assessing the impact of mid holocene insolation on the atmosphere ocean system a sequence of numerical simulations was performed in order to assess the role of early to mid holocene orbital forcing on the coupled atmosphere ocean system results from both an atmosphere only general circulation model gcm forced by specified sea surface temperatures and a coupled atmosphere ocean gcm consistently suggest a stronger south asian monsoon and a strengthened pacific walker circulation the latter feature interacts dynamically with the equatorial ocean in the coupled model to produce enhanced pacific upwelling a more pronounced cold tongue and an even stronger monsoon results suggest that the climate of the equatorial pacific was more similar to the la nina phase of the modern southern oscillation rather than the el niiio phase

millennial scale climate variability of the northeast pacific ocean and northwest north america based on radiolaria and pollen abstract radiolaria and pollen abundances in marine sediment cores from the northeast pacific are used to reconstruct oceanographic and continental climate change during the past glacial cycle 0 150 kyr these data allow direct comparison of the climate response of continental and oceanic systems detailed 18 o and ams 14 c measurements provide a link into global stratigraphic frameworks canonical correlation analysis extracts two modes of variation common to both the radiolaria and pollen records the first mode of variation correlates an assemblage of radiolaria associated with coastal upwelling with increased redwood western hemlock and alder pollen this association is consistent with the modern relationship between coastal upwelling coastal fog and redwood forests a second canonical mode relates an oceanic fauna now found in highest abundance in the far north pacific with reduced pine and western hemlock pollen abundance comparison of these records to an ice core 18 o record suggests that at wavelengths 3000 years warm events in greenland are correlated to intervals of increased coastal upwelling off oregon decreases in importance of very cold north pacific fauna suggesting warming and increases in pollen associated with wetter coastal environments radiolarian based sea surface temperature estimates suggest that the variability of the northeast pacific on this time scale is about 2 c warming in the coastal regions reflects reduced advection of the california current but is moderated by increases in cool coastal upwelling we infer that the response of the northeast pacific to millennial scale climate changes is related to changes in atmospheric circulation at mid to high latitudes preliminary analysis suggests that oceanic variability off oregon at wavelengths 18 o records this variability is associated with changes in subtropical faunal elements without similar changes in other faunal elements this finding suggests that unlike longer period millennial scale events the propagation of the shorter wavelength events to the northeast pacific is through subtropical or tropical teleconnections

a modern analogue of western united states summer palaeoclimate at 6000 years before present a modern summer climate analogue of western united states 6 ka palaeoclimate based on broad scale patterns from proxy data and general circulation model simulations provides details concerning its sum mer synoptic climatology august 1955 serves as a close analogue featuring an amplified subtropical ridge aloft and anticyclonic flow near the surface over the central united states and stronger westerly flow along the canadian northwest united states border the august 1955 summer temperature pattern similar to those from general circulation model simulations exhibits positive anomalies throughout most of the continental interior the august 1955 precipitation pattern corresponds very closely with patterns of effective moisture from proxy data showing widespread aridity and some stronger monsoonal activity in the southwest the analogue suggests that the amplified subtropical ridge is a very important feature of 6 ka palaeoclimate with the circulation pattern differing from that in the early holocene implying interactions between both milankov itch and sub milankovitch forcing factors

anomalous north pacific atmospheric circulation and large winter floods in the southwestern united states abstract specific anomalous atmospheric circulation conditions over the north pacific are conducive to the occurrence of the largest winter floods 10 yr return period on rivers in six hydroclimatic subregions of arizona and southern utah nevada and california composite maps of anomalies in daily 700 mb heights indicate that floods in all of the subregions are associated with a low pressure anomaly off the california coast and a high pressure anomaly in the vicinity of either alaska or the aleutian islands of these two major circulation features the presence of the low is the controlling factor in determining whether large floods will occur shifts in the locations of the low and high pressure anomalies over the north pacific appear to control which subregions experience floods with high elevation topographic features and proximity to air masses forming a major influence over the specific atmospheric circulation conditions that generate large floods in each hydroclimatic region concerning the int

