sediments geomorphology magnetostratigraphy and vertebrate paleontology in the san pedro valley arizona the san pedro valley in southern arizona was probably formed by block faulting during the miocene sediments that filled the valley primarily the st david formation have yielded a robust vertebrate fossil record calibrated during the last 25 years by magnetostratigraphy and isotopic dating of volcanic ejecta this chronologic framework is combined with new sedimentologic paleomagnetic and geomorphic data to evaluate the influence of subsequent tectonism within the basin we conclude that tectonism in the san pedro valley has been relatively quiescent following the initial block faulting and suggest that ensuing rates of sedimentation and incision were most likely controlled by climatic factors rather than tectonic geologic ones vertebrate datum planes established in 1975 are revised on the basis of new paleontologic and geologic isotopic data we now recognize only three faunal datum planes in the st david formation and have abandoned the youngest lepus datum plane

mechanisms controlling the sensitivity of the atlantic thermohaline circulation to the parameterization of eddy transports in ocean gcms abstract the authors identify and describe the important dynamical mechanisms that explain the significant sensitivity of the atlantic thermohaline circulation to the parameterization of heat and salt transports by mesoscale eddies in numerical models in particular the effects of the gent mcwilliams gm scheme which has a strong flattening effect on isopycnals and a simple horizontal diffusion scheme are considered and compared two control runs one with each scheme exhibit very different circulations and density structures to analyze the dynamical reasons for the differences between the control runs a number of numerical experiments with regionally varying diffusion coefficients are carried out emphasizing the effects of different schemes in key regions the main effect of eddies in the southern ocean in nature is to shoal the subsurface isopycnal surfaces thus increasing the density of the northward inflow of relatively dense intermediate waters into the atlantic as will be seen this is more

the stable isotopic composition of modern soil carbonate and its relationship to climate the oxygen isotopic composition of modern soil carbonate is well correlated with the isotopic composition of local meteoric water the carbon isotopic cycle for co 2 in soils can be described in terms of the proportion of biomass using the c 4 photosynthetic pathway and the co 2 respiration rate of the soil at low soil respiration rates significant atmospheric co 2 mixing can occur in general the carbon isotopic composition of soil carbonate is related to the proportion of c 4 biomass present in soil but soils that freeze to the depth of carbonate formation often have a significant atmospheric component this suggests that freezing of the soil solution should be considered as another important mechanism for soil carbonate formation because of these relationships the isotopic composition of soil carbonate may be a paleoclimatic and paleoecologic indicator in cases in which diagenetic alteration has not occurred

global thermohaline circulation part i sensitivity to atmospheric moisture transport a global ocean general circulation model of idealized geometry combined with an atmospheric model based on observed transports of heat momentum and moisture is used to explore the sensitivity of the global conveyor belt circulation to the surface freshwater fluxes in particular the effects of meridional atmospheric moisture transports the numerical results indicate that the equilibrium strength of the north atlantic deep water nadw formation increases as the global freshwater transports increase however the global deep water formation that is the sum of the nadw and the southern ocean deep water formation rates is relatively insensitive to changes of the freshwater flux perturbations to the meridional moisture transports of each hemisphere identify equatorially asymmetric effects of the freshwater fluxes the results are consistent with box model results that the equilibrium nadw formation is primarily controlled by the magnitude of the southern hemisphere freshwater flux however the results show that the northern hemisphere freshwater flux has a strong impact on the transient behavior of the north atlantic overturning increasing this flux leads to a collapse of the conveyor belt circulation but the collapse is delayed if the southern hemisphere flux also increases the perturbation experiments also illustrate that the rapidity of collapse is affected by random fluctuations in the wind stress field

interdecadal changes in the enso teleconnection to the caribbean region and the north atlantic oscillation abstract the el nino southern oscillation enso phenomenon and variability in the subtropical north atlantic high sea level pressure slp are known to affect rainfall in the caribbean region an el nino event is associated with drier than average conditions during the boreal summer of year 0 and wetter than average conditions during the spring of year 1 dry conditions during the summer of year 0 of an el nino are associated with the locally divergent surface circulation engendered by the eastward shift of deep convection in the pacific ocean wet conditions during the spring of year 1 of an el nino are associated with the lagged warming of the tropical north atlantic ocean variability in the strength of the north atlantic high is governed mainly by the north atlantic oscillation nao with a positive nao phase implying a stronger than normal high and vice versa the nao is negatively correlated with caribbean rainfall indirectly via anomalous sea surface temperatures sst associated with a

periodic changes of stream flow in the last 40 years in tarim river basin xinjiang china using the annual runoff series for the last 40 years from the tarim river basin their periodic properties were analysed and their future trends predicted runoff data were collected at five hydrological gauging stations in the three main branches of the tarim river an extrapolation method and variance analysis were used to identify periods in annual runoff and a trend superposition model to predict future changes results show that there is a common period of 17 years in annual runoff changes for all three branches with hotan river showing an additional period of 10 years based on this trend it is suggested that the annual runoff of the tarim river should decrease in the period of 2006 2008 but increase ill year 2009 a and the how may possibly begin to decrease significantly in veal 2010 the long term trend of runoff in tarim basin has followed the global prediction of gcms i e began to increase in accordance with global increase of air temperature and precipitation in 1990 however it has shown a local feature of uneven changes among the head streams in the same basin which needs to be further investigated copyright c 2008 john wiley a sons ltd

the selection of sites for paleovegetational studies the judicious selection of sites for paleovegetational and paleoclimatic studies permits paleoecologists to answer specific research questions that go beyond primary descriptions of past vegetation we present a model that describes the relationship between basin size and pollen source area and predicts the proportions of local extralocal and regional pollen sampled by lake basins of different size the distinctive sampling properties of lakes peats and small hollows can be exploited to provide details of pattern in paleovegetation so long as attention is given to the limitations and problems of these types of sites combinations of site types in a single study most fully exploit the information contained in sediments

