signature of the gold cove event 10 2 ka in the labrador sea abstract one of the least known catastrophic meltwater and sediment release events of the laurentide ice sheet during the early holocene is the so called gold cove proglacial lake outbursts a drainage event through the hudson strait this event may have disturbed circulation in the labrador sea by releasing freshwater here we report data from four sediment cores and re analysis of six published records spatially covering 2000 km from the mouth of hudson strait to the northeastern sohm abyssal plain abundant detrital carbonate dc and less positive 18o values in neogloboquadrina pachyderma sinistral suggest a short lived meltwater event in the hudson strait region this discharge transported detrital carbonate rich sediment and shallow water benthic forams southward along the labrador margin mixing with the north atlantic current near the grand banks sediments associated with this event were guided through the deep labrador sea from the mouth of hudson strait and the northwest atlantic mid ocean channel namoc to the sohm abyssal plain furthermore we report less positive 13c values in the epibenthic foram cibicidoides wuellerstorfi from the southern labrador sea during this event our twenty one 14c ams dates in addition to earlier published radiocarbon dates from the labrador margin suggest that this dc event was coeval with the gold cove proglacial lake outbursts related to re advance of the laurentide ice sheet

dynamics of decadal variability in the atlantic subpolar gyre a stochastically forced oscillator internal variability of the atlantic subpolar gyre is investigated in a 600 years control simulation of a comprehensive coupled climate model the subpolar gyre shows irregular oscillations of decadal time scale with most spectral power between 15 and 20 years positive and negative feedback mechanisms act successively on the circulation leading to an internal oscillation this involves periodically enhanced deep convection in the subpolar gyre center and intermittently enhanced air sea thermal coupling as a result anomalies of the large scale atmospheric circulation can be transferred to the ocean on the ocean s intrinsic time scale exciting the oscillator stochastically a detailed understanding of oscillatory mechanisms of the ocean and their sensitivity to atmospheric forcing holds considerable potential for decadal predictions as well as for the interpretation of proxy data records

variability of el nino southern oscillation activity at millennial timescales during the holocene epoch the variability of el nino southern oscillation enso during the holocene epoch in particular on millennial timescales is poorly understood palaeoclimate studies have documented enso variability for selected intervals in the holocene but most records are either too short or insufficiently resolved to investigate variability on millennial scales1 2 3 here we present a record of sedimentation in laguna pallcacocha southern ecuador which is strongly influenced by enso variability and covers the past 12 000 years continuously we find that changes on a timescale of 2 8 years which we attribute to warm enso events become more frequent over the holocene until about 1 200 years ago and then decline towards the present periods of relatively high and low enso activity alternating at a timescale of about 2 000 years are superimposed on this long term trend we attribute the long term trend to orbitally induced changes in insolation and suggest internal enso dynamics as a possible cause of the millennial variability however the millennial oscillation will need to be confirmed in other enso proxy records

moisture and temperature changes associated with the mid holocene tsuga decline in the northeastern united states abstract a decline of hemlock tsuga populations at ca 5 5 ka thousands of calibrated radiocarbon years before 1950 ad stands out as the most abrupt vegetation change of the holocene in north america but remains poorly understood after decades of study recent analyses of fossil pollen have revealed a concurrent abrupt oak quercus decline and increases in the abundance of beech fagus and pine pinus on cape cod in eastern massachusetts but the replacement of drought tolerant oaks by moisture sensitive beeches appears inconsistent with low lake levels in the region at the same time the oak and beech changes are also limited to coastal areas and the coastal inland differences require an explanation here we develop a new lake level reconstruction from deep pond cape cod by using a transect of sediment cores and ground penetrating radar gpr profiles to constrain the past elevations of the sandy littoral zone of the pond the reconstruction shows that a series of multi century episodes of low water coincide with the abrupt hemlock and oak declines and interrupt subsequent phases of hemlock recovery the lake level variations equal precipitation deficits of 100 mm superimposed on a holocene long moisture increase of 400 mm however because moisture deficits do not easily explain the oak and beech changes we also evaluate how the climate preferences of the regional vegetation changed over time by matching the fossil pollen assemblages from deep pond with their modern equivalents reconstructions of the precipitation requirements of the vegetation correlate well even in detail with the lake level record r 0 88 at deep pond and indicate close tracking of effective moisture precipitation minus evapotranspiration by the vegetation despite the abrupt species declines which could have decoupled climate and vegetation trends reconstructions of the temperature preferences of the vegetation indicate that coastal sites may have cooled by 0 5 2 5 c after ca 5 5 ka while inland sites warmed by 0 5 1 c the change in coastal temperature preferences agrees with sea surface cooling in the western atlantic ocean of 1 c consequently the persistence of low hemlock abundance after 5 5 ka in the northeast u s may have resulted from oceanic changes that produced multi century droughts and thus delayed the post decline recovery of hemlock populations

evidence for a reduction in the carbonate ion content of the deep sea during the course of the holocene the paleo carbonate ion proxy proposed by broecker et al 1999 is applied in a search for trends in the holocene acidity of waters in the transition zone between north atlantic deep water and antarctic bottom water aabw a clear signal emerges that the carbonate ion content of waters in this zone declined during the past 8000 years in order to determine whether this decline represents a strengthening of the northward penetrating tongue of low co3 content aabw or a global reduction of co3 ion measurements were made on a core from the ontong java plateau in the western equatorial pacific evidence for a similar decline in co3 ion over the course of the holocene was obtained lending support of the latter explanation such a drop is consistent with the recent finding by indermuhle et al 1999 that the co2 content of the atmosphere as recorded in the taylor dome antarctica ice core rose by 20 25 ppm during the past 8000 years

surface melt driven laurentide ice sheet retreat during the early holocene received 21 september 2009 revised 3 november 2009 accepted 30 november 2009 published 30 december 2009 1 to better understand mechanisms of ice sheet decay we investigate the surface mass balance of the laurentide ice sheet lis during the early holocene a period of known rapid lis retreat we use a surface energy mass balance model embm driven with conditions derived from an equilibrium atmosphere ocean general circulation model 9 kilo years ago simulation our embm indicates a net lis surface mass balance of 0 67 0 13 m yr 1 with losses primarily due to enhanced boreal summer insolation and warmer summers this rate of loss compared to lis volume reconstructions suggests that surface ablation accounted for 74 22 of the lis mass loss with the remaining loss likely driven by dynamics resulting in basal sliding and calving thus surface melting likely played a governing role in the retreat and disappearance of this ice sheet citation carlson a e f s anslow e a obbink a n legrande d j ullman and j m licciardi 2009 surface melt driven laurentide ice sheet retreat during the early holocene geophys res lett 36 l24502 doi 10 1029 2009gl040948

