dynamical mechanisms for african monsoon changes during the mid holocene 1 during the mid holocene about 6 ka bp the african monsoon was stronger and extended farther north than in present day climate associated with differences in orbital parameters affecting insolation earlier research suggests that land surface vegetation interaction and atmosphere ocean coupling may be important to the northward extension of the holocene monsoon rainfall but different climate models produce different results in terms of rainfall distribution this study examines the dynamical mechanisms that modulate the orbital forcing effect on the african monsoon convection a series of simulations with 6 ka bp orbital parameters modified land surface albedo that mimics grassland extending over north africa and alterations of the model dynamics are used it is found that the ventilation mechanism inflow of low moist static energy air from oceanic or poleward regions is a key process in setting the northern rainfall boundary for the african monsoon in particular advection and diffusion of less moist air oppose the increase of moisture that would be required to meet the increased convective instability criteria set by the warmer tropospheric air above the continent while changes in heat fluxes into the atmospheric column favor convection in principle the ventilation mechanism limits the poleward extent of the region that can actually convect

orographic effects of the tibetan plateau on the east asian summer monsoon an energetic perspective this paper investigates the dynamical processes through which the tibetan plateau tp influences the east asian summer monsoon easm within the framework of the moist static energy mse budget using both observations and atmospheric general circulation model agcm simulations the focus is on the most prominent feature of the easm the so called meiyu baiu mb which is characterized by a well defined southwest northeast elongated quasi stationary rainfall band spanning from eastern china to japan and into the northwestern pacific ocean between mid june and mid july observational analyses of the mse budget of the mb front indicate that horizontal advection of moist enthalpy and primarily of dry enthalpy sustains the front in a region of otherwise negative net energy input into the atmospheric column a decomposition of the horizontal dry enthalpy advection into mean transient and stationary eddy fluxes identifies the longitudinal thermal gradient due to zonal asymmetries and the meridional stationary eddy velocity as the most influential factors determining the pattern of horizontal moist enthalpy advection numerical simulations in which the tp is either retained or removed show that the tp influences the stationary enthalpy flux and hence the mb front primarily by changing the meridional stationary eddy velocity with reinforced southerly wind over the mb region and northerly wind to its north changes in the longitudinal thermal gradient are mainly confined to the near downstream of the tp with the resulting changes in zonal warm air advection having a lesser impact on the rainfall in the extended mb region

florida straits deglacial temperature and salinity change implications for tropical hydrologic cycle variability during the younger dryas temperature and d 18 osw a proxy for surface salinity records based on mg ca ratios and oxygen isotope measurements in the planktonic foraminifera globigerinoides ruber from a sediment core located on the western margin of the florida straits as a proxy for meltwater discharge influence on florida straits surface water salinity we also measured ba ca ratios in g ruber from the same core results show that riverine influence on florida straits surface water started by 17 2 ka bp and ended by 13 6 ka bp 600 years before the start of the younger dryas yd cold interval the initiation of the yd is marked by an abrupt increase in florida straits d 18 osw values indicating a shift to elevated sea surface salinity occurring in 130 years most likely resulting from increased regional aridity and or reduced precipitation in order to resolve the timing of tropical atmospheric circulation change relative to amoc variability across this transition we compare the timing of surface water changes to a recently published record of florida current variability in the same core reconstructed from benthic oxygen isotope measurements we find synchronous changes in atmospheric and ocean circulation on the transition into the yd consistent with an abrupt reduction in amoc as the driver of tropical atlantic atmospheric circulation change at this time

late quaternary paleohydrological conditions in the drylands of northern mexico a summer precipitation proxy record of the last 80 cal ka bp abstract we present a late quaternary multi proxy record from the paleolake babicora in order to understand the influence of north american monsoon nam and westerly winter storms on the paleohydrological budget of the drylands of northern mexico stratigraphy 14 c chronology mineral magnetism and geochemistry of the 976 cm long sediment core were used to reconstruct the hydrological conditions over the last 80 cal ka bp the inverse relationship between the proxy records of runoff into babicora and winter precipitation in southwestern usa indicate the minimal influence of westerly winter storms at 29 n during the last glacial period westerly winds transported minimally chemically altered sediments from the dry watershed during the cold stadials and the basin received more than average runoff as a result of a stronger nam during the warm interstadials the highest pluvial discharge occurred prior to 58 cal ka bp and terrestrial plants became the major contributor to organic matter deposited between 71 and 53 cal ka bp over the last 40 cal ka bp the high amplitude fluctuations in runoff and lake water salinity mirrored an unstable summer rainfall regime

