development of partial rock veneers by root throw in a subalpine setting rock veneers stabilize hillslope surfaces occur especially in areas of immature soil and form through a variety of process sets that includes root throw near westcliffe colorado usa data were collected from a 20 500 m transect on the east slope of the sangre de cristo mountains ages of pit mound complexes with rock fragments exposed at the surface by root throw ranged from recent freshly toppled tree to unknown complete tree decay calculations based on dimensions of the pit mound complexes estimated time of tree toppling sizes of exposed rock fragments and percentage rock covers at pit mound complexes as well as within the transect area indicate that recent rates of root throw have resulted in only partial rock veneering since late pleistocene deglaciation weathering of rock fragments prevents development of an extensive rock veneer and causes a balance achieved within an estimated 700 years between the rates of rock fragment exposure by root throw and clast disintegration by chemical reduction the estimated rate of rock fragment reduction accounts for part of the fluvial sediment yields observed for forested subalpine areas of western north america copyright 2005 john wiley sons ltd

rapid oscillations in vostok and grip ice cores we investigate the spectral properties of climatic times series derived from two recent ice cores in greenland and east antarctica we find that the signals behave in a similar way in the high frequency part of their spectra the rapid oscillations found in the grip ice core were closely correlated to heinrich events a comparable spectral feature is detected in the vostok ice core we discuss the possibilities of connections between the two hemispheres and proper ice sheet oscillations in the light of simple oscillating climate models

lake devlin and pinedale glacial history front range colorado abstract glacial lake devlin was an ice margin lake that formed during pinedale time when the north boulder creek valley glacier blocked the mouth of a tributary near the downvalley limit of glaciation the lake was about 2 km long and as much as 60 m deep and was fed by meltwater from small valley glaciers it accumulated sediment apparently without interruption for about 10 000 yr the inception of lake devlin between 23 500 and 21 000 yr b p provides a limiting date for the maximum expansion of pinedale ice because the lake could not form until the north boulder creek glacier had advanced to within 2 km of its downvalley limit the maximum lowering of timberline which occurred about 19 000 yr b p is assumed to coincide with the pinedale glacial maximum apparent timberline elevations determined from picea pinus pollen ratios suggest that ice front positions during the time between 23 000 and 19 000 yr b p may have been only slightly less extensive than at the pinedale maximum lake devlin drained catastrophically about 13 000 yr b p which was after ice had receded from the outlet area pinedale glaciation in the front range began about or after 30 000 yr b p and final deglaciation occurred between 15 000 and 12 000 yr b p

late pleistocene and holocene palaeohydrology of lake rukwa tanzania inferred from diatom analysis diatom analysis has been used to provide information on the lake level fluctuations of lake rukwa during the last 21 7 ka diatom inferred chemical variables conductivity ph anion and cation ratios indicate that at the lgm lake level was low probably driven by a low p e ratio but with sufficient water to avoid desiccation following the lgm deep water conditions were obtained through a series of steps until maximum water depth was reached by at least 13 5 ka the high lake level phase lasted through the early holocene until after 6 7 ka a shallow saline lake has occupied the basin from ca 5 5 ka until the present day the pattern of reconstructed lake level fluctuation in lake rukwa is similar to that from lake tanganyika and other lakes from the equatorial and northern tropical zone we can confirm that the n tropical mode of millennial scale lake level change since the lgm extended to at least 8 s

persistent millennial scale climatic variability over the past 25 000 years in southern africa abstract data from stalagmites in the makapansgat valley south africa document regional climatic change in southern africa in the late pleistocene and holocene a new tims u series dated stalagmite indicates speleothem growth from 24 4 to 12 7 ka and from 10 2 to 0 ka interrupted by a 2 5 ka hiatus high resolution oxygen and carbon stable isotope data suggest that postglacial warming was first initiated 17 ka was interrupted by cooling probably associated with the antarctic cold reversal and was followed by strong warming after 13 5 ka the early holocene experienced warm evaporative conditions with fewer c4 grasses cooling is evident from 6 to 2 5 ka followed by warming between 1 5 and 2 5 ka and briefly at ad 1200 maximum holocene cooling occurred at ad 1700 the new stalagmite largely confirms results from shorter holocene stalagmites reported earlier the strongest variability superimposed on more general trends has a quasi periodicity between 2 5 and 4 0 ka also present are weaker 1 0 ka and 100 year oscillations the latter probably solar induced given similarities to the antarctic records the proximate driving force producing millennial and centennial scale changes in the makapansgat record is postulated to be atmospheric circulation changes associated with change in the southern hemisphere circumpolar westerly wind vortex

timing of millennial scale climate change at siple dome west antarctica during the last glacial period using atmospheric methane and the isotopic composition of o2 as correlation tools we place the dd record of ice from the siple dome west antarctica ice core on a precise common chronology with the gisp2 greenland ice core for the period from 9 to 57 ka the onset of major millennial warming events in siple dome preceded major abrupt warmings in greenland and the pattern of millennial change at siple dome was broadly similar though not identical to that previously observed for the byrd ice core also in west antarctica the addition of siple dome to the database of well dated antarctic paleoclimate records supports the case for a coherent regional pattern of millennial scale climate change in antarctica during much of the last ice age and glacial interglacial transition

the geochemistry of carbonate replacement pb zn ag mineralization in the lavrion district attica greece fluid inclusion stable isotope and rare earth element studies strata bound carbonate replacement pb zn ag deposits in the lavrion district greece are spatially related to a late miocene granodiorite intrusion 7 10 ma and various sills and dikes of mafic to felsic composition the plaka granodiorite contains porphyry molybdenum mineralization and is locally associated with a ca fe skarn carbonate replacement deposits occur predominantly in marbles upper and lower marble of the basal unit kaesariani schists and along a major detachment fault that separates the basal unit from the upper unit orebodies are mainly strata bound carbonate replacement although sulfides also occur in veins the mineralogy of carbonate replacement deposits is dominated by base metal sulfides and sulfosalts of ag bi sn sb as and pb particularly at plaka and kamariza carbonates are intergrown with earlier formed sulfides and sulfosalts but are more abundant late in the paragenetic sequence with fluorite and barite fluid inclusion studies of sphalerite fluorite calcite and quartz in carbonate replacement deposits suggest that they were deposited from 132 to 365 c from co2 poor low to high salinity fluids 1 20 wt nacl equiv carbon and oxygen isotope compositions of calcite 13c 15 6 to 1 5 and 18o 9 2 to 17 3 intergrown with sulfides reflect variable exchange of the ore bearing fluid with the upper and lower marbles and proximity to the plaka granodiorite post archean australian shale paas normalized rare earth and yttrium patterns of the upper and lower marbles and calcite intergrown with sulfides show positive eu and negative ce anomalies as well as y ho ratios between 40 and 80 normalized rare earth and yttrium patterns of fluorite also have positive eu and negative ce anomalies such anomalies for both the carbonates and fluorite reflect the high ph or high f o2 conditions of the late stage hydrothermal fluids and the likely derivation of calcium from marine carbonates precursors of the upper and lower marbles the range of sulfur isotope compositions for sulfides 34s 4 9 to 5 3 with one outlier of 9 4 in carbonate replacement and vein deposits is due likely to a magmatic sulfur source with a contribution of reduced seawater sulfate sulfur isotope compositions of barite from carbonate replacement range from 34s 17 2 to 23 7 per mil and reflect miocene seawater sulfate values if a magmatic source of sulfur is assumed along with an average temperature of 250 c for the ore forming fluids as based on fluid inclusion studies sulfides in carbonate replacement deposits were deposited at values of log f o2 41 to 36 and a ph 5 8 to 9 1 however the range of sulfur isotope values does not rule out the possibility that sulfur in sulfides could have been produced by the reduction of seawater sulfate with no contribution from a magmatic source the carbonate replacement deposits resemble manto type sulfide deposits in mexico central colorado south korea nevada and northern greece

major features and forcing of high latitude northern hemisphere atmospheric circulation using a 110 000 year long glaciochemical series the greenland ice sheet project 2 glaciochemical series sodium potassium ammonium calcium magnesium sulfate nitrate and chloride provides a unique view of the chemistry of the atmosphere and the history of atmospheric circulation over both the high latitudes and mid low latitudes of the northern hemisphere interpretation of this record reveals a diverse array of environmental signatures that include the documentation of anthropogenically derived pollutants volcanic and biomass burning events storminess over marine surfaces continental aridity and biogenic source strength plus information related to the controls on both high and low frequency climate events of the last 110 000 years climate forcings investigated include changes in insolation of the order of the major orbital cycles that control the long term behavior of atmospheric circulation patterns through changes in ice volume sea level events such as the heinrich events massive discharges of icebergs first identified in the marine record that are found to operate on a 6100 year cycle due largely to the lagged response of ice sheets to changes in insolation and consequent glacier dynamics and rapid climate change events massive reorganizations of atmospheric circulation that are demonstrated to operate on 1450 year cycles changes in insolation and associated positive feedbacks related to ice sheets may assist in explaining favorable time periods and controls on the amplitude of massive rapid climate change events explanation for the exact timing and global synchroneity of these events is however more complicated preliminary evidence points to possible solar variability climate associations for these events and perhaps others that are embedded in our ice core derived atmospheric circulation records

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

milankovitch forcing of bioturbation intensity in deep marine thin bedded siliciclastic turbidites abstract trace fossils have been used to infer a milankovitch type control on pelagic and hemipelagic sediments but there have been no comparable studies in deep marine siliciclastic turbidite successions here we present the results of a quantitative analysis of trace fossil abundance and intensity in an essentially continuous middle eocene 230 m long well a6 core comprising very thin and thin bedded siliciclastic turbidites in the deep marine ainsa basin spanish pyrenees after removing the sediment slides and debris flow deposits as these represent geologically instantaneous events approximately two orders of magnitude thicker than the typical laminated turbiditic sediments in the core spectral analysis was conducted on the bioturbation intensity data spectral analysis of bioturbation intensity in the a6 core suggests for the first time from a siliciclastic turbidite succession at a tectonically active plate margin thrust top basin in a foreland basin a cyclicity that is interpreted to reflect the 41 kyr and an 112 kyr possibly an average of the 95 and 125 kyr milankovitch frequencies we propose that this drove environmental changes in bottom water conditions in the ainsa basin most likely leading to critically varying levels of oxygenation stratification of bottom waters our age model for the ainsa basin 4 km of deep marine sediments in 10 myr yields an average sediment accumulation rate of 40 cm kyr 1 that is consistent with that inferred from the spectral analysis 30 cm kyr 1 for fine grained sedimentation an implication of these results is that global environmental change probably glacio eustasy acted as a driver on deep marine siliciclastic sedimentation patterns during the middle eocene

temperature dependence of the oxygen isotopic fractionation between diatom silica and water abstract we present a new paleotemperature scale based on the oxygen isotopic ratio of the non exchangeable fraction of the oxygen from diatom silica the equation t 17 2 2 4 18osilica 18owater 40 0 2 18osilica 18owater 40 2 was derived using recent sediment samples from different oceanic areas the temperature and isotopic composition of the local surface water comparison of our results with other relationships established for quartz water or amorphous silica water at higher temperature suggests no difference in isotopic fractionation between quartz water and biogenic silica water couples

millennial band climate spectrum resolved and linked to centennial scale solar cycles abstract spectral comparison of the hulu cave grip and gisp2 18o records suggests that the single dominant period centered on 1470 yr in the gisp2 spectrum the so called 1500 yr cycle is an artifact of the age model the 230th dated hulu cave record indicates a millennial band spectrum with a weak spectral peak near this period flanked by two stronger peaks centered on 1667 and 1190 yr similar to the grip 18o record all three millennial band cycles may derive from heterodynes of centennial band solar cycles as observed in the holocene record of atmospheric 14c production this suggests a link between centennial scale solar variability and abrupt climate change at the millennial time scale

interhemispheric synchrony of the last deglaciation inferred from alkenone palaeothermometry the relative timings of the last deglacial warming in the southern and northern hemispheres are not well constrained but are a crucial component in understanding the mechanisms of deglaciation1 a clearer picture of the degree of interhemispheric synchrony has been obscured by a dearth of high resolution temperature records that can be tied to the absolute calendar timescale moreover the quantification of tropical temperatures during the last glacial cycle is controversial2 8 here we apply the alkenone method of sea surface temperature reconstruction9 10 to several high resolution sediment cores recovered from the tropical indian ocean between 20 n and 20 s the inferred initial sea surface temperature warming 15 000 calendar years ago at 20 s is in phase with northern hemisphere sea this study and air11 temperature changes but lags antarctic warming12 14 by several millennia this finding along with the results of recent modelling studies15 16 provides strong support for the idea that changes in the ocean s global thermohaline circulation were not the only cause of interhemispheric climate teleconnection during the last deglaciation

importance of the indian ocean for simulating rainfall anomalies over eastern and southern africa the relative contributions of the indian ocean and pacific ocean sea surface temperatures ssts to the rainfall variability over eastern central and southern africa during the austral spring summer are examined the variability of african rainfall is statistically related to both oceans but the variability in the two oceans is also related to separate the effects of the indian and pacific oceans a suite of numerical model simulations is presented goga the atmosphere is forced by observed ssts globally ioga the atmosphere is forced by observed ssts only in the indian ocean basin and poga the atmosphere is forced by observed ssts only in the tropical pacific basin while the sst variability of the tropical pacific exerts some influence over the african region it is the atmospheric response to the indian ocean variability that is essential for simulating the correct rainfall response over eastern central and southern africa analyses of the dynamical response s seen in the numerical experiments and in the observations indicate that the pacific and indian oceans have a competing influence over the indian ocean african region this competition is related to the influence of the two oceans on the walker circulation and the consequences of that variability on low level fluxes of moisture over central and southern africa finally given the high correlation found between sst variability in the indian and pacific oceans with the pacific leading by 3 months we speculate on an approach to long lead dynamical climate prediction over central east and southern africa

spatial and temporal patterns of late quaternary eolian deposition eastern colorado u s a abstract eolian sediment covers about 60 of colorado east of the rocky mountains about 30 of the sediment is sand and 70 is loess initially flood plains were the principal sources of eolian sediment but during the holocene dunes formed from older eolian sand and alluvium on uplands since latest pleistocene time dominant dune forming winds have been northwesterly in the northern part of the region and southwesterly in the southern part at present sand sheets and dunes mainly parabolic types are stable and covered with vegetation in dunes sand is commonly 20 30 m thick but elsewhere averages

hydrogen isotope ratios of recent lacustrine sedimentary n alkanes record modern climate variability abstract hydrogen isotope ratios were measured on n alkanes n c12 to n c31 extracted from recent lake surface sediments along a n s european transect to test if modern climate variability is recorded in these biomarkers d values of the n alkanes are compared to d values of meteoric water from the iaea gnip database spanning a range from 119 in northern sweden to 41 in southern italy to lake water d values and to mean annual temperatures varying between 2 0 c in the north and 13 7 c in the south d values of the short chained n alkanes n c12 to n c20 excluding algal derived n c17 and n c19 are higher in the north and lower in the south the isotopic fractionation e for hydrogen between meteoric water and the short chained n alkanes is increasing from n to s by more than 100 and is significantly correlated to mean annual temperature for n c16 and n c18 this suggests that these n alkanes may originate from a different source in the northern lakes possibly due to petroleum contamination or are synthesized using a different biochemical pathway the n c17 and n c19 alkanes of algal origin the n c21 and n c23 alkanes originating from water plants and the long chain n alkanes n c25 n c27 n c29 and n c31 of terrestrial origin clearly correlate with d values of meteoric water lake water and mean annual temperature indicating that they excellently record the d value of meteoric water the mean hydrogen isotope fractionation ec17 w of 157 sd 13 between n c17 and meteoric water is fairly constant over the wide range of different climates and lake environments suggesting only minor influence of environmental factors on this biochemical fractionation this suggests that d values of n c17 are suitable to reconstruct the isotopic composition of source water the mean fractionation between the long chain n alkanes and water is 128 sd 12 the mean difference of 31 between both e values is likely due to evaporative enrichment of deuterium in the leaf water if this is the only influence on the enrichment the difference between the d values of terrestrial and aquatic compounds might be suitable to reconstruct terrestrial evapotranspiration of the lake environment

water level fluctuations of lake tanganyika in phase with oceanic changes during the last glaciation and deglaciation there has been considerable controversy about the magnitude of fluctuations of the levels of lake tanganyika the earth s second deepest lake 1 470 m following the discovery of submerged valleys extending down to 550 m below present lake levels1 these fluctuations register changes in the precipitation evaporation evapotranspiration ratio in a large equatorial tropical area of catchment south of the equator here we report new palaeohydrological data back to 40 kyr bp from carbon dating of the total organic matter in two diatomaceous cores the results constrain the vertical lake level fluctuations more than has hitherto been possible2 5 and show that the fluctuations from 26 kyr bp are correlated with changes in global sea level and ice volume surpris ingly fluctuations seem to be in phase with those of the african lakes north of the equator which are clearly linked to the milankovitch mechanisms

timing of the younger dryas event in east africa from lake level changes the last deglaciation was interrupted by an abrupt cooling event the younger dryas at 11 000 ndash 10 000 yr bp uncalibrated radiocarbon timescale 1 originally recognized in climate records from northwest europe the younger dryas has now been identified in marine and ice core records worldwide2 6 in the tropics a broadly contemporaneous change in climate is recorded by decreases in water levels and increased salinity of lakes7 9 14 indicating a period of arid climate caused by a reduction in ocean to land moisture flux the exact timing of these changes in relation to the younger dryas event in high latitude records has remained unclear however here we present climate records based on analyses of diatom assemblages geochemistry and magnetic mineralogy of radiocarbon dated sequences of laminated lake sediments from lake magadi in the east african rift these records provide a detailed record of climate change in lowland equatorial africa throughout the last deglaciation 12 800 ndash 10 000 14c yr bp we find that lake level and humidity maxima coincide with the most rapid phases of ice melting in the northern hemisphere and that the climate changes including the younger dryas event were synchronous at low and high latitudes thus the effects of abrupt climate change appear to be felt at both high and low latitudes without a significant time lag

the probable importance of snow and sediment shielding on cosmogenic ages of north central colorado pinedale and pre pinedale moraines abstract eight uncorrected 36 cl ages for pinedale boulders in north central colorado fall in the range 16 5 to 20 9 kyr 10 be age determinations on four of five boulders are in close agreement 6 difference with 36 cl determinations hypothetical corrections for snow shielding increased the 36 cl ages of pinedale boulder surfaces by an average of 12 most ages for pre pinedale bull lake boulders fall within marine isotope stage mis 5 a time when continental and sierran ice accumulations were small or nonexistent under the assumption that these boulders were deposited on moraines that formed before the end of mis 6 140 kyr bp calculations indicated that rock surface erosion rates would have had to range from 5 9 to 10 7 mm kyr 1 to produce the observed 36 cl values when compared to rates that have been documented for the past 20 kyr these erosion rates are extremely high snow shielding accounts for 0 48 of the additional years needed to shift pre pinedale dates to mis 6 this suggests that some combination of snow shielding sediment shielding or 36 cl leakage has greatly decreased the apparent ages of most pre pinedale boulders inability to account for the effects of these processes seriously hinders the use of cosmogenic ages of pre pinedale boulders as estimators of the timing of alpine glaciation

numerical modeling of cosmogenic deglaciation records front range and san juan mountains colorado 1 we use cosmogenic radionuclide crn exposure ages from polished striated bedrock to constrain numerical simulations of deglaciation in the middle boulder creek valley colorado front range and the animas river valley san juan mountains colorado in both valleys the cosmogenic ages suggest initiation of deglaciation 20 ka and ongoing retreat until 12 13 ka while the first order trend in crn concentrations in each valley suggests a monotonic glacial retreat we evaluate other retreat scenarios with different implications for post last glacial maximum regional climate we use a 2 d numerical glacier simulation with a crn layer to investigate how crn based deglaciation records are affected by retreat histories that are punctuated by periods of glacier readvance the crn layer simulates both production during periods of exposure and reduction by glacial erosion during readvances we simulate glacial occupation of the valleys as they respond to equilibrium line altitude ela histories characterized by stepwise change gradual rise or a rise punctuated by short periods of lowering each scenario generates a distinct spatial pattern of concentrations in the crn layer these results and the spatial pattern of measured concentrations in bedrock constrain the range of ela histories that reproduce the crn pattern in each valley in the animas river valley the exposure ages are well explained by a linear ela rise from full glacial to deglacial conditions ages in middle boulder creek valley are best explained by a deglaciation history including a stillstand or partial readvance between 16 and 14 ka followed by rapid retreat

the palaeolimnology of northern lake malawi over the last 25 ka based upon the elemental and stable isotopic composition of sedimentary organic matter abstract sediments accumulating in offshore areas of northern lake malawi typically contain 1 4 organic carbon this organic matter om is of mainly phytoplankton origin with varying mixtures of terrestrial and degraded reworked material high resolution stable carbon and nitrogen isotopic elemental and rock eval pyrolysis analyses of the bulk om contained in three cores have been used to characterise the om and provide detailed insights into the palaeolimnology and palaeoclimatology of northern lake malawi over the last 25 ka preservation of the om as reflected by its rock eval hydrogen index hi is highly variable periods characterised by notably low hi 15 n during the mid and late holocene suggest intervals of stable stratification due to significantly reduced wind driven mixing in the northern part of the lake when n fixing cyanophytes formed a major component of the phytoplankton the geochemical data support recent suggestions of lowstand conditions in lake malawi at the time of the lgm and provide additional evidence of periods of intense but localised aridity in the terminal pleistocene earliest holocene and at the time of the little ice age the change in mixing regime at ca 11 8 ka seems to have been the result of a radical reorganisation of the dominant surface wind flow over south eastern africa the transition was within the resolution of currently available chronologies apparently contemporaneous with marked changes in the circulation patterns of adjacent oceans and the climate of more distant parts of the northern hemisphere

comparison of oxygen isotope records from the gisp2 and grip greenland ice cores recent results1 2 from the greenland ice core project grip summit ice core suggest that the climate in greenland has been remarkably stable during the holocene but was extremely unstable for the time period represented by the rest of the core spanning the last two glaciations and the intervening eemian inter glacial here we present the complete oxygen isotope record for the greenland ice sheet project 2 gisp2 core drilled 28 km west of the grip core we observe large rapid climate fluctuations throughout the last glacial period which closely match those reported for the grip core however in the bottom 10 of the cores spanning the eemian interglacial and the previous glacia tion there are significant differences between the two records it is possible that ice flow may have altered the chronological sequences of the stratigraphy for the bottom part of one or both of the cores considerable further work will be necessary to evaluate the likelihood of this and the extent to which it will still be possible to extract meaningful climate information from the lowest sections of the cores

surface exposure ages of front range moraines that may have formed during the younger dryas 8 2 cal ka and little ice age events abstract surface exposure 10 be ages have been obtained on boulders from three post pinedale end moraine complexes in the front range colorado boulder rounding appears related to the cirque to moraine transport distance at each site with subrounded boulders being typical of the 2 km long chicago lakes glacier subangular boulders being typical of the 1 km long butler gulch glacier and angular boulders being typical of the few hundred m long isabelle glacier surface exposure ages of angular boulders from the isabelle glacier moraine which formed during the little ice age lia according to previous lichenometric dating indicate cosmogenic inheritance values ranging from 0 to 3 0 10 be ka 1 subangular boulders from the butler gulch end moraine yielded surface exposure ages ranging from 5 to 10 2 10 be ka we suggest that this moraine was deposited during the 8 2 cal ka event which has been associated with outburst floods from lake agassiz and lake ojibway and that the large age range associated with the butler gulch end moraine is caused by cosmogenic shielding of and or spalling from boulders that have ages in the younger part of the range and by cosmogenic inheritance in boulders that have ages in the older part of the range the surface exposure ages of eight of nine subrounded boulders from the chicago lakes area fall within the 13 0 11 7 10 be ka age range and appear to have been deposited during the younger dryas interval the general lack of inheritance in the eight samples probably stems from the fact that only a few thousand years intervened between the retreat of the pinedale glacier and the advance of the chicago lakes glacier in addition bedrock in the chicago lakes cirque area may have remained covered with snow and ice during that interval thus partially shielding the bedrock from cosmogenic radiation

the two step shape and timing of the last deglaciation in antarctica the two step character of the last deglaciation is well recognized in western europe in greenland and in the north atlantic for example in greenland a gradual temperature decrease started at the bolling b around 14 5 ky bp spanned through the allerod a and was followed by the cold younger dryas yd event which terminated abruptly around 11 5 ky bp recent results suggest that this ba yd sequence may have extended throughout all the northern hemisphere but the evidence of a late transition cooling is still poor for the southern hemisphere here we present a detailed isotopic record analyzed in a new ice core drilled at dome b in east antarctica that fully demonstrates the existence of an antarctic cold reversal acr these results suggest that the two step shape of the last deglaciation has a worldwide character but they also point to noticeable interhemispheric differences thus the coldest part of the acr which shows a temperature drop about three times weaker than that recorded during the yd in greenland may have preceded the yd antarctica did not experienced abrupt changes and the two warming periods started there before they started in greenland the links between southern and northern hemisphere climates throughout this period are discussed in the light of additional information derived from the antarctic dust record

potential glacial evidence for the younger dryas event in the cordillera of north and south america there is debate as to whether or not the younger dryas yd climatic event affected areas outside of the north atlantic region the potential of alpine glacial deposits to record the yd is of interest because alpine glaciers would be likely to leave a morainal record even during minor short lived advances caused by a period of deteriorating climate in the cordilleran ranges of north and south america there are several candidate moraines many bracketed between radiocarbon ages of ca 12 and 10 ka which may be yd in age the best examples are in the canadian and american rockies the ecuadorian peruvian and bolivian andes and around the patagonian ice cap in most cases temporal brackets are not narrow enough and some radiocarbon ages may not be accurate enough to allow correlation at the present time there are two cases of lateglacial readvance whose ages are interpreted to match or fall within the yd interval an advance of the crowfoot glacier in canada is interpreted to have occurred within the period ca 11 3 and 10 14c ka bp and an advance of the reschreiter glacier in ecuador is interpreted to have culminated in the period ca 11 to 10 6 14c ka bp the latter interpretation depends on an assumption that lake level variations were caused by glacier advances and retreats

hydrogen isotope ratios of palmitic acid in lacustrine sediments record late quaternary climate variations the rich paleoclimate information preserved in lacustrine sedimentary organic matter can be difficult to extract because of the mixed terrestrial and aquatic inputs herein we demonstrate that compound specific hydrogen isotope analysis of palmitic acid pa a ubiquitous compound in lacustrine sediments captures the d signals of lake water samples collected across a diverse range of 33 north american lakes show a strong correlation between water and dpa values at crooked pond massachusetts the dpa changes in a 14 k y sediment record parallel temperature trends inferred from fossil pollen downcore changes reveal differences between climatic trends in new england and in greenland that are consistent with important regional differences in climate controls

a climatic record from l4c dated wood fragments from southwestern colorado deuterium concentration in trees are related to the climatic temperature at which the trees grew deuterium analyses were made on all available 39 14 c dated all 14 c dates cited are uncorrected wood fragments collected from lake emma sediments the 14 c dates range from 9600 to 5400 b p tree line was above lake emma at 9600 b p was at lake emma at about 5000 b p and is 80 m below lake emma at the present time the isotopic records at the various intervals of time coincide very well with this history the range of d values is maximum at 9600 b p and is minimum at about 5400 b p these data allow us to estimate the temperature range for the area between tree line and lake emma between these times these results confirm previously observed cooling trends from several sources in the western hemisphere

asynchronous maximum advances of mountain and continental glaciers the last maximum glacier advance in many mountain ranges appears to have predated the last maximum advance of the wisconsinan continental ice sheets 20 000 years b p published evidence from widely spaced localities in north and south america europe asia and hawai i suggests that some mountain glaciers extended farther during one or more stades early in the last glaciation roughly 115 000 30 000 years b p than at its end when low global temperature and high precipitation presumably were most favorable for the growth of glaciers variations in the timing of glacier advances in different mountain ranges suggest that alpine glaciers are dependent on regional climate as well as global conditions their study might provide sensitive tests for theories of global climate change glacial deposits are often used as time markers in neotectonic and geomorphic studies and their ages sometimes are inferred from distant measurements or from proxy histories of global climate change undated geomorphic features such as moraines sharply incised streams and river terraces which commonly have been associated with the last maximum glacier advance 20 000 years b p or last deglaciation 13 000 years b p may actually be older by a factor of 2 to 4 or more conversely deposits assigned to the maximum advances of the penultimate global glaciation 130 000 years b p may actually date from early in the most recent glacial cycle and be younger by a comparable factor rates of faulting or folding estimated from displacements or deformation and the assumed ages of such geomorphic features might be too great or too small by corresponding factors

century to millennial scale climatic variability in lake malawi revealed by isotope records diatom based oxygen isotope data 18odiatom from lake malawi show multi centennial scale wet dry intervals spaced approximately every 2 3 ka throughout a 25 ka sequence the 18odiatom record is supported by a lower resolution deuterium dpa isotope curve derived from palmitic acid we interpret these isotope data in terms of major shifts in precipitation and evaporation moderated by seasonal controls on the host organisms dry periods marked by relatively positive isotope values represent the extension of abrupt holocene events noted from northern and equatorial africa to 10 15 s these events in lake malawi correspond to cool episodes in greenland thereby demonstrating teleconnections generated by meridional temperature gradients sea surface temperatures are likely to be the primary transmitter of deglacial climate changes although trade wind strength and direction is critical in controlling precipitation patterns in tropical regions conversely the global hydrological cycle driven by low latitude regions represents an important positive feedback amplifying deglacial processes

a high resolution paleoclimate record spanning the past 25 000 years in southern east africa high resolution profiles of the mass accumulation rate of biogenic silica and other geochemical proxies in two piston cores from northern lake malawi provide a climate signal for this part of tropical africa spanning the past 25 000 years the biogenic silica mass accumulation rate was low during the relatively dry late pleistocene when the river flux of silica to the lake was suppressed millennial scale fluctuations due to upwelling intensity in the late pleistocene climate of the lake malawi basin appear to have been closely linked to the northern hemisphere climate until 11 thousand years ago relatively cold conditions in the northern hemisphere coincided with more frequent north winds over the malawi basin perhaps resulting from a more southward migration of the intertropical convergence zone

modeling past and future alpine permafrost distribution in the colorado front range rock glaciers a feature associated with at least discontinuous permafrost provide important topoclimatic information active and inactive rock glaciers can be used to model current permafrost distribution relict rock glacier locations provide paleoclimatic information to infer past conditions future warmer climates could cause permafrost zones to shrink and initiate slope instability hazards such as debris flows or rockslides thus modeling change remains imperative this research examines potential past and future permafrost distribution in the colorado front range by calibrating an existing permafrost model using a standard adiabatic rate for mountains 0 5 c per 100 m for a 4 c range of cooler and warmer temperatures according to the model permafrost currently covers about 12 per cent 326 1 km2 of the entire study area 2721 5 km2 in a 4 c cooler climate 73 7 per cent 2004 4 km2 of the study area could be covered by permafrost whereas in a 4 c warmer climate almost no permafrost would be found permafrost would be reduced severely by 93 9 per cent a loss of 306 2 km2 in a 2 0 c warmer climate however permafrost will likely respond slowly to change relict rock glacier distribution indicates that mean annual air temperature maat was once at least some 3 0 to 4 0 c cooler during the pleistocene with permafrost extending some 600 700 m lower than today the model is effective at identifying temperature sensitive areas for future monitoring however other feedback mechanisms such as precipitation are neglected copyright 2005 john wiley sons ltd

paleocean circulation during the last deglaciation a bipolar seesaw hughen et al 1998 have documented that during the first 200 years of younger dryas time the 14c content of atmospheric co2 increased by 50 and that during the remainder of this 1200 year duration cold event it steadily declined the initial increase in 14c c was likely the result of a reduction in the atlantic s conveyor circulation however were the subsequent radiocarbon decline due to the rejuvenation of this potent heat pump then it is difficult to understand why the climate conditions in the northern atlantic basin remained cold throughout the younger dryas modeling exercises by stocker and wright 1996 mikolajewicz 1998 and schiller et al 1998 show that if the conveyor is terminated the transfer of radiocarbon into the deep sea shifts to the southern ocean thereby stabilizing the atmospheric 14c c ratio paleoclimatic evidence from the antarctic continent suggests that this model based scenario might have been played out in the real world while the younger dryas cooling has been documented in many places around the world including new zealand denton and hendy 1994 sowers and bender 1995 using their 18o in o2 based correlation between the ice core 18o in ice records for antarctica and greenland have demonstrated that in antarctica the younger dryas was a time of maximum warming the point of this paper is that the steep rise in 18o rise in antarctic ice which commenced close to the onset of the younger dryas might have been caused by heat released to the atmosphere in response to an increase in deep sea ventilation in the southern ocean

revised age of deglaciation of lake emma based on new radiocarbon and macrofossil analyses abstract previous radiocarbon ages of detrital moss fragments in basal organic sediments of lake emma indicated that extensive deglaciation of the san juan mountains occurred prior to 14 900 yr b p carrara et al 1984 paleoecological analyses of insect and plant macrofossils from these basal sediments cast doubt on the reliability of the radiocarbon ages subsequent accelerator radiocarbon dates of insect fossils and wood fragments indicate an early holocene age rather than a late pleistocene age for the basal sediments of lake emma these new radiocarbon ages suggest that by at least 10 000 yr b p deglaciation of the san juan mountains was complete the insect and plant macrofossils from the basal organic sediments indicate a higher than present treeline during the early holocene the insect assemblages consisted of about 30 bark beetles which contrasts markedly with the composition of insects from modern lake sediments and modern specimens collected in the lake emma cirque in which bark beetles comprise only about 3 of the assemblages in addition in the fossil assemblages there were a number of flightless insect species not subject to upslope transport by wind indicative of coniferous forest environments these insects were likewise absent in the modern assemblage

hydrogen isotope ratios of lacustrine sedimentary n alkanes as proxies of tropical african hydrology insights from a calibration transect across cameroon abstract hydrogen isotope values d of sedimentary aquatic and terrestrial lipid biomarkers originating from algae bacteria and leaf wax have been used to record isotopic properties of ancient source water i e precipitation and or lake water in several mid and high latitude lacustrine environments in the tropics however where both processes associated with isotope fractionation in the hydrologic system and vegetation strongly differ from those at higher latitudes calibration studies for this proxy are not yet available to close this gap of knowledge we sampled surface sediments from 11 lakes in cameroon to identify those hydro climatological processes and physiological factors that determine the hydrogen isotopic composition of aquatic and terrestrial lipid biomarkers here we present a robust framework for the application of compound specific hydrogen isotopes in tropical africa our results show that the d values of the aquatic lipid biomarker n c17 alkane were not correlated with the d values of lake water carbon isotope measurements indicate that the n c17 alkane was derived from multiple source organisms that used different hydrogen pools for biosynthesis we demonstrate that the d values of the n c29 alkane were correlated with the d values of surface water i e river water and groundwater which on large spatial scales reflect the isotopic composition of mean annual precipitation such a relationship has been observed at higher latitudes supporting the robustness of the leaf wax lipid d proxy on a hemispheric spatial scale in contrast the d values of the n c31 alkane did not show such a relationship but instead were correlated with the evaporative lake water d values this result suggests distinct water sources for both leaf wax lipids most likely originating from two different groups of plants these new findings have important implications for the interpretation of long chain n alkane d records from ancient lake sediments in particular a robust interpretation of palaeohydrological data requires knowledge of the vegetation in the catchment area as different plants may utilise different water sources our results also suggest that the combination of carbon and hydrogen isotopes does help to differentiate between the metabolic pathway and or growth form of organisms and therefore the source of hydrogen used during lipid biosynthesis

exploring weathering and regolith transport controls on critical zone development with models and natural experiments the architecture of the critical zone including mobile regolith thickness and depth to the weathering front is first order controlled by advance of a weathering front at depth and transport of sediment at the surface differences in conditions imposed by slope aspect in the gordon gulch catchment of the boulder creek critical zone observatory present a natural experiment to explore these interactions the weathering front is deeper and saprolite more decayed on north facing than on south facing slopes simple numerical models of weathering front advance mobile regolith production and regolith transport are used to test how weathering and erosion rates interact in the evolution of weathered profiles as the processes which attempt are being made to mimic are directly tied to climate variables such as mean annual temperature the role of quaternary climate variation in governing the evolution of critical zone architecture can be explored with greater confidence

holocene glacier fluctuations in the american cordillera abstract in many areas of the american rocky mountains colorado wyoming utah idaho montana cascade range washington oregon and the sierra nevada california radiocarbon ages suggest that ice receded to near present limits before 10 ka bp a pre altithermal readvance or stillstand left moraines ca 1 3 km beyond and ca 50 300 m below present glacier margins at one locality on glacier peak in washington these deposits are perhaps dated to the early holocene but in general these deposits are probably at least 10 ka old glacial advances during the altithermal ca 8 to 5 ka bp elsewhere are rare radiocarbon evidence for altithermal glacial advances in the colorado front range is questionable the earliest radiocarbon dated neoglacial advances occurred about 5 ka bp in the washington cascades moraines and related deposits believed to be of early neoglacial age i e 5 to 3 ka bp elsewhere are poorly dated for example moraines believed to date to the early neoglacial in colorado triple lakes and wyoming temple lake are actually late pleistocene age based on radiocarbon ages derived from lake sediments at the type localities although relative age data i e lichens rock weathering soils from many areas suggest a three fold neoglacial sequence little supporting radiocarbon evidence is available however tephrochronology distinguishes three neoglacial advances near mount rainier in washington in most mountain ranges of the western united states fresh unweathered sharp crested moraines usually adjacent to present ice margins or near headwalls in empty cirques date to the little ice age of the last several centuries however detailed photographic or other historical records of glacial fluctuations during the last century are rare and detailed mass balance studies rarer still whether or not pre little ice age glacial fluctuations in the american cordillera are synchronous must await better radiometric dating of local moraine sequences

a review of the oxygen isotope composition of lacustrine diatom silica for palaeoclimate reconstruction diatom silica frustules are a form of biogenic opal and contain oxygen isotopes that are being increasingly used in palaeoclimate studies frustules are comprised of an inner tetrahedrally bonded silica skeleton si o si with an outer hydrous layer the hydrous layer is freely exchangeable and must be removed prior to oxygen isotope measurement using oxidising reagents and or high temperatures analysis of the oxygen isotope composition of diatom silica requires samples that are almost pure diatomite since extraction techniques will liberate oxygen from all the components in the sediment there is a generally acceptable protocol involving chemistry sieving and settling techniques and more recently laminar flow separation recent studies of lacustrine diatoms have shown that even a small proportion of contaminant can have a significant influence on the oxygen isotope value all lake sediments require their own specific procedure and every sample must be scrutinised by microscopy to check for the level of contamination prior to analysis where sediment cannot be purified sufficiently a semi quantitative assessment of the diatom content can facilitate mass balance techniques when robust preparation methods are used diatom silica offers an important palaeoclimate proxy providing an additional complementary rather than an alternative host of oxygen isotopes in carbonates in lacustrine isotope studies it is often assumed but rarely proven that diatom silica form in isotope equilibrium and hence empirically derived palaeotemperature equations usually made in vitro can be used most studies lack the detailed investigation of contemporaneous materials from natural waters of the region under study to test these equations further investigations need to be conducted in order to understand the systematic relationship between temperature water isotope composition and diatom isotope composition to enable quantitative interpretation of the sediment record experiments to test for vital effects and inter specific differences have so far revealed little variation it is useful to know when and where the diatom silica is formed in the modern environment so details of diatom ecology are important in the interpretation of isotope data here we show that valuable palaeoclimate data can be gained from the oxygen isotope composition of diatom silica 18odiatom especially since diatoms are abundant in many lakes sensitive to climate variation where other hosts e g carbonates are absent to date the most successful studies have been conducted in areas where the 18odiatom registers changes in the oxygen isotope composition of the lake water rather than temperature which is then related to other aspects of climate in these studies the range of values obtained is often greater than the error introduced by contaminating materials

orbital forcing of glacial interglacial variations in chemical weathering and silicon cycling within the upper white nile basin east africa stable isotope and biomarker evidence from lakes victoria and edward on quaternary time scales the global biogeochemical cycle of silicon is interlocked with the carbon cycle through biotic enhancement of silicate weathering and uptake of dissolved silica by vascular plants and aquatic microalgae notably diatoms for which si is an essential nutrient large tropical river systems dominate the export of si from the continents to the oceans here we investigate variations in si cycling in the upper white nile basin over the last 15 ka using sediment cores from lakes victoria and edward coupled measurements of stable o and si isotopes on diatom separates were used to reconstruct past changes in lake hydrology and si cycling while the abundances of lipid biomarkers characteristic of terrestrial emergent higher plants submerged floating aquatic macrophytes and freshwater algae document past ecosystem changes during the late glacial to mid holocene 15 5 5 ka bp orbital forcing greatly enhanced monsoon rainfall forest cover and chemical weathering riverine inputs of dissolved silica from the lake catchments exceeded aquatic demand and may also have had lower si isotope values since 5 5 ka bp increasingly dry climates and more open vegetation reinforced by the spread of agricultural cropland over the last 3 4 ka have reduced dissolved silica inputs into the lakes centennial to millennial scale dry episodes are also evident in the isotopic records and merit further investigation

spatial and temporal variations in meteoric 10be inventories and long term deposition rates colorado front range abstract atmospherically derived cosmogenic nuclides such as meteoric 10 be offer great potential in quaternary science because they can be used as landform geochronometers and as sediment tracers use of meteoric 10 be inventories for determining landform age soil residence time or erosion rates requires knowing the long term rate of deposition for this cosmogenic nuclide to the earth s surface here we present meteoric 10 be inventories measured at 6 dated sites near the boulder creek critical zone observatory front range colorado each site lies on a relatively stable landform broad moraine or fluvial terrace at slopes between 10 be deposition precipitation dryfall and dust deposition calculated from measured inventories at these sites range from 0 4 to 5 0 10 6 atoms cm 2 yr 1 none of the inventories match expected primary meteoric 10 be deposition rates that global latitude based models or precipitation specific models predict for this region meteoric 10 be inventories and calculated deposition rates at the three older 90 ka sites fall below 0 3 0 7 model predictions inventories and rates at the three pinedale age sites lgm 21 15 ka are higher 1 5 4 than model predictions previous analysis of soils in the front range over the same timeframe as these sites indicate that long term dust accumulation rates are low and associated secondary meteoric 10 be deposition is unlikely to account for the deviation from model predictions at the pinedale age sites in particular long term dustfall would have to be 1 2 orders of magnitude higher to add the meteoric 10 be required to account for measured inventories low inventories at older sites are consistent with loss of meteoric 10 be through physical erosion or selective removal of near surface soil enriched in meteoric 10 be time averaged rates of total meteoric 10 be deposition for these sites are minima we speculate that higher inventories at pinedale age sites reflect the addition of meteoric 10 be due to snowdrifts and or interflow associated with landform location increasing the effective precipitation related primary meteoric 10 be deposition at the sampled sites given these constraints and the spatial variation observed around the front range we suggest that 1 5 0 2 10 6 atoms cm 2 yr 1 is an appropriate long term meteoric 10 be deposition rate to use across much of the front range region that receives 45 55 cm of precipitation a year this long term total meteoric 10 be deposition rate is 30 50 higher than that predicted by global or annual precipitation specific models emphasizing that local calibration of deposition rates is essential for geomorphic studies around the globe

widespread inclination shallowing in permian and triassic paleomagnetic data from laurentia support from new paleomagnetic data from middle permian shallow intrusions in southern illinois usa and virtual geomagnetic pole distributions abstract recent paleomagnetic work has highlighted a common and shallow inclination bias in continental redbeds the permian and triassic paleomagnetic records from laurentia are almost entirely derived from such sedimentary rocks so a pervasive inclination error will expectedly bias the apparent polar wander path of laurentia in a significant way the long standing discrepancy between the apparent polar wander paths of laurentia and gondwana in permian and triassic time may be a consequence of such a widespread data pathology here we present new middle permian paleomagnetic data from igneous rocks and a contact metamorphosed limestone from cratonic laurentia the exclusively reversed middle permian magnetization is hosted by low ti titanomagnetite and pyrrhotite and yields a paleomagnetic pole at 56 3 s 302 9 e a95 3 8 n 6 this pole which is unaffected by inclination shallowing suggests that a shallow inclination bias may indeed be present in the laurentian records to further consider this hypothesis we conduct a virtual geomagnetic pole distribution analysis comparing theoretical expectations of a statistical field model tk03 gad against published data sets this exercise provides independent evidence that the laurentian paleomagnetic data is widely biased likely because of sedimentary inclination shallowing we estimate the magnitude of this error from our model results and present and discuss several alternative corrections

effect of aridity and rainfall seasonality on vegetation in the southern tropics of east africa during the pleistocene holocene transition fossil pollen analyses from northern lake malawi southeast africa provide a high resolution record of vegetation change during the pleistocene holocene transition 18 9 ka recent studies of local vegetation from lowland sites have reported contrasting rainfall signals during the younger dryas yd the lake malawi record tracks regional vegetation changes and allows comparison with other tropical african records identifying vegetation opening and local forest maintenance during the yd our record shows a gradual decline of afromontane vegetation at 18 ka around 14 5 ka tropical seasonal forest and zambezian miombo woodland became established at 13 ka drier more open formations gradually became prevalent although tropical seasonal forest taxa were still present in the watershed during the yd this drought intolerant forest type was likely restricted to areas of favorable edaphic conditions along permanent waterways the establishment of drought tolerant vegetation followed the reinforcement of southeasterly tradewinds resulting in a more pronounced dry winter season after 11 8 ka the onset of the driest most open vegetation type was coincident with a lake low stand at the beginning of the holocene this study demonstrates the importance of global climate forcing and local geomorphological conditions in controlling vegetation distribution

late pleistocene glacial equilibrium line altitudes in the colorado front range a comparison of methods abstract six methods for approximating late pleistocene pinedale equilibrium line altitudes elas are compared for rapidity of data collection and error rmse from first order trend surfaces using the colorado front range trend surfaces computed from rapidly applied techniques such as glaciation threshold median altitude of small reconstructed glaciers and altitude of lowest cirque floors have relatively high rmses 97 186 m because they are subjectively derived and are based on small glaciers sensitive to microclimatic variability surfaces computed for accumulation area ratios aars and toe to headwall altitude ratios thars of large reconstructed glaciers show that an aar of 0 65 and a thar of 0 40 have the lowest rmses about 80 m and provide the same mean ela estimate about 3160 m as that of the more subjectively derived maximum altitudes of pinedale lateral moraines rmse 149 m second order trend surfaces demonstrate low elas in the latitudinal center of the front range perhaps due to higher winter accumulation there the mountains do not presently reach the ela for large glaciers and small front range cirque glaciers are not comparable to small glaciers existing during pinedale time therefore pleistocene ela depression and consequent temperature depression cannot reliably be ascertained from the calculated ela surfaces

evidence for cirque glaciation in the colorado front range during the younger dryas chronozone abstract late pleistocene glacial chronologies developed for the front range mountains of colorado include two or more cirque glacier advances locally known as the satanta peak advances sediment cores were recovered from sky pond an alpine lake 3320 m located less than 100 m downvalley from a moraine that exhibits late pleistocene to early holocene relative age features and appears to correlate to the satanta peak deposits one of the cores penetrated 0 5 m of basal diamict and recovered 3 3 m of overlying sediments that are predominantly gyttja an accelerator mass spectrometry ams age of 12 040 60 yr14c b p was obtained from directly above the basal diamict and is similar to other reported ages for cirque deglaciation in the front range mountains the lower portion of the gyttja contains an interval of clastic sediments that show characteristics consistent with glacial activity in alpine catchments radiocarbon ages obtained from below and near the upper contact of this clastic interval are 11 070 50 and 9970 8014c yr b p respectively an additional ams age of 10 410 9014c yr b p was obtained from within the clastic interval in a second core the most likely source for this interval of clastic sediments is a moraine situated directly upvalley from sky pond and consequently it appears that the deposition of this moraine was coeval with the european younger dryas event 11 000 10 00014c yr b p similarities in soil development weathering features and altitude between this moraine and the type satanta peak moraines suggest that the moraines are correlative these findings are in agreement with a growing body of evidence that suggests a relatively minor advance of alpine glaciers occurred in the north american rockies during the younger dryas chron

extensive early and middle wisconsin glaciation on the western olympic peninsula washington and the variability of pacific moisture delivery to the northwestern united states abstract large glaciers descended western valleys of the olympic mountains six times during the last wisconsin glaciation terminating in the pacific coastal lowlands the glaciers constructed extensive landforms and thick stratigraphic sequences which commonly contain wood and other organic detritus the organic material coupled with stratigraphic data provides a detailed radiocarbon chronology of late pleistocene ice margin fluctuations the early wisconsin lyman rapids advance which terminated prior to ca 54 000 14c yr b p represented the most extensive ice cover subsequent glacier expansions included the hoh oxbow 1 advance which commenced between ca 42 000 and 35 000 14c yr b p the hoh oxbow 2 advance ca 30 800 to 26 300 14c yr b p the hoh oxbow 3 advance ca 22 000 19 300 14c yr b p the twin creeks 1 advance 19 100 18 300 14c yr b p and the subsequent undated twin creeks 2 advance the hoh oxbow 2 advance represents the greatest ice extent of the last 50 000 yr with the glacier extending 22 km further downvalley than during the twin creeks 1 advance which is correlative with the global last glacial maximum local pollen data indicate intensified summer cooling during successive stadial events because ice extent was diminished during colder stadial events precipitation not summer temperature influenced the magnitude of glaciation most strongly regional aridity independently documented by extensive pollen evidence limited ice extent during the last glacial maximum the timing of glacier advances suggests causal links with north atlantic bond cycles and heinrich events

fluorine concentrations of ore fluids in the illinois kentucky district evidence from sem eds analysis of fluid inclusion decrepitates abstract the illinois kentucky district is an atypical occurrence of mississippi valley type mvt mineralization that consists predominantly of fluorite rather than metal sulfide minerals a long standing assumption for the predominance of fluorite in the illinois kentucky district is that the ore fluids there were anomalously rich in dissolved fluorine compared to typical sedimentary brines and other mvt ore fluids this hypothesis is based on the unusual close temporal and spatial association of fluorine rich ultramafic igneous rocks to mvt mineralization in the district high k and sr concentrations in the igneous rocks and in mvt ore hosted fluid inclusions a significant mantle 3he 4he component in ore hosted fluid inclusions and reaction path models that show titration of a hf rich fluid into sedimentary brine is capable of producing a fluorite dominated mvt ore mineral assemblage the purpose of the present study was to test this hypothesis more directly by determining the fluorine concentration of the illinois kentucky ore fluid through sem eds analysis of evaporative solute mounds resulting from thermal decrepitation of fluid inclusions hosted in sphalerite all 26 evaporative solute mounds from illinois kentucky sphalerite samples analyzed contained detectable concentrations of fluorine of 1 4 weight percent based on calibration to standard solutions and previously published fluid inclusion major element concentrations these solute mound fluorine concentrations correspond to fluid inclusion fluorine concentrations of about 680 4300 ppm indicating that the illinois kentucky ore fluids were quite rich in fluorine compared to typical sedimentary brines which have fluorine concentrations mainly on the order of 1 s to 10 s of ppm in contrast solute mounds from sphalerite hosted fluid inclusions from the tri state district did not contain fluorine in excess of the detection limit the detection limit equates to an aqueous fluorine concentration between 87 and 169 ppm the high apparent fluorine concentrations of the illinois kentucky fluid inclusions suggest that the ore fluids were very acidic with a ph ranging between 0 and 1 4 much lower than the typical ph range of mvt ore fluids of 4 5 5 such a low ph would have greatly increased the solubility of metal sulfide minerals suppressing their precipitation and contributing further to the predominance of fluorite in the illinois kentucky district

timing of late quaternary glaciations in the western united states based on the age of loess on the eastern snake river plain idaho the most viable sources for ubiquitous loess deposits on the eastern snake river plain idaho are aggraded river valleys active alluvial fans and fluctuating pluvial lake margins associated with regional late pleistocene glaciation of the northern rocky mountains stratigraphic studies on the idaho national engineering laboratory document two distinct loess deposits separated by a well developed paleosol resting on basaltic lava baked sediments beneath this lava yielded thermoluminescence tl age estimates of 108 000 13 000 and 101 000 7000 yr and baked organic matter gave radiocarbon ages of 32 000 yr b p consistent with an earlier k ar age for the flow of 95 000 25 000 yr the overlying two loess deposits yielded tl age estimates of 74 000 6000 and 28 000 3000 yr the available geochronology indicates that the latest period of loess deposition commenced ca 40 000 to 35 000 yr ago and ceased approximately 10 000 yr ago which is generally coincident with the inferred timing of regional pinedale glaciation and pluvial lake expansion we estimate that the penultimate loess depositional episode dates between 80 000 and 60 000 yr ago which is significantly younger than previous age estimates of 140 000 to 150 000 yr based on stratigraphic position we speculate that this period of loess deposition may correlate with documented early to middle wisconsinan glaciation and a high stand of pluvial lakes in the basin and range province

constraints from fluid inclusion compositions on the origin of mississippi valley type mineralization in the illinois kentucky district fluid inclusions hosted by mississippi valley type mvt ores in the illinois kentucky district were analyzed using microthermometry laser ablation inductively coupled plasma mass spectrometry la icp ms and raman spectroscopy to characterize the composition and temperature of mineralizing fluids in order to seek insights into the process of ore formation results from the present study support the interpretation of previous microthermometry studies that relate mineralization to mixing of two higher temperature brines and a cooler dilute meteoric fluid fluid inclusions in the illinois kentucky district have overall higher atomic ratios of ba na ca mg k na sr na and lower atomic ratios of mg na compared to inclusions in the ozark mvt districts and are thus compositionally distinct these differences may indicate that the illinois kentucky and ozark mvt ore fluids were derived largely from different source basins or may reflect input from igneous rocks in the illinois kentucky district low ca mg ratios of illinois kentucky fluid inclusions indicate that mineralizing fluids were dolomitizing and therefore could not have traveled far through the limestone ore host formations but instead traveled through deeper nonlimestone units before rising along faults and fractures in the district some fluid inclusions in illinois kentucky mvt mineralization yielded strong la icp ms responses for ore metals a single sphalerite hosted fluid inclusion out of 18 analyzed contained detectable pb corresponding to a concentration of 420 ppm whereas numerous fluorite hosted fluid inclusions contained detectable pb zn and cu corresponding to maximum concentrations on the order of 1 000s of ppm the high and similar concentrations of cu pb and zn in some fluorite hosted fluid inclusions in the illinois kentucky district are problematic because cu sulfide mineralization should be much more abundant than pb and zn sulfide mineralization owing to the lower solubility of cu relative to pb and zn in the presence of reduced sulfur which is not observed in the field this raises the possibility that ore metals may exist as solid phases in suspension or adhering to fluid inclusion walls in some inclusions rather than as dissolved species in aqueous solution raman microprobe analyses of fluid inclusions hosted by minerals occurring throughout the paragenetic sequence detected methane concentrations ranging from about 40 to 1 200 ppm with most on the order of 100s ppm even for the lowest methane concentration determined redox conditions would have been reducing at a log of 55 4 at a temperature of 100 c at such reducing conditions sulfide would predominate over sulfate this is supported by high concentrations of ba in the fluid inclusions the overall reducing conditions throughout the paragenetic sequence and the possible presence of both metal poor and rich fluid inclusions would suggest that mixing between a metal rich sulfide poor fluid and a metal poor sulfide rich fluid were an important mechanism for sulfide mineral precipitation minimum depths of mineralization estimated from methane concentrations in fluid inclusions range to about 1 500 m and possibly 2 500 m which would not have been reached until late pennsylvanian to permian time i e during the alleghanian orogeny

history of diagenetic fluids in a distant foreland area middle and upper pennsylvanian cherokee basin kansas usa fluid inclusion evidence abstract analysis of fluid inclusion data in diagenetic cements from pennsylvanian limestones and sandstones of the cherokee basin in southeastern kansas reveals the succession of diagenetic fluids in a distant foreland of the arkoma ouachita system this succession includes early low salinity 0 0 2 4 wt nacl eq fluids of meteoric affinity fluid i followed by low temperature na ca cl brines fluid ii with salinities between 8 4 and 24 1 wt nacl eq fluids i and ii were present in the system during precipitation of early stage calcite cements at temperatures less than about 50 c another na ca cl brine fluid iii with salinity up to 25 wt nacl eq was present in the system later at temperatures of maximum burial at least 80 85 c and higher fluid iii is followed by a na cl brine fluid iv with salinities about 19 21 wt nacl eq characterized by temperatures distinctly higher than maximum burial up to 150 c fluid iii and fluid iv were entrapped during precipitation of late stage baroque dolomite and fe dolomite in pennsylvanian limestones and late stage fe dolomite and ankerite in pennsylvanian sandstones the record of progression from fluid iii to fluid iv may have been partially obscured by thermal re equilibriation of some inclusions during migration of fluid iv fluids with na ca cl chemistry fluid ii and iii were either indigenous subsurface fluids of the cherokee and arkoma basins or might have originated as reflux fluids in a permian evaporitic basin of central kansas later na cl brine fluid iv originated in deeper parts of the arkoma ouachita system and might have acquired their salinity by dissolution of hypothetical salts buried beneath the ouachitas temperatures recorded by fluid inclusions in late diagenetic carbonates are 20 60 c higher than those calculated for the maximum burial of the studied section this thermal anomaly suggests an advective heat transfer from the arkoma ouachita system onto the shelf of the cherokee basin related to the invasion of late diagenetic fluids

thermoluminescence chronology of sand profiles in the mu us desert china abstract thermoluminescence tl is used to calculate the ages of two sand loess profiles in the marginal zone of the mu us desert china alternating units of dune sand and loess with palaeosols in the two profiles imply episodes of dune formation separated by periods of loess accumulation and pedogenesis our results indicate that there are three layers of dune sand intercalated in sediments of the last full glacial cycle suggesting the occurrence of three arid episodes during this time the first arid episode within this period occurred between

late quaternary eolian sand depositional record for southwestern kansas landscape sensitivity to droughts abstract eolian deposits and landforms are ubiquitous in western kansas particularly south of the arkansas river with the presence of a stabilized dune field stratigraphic studies and associated optical dating reveal a complex depositional history for this dune field spanning the late quaternary the oldest eolian deposits from ca 16 to 12 ka completely or partially bury high terraces with ages of ca 16 and 33 ka it is unknown whether these eolian deposits reflect regional aridity and or a change in sediment availability with the transition of the arkansas river from a single channel to a braided system there is pervasive evidence for episodic eolian erosion and deposition ca 9 8 to 6 3 ka generally coincident with loess deposition on upland surfaces olson c g nettleton w d porter d a brasher b r 1997 middle holocene aeolian activity on the high plains of west central kansas holocene 7 3 255 261 and thus it is inferred to reflect regional aridity sites within the dune field show a sequence of eolian sands and weak buried soils reflecting either dune migration or sand sheet accretion at ca 1490 430 380 320 180 and 70 yr ago which correspond well with continental scale droughts in the tree ring record eolian sand in the arkansas river dune field may be derived principally with variability in fluvial activity and in climