b the anthropogenic greenhouse era began thousands of years ago climatic change 61 261 293 could be explained by a more efficient thermohaline circulation around the mid holocene with an anthropogenic effect initiated at not vert similar2500 bp as shown by the inter hemispheric gradient

stochastic resonance in the north atlantic large abrupt changes between warm and cold modes of north atlantic climate exhibit spectral power at 1500 years yet some climatic changes were linked to outburst floods and other events that are unlikely to have been truly periodic we hypothesize that a weak periodic forcing has combined with noise from ice sheet related events to cause the observed mode switches this stochastic resonance hypothesis predicts a recurrence pattern between warmings that is distinct from the predictions of simple periodic and stochastic models and at least some other models the ice isotopic data from central greenland ice cores are consistent with the stochastic resonance hypothesis but not with other models we have tested we thus support arguments for the existence of a periodicity of 1500 years in the north atlantic climate system and for the importance of ice sheet events in forcing north atlantic changes

analysis and modelling of gravity and piston coring based on soil mechanics the effects of gravity and piston corers on the dimensional accuracy of marine sediment cores is analysed using principles of soil mechanics a model for the coring process is built around the feedback that arises and develops between the core barrel and the sampled sediment this model for sediment response is applied to different hypothetical coring scenarios which are then compared to real examples providing insights into the specific effects of each sampling method and the development of these effects down core four cores from a single location on the iberian margin are found to contain stratigraphically intact successions that differ in length by a factor of up to 2 7 due solely to the different effects of each coring method these dimensional discrepancies are attributed to the combined effects of over sampling in the upper portions of the piston cores due to cable rebound causing upward piston acceleration and under sampling dominant in the basal portions of the open barrel gravity type cores it is suggested that heavier piston corers deployed on longer lighter cables are prone to greater over sampling ratios over longer stratigraphic intervals due to the increased likelihood and extent of cable rebound cable rebound may also give rise to double penetration of gravity corers resulting in repeated stratigraphic intervals knowledge of the dimensional accuracy of marine sediment cores is essential to an evaluation of past sedimentation rates and hence interpretations of past depositional processes it is therefore essential that we recognise the sampling effects of each coring method and their variability down core lest coring artefacts be interpreted as sedimentary signals different core types may be more suited to different palaeoceanographic investigations hence failing the development of a practical cable deployed recoilless piston corer a combination of a variety of core types will permit the best acquisition of the in situ stratigraphic truth our results suggest that a large diameter dc 20 30 cm square barrel gravity corer for the top 10 12 m combined with a cylindrical piston corer below 10 m may provide the least deformed material

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

the effect of ocean dynamics in a coupled gcm simulation of the last glacial maximum general circulation models gcms of the climate system are powerful tools for understanding and predicting climate and climate change the last glacial maximum lgm provides an extreme test of the model s ability to simulate a change of climate and allows us to increase our understanding of mechanisms of climate change we have used a coupled high resolution ocean atmosphere gcm hadcm3 to simulate the equilibrium climate at the lgm the effect of ocean dynamics is investigated by carrying out a parallel experiment replacing the dynamic three dimensional ocean gcm with a static thermodynamic mixed layer ocean model changes to the ocean circulation and feedbacks between the ocean atmosphere and sea ice have an important influence on the surface response and are discussed the coupled model produces an intensified thermohaline circulation and an increase in the amount of heat transported northward by the atlantic ocean equatorward of 55 n which is at odds with the interpretation of some proxy records such changes which the thermodynamic mixed layer ocean model cannot produce have a large impact around the north atlantic region and are discussed in the study

climate and hydrographic variability in the indo pacific warm pool during the last millennium planktonic foraminiferal mg ca and delta o 18 derived sea surface temperature and salinity records from the makassar strait indonesia show a long term cooling and freshening trend as well as considerable centennial scale variability during the last millennium the warmest temperatures and highest salinities occurred during the medieval warm period mwp while the coolest temperatures and lowest salinities occurred during the little ice age lia these changes in the western pacific along with observations from other high resolution records indicate a regionally coherent southern displacement of the inter tropical convergence zone during the lia with more arid conditions in the northern tropics and wetter conditions in the southern tropics

abrupt onset of the little ice age triggered by volcanism and sustained by sea ice ocean feedbacks 1 northern hemisphere summer temperatures over the past 8000 years have been paced by the slow decrease in summer insolation resulting from the precession of the equinoxes however the causes of superposed century scale cold summer anomalies of which the little ice age lia is the most extreme remain debated largely because the natural forcings are either weak or in the case of volcanism short lived here we present precisely dated records of ice cap growth from arctic canada and iceland showing that lia summer cold and ice growth began abruptly between 1275 and 1300 ad followed by a substantial intensification 1430 1455 ad intervals of sudden ice growth coincide with two of the most volcanically perturbed half centuries of the past millennium a transient climate model simulation shows that explosive volcanism produces abrupt summer cooling at these times and that cold summers can be maintained by sea ice ocean feedbacks long after volcanic aerosols are removed our results suggest that the onset of the lia can be linked to an unusual 50 year long episode with four large sulfur rich explosive eruptions each with global sulfate loading 60 tg the persistence of cold summers is best explained by consequent sea ice ocean feedbacks during a hemispheric summer insolation minimum large changes in solar irradiance are not required