role of seasonal transitions and westerly jets in east asian paleoclimate abstract the summer rainfall climate of east asia underwent large and abrupt changes during past climates in response to precessional forcing glacial interglacial cycles as well as abrupt changes to the north atlantic during the last glacial however current interpretations of said changes are typically formulated in terms of modulation of summer monsoon intensity and do not account for the known complexity in the seasonal evolution of east asian rainfall which exhibits sharp transition from the spring regime to the meiyu and then again from the meiyu to the summer regime we explore the interpretation that east asian rainfall climate undergoes a modulation of its seasonality during said paleoclimate changes following previous suggestions we focus on role of the westerly jet over asia namely that its latitude relative to tibet is critical in determining the stepwise transitions in east asian rainfall seasons in support of this linkage we show from observational data that the interannual co variation of june july august rainfall and upper tropospheric zonal winds show properties consistent with an altered timing of the transition to the meiyu summer and with more northward shifted westerlies for earlier transitions we similarly suggest that east asian paleoclimate changes resulted from an altered timing in the northward evolution of the jet and hence the seasonal transitions in particular the transition of the jet from south of the plateau to the north that determines the seasonal transition from spring rains to the meiyu in an extreme scenario which we speculate the climate system tended towards during stadial cold phases of d o stadials and periods of low northern hemisphere summer insolation the jet does not jump north of the plateau essentially keeping east asia in prolonged spring conditions we argue that this hypothesis provides a viable explanation for a key paleoproxy signature of d o stadials over east asia namely the heavier mean 18o of precipitation as recorded in speleothem records the southward jet position prevents the low level monsoonal flow which is isotopically light from penetrating into the interior of east asia as such precipitation there will be heavier consistent with speleothem records this hypothesis can also explain other key evidences of east asian paleoclimate changes in particular the occurrence of dusty conditions during north atlantic stadials and the southward migration of the holocene optimal rainfall

ice sheet modulation of deglacial north american monsoon intensification the north american monsoon the dominant source of rainfall for much of the arid us southwest remains one of the least understood monsoon systems the late pleistocene evolution of this monsoon is poorly constrained largely because glacial changes in winter rainfall obscure summer monsoon signatures in many regional proxy records here we develop deglacial records of monsoon strength from isotopic analyses of leaf wax biomarkers in marine sediment cores reconstructions indicate a regional decrease in monsoon rainfall during the last glacial maximum and that the deglacial trajectory of the north american monsoon closely tracks changes in north american ice cover in climate model simulations north american ice cover shifts the westerlies southwards favouring the mixing of cold dry air into the us southwest this process known as ventilation weakens the monsoon by diluting the energy fluxes required for convection as the ice sheet retreats northwards the monsoon strengthens and local ocean conditions may play a larger role in regulating its intensity we conclude that on glacial interglacial timescales ice sheet induced reorganizations of atmospheric circulation have a dominant influence on the north american monsoon the intensity of the north american summer monsoon was modified by changes in the extent of the laurentide ice sheet during the last glacial maximum and subsequent deglaciation according to isotope records and numerical simulations