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

coupled north atlantic slope water forcing on gulf of maine temperatures over the past millennium to investigate ocean variability during the last millennium in the western gulf of maine gom we collected a 142 year old living bivalve arctica islandica l in 2004 and three fossil a islandica shells calibrated 14cams 1030 78 ad 1320 45 ad 1357 40 ad for stable isotope and growth increment analysis a statistically significant relationship exists between modern gom temperature records shell isotope derived 30 m r 0 79 p 0 007 prince 5 50 m r 0 72 p 0 019 boothbay harbor sst r 0 76 p 0 011 and labrador current lc transport data from the eastern newfoundland slope during 1993 2003 in all cases as lc transport increased gom water temperatures decreased the following year decadal trends in the north atlantic oscillation nao and the atlantic multidecadal oscillation amo influence gom water temperatures in the most recent period with water temperatures decreasing during nao and amo negative modes most likely linked to lc transport and gulf stream interaction mean shell derived isotopic changes 18oc during the last 1 000 years were 0 47 and likely reflect a 1 2 c cooling from 1000 ad to present based on these results we suggest that observed cooling in the gom during the last millennium was due to increased transport and or cooling of the lc and decreased gulf stream influence on the gom

alkenone paleothermometry in the north atlantic a review and synthesis of surface sediment data and calibrations despite a clear correlation of alkenone unsaturation and sea surface temperatures sst throughout most parts of the ocean scatter of the regression for various calibration equations has been shown to increase significantly at low ssts in this study we combine previously published n 101 and new n 51 surface sediment data from the northern north atlantic to constrain uncertainties of alkenone paleothermometry at low ssts and to discuss possible sources of the increased scatter in the regression the correlation between alkenone unsaturation and ssts is strongest in particular at the cold end ssts 10 c when the tetra unsaturated alkenones c37 4 are included in the unsaturation index expressed as inline image and regressed against spring summer temperature surface ocean salinity and sea ice cover are not correlated with inline image per se however samples located in regions of permanent winter sea ice cover exhibit a significant warm bias deviation from the linear regression is posited to be related to a number of additional non exclusive factors such as advection of allochthonous material local temperature stratification and uncertainty in the absolute age of surface sediment samples assumed to be equivalent to modern conditions we conclude that alkenone unsaturation allows accurate reconstruction of sst records from many regions of the north atlantic if the factors confounding alkenone paleothermometry detailed here can be excluded

low oxygen events in the laurentian channel during the holocene geochemical and micropaleontological analyses were carried out on a 35 cm box core cr06 tce spanning the last 6000 years in the esquiman channel a northeast arm of the laurentian channel in the gulf of st lawrence a 0 6 decrease of 18o in benthic foraminifer globobulimina auriculata shells characterizes the upper 10 cm of the core and suggests a warming of the bottom waters this change is concomitant with increased percentages of the low oxygen tolerant benthic foraminifer species brizalina subaenariensis and the atlantic water species oridorsalis umbonatus although a precise timing cannot be established notably because of the smoothing effect of bioturbation the amplitude of the trend recorded in the esquiman channel is coherent with that of the regional warming observed in the bottom water of the main axis of the laurentian channel over the last century warm bottom water conditions however are not exclusive to the recent time interval as shown by data from the lower part of the core which are also characterized by low 18o values in g auriculata and occurrence of both b subaenariensis and o umbonatus such data suggest the existence of low oxygen and relatively high temperature conditions in the bottom water of the esquiman channel about 4 to 6 kyrs ago likely related to enhanced inflow of atlantic water in the gulf of st lawrence through the cabot strait and the laurentian channel these results highlight the sensitivity of bottom water properties in the gulf of st lawrence to changes in the western north atlantic circulation

early holocene large scale meltwater discharge from greenland documented by foraminifera and sediment parameters records of foraminiferal assemblages combined with lithological properties grain size magnetic parameters and xrf data of marine sediment cores from west greenland coastal waters and the adjacent labrador sea document widespread early holocene meltwater discharge this discharge is concluded to originate from large scale melting of the greenland ice sheet gis having started prior to 8600 cal yr bp and ended at about 7700 7500 cal yr bp when the gis margin had withdrawn from the fjords and become mainly land based the benthic foraminiferal record from one of the coastal sites mainly reflects west greenland current wgc subsurface water properties and to a minor degree surface productivity the most significant feature in this record is an abrupt shift to a higher productivity regime around 7700 cal yr bp we suggest that the cessation of a widespread gis meltwater discharge at that time favoured an increased influence of sub surface water of atlantic origin and initiation of modern subpolar gyre circulation enabling labrador sea deep convection further offshore a record of planktonic foraminiferal assemblages shows an oceanographic change at ca 9500 cal yr bp whilst a gradual but marked change in the planktonic foraminiferal assemblage between 8800 and 7000 cal yr bp may be related to a narrowing of the wgc low salinity surface water belt the oceanic regime off west greenland prior to 7800 cal yr bp was thus characterised by the presence of a permanent and widespread meltwater surface layer presumably preventing deep convection in this region apart from indications of a slight decrease in meltwater discharge by the benthic foraminiferal fauna data neither of the records show any clear signal of a regionally important 8 2 ka event

late quaternary sedimentation in the western north atlantic stratigraphy and paleoceanography abstract a detailed analysis of stratigraphic indices was undertaken to study paleoclimatic change in the western north atlantic three stratigraphic indicators carbonate content globorotalia menardii and uvigerina peregrina were tested in ten radiocarbon dated cores from the north american continental margin of these carbonate content appears to be most accurate carbonate content exhibits two major increases from late glacial time to the present estimated ages of the average of the mid point of these increase are 13 200 b p and 8600 b p an absolute chronology was established down fifteen cores using these stratigraphic indicators and the radiocarbon dates sea surface temperature sst estimates derived from planktonic foraminifera were mapped at 2000 yr intervals from 2000 b p to 14 000 b p sst warms rapidly from 12 000 to 8000 b p reaches a maximum between 8000 and 6000 b p and gradually cools after 6000 b p bottom waters were monitored with uvigerina peregrina and dissolution indices the disappearance of uvigerina suggests that the water type it lives in low o 2 started to thin from both the top and bottom by 13 000 b p and by 7300 b p had disappeared fragmentation of planktonic foraminifera was used as an index of dissolution in most cores dissolution was intense during late glacial time lessened during early postglacial time and resumed in the late postglacial

the holocene labrador current changing linkages to atmospheric and oceanographic forcing factors the role of labrador current lc in providing freshwater to the north atlantic and hence modulating the atlantic meridional overturning circulation amoc has been debated with very few data this study provides the first data on the relative speed of the lc for the past 9 2 ka using mean size of the sortable silt of sediments as a proxy from a mid shelf basin in the western labrador sea lc speed progressively intensified from 9 2 5 ka during which two meltwater events had minor influence the lc was fastest between 5 ka and 3 1 ka and gradually weakened after 3 1 ka the mean size of the sortable silt data shows overall co variations with similar existing data from deep water areas of the greater north atlantic suggesting an overarching link that modulates changes in both the near surface and deep north atlantic a dynamical link between the lc and amoc vigor is hypothesized in which high meltwater discharge in the labrador sea oriented the subpolar gyre spg more north south meridional weakening the spg and thus decelerating the upper limb of the amoc conversely reduction in lc transport during the late holocene due to the greater export of sea ice oriented the spg more east west