lower timberline in central colorado during the past 15 000 yr a 15 000 yr paleoenvironmental record at the elevation of lower timberline in central colorado suggests two major paleoclimatic changes during that interval 1 at 10 000 yr b p from cool moist conditions to warm moist conditions and 2 at 4 000 yr b p to warm dry conditions this sequence can be interpreted as a shift from precipitation predominantly originating from pacific winter storms before 10 000 yr b p to a predominantly monsoonal type regime lasting until 4 000 yr b p to the present pattern contemporaneity of higher upper timberline and lower lower timberline during the 10 000 to 4 000 yr b p interval is not contradictory but a result of changes in those circulation patterns

thermohaline circulation stability a box model study part ii coupled atmosphere ocean model abstract a thorough analysis of the stability of a coupled version of an interhemispheric three box model of thermohaline circulation thc is presented this study follows a similarly structured analysis of an uncoupled version of the same model presented in part i of this paper the model consists of a northern high latitude box a tropical box and a southern high latitude box which can be thought of as corresponding to the northern tropical and southern atlantic ocean respectively this paper examines how the strength of thc changes when the system undergoes forcings representing global warming conditions since a coupled model is used a direct representation of the radiative forcing is possible because the main atmospheric physical processes responsible for freshwater and heat fluxes are formulated separately each perturbation to the initial equilibrium is characterized by the total radiative forcing realized by the rate of increase and by the north south asymmetry although only weakly asymm

boundary mixing and the dynamics of three dimensional thermohaline circulations boundary mixing is implemented in an ocean general circulation model such that the vertical mixing coefficient ky is nonzero only near side boundaries and in convection regions the model is used in a highly idealized configuration with no wind forcing and very nearly fixed surface density to investigate the three dimensional dynamics of the thermohaline circulation for ky 5 20 3 1024 m2 s21 and lower the meridional overturning strength to great accuracy is proportional to meridional heat transport is proportional to the circulation 2 3 1 2 kk y y patterns resemble those from runs with uniform vertical mixing but vertical motion is entirely confined to the boundary regions near the western boundary there is upwelling everywhere near the eastern boundary there is a consistent pattern of downwelling above upwelling with downwelling reaching deeper at high latitudes this pattern is explained by convection and vertical advective diffusive balance underneath for ky 5 30 3 1024 m2 s21 and higher no steady solutions have been found the meridional overturning oscillates on a timescale of about 25 years a time averaged thermally direct overturning cell is not supported dynamically because convection extends longitudinally across the entire basin and upwelling near the western boundary does not lead to densities higher than at the eastern boundary assuming uniform upwelling in the west level isopycnals near the equator and level isopycnals along the eastern boundary south of the outcropping latitude permits the analytic determination of convection depth at the eastern wall and hence the density difference between the eastern and western walls this difference is at most one quarter the surface density difference between high and low latitudes and agrees in magnitude and latitudinal dependence with the numerical experiments scaling arguments estimate overturning strength as of the order of 10 3 106 m3 s21 and confirm the 2 3 power dependence onky the derivation also gives a dependence of overturning strength with latitude that agrees qualitatively with the numerical results the scaling for the dependence of meridional heat transport on latitude agrees well with the model results scaling for heat transport amplitude agrees less well but correctly predicts a weaker dependence on ky than maximum overturning

a wind induced thermohaline circulation hysteresis and millennial variability regimes abstract the multiple equilibria of the thermohaline circulation thc used here in the sense of the meridional overturning circulation as function of the surface freshwater flux has been studied intensively following a stommel paper from 1961 it is shown here that multistability and hysteresis of the thc also exist when the wind stress amplitude is varied as a control parameter both the massachusetts institute of technology ocean general circulation model mitgcm and a simple three box model are used to study and explain different dynamical regimes of the thc and thc variability as a function of the wind stress amplitude starting with active winds and a thermally dominant thermohaline circulation state the wind stress amplitude is slowly reduced to zero over a time period of 40 000 yr 40 kyr and then increased again to its initial value over another 40 kyr it is found that during the decreasing wind stress phase the thc remains thermally dominant until very low wind stress amplitude at which

dramatic increase in late cenozoic alpine erosion rates recorded by cave sediment in the southern rocky mountains abstract apparent increases in sedimentation rates during the past 5 ma have been inferred at sites around the globe to document increased terrestrial erosion rates but direct erosion rate records spanning this period are sparse modern and paleo erosion rates for a small alpine catchment 3108 m above sea level in the southern rocky mountains are measured using the cosmogenic radionuclides crns 10be and 26al in cave sediment bedrock on the overlying landscape surface and coarse bedload in a modern fluvial drainage the unique setting of the marble mountain cave system allows the inherited erosion rates to be interpreted as basin averaged erosion rates resulting in the first crn based erosion rate record from the rocky mountains spanning 5 myr pliocene erosion rates derived from the oldest cave sample 4 9 0 4 ma for the landscape above the cave are 4 9 1 1 m myr 1 mid pleistocene erosion rates are nearly an order of magnitude higher 33 1 2 7 to 41 3 3 9 m myr 1 and modern erosion rates are similar due to the effects of snow shielding these erosion rate estimates are likely higher than actual rates by 10 15 the most likely explanation for this dramatic increase in erosion rates which likely occurred shortly before 1 2 ma is an increase in the effectiveness of periglacial weathering processes at high elevations related to a cooler and wetter climate during the pleistocene providing support for the hypothesis that changes in late cenozoic climate are responsible for increased continental erosion