orbital pacing and ocean circulation induced collapses of the mesoamerican monsoon over the past 22 000 y the dominant controls on global paleomonsoon strength include summer insolation driven by precession cycles ocean circulation through its influence on atmospheric circulation and sea surface temperatures however few records from the summer north american monsoon system are available to test for a synchronous response with other global monsoons to shared forcings in particular the monsoon response to widespread atmospheric reorganizations associated with disruptions of the atlantic meridional overturning circulation amoc during the deglacial period remains unconstrained here we present a high resolution and radiometrically dated monsoon rainfall reconstruction over the past 22 000 y from speleothems of tropical southwestern mexico the data document an active last glacial maximum 18 24 cal ka b p monsoon with similar 18o values to the modern and that the monsoon collapsed during periods of weakened amoc during heinrich stadial 1 ca 17 ka and the younger dryas 12 9 11 5 ka the holocene was marked by a trend to a weaker monsoon that was paced by orbital insolation we conclude that the mesoamerican monsoon responded in concert with other global monsoon regions and that monsoon strength was driven by variations in the strength and latitudinal position of the intertropical convergence zone which was forced by amoc variations in the north atlantic ocean the surprising observation of an active last glacial maximum monsoon is attributed to an active but shallow amoc and proximity to the intertropical convergence zone the emergence of agriculture in southwestern mexico was likely only possible after monsoon strengthening in the early holocene at ca 11 ka

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

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

pore fluid constraints on the temperature and oxygen isotopic composition of the glacial ocean pore fluids from the upper 60 meters of sediment 3000 meters below the surface of the tropical atlantic indicate that the oxygen isotopic composition 18 o of seawater at this site during the last glacial maximum was 0 8 0 1 per mil higher than it is today combined with the 18 o change in benthic foraminifera from this region the elevated ratio indicates that the temperature of deep water in the tropical atlantic ocean was 4 c colder during the last glacial maximum extrapolation from this site to a global average suggests that the ice volume contribution to the change in 18 o of foraminifera is 1 0 per mil which partially reconciles the foraminiferal oxygen isotope record of tropical sea surface temperatures with estimates from barbados corals and terrestrial climate proxies

high latitude climate sensitivity to ice sheet forcing over the last 120 kyr abstract interpretation of ice core records is currently limited by paucity of modelling at adequate temporal and spatial resolutions several key questions relate to mechanisms of polar amplification and inter hemispheric coupling on glacial interglacial timescales here we present the first results from a large set of global ocean atmosphere climate model snap shot simulations covering the last 120 000 years using the hadley centre climate model hadcm3 at up to 1 kyr temporal resolution two sets of simulations were performed in order to examine the roles of orbit and greenhouse gases versus ice sheet forcing of orbital scale climate change a series of idealised heinrich events were also simulated but no changes to aerosols or vegetation were prescribed this paper focuses on high latitudes and inter hemispheric linkages the simulations reproduce polar temperature trends well compared to ice core reconstructions although the magnitude is underestimated polar amplification varies with obliquity but this variability is dampened by including variations in land ice coverage while the overall amplification factor increases the relatively constant amplification of antarctic temperatures with ice sheet forcing included suggests it is possible to use antarctic temperature reconstructions to estimate global changes which are roughly half the magnitude atlantic ocean overturning circulation varies considerably only with the introduction of northern hemisphere ice sheets but only weakens in the north atlantic in the deep glacial when ocean sea ice feedbacks result in the movement of the region of deep convection to lower latitudes and with the introduction of freshwater to the surface north atlantic in order to simulate heinrich events

steering of westerly storms over western north america at the last glacial maximum the last glacial maximum hydroclimate over western north america differed from the modern climate a proxy model comparison suggests that the glacial storm track was squeezed and steered by atmospheric high pressure systems

response of the north american monsoon to regional changes in ocean surface temperature 1 the north american monsoon nam an onshore wind shift occurring between july and september has evolved in character during the holocene largely due to changes in northern hemisphere insolation published paleoproxy and modeling studies suggest that prior to 8000 cal years bp the nam affected a broader region than today extending westward into the mojave desert of california holocene proxy sst records from the gulf of california goc and the adjacent pacific provide constraints for this changing nam climatology prior to 8000 cal years bp lower goc ssts would not have fueled northward surges of tropical moisture up the goc which presently contribute most of the monsoon precipitation to the western nam region during the early holocene the north pacific high was further north and ssts in the california current off baja california were warmer allowing monsoonal moisture flow from the subtropical pacific to take a more direct northwesterly trajectory into an expanded area of the southwestern u s west of 114 w a new upwelling record off southwest baja california reveals that enhanced upwelling in the california current beginning at 7500 cal year bp may have triggered a change in nam climatology focusing the geographic expression of nam in the southwest usa into its modern core region east of 114 w in arizona and new mexico holocene proxy precipitation records from the southwestern u s and northwestern mexico including lakes vegetation pollen and caves are reviewed and found to be largely supportive of this hypothesis of changing holocene nam climatology

