reliable late pleistocene stratigraphic ages and shorter groundwater travel times from14c in fossil snails from the southern great basin both aquatic and land snails are common in the geologic record but their utility in dating is greatly restricted by their well documented tendency to yield14c dates inconsistent with true14c ages in this study we examine the use of14c ages from 1 small previously unstudied terrestrial snails to date hosting spring deposits and from 2 cooccuring aquatic snails to constrain groundwater travel times during the last glacial period our study area in the southern great basin encompasses yucca mountain site of the proposed high level nuclear waste repository where information on the age and extent of past high water tables and on groundwater flow times is crucial to several licensing issues our results show that shells of small terrestrial snails belonging tovalloniasp yield14c dates consistent with14c ages of associated carbonized wood these results imply that these taxa can provide reliable14c age control on the broadly distributed deposits in which they have been described in contrast cooccurring aquatic snails from fossil spring deposits yield14c ages generally greater than the control age this is because the aquatic shells often formed in spring waters that had an initial14c deficiency however the magnitude of the deficiency is much less than that observed in nearby modern springs arguing for much higher average14c contents in late pleistocene groundwaters in these basins if representative this implies shorter groundwater travel times through aquifers in southern nevada during late glacial time

the isotopic climatic records in the allerod bolling younger dryas and post younger dryas events the combined isotopic records in the greenland ice cores in modern and ancient wood samples and in marine foraminifera provide a climatic record for the allerod bolling younger dryas and post younger dryas periods that is not compatible with the usual climatic interpretation of the 2h 1h ratios in the greenland ice cores for example the younger dryas was not solely a north atlantic event because evidence for it is also found in the 2h 1h record in wood samples which grew on the north slope of alaska the younger dryas was not necessarily a time of cooling over the ice caps moreover it might have been a warming period in the temperate zones of the northern hemisphere continents a better understanding of the climatic scenarios during these periods of time could be ascertained from additional isotopic data in plant remains which grew during these intervals and which cover a wider climatic range on the continents

late glacial stage and holocene tropical ice core records from huascaran peru two ice cores from the col of huascaran in the north central andes of peru contain a paleoclimatic history extending well into the wisconsinan wurm glacial stage and include evidence of the younger dryas cool phase glacial stage conditions at high elevations in the tropics appear to have been as much as 8 to 12 c cooler than today the atmosphere contained about 200 times as much dust and the amazon basin forest cover may have been much less extensive differences in both the oxygen isotope ratio 18o 8 per mil and the deuterium excess 4 5 per mil from the late glacial stage to the holocene are comparable with polar ice core records these data imply that the tropical atlantic was possibly 5 to 6 c cooler during the late glacial stage that the climate was warmest from 8400 to 5200 years before present and that it cooled gradually culminating with the little ice age 200 to 500 years before present a strong warming has dominated the last two centuries

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

the influence of diet on the δ13c of shell carbon in the pulmonate snail helix aspersa abstract the influence of diet and atmospheric co2 on the carbon isotope composition of shell aragonite and shell bound organic carbon in the pulmonate snail helix aspersa raised in the laboratory was investigated three separate groups of snails were raised on romaine lettuce c3 plant 13c 25 8 corn c4 plant 13c 10 5 and sour orange 12c enriched c3 plant 13c 39 1 the isotopic composition of body tissues closely tracked the isotopic composition of the snail diet as demonstrated previously however the isotopic composition of the acid insoluble organic matrix extracted from the aragonite shells does not track diet in all groups in snails that were fed corn the isotopic composition of the organic matrix was more negative than the body by as much as 5 whereas the matrix was approximately 1 heavier than the body tissues in snails fed a diet of c3 plant material these results indicate that isotopic composition of the organic matrix carbon cannot be used as an isotopic substrate for paleodietary reconstructions without first determining the source of the carbon and any associated fractionations the isotopic composition of the shell aragonite is offset from the body tissues by 12 3 in each of the culture groups this offset was not influenced by the consumption of carbonate and is not attributable to the diffusion of atmospheric co2 into the hemolymph the carbon isotopic composition of shell aragonite is best explained in terms of equilibrium fractionations associated with exchange between metabolic co2 and hco3 in the hemolymph and the fractionation associated with carbonate precipitation these results differ from previous studies based primarily on samples collected in the field that have suggested atmospheric carbon dioxide contributes significantly to the shell 13c the culture results indicate that the 13c of aragonite is a good recorder of the isotopic composition of the snail body tissue and therefore a better recorder of diet than is the insoluble shell organic carbon because the systematic fractionation of carbon isotopes within the snail is temperature dependent the 13c of the shell could provide an independent technique for estimating paleotemperature changes

holocene trends in 18o in land snail shells from the negev desert and their implications for changes in rainfall source areas abstract the 18o value of the carbonate of land snail shells is related to the 18o content of precipitation which in turn relates to the source region and trajectory of the rain bearing air masses analyses of 18o of the shell carbonate of 76 radiocarbon dated holocene samples of the land snail trochoidea seetzeni from the northern negev desert in southern israel were carried out and the results were compared to modern snails from the same region early holocene 18o values are similar to modern but during the period centered around 6500 6000 yr b p a depletion of some 2 below modern is observed a change in the atmospheric circulation pattern for this period is thus indicated most likely an increase in the frequency of storm systems reaching the region from north eastern africa by 3500 yr b p 18o values had reached modern levels and indicate a stable pattern of atmospheric circulation since that time

response of interior north america to abrupt climate oscillations in the north atlantic region during the last deglaciation several broad scale climatic oscillations during the last deglaciation are well documented in regions around the north atlantic ocean this paper reviews empirical evidence for these deglacial climatic oscillations from non coastal north america and discusses implications for testing climatic simulations and for understanding the cause and transmittal mechanisms paleoclimatic interpretation of oxygen isotope records from several small sites in the eastern great lakes region indicates a classic deglacial climatic sequence that is comparable with records from europe and greenland the climatic events as interpreted from crawford lake oxygen isotopes include the bolling allerod boa warming at 12 700 14c bp a warm boa at 12 500 10 920 14c bp an intra allerod cold period shortly before 11 000 14c bp a cold younger dryas yd climate reversal at 10 920 10 000 14c bp the holocene warming at 10 000 14c bp a brief preboreal oscillation pb at 9650 14c bp and an early holocene cold event at 7500 14c bp 8200 cal bp some of these events were also evident from changes in upland and aquatic vegetation and sediment lithology the pronounced yd climatic reversal has been documented from pollen records along the ecotones at this time and from glacier readvances in the great lakes region along the rocky mountains from alberta to colorado the yd event is indicated by alpine glacier advance and or shift in timberline vegetation in minnesota and upper new york early holocene climatic instability is also suggested by oxygen isotope records and or varve thickness the regional variations in evidence for the yd and other events in north america suggest that climatic oscillations may have different expressions in paleo records depending on geographic location and characteristics of a particular site the extent and magnitude of these climatic oscillations across north america suggest that these oscillations are an expression of climatic change that was probably widespread rather than locally induced by a nearby glacier the location of these sites implies that climatic signals were likely carried over the northern hemisphere through the atmosphere as indicated by general circulation models we hypothesize that the lack of evidence for a cold yd in interior north america west of the great lakes region and east of the rocky mountains was caused by the trapping of cold arctic air mass north of the laurentide ice sheet and by uninterrupted northward expansion of warm caribbean air this strongly contrasted climate also produced non analogous biological assemblages

stable carbon isotope record of middle to late holocene climate changes from land snail shells at hinds cave texas abstract stable carbon isotope ratios of shell carbonate and shell organic matter from land snails rabdotus alternatus from hinds cave texas are used to reconstruct the climate history of the drier western part of the southern great plains present mean annual rainfall of 400 mm over the last ca 5000 yr ams radiocarbon analyses of four individual shells from each of two strata indicate that there are no problems of reworking or age mixtures within strata however burning is evident in many shells amino acid composition and racemization were used to identify shells that had been significantly heated and these were rejected for isotopic analysis because of possible alteration of the isotopic signal fourteen samples of snails from hinds cave each consisting of 2 6 individual shells from the same provenience were radiocarbon dated by ams and analyzed for stable carbon isotopes in addition a sample collected alive ca ad 1890 from a nearby site was analyzed to represent modern conditions the shells show no decrease in organic content with age indicating good preservation of shell organic matter both the organic and carbonate isotope values show similar temporal trends the oldest samples 4100 4600 cal yr bp are relatively depleted in 13 c maximum 13 c values are reached at ca 3500 cal yr bp thereafter there is a regular depletion in 13 c with the recent sample showing the lowest 13 c value the change in the isotopic values over time is quite large range of 4 5 for organic matter and indicates substantial climatic changes the variations in isotopic values are interpreted as resulting primarily from changes in moisture conditions affecting the abundance of drought adapted cam crassulacean acid metabolism plant species enriched in 13 c thus relatively moist mid holocene conditions are indicated with increasingly dry conditions reaching a maximum at ca 3500 cal yr bp followed by progressively moister conditions through the late holocene

evidence for a younger dryas like cooling event on the british columbia coast two independent paleoclimatic records from the pacific coast of canada indicate that a late glacial warming trend was interrupted by a return to colder conditions between about 11 000 and 10 200 radiocarbon yr b p correlative with the classical younger dryas chronozone of the north atlantic region fossil benthic foramnifera from three cores from the continental shelf dated by accelerator mass spectrometry show peak abundances of the cold water indicator species cassidulina reniforme at this time fossil pollen spectra from two sites on the queen charlotte islands record a shift from forest to open herb rich vegetation after 11 100 yr b p probably in response to colder and wetter conditions identified by pollen climate transfer functions these preliminary data for a cold oscillation between ca 11000 and 10000 yr ago in the northeast pacific argue that this deglacial phenomenon was not restricted to the north atlantic but was a hemispheric and possibly global event

the use of land snail shells in paleoenvironmental reconstruction fossil land snail shells constitute a valuable source of paleoenvironmental information for the quaternary they can be dated by a variety of methods including radiocarbon amino acid racemization and epimerization and perhaps also thu and esr the vast majority of paleoenvironmental studies based on land snail shells have examined the faunal composition of fossil assemblages from which a variety of paleoenvironmental characteristics such as biome temperature and moisture conditions have been reconstructed still there are a number of problems involved in using this approach and these are discussed shell morphology has occasionally been used to reconstruct such factors as rainfall and temperature stable isotope studies on quaternary land snails include analysis of 18o values of organic matter in the shells to reconstruct c4 plant distributions from which rainfall amounts can be inferred and analysis of 18o values of shell carbonate from which trends in the oxygen isotope composition of rainfall can be reconstructed stable carbon isotopes of shell carbonate have also been studied but their interpretation is not clear amino acid epimerization analysis dalloisoleucinel isoleucine ratios of land snail shells has been used for estimation of paleotemperatures some potential uses of land snail shells for paleoenvironmental reconstruction include the study of stable isotopes of h and n periodic growth lines and deposits of pedogenic carbonates on the shells

age of the crowfoot advance in the canadian rocky mountains a glacial event coeval with the younger dryas oscillation a suite of sediment core samples was recovered from two lakes crowfoot and bow lakes that are adjacent to the crowfoot moraine type locality to identify and radiocarbon date sediments related to the crowfoot advance the crowfoot moraine system widely recognized throughout northwestern north america represents a glacial advance that is post wisconsin and pre mazama tephra in age an interval of inorganic sediments bracketed by accelerator mass spectrometry radiocarbon ages of ca 11 330 and 10 100 sup 14 c yr b p is associated with the crowfoot moraine the crowfoot advance is therefore approximately synchronous with the european younger dryas cold event ca 11 000 10 000 sup 14 c yr b p furthermore the termination of the crowfoot advance also appears to have been abrupt these findings illustrate that the climatic change responsible for the european younger dryas event extended beyond the northern atlantic basin and western europe equilibrium line altitude ela depressions associated with the crowfoot advance are similar to those determined for the little ice age advance whereas younger dryas ela depressions in europe significantly exceed little ice age ela depressions 26 refs 3 figs 1 tab

the gisp2 δ18o climate record of the past 16 500 years and the role of the sun ocean and volcanoes measured 18o 16o ratios from the greenland ice sheet project 2 gisp2 ice core extending back to 16 500 cal yr b p provide a continuous record of climate change since the last glaciation high resolution annual 18o 16o results were obtained for most of the current millennium a d 818 1985 and record the medieval warm period the little ice age and a distinct 11 yr 18o 16o cycle volcanic aerosols depress central greenland annual temperature 1 5 c maximally and annual 18o 16o for about 4 yr after each major eruptive event on a bidecadal to millennial time scale the contribution of solar variability to holocene greenlandic temperature change is 0 4 c the role of thermohaline circulation change on climate problematic during the holocene is more distinct for the 16 500 10 000 cal yr b p interval analogous to 14c age calibration terminology we express time in calibrated cal yr b p a d 1950 0 cal yr b p the oldest dryas bolling older dryas allerod younger dryas sequence appears in great detail bidecadal variance in 18o 16o but not necessarily in temperature is enhanced during the last phase of lateglacial time and the younger dryas interval suggesting switches of air mass transport between jet stream branches the branched system is nearly instantaneously replaced at the beginning of the bolling and holocene at 14 670 and 11 650 cal yr b p respectively by an atmospheric circulation system in which 18o 16o and annual accumulation initially track each other closely thermodynamic considerations of the accumulation rate temperature relationship can be used to evaluate the 18 o 16o temperature relationship the gisp2 ice layer count years of major gisp2 climate transitions also support the use of coral 14c ages for age calibration

oxygen isotope and palaeotemperature records from six greenland ice core stations camp century dye 3 grip gisp2 renland and northgrip oxygen isotope variations spanning the last glacial cycle and the holocene derived from ice core records for six sites in greenland camp century dye 3 grip gisp2 renland and northgrip show strong similarities this suggests that the dominant influence on oxygen isotope variations reflected in the ice sheet records was regional climatic change differences in detail between the records probably reflect the effects of basal deformation in the ice as well as geographical gradients in atmospheric isotope ratios palaeotemperature estimates have been obtained from the records using three approaches i inferences based on the measured relationship between mean annual 18o of snow and of mean annual surface temperature over greenland ii modelled inversion of the borehole temperature profile constrained either by the dated isotopic profile or iii by using monte carlo simulation techniques the third of these approaches was adopted to reconstruct holocene temperature variations for the dye 3 and grip temperature profiles which yields remarkably compatible results a new record of holocene isotope variations obtained from the northgrip ice core matches the grip short term isotope record and also shows similar long term trends to the dye 3 and grip inverted temperature data the northgrip isotope record reflects i a generally stronger isotopic signal than is found in the grip record ii several short lived temperature fluctuations during the first 1500 yr of the holocene iii a marked cold event at ca 8 2 ka the 8 2 ka event iv optimum temperatures for the holocene between ca 8 6 and 4 3 ka a signal that is 0 6 stronger than for the grip profile v a clear signal for the little ice age and vi a clear signal of climate warming during the last century these data suggest that the northgrip stable isotope record responded in a sensitive manner to temperature fluctuations during the holocene copyright 2001 john wiley sons ltd

rainfall in the negev desert during the middle holocene based on 13c of organic matter in land snail shells abstract analysis of stable carbon isotope ratios 13 c 12 c of organic matter in land snail shells is used to infer middle holocene rainfall amounts in the negev desert by reconstructing the distribution of c 4 plants in the family chenopodiaceae the organics are derived from the diet of the snails which consists of plant material and are enriched in 13 c where c 4 plants are present a survey of modern plant communities indicates that in areas receiving 300 mm mean annual rainfall nearly all plant communities consist of c 3 species only no c 4 chenopodes whereas in areas under 230 mm rainfall most plant communities contain one or more c 4 chenopode species in between is a transition zone consisting of a mosaic of both pure c 3 and mixed c 3 c 4 plant communities isotopic results for fossil land snails indicate a consistent geographic pattern throughout the middle holocene from ca 6500 to 3000 yr b p with the transition zone located ca 20 km south of its present position this implies a near doubling of rainfall within this region as compared to present

the el abra stadlal in the eastern cordillera of colombia south america abstract the el abra stadial is a climatic cooling event defined in the eastern cordillera of colombia which corresponds both in timing and intensity to the younger dryas climatic oscillation of the northern north atlantic region the stadial was preceded by the relatively warm guantiva interstadial and followed by the warm holocene colder climatic conditions during the el abra stadial caused a 400 m lowering of the upper andean forest limit below its altitudinal location during the preceding guantiva interstadial translated in temperature change this represents a drop in mean annual temperature of approximately 2 to 3 c the lowering of the upper forest limit and inferred temperature change for the el abra stadial compared to late holocene conditions are in the order of 600 m and 4 c respectively the transition of the guantiva interstadial to the el abra stadial has been radiocarbon dated at 11 210 90 bp and 10 820 60 bp climatic warming may have started shortly after 10 380 90 bp and near present day temperatures were reached at least as early as 8920 100 bp maximum cooling was probably attained between 10 800 bp and 10 300 bp further ams radiocarbon dating is planned to establish more exactly the timing of the el abra stadial a glacial advance of el abra age the bocatoma stade has been recognized in the highest part of the colombian eastern cordillera the sierra nevada del cocuy during this advance glaciers extended some 700 m lower than at present a climatic cooling event of el abra age associated with shifts of the upper andean forest limit and glacial advances has also been recognized in other parts of andean colombia including the central cordillera and the sierra nevada de santa marta

late pleistocene c4plant dominance and summer rainfall in the southwestern united states from isotopic study of herbivore teeth abstract patterns of climate and c4plant abundance in the southwestern united states during the last glaciation were evaluated from isotopic study of herbivore tooth enamel enamel 13c values revealed a substantial eastward increase in c4plant consumption formammuthusspp bisonspp equusspp andcamelopsspp the 13c values were greatest inbisonspp 6 9 to 1 7 andmammuthusspp 9 0 to 0 3 and in some locales indicated c4 dominated grazing the 13c values of antilocaprids were lowest among taxa 12 5 to 7 9 and indicated c3feeding at all sites on the basis of modern correlations between climate and c4grass abundance the enamel data imply significant summer rain in parts of southern arizona and new mexico throughout the last glaciation enamel 18o values range from 19 0 to 31 0 and generally increase to the east this pattern could point to a tropical or subtropical source of summer rainfall at a synoptic scale the isotope data indicate that interactions of seasonal moisture temperature and lowered atmospheric pco2determined glacial age c4abundance patterns

stable carbon and oxygen isotopes in quaternary soil carbonates as indicators of ecogeomorphic changes in the northern chihuahuan desert usa stable carbon and oxygen isotopes in soil carbonates provide an additional technique for investigating quaternary ecogeomorphic changes in arid and semiarid regions this study presents 13c and 18o values in surface and buried soils in a basin and range region of southern new mexico and west texas three landform categories were investigated for their isotopic content 1 middle piedmont slopes 2 a lower piedmont slope and adjacent playa depressions and 3 an intermontane basin floor soils of the middle piedmont slopes had greatest 13c variability ranging from 0 6 in buried soils to 11 1 pdb in surface soils soils of the basin floor had greatest variability in 18o values ranging from 0 6 to 7 6 pdb fossil pollen patterns roughly paralleled 13c shifts with cheno am pollen exhibiting greatest changes both isotope and pollen data across stratigraphic discontinuities suggest that a conversion from c4 grasses to c3 desert shrubs accompanied holocene erosion

climatic and oceanic controlled deposition of late ordovician early silurian black shales on the north yangtze platform south china abstract black shales deposited across the late ordovician and early silurian transition are considered to be the most important source rock and shale gas reservoirs in the yangtze region of south china however the origin of these sediments remains contentious in this study we investigated the systematic changes in total organic carbon toc organic carbon isotopes 13corg major elements and trace elements on the ordovician silurian boundary strata from qiaoting north yangtze sea to reconstruct the climatic conditions redox changes primary productivity and other factors there promoting the understanding for the formation mechanism of the organic rich black shales element compositions and their ratios suggest that the black shales studied here were dominantly sourced by the felsic igneous rock which is similar to granodiorite in composition paleoclimatic proxies cia ciw 13corg suggest that the global climate system experienced significant changes from warm humid to cold dry and then to warm humid climates during the ordovician silurian transition paleoredox indices s c u th ni co and v v ni ratios also demonstrate fluctuating redox variations from anoxic to oxygenated and then to anoxic states during this interval in the yangtze sea which describe an oxygenated guanyinqiao strata sandwiched between the stagnant and anoxic wufeng and longmaxi black shales paleoproductivity parameters toc babio contents and p al ratio suggest that the organic rich wufeng and longmaxi black shales were deposited with a high biological productivity while the organic lean guanyinqiao sediments were deposited with a low biological productivity these data demonstrate large climatic and oceanic fluctuations during the ordovician silurian transition providing essential controlling factors on the oceanic anoxia primary productivity and subsequent organic rich black shale depositions in the yangtze region during the late ordovician and early silurian intervals

a continuous high resolution record of late pleistocene climate variability from the estancia basin new mexico lake sediments that accumulated in the estancia basin central new mexico provide a detailed record of submillennial latest pleistocene climatic oscillations sedimentologic biologic and geochemical proxies for changes in salinity and lake level were used to identify episodes of wet and dry climate that occurred between 24 and 12 k y b p as dated by 14c by means of accelerator mass spectrometry from shoreline and basin center deposits these dates determine the timing and duration of the episodes of wet and dry climate the lake expanded and contracted repeatedly during the last glacial maximum and fluctuated near its highest elevation several times during the interval ca 20 to 15 k y b p a pronounced lowstand lasting 1 k y occurred between ca 15 and 14 k y b p followed by two more highstands of the lake between ca 14 and 12 5 k y b p desiccation of the perennial lake after 12 k y b p was followed by a final poorly dated highstand at ca 10 k y b p the record of lake level fluctuations from the estancia basin provides a high resolution record of changes in water budget in a basin the comparatively simple physiography and hydrogeology of which ensured a direct response to the strong climatic fluctuations that occurred during the last glacial maximum and termination

oxygen isotopes in fresh water biogenic opal northeastern us allerod younger dryas temperature shift the first oxygen isotope analysis of biogenic opal from lake sediments from the allerod younger dryas transition in a core from linsley pond connecticut gives an average estimate of a 6 c drop in temperature during the younger dryas this shift represents temperatures during the bloom season and may be less than the winter temperature drop the sharp transition itself with a duration of about 200 years suggests that the temperature decrease may have been as large as 12 c previous estimates of the allerod younger dryas temperature shifts are controversial and range from 3 20 c suggesting that further interdisciplinary research on the same samples is warranted

c4 plant productivity and climate co2 variations in south central texas during the late quaternary abstract a continuous record of organic carbon 13c from a buried soil sequence in south central texas demonstrates 1 strong coupling between marine and adjacent continental ecosystems in the late pleistocene as a result of glacial meltwater entering the gulf of mexico and 2 ecosystem decoupling in the holocene associated with a reduction of meltwater and a shift in global circulation patterns in the late pleistocene reduction in c4 plant productivity correlates with two well documented glacial meltwater pulses 15 000 and 12 000 14c yr b p indicating a cooler than present adjacent continental environment increased c4 production between 11 000 and 10 000 14c yr b p suggests that the younger dryas was a warm interval responding to the diversion of glacial meltwater away from the mississippi river with waning meltwater flow c4 productivity generally increased throughout the holocene culminating in peak warm intervals at 5000 and 2000 14c yr b p shifts in the abundances of c3 c4 plants through the late quaternary show no correlation to ecophysiological responses to atmospheric co2 concentration

the abrupt termination of the younger dryas climate event previous studies on two deep greenland ice cores have shown that a long series of climate oscillations characterized the late weichselian glaciation in the north atlantic region1 and that the last glacial cold period the younger dryas ended abruptly 10 700 years ago2 here we further focus on this epoch defining event and present detailed heavy isotope and dust concentration profiles which suggest that in less than 20 years the climate in the north atlantic region turned into a milder and less stormy regime as a consequence of a rapid retreat of the sea ice cover a warming of 7 c in south greenland was completed in about 50 years

paleoenvironment of the folsom archaeological site new mexico usa approximately 10 500 14c yr b p as inferred from the stable isotope composition of fossil land snail shells abstract well preserved aragonitic land snail shells vallonia from late pleistocene eolian sediment in the folsom archaeological site in new mexico exhibit an overall decrease of 18opdb from maximum values of 2 7 more positive than modern to younger samples with lower average values of about 3 6 within the modern range the age of the samples approximately 10 500 14c yr b p suggests that the decrease in 18o may manifest climatic changes associated with the younger dryas some combination of increased relative humidity and cooler temperatures with decreased 18o of precipitation during the times of snail activity can explain the decrease in shell 18o a well known paleoindian bison kill occurred at the folsom site during this inferred environmental transition average 13c values of the aragonite shells of the fossil vallonia range from 7 3 to 6 0 among different archaeological levels and are not as negative as modern values this suggests that the proportion of c4 vegetation at the folsom site approximately 10 500 14c yr b p was greater than at present a result which is consistent with other evidence for higher proportions of c4 plants in the region at that time

stable isotope analyses on the last 30 ka molluscan fauna from pampa grassland bonaerense region argentina abstract fossil and living shells of terrestrial and freshwater molluscs collected from various sites in the bonaerense and patagonia regions of argentina were analysed for their stable isotope content as a possible tool for palaeoclimatic and palaeoenvironmental reconstruction the oxygen isotope composition of calcareous shell from freshwater samples was interpreted in terms of a change in moisture condition and the related evaporative effect on surficial bodies of water linked to a climatic shift the carbon isotope composition mainly records changes in biological productivity interpretation of the isotopic composition of the land snail carbonate is more complex and worldwide data from living shells have been used to decipher to some extent the palaeoenvironmental record contained in their isotopic composition overall the interpretations deduced from isotopic data of land snails as well as from nearby freshwater molluscs are quite comparable the isotopic data suggest that arid conditions occurred between 35 and 15 ka b p and warmer conditions around 35 25 ka b p a shift toward a new phase comparable with the present meteorological and climatic situation probably began around 9 ka b p and it was linked to some short but severe climatic fluctuations thus suggesting that high climatic stress characterised this period moreover a trend toward slightly arid conditions is suggested by isotopic data possibly sometimes after 5 ka the differences arising from the geographical location and the short duration of each single series studied still hamper an extensive interpretation of the isotopic data

precise cosmogenic 10be measurements in western north america support for a global younger dryas cooling event the inner titcomb lakes moraine in the wind river mountains wyoming has been dated with a precision of 5 using accelerator mass spectrometric measurements of cosmogenic 10be in surface samples from nine boulders resting on its crest the age of this moraine is 11 4 0 5 to 13 8 0 6 ka the range in age is due to the uncertainty in the 10be production rate and uncertainties in rock erosion rate and shielding of cosmic rays by snow additional exposure ages on bedrock and boulder surfaces that are stratigraphically younger and older than the inner titcomb lakes moraine conform with this age the exposure ages of the inner titcomb lakes moraine overlap the north atlantic younger dryas event 12 8 0 2 to 11 5 0 3 ka

stable carbon and oxygen isotopes of pedogenic carbonates ajo mountains southern arizona implications for paleoenvironmental change abstract in this study we compare the stable isotope composition of late holocene pedogenic carbonate from a late holocene soil developed on the pediment below the ajo mountains in southern arizona usa with measurements of the 18 o of the soil water throughout the seasonal cycle and with 18 o and 13 c measurements on older dated soils the comparison with the soil water composition shows that the pedogenic carbonate is forming in equilibrium with highly evaporated soil water at the highest summer temperatures the 13 c of the carbonate apparently reflects equilibrium with co 2 of an isotopic composition consistent with the present mixture of c3 desert shrubs and cam cactus comparison with glacial period soil carbonates shows that there has been little change in the 18 o but that the 13 c was about 4 5 heavier independent evidence indicates that glacial period summer temperatures were probably much cooler and that the 18 o of precipitation was also lighter the increase in the carbonate water fractionation factor with decreasing temperature probably counteracted the decrease in the 18 o of precipitation producing little net change in the 18 o of the soil carbonate we attribute the decrease in the 13 c to replacement of a glacial period c4 grassland by holocene c3 cam desert shrubs and succulents this replacement was probably in response to an increase in temperature and reduction in summer precipitation at the end of the last glacial period the 13 c of older soil carbonates indicates that landscape was dominated by grassland for most of the past 700 ka and that the present vegetation likely represents a relatively brief anomaly

geochemical and geophysical implications of the radiocarbon calibration abstract a precise and accurate chronological framework is crucial to study the dynamics of a variety of phenomena which occurred during the last 45 000 years although the 14 c dating method has been widely applied since the 1950s it is recognized that the atmospheric 14 c 12 c ratio has not been stable during the past in order to calculate accurate ages these fluctuations have to be corrected by means of a calibration curve obtained by comparing raw 14 c measurements with true calendar ages provided by independent dating methods the calibration curve obtained so far is characterized by a long term trend with raw 14 c ages being significantly younger than calendar ages during most of the last 45 000 years abrupt 14 c shifts which occurred over centuries to millennia are superimposed on this long term trend of decreasing atmospheric 14 c 12 c ratio to a certain extent it is possible to outline the different causes of atmospheric 14 c variations by considering complementary information obtained from other cosmogenic nuclides studied at different latitudes 1 most high frequency changes in the atmospheric 14 c 12 c ratio are linked to magnetic fluctuations of solar origin as revealed by studying the last three centuries for which direct observations of the sun are available a similar conclusion is derived by comparing 14 c 12 c events with 10 be and 36 cl concentration maxima in polar ice cores 2 the long term trend shift of 14 c ages is due to a long period of decreased shielding effect of the geomagnetic dipole field which occurred over the interval between 10 000 and 40 000 years bp this interpretation is supported by paleomagnetic measurements performed on volcanic and sedimentary rocks and by 10 be and 36 cl analysed in low and high latitude records 3 a prominent and rapid atmospheric 14 c 12 c excursion occurred between 13 000 and 11 500 cal yr bp thus corresponding to the younger dryas cold period by contrast with the 14 c variability mentioned above which is linked to 14 c production changes this so called 14 c age plateau is probably due to an abrupt variation in the rates of exchange within the global carbon cycle this interpretation is supported by independent geochemical proxies and by numerical modelling of the carbon cycle

geomorphological evidence of carbonate build up demise on equatorial margins a case study from offshore northwest australia abstract the demise of miocene carbonate build ups in the browse basin northwest australia has been explained as relating to geological and oceanographic processes these include accelerated tectonic subsidence driven by subduction ocean cooling following the mid miocene climate optimum nutrient excess poisoning by sediment drifts and local erosion driven by current winnowing occurring discretely or simultaneously here we critically assess the evidence for these different mechanisms using a combination of high resolution 3 d seismic data regional 2 d seismic profiles and numerical stratigraphic forward modelling seismic interpretation and numerical modelling found that the proposed uniform subsidence rate of 125 m ma between 16 5 ma and 5 33 ma for the northern browse basin belde et al 2017 when combined with the published estimate of eustatic sea level in miller et al 2005 was insufficient to drown the miocene carbonate sequence and generate the geomorphological changes barrier reef to isolated carbonate build ups observed on seismic data instead a subsidence profile comprising pulses of rapid and slow subsidence is required significantly our results suggest that subsidence rates exceeded 400 m ma in the northern browse basin and that parts of the basin record the accumulation of sediment drifts these sediment drifts are interpreted to have buried some carbonate build ups while suspended sediment reduced light transmissivity inhibiting carbonate production thus we postulate that current activity and excess nutrient supply are key but often overlooked oceanographic processes that lead to the demise of carbonate build ups

carbon and oxygen isotope composition of shell carbonate of desert land snails abstract variation in 13 c 12 c and 18 o 16 o ratios in the shell carbonate of several species of land snails was studied along a climatic gradient in semi arid to arid areas in the southern levant 13 c was found to be enriched in snails from communities having plants with a c 4 photosynthetic pathway depleted 13 c values were found in areas with high mean annual rainfall apparently due to higher input of metabolic co 2 as a result of greater snail activity shell carbonate 18 o values show a weak relation to the 18 o values of rainwater the shell 18 o values are enriched by 2 8 relative to isotopic equilibrium with environmental waters enrichment is suggested to result from metabolic effects on body water with lower activity producing greater enrichment but evaporation could also be a factor consistent differences in both 13 c and 18 o were found among species and may relate to the time when shell deposition occurs as with most paleoenvironmental indicators the application of shell isotopes is complicated by the multiplicity of controls of isotopic composition

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

stable carbon and oxygen isotopic variations in modern rabdotus land snail shells in the southern great plains usa and their relation to environment abstract variations of stable isotopic ratios of carbon 13c 12c and oxygen 18o 16o were investigated in modern shells of two species of rabdotus land snails r dealbatus and r alternatus in the southern great plains geographic variation in relation to climate and vegetation microgeographic variation variability among individuals and detailed records of seasonal variations within individual shells were studied stable carbon isotopic ratios in shell carbonate are primarily a function of the isotopic composition of the diet of the snails as represented by the isotopic composition of shell organic matter this in turn reflects the presence or absence of cam crassulacean acid metabolism or c4 plants vegetation density may have a small effect on the carbon isotope ratios microgeographic variation samples within 25 to 300 m is greater than that seen across different climatic regions and points to very local control of isotopic variations predominantly related to vegetation seasonal variations as assessed through serial analysis of individual shells up to 35 samples per shell may provide a means for distinguishing between isotopic influences of perennial cam vs annual c4 plants carbon isotopic variations in time series of shells from a site provide a means of reconstructing temporal changes in environment and climate oxygen isotopic values of shell carbonate are uniform across the region and also show no significant microgeographic variation the oxygen isotopic composition appears to be mainly a function of the rainwater isotopic composition with no direct influence of rainfall amount or evaporative effects the 18o values are only 2 enriched relative to estimated equilibrium with rainwater variability is low sd of 0 8 among sites so the isotopic composition of fossil rabdotus shells can provide a precise record of changes in the isotopic composition of rain over time

rapid response of alpine timberline vegetation to the younger dryas climate oscillation in the colorado rocky mountains usa paleobotanical records from two high altitude 3 300 m sites in colorado show a clear and immediate response to the younger dryas climate oscillation the black mountain lake and sky pond records indicate that alpine timberline migrated upslope to near modern elevations during the late bolling allerod 13 6 12 9 ka subsequent declines in arboreal pollen percentages and accumulation rates during the younger dryas interval 12 9 11 7 ka reflect a downslope displacement of the alpine timberline ecotone of 60 120 m in elevation this change translates to a cooling of summer temperature by approximately 0 4 0 9 c and is consistent with proposed younger dryas advances of alpine glaciers in the rocky mountains to positions close to little ice age maxima alpine timberline readvanced upslope to elevations above both sites between 11 7 and 11 4 ka the concomitant response of temperature sensitive alpine timberline vegetation in colorado and late glacial changes in north atlantic thermohaline circulating implicates a rapid widespread atmospheric transmission of the younger dryas climate oscillation