high elevation late pleistocene mis 6 5 vertebrate faunas from the ziegler reservoir fossil site snowmass village colorado abstract the vertebrate record at the ziegler reservoir fossil site zrfs near snowmass village colorado ranges from 140 to 77 ka spanning all of marine oxygen isotope stage mis 5 the site contains at least 52 taxa of macro and microvertebrates including one fish three amphibian four reptile ten bird and 34 mammal taxa the most common vertebrate is ambystoma tigrinum tiger salamander which is represented by 22 000 elements representing the entire life cycle the mastodon mammut americanum is the most common mammal and is documented by 1800 skeletal elements making the zrfs one of the largest accumulations of proboscidean remains in north america faunas at the zrfs can be divided into two groups a lake margin group dating to 140 100 ka that is dominated by woodland taxa and a lake center group dating to 87 77 ka characterized by taxa favoring more open conditions the change in faunal assemblages occurred between mis 5c and 5a vertebrates were absent from mis 5b deposits which were times of significant environmental change at the zrfs furthermore the zrfs provides a well dated occurrence of the extinct bison latifrons which has implications for the timing of the rancholabrean mammal age in the region

pliocene and pleistocene geologic and climatic evolution in the san luis valley of south central colorado rogers k l larson e e smith g katzman d smith g r cerling t wang y baker r g lohmann k c repenning c a patterson p and mackie g 1992 pliocene and pleistocene geologic and climatic evolution in the san luis valley of south central colorado palaeogeogr palaeoclimatol palaeoecol 94 55 86 sediments of the alamosa formation spanning the upper part of the gauss and most of the matuyama chrons were recovered by coring in the high 2300 m san luis valley of south central colorado the study site is located at the northern end of the rio grande rift lithologic changes in the core sediments provide evidence of events leading to integration of the san luis drainage basin into the rio grande the section which includes the huckleberry ridge ash 2 02 ma and spans the entire matuyama chron contains pollen and invertebrate and vertebrate fossils stable isotope analyses of inorganic and biogenic carbonate taken over most of the core indicate substantially warmer temperatures than occur today in the san luis valley at the end of the olduvai subchron summer precipitation decreased summer pan evaporation increased and temperatures increased slightly compared to the earlier climate represented in the core by the end of the jaramillo subchron however cold wet and warm dry cycles become evident and continue into the cold wet regime associated with the deep sea oxygenisotope stage 22 glaciation previously determined from outcrops at the same locality correspondence between the hansen bluff climatic record and the deep sea oxygen isotope record oxygen isotope stages from about 110 18 is apparent indicating that climate at hansen bluff was responding to global climatic changes

paleomagnetism of two late pleistocene lake basins in colorado an evaluation of detrital remanent magnetization as a recorder of the geomagnetic field paleomagnetic studies of magnetite bearing lacustrine sediments from two late pleistocene sections age range about 12 000 23 000 14c years one at devlins park in the front range the other at triangle park in the northern end of the sawatch range colorado indicate that detrital remanent magnetization of fine grained silty clay sediments at both locations was capable of recording changes in the magnetic field over periods as short as 100 200 years whereas coarser grained silts in the triangle park section seem to be less efficient recorders of the geomagnetic field the remanence of the fine grained sediments contains an apparent inclination error of about 9 at both localities in addition locally operative depositional and or postdepositional processes have produced both intrabasinal and interbasinal systematic differences in the remanence records of up to about 30 in declination and 10 in inclination comparison of the directional data from devlins park with those from four other broadly contemporaneous north american lake basins reveals that although there are abundant minor and several major discrepancies there are general similarities in the longer wavelength features particularly of declination the differences in the records are attributable to depositional and postdepositional factors variable and secondary overprinting local field differences sampling techniques and incorrect stratigraphic correlations the overall similarity in the records probably reflects the influence of the geomagnetic field the greatest difficulty in determining the significance of the similarities and differences between the records from the various lakes is the present lack of precise age and stratigraphic control

evolution of atlantic pacific δ13c gradients over the last 2 5 m y the evolution of interocean carbon isotopic gradients over the last 2 5 m y is examined using high resolution 13c records from deep sea cores in the atlantic and pacific oceans over much of the northern hemisphere ice ages relative reductions in north atlantic deep water production occur during ice maxima from 2 5 to 1 5 ma glacial reductions in nadw are less than those observed in the late pleistocene glacial suppression of nadw intensified after 1 5 ma earlier than the transition to larger ice sheets around 0 7 ma at a number of times during the pleistocene 13c values at dsdp site 607 in the north atlantic were indistinguishable from eastern equatorial pacific 13c values from approximately the same depth odp site 677 indicating significant incursions of low 13c water into the deep north atlantic atlantic pacific 13c values converge during glaciations between 1 13 1 05 m y 0 83 0 70 m y and 0 46 0 43 m y this represents a pseudo periodicity of approximately 300 kyr which cannot easily be ascribed to global ice volume or orbital forcing this partial decoupling at low frequencies of the 18o and 13c signals at site 607 indicates that variations in north atlantic deep water circulation cannot be viewed simply as a linear response to ice sheet forcing

a 420 000 year deuterium excess record from east antarctica information on past changes in the origin of precipitation at vostok here we present and analyze the complete vostok ice cores deuterium excess record which spans the last four climatic cycles back to 420 000 years b p to extract paleoclimate information from this record we use isotopic modeling showing that changes in deuterium excess d d 8 18o of vostok precipitation reflect changes in the average temperature of oceanic moisture sources over the last 250 000 years the deuterium excess is strongly anticorrelated with changes in earth s obliquity this reflects changes in the relative contribution of low and high latitudes to the vostok precipitation resulting from changes in the latitudinal annual mean insolation gradient governed by the obliquity however this modulation by obliquity is not observed prior to 250 000 years b p we attribute this difference to the ice flow the deeper ice under vostok station having accumulated in a location upstream vostok receiving precipitation from different oceanic origins despite this difference between the earlier and the later portions of the record the deuterium excess changes during all the glacial inceptions are similar in amplitude and timing relative to the deuterium decrease the glacial inceptions are characterized by high deuterium excess indicating the significant role of the tropics in supplying moisture to the already cold poles