holocene temperature and salinity variability of the atlantic water inflow to the nordic seas piston core link14 from 346 m water depth on the eastern shelf of the faroe islands in the faroe shetland channel has been investigated in order to reconstruct temperature and salinity changes of the atlantic surface water during the holocene we have analyzed the distribution of benthic and planktic foraminifera stable isotopes and calculated by transfer functions absolute surface subsurface and bottom water temperatures and salinities the summer sea surface temperature sst shows a stepwise decrease from an early holocene warm phase 10 300 8300 yr bp with temperatures around 12 c over a mid holocene cooler phase 8300 4000 yr bp to a late holocene relatively cool phase with sst around or slightly below 11 c bottom water temperatures at 346 m water depth show much less variation than the sst the late holocene sea surface cooling was probably caused by increase in the influence of the polar east icelandic current a distinct decrease in the subsurface salinity from about 8300 to 6800 yr bp is proba

a holocene record of atmospheric dust deposition on the penny ice cap baffin island canada a holocene record of atmospheric dust deposition on the penny ice cap baffin island canada

late holocene variability of upper north atlantic deep water temperature and salinity 1 magnesium calcium ratios in benthic foraminifera cibicidoides pachyderma from a sediment core on the laurentian slope 1854 m exhibit strong millennial scale fluctuations during the past 4000 years we convert these data to seawater paleotemperatures using a new monospecific linear equation results suggest that the temperature of upper north atlantic deep water dominated by labrador seawater today has varied by at least 2 c during the late holocene millennial scale coolings coincide with previously identified periods of increased drift ice and regional glacier advances including the little ice age paired oxygen isotope measurements indicate that salinity and perhaps density were reduced during the cold periods we discuss possible mechanisms for transmitting this cold fresh signal from surface waters to intermediate depths our reconstructed late holocene ranges in upper north atlantic deep water properties greatly exceed those of the instrumental record and imply that large changes may be yet to come

on the coherency of ocean and land surface specular scattering for gnss r and signals of opportunity systems an analysis of the coherency properties of specular scattering from ocean and land surfaces as observed in global navigation satellite system reflectometry gnss r and signals of opportunity systems is presented this analysis applies existing approximate models for the coherent and incoherent contributions approximate expressions are developed for when one component of the return is likely to dominate as a function of surface and observing system properties the model developed is then applied for sea surface returns and the relative contribution of the coherent term is expressed as a function of the receiver height frequency incidence angle and wind speed for l band spaceborne measurements it is shown that coherence is expected only for wind speeds less than 2 3 m s while for p band spaceborne measurements coherence can dominate returns for wind speeds up to 5 7 m s for land surface measurements from space it is shown that the surface rms height needs to be sufficiently low for coherent components to dominate returns coherence dominates for roughness values not exceeding a range of 5 7 cm for the l band and 15 30 cm for the p band for the l band these conditions over land are likely to be created primarily by inland water bodies a model for the specular scattering from a water body including earth curvature effects is then developed to highlight the strong dependence of the resulting coherent field on the shape of the water body and any offset in its location from the specular point these results further clarify the significant variability that should be expected in coherent scattering from inland water bodies

possible reverse trend in asian summer monsoon strength during the late holocene holocene climate change is characterized as generally cooling in high latitudes and drying in tropical and asian summer monsoonal regions following the gradual decrease in northern hemisphere summer insolation over the last 12 000 years however some recent high resolution well dated monsoon reconstructions seem to suggest an abnormal increase in asian summer monsoon strength during the late holocene against the generally weakening holocene trend here we synthesize marine and terrestrial moisture records from asian monsoonal regions that span most of the holocene period late holocene strengthening of asian summer monsoon identified from a wealth of the synthesized monsoon records appears to be a robust feature which warrants further consideration of its possible causes the possible reverse trend in asian summer monsoon strength preceding insolation minima seems to have also occurred during previous interglacial periods based on speleothem records we further show a similar late holocene reverse trend in tropical hydrological changes suggesting that the asian summer monsoon behavior might be internally linked to the movement of the average position of the itcz and enso variability during the late holocene on the other hand we suggest that even though several holocene temperature records indeed show a reverse trend in the late holocene the overall evidence for a link between the late holocene reverse trend in asian summer monsoon and global temperature changes is insufficient the reverse trend in asian summer monsoon during the late holocene is difficult to be explained with the traditional boreal insolation driven view we suggest that this phenomenon might be linked to austral summer insolation changes and or greenhouse gas increase however we caution that additional paleoclimate reconstructions and model simulations are needed to systematically study the spatial pattern and understand underlying mechanism of the late holocene reverse trend in asian summer monsoon strength

sea ice detection using u k tds 1 gnss r data a sea ice detection algorithm developed using the u k techdemosat 1 u k tds 1 global navigation satellite systems gnsss reflectometry data over the arctic and antarctic regions is presented it is based on measuring the similarity of the received gnss reflected waveform or delay doppler map ddm to the coherent reflection model waveform over the open ocean the scattered signal has a diffusive incoherent nature it is described by the rough surface scattering model based on the geometric optics and the gaussian statistics for the ocean surface slopes over sea ice and in particular newly formed sea ice the scattered signal acquires a coherence which is characteristic for a surface with large flat areas in order to measure the similarity of the received waveform or ddm to the coherent reflection model three different estimators are presented the normalized ddm average the trailing edge slope tes and the matched filter approach here a probabilistic study is presented based on a bayesian approach using two different and independent ground truth data sets this approach allows one to thoroughly assess the performance of the estimators the best results are achieved for both the tes and the matched filter approach with a probability of detection of 98 5 a probability of false alarm of 3 6 and a probability of error of 2 5 however the matched filter approach is preferred due to its simplicity data from amsr2 processed using the arctic radiation and turbulence interaction study sea ice algorithm and from an special sensor microwave imager sounder radiometer processed by ocean and sea ice saf have been used as ground truth a pixel has been classified as a sea ice pixel if the sea ice concentration sic in it was larger than 15 the measurement of the sic is also assessed in this paper but the nature of the u k tds 1 data lack of calibrated data does not allow to make any specific conclusions about the sic

contrasting trends in north atlantic deep water formation in the labrador sea and nordic seas during the holocene the holocene north atlantic deep water formation is studied in a 9 000 year long simulation with a coupled climate model of intermediate complexity forced by changes in orbital forcing and atmospheric trace gas concentrations during the experiment deep water formation in the nordic seas is reduced due to an enhanced influx of sea ice from the central arctic decreasing both surface salinity and density whereas deep water formation in the labrador sea increases due to surface cooling this leads to changes in the distribution of oceanic heat transported northwards by the atlantic ocean with less heat released 120 wm 2 in february in the nordic seas amplifying the surface cooling and increasing the sea ice cover in the labrador sea the oceanic heat release increases slightly 14 wm 2 thus dampening locally the cooling trend the overall atlantic overturning strength remains constant throughout the experiment over the nordic seas reduced evaporation contributes to the surface freshening

