pliocene climate history of the western united states derived from lacustrine ostracodes abstract pliocene sediments from several sites in western north america contain ostracodes indicating deposition in lacustrine and wetland settings the ostracodes offer a means of reconstructing the aquatic paleoenvironment because water temperature chemistry and lake volume are coupled to climate reconstruction of these parameters provides a direct insight into pliocene climate the site ages were determined from tephrochronology paleomagnetics and associated mammals the morphology of many ostracode species also provides direct information about the paleoenvironment in which they lived during the pliocene about 3 5 2 5 ma some species have unusually ornate carapace morphology indicative of large geologically stable lakes which must have required a stable climate to sustain them north american pliocene climate changed from a modern like state 4 5 3 5 ma to a period with greater precipitation and less evaporation than today 3 5 2 5 ma this wetter period inferred from the large geologically long lived lakes implies a stable atmospheric circulation pattern the stable circulation pattern collapsed around 2 5 ma and climate returned to a modern like situation

quantifying climatic change through the last glacial interglacial transition based on lake isotope palaeohydrology from central turkey questions remain as to the nature of climatic change through the last glacial interglacial transition in the eastern mediterranean region particularly the relative contribution of evaporation and precipitation to regional water balance here changes in oxygen isotope values through this time period from eski acigol a crater lake in central turkey are investigated using hydrological and isotope mass balance models these allow changes in evaporation and precipitation to be quantified and their relative importance evaluated we show that it is the volumetric flux rate of water passing through the lake system and not the precipitation to evaporation ratio per se which controlled the stable isotope record in eski acigol early holocene precipitation is shown to be much greater than that during both the latter part of the last glaciation and the present day we test these calculated values against other records in the same region firstly with other lake records in anatolia the konya basin and lake van and secondly with isotope inferred palaeo precipitation data from soreq cave in israel this reveals a contrast between pre and post lgm precipitation values in turkey wetter and drier respectively and also suggests that during the last glacial interglacial transition there was a more marked precipitation gradient than at present between northern interior and southern coastal parts of the eastern mediterranean region

estimating groundwater exchange with lakes 1 the stable isotope mass balance method groundwater inflow and outflow contributions to the hydrologic budget of lakes can be determined using a stable isotope 18o 16o mass balance method the stable isotope method provides a way of integrating the spatial and temporal complexities of the flow field around a lake thereby offering an appealing alternative to the traditional time and labor intensive methods using seepage meters and an extensive piezometer network in this paper the method is applied to a lake in northern wisconsin demonstrating that it can be successfully applied to lakes in the upper midwest where thousands of similar lakes exist inflow and outflow rates calculated for the wisconsin lake using the isotope mass balance method are 29 and 54 cm yr respectively which compare well to estimates derived independently using a three dimensional groundwater flow and solute transport model of 20 and 50 cm yr such a favorable comparison lends confidence to the use of the stable isotope method to estimate groundwater exchange with lakes in addition utilization of stable isotopes in studies of groundwater lake systems lends insight into mixing processes occurring in the unsaturated zone and in the aquifer surrounding the lake and verifies assumed flow paths based on head measurements in piezometers

ocean atmosphere forcing of centennial hydroclimate variability in the pacific northwest reconstructing centennial timescale hydroclimate variability during the late holocene is critically important for understanding large scale patterns of drought and their relationship with climate dynamics we present sediment oxygen isotope records spanning the last two millennia from 10 lakes as well as climate model simulations indicating that the little ice age was dry relative to the medieval climate anomaly in much of the pacific northwest of north america this pattern is consistent with observed associations between the el nino southern oscillation enso the northern annular mode and drought as well as with proxy based reconstructions of pacific and atlantic ocean atmosphere variations over the past 1000 years the large amplitude of centennial variability indicated by the lake data suggests that regional hydroclimate is characterized by longer term shifts in enso like dynamics and that an improved understanding of the centennial timescale relationship between external forcing and drought is necessary for projecting future hydroclimatic conditions in western north america

quaternary palaeolakes in the evolution of semidesert basins with special emphasis on lake bonneville and the great basin u s a abstract this paper reviews attributes of quaternary lakes and lake basins which are often important in the environmental prehistory of semideserts basin floor and basin closure morphometry have set limits on palaeolake sizes lake morphometry and basin drainage patterns have influenced lacustrine processes and water and sediment loads have influenced basin neotectonics information regarding inundated runoff producing and extra basin spatial domains is acquired directly from the palaeolake record including the littoral morphostratigraphic record and indirectly by reconstruction increasingly detailed hypotheses regarding lake bonneville the largest late pleistocene palaeolake in the great basin are subjects for further testing and refinement oscillating transgression of lake bonneville began about 28 ka the highest stage occurred about 15 ka and termination occurred abruptly about 13 ka a final resurgence of perennial lakes probably occurred in many subbasins of the great basin between 11 and 10 ka when the highest stage of great salt lake successor to lake bonneville developed the gilbert shoreline and when the russell shoreline formed 500 600 km away in the carson desert of the lake lahontan basin the highest post gilbert stage of great salt lake which has been one of the few permanent lakes in the great basin during holocene time probably occurred between 3 and 2 ka a set of eighteen general observations regarding geologic aspects of the palaeolake record in semidesert basins is helping to guide quaternary studies in the great basin

cyclostratigraphy and orbital tuning of the terrestrial upper santonian lower danian in songliao basin northeastern china abstract the songke 1 north sk 1n borehole recovered a continuous 1541 66 m late santonian early danian terrestrial succession in songliao basin sb northeastern china it provides a unique record for improving our understanding of continental paleoclimate and ecological system in cretaceous greenhouse world here we use thorium th logging data as a paleoenvironmental and paleoclimatic proxy to conduct a detailed cyclostratigraphic study on the sk 1n core power spectra evolutionary fast fourier transformation and wavelet analysis all reveal significant decameter to meter scale sedimentary cycles in the nenjiang k 2 n sifangtai k 2 s and mingshui k 2 m formations the ratios of cycle wavelengths in these stratigraphic units are 20 5 2 1 and are interpreted as milankovitch cycles of 405 kyr and 100 kyr eccentricity 38 4 kyr obliquity and 20 kyr precession cycles respectively an astronomical time scale ats is established by tuning filtered 405 kyr eccentricity cycles to a target curve of the astronomical solution la2010d based on the magnetostratigraphic time framework of the sk 1n borehole this ats provides precise numerical ages for stratigraphic boundaries biozones geological and geophysical events and serves as a basis for correlation of strata and events between marine and terrestrial systems the cretaceous paleogene k pg campanian maastrichtian santonian campanian boundaries are estimated at core depths of 318 m 752 8 m and 1751 1 m respectively a 3 8 myr long hiatus between the nenjiang k 2 n and sifangtai k 2 s formations occurs from 76 1 to 79 9 million years ago the ages and durations of magnetochrons c33r to c30n are precisely estimated and provide new constraints on the late cretaceous geomagnetic polarity time scale gpts and south atlantic sea floor spreading rates

vegetation zones and diagnostic pollen profiles of a coastal peat swamp bocas del toro panama abstract a survey of the dominant vegetative cover of a large domed coastal swamp near changuinola in the province of bocas del toro panama has been undertaken as an initial step in reconstructing the holocene history of peat accumulation on this coast seven phasic communities of peat forming vegetation are defined and mapped 1 rhizophora mangle mangrove swamp 2 mixed back mangrove swamp 3 raphia taedigera palm swamp 4 mixed forest swamp 5 campnosperma panamensis forest swamp 6 sawgrass stunted forest swamp 7 myrica cyrilla bog plain pollen extracted from surface peat samples and collected from dominant vegetation at representative sites is used to prepare a pollen profile of each phasic community these profiles are then compared to pollen distribution in 2 peat cores one from the deep central part of the deposit and the second from a site near the marine margin in order to construct a history by floral succession of the 4000 year evolution of the deposit the changuinola mire originated as freshwater palm and hardwood forest swamps that developed in close proximity to both the changuinola river mouth probably behind a barrier bar and freshwater lagoon system adjacent to a low energy mangrove dominated bay the early swamp was likely drained to the southeast by brackish blackwater creeks much as it is today and formerly extended considerably farther in the direction of almirante bay the palm swamp was succeeded by hardwood forest swamp dominated by a very limited number of specialized species only one of which campnosperma panamensis is prone to forming monospecific stands increasing accumulation of woody peat promoted by the everwet climate impeded drainage of the mire leading to doming increased oligotrophy and establishment of bog plain conditions in a manner similar to that described by the anderson model of succession in the coastal swamps of malaysia and indonesia development of the changuinola mire did not require the initial mangrove phase which is common to the peat swamps of southeast asia as the palm raphia taedigera is able to colonise and institute peat accumulation in a variety of freshwater and brackish environments

evaporation induced 18o and 13c enrichment in lake systems a global perspective on hydrologic balance effects growing pressure on sustainable water resource allocation in the context of global development and rapid environmental change demands rigorous knowledge of how regional water cycles change through time one of the most attractive and widely utilized approaches for gaining this knowledge is the analysis of lake carbonate stable isotopic compositions however endogenic carbonate archives are sensitive to a variety of natural processes and conditions leaving isotopic datasets largely underdetermined as a consequence isotopic researchers are often required to assume values for multiple parameters including temperature of carbonate formation or lake water delta o 18 in order to interpret changes in hydrologic conditions here we review and analyze a global compilation of 57 lacustrine dual carbon and oxygen stable isotope records with a topical focus on the effects of shifting hydrologic balance on endogenic carbonate isotopic compositions through integration of multiple large datasets we show that lake carbonate delta o 18 values and the lake waters from which they are derived are often shifted by 10 parts per thousand relative to source waters discharging into the lake the global pattern of delta o 18 and delta c 13 covariation observed in 70 of the records studied and in several evaporation experiments demonstrates that isotopic fractionations associated with lake water evaporation cause the heavy carbon and oxygen isotope enrichments observed in most lakes and lake carbonate records modeled endogenic calcite compositions in isotopic equilibrium with lake source waters further demonstrate that evaporation effects can be extreme even in lake records where delta o 18 and delta c 13 covariation is absent aridisol pedogenic carbonates show similar isotopic responses to evaporation and the relevance of evaporative modification to paleoclimatic and paleotopographic research using endogenic carbonate proxies are discussed recent advances in stable isotope research techniques present unprecedented opportunities to overcome the underdetermined nature of stable isotopic data through integration of multiple isotopic proxies including dual element c 13 excess values and clumped isotope temperature estimates we demonstrate the utility of applying these multi proxy approaches to the interpretation of paleohy droclimatic conditions in ancient lake systems understanding past present and future hydroclimatic systems is a global imperative significant progress should be expected as these modern research techniques become more widely applied and integrated with traditional stable isotopic proxies c 2015 elsevier ltd all rights reserved

geochemistry of ostracode calcite part 2 the effects of water chemistry and seasonal temperature variation on candona rawsoni abstract reconstruction of lake paleochemistry from the isotope and trace element composition of fossil ostracodes requires an understanding of geochemical variations induced by the seasonality of shell formation monthly field collections of live candona rawsoni from two hyposaline lakes in the eastern dakotas reveal large variations in 18 o values 2 5 o and mg ca and sr ca ratios up to 35 among individual adults within the same assemblage because the measured variations in water chemistry were small the amplitude of ostracode 18 o and mg ca variations must result from seasonal temperature variation ostracode sr ca ratios which should be temperature independent shows strong positive covariance with shell mg ca these results imply that sr uptake in ostracode calcite increases with the mg content of the shell the partitioning coefficient for mg on the other hand appears to decreases at high values of mg ca in the host water the geochemical variability in the field collections indicates that c rawsoni populations are composed of multiple overlapping generations that attain maturity at different times many juveniles apparently undergo several molts in mid summer and persist as penultimate instars a 1 into the following spring geochemical analysis of late instar juvenile c rawsoni from lacustrine sedimentary records is likely to provide information on mid summer conditions large numbers of fossil ostracodes should be analyzed from each stratigraphic interval to eliminate seasonal noise from the paleochemistry records of temperate region lakes

distinct patterns of chemical weathering in the drainage basins of the huanghe and xijiang river china evidence from chemical and sr isotopic compositions abstract aquatic geochemistry on watershed background helps elucidate the influence of natural processes and anthropogenic activities to the environment the 87sr 86sr ratios and dissolved loads were measured in 35 water samples from the huanghe and xijiang river drainage basins the dissolved loads and 87sr 86sr ratios differed considerably between the two drainage basins the 87sr 86sr ratios of the xijiang river are between 0 70856 and 0 70936 with an average of 0 70894 reflecting weathering of carbonate minerals as the dominant source and anthropogenic pollution as the secondary one especially in the upper reach tributaries while the sr isotopic ratios in the huanghe river range from 0 70986 to 0 71139 averaging 0 71118 that results primarily from evaporite dissolution precipitation contributes to 2 8 and 5 7 to the chemistry of the xijiang and huanghe river respectively evaporite dissolution and silicate weathering account for 79 4 and 11 3 respectively in the huanghe river while carbonate dissolution and silicate weathering account for 75 4 and 14 6 respectively in the xijiang river the chemical weathering rate is 107 6 t km2 yr in the xijiang river drainage basin and 34 2 t km2 yr in the huanghe river drainage basin the co2 consumption budget is 305 2 109 mol yr in the xijiang river drainage basin and 147 9 109 mol yr in the huanghe river drainage basin

palaeohydrological significance of late quaternary strontium isotope ratios in a tropical lake ostracods preserved in late quaternary sediments of wallywash great pond a fresh coastal lake in sw jamaica record temporal variations in the strontium isotope composition of lake water oxygen isotope and sr ca ratios in ostracods reveal temporal variations in the lake s hydrology related to effective precipitation and in its salinity related to varying marine saline groundwater input from changes in relative sea level evaluation of isotopic and trace element data indicates that the stratigraphic variations in 87sr 86sr ratios during the late quaternary are best explained by climatically controlled hydrological changes during wetter periods the lake s sr budget was dominated by springwater input with relatively low 87sr 86sr ratio whereas during drier times reduced springflow possibly coupled with input of more radiogenic sr from other sources such as sea spray aerosols and perhaps saharan dust led to an increase in the sr isotope ratio of the lake water despite proximity of the lake to the sea and evidence for slight intrusion of marine saline groundwater in the past however the extent of marine input appears to have had limited influence on the lake s sr isotope ratios whereas the 87sr 86sr ratios cannot be used as a palaeosalinity proxy in this particular lake they do provide valuable information about the mechanisms underlying hydrological change

lake level reconstruction for 12 8 2 3 ka of the ngangla ring tso closed basin lake system southwest tibetan plateau article i nfo we present a shoreline based millennial scale record of lake level changes spanning 12 8 2 3 ka for a large closed basin lake system on the southwestern tibetan plateau fifty three radiocarbon and eight u th series ages of tufa and beach cement provide age control on paleoshorelines ringing the basin supplemented by nineteen ages from shell and aquatic plant material from natural exposures generally recording lake regressions our results show that paleo ngangla ring tso exceeded modern lake level 4727 m asl continuously between 12 8 and 2 3 ka the lake was at its highstand 135 m 4862 m asl above the modern lake from 10 3 ka to 8 6 ka this is similar to other closed basin lakes in western tibet and coincides with peak northern hemisphere summer insolation and peak indian summer monsoon intensity the lake experienced a series of millennial scale oscillations centered on 11 5 10 8 8 3 5 9 and 3 6 ka consistent with weak monsoon events in proxy records of the indian summer mon soon it is unclear whether these events were forced by north atlantic or indian ocean conditions but based on the abrupt lake level regressions recorded for ngangla ring tso they resulted in significant periodic reductions in rainfall over the western tibetan plateau throughout the holocene