thermohaline oscillations induced by strong steady salinity forcing of ocean general circulation models abstract a series of numerical experiments is conducted with a three dimensional ocean general circulation model and a two dimensional counterpart both designed for efficient integration over diffusive millennial time scales with strong steady salinity fluxes salting at low latitudes and freshening at high basin mean temperature and several other diagnostics show a series of self sustaining oscillations the oscillations termed deep decoupling oscillations exhibit halocline catastrophes at regular intervals followed by warming deep decoupled phases when the deep overturning is weak cooling flushes and in the lower range of salinity forcing a coupled phase when the deep ocean advective diffusive heat balance is almost but not quite met it is suggested that oscillations arise when a steady overturning circulation encounters a contradiction the poleward salt and heat transport needed to maintain convection in the polar ocean requires more overturning than is consistent with the reduced therm

is ocean thermohaline circulation linked to abrupt stadial interstadial transitions abstract are the abrupt stadial interstadial s is climate transitions observed in the greenland ice cores also seen in marine climate records literature which shows that the s is events have a large footprint in ocean surface marine properties encompassing the entire northern hemisphere has been reviewed whether this influence extends to the deep circulation is more equivocal on most recent evidence several of the heinrich ice rafting debris events are clearly associated with elimination of north atlantic source waters from the deep atlantic but most cores studied do not resolve the other s is events except for one new record from the bermuda rise which shows all of the major s is cycles in benthic foraminiferal deep sea corals can tell us how rapid these events are in the deep sea

pleistocene paleoenvironmental evolution at continental middle latitude inferred from carbon and oxygen stable isotope analysis of ostracodes from the guadix baza basin granada se spain abstract a representative paleoenvironmental reconstruction of continental middle latitude from ca 2 my to the upper part of middle pleistocene 279 77 ky was obtained from the carbon and oxygen stable isotopes analyzed in ostracode shells cyprideis torosa recovered in the guadix baza basin se spain an intramontaneous closed depression filled by alluvial and lacustrine sediments this study was performed along a 356 m thick composite section dated previously by paleomagnetism and the amino acid racemization method 13c and 18o profiles reflected changes in temperature the evaporation infill ratio in the water bodies and the amount of rain 13c is also affected by changes in plant biomass periods with high 13c and 18o values are associated with warm and dry regimes and with less vegetation which in some cases coincide with the development of displacive gypsum crystals whereas low 13c and 18o values correlate with cold and humid episodes which cause more vegetation biomass and therefore increasing the input of isotopically light carbon intermediate 18o values are linked to temperate dry or humid episodes when they coincide with high or low 13c values respectively 86 paleoclimatic events were distinguished in the pleistocene record from the 13c and 18o profiles from both the statistical analysis of the geochemical data and the geological observations four cold and humid long periods low 18o and four warm and dry long periods high 18o were defined this differs with respect to the paleoclimatological behavior established for the northern hemisphere where during cold periods glacial no water was available while permafrost conditions persisted whereas in warm episodes interglacial higher precipitation rates occurred good correspondences between the guadix baza basin paleoclimatic record and a marine oxygen isotope sequence two continental cores and other long mediterranean paleoenvironmental records pollen sequences from israel were found which suggested that climate changes in the guadix baza basin were in tune with global climatic changes

a chronology of post glacial landslides suggests that slight increases in precipitation could trigger a disproportionate geomorphic response large deep seated landslides are common throughout the southeastern san juan mountains of colorado and new mexico but their timing and initiation are not well understood determining when the landslides occurred would aid in clarifying the mechanisms for initiating landslides in the region and would help us to understand post glacial landscape evolution we studied seven prehistoric landslides located within the tertiary volcanic rocks of the san juan volcanic field the landslides range in area from 0 8 km2 to 11 3 km2 and most are located in areas that were previously mapped as having been ice covered during the last glaciation landslide deposits were dated using a variety of methods including surface exposure dating 36cl radiocarbon dating of basal bog sediments and organic material buried in soils and relative soil development the resulting limiting ages range from approximately 14 ka to 2 ka and show that deep seated landsliding has occurred throughout the post glacial period this broad range in ages is inconsistent with our initial hypothesis which proposed that landslides were likely the result of debuttressing of glacial walls during glacial retreat furthermore the timing of landslides does not seem to correlate with documented post glacial climatic shifts therefore we conclude that landsliding in the region was the result of wetter than normal periods lasting months to years acting on weak bedrock pre conditioned to failure and prepared by glacial debuttressing our findings suggest that the study area is likely still susceptible to deep seated landsliding and may become even more prone to large scale slope failure if future climate change increases precipitation in the san juan mountains

antarctic sea ice and the control of pleistocene climate instability a hypothesis is presented for the origin of pleistocene climate instability based on expansion of antarctic sea ice and associated changes in the oceans salinity structure the hypothesis assumes that thermohaline overturning is dominated by the reconfigured conveyor of toggweiler and samuels 1993b in which deepwater upwelling is restricted to high southern latitudes the reconfigured conveyor is shown to be potentially stabilized in an on mode by precipitation at high southern latitudes and potentially destabilized into on and off modes by the counteracting influence of antarctic sea ice the mechanism is clarified by the use of a hydraulic analogue we hypothesize that this mechanism accounts for dominant patterns of thermohaline overturning and climate instability between pleistocene warm and cold periods the hypothesis is shown to be consistent with a range of paleoceanographic evidence and to potentially account for details of observed rapid climate changes during glacial and interglacial periods including aspects of interhemispheric timing

stochastic models of the meridional overturning circulation time scales and patterns of variability the global meridional overturning circulation moc varies over a wide range of space and time scales in response to fluctuating weather perturbations that may be modelled as stochastic forcing th