reconstruction of sea surface temperature salinity and sea ice cover in the northern north atlantic during the last glacial maximum based on dinocyst assemblages past sea surface conditions over the northern north atlantic during the last glacial maximum were examined from the study of 61 deep sea cores the last glacial maximum time slice studied here corresponds to an interval between heinrich layers h2 and h1 and spanningabout 20 16 ka on a 14c time scale transfer functions based on dinocyst assemblages were used to reconstruct sea surface temperature salinity and sea ice cover the results illustrate extensive sea ice cover along the eastern canadian margins and sea ice spreading only during winter over most of the northern north atlantic on the whole much colder winter prevailed despite relatively mild conditions in august 10 15 c at most offshore sites thus suggesting a larger seasonal contrast of temperatures than today lower salinity than at present is reconstructed especially along the eastern canadian and scandinavian margins likely because of meltwater supply from the surrounding ice sheets these reconstructions contrast with those established by climap on the basis of planktonic foraminifera these differences are discussed with reference to the stratigraphical frame of the last glacial maximum which was not the coldest phase of the last glacial stage the respective significance of dinocyst and foraminifer records is also examined in terms of the thermohaline characteristics of surface waters and the vertical structure of upper water masses which was apparently much more stratified than at present in the northern north atlantic thus preventing deep water formation

mid to late holocene strengthening of the east greenland current linked to warm subsurface atlantic water the relatively fresh and cold east greenland current egc connects the arctic with the subpolar north atlantic ocean its strength and influence on the freshwater balance in the north atlantic affects both the subpolar gyre dynamics and deep convection in the labrador sea enhanced freshwater and sea ice expansion in the subpolar north atlantic is suggested to modify the northward heat transport within the north atlantic current high resolution palaeoceanographic reconstructions based on planktic and benthic foraminifera assemblage data from the central east greenland shelf foster bugt reveal distinct centennial to millennial scale oceanographic variability that relates to climatic changes during the mid to late holocene the last c 6 3 ka bp our data highlight intervals of cooling and freshening of the polar surface egc waters that accompany warming in the subsurface atlantic waters which are a combination of chilled atlantic intermediate water aiw from the arctic ocean and of the return atlantic current rac from the west spitsbergen current wsc mid holocene thermal optimum conditions prevailed until c 4 5 ka bp a thin absent surface polar water layer low drift sea ice occurrence and strong contribution of recirculating warm atlantic waters at the subsurface suggest a relatively weak egc during this period subsequently between 1 4 and 4 5 ka bp the water column became well stratified as the surface polar water layer thickened and cooled indicating a strong egc this egc strengthening parallelled enhanced subsurface chilled aiw contribution from the arctic ocean after c 4 5 ka bp which culminated from 1 4 to 2 3 ka bp this coincides with warming identified in earlier work of the north atlantic current the irminger current and the west greenland current we link the enhanced contribution of chilled atlantic water during this period to the time of the roman warm period the observed warming offshore east greenland centred at c 1 8 ka bp likely occurred in response to changes in the interactions of i a weakened subpolar gyre ii increased northward heat advection in the north atlantic current and iii a predominant positive north atlantic and arctic oscillation mode prevailing during the time of the roman warm period

holocene sea surface conditions in the western north atlantic spatial and temporal heterogeneities 1 holocene records of sea surface conditions in the western nordic seas were obtained from quantitative reconstructions based on dinoflagellate cyst assemblages two sediment cores from the east greenland and the north iceland shelves provide a detailed account of the long and short term dynamics of the opposing flows of arctic versus atlantic waters both marker species and quantitative reconstructions depict an overall trend toward warmer winter temperatures saltier surface waters and decreased sea ice extent the latter is supported by a close relationship between relative abundances of an atlantic dinocyst species nematosphaeropsis labyrinthus and ice rafted debris ird records we propose that the late holocene increased ird delivery in the denmark strait region was primarily induced by a combination of less extensive sea ice cover under increased atlantic water inflow and sustained iceberg calving tied to the readvance of the greenland ice sheet our records thus suggest diminishing polar water supplies throughout the holocene although the timing of regime changes differs between the western and eastern sides of the denmark strait finally comparison of our records with a core from southern greenland points to very heterogeneous sea surface conditions in the western north atlantic which could be explained by the decoupled dynamics of the two irminger current branches similarities between the southern greenland marine record and a continental record nearby suggest a close coupling with atmospheric processes reminiscent of a north atlantic oscillation like climate pattern

evidence from driftwood records for century to millennial scale variations of the high latitude atmospheric circulation during the holocene different holocene sea ice drift patterns in the arctic ocean have been hypothesized by dyke et al from radiometric analyses of driftwood collected in the canadian arctic archipelago a dynamic thermodynamic sea ice model is used to simulate the modes of arctic ocean ice circulation for different atmospheric forcings and hence determine the atmospheric circulations which may have accounted for the inferred ice drift patterns the model is forced with the monthly mean wind stresses from 1968 a year with very large ice export and 1984 very low ice export two years with drastically different winter sea level pressure patterns and with different phases of the nao index the simulations show that for the 1968 wind stresses a weak beaufort gyre with a broad transpolar drift stream tds shifted to the east are produced leading to a large ice export from the arctic similarly the 1984 wind stresses lead to an expanded beaufort gyre with a weak tds shifted to the west and a low ice export these results correspond to the patterns inferred by dyke et al based on the simulations the driftwood record suggests that for centuries to millennia during the holocene the high latitude average atmospheric circulation may have resembled that of 1968 1984 and today s climatology with abrupt changes from one state to the other

apparent long term cooling of the sea surface in the northeast atlantic and mediterranean during the holocene reconstructions of upper ocean temperature t during the holocene 10 0 ka b p were established using the alkenone method from seven high accumulation sediment cores raised from the northeast atlantic and the mediterranean sea 361n 751n all these paleo t records document an apparent long term cooling during the last 10 kyr in records with indication of a constant trend the apparent cooling ranges from 0 27 to 0 151c kyr 1 records with indication of time variable trend show peak to peak amplitudes in apparent temperatures of 1 2 2 91c a principal component analysis shows that there is one factor which accounts for a very large fraction 67 of the total variance in the biomarker paleo t records and which dominates these records over other potential secondary influences two possible contributions are 1 a widespread surface cooling which may be associated with the transition fromthe hypsithermal interval b9 5 7 ka b p to the neoglaciation b5 7 0 ka b p and 2 a change in the seasonal timing and or duration of the growth period of alkenone producers prymnesiophyte algae the first contribution is consistent with many climate proxy records from the northeast atlantic area and with climate model simulations including milankovitch forcing the second contribution is consistent with the divergence between biomarker and summer faunal paleo t fromearly to late holocene observed in two cores further work is necessary and in particular the apparent discordance between biomarker and faunal t records for the relative stable holocene period must be understood to better constrain the climatic and ecological contributions to the apparent cooling observed in the former records r 2002 elsevier science ltd all rights reserved

estimating the regional climate signal in a late pleistocene and early holocene lake sediment δ18o record from vermont usa abstract we present a new oxygen isotope 18o record from carbonate rich lake sediments from central vermont the record from twin ponds spans from 13 5 cal ka bp 1950 ad to present but contains a 6 ka long hiatus starting shortly after 7 5 cal ka bp we compare the record for ca 13 5 7 5 cal ka bp with published 18o data from the region after using a bayesian approach to produce many possible chronologies for each site principal component analysis then identified chronologically robust multi site oxygen isotope signals including negative values during the younger dryas but no significant deviations from the early holocene mean of the regional records however differences among sites indicate significant trends that likely relate to interacting changes in the regional gradients of seasonal temperatures and precipitation as well as moisture sources moisture pathways and aridity that were controlled by large scale climatic controls such as insolation the progressive decline of the laurentide ice sheet and changes in oceanic circulation centennial shifts punctuate these trends at ca 9 3 and 8 2 cal ka bp and reveal that the local character of these short lived features requires a detailed understanding of lake hydrology and regional isotopic gradients to yield reliable information for regional climate reconstructions