variable winter moisture in the southwestern united states linked to rapid glacial climate shifts the last glacial period was characterized by large rapid climate fluctuations an analysis of a speleothem from new mexico shows that the coldest conditions over greenland coincide with increased winter precipitation in the southwestern united states which can be attributed to a southward displacement of the polar jet stream and the north american storm track

glacial interglacial variability in upwelling and bioproductivity off nw mexico implications for quaternary paleoclimate sedimentary accumulation of biogenic components organic carbon opal and biogenic barium on the northwestern mexican margin declined during every glacial interval of the past 140 kyr indicating decreases in upwelling induced productivity during cold periods the glacial interglacial contrasts in upwelling on this margin are attributed to reversals in land ocean thermal contrast the waxing and waning of the laurentide ice sheet and consequent responses of the western hemisphere wind fields this scenario is consistent with three independent lines of evidence terrestrial paleoclimatic data general circulation model results and our marine records this pattern of glacial interglacial variability in upwelling off nw mexico is opposite to that observed in other low latitude and midlatitude upwelling areas such as the eastern equatorial pacific these results add to a growing pool of observations that the response of oceanic upwelling to glacial climatic forcing has been regionally variable

moisture variability in the southwestern united states linked to abrupt glacial climate change the atmospheric response to millennial scale circulation changes in the north atlantic ocean during the last glacial period has been difficult to constrain cave deposits from southwestern north america reveal that atmospheric moisture in this region increased in response to slowdowns of the atlantic meridional overturning circulation

oceanic link between abrupt changes in the north atlantic ocean and the african monsoon abrupt changes in the african monsoon which have been recorded throughout the late pleistocene and early holocene epochs tend to coincide with changes in north atlantic thermohaline circulation a numerical simulation shows that the interaction between thermohaline circulation in the north atlantic ocean and wind driven currents in the topical atlantic ocean contributes to the rapidity of african monsoon transitions during abrupt climate change events

high resolution heat flow density in the lower congo basin 1 heat flow density hfd has been derived in the lower congo basin from 293 conventional probe measurements 424 oil exploration data and about 1 000 000 bottom simulating reflector bsr values each set of data provides independent estimates of temperature gradients associated with specific uncertainties and geographic distributions but a statistical comparison in overlapping domains shows a relatively good consistency in situ conductivity has been measured in conventional surveys and estimated from rock composition and porosity in oil exploration boreholes in the deep offshore measurements and estimates are consistently low and increase significantly toward the shelf domain thermal conductivity does not change significantly in the interval between the water bottom and the bsr and can be estimated from the surface measurements to provide an inference of hfd the combined sets of hfd in the lower congo basin cover the overall domain of the continental margin and a part of the oceanic domain high resolution imaging of the bsr by three dimensional 3 d seismic studies provides in addition good quality nominal resolution of 500 m in the deep offshore at short length scales on the order of kilometers large variations of hfd from 30 to 140 mwm 2 exist above salt diapirs and canyons related to heat refraction on the other hand no small scale effect related to fluid flow has been detected although active venting is known at larger length scale hfd in the deep offshore is different on both sides of the congo submarine channel 54 8 mwm 2 to the north and 48 12 mwm 2 to the south which are also characterized by different basin tectonic styles at the margin scale hfd is significantly different in the oceanic domain 42 3 mwm 2 the deep continental offshore 52 10 mwm 2 and the continental shelf 65 15 mwm 2 a progressive reduction of the radiogenic heat contribution in the extended continental crust is the more likely the cause of these differences mantle heat flow below the continental margin is consequently high compared to that expected 15 mwm 2 for old shields such as congo