late glacial to holocene changes in winds upwelling and seasonal production of the northern california current system abstract a core 120 km off the coast of southern oregon was examined for changes in lithology diatoms and pollen over the past 30 000 yr primary production during the late pleistocene was about half that of the holocene evidence from diatoms and pollen indicates that summer upwelling was much weaker implying an absence of strong northerly winds early pliocene diatoms found throughout the late pleistocene section were probably derived from diatomites east of the cascades and provide evidence for strong easterly winds over a dry continental interior the findings verify predictions of a climate model based on glacial maximum conditions there is no compelling evidence for a climatic reversal corresponding to the european younger dryas chron during the early holocene 9000 7000 yr b p there may have been years when winds were insufficiently strong to support upwelling so that warm stratified waters lay closer to the coast

biomarker temperature estimates for modern and last glacial surface waters of the california current system between 33 and 42 n sea surface temperature sst estimates were made using data for c37 39 alkenones analyzed in modern and glacial age intervals of sediment cores collected along an east west 42 n from 125 to 132 w and a north south 41 33 n transect of the california current system ccs the prymnesiophyte biomarker results suggest that surface waters warmed significantly throughout this region since the last glacial maximum lgm but the magnitude of warming varied spatially stratigraphic profiles from four sediment cores along the north south transect indicate the warming period was confined to the glacial interglacial transition 15 10 ka with sst reaching a maximum value at 10 ka and maintaining a uniformly high value throughout the holocene comparison of estimates derived from analysis of modern and lgm sediment intervals indicates the magnitude of the sst change was minimal for locations south of about 36 n 1 2 c and increased significantly 3 5 c north of this latitude using a simple heat balance model we calculate from the latitudinal gradient in sst that southward flow in the california current system during the lgm was about 60 of that measured today at 36 n our findings support the conclusion of others based on pollen data that coastal upwelling in the region of the northeast pacific off northern california was significantly reduced or even completely shutdown during the lgm

evidence for temporal fluctuations in marine radiocarbon reservoir ages in the santa barbara channel southern california abstract differences in the 14 c ages of closely associated marine shell and carbonized plant material from stratified archaeological deposits on san miguel island california suggest holocene 10 000 present fluctuations in marine 14 c reservoir ages these fluctuations coincide with 18 o and 13 c shifts measured in mytilus californianus shells from the same stratigraphic contexts and general atmospheric oceanic circulation models for the region based on these data we make three primary observations 1 significant changes appear to have occurred in the radiocarbon reservoir during the holocene 2 these fluctuations appear to correlate with regional oceanographic changes and 3 high resolution 14 c dating of marine shells may require different r values for different periods of time

high resolution climatic evolution of coastal northern california during the past 16 000 years holocene and latest pleistocene oceanographic conditions and the coastal climate of northern california have varied greatly based upon high resolution studies ca every 100 years of diatoms alkenones pollen caco 3 and total organic carbon at ocean drilling program odp site 1019 41 682 n 124 930 w 980 m water depth marine climate proxies alkenone sea surface temperatures ssts and caco 3 behaved remarkably like the greenland ice sheet project gisp 2 oxygen isotope record during the balling allerod younger dryas yd and early part of the holocene during the yd alkenone ssts decreased by 3 c below mean bolling allerod and holocene ssts the early holocene ca 11 6 to 8 2 ka was a time of generally warm conditions and moderate caco 3 content generally 4 the middle part of the holocene ca 8 2 to 3 2 ka was marked by alkenone ssts that were consistently 1 2 c cooler than either the earlier or later parts of the holocene by greatly reduced numbers of the gyre diatom pseudoeunotia doliolus 10 and by a permanent drop in caco 3 to 3 starting at ca 5 2 ka coastal redwood and alder began a steady rise arguing for increasing effective moisture and the development of the north coast temperate rain forest at ca 3 2 ka a permanent ca 1 c increase in alkenone sst and a threefold increase in p doliolus signaled a warming of fall and winter ssts intensified higher amplitude and more frequent cycles of pine pollen alternating with increased alder and redwood pollen are evidence that rapid changes in effective moisture and seasonal temperature enhanced el nino southern oscillation enso cycles have characterized the site 1019 record since about 3 5 ka

otolith δ18o record of mid holocene sea surface temperatures in peru peruvian sea catfish galeichthys peruvianus sagittal otoliths preserve a record of modern and mid holocene sea surface temperatures ssts oxygen isotope profiles in otoliths excavated from ostra 6010 90 years before the present yr b p 8 degrees 55 s indicate that summer ssts were approximately 3 degrees c warmer than those of the present siches otoliths 6450 110 yr b p 4 degrees 40 s recorded mean annual temperatures approximately 3 degrees to 4 degrees c warmer than were measured under modern conditions trophic level and population diversity and equitability data from these faunal assemblages and other peruvian archaeological sites support the isotope interpretations and suggest that upwelling of the peru chile current intensified after approximately 5000 yr b p

apparent synchroneity of submillennial scale climate events between greenland and santa barbara basin california from 30 10 ka abstract evidence is presented from odp hole 893a santa barbara basin for possible synchroneity between greenland and california margin submillennial climate change records during termination 1 and the last glacial maximum lgm planktonic foraminiferal isotope and faunal records are compared to the gisp2 18o records greenland for the interval between 30 and 10 ka using a new chronology based on 38 planktonic 14c dates apparent synchronous climatic events include interstadial is events 2 3 and 4 a broad warming trend between 21 and 19 ka and a broad cooling trend between 19 and 17 ka bolling and allerod warm episodes separated by the cooler older dryas event younger dryas cool episode and the preboreal the resulting tight correlation supports an intimate teleconnection between santa barbara basin surface waters and greenland air temperatures differences between greenland and santa barbara basin suggest that some regional variations were superimposed on the global pattern of climate change e g the pre bolling warming

assessment of sea surface temperature at 42 n in the california current over the last 30 000 years assessment of changes in surface ocean conditions in particular sea surface temperature sst is essential to understand long term changes in climate especially in regions where continental climate is strongly influenced by oceanographic processes to evaluate changes in sst in the northeast pacific we have analyzed long chain alkenones of prymnesiophyte origin at 38 depths in a piston and associated trigger core collected beneath the contemporary core of the california current system at 42 n 270 km off the coast of oregon california the samples span 30 000 years of deposition at this location unsaturation patterns u37k in the alkenone series display a statistically significant difference p 0 001 between interglacial 0 44 0 02 n 11 and glacial 0 29 0 04 n 20 intervals of the cores detailed examination of other compositional features of the c37 c38 c39 alkenone series and a related c36 alkenoate series measured downcore suggests the published u37k temperature calibration u37k 0 034 t 0 039 defined for cultures of a strain of emiliania huxleyi isolated from the subarctic pacific provides best estimates of winter sst at our study site this inference is purely statistical and does not imply however that the phytoplankton source of these biomarkers is most productive in winter or at the ocean surface the temperature record for u37k implies 1 an 4 c shift occurred in winter sst from 7 5 1 1 c at the last glacial maximum to 11 7 0 7 c in the present interglacial period and 2 this warming trend was confined to the time frame 14 10 ka within the glacial to interglacial transition period these conclusions are corroborated entirely by results from an independent sst transformation of radiolarian species assemblage data obtained from the same core materials

modeling climate shift of el nino variability in the holocene a coupled ocean atmosphere general circulation model is used to investigate climatic shift of el nino in the holocene the model simulates a reduced enso intensity in the early and mid holocene in agreement with paleoclimate record the enso reduction is proposed to be caused by both an intensified asian summer monsoon and a warm water subduction from the south pacific into the equatorial thermocline

radiocarbon dating of marine samples from gulf of cadiz the reservoir effect abstract the 14 c content of marine shells can be used as a proxy for past coastal upwelling intensity the iberian coast of the gulf of cadiz can be divided in two regions with different oceanographic conditions the southern portuguese coast with an active upwelling influenced by the northeastern atlantic circulation and the southwestern andalusian coast where due to its configuration a wind driven coastal upwelling is absent because these oceanographic conditions r a parameter that quantifies the marine reservoir effect has positive values with a weighted mean of 65 20 14 c yr southern portuguese coast or negative values with a weighted mean of 135 20 14 c yr andalusian coast nevertheless between 4400 bp and 4000 bp r values are strongly positive suggesting a very strong upwelling existed in the entire region of the gulf of cadiz this does not reflect wind driven processes but probably an eastward extension of the azores front which is characterized by locally intense upwelling along the azores current that penetrated into the gulf of cadiz the study highlights the importance of using appropriate calibration methods when developing 14 c chronologies from the gulf of cadiz as well as the potential of r for reconstructing palaeoceanographic conditions