holocene and eemian sea surface temperature trends as revealed by alkenone and mg ca paleothermometry in this study we review a global set of alkenone and foraminiferal mg ca derived sea surface temperatures sst records from the holocene and compare them with a suite of published eemian sst records based on the same approach for the holocene the alkenone sst records belong to the actualized ghost database kim j h schneider r r 2004 ghost global database for alkenone derived holocene sea surface temperature records available from http www pangaea de projects ghost while the mg ca derived sst database represents a new compilation the actualized ghost database not only confirms the sst changes previously described but also documents the holocene temperature evolution in new oceanic regions such as the northwestern atlantic the eastern equatorial pacific and the southern ocean a comparison of holocene sst records stemming front the two commonly applied paleothermometry methods reveals contrasting sometimes divergent sst evolution particularly at low latitudes where sst records are abundant enough to infer systematic discrepancies at a regional scale opposite sst trends at particular locations could be explained by out of phase trends in seasonal insolation during the holocene this hypothesis assumes that a strong contrast in the ecological responses of coccolithophores and planktonic foraminifera to winter and summer oceanographic conditions is the ultimate reason for seasonal differences in the origin of the temperature signal provided by these organisms as a simple test for this hypothesis eemian sst records are considered because the holocene and eemian time periods experienced comparable changes in orbital configurations but had a higher magnitude in insolation variance during the eemian for several regions sst changes during both interglacials were of a similar sign but with higher magnitudes during the eemian as compared to the holocene this observation suggests that the ecological mechanism shaping sst trends during the holocene was comparable during the penultimate interglacial period although this ecology hypothesis fails to explain all of the available results we argue that any other mechanism would fail to satisfactorily explain the observed sst discrepancies among proxies c 2010 elsevier ltd all rights reserved

application of the cbω model to the calculation of diffusion parameters of si in silicates silicon diffusion in major mantle minerals plays an important role in understanding a number of physical and chemical processes in the earth s interior inspection of existing experimental data reveals linear compensation law between the preexponential factors and the activation energies for si diffusion in various minerals by focusing on those of geophysical interest on the basis of the observed compensation relationship here we propose a thermodynamic model the so called cb model that interconnects point defect parameters with the bulk properties to reproduce the si self diffusion coefficients in different rock forming minerals when the uncertainties are considered the predicted results show that the temperature and pressure dependences of self diffusion coefficients concur with existing experimental data and theoretical calculations in most cases

analysis of scattering characteristics from inland bodies of water observed by cygnss abstract measurements made by spaceborne global navigation satellite system reflectometry gnss r instruments have shown strong reflected power over inland waters that has been attributed to coherent scattering coming from the first fresnel zone previous work in the field has shown the ability of gnss r to observe the global surface water distribution by generating dynamic maps of wetlands and other inundated areas these maps can be generated by leveraging the large difference in received power of the gnss signals as they reflect from water surfaces compared to land in this paper we utilize a full forward scattering model approach to evaluate the accuracy of these maps the cygnss end to end simulator e2es was extended to include the contributions from coherent surface scattering in heterogeneous regions where the area around the specular point contains both water and land in complex geometries the simulator is then used to analyze the accuracy of a current approach to estimate the surface water content in the first fresnel zone from a single measurement called a fractional water in footprint approach we find that contributions to the total received power by the scattering from outside the first fresnel zone as well as cygnss instrument effects impact the accuracy of this approach furthermore the measured signal from calibration scattering targets is compared to the results of simulation to validate the scattering model the work shows that the variability of the peak reflected power over inland bodies of water due to the scene heterogeneities should be accounted for in designing retrieval algorithms to map wetland extent

effect of sea roughness on bistatically scattered range coded signals from the global positioning system a series of aircraft experiments was performed using a specialized gps receiver and a nadir oriented left hand circularly polarized antenna this apparatus received reflections of the gps signals from water surfaces under a variety of sea states the cross correlation between the reflected signal and a reference pseudo random noise code was recorded as a function of the relative time delay the shape of this function showed a dependence on the roughness of the reflecting surface this dependence generally followed that predicted by theory for bistatic scattering of range coded signals use of this information as a remote sensing technique for the determination of sea state is discussed

reconciling divergent trends and millennial variations in holocene temperatures cooling during most of the past two millennia has been widely recognized and has been inferred to be the dominant global temperature trend of the past 11 700 years the holocene epoch however long term cooling has been difficult to reconcile with global forcing and climate models consistently simulate long term warming the divergence between simulations and reconstructions emerges primarily for northern mid latitudes for which pronounced cooling has been inferred from marine and coastal records using multiple approaches here we show that temperatures reconstructed from sub fossil pollen from 642 sites across north america and europe closely match simulations and that long term warming not cooling defined the holocene until around 2 000 years ago the reconstructions indicate that evidence of long term cooling was limited to north atlantic records early holocene temperatures on the continents were more than two degrees celsius below those of the past two millennia consistent with the simulated effects of remnant ice sheets in the climate model community climate system model 3 ccsm3 ccsm3 simulates increases in growing degree days a measure of the accumulated warmth above five degrees celsius per year of more than 300 kelvin days over the holocene consistent with inferences from the pollen data it also simulates a decrease in mean summer temperatures of more than two degrees celsius which correlates with reconstructed marine trends and highlights the potential importance of the different subseasonal sensitivities of the records despite the differing trends pollen and marine based reconstructions are correlated at millennial to centennial scales probably in response to ice sheet and meltwater dynamics and to stochastic dynamics similar to the temperature variations produced by ccsm3 although our results depend on a single source of palaeoclimatic data pollen and a single climate model simulation they reinforce the notion that climate models can adequately simulate climates for periods other than the present day they also demonstrate that amplified warming in recent decades increased temperatures above the mean of any century during the past 11 000 years

reconciling discrepancies between uk37 and mg ca reconstructions of holocene marine temperature variability significant discrepancies exist between the detrended variability of late holocene marine temperatures inferred from mg ca and uk37 proxies with the former showing substantially more centennial scale variation than the latter discrepancies exceed that attributable to differences in location and persist across various calibrations indicating that they are intrinsic to the proxy measurement we demonstrate that these discrepancies can be reconciled using a statistical model that accounts for the effects of bioturbation sampling and measurement noise and aliasing of seasonal variability the smaller number of individual samples incorporated into mg ca measurements relative to uk37 measurements leads to greater aliasing and generally accounts for the differences in the magnitude and distribution of variability an inverse application of the statistical model is also developed and applied in order to estimate the spectrum of marine temperature variability after correcting for proxy distortions the correction method is tested on surrogate data and shown to reliably estimate the spectrum of temperature variance when using high resolution records applying this inverse method to the actual mg ca and uk37 data results in estimates of the spectrum of temperature variance that are consistent this approach provides a basis by which to accurately estimate the distribution of intrinsic marine temperature variability from marine proxy records

holocene water mass changes in the labrador current the labrador current is part of the anticlockwise subpolar gyre and plays a major role in the formation of north atlantic deep water it is influenced by the west greenland and baffin currents supp

scattering of gps signals from the ocean with wind remote sensing application a theoretical model that describes the power of a scattered global positioning system gps signal as a function of geometrical and environmental parameters has been developed this model is based on a bistatic radar equation derived using the geometric optics limit of the kirchhoff approximation the waveform i e the time delayed power obtained in the delay mapping technique depends on a wave slope probability density function which in turn depends on wind waveforms obtained for aircraft altitudes and velocities indicate that altitudes within the interval 5 15 km are the best for inferring wind speed in some regimes an analytical solution for the bistatic radar equation is possible this solution allows converting trailing edges of waveforms into a set of straight lines which could be convenient for wind retrieval a transition to satellite altitudes together with satellite velocities makes the peak power reduction and the doppler spreading effect a significant problem for wind retrieval based on the delay mapping technique at the same time different time delays and different doppler shifts of the scattered gps signal could form relatively small spatial cells on sea surface suggesting mapping of the wave slope probability distribution in a synthetic aperture radar sar fashion this may allow more accurate measurements of wind velocity and wind direction

high resolution holocene palynological record from the scotian shelf abstract because of their location at the confluence of polar and subtropical airmasses and near a transition zone between the cold labrador current and the gulf stream the atlantic provinces experience some of the most dynamic climate conditions in canada major climate changes occurred during the holocene as shown by pollen records from lakes but previous paleoceanographic studies based on low resolution proxy data records do not show major changes during the past 8000 years therefore the holocene history of canada s atlantic region was examined using a high resolution palynological record from the scotian shelf la have basin sea surface conditions were reconstructed using proxy data from dinoflagellate cysts and paleobioclimatic transfer functions ocean atmosphere interactions are determined by onshore offshore correlation of marine and pollen records from nova scotia results show a succession of major paleoceanographic events sea surface temperatures sst february and august up to 5 c higher than today s average and slightly higher salinity are reconstructed between 10 5 and 8 5 ka the last pulse of meltwater from the residual ice sheet affected the shelf waters between 8 5 and 6 5 ka by lowering the sst in february and the salinity most previous studies failed to record this event since 6 5 ka august temperature generally remained around today s value while february temperature was generally 2 c above it except for recurring colder and lower salinity intervals these cold intervals have a recurrence of about 1000 years a slight cooling of summer sst is recorded in the last 500 years comparison with climatic reconstructions from nova scotia pollen records shows a difference in timing between ocean and atmosphere the onset of the climatic optimum hypsithermal in nova scotia lags by about 2000 years until 8 ka relative to the ocean but it lasted longer the neoglacial cooling in atlantic canada however started earlier 2 ka than the late holocene ocean cooling la have basin s holocene paleoceanography presents some differences from most other studies from the region 1 the hypsithermal started earlier 2 the last pulse of meltwater is recorded 3 the last 6500 years are punctuated by colder intervals increased productivity and blooms of toxic algae in the early holocene are probably due to a combination of factors increased nutrients and a greater stability of the water column because of meltwaters higher sst and increased upwelling or storm activity

multicentennial variability of the sea surface temperature gradient across the subpolar north atlantic over the last 2 8 kyr a 2800 year long august sea surface temperature asst record based on fossil diatom assemblages is generated from a marine sediment core from the northern subpolar north atlantic the record is compared with the asst record from the norwegian sea to explore the variability of the asst gradient between these areas during the late holocene the asst records demonstrate the opposite climate tendencies towards a persistent warming in the core site in the subpolar north atlantic and cooling in the norwegian sea at the multicentennial scale of asst variability of 600 900 years the records are nearly in anti phase with warmer colder periods in the subpolar north atlantic corresponding to the colder warmer periods in the norwegian sea at the shorter time scale of 200 450 years the records display a phase locked behaviour with a tendency for the positive asst anomalies in the norwegian sea to lead by 30 years the negative asst anomalies in the subpolar north atlantic this apparent asst seesaw might have an effect on two major anomalies of the european climate of the past millennium medieval warm period mwp and the little ice age lia during the mwp warming of the sea surface in the norwegian sea occurred in parallel with cooling in the northern subpolar north atlantic whereas the opposite pattern emerged during the lia the results suggest that the observed asst seesaw between the subpolar north atlantic and the norwegian sea could be a surface expression of the variability of the eastern and western branches of the atlantic meridional circulation amoc with a possible amplification through atmospheric feedback

a high resolution hydrogen isotope record of behenic acid for the past 16 kyr in the northeastern united states abstract d h ratios of leaf lipids from lacustrine aquatic macrophytes have been shown to record precipitation d values which can reflect precipitation weighted mean annual temperature mat in the northeastern united states here we report a high resolution hydrogen isotopic record from little pond massachusetts usa which we compare with other paleoclimate data from the region including a similar d record from blood pond massachusetts together the two datasets provide a 16 ka record of d variability in the region affording new insights into holocene climate history first the long term trends in d correlate significantly with regional temperatures inferred from alkenone records from nearby areas of the north atlantic and lake level inferred changes in precipitation and evaporation the long term d trends reflect a period of maximum regional warmth at ca 8 6 ka after the collapse of the laurentide ice sheet second unlike the positive relationship between temperature and d observed over the long term and during early events like the younger dryas we find that a series of warm and dry events at 4 9 4 6 4 2 3 9 2 9 2 1 and 1 3 1 2 cal kya bp coincide with negative d excursions from the long trends these events were likely driven by summer drought dynamics

oxygen 18 16 variability in greenland snow and ice with 10 3 to 105 year time resolution the 3 km long greenland ice sheet project 2 gisp2 ice core presents a 100 000 year detailed oxygen isotope profile covering almost a full glacial interglacial cycle measuranents of isotopic fluctuations in snow frost and atmospheric water vapor samples collected during summer field seasons up to 20 0 are compatible with the large and abrupt 80 160 changes observed in accumulated tim snow pit 15 80 profiles from the gisp2 summit area however show rapid smoothing of the 180 160 signal near the surface beyond about 2 m depth the smoothedi5180 signal is fairly well preserved and can be interpreted in terms of average local weather conditions and climate the longer climate fluctuations also have regional and often global significance in the older part of the record corresponding to marine isotope stages mis 5a to 5d the effect of orbital climate forcing via the 19 and 23 kyr precession cycles and the 41 kyr obliquity cycle is obvious from the end ofmis 5a at about 75 000 years b p till the end of the glacial at the younger dryas preboreal transition at 11 650 years b p the o180 160 record shows frequent rapid switches between intermediate interstadial and low stadial values fourier spectra of the oscillations that are superimposed on the orbitally induced changes contain a strong periodicity at 1 5 kyr a broad peak at 4 0 kyr and additional shorter periods detailed comparison of the gisp2 180 160 record with the vostok antarctica 15d record pacific ocean foraminiferal 180 160 grande pile france tree pollen and insolation indicates that a counterpart to many of the rapid 180 160 fluctuations of gisp2 can be found in the other records and that the gisp2 isotopic changes clearly are the local expression of climate changes of worldwide extent correlation of events on the independent gisp2 and specmap time scales for the interval 10 000 50 000 years b p shows excellent chronometric agreement except possibly for the event labeled 3 1 the glacial to interglacial transition evidently started simultaneously in the arctic and the antarctic but its development and its expression in greenland isotopes was later suppressed by the influence of meltwater especially from the barents sea ice sheet on deep water formation and ocean circulation meltwaters from different ice sheets bordering the north atlantic also influenced ocean circulation during the bolling allerod interstadial complex and the younger dryas and led to a distinct development of european climate and greenland 180 160 values the holocene interval with long term stable mean isotopic values contains several fluauations with periods from years to millennia dominant is a 6 3 year oscillation with amplitude up to 3 to 4 0 periodicities of 11 and 210 years also found in the solar modulated records of the cosmogenic isotopes 1obe and 14c suggest solar processes as the cause of these cycles depression of180 160 values cooling by volcanic eruptions is observed in stacked gisp215 80 records but the effect is small and not likely to trigger major climate changes

holocene thermal maximum in the western arctic 0 180 w the spatio temporal pattern of peak holocene warmth holocene thermal maximum htm is traced over 140 sites across the western hemisphere of the arctic 0 180 w north of 60 n paleoclimate inferences based on a wide variety of proxy indicators provide clear evidence for warmer than present conditions at 120 of these sites at the 16 terrestrial sites where quantitative estimates have been obtained local htm temperatures primarily summer estimates were on average 1 6 0 8 c higher than present approximate average of the 20th century but the warming was time transgressive across the western arctic as the precession driven summer insolation anomaly peaked 12 10 ka thousands of calendar years ago warming was concentrated in northwest north america while cool conditions lingered in the northeast alaska and northwest canada experienced the htm between ca 11 and 9 ka about 4000 yr prior to the htm in northeast canada the delayed warming in quebec and labrador was linked to the residual laurentide ice sheet which chilled the region through its impact on surface energy balance and ocean circulation the lingering ice also attests to the inherent asymmetry of atmospheric and oceanic circulation that predisposes the region to glaciation and modulates the pattern of climatic change the spatial asymmetry of warming during the htm resembles the pattern of warming observed in the arctic over the last several decades although the two warmings are described at different temporal scales and the htm was additionally affected by the residual laurentide ice the similarities suggest there might be a preferred mode of variability in the atmospheric circulation that generates a recurrent pattern of warming under positive radiative forcing unlike the htm however future warming will not be counterbalanced by the cooling effect of a residual north american ice sheet

cooling of northwest atlantic slope waters during the holocene 2 greenland ice cores depict a holocene climate that was warm and stable compared to the preceding 100 kyr yet most elements of the climate system were undergoing large secular changes during the last 11 kyr summer insolation in the northern hemisphere declined to last glacial maximum lgm levels atmospheric co2 increased 20 ppmv and the carbonate ion concentration of the deep sea decreased 5 10 mmol kg broecker et al 1999 signaling a large shift in the global carbon cycle the oxygen isotope composition of atmospheric o2 an indicator of global productivity and hydrology increased 0 5 retracing roughly 50 of the non ice volume deglacial decrease bender et al 1999 the monsoons of asia and

molecular stratigraphy a new tool for climatic assessment variations in sea surface temperatures over the past 500 000 years are inferred from the relative abundance behaviour of two organic compounds c37 alkenones over the upper 8 metres of a sediment core from the eastern equatorial atlantic this molecular record ascribed to contributions from prymnesiophyte algae correlates well with the variations in the 18 signal for the calcareous skeletons of certain planktonic foraminifera thus providing the first demonstration of a new stratigraphical technique which may be especially valuable where methods based on carbonate 18 fail

ocean gyre circulation changes associated with the north atlantic oscillation abstract observational evidence is presented for interannual to interdecadal variability in the intensity of the north atlantic gyre circulation related to the atmospheric north atlantic oscillation nao patterns a two point baroclinic pressure difference between the subtropical and subpolar gyre centers an oceanic analogue to the much used sea level pressure slp based atmospheric nao indices is constructed from time series of potential energy anomaly pea derived from hydrographic measurements in the labrador basin and at station s near bermuda representing the upper 2000 db eastward baroclinic mass transport between the two centers the transport index indicates a gulf stream and north atlantic current that gradually weakened during the low nao period of the 1960s and then intensified in the subsequent 25 years of persistently high nao to a record peak in the 1990s the peak to peak amplitude difference was 15 20 megatons per second mt s 1 with a 43 yr mean of about 60 mt s 1 a change of 25 33

the structure of holocene climate change in mid latitude north america abstract a sequence of long term and rapid changes during the holocene appears in a network of 40 well resolved paleoclimate datasets from mid latitude north america including records of pollen inferred temperatures alkenone derived sea surface temperatures ssts lake level changes dust accumulation and lake isotopes from idaho to maine statistical analyses reveal that changes in insolation and the laurentide ice sheet explain 51 7 of the variance in the records especially multi millennial trends but peak rates of change indicate additional rapid changes at ca 10 8 9 4 8 3 7 0 5 5 5 2 4 7 2 1 and 0 9 ka step changes between 9 4 and 8 3 ka relate to ice sheet dynamics that warmed much of the region and changes at 5 5 ka were the largest since the demise of the ice sheet the shift at 5 5 ka initiated widespread cooling and increases in effective moisture which culminated in the coolest wettest millennia in most areas after 2 1 ka replicated evidence from multiple records also shows a spatially varied set of multi century fluctuations including 1 low temperatures and high effective moisture at 5 5 4 8 ka in the mid continent and 2 repeated phases of low ssts cool summers and drought superimposed upon long cooling moistening trends in eastern north american since 5 5 ka

holocene forcing of the indian monsoon recorded in a stalagmite from southern oman a high resolution oxygen isotope record from a thorium uranium dated stalagmite from southern oman reflects variations in the amount of monsoon precipitation for the periods from 10 3 to 2 7 and 1 4 to 0 4 thousand years before the present ky b p between 10 3 and 8 ky b p decadal to centennial variations in monsoon precipitation are in phase with temperature fluctuations recorded in greenland ice cores indicating that early holocene monsoon intensity is largely controlled by glacial boundary conditions after approximately 8 ky b p monsoon precipitation decreases gradually in response to changing northern hemisphere summer solar insolation with decadal to multidecadal variations in monsoon precipitation being linked to solar activity