controls on trace element sr mg compositions of carbonate cave waters implications for speleothem climatic records abstract at two caves clamouse s france and ernesto ne italy cave drip and pool waters were collected and sampled at intervals over a 2 3 year period mg ca and sr ca concentration ratios corrected for marine aerosols are compared with those of bedrocks and in some cases aqueous leachates of soils and weathered bedrocks cave waters do not lie along mixing lines between calcite and dolomite of bedrock carbonate but typically show enhanced and covarying mg ca and sr ca four factors are considered as controlling processes 1 the much faster dissolution rate of calcite than dolomite allows for the possibility of increase of mg ca if water rock contact times are increased during drier conditions a theoretical model is shown to be comparable to experimental leachates 2 prior calcite precipitation along a flow path is a powerful mechanism for generating enhanced and covarying mg ca and sr ca ratios this mechanism requires the solution to lose co 2 into pores or caverns 3 incongruent dolomite dissolution has only limited potential and is best regarded as two separate processes of dolomite dissolution and calcite precipitation 4 selective leaching of mg and sr with respect to ca is shown to be important in leachates from ernesto where it appears to be a phenomenon of calcite dissolution in general selective leaching can occur whenever ca is sequestered into precipitates due to freezing or drying of soils or if there is derivation of excess sr and mg from non carbonate species the ernesto cave has abundant water supply which in the main chamber is derived from a reservoir with year round constant p co 2 of around 10 2 4 and no evidence of calcite precipitation in the karst above the cave two distinct but overlying trends of enhanced and covarying mg ca and sr ca away from the locus of bedrock compositions are due to calcite precipitation within the cave and at a variable drip site due to enhanced selective leaching at slow drip rates mg enhancement in the first chamber is due to a more dolomitic bedrock and longer residence times the clamouse site has a less abundant water supply and presents geochemical evidence of prior calcite precipitation both in the cave and in overlying porous dolomite dedolomitized limestone bedrock initial p co 2 values as high as 10 1 are inferred experimental incubations of clamouse soils which generated enhanced p co 2 and precipitated caco 3 had compositions similar to the karst waters calcite precipitation is inferred to be enhanced in drier conditions hydrological controls on cave water chemistry imply that the trace element chemistry of speleothems may be interpretable in palaeohydrological terms drier conditions tends to promote not only longer mean residence times enhancing dolomite dissolution and hence mg ca but also enhances degassing and calcite precipitation leading to increased mg ca and sr ca

stable isotope paleoclimatology of brine inclusions in halite modeling and application to searles lake california abstract the isotopic composition of brine inclusions in halite can be used as paleohydrologic and paleoclimatic indicators in arid environments for this purpose steady state isotopic models of the brine inclusions in halite were developed for three hydrologic regimes the evaporation pan terminal lake and through flow lake or strait exchange sea the model calculations predict that the steady state isotopic compositions are sensitive to the hydrologic regime to the normalized relative humidity and to the isotopic compositions of atmospheric water vapor and inflow water short term fluctuations of the isotopic parameters in the steady state equations are not important and in general only the long term mean values are recorded in the brine inclusions in halite modern and old up to 1 95 ma samples of nonmarine halite from searles lake california were analyzed for the chemical and isotopic compositions of the brine inclusions from the modern surface halite the isotopic composition of the atmospheric water probably of the winter time and the probable seasons when the halite samples were deposited were obtained from brine inclusions of the 1 3 ma halite the annual mean isotopic compositions of atmospheric water vapor and inflow water were calculated using the steady state terminal lake model the calculated isotopic composition of the 1 3 ma inflow water shows a d enrichment of about 60 compared to the modern counterpart the amount of this d enrichment is similar to that found from the calcite veins at the spring mountain in nevada wionograd et al 1985 which was attributed to the effect of the orogeny of the sierra nevada the chemical composition of the brine inclusions indicates that the owens river was more enriched in hco 3 co 3 2 10 000 and 28 000 a ago than at present probably due to larger contribution of long valley caldera hot spring waters

magnesium content of non marine ostracod shells a new palaeosalinometer and palaeothermometer abstract ostracods are microcrustaceans with calcitic carapaces each consisting of two valves or shells that grow by moulting and regeneration of sequentially larger valves up to nine times before reaching maturity the mg content of these valves is species dependent and is a function of their growth stage and the temperature salinity mg content and mg ca ratio of their host water newly formed partly calcified valves of the non marine nektic ostracod mytilocypris henricae have high mg contents 100 000 ppm mg in caco3 at individual shell weights of 2 g this relative mg content decreases as calcification proceeds until shell weights exceed 120 g whereupon the mg content is constant and useful as an indicator of the ostracod s environment mg ca ratios in individual ostracod valves from several species of the genera mytilocypris and australocypris from culture experiments and a series of australian lakes indicate that the valve s mg content increases as a function of increasing temperature and salinity within the range 11 25 c and for waters containing 24 2325 ppm mg the molar mg ca ratio in these ostracods fully calcified valves increases by 0 0015 or 370 ppm mg in caco3 per 1 c and per change of 125 ppm mg in solution for most mainland australian lakes a change in water composition of 125 ppm mg corresponds to a change of 3 5 in salinity for a given mg content of a lake the mg ca ratio of the water may vary within the range of common lacustrine values without significantly affecting mg uptake in the ostracod valves these relationships can provide palaeosalinities and or palaeotemperatures for ostracods recovered from continental sediments for mg partitioning an increase in temperature and salinity produce increased mg content in ostracods whereas for 18o fractionation in biogenic calcite an increase in temperature produces a decrease in 18o of the ostracod and an increase in salinity leads to an increase in 18o of the ostracod thus a combination of 18o and mg measurements on non marine ostracods should enable unique resolution of palaeotemperature and palaeosalinity variations

chronology of the terrestrial upper cretaceous in the songliao basin northeast asia the cretaceous was a period of great environmental biological and geodynamical change but a paucity of an accurate time frame for continental strata has prevented our understanding of the terrestrial processes and the integration of marine and terrestrial records here we present an integrated chronology for the terrestrial upper cretaceous in east asia based on new high resolution magnetostratigraphic results and previously published sims u pb zircon analyses of the cretaceous continental scientific drilling borehole sequence from the songliao basin in northeast china hereafter termed ccsd sk i which consists of a sedimentary sequence from the upper quantou formation k2q to the mingshui formation k2m correlation to the geomagnetic polarity timescale was achieved by combining magnetostratigraphic and sims u pb zircon geochronologic data and lithostratigraphy our correlation suggests that the ccsd sk i sedimentary sequence spans from the upper chron c34n to chron c29r the age of the sedimentary sequence from the upper quantou formation k2q to the mingshui formation k2m in the songliao basin can thus be constrained to an interval from the turonian to the maastrichtian ages in addition our correlation puts the cretaceous paleogene boundary in the upper part of member 2 of the mingshui formation k2m2 the constructed comprehensive chronology allows the precise correlation of the terrestrial upper cretaceous of the songliao basin to marine strata as well as other continental sequences

role of the westerlies in central asia climate over the cenozoic abstract there remains substantial debate concerning the relative roles of tectonics and of global climate in pacing the evolution of aridity in central asia over the cenozoic tibetan plateau uplift variable monsoonal strength paratethys retreat and reduced moisture transport from the north atlantic have all been hypothesized to drive aridity in central asia distinguishing between these mechanisms requires knowledge of moisture transport pathways to central asia through time presently central asia receives recycled high 18 o moisture that has been transported across eurasia by the westerlies while southern tibet receives low 18 o moisture distilled by passage over the himalaya here we reconstruct the spatial distribution of oxygen isotopes in precipitation since the early eocene using a compilation of 18 o data from 2750 sedimentary carbonate samples across asia the spatial distribution of paleo precipitation 18 o remains remarkably similar through time with low 18 o values in the lee of the himalaya in southern tibet constant high 18 o values in central asia and intermediate values on the central plateau despite independent evidence for substantial changes in both topography and climate through time these results suggest that a long standing topographic feature has continuously blocked southerly moisture and subsequent progressive uplift of the tibetan plateau has had little impact on central asian climate in turn southerly monsoonal moisture has never persistently extended northward of the central plateau as a result the westerlies have remained the dominant moisture source in central asia since at least the early eocene therefore sedimentary aridity indicators in central asia such as loess deposition are controlled by the trans eurasian westerly moisture flux rather than tibetan plateau uplift

δ13c δ18c covariance as a paleohydrological indicator for closed basin lakes the relationship between 18o and 13c in a closed basin lake is a function of hydrological change vapor exchange lake productivity and total co2 or carbonate alkalinity concentration covariance of 18o and 13c usually occurs in lake sediments under the condition of hydrological closure for time periods of the order of 5000 yr or longer on shorter time scales however certain subtleties in the use of the 13c 18o covariance as a hydrology history indicator must be recognized the covariant trend may not be found in hyper alkaline lakes because of the insensitivity of 13c to lake volume changes for less alkaline closed basin lakes covariance of 13c 18o will result from relatively rapid increase or decrease of lake volume with the rapid decrease condition giving rise to heavy and narrow ranged isotopic values for both 13c and 18o due to the coupled evaporation productivity effect when the lake volume remains stable as is the case for an over flowing open lake poor 13c 18o covariance will ensue due to the effect of vapor exchange with the atmosphere we have used the measured 13c 18o covariance in carbonate sediments of mono lake california to deduce paleo hydrological and lake water alkalinity and salinity variations and found the results in consonance with holocene lake level history derived from the 18o and other information

two different r n geomagnetic reversals with identical vgp paths recorded at the same site we have studied two reverse normal r n geomagnetic reversals recorded at different stratigraphical levels of a single section of tortonian marine clays in western crete we report here that the virtual geomagnetic pole vgp paths associated with these two transitions which are separated in time by a period of 1 myr are identical although the possibility of an accidental coincidence cannot be rejected this result suggests that the basic mechanisms in the geodynamo leading to a reversal might persist unchanged over a long period of time moreover the two vgp paths are largely constrained in longitude along a mean great circle located 80 w of the site so that the transitional field cannot be correctly described in terms of pure axial symmetry

quantifying closed basin lake temperature and hydrology by inversion of oxygen isotope and trace element paleoclimate records lake systems are important paleoclimate archives that preserve ecosystem and hydrologic responses to critical periods in earth history such as carbon cycle perturbations and glacial interglacial cycles geochemical measurements of biogenic carbonate for example 18o 13c 87sr 86sr li u sr and mg are indicators of hydrologic variability in lake systems throughout the geologic record in this study we present a new closed basin lake modeling approach hybim the hydrologic balance inverse model that employs a system of total differential equations and uses the measured 18o sr ca and mg ca of biogenic carbonate to determine changes in temperature runoff and lake evaporation using equally spaced time steps these equations are simultaneously solved to constrain the hydrologic parameters of the lake as recorded in biogenic carbonate we use a monte carlo approach to account for uncertainty in the input parameters such as 18o temperature relationships partition coefficient uncertainty and watershed solute chemistry for illustrative purposes we apply the model to two ostracod valve datasets covering different timescales 1 the cretaceous songliao basin northeast china and 2 holocene lake miragoane haiti modern water measurements of water isotopes and cation concentrations from each location are required as model inputs we compare our modeling results with author interpretations and geologic observations the modeling approach presented in this study can be applied to other closed basin lake records can be modified for other calcifying species for example gastropods or mollusks or with calibration to inorganic lacustrine carbonate in addition this approach holds promise for extension with additional proxy measurements that is d u ca or li ca and changing source area on tectonic timescales using proxies that reflect changing source lithology that is sr and pb isotopes future incorporation of age model uncertainty in the monte carlo approach will also provide utility by quantifying temporal uncertainty on the hydrologic response recorded by lake sediments

variability in ostracod partition coefficients d sr and d mg implications for lacustrine palaeoenvironmental reconstructions living and sub recent ostracods were collected from four lakes and foul springs located in belgium france and spain characterized by low water mg ca ratios from 0 08 to 0 65 valves of ostracods belonging to cyprideis torosa candona neglecta c marchica and c candida have been extensively analysed for their magnesium and strontium contents with respect to calcium comparisons of these results are made using the same elements from the waters in which the ostracod valves calcified our results offer new and somewhat different interpretations with respect to the uptake of mg and sr in the calcitic ostracod valves these findings include the following 1 the partition coefficient l sr for ostracods belonging to the same genus candona is not a constant the observed variability in d sr values could be due to the rare of biological calcification although more investigations are necessary to assess the influence of this factor it is possible that some taxa grouped under the genus candona belong to a different genus 2 even at the species level cyprideis torosa the ostracod d sr cannot be considered to be constant at low water mg ca ratio 0 65 our d sr value for c torosa is higher than those determined by others for c torosa calcified in evolved marine water 3 the mg ca ratio of the water appears as a major factor influencing d mg for ostracods for low water mg ca values similar to 2 d mg drastically increases this phenomenon is also effective for inorganic calcite implying a non biologic effect that could be linked to mg2 adsorption consequently large variations in me content of fossil ostracod valves from selected lacustrine sequences with expected low water mg ca similar to 2 can be due to small variations in water mg ca c 1998 elsevier science b v all rights reserved

a late quaternary palaeolimnological record from jamaica based on trace element chemistry of ostracod shells abstract the trace element chemistry of ostracod shells provides one of the most promising methods for palaeolinmological reconstruction in this paper molar sr ca and mg ca ratios in shells of the ostracod cypretta brevisaepta furtos 1934 were used to reconstruct the late quaternary palaeolimnology of wallywash great pond a small freshwater coastal lake in jamaica analyses of modern jamaican ostracods were undertaken to establish molar distribution coefficients kd d m s for this species k d sr 0 306 0 069 k d mg 0 0142 0 0088 these values were used to reconstruct past levels of sr ca water and mg ca water for a 9 23 m lake sediment core from wallywash great pond the results were coupled with previously published stable isotope 18 o and 13 c determinations on fine grained calcite to reconstruct the salinity palaeohydrology and palaeoproductivity of the lake over the last 125 ka b p 10 3 calendar years before present three major highstands with slightly elevated salinity occurred during marine isotope stage 5 under elevated sea level and a humid climate the lake dried out 93 5 ka b p as sea level felll and the climate became drier it refilled around 10 7 ka b p three separate highstands during the holocene were sustained by humid conditions and high relative sea level although there were small increases in salinity during the holocene when sea level was close to the deepest part of the lake significant saltwater intrusion appears to have been prevented by freshwater discharge from inland throughout the late quaternary the salinity of the great pond showed low absolute variations from 0 3 to 0 6 o the long term trends in lake level reflect both orbitally induced changes in insolation and eustatic sea level water level fluctuations during the holocene however reflect climatic changes on the millennial timescale rather than orbital forcing or sea level change

isotopic and hydrologic responses of small closed lakes to climate variability comparison of measured and modeled lake level and sediment core oxygen isotope records abstract simulations conducted using a coupled lake catchment isotope mass balance model forced with continuous precipitation temperature and relative humidity data successfully reproduce within uncertainty limits long term i e multidecadal trends in reconstructed lake surface elevations and sediment core oxygen isotope 18 o values at castor lake and scanlon lake north central washington error inherent in sediment core dating methods and uncertainty in climate data contribute to differences in model reconstructed and measured short term i e sub decadal sediment i e endogenic and or biogenic carbonate 18 o values suggesting that model isotopic performance over sub decadal time periods cannot be successfully investigated without better constrained climate data and sediment core chronologies model reconstructions of past lake surface elevations are consistent with estimates obtained from aerial photography simulation results suggest that precipitation is the strongest control on lake isotopic and hydrologic dynamics with secondary influence by temperature and relative humidity this model validation exercise demonstrates that lake catchment oxygen isotope mass balance models forced with instrumental climate data can reproduce lake hydrologic and isotopic variability over multidecadal or longer timescales and therefore that such models could potentially be used for quantitative investigations of paleo lake responses to hydroclimatic change

controls on the spatial and temporal variability of vadose dripwater geochemistry edwards aquifer central texas abstract a 4 yr study of spatial and temporal variability in the geochemistry of vadose groundwaters from caves within the edwards aquifer region of central texas offers new insights into controls on vadose groundwater evolution the relationship between vadose and phreatic groundwaters and the fundamental influence of soil composition on groundwater geochemistry variations in sr isotopes and trace elements mg ca and sr ca ratios of dripwaters and soils from different caves as well as phreatic groundwaters provide the potential to distinguish between local variability and regional processes controlling fluid geochemistry and a framework for understanding the links between climatic and hydrologic processes the sr isotope compositions of vadose cave dripwaters mean 87sr 86sr 0 7087 and phreatic groundwaters mean 87sr 86sr 0 7079 generally fall between values for host carbonates mean 87sr 86sr 0 7076 and exchangeable sr in overlying soils mean 87sr 86sr 0 7088 dripwaters have lower mg ca and sr ca ratios and higher 87sr 86sr values than phreatic groundwaters dripwater 87sr 86sr values also inversely correlate with both mg ca and sr ca ratios mass balance modeling combined with these geochemical relationships suggest that variations in fluid compositions are predominantly controlled by groundwater residence times and water rock interaction with overlying soils and host aquifer carbonate rocks consistent differences in dripwater geochemistry i e 87sr 86sr mg ca and sr ca between individual caves are similar to compositional differences in soils above the caves while these differences appear to exert significant control on local fluid evolution geochemical and isotopic variations suggest that the controlling processes are regionally extensive temporal variations in 87sr 86sr values and mg ca ratios of dripwaters from some sites over the 4 yr interval correspond with changes in both aquifer and climatic parameters these results have important implications for the interpretation of trace element and isotopic variations in speleothems as paleoclimate records as well as the understanding of controls on water chemistry for both present day and ancient carbonate aquifers

authigenic calcium carbonate flux in groundwater controlled lakes implications for lacustrine paleoclimate records groundwater dominated lakes are an important feature of many landscapes their sediments are a particularly valuable source of paleoenvironmental information in semiarid regions where perennial lakes may otherwise be scarce where groundwater and lake composition are favorable carbonate mineral precipitation evaporative concentration of lake water and microbial processes can combine to strongly deplete dissolved ca relative to influent groundwaters the authigenic carbonate flux acf can then become limited by water column cation availability and thereby be coupled to groundwater inflow rates and aquifer recharge here we analyze sedimentary records from two marl producing groundwater controlled lakes and demonstrate a link between one dimensional acf and the palmer drought severity index pdsi a measure of land surface wetness in a restricted outflow lake with high carbonate alkalinity acf is enhanced during historically wet climatic periods in response to increased aquifer recharge rates acf in this lake declines during droughts a neighboring dilute lake with a high rate of groundwater outflow shows comparatively weak coupling between acf and pdsi history ionic chemistry carbonate mineral equilibria and 13 c patterns of dissolved inorganic carbon show that the sensitivity of the acf signal depends on the degree of evaporative evolution of lake water and the mineral saturation state of the water column under conditions of stratification and ice cover copyright 2005 elsevier ltd

the use of total lake surface area as an indicator of climatic change examples from the lahontan basin abstract variation in the size of lakes in the lahontan basin is topographically constrained river diversion also has played a major role in regulating lake size in lahontan subbasins the proper gage of lake response to change in the hydrologic balance is neither lake depth level nor lake volume but instead lake surface area normalization of surface area is necessary when comparing surface areas of lakes in basins having different topographies to a first approximation normalization can be accomplished by dividing the paleosurface area of a lake by its mean historical reconstructed surface area

lacustrine cave carbonates novel archives of paleohydrologic change in the bonneville basin utah usa records of past changes in lake levels and lake water isotopic composition in closed basins provide key insights into past variations in the hydrological cycle however these records are often limited by dating precision and temporal resolution here we present data from lacustrine cave carbonates a previously unexplored class of carbonates that comprise a promising new archive of past hydrologic changes in the bonneville basin of the northeastern great basin usa these dense carbonates precipitated within caves crevices and other protected spaces flooded by lake bonneville during its highstand in the last glacial period we report on deposits in cathedral and craners caves located 50 km apart at similar elevations approximately 100 m above the modern great salt lake and almost 200 m below lake bonneville s highstand shoreline carbonates from the two caves show similar chronologies mineralogical transitions isotopic compositions and uranium concentrations these findings suggest that lacustrine cave carbonates record changes in lake level and in the isotopic composition and chemistry of lake water importantly the deposits can be precisely dated by u th methods providing the first records of lake bonneville s water balance changes tied to precise u th ages close agreement between paired u th and calibrated 14 c ages in the deposits suggests a minimal o200 a carbon reservoir effect in the lake and allows 14 c dating to be used for age control in portions of the deposits less suitable for u th dating we use ages for the onset and cessation of lacustrine cave carbonate deposition to offer new constraints on past changes in lake level and the carbonate saturation state of lake water we also present precisely dated high resolution oxygen and uranium isotope records from the deposits within a first phase of deposition reflecting the lake s transgression between 26 and 18 ka our isotopic data suggest a large influx of freshwater during heinrich stadial 2 a hiatus in deposition beginning at 18 270 3 ka may be the result of freshening related to the lake s overflow calcite deposition resumes at cathedral cave at 16 470 2 ka suggesting that basin overflow had ceased by this time and that the lake s calcite saturation state had increased andd 18 o values increase markedly after 15 9 ka consistent with drying at this time these data imply that the lake s deglacial regression began well before the bolling warming cessation of this second phase of deposition at 14 770 2 ka coincident with the bolling warming may reflect the lake s drop below cathedral cave s elevation a final stage of aragonite deposition between 13 8 and 13 6 ka differs from the underlying facies and may not reflect deposition from lake waters filling the cave

temporal variations in lake water temperature paleoenvironmental implications of lake carbonate δ18o and temperature records abstract 18 o and clumped isotope measurements of lacustrine carbonates provide important records of past terrestrial climate and paleoelevation however one of the primary challenges interpreting these data is constraining the relationship between mineral formation temperature and seasonal and mean annual climate we examined surface water temperature records from 88 lakes across the globe to develop transfer functions that relate seasonal water surface temperature to mean annual air temperature these transfer functions provide a means of reconciling proxy measures of water temperature with annual climatic conditions mean annual surface water temperature is related to mean annual air temperature maat 0 0318 t water 2 2 195 t water 12 607 r 2 0 96 independent of lake size with consistently higher mean water temperatures than air n hemisphere spring april june mean lake surface temperatures maat 0 0097 t water 2 1 379 t water 8 227 r 2 0 94 are only slightly warmer than maat for lakes in all climate zones while summer water temperatures jja maat 0 0055 t water 2 1 476 t water 18 915 r 2 0 90 may be 10 20 c warmer than maat for cold climate lakes comparison of carbonate stable isotopic data from sites with different timescales of carbonate formation such as found along latitude or elevation gradients can lead to large over or underestimates of past temperature accurate paleoclimatic interpretation of isotopic proxies or use of temperature proxy constraints i e fossil leaves to estimate lake temperature and calculate water 18 o thus requires appropriate application of water temperature air temperature transfer functions particularly when considering ancient cool high elevation lake systems

general circulation model simulation of the δ18o content of continental precipitation in the middle cretaceous a model proxy comparison we use the genesis atmospheric general circulation model gcm with water isotopic transport and fractionation capabilities to quantify the influence of atmospheric co 2 sea level and elevation of the western cordillera on the 18 o of middle cretaceous precipitation the model predicts a systematic increase of nearly 3 in the 18 o of north american precipitation due to warming associated with an increase in co 2 from 2 to 12 times pre industrial levels in contrast the specification of lowstand conditions and a high ancestral western cordillera reduces the 18 o of north american precipitation locally by as much as 6 and 8 we compare the simulated 18 o of precipitation with the 18 o of paleosol siderite spherules and find good agreement only when the model includes lowstand conditions and a high ancestral western cordillera our results imply either that cretaceous high latitude paleosol 18 o was influenced by orographic precipitation and the western interior seaway or that the gcm9s hydrological cycle is deficient at high p co 2 additional paleosol data are needed to resolve this issue

highest pluvial lake shorelines and pleistocene climate of the western great basin abstract shoreline altitudes of several pluvial lakes in the western great basin of north america record successively smaller lakes from the early to the late pleistocene this decrease in lake size indicates a long term drying trend in the regional climate that is not seen in global marine oxygen isotope records at 70 m above its late pleistocene shoreline lake lahontan in the early middle pleistocene submerged some basins previously thought to have been isolated other basins known to contain records of older pluvial lakes that exceeded late pleistocene levels include columbus fish lake lake columbus rennie kobeh diamond lakes jonathan and diamond newark long lake hubbs and clover very high stands of some of these lakes probably triggered overflows of previously internally drained basins adding to the size of lake lahontan simple calculations based on differences in lake area suggest that the highest levels of these pluvial lakes required a regional increase in effective moisture by a factor of 1 2 to 3 relative to late pleistocene pluvial amounts assuming that effective moisture is directly proportional to the hydrologic index or lake area tributary basin area these previously unknown lake levels reflect significant changes in climate tectonics and or drainage basin configurations and could have facilitated migration of aquatic species in the great basin

upper permian lacustrine organic facies evolution southern junggar basin nw china abstract the jingjingzigou lucaogou and hongyanchi formations of the southern junggar basin together comprise over 1000 m of organic rich lacustrine facies they record an overall evolution from relatively shallow evaporative lakes to freshwater lakes associated with fluvial systems the jingjingzigou formation is characterized by low toc and hi and biomarker features such as abundant carotane consistent with a specialized saline or hypersaline biota biomarker distributions in jingjingzigou formation extracts most closely resemble oils from the giant karamay oilfield the lucaogou formation represents one of the richest and thickest lacustrine source rock intervals in the world yet it contradicts conventional lacustrine source rock models in at least two important aspects first deposition occurred at middle paleolatitudes 39 43 n rather than in the tropics second limited nutrient supply in a drainage basin dominated by intermediate volcanic rocks appears to have caused low to moderate primary productivities stable salinity stratification and low inorganic sedimentation rates in a deep lake nonetheless resulted in deposits with up to 20 toc and hi near 800 the overlying hongyanchi formation has 1 5 toc but low hi and was deposited in freshwater oxic to sub oxic lakes

ostracod shell chemistry as proxy for paleoenvironmental change abstract the application of ostracod shell chemistry as a paleoenvironmental tool has grown within the past decades most studies have investigated mg and sr in ostracod shells as proxies for temperature and salinity and the use of a wide range of trace elements as prospective paleoenvironmental indicators has yet to be developed for lacustrine systems only a few preliminary studies have used trace metals in paleolimnological studies such as cd ba and zn as paleonutrient indicators or mn fe and u as redox and oxygenation indicators this paper reviews the state of the art of geochemical analyses in microfossils such as ostracods foraminifera and corals and provides insights in new trace element proxies with the goal to promote the use of trace elements in ostracod shells as paleoenvironmental proxies in paleoceanography foraminifera and corals are most prominently used to reconstruct past climate conditions well established proxies such as 18o mg ca and sr ca provide information about changes in sea surface temperatures in addition a great number of new proxies have been developed recently such as radiogenic isotopes and redox sensitive trace elements in paleolimnology ostracod shell chemistry is widely used to assess paleohydrological changes reconstruction of temperature and salinity changes in lake environments is often achieved by oxygen isotopes as well as mg ca and sr ca ratios but depending on the hydrological and geological settings of the lake system local calibrations are needed to assess which proxy is suited to reflect which processes new proxies need to be tested by novel techniques that recently have become available compared to conventional instrumentation used in ostracod shell chemistry methods such as laser ablation icp ms and nanosims allow single shell analysis and provide high resolution data the potential of ostracods in paleolimnology is not yet fully assessed but can be developed by learning from paleoceanographic studies

a late pleistocene and holocene vegetation and environmental record from lake wangoom western plains of victoria australia abstract pollen microfaunal and sedimentological evidence from the top 20 m of sediment in a closed volcanic crater lake is used to construct a detailed record of vegetation and environmental conditions through the holocene and a substantial part of the late pleistocene radiocarbon dating suggests that the sequence covers tha last 51 000 yr or so high lake levels and the presence of forest or woodland vegetation indicate that the holocene and the basal few thousand years of the record experienced wet and warm conditions the earliest period was succeeded by a long phase of lower but variable moisture levels before more arid conditions resulted in the replacement of forest and woodland by herbaceous vegetation and frequent lake drying periods of slight amelioration occurred between about 27 000 and 19 000 and before 15 000 radiocarbon yr ago maximum aridity occurred between about 19 000 and 10 000 yr b p a period incorporating the height of the last glacial during the pleistocene the lake became gradually more saline but has been fresh during the holocene even under low lake levels an increase in charcoal values is recorded probably around 20 000 years ago and this could have resulted from increased burning as a result of the activities of aboriginal people this may have also caused a change in understorey vegetation from one composed largely of asteraceae to grassland the record is compared with others from southeastern australia and there is good correspondence through the last 20 000 or 30 000 yr but before this problems of dating and the condensed nature of most sequences prevent detailed correlation

continental climate response to orbital forcing from biogenic silica records in lake baikal changes in insolation caused by periodic changes in the earth s orbital parameters provide the primary forcing for global ice ages1 6 but it is not clear to what extent the climates in continental interiors are controlled directly by regional variations in insolation and to what extent they are driven instead by the highly nonlinear response of the oceans and ice sheets here we investigate this question using the record of biogenic silica in lake baikal as a proxy for climate change in this high latitude mid continental region we find a good correlation between this record and that of marine oxygen isotopes4 over the past 250 kyr the baikal record exhibits both a strongly nonlinear component manifested in a 100 kyr periodicity and weaker direct insolation components manifested in the 41 kyr obliquity and 23 and 19 kyr precession orbital cycles these results show that even though extreme continental climates such as this are influenced directly by insolation variations they are dominated by the nonlinear rhythm of the oceans and ice sheets

influence of eolian deposition and rainfall amounts on the u isotopic composition of soil water and soil minerals 230th u dating of secondary deposits such as speleothems and pedogenic carbonate and opal provides information about the original 234u 238u composition of the waters the minerals precipitated from as a result these types of deposits often display temporal variations in initial 234u 238u that could reflect environmental variability past variations in soil water 234u 238u may reflect changes in the flux of water through the soil weathering rates or dust inputs in order to evaluate these potential influences we analyzed modern soil water chemistry and u isotopic compositions of fluids and solids from three soil sequences in nevada modern soil pore waters from diamond valley newark valley and fish lake valley nevada show significant 234u 238u differences between sites as a result of differing soil parent material newark valley pore waters are more heavily influenced by carbonate weathering and display much more enriched 234u 238u values than diamond and fish lake valley pore waters which are dominated by silicate weathering newark valley and diamond valley although located very near to each other have significantly different pore water 234u 238u values suggesting that deposition of distantly sourced dust does not exert a dominant influence on pore water 234u 238u at these sites pore water 234u 238u values also vary systematically along a rainfall gradient at fish lake valley where sites with lower rainfall display higher soil water 234u 238u values the results of this modern soil water chemistry study were used to parameterize two models that investigate the relative influence of dust content recoil and weathering on pore water 234u 238u in the upper layer of the soil and at depth under changing infiltration rates these models suggest that while the composition of soluble dust entering a soil profile is an important consideration the pore water 234u 238u at depth and thus the initial 234u 238u of minerals that precipitate from pore waters should be highly sensitive to changes in the infiltration flux through the soils as infiltration flux is closely tied to rainfall the initial 234u 238u of soil minerals at these sites should provide information about past rainfall variability

evolution of the cenozoic carbon cycle the roles of tectonics and co2 fertilization cenozoic carbon fluxes associated with rock weathering sediment burial and volcanic degassing are calculated from the mass balance equations coupling marine isotopic records of carbon both organic and inorganic strontium and osmium the result is confirmed by the good match between modeled carbonate sedimentation rates and carbonate sedimentation rates previously integrated from ocean basins worldwide the coevolution between weathering and burial of carbonate suggests that marine carbonate accumulation was regulated mainly by the recycling of carbonate rocks which mediated the bicarbonate ion concentration of the oceans a reduction in co2 effusion to the atmosphere caused by reduced volcanic degassing from 52 to 15 ma and tectonically enhanced organic rock exhumation since 15 ma is also observed these changes in co2 effusion are balanced by concomitant changes in co2 sequestration by silicate weathering and organic carbon burial importantly we demonstrate a clear decoupling of modeled silicate weathering rates from global climate over the last 15 ma this observation is inconsistent with temperature mediated mineral dissolution acting as the key mechanism facilitating the co2 silicate weathering feedback process however we instead observe a clear coupling positive correlation between modeled silicate weathering organic carbon burial and atmospheric co2 concentration we suggest that co2 fertilization effects on terrestrial biomass productivity and plant weathering could have represented a major negative feedback process helping to balance atmospheric co2 at least during the cenozoic ice house periods

climatic change recorded by stable isotopes and trace elements in a british holocene tufa combined stable isotope d18o and d13c and trace element mg sr geochemistry from bulk tufa calcite and ostracod shell calcite from an early holocene british tufa reveal clear records of holocene palaeoclimatic change variation in d18o is caused principally by change in the isotopic composition of holocene rainfall recharge itself caused mainly by change in air temperature the d13c variability through much of the deposit reflects increasing influence of soil zone co2 owing to progressive woodland soil development bulk tufa mg ca and sr ca are controlled by their concentrations in the spring water importantly mg ca ratios are not related to d18o values and thus show no temperature dependence first order sympathetic relationships between d13c values and mg ca and sr ca are controlled by aquifer processes residence times co2 degassing and calcite dissolution reprecipitation and probably record intensity of palaeorainfall recharge effects stable isotope records from ostracod shells show evidence of vital effects relative to bulk tufa data the ostracod isotopic records are markedly spiky because the ostracods record snapshots of relatively short duration years whereas the bulk tufa samples record averages of longer time periods probably decades the d18o record appears to show early holocene warming a thermal maximum at ca 8900 cal yr bp and the global 8200 yr bp cold event combined d13c mg ca and sr ca data suggest that early holocene warming was accompanied by decreasing rainfall intensity the mg ca data suggest that the 8200 yr bp cold event was also dry warmer and wetter conditions were re established after the 8200 yr bp cold event until the top of the preserved tufa sequence at ca 7100 cal yr bp

lake baikal record of continental climate response to orbital insolation during the past 5 million years the sedimentary record of biogenic silica from lake baikal in south central siberia suggests that this region of central asia was impacted by two major cooling episodes at 2 8 to 2 6 and 1 8 to 1 6 million years ago the spectral evolution of this continental interior site parallels the evolutionary frequency spectra for various marine oxygen isotope records in the baikal record the 41 000 year obliquity cycle is particularly strong from 1 8 to 0 8 million years ago variance in the 100 000 year eccentricity band increases during the past 0 8 million years the expected precession frequency of 23 000 years is highest during the past 400 000 years the modulation of the predicted 23 000 and 41 000 year insolation forcing by the 100 000 and 400 000 year eccentricity bands indicates that the transfer of variance from the precession and obliquity frequencies to the eccentricity part of the spectrum occurred in the eurasian continental interior as well as in tropical and high latitude ocean sites

evaluating the ca isotope proxy the use of ca isotopes as a proxy for mass flux imbalances in the ca cycle is evaluated critically a compiled ca isotope record for the last 45 ma derived from bulk nannofossil ooze and with a temporal resolution of 0 5 ma and an interpretation of the record are presented in the context of the global ca cycle this analysis which assumes that nannofossil ooze records isotopic variations in seawater indicates a dynamic ca cycle in the cenozoic such dynamic behavior has serious implications for the c cycle and suggests feedbacks between the ca and c cycles to stabilize or buffer the oceanic carbon reservoir alternative applications of the ca isotope proxy are investigated using numerical models to determine the extent to which ca isotopes are sensitive to other aspects of the ca cycle the results of the simulations are applied to specific cases in the cenozoic the simulations illustrate how variations in the global fractionation factor between calcium carbonate and seawater can produce trends similar to those observed when comparing previously published ca isotopic compositions of marine barite to the nannofossil ooze record the large drop in the 44 ca value of bulk nannofossil ooze near the eocene oligocene boundary can be reconciled in two ways either as a substantial increase in weathering relative to sedimentation or as an indicator of changing depositional mode within the ocean though the preferred interpretation is not clear at present it is evident that ca isotopes stand to be a unique proxy for ca cycling once the isotope systematics are elucidated

fluvial tufa evidence of late pleistocene wet intervals from santa barbara california u s a abstract past pluvials in the western united states provide valuable context for understanding regional hydroclimate variability here we report evidence of conditions substantially wetter than today from fluvial tufa deposits located near zaca lake santa barbara county california that have been dated by radiocarbon 14c and infra red stimulated luminescence irsl two successions of tufa deposition occur within a small catchment that drains miocene monterey formation bedrock 1 a fluvial deposit 0 0 5 m thick 200 m in extent that formed along a narrow valley below a modern spring and 2 a perched deposit 10 m higher 2 m thick 15 m in extent irsl and radiocarbon dating of the perched carbonates suggest at least two episodes of carbonate growth one at 19 4 2 4 1 through 17 8 2 8 1 ka and another at 11 9 1 5 1 ka verified with a charcoal 14c age of 10 95 0 12 2 cal ka bp the relationship between the perched and fluvial spring deposits is inferred to represent a drop in the water table of more than 10 m associated with a transition from a wet climate in the late glacial to a dry holocene today the wet period indicated by the perched tufa deposit between 19 4 and 17 8 ka is relatively consistent with other california climate records both north and south of zaca lake however tufa growth ca 12 to 11 ka demonstrates wet conditions occurred as far south as zaca lake during the younger dryas event in contrast to climate records farther south in lake elsinore that indicate a drying trend through this interval a small shift north in the average position of the winter season storm track could explain wet winters at zaca while at the same time generating dry winters at lake elsinore 275 km southwest of zaca if true these data indicate that rather small latitudinal shifts in the average winter season storm track can produce large changes in regional hydroclimate

uranium isotopes in soils as a proxy for past infiltration and precipitation across the western united states the intermittent presence of large pleistocene lakes in the southwestern interior of north america a region that is now a semi arid desert suggests repeated oscillations between profoundly different climatic conditions the origin of these shifts is still unresolved due to inconsistencies in existing climate proxy data for example pollen lake levels and oxygen isotopes in speleothems to resolve the inconsistencies in the water balance over the last 10 to 60 kyr we use uranium isotopic variations in secondary soil minerals to quantify net infiltration and precipitation along a north south transect in western north america we show that winter infiltration increased by 30 to 100 percent and precipitation by a lesser amount in the valleys of the great basin and mojave deserts between 60 and 26 ka this increase in infiltration and precipitation preceded the last glacial maximum lgm and the timing of most lake highstands in the region by 5 to 10 kyr respectively suggesting a possible last precipitation maximum lpm that coincided with a minimum in winter insolation subsequent decreases in infiltration and precipitation after the lgm can be reconciled with the timing of lake highstands if colder summer temperatures due to a minimum in summer insolation reduced lake evaporation the soil records combined with a range of proxy data suggest that seasonal insolation is the long term driver for large shifts in both precipitation and surface water variability in the region

paleoclimatic implications of an 850 year oxygen isotope record from the northern tibetan plateau oxygen isotope records from the sediments of hydrologically closed lakes are commonly interpreted in terms of changing effective precipitation we compare an 850 year long oxygen isotope record derived from ostracode carbonate from the sediments of sugan lake in the northern tibetan plateau with tree ring and ice core evidence for changing temperature precipitation and isotopic composition of the lake s inflow taking into account all of these independent records we show that variations in the carbonate delta o 18 values could not have been the result of varying effective precipitation alone changes in water temperature and in the delta o 18 of source waters also played a significant role where independent records of temperature precipitation or the isotopic composition of input waters are unavailable care should be taken to avoid simplistic interpretations of carbonate stable isotope records as these may contribute to incorrect paleoclimatic reconstructions

mid holocene drying of the u s great basin recorded in nevada speleothems lake level records point to dramatic changes in great basin water balance over the last 25 ka but the timing and pace of holocene drying in the region remains poorly documented here we present stable isotope and trace metal data from two lehman caves nv speleothems that provide a well dated record of latest pleistocene to mid holocene hydroclimate in the u s great basin together the stalagmites span the interval between 16 4 ka and 3 8 ka with a hiatus from 15 0 ka to 12 7 ka mg ca and d 13 c covary throughout the records consistent with control by the extent of degassing and prior calcite precipitation pcp measurements of modern cave and soil waters support pcp as the primary control on drip water trace element composition we therefore interpret mg ca and d 13 ca s reflecting infiltration rates with higher values corresponding to drier periods both mg ca and d 13 c indicate a wet period at the beginning of the record 12 7e8 2 ka followed by pronounced drying after 8 2 ka this mid holocene drying is consistent with records from around the western united states including a new compilation of great basin lake level records the strong temporal correspondence with the collapse of the laurentide ice sheet over hudson bay suggests that this drying may have been triggered by northward movement of the winter storm track as a result of ice sheet retreat however we cannot rule out an alternative hypothesis that wet early holocene conditions are related to equatorial pacific sea surface temperature regardless our results suggest that great basin water balance in the early holocene was driven by factors other than orbital changes

are two elements better than one dual isotope ratio detrending of evaporative effects on lake carbonate paleoelevation proxies 1 stable isotope based proxy methods enhance our ability to interpret geodynamical histories for tectonic provinces via paleoelevational reconstructions these methods require that the unmodified isotopic composition of meteoric water is recorded by authigenic minerals a critical assumption that has not been tested across wide ranging environmental and topographical contexts here we show that quaternary lake carbonate 18o values are not strongly nor significantly correlated with regional elevation due to the isotopic modification of in flow waters following entry into the lake environment these modifications are largely caused by surface water evaporation and can result in 3 km errors in paleoelevation estimates if not accounted for however our analysis suggests that positive shifts in surface water 18o are accompanied by similar magnitude shifts in 13c dic this positive co variation in 18o and 13c may be used to detrend lake carbonate compositions for the effects of surface water evaporation using a parameter we define here as the 13c excess when tibetan lakes are excluded from the data set 13c excess values are significantly correlated with mean up slope hypsometric altitude with an error of 500 m application of the13c excess approach to cenozoic lake carbonate records from the western u s cordillera both challenges and reinforces previous paleoelevational interpretations based on 18o alone

constraints on paleoclimate from 11 5 to 5 0 ka from shoreline dating and hydrologic budget modeling of baqan tso southwestern tibetan plateau abstract 14c dating of shoreline deposits of closed basin lake baqan tso in the western tibetan plateau shows that lake level regressed from the undated highstand 46 m above modern 4 3 modern surface area of likely earliest holocene age by 11 5 ka and remained larger than modern until at least 5 0 ka the shoreline record broadly matches other regional climate records with lake level closely following northern hemisphere summer insolation overprinted by sub millennial lake level oscillations a model coupling modern land runoff and lake surface heat closely reproduces estimated modern precipitation of 240 mm yr we estimate that the baqan tso basin required 380 mm yr precipitation to sustain the maximum early holocene lake area a 55 increase over modern precipitation increases not glacial meltwater drove lake level changes as baqan tso basin was not glaciated during the holocene our estimate assumes early holocene insolation 1 3 overall increase and mean annual increases of 2 c in temperature and 37 in relative humidity we additionally developed a holocene precipitation history for baqan tso using dated paleolake areas using the modern and early holocene model results as end members we estimate precipitation in the western tibetan plateau which was 300 380 mm yr between 5 0 and 11 5 ka with error of 29 57 mm yr 12 15

estimating groundwater exchange with lakes 2 calibration of a three dimensional solute transport model to a stable isotope plume a three dimensional groundwater flow and solute transport model was calibrated to a plume of water described by measurements of 18o and used to calculate groundwater inflow and outflow rates at a lake in northern wisconsin the flow model was calibrated to observed hydraulic gradients and estimated recharge rates calibration of the solute transport submodel to the configuration of a stable isotope 18o plume in the contiguous aquifer on the downgradient side of the lake provides additional data to constrain the model a good match between observed and simulated temporal variations in plume configuration indicates that the model closely simulated the dynamics of the real system the model provides information on natural variations of rates of groundwater inflow lake water outflow and recharge to the water table inflow and outflow estimates compare favorably with estimates derived by the isotope mass balance method krabbenhoft et al this issue model simulations agree with field observations that show groundwater inflow rates are more sensitive to seasonal variations in recharge than outflow

pluvial lakes in the great basin of the western united states a view from the outcrop abstract paleo lakes in the western united states provide geomorphic and hydrologic records of climate and drainage basin change at multiple time scales extending back to the miocene recent reviews and studies of paleo lake records have focused on interpretations of proxies in lake sediment cores from the northern and central parts of the great basin in this review emphasis is placed on equally important studies of lake history during the past 30 years that were derived from outcrop exposures and geomorphology in some cases combined with cores outcrop and core records have different strengths and weaknesses that must be recognized and exploited in the interpretation of paleohydrology and paleoclimate outcrops and landforms can yield direct evidence of lake level facies changes that record details of lake level fluctuations and geologic events such as catastrophic floods drainage basin changes and isostatic rebound cores can potentially yield continuous records when sampled in stable parts of lake basins and can provide proxies for changes in lake level water temperature and chemistry and ecological conditions in the surrounding landscape however proxies such as stable isotopes may be influenced by several competing factors the relative effects of which may be difficult to assess and interpretations may be confounded by geologic events within the drainage basin that were unrecorded or not recognized in a core the best evidence for documenting absolute lake level changes lies within the shore nearshore and deltaic sediments that were deposited across piedmonts and at the mouths of streams as lake level rose and fell we review the different shorezone environments and resulting deposits used in such reconstructions and discuss potential estimation errors lake level studies based on deposits and landforms have provided paleohydrologic records ranging from general changes during the past million years to centennial scale details of fluctuations during the late pleistocene and holocene outcrop studies have documented the integration histories of several important drainage basins including the humboldt amargosa owens and mojave river systems that have evolved since the miocene within the active tectonic setting of the great basin these histories have influenced lake levels in terminal basins many pre late pleistocene lakes in the western great basin were significantly larger and record wetter conditions than the youngest lakes outcrop based lake level data provide important checks on core based proxy interpretations we discuss four such comparisons in some cases such as for lakes owens and manix outcrop and core data synthesis yields stronger and more complete records in other cases such as for bonneville and lahontan conflicts point toward reconsideration of confounding factors in interpretation of core based proxies

the impact of neogene grassland expansion and aridification on the isotopic composition of continental precipitation the late cenozoic was a time of global cooling increased aridity and expansion of grasslands in the last two decades numerous records of oxygen isotopes have been collected to assess plant ecological changes understand terrestrial paleoclimate and to determine the surface history of mountain belts the 18o values of these records in general increase from the mid miocene to the recent we suggest that these records record an increase in aridity and expansion of grasslands in midlatitude continental regions we use a nondimensional isotopic vapor transport model coupled with a soil water isotope model to evaluate the role of vapor recycling and transpiration by different plant functional types this analysis shows that increased vapor recycling associated with grassland expansion along with biomechanistic changes in transpiration by grasses themselves conspires to lower the horizontal gradient in the 18o of atmospheric vapor as an air mass moves into continental interiors the resulting signal at a given inland site is an increase in 18o of precipitation with the expansion of grasslands and increasing aridity matching the general observed trend in terrestrial cenozoic 18o records there are limits to the isotopic effect that are induced by vapor recycling which we refer to here as a hydrostat in the modern climate this hydrostatic limit occurs at approximately the boundary between forest and grassland ecosystems