depth and seasonality of alkenone production along the california margin inferred from a core top transect alkenone unsaturation indices uk 37 of marine sediment could prove particularly useful on organic rich continental margins where carbonate dissolution hampers the use of other paleoclimatic proxies mccaffrey et al 1990 kennedy and brassell 1992 forty core top samples of recent sediment from a latitudinal transect 23 40 n along the california margin yield uk 37 values that correlate linearly with modern mean annual sea surface temperatures ssts in the range of 12 23 c reproducibility of the unsaturation value in closely spaced cores is near analytical error uk 37 data define a relationship to temperature nearly identical to the prahl et al 1988 laboratory cultures of emiliania huxleyi the close agreement is particularly significant in light of the nannofossil composition of the sediments where the abundance of the coccolith taxon gephyrocapsa oceanica known to synthesize alkenones equals or exceeds that of e huxleyi comparison with seasonal temperature variations at different depths indicates that little if any alkenone production occurs at depths 30 m along the continental margin water depths 2 km sediments in more pelagic locations exhibit small but consistent biases toward winter and or subsurface production similar to previously reported sediment trap and core top data from the oregon margin prahl et al 1993 doose et al 1997

simulated changes in extreme temperature and precipitation events at 6 ka abstract we have employed a high resolution regional climate model rcm to test the sensitivity of extreme climate events to 6 ka orbital forcing using western north america as a case study calculated differences 6 ka control in annual extreme precipitation event frequency were mostly positive throughout the rcm domain as were differences in extreme event contribution to total annual precipitation these annual mean differences were driven by changes in the seasonal distribution of extreme precipitation events at 6 ka which was in turn linked to changes in the daily variability of upper level zonal wind speeds over the northeast pacific positive differences in the frequency of extreme maximum daily temperature values occurred inland in the rcm domain with peak differences of 28 days year over the great basin likewise days in which the maximum daily temperature exceeded 32 c were 24 more frequent in the 6 ka integration than in the control integration and heat waves lasted up to 12 days longer mean first and last freeze dates were generally earlier in the 6 ka integration along the pacific coast and over the southern great basin and later over the northern great basin and interior of the pacific northwest we propose that by exerting controls on paleoclimate archives unique from changes in the mean climate state such changes in the frequency and duration of extreme climate events could have played a substantial role in shaping the mid holocene paleoclimate record of the region and that extreme climate events may have been similarly important in other regions and during other time periods

coastal upwelling in the california current system abstract coastal upwelling in the california current system has been the subject of large scale studies off california and baja california and of small scale studies off oregon recent studies of the winds along the entire coast from 25 n to 50 n indicate that there are significant along shore variations in the strength of coastal upwelling which are reflected in the observed temperature distribution active upwelling appears to be restricted to a narrow coastal band about 10 25 km wide along the entire coast but the region influenced by coastal upwelling may be much wider intensive observations of the upwelling zone during summer off oregon show the presence of a southward coastal jet at the surface a mean vertical shear a poleward undercurrent along the bottom and persistently sloping isopycnals over the continental shelf most of the upwelling there occurs during relatively short periods several days long of upwelling favorable winds during the upwelling season off oregon the offshore ekman transport is carried by the surface ekman layer and the onshore return flow occurs through a quasi geostrophic interior it is not known whether the structure and dynamics observed off oregon are typical of the upwelling zone along the entire coast though some of the same features have been observed off baja california current and future research will eventually show whether the oregon results are also applicable in the region of persistently strong upwelling favorable winds off northern california and in the region of complex bathymetry off central and southern california

geoarchaeological evidence from peru for a 5000 years b p onset of el nino for the tropical west coast of south america where el nino southern oscillation enso is most pronounced archaeological and associated paleontological deposits in northern peru revealed a major climate change at about 5000 years before the present yr b p the data implied the presence of stable warm tropical water as far south as 10 s during the early mid holocene about 8000 to 5000 yr b p these data suggest that enso did not occur for some millennia preceding 5000 yr b p when global and regional climate was warmer than today