handbook of environmental isotope geochemistry vol 1 the terrestrial environment a p fritz and j ch fontes editors elsevier amsterdam 1980 xii 532 pp u s 90 25 dfl 185 00

pleistocene irvingtonian cudahyan vertebrates from the texas panhandle and their geographic and paleoecologic significance abstract the cudahy fauna was named by c w hibbard for a fossil assemblage recovered from silty clays beneath the pearlette volcanic ash at the type cudahy ash pit and other sites in meade county kansas the pearlette ash comprises several variously dated yellowstone ashes the cudahy lava creek b ash is most recently dated at 0 639 0 002 ma and is an important geochronologic marker in the great plains the term cudahy ash pit local fauna is now used for that of the type locality although other cudahy type cudahyan local faunas have been described from 1968 to 1971 30 tons of silty clay and diatomite were collected from below outcrops of lava creek b ash at elevations of 988 1058 m 3240 3470 ft in the high plains of the texas panhandle screenwashing yielded about 30 mammal taxa and numerous small amphibians reptiles birds and mollusks four local faunas woody draw bull draw deadman s creek and mayfield ranch are recognized these faunas dominated by shrews and microtine arvicoline rodents are compositionally similar to each other and to the cudahy and sunbrite ash pit local faunas of meade county even though the panhandle sites are 300 340 km 186 211 miles southwest of the kansas sites and are 213 280 m 700 920 ft higher in elevation the panhandle faunas contain two new species of sorex as well as sorex cf cinereus sorex cudahyensis sorex lacustris sorex microsorex pratensis sorex megapalustris blarina cynomys spermophilus geomys tobinensis thomomys cf talpoides peromyscus cragini reithrodontomys moorei ondatra annectens mictomys meltoni microtus llanensis rare microtus paroperarius and microtus meadensis the large sample size of the latter two taxa permits statistical comparisons of teeth with those from kansas the texas faunas further substantiate the cudahyan as a subage of the irvingtonian between the older sappan and younger sheridanian subages and equivalent to irvingtonian ii based on microtine arvicoline rodents local habitats consisted of ponds or playas with marshy areas suitable for shrews muskrats and bog lemmings moist grassy meadows nearby would have been inhabited by voles mice and some of the shrews the less abundant gophers ground squirrels and prairie dog would have preferred drier grassy uplands with well drained soils several mammals e g bog lemming show affinities with extant boreal species that range through canada and the northern united states the collective evidence suggests that the texas panhandle enjoyed cooler summers with more effective rainfall during cudahy time

on the origin and age of the great sand dunes colorado abstract over the past 100 yr several hypotheses have been proposed for the origin and age of the great sand dunes these hypotheses differ widely in the descriptions of dune morphometry the immediate source of eolian sand and when sand transport occurred the primary purpose of this paper is to evaluate these hypotheses and where warranted to present new ideas about the origin and age of the great sand dunes to evaluate the previous hypotheses we had to develop more detailed information about the surficial geology of the northern san luis valley thus we mapped the surficial geology of an area extending several tens of kilometers north south and west of the great sand dunes and examined subsurface stratigraphy in more than 200 wells and borings in addition we used relative dating criteria and several radiocarbon and osl ages to establish the chronology of surficial deposits and we determined the u pb ages of detrital zircons to obtain information about the sources of the sand in the great sand dunes the first principal finding of this study is that the lower part of the closed basin north of the rio grande referred to here as the sump is the immediate source of the sand in the great sand dunes rather than the late pleistocene flood plain of the rio grande the most widely accepted hypothesis a second principal finding is that the great sand dunes are older than late pleistocene they postdate the draining of lake alamosa which began 440 ka and predate the time when streams draining the west flank of the sangre de cristo mountains were deflected by incipient dunes that formed near the mountain front geomorphic and stratigraphic evidence indicate that this deflection occurred prior to the end of the next to last glaciation bull lake i e prior to 130 ka

stable isotopes in pocket gopher teeth as evidence of a late matuyama climate shift in the southern rocky mountains abstract previous work in the san luis valley of south central colorado and northern new mexico at the hansen bluff and sam cave localities has allowed reconstruction of the paleoclimate in the region during the interval from 0 6 myr to 2 6 myr surface exposures at hansen bluff have been correlated to deep sea oxygen isotope core stages 18 23 and one of the glacial periods contained therein to the oldest nebraskan till in southwestern iowa deposits in sam cave correlate to hansen bluff on the basis of paleomagnetics climate interpretation and microtine rodent biochronology in this paper carbon and oxygen stable isotope data of herbivorous rodent teeth are used as indicators of change in the predominance of c3 c4 and cam plants and of changes in temperature and precipitation taken together with other previously analyzed data from these localities the evidence indicates a shift in the severity of glacial events that is mainly related to the hydrologic characteristics of the climate this shift may have been caused by broad regional uplift that may have been instrumental in shifting global climate cycles from the 41 000 yr matuyama cycles to the brunhes 100 000 yr dominated cycles