regional climate variability in the western subtropical north atlantic during the past two millennia 1 western subtropical north atlantic oceanic and atmospheric circulations connect tropical and subpolar climates variations in these circulations can generate regional climate anomalies that are not reflected in northern hemisphere averages assessing the significance of anthropogenic climate change at regional scales requires proxy records that allow recent trends to be interpreted in the context of long term regional variability we present reconstructions of gulf stream sea surface temperature sst and hydrographic variability during the past two millennia based on the magnesium calcium ratio and oxygen isotopic composition of planktic foraminifera preserved in two western subtropical north atlantic sediment cores reconstructed sst suggests low frequency variability of 1 c during an interval that includes the medieval climate anomaly mca and the little ice age lia a warm interval near 1250 a d is distinct from regional and hemispheric temperature possibly reflecting regional variations in ocean atmosphere heat flux associated with changes in atmospheric circulation e g the north atlantic oscillation or the atlantic meridional overturning circulation seawater d 18 o which is marked by a fresher mca and a more saline lia covaries with meridional migrations of the atlantic intertropical convergence zone the northward advection of tropical salinity anomalies by mean surface currents provides a plausible mechanism linking carolina slope and tropical atlantic hydrology

oxygen isotope records of holocene climate variability in the pacific northwest abstract oxygen isotope 18o measurements of authigenic carbonate from cleland lake southeastern british columbia paradise lake central british columbia and lime lake eastern washington provide a 9000 year holocene record of precipitation evaporation balance variations in the pacific northwest both cleland lake and paradise lake are small surficially closed basin systems with no active inflows or outflows lime lake is surficially open with a seasonally active overflow water isotope values from cleland and paradise plot along the local evaporation line indicating that precipitation evaporation balance is a strong influence on lake hydrology in contrast lime lake water isotope values plot on the local meteoric water line signifying minimal influence by evaporation to infer past hydrologic balance variations at a high temporal resolution we sampled the cleland paradise and lime lake sediment cores at 1 60 mm intervals 3 33 years per sample on average and measured the isotopic composition of fine grained

asynchronous variation in the east asian winter monsoon during the holocene the east asian winter monsoon eawm is one of the most active components of the global climate system climate anomalies associated with the eawm differ between extratropical and tropical regions due to the eawm s meridional extent spatial differences in the eawm variability on centennial and millennial time scales during the holocene have not been well understood this study describes holocene spatiotemporal features of the eawm based on comparisons of proxy records and climate simulations the analysis specifically compared four proxy records located throughout china to assess the eawm s spatial variability during the holocene these records indicate a stronger eawm during the early holocene than that during the late holocene the eawm also shows a rapid asynchronous decline from northwestern to southeastern china the eawm declined in northwestern china from 10 to 7 5 ka b p whereas the decline did not occur in southern china and the eastern tibetan plateau until 6 4 5 ka b p coupled equilibrium and transient simulations of climate evolution during the holocene indicate that the decline of the eawm from 10 to 7 5 ka b p was probably caused by melting of northern hemisphere nh ice sheets and enhanced atlantic meridional overturning circulation amoc the decline of the eawm from 6 to 4 5 ka b p over the eastern tibetan plateau and southern china is related to an abrupt increase in sea surface temperatures ssts of the tropical western indian ocean we therefore argue that the regional shift in eawm intensity was probably related to a distinguishing response to high latitude nh ice sheets and amoc and low latitude tropical ssts forcings

centennial to millennial hydroclimatic fluctuations in the humid northeast united states during the holocene paleoclimate records indicate that the hydroclimate of the northeast united states changed continuously during the holocene but the signals of multi century variations have been difficult to distinguish from local effects and noise systematic replication of the signals can help diagnose the patterns of change here we use ground penetrating radar gpr and sediment core analyses to extend and compare the regional network of lake level records we reconstruct the histories of two lakes in northeast pennsylvania which show that multi century hydrologic changes observed in coastal new england extended to the susquehanna river watershed correlations with isotopic and marine temperature records r 0 65 indicate that high temperatures coincided with low water at 4 9 3 8 and 2 8 2 0 ka widely recognized holocene events at ca 4 2 and 2 7 ka therefore may have shared similar ocean atmosphere dynamics in this region low water levels in pennsylvania from ca 5 5 4 9 ka however demonstrate that other multi century changes had different patterns at ca 5 5 ka anti phased inland and coastal hydrologic changes followed a sharp temperature decline and produced drought possibly as far inland as the great lakes the long term increase in water levels since then underscores that current pluvial conditions in the region probably lack a holocene precedent

subtropical north atlantic temperatures 60 000 to 30 000 years ago a reconstruction of sea surface temperature based on alkenone unsaturation ratios in sediments of the bermuda rise provides a detailed record of subtropical climate from 60 000 to 30 000 years ago northern sargasso sea temperatures changed repeatedly by 2 to 5 c covarying with high latitude temperatures that were previously inferred from greenland ice cores the largest temperature increases were comparable in magnitude to the full glacial holocene warming at the site abrupt cold reversals of 3 to 5 c lasting less than 250 years occurred during the onset of two such events greenland interstadials 8 and 12 suggesting that the largest most rapid warmings were especially unstable

an evaluation of 14c age relationships between co occurring foraminifera alkenones and total organic carbon in continental margin sediments 1 radiocarbon age relationships between co occurring planktic foraminifera alkenones and total organic carbon in sediments from the continental margins of southern chile northwest africa and the south china sea were compared with published results from the namibian margin age relationships between the sediment components are site specific and relatively constant over time similar to the namibian slope where alkenones have been reported to be 1000 4500 years older than co occurring foraminifera alkenones were significantly similar to 1000 years older than co occurring foraminifera in the chilean margin sediments in contrast alkenones and foraminifera were of similar age within 2 sigma error or better in the nw african and south china sea sediments total organic matter and alkenone ages were similar off namibia age difference toc alkenones 200 700 years chile 100 450 years and nw africa 360 770 years suggesting minor contributions of preaged terrigenous material in the south china sea total organic carbon is significantly 2000 3000 years older owing to greater inputs of preaged terrigenous material age offsets between alkenones and planktic foraminifera are attributed to lateral advection of organic matter physical characteristics of the depositional setting such as seafloor morphology shelf width and sediment composition may control the age of co occurring sediment components in particular offsets between alkenones and foraminifera appear to be greatest in deposition centers in morphologic depressions aging of organic matter is promoted by transport age offsets are correlated with organic richness suggesting that formation of organic aggregates is a key process

the 8200 year b p event in the slope water system western subpolar north atlantic 1 stable isotope trace metal alkenone paleothermometry and radiocarbon methods have been applied to sediment cores in the western subpolar north atlantic between hudson strait and cape hatteras to reveal the history of climate in that region over the past 11 kyr we focus on cores from the laurentian fan which is known to have rapid and continuous accumulation of hemipelagic sediment although results among our various proxy data are not always in agreement the weight of the evidence alkenone sea surface temperature sst 18o and abundance of globigerinoides ruber indicates a continual cooling of surface waters over laurentian fan from about 18 c in the early holocene to about 8 c today alternatively mg ca data on planktonic foraminifera indicate no systematic change in holocene sst the inferred long term decrease in sst was probably driven by decreasing seasonality of northern hemisphere insolation two series of proxy data show the gradual cooling was interrupted by a two step cold pulse that began 8500 years ago and lasted about 700 years although this event is associated with the final deglaciation of hudson bay there is no 18o minimum anywhere in the labrador sea yet there is some evidence for it as far south as cape hatteras finally although the 8200 year b p event has been implicated in decreasing north atlantic ventilation and hence widespread temperature depression on land and at sea we find inconsistent evidence for a change at that time in deep ocean nutrient content at 4 km water depth