orbital suppression of wind driven upwelling in the california current at 6 ka 1 marine records clearly show activity in the california current cc system to have been altered during the mid holocene however the specific nature of cc activity during this period remains ambiguous as proxies that suggest cooler than present sea surface temperatures co occur in mid holocene marine sediments with multiple proxies that suggest decreased coastal upwelling to test the sensitivity of wind driven upwelling in the cc to mid holocene orbital forcing we have employed a high resolution regional climate model rcm the seasonality of modern cc wind driven coastal upwelling simulated by the rcm is in strong agreement with observational data further we show that changes in the seasonality of solar insolation induced by mid holocene milankovitch forcing decrease early and peak season coastal upwelling in the cc along with increasing late season coastal upwelling these results are in agreement with several marine and terrestrial proxies suggesting that relative to present cc activity during the mid holocene was characterized by a longer and less vigorous upwelling season with decreased seasonal contrast

differences in radiocarbon age between shell and charcoal from a holocene shellmound in northern california abstract the west berkeley shellmound the oldest well dated archaeological site in the san francisco bay region contains shell and charcoal ranging in age from ca 1200 to 5700 cal yr b p radiocarbon ages of marine shell and charcoal collected from fifteen stratigraphic levels in the west berkeley shellmound suggest changes in the 14 c content of san francisco bay surface waters relative to the atmosphere the oceanic reservoir age over the past 5000 yr the reservoir age of san francisco bay waters fluctuated between 870 and 170 14 c yr over the past 5000 yr with the lowest values occurring 2900 to 3800 cal yr b p and the highest values between 1200 and 2000 cal yr b p changes in the radiocarbon reservoir age may be due to changes in the strength of seasonal wind driven upwelling off coastal california where upwelling brings 14 c depleted waters to the surface the period of lowest r values at 3500 to 3900 cal yr b p is coincident with relatively low salinity in san francisco bay indicating high freshwater inflow and wet climate in california based on lake level records the period of high r values 1200 2000 cal yr b p is coincident with one of the driest periods in california during the late holocene these data suggest a link between coastal upwelling and precipitation over central california the age of the top of west berkeley mound and several other mounds in the san francisco bay region 1100 to 1300 cal yr b p coincides with a prolonged dry period in california and low river inflow to san francisco bay perhaps the sites were abandoned because of the drought

seasonal cycles of currents temperatures winds and sea level over the northeast pacific continental shelf 35 n to 48 n seasonal cycles of coastal wind stress adjusted sea level asl shelf currents and water temperatures off the west coast of north america 35 n to 48 n are estimated by fitting annual and semiannual harmonics to data from 1981 1983 longer records 9 34 years of monthly asl indicate that these two harmonics adequately represent the long term monthly average seasonal cycle and that the current measurement period is long enough to estimate the seasonal cycles we characterize the differences between fall winter and spring summer as follows for fall winter monthly mean winds north of 35 n are northward for 3 6 months longer in the north than in the south south of 35 n the mean winds are near zero or weakly southward monthly mean alongshore currents are northward over midshelf and shelf break at all locations sampled at depths of 35 m and deeper and are associated with high coastal sea levels and relatively warm water temperatures for spring summer monthly mean wind stresses are southward at all latitudes for 3 6 months longer in the south than in the north sea levels are low and water temperatures are relatively cool monthly mean currents at 35 m depth over the shelf are southward for 1 6 months longer at the shelf break than over midshelf and longer in the north than in the south while the deeper currents are less southward or northward the magnitudes of the seasonal cycles of all variables are maximum between approximately 38 n and 43 n generally decreasing slightly to the north and greatly to the south at each location the seasonal cycle of the alongshore current from 35 m depth at midshelf leads the sea level slightly and both lead the wind stress and temperatures by 1 2 months the seasonal cycles of all variables show a south to north progression south leads north by 1 2 months at 48 n annual mean currents at 50 m depth over the shelf break oppose the annual mean wind northward wind and southward current similarly at 35 n annual mean currents at 35 m depth over both midshelf and shelf break are opposed to the annual mean wind southward wind and northward current from 35 n to 43 n both summer and winter regimes are dominated by strongly fluctuating currents