dansgaard oeschger oscillations in a coupled atmosphere ocean climate model abstract a reduced model of the global thermohaline circulation has been asynchronously coupled to a simple energy balance climate model in order to investigate the natural variability of the overturning circulation that may have been characteristic of late glacial conditions previous analyses with the ocean only component of the model have suggested that the nature of the internal variability of the thermohaline circulation was a strong function of the surface boundary conditions on temperature and salinity flux when the boundary conditions were altered from those corresponding to modern conditions to those appropriate to full glacial conditions then the internal variability was shown to be radically transformed under modern conditions the ocean only version of the model delivered an overturning circulation that was only weakly time dependent with a characteristic period of centuries under full glacial boundary conditions however the circulation became strongly time dependent with a characteristic

sea level and deep water temperature changes derived from benthic foraminifera isotopic records we show that robust regressions can be established between relative sea level rsl data and benthic foraminifera oxygen isotopic ratios from the north atlantic and equatorial pacific ocean over the last climatic cycle we then apply these regressions to long benthic isotopic records retrieved at one north atlantic and one equatorial pacific site to build a composite rsl curve as well as the associated confidence interval over the last four climatic cycles our proposed reconstruction of rsl is in good agreement with the sparse rsl data available prior to the last climatic cycle we compute bottom water temperature changes at the two sites and at one southern indian ocean site taking into account potential variations in north atlantic local deep water 18o our results indicate that a last glacial maximum lgm enrichment of the ocean mean oxygen isotopic ratio of 0 95 is the lowest value compatible with unfrozen deep waters in the southern indian ocean if local deep water 18o did not increase during glacials with respect to present such a value of the lgm mean ocean isotopic enrichment would impose a maximum decrease in local bottom water 18o at the north atlantic site of 0 30 during glacials

east asian monsoon forcing of suborbital variability in the sulu sea during marine isotope stage 3 link to northern hemisphere climate 1 we have generated a new high resolution record of variations in planktonic foraminiferal oxygen isotopes 18o and mg ca from a sediment core images 97 2141 in the sulu sea located in the philippine archipelago of western tropical pacific this record reveals distinct suborbital scale 18o changes most notably during marine isotope stage 3 mis3 30 000 to 60 000 years b p the amplitudes of these 18o fluctuations 0 4 to 0 7 exceed that which can be attributed to sea level changes and must be due to changes in sea surface conditions in the same interval variations in planktonic foraminifera mg ca suggest that suborbital surface ocean temperature variations of 1 to 1 5 c in the sulu sea were not in phase with 18o combined this evidence indicates that the mis3 millennial 18o events in the sulu sea were primarily the result of changes in surface water salinity which today is directly related to the east asian monsoon eam and its influence on the balance between surface water contributions from the south china sea and western pacific warm pool wpwp within dating uncertainties the mis3 sulu sea 18o suborbital variability indicates that times of fresher surface conditions in the sulu sea coincide with similar conditions in the wpwp stott et al 2002 and also with intensifications of the summer eam as recorded in the u th dated chinese hulu cave speleothem 18o record wang et al 2001 and thus by inference with interstadials in the greenland ice core records combined these results indicate that pronounced suborbital variability in the summer eam and intertropical convergence zone itcz during mis3 was tightly coupled with climate conditions in the northern high latitudes

lake sediment records of holocene hydroclimate and impacts of the mount mazama eruption north central washington usa abstract multi proxy sediment records from castor lake and scanlon lake north central washington provide a late quaternary perspective on lake catchment hydrologic and ecosystem responses to climate change and the mazama volcanic ashfall event analyses of authigenic carbonate mineral oxygen and carbon isotope values organic carbon and nitrogen content and sedimentological facies were conducted on sediment from castor lake in order to reconstruct lake catchment hydrologic balance the source and abundance of organic matter and variations in lake level sedimentary facies characterization was conducted on the scanlon lake sediment in order to provide supporting evidence for the lake level shifts inferred from the castor lake data marked changes in castor lake proxy values occur from 12 860 to 11 440 calendar years before present yr bp suggesting lake catchment responses to the younger dryas cold reversal including higher lake levels substantial variability in productivity and increased catchment erosion high 18o values at 9630 yr bp indicate that lake levels were lower than at present and that the early holocene was dry sedimentological analyses and 18o data demonstrate that subsequent to the mazama climactic eruption 7600 yr bp a transition to the lowest lake levels of the holocene occurred over several centuries with maximum 18o values at 7290 90 uncertainty range 7020 7500 yr bp and low lake levels persisting until 6190 5960 6410 yr bp lithological changes in the scanlon lake record support these inferences the lowest 18o values of the holocene occur in sediment from 5000 yr bp indicating high lake levels at this time after which a secular trend toward higher 18o values and lower lake levels occurred the prolonged lowstand in the centuries following the mazama eruption suggests that catchment hydrologic characteristics were strongly impacted by the ashfall through effects on soil water retention capacity that similar responses may have occurred in comparable settings within the mazama ashfall zone and that such volcanic events have the potential confound the climate signal in lake sediment records were a similar event to happen in the future catchment hydrology stream river discharge and lake catchment ecosystem dynamics could be affected for centuries

palaeo oceanography deepwater variability in the holocene epoch the conversion of surface water to deep water in the north atlantic results in the release of heat from the ocean to the atmosphere which may have amplified millennial scale climate variability during glacial times1 and could even have contributed to the past 11 700 years of relatively mild climate known as the holocene epoch 2 3 4 here we investigate changes in the carbon isotope composition of benthic foraminifera throughout the holocene and find that deep water production varied on a centennial millennial timescale these variations may be linked to surface and atmospheric events that hint at a contribution to climate change over this period