mineral assemblages fluid evolution and genesis of polymetallic epithermal veins glojeh district nw iran abstract the glojeh district contains silver and base metal rich epithermal veins and is one of the most highly mineralized locations in the tarom hashtjin metallogenic province northwestern iran it consists of four major epithermal veins which are located in the south glojeh and north glojeh areas alteration in the glojeh district consists of propylitic sericitic and argillic assemblages as well as extensive silicification the ore bearing veins comprise three paragenetic stages 1 early cu au as sb fe bearing minerals 2 middle stage pb zn cu cd ag bearing minerals and 3 late hematite ag bi au pb mineralogy the veins are best classified as the product of an early high sulfidation hydrothermal system which was overprinted by an intermediate sulfidation system that was rich in ag and base metals hematite is locally altered to goethite in zones of as much as 40 m in width during supergene alteration and the goethite is an important exploration tool fluid inclusions from the early middle and late stages respectively have salinities and homogenizations temperatures ranging from 5 to 11 wt nacl eq and 220 c to 340 c to 1 to 8 wt nacl eq and 200 c to 290 c and finally to 0 1 to 2 wt nacl eq and 150 c to 200 c the oxygen isotope values in quartz range from 8 8 to 13 3 and most calculated fluid 18 o values are between 4 and 8 suggesting a magmatic fluid with some meteoric water contamination sulfur isotope values for chalcopyrite pyrite sphalerite and galena are mainly 7 3 to 1 3 and 0 3 to 8 4 for north glojeh and south glojeh respectively sulfur isotope data suggest a magmatic origin boiling isothermal mixing and dilution are the main mechanisms for ore deposition in the glojeh veins recent 40 ar 39 ar age measurements of 42 20 0 34 ma and 42 56 1 47 ma for the north glojeh and south glojeh veins respectively overlap with the 41 87 1 58 ma age of the goljin intrusion in the northern part of the district which we interpret as the main heat source controlling the hydrothermal systems

mid to late wisconsin pollen record of san felipe basin baja california abstract a lacustrine sequence from laguna seca san felipe baja california in the southwestern sonoran desert provides the first record of mid to late wisconsin vegetation in one of the driest regions of north america pollen analysis was performed in the intermediate part of the sequence from ca 44 000 to ca 13 000 14 c yr b p according to six 14 c radiocarbon dates sedimentation of eolian sands prior to 44 000 14 c yr b p indicates dry conditions high pollen concentration of montane and chaparral species in the pollen assemblages contrast with the modern desert vegetation sixty four pollen types were identified the source of mid to late wisconsin sedimentation must have been the plant communities in the surrounding highlands of the basin mid wisconsin pollen assemblages dominated by pines junipers and artemisia reflect humid conditions in the area by late wisconsin time a significant increment in junipers indicates a lowering in the altitudinal ranges of woodlands chaparral pollen from wind pollinated species is abundant while desert plants specifically animal or insect pollinated species are poorly represented the san felipe record does not identify the last glacial maximum as the time of greatest effective moisture as suggested by pluvial lake levels and other paleoclimatic evidence in the soutwestern united states

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

silicic acid biogeochemistry in the gulf of california insights from sedimentary si isotopes reflects transient iron limitation our new d 30 si record from the guaymas basin shows dramatic variations at millennial timescales low d 30 si values synchronous with heinrich events are interpreted as resulting from the decline in si oh 4 utilization at times of decreased upwelling strength while nearly complete si oh 4 utilization was observed at times of invigorated upwelling and increased opal burial during the holocene the bolling allerod and the last glacial period we attribute the complete utilization of si oh 4 to the occurrence of transient fe limitation at these times our study highlights the importance of fe limitation on si and c cycling in coastal upwelling regions and suggests that upwelling dynamics in combination with fe availability have the potential to modulate marine si distribution and opal burial even at short timescales citation pichevin l r s ganeshram b c reynolds f prahl t f pedersen r thunell and e l mcclymont 2012 silicic acid biogeochemistry in the gulf of california insights from sedimentary si isotopes paleoceanography 27 pa2201