high resolution climate simulations of oxygen isotope stage 3 in europe abstract oxygen isotope stage 3 ois 3 climate and its variations are the focus of the stage 3 project the objective of the ois 3 modeling effort is twofold 1 to explore the importance of different boundary conditions on the climate of europe and 2 to develop climate simulations that best reproduce the wealth of ois 3 observations given the complexity of the topography and coastlines the modeling effort is based on a nested general circulation model gcm and mesoscale model regcm2 with climate simulations for europe on a 60 km grid spacing the key conclusions are as follows 1 the mesoscale model driven by gcm output does a reasonable job of reproducing the modern european climate 2 ois 3 variations in orbit co2 and ice sheet size are of little significance in explaining the observed climate variability 3 the model results focus attention on north atlantic sea surface temperatures sst as a major factor in explaining ois 3 climates 4 experiments for different sst values capture a number of systematic changes in sea level pressure and precipitation 5 climate models simulate substantial european cooling and significant changes in precipitation but they do not explain large differences between ois 3 warm and cold episodes

large scale thermal anomalies in the california current during the 1982 1983 el nino the large scale thermal structure of the california current during 1982 83 shows several anomalous conditions warm sea surface temperature anomalies 1 2 c depression of the thermocline by 50 m or more and pronounced subsurface warming 3 4 c relative to the 30 year mean the subsurface anomaly is much greater than the surface anomaly these persistent 6 months structures coupled with unusually high sea levels along the north american coast show that a major californian el nino occurred during 1982 83 the data support the conclusion that the expansion and intensification of the aleutian low and the decrease in strength of the pacific high produced an anomalous basin wide atmospheric circulation which coupled directly to the large scale oceanic wind driven circulation to produce the californian el nino the enhanced transport from the west offshore california and south baja california which such a circulation would produce is consistent with the observed water properties the equatorial 1982 83 el nino is probably related to the californian el nino through an atmospheric teleconnection between equatorial sea surface temperature and the atmospheric hadley circulations

sea surface temperature changes in the northeastern pacific ocean during the past 20 000 years and their relationship to climate change in northwestern north america abstract modern ocean atmosphere interactions in the northeastern pacific ocean have a significant effect on the climate of the west coast of north america we present radiolarian microfossil based temperature reconstructions for the eastern north pacific spanning the past 20 000 yr to examine possible correlations and linkages between continental climate change and changes in sea surface temperature sst in the northeastern pacific ocean on millennial time scales the reconstructions indicate that the regional pattern of ocean circulation off the west coast of north america was further south 15 000 cal yr b p than it is today and reached its present location 13 000 cal yr b p the north pacific drift and transition zone were further south as a result of a more southerly north pacific high pressure cell prior to 13 000 cal yr b p while two continental paleoclimate records from northwestern north america show regional differences they also can be correlated to the sst changes a coastal site at 48 n shows similar patterns in summer temperatures as observed in offshore marine records of ssts however an inland continental record seems to reflect more regional scale changes in sea surface conditions showing a thermal maximum centered at 10 000 cal yr b p which is observed in the marine transect south of 42 n we conclude based on the pattern of oceanographic change as reflected in radiolarian assemblages that changes in the past latitudinal position of the north pacific drift played a significant role in controlling continental climate immediately to its east as it does in the present environment we also conclude that during the past 20 000 yr much of the evolution of oceanographic change is related to the migration of the atmospheric pressure cells the north pacific high and aleutian low of the northeastern pacific

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

collapse of the california current during glacial maxima linked to climate change on land time series of alkenone unsaturation indices gathered along the california margin reveal large 4 to 8 c glacial interglacial changes in sea surface temperature sst over the past 550 000 years interglacial times with ssts equal to or exceeding that of the holocene contain peak abundances in the pollen of redwood the distinctive component of the temperate rainforest of the northwest coast of california in the region now dominated by the california current ssts warmed 10 000 to 15 000 years in advance of deglaciation at each of the past five glacial maxima ssts did not rise in advance of deglaciation south of the modern california current front glacial warming along the california margin therefore is a regional signal of the weakening of the california current during times when large ice sheets reorganized wind systems over the north pacific both the timing and magnitude of the sst estimates suggest that the devils hole nevada calcite record represents regional but not global paleotemperatures and hence does not pose a fundamental challenge to the orbital milankovitch theory of the ice ages