tropical atlantic sea surface temperature variability and its relation to el nino southern oscillation past analyses of tropical atlantic sea surface temperature variability have suggested a dipole behavior between the northern and southern tropics across the intertropical convergence zone itcz by analyzing an improved 43 year 1950 1992 record of sst smith et al 1996 and other data derived from the comprehensive ocean atmosphere data set coads it is shown that the regions north and south of the itcz are statistically independent of each other at the seasonal to interannual timescales dominating the data confirming the conclusions of houghton and tourre 1992 some dipole behavior does develop weakly during the boreal spring season when there is a tendency for sst anomaly west of angola to be opposite of that in the tropical north atlantic it is further shown that tropical atlantic sst variability is corre lated with pacific e1 nifio southern oscillation enso variability in several regions the ma jor region affected is the north atlantic area of ne trades west of 40ow along 10on 20on and extending into the caribbean there about 50 80 of the anomalous sst variability is associ ated with the pacific enso with atlantic warmings occurring 4 5 months after the mature phases of pacific warm events an analysis of local surface flux fields derived from coads data shows that the enso related atlantic warmings occur as a result of reductions in the surface ne trade wind speeds which in turn reduce latent and sensible heat losses over the region in ques tion as well as cooling due to entrainment this enso connection is best developed during the boreal spring following the most frequent season of maximum enso anomalies in the pacific a region of secondary covariability with enso occurs along the northern edge of the mean itcz position and appears to be associated with northward migrations of the itcz when the north at lantic warmings occur although easterly winds are intensified in the western equatorial atlantic in response to pacific warm events they do not produce strong local changes in sst contrary to expectations from studies based on equatorial dynamics these teleconnected wind anomalies do not give rise to significant correlations of sst in the gulf of guinea with the pacific enso as the teleconnection sequence matures strong se trades at low southern latitudes follow the de velopment of the north atlantic sst anomaly and precede by several months the appearance of weak negative sst anomalies off angola and stronger positive anomalies extending eastward from southern brazil along 15o 30os

variability and multiple equilibria of the thermohaline circulation associated with deep water formation abstract in this study the variability of the thermohaline circulation on decadal and centennial time scales that is related to the process of deep water formation is investigated this is done within the context of a simple geostrophic three layer ocean model with a rectangular closed basin when slowly varying the atmospheric forcing the model response shows sudden transitions characterized by local changes in convective activity many different equilibria were found within the thermally driven regime in stommel s sense i e downwelling occurs near the poles next the deep water formation process was analyzed with a one dimensional box model in this box model four different regimes can be identified convective nonconvective periodic and a regime where both convection and no convection are possible with the same mixed boundary conditions these regimes were identified in the circulation of the ocean model a region was traced where convection is possible according to the authors analysis but

a reassessment of the age of the fauna from cumberland bone cave maryland middle pleistocene using coupled u series and electron spin resonance dating esr

ocean gyres and abrupt change in the thermohaline circulation a conceptual analysis the implications are investigated of representing ocean gyre circulations by a diffusion term in the stommel and rooth box models of the thermohaline circulation thc in one and two hemispheres respectively the approach includes mostly analytical solution and study of the bifurcation structure but also numerical integration and feedback analysis sufficient diffusion gyre strength eliminates multiple equilibria from either model highlighting the need for accurate gyre circulation strength in general circulation models gcms when considering the potential for abrupt climate change associated with thc shutdown with diffusion steady state flow strength in the rooth model depends on freshwater forcing i e implied atmospheric water vapor transport in both hemispheres not only on that in the upwelling hemisphere as in the nondiffusive case with asymmetric freshwater forcing two solutions strong stable and weak unstable are found with sinking in the hemisphere with stronger forcing and one solution with sinking in the other hemisphere under increased freshwater forcing the two solutions in the hemisphere with stronger forcing meet in a saddle node bifurcation if diffusion is sufficiently strong to prevent a subcritical hopf bifurcation first followed by flow reversal thus the bifurcation structure with respect to freshwater forcing of the diffusive rooth model of two hemisphere thc is similar to that of the stommel model of single hemisphere thc albeit with a very different dynamical interpretation gyre circulations stabilize high latitude sinking in the stommel model in the rooth model gyre circulations only stabilize highlatitude sinking if the freshwater forcing is weaker in the sinking hemisphere than in the upwelling hemisphere by an amount that increases with diffusion the values of diffusion and freshwater forcing at which qualitative change in behavior occurs correspond to the range of the values used in and obtained with gcms suggesting that this analysis can provide a conceptual foundation for analyzing the stability of the interhemispheric thc and also for the potential of the atlantic thc to undergo abrupt change

multiple equilibria of the global thermohaline circulation abstract a general circulation model with a highly idealized geometry is used to investigate which fundamentally different equilibria of the global thermohaline circulation may exist the model comprises two identical basins representing the atlantic and pacific oceans which are connected by a circumpolar channel in the south the model circulation is driven in addition to wind forcing by restoring the sea surface temperature to prescribed values and specified freshwater fluxes in the surface salinity budget mixed boundary conditions the boundary conditions are symmetric with respect to the equator and identical for both oceans four fundamentally different stable steady states are found under the same set of boundary conditions two of the equilibria show both oceans in the same state with high altitude deep water formation occuring either in both northern or in both southern oceans respectively two additional equilibria exist in which the thermohaline circulations of the basins differ fundament

on the freshwater forcing of the thermohaline circulation in the limit of low diapycnal mixing 1 a conjecture is offered on the stability characteristics of the thermohaline circulation in the limit of very low diapycnal mixing in this limit the action of the winds on the antarctic circumpolar current acc can sustain a deep overturning pattern known as the reconfigured conveyor consisting of upwelling around antarctica and sinking in the north atlantic as shown by the work of toggweiler and others it is conjectured that in this limit northern sinking should be stabilized in an on state because of the penetration of freshwater into the ocean interior via isopycnal layers that outcrop to the surface within and south of the acc this conjecture is supported by qualitative arguments and by a hydraulic model for the reconfigured conveyor the hydraulic model takes into account the freshwater budgets of the atlantic basin antarctic surface waters and the remaining oceans it also takes into account in simple terms wind driven antarctic upwelling eddy transports and mixing within the acc changes in pycnocline depth the role of temperature forcing and advective feedbacks on salinity the hydraulic model suggests that multiple on off states of the reconfigured conveyor are possible but only if the deep waters that form in the northern hemisphere are fresher than the intermediate waters that form in the vicinity of the acc in the southern hemisphere a condition that is not satisfied in the modern ocean

middle pleistocene formation of the rio grande gorge san luis valley south central colorado and north central new mexico usa process timing and downstream implications abstract the rio grande is the fourth longest river in north america extending over 3 000 km from the rocky mountains to the gulf of mexico the pleistocene evolution of this river from individual subbasins into a coalesced fluvial system has been long debated herein we constrain the middle pleistocene evolution of the northernmost and largest rio grande basin the san luis basin and the timing of incision of the rio grande gorge based on new geologic mapping 3he surface exposure dating and u series dating on pedogenic carbonates 3he dating of shoreline deposits of closed basin lake alamosa and fluvially scoured bedrock along the rio grande gorge rim indicate the san luis basin was connected with southerly rio grande basins and progressively incised since 400 ka waning tectonic activity coupled with the hydrologic response to intensified middle pleistocene glaciation drove southward spillover of lake alamosa 400 ka expanding the river s drainage basin by nearly 24 000 km2 and adding recharge areas in the high altitude glaciated san juan tusas and sangre de cristo mountains geologic barriers to streamflow on the taos plateau were breached and basin tributary drainages were integrated and inset into their present canyons 250 200 ka with approximately 120 m of incision during 400 200 ka previous work demonstrates that southerly streamflow through basins south of the slb terminated into a large rift related bolson complex until the middle pleistocene and most axial rio grande river incision has occurred since 630 ka we propose that middle pleistocene integration of the slb was the major geomorphic event that eroded plio pleistocene tectonic and volcanic barriers and re established flow of the rio grande from the rocky mountains to the gulf of mexico

constraints on the age of the great sand dunes colorado from subsurface stratigraphy and osl dates abstract the age of the great sand dunes has been debated for nearly 150 yr seven ages ranging from miocene to late holocene have been proposed for them this paper presents new information chiefly subsurface stratigraphic data osl dates and geomorphic evidence that indicates that the great sand dunes began to form in the latter part of the middle pleistocene the dunes overlie a thick wedge of piedmont slope deposits which in turn overlies sediment of lake alamosa a paleolake that began to drain about 440 ka the wedge of piedmont slope deposits extends westward for at least 23 km and is as much as 60 m thick at a distance of 10 km from the sangre de cristo range ostracodes from one well indicate that the eastern shoreline of lake alamosa extended to within 4 3 km of where the great sand dunes eventually formed the time represented by the wedge of piedmont slope deposits is not known exactly but the wedge post dates 440 ka and was in place prior to 130 ka because by then the dunes overlying it were sufficiently close and tall enough to obstruct streams draining from the sangre de cristo range

late glacial and holocene record of climatic change in the southern rocky mountains from sediments in san luis lake colorado usa abstract large rapid climate changes occurred over the last glacial cycle in the southwestern united states and elsewhere in many regions of the world some of these changes were attributed to alternations between stadial and interstadial conditions in the north atlantic but intense debate exists on how climate anomalies in the north atlantic transmit to the southwest here we report a sediment record from san luis lake in southern colorado through analyses of grain size magnetic susceptibility mg ca total inorganic carbon 18 o and 13 c to indicate climatic and environmental changes in the southern rocky mountains over the last 16 5 ka we found that san luis lake remained hydrologically closed most of the time but overflowed during the second half of the mystery interval the big wet 15 7 14 9 ka and the latter part of the mid holocene the neopluvial 4 3 ka over the course of the last deglaciation san luis lake underwent a series of large millennial scale hydroclimatic changes such as the big dry 16 5 15 7 ka the big wet the bolling allerod dry 14 9 12 7 ka and the younger dryas wet 12 8 11 6 ka corresponding to warm cold phases in the high latitude northern hemisphere the north american monsoon waxed during the pre boreal interval 11 6 10 5 ka and waned through the holocene in phase with northward and southward displacement of the intertropical convergence zone itcz the san luis lake basin was relatively dry in the early holocene 10 5 6 7 ka wet and fluctuating in the mid holocene 6 7 2 6 ka and dry and less variable in the late holocene 2 6 0 ka we found evidence that extreme pluvial episodes of the southern rocky mountains and elsewhere in the american southwest were coeval with cold phases of the north pacific our results highlight the role of the north pacific in modulating atmospheric circulations over the region on millennial timescales

semi adiabatic model of the deep stratification and meridional overturning abstractan analytical model of the atlantic deep stratification and meridional overturning circulation is presented that illustrates the dynamic coupling between the southern ocean and the midlatitude gyres the model expressed here in terms of the two and a half layer framework predicts the stratification and meridional transport as a function of the mechanical and thermodynamic forcing at the sea surface the approach is based on the classical elements of large scale circulation theory ideal thermocline inertial western boundary currents and eddy controlled antarctic circumpolar current acc models which are combined to produce a consistent three dimensional view of the global overturning the analytical tractability is achieved by assuming and subsequently verifying that the pattern of circulation in the model is largely controlled by adiabatic processes the time mean and eddy induced isopycnal advection of buoyancy the mean stratification of the lower thermocline is determined by the surface fo

a half million year record of paleoclimate from the lake manix core mojave desert california abstract pluvial lakes in the southwestern u s responded sensitively to past climate through effects on rainfall runoff and evaporation although most studies agree that pluvial lakes in the southwestern u s reached their highest levels coeval with glacial stages the specific timing of increased effective moisture and lake level rise is debated particularly for the southwesternmost lakes we obtained a 45 m core of lacustrine sediment from lake manix the former terminus of the mojave river prior to about 25 ka and supplemented data from the core with outcrop studies these sediments provide a robust record of mojave river discharge over the last half million years lake manix persisted from ois 12 through early ois 2 including during interstadial ois 3 and interglacials ois 5 7 and 9 the ostracode faunal record displays a shift from an unexpectedly warm summer dominated lake hydrology during ois 12 to predominantly colder winter dominated conditions afterwards the ostracode based stable isotope record displays a large degree of intra sample variability and does not mimic other well known isotopic records of climate change evaporation likely buffered the manix 18o record from most of the expected isotopic differences between interglacial and glacial interval discharge isotopically depleted and stable lakes occurred only four to six times most notably during ois 7 and ois 9 internal drainage basin changes also affected the isotopic record persistence of lakes in the manix basin during interglacials requires atmospheric or oceanic circulation controls on the mean position of the pacific storm track other than large ice sheets we propose that the relative strength and sign of the northern annular mode nam and its influence on atmospheric river derived precipitation is a potential explanation

an adjoint analysis of the meridional overturning circulation in an ocean model abstract using the adjoint of a fully three dimensional primitive equation ocean model in an idealized geometry spatial variations in the sensitivity to surface boundary forcing of the meridional overturning circulation s strength are studied steady state sensitivities to diapycnal mixing wind stress freshwater and heat forcing are examined three different commonly used boundary forcing scenarios are studied both with and without wind forcing almost identical circulation is achieved in each scenario but the sensitivity patterns show major quantitative and qualitative differences sensitivities to surface forcing and diapycnal mixing are substantially larger under mixed boundary conditions in which fluxes of freshwater and heat are supplemented by a temperature relaxation term or under flux boundary conditions in which climatological fluxes alone drive the circulation than under restoring boundary conditions the sensitivity pattern to diapycnal mixing which peaks in the tropics is similar

an interhemispheric four box model of the meridional overturning circulation abstract the authors introduce a four box interhemispheric model of the meridional overturning circulation a single box represents high latitudes in each hemisphere and in contrast to earlier interhemispheric box models low latitudes are represented by two boxes a surface box and a deep box separated by a thermocline in which a balance is assumed between vertical advection and vertical diffusion the behavior of the system is analyzed with two different closure assumptions for how the low latitude upwelling depends on the density contrast between the surface and deep low latitude boxes the first is based on the conventional assumption that the diffusivity is a constant and the second on the assumption that the energy input to the mixing is constant there are three different stable equilibrium states that are closely analogous to the three found by bryan in a single basin interhemispheric ocean general circulation model one is quasi symmetric with downwelling in high latitudes of both hemispheres a

diminished groundwater recharge and circulation relative to degrading riparian vegetation in the middle tarim river xinjiang uygur western china riparian vegetation in the middle tarim river is being degenerated and groundwater in the riparian system is a key factor controlling this process vegetation degeneration is related to the reduced inflow from the upstream reduced recharge to riparian groundwater lowered water table and increased salinity of the groundwater system systematic chemical and isotopic sampling of river water and groundwater transects perpendicular to the river show that riparian groundwater is fed by the river river water picks up salts from the river bed and or from irrigation returns as it flows downstream and groundwater from riverbank sediments mirror this pattern mineral dissolution and evapotranspiration are the main mechanisms of salt accumulation in groundwater reduced recharge from the river has resulted in a distinctive zoning pattern in groundwater salinity groundwater residence times derived from tritium contents indicate that the extent of modern recharge since the 1960s is limited to approximately 1500 m from the riverbank in the middle reaches of the tarim river vegetation grows better in habitats with modern groundwater than in other areas the embankment built in 2001 along one side of the tarim river appears to have reduced groundwater recharge causing the water table to drop and ecosystem to degenerate copyright c 2009 john wiley sons ltd

the southern oscillation part i global associations with pressure and temperature in northern winter abstract we describe the global correlations between a measure of the southern oscillation and sea level pressure and surface air temperature in the northern winter the stability of these correlations were tested on the northern hemisphere for an 80 year period and it turned out that most stable correlation coefficients were found over india the north pacific ocean the rocky mountains and the central and western north atlantic ocean on the southern hemisphere most records are too short for a similar test but the following may tentatively be said about the southern oscillation in middle and high southern latitudes when pressure is low in lower latitudes over the south pacific ocean it tends to be high at higher latitudes of that ocean high over east antarctica and low in the belt of westerlies in the indian and south atlantic oceans in the zonal average on both hemispheres the pressure gradients in this extreme of the oscillation tend to be steeper at lower latitudes and flatter at higher latitud

substantial contribution to sea level rise during the last interglacial from the greenland ice sheet during the last interglacial period the eemian global sea level was at least three metres and probably more than five metres higher than at present1 2 complete melting of either the west antarctic ice sheet or the greenland ice sheet would today raise sea levels by 6 7 metres but the high sea levels during the last interglacial period have been proposed to result mainly from disintegration of the west antarctic ice sheet3 with model studies attributing only 1 2 m of sea level rise to meltwater from greenland4 5 this result was considered consistent with ice core evidence4 although earlier work had suggested a much reduced greenland ice sheet during the last interglacial period6 here we reconsider the eemian evolution of the greenland ice sheet by combining numerical modelling with insights obtained from recent central greenland ice core analyses our results suggest that the greenland ice sheet was considerably smaller and steeper during the eemian and plausibly contributed 4 5 5 m to the sea level highstand during that period we conclude that the high sea level during the last interglacial period most probably included a large contribution from greenland meltwater and therefore should not be interpreted as evidence for a significant reduction of the west antarctic ice sheet

parameter sensitivity of primitive equation ocean general circulation models abstract experiments with a low resolution primitive equation ocean general circulation model with idealized basin geometry and surface forcing have been carried out in order to identify the processes controlling the climatically important aspects of the circulation emphasis was placed on the sensitivity of the model solutions to the magnitude of the vertical diffusivity scaling arguments suggest and the numerical experiments confirm that the solutions are most sensitive to the magnitudes of the wind stress curl and the vertical diffusivity for small vertical diffusivity wind forcing dominates the solution the vertical scale of the thermocline is set by the strength of the ekman pumping and there is a multiple gyre circulation in the upper layers for large vertical diffusivity diabatic surface forcing dominates the solution vertical diffusion controls the vertical scale of the thermocline and there is a single large anticyclonic gyre in the upper layers both the meridionally and zonally inte