the influence of backstop dip and convergence velocity in the growth of viscous doubly vergent orogenic wedges insights from thermomechanical laboratory experiments abstract we investigate the effects of convergence velocity and backstop geometry on the evolution of experimental viscous doubly vergent orogenic wedges experiments are performed in a thermomechanical shortening box using the newtonian properties of 52 54 en type paraffin as analogue to the natural strength distribution in the continental crust a velocity discontinuity is imposed at the subduction slot where the base plate is pulled below a rigid backstop the mobile base plate dips 5 two different backstop dips 60 and 30 are used in combination with constant convergence velocities of 2 and 10 cm h 1 results point out that backstop geometry exerts a first order control on the evolution of doubly vergent experimental wedges steeply dipping backstops cause the stationary uplift of the wedge axial region above the subduction slot whereas shallow dipping backstops allow its retroward translation in concomitance with a more pronounced deformation in the retrowedge convergence velocity controls the uplift in the wedge axial region and the amount of the outward migration of the deformation front in both the prowedge and retrowedge regions for fast convergence velocity and shallow dipping backstop deformation in the retrowedge is favoured an increase in the backstop dip and or a decrease in the convergence velocity favour deformation in the prowedge both prowedge and retrowedge tend to grow in self similar conditions with an aspect ratio depending upon the backstop dip and convergence velocity these results may have important consequences for the evolution and deformation style of ductile dominated orogenic wedges suggesting that location of the axial region and deformation progression in natural orogens can be strongly controlled by boundary and kinematic conditions

sampling armor layer sediments a method is presented for interpreting and describing the size distribution and the packing of particles in armor layers formed on the surface of non uniform bed sediments an armor layer thickness is defined in terms of the larger particle size fraction constituting the armor layer

simulated influences of lake agassiz on the climate of central north america 11 000 years ago eleven thousand years ago large lakes existed in central and eastern north america along the margin of the laurentide ice sheet the large scale north american climate at this time has been simulated with atmospheric general circulation models1 2 but these relatively coarse global models do not resolve potentially important features of the mesoscale circulation that arise from interactions among the atmosphere ice sheet and proglacial lakes here we present simulations of the climate of central and eastern north america 11 000 years ago with a high resolution regional climate model nested within a general circulation model the simulated climate is in general agreement with that inferred from palaeoecological evidence our experiments indicate that through mesoscale atmospheric feedbacks the annual delivery of moisture to the laurentide ice sheet was diminished at times of a large cold lake agassiz relative to periods of lower lake stands the resulting changes in the mass balance of the ice sheet may have contributed to fluctuations of the ice margin thus affecting the routing of fresh water to the north atlantic ocean a retreating ice margin during periods of high lake level may have opened an outlet for discharge of lake agassiz into the north atlantic a subsequent advance of the ice margin due to greater moisture delivery associated with a low lake level could have dammed the outlet thereby reducing discharge to the north atlantic these variations may have been decisive in causing the younger dryas cold event3 4

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

intercomparison of simulated global vegetation distributions in response to 6 kyr bp orbital forcing abstract the response of ten atmospheric general circulation models to orbital forcing at 6 kyr bp has been investigated using the biome model which predicts equilibrium vegetation distribution as a diagnostic several common features emerge a reduced tropical rain forest as a consequence of increased aridity in the equatorial zone b expansion of moisture demanding vegetation in the old world subtropics as a consequence of the expansion of the afro asian monsoon c an increase in warm grass shrub in the northern hemisphere continental interiors in response to warming and enhanced aridity and d a northward shift in the tundra forest boundary in response to a warmer growing season at high northern latitudes these broadscale features are consistent from model to model but there are differences in their expression at a regional scale vegetation changes associated with monsoon enhancement and high latitude summer warming are consistent with palaeoenvironmental observations but the simulated sh

suppression of el nino during the mid holocene by changes in the earth s orbit a number of recent reports have interpreted paleoproxy data to describe the state of the tropical pacific especially changes in the behavior of the el nino southern oscillation enso over the holocene these interpretations are often contradictory especially for the eastern tropical pacific and adjacent areas of south america here we suggest a picture of the mid holocene tropical pacific region which reconciles the data enso variability was present throughout the holocene but underwent a steady increase from the mid holocene to the present in the mid holocene extreme warm el nino events were smaller in amplitude and occurred less frequently about a mean climate state with a cold eastern equatorial pacific and largely arid coastal regions as in the present climate this picture emerges from an experiment in which a simple numerical model of the coupled ocean atmosphere system in the tropical pacific was driven by orbital forcing we suggest that the observed behavior of the tropical pacific climate over the mid to late holocene is largely the response to orbitally driven changes in the seasonal cycle of solar radiation in the tropics which dominates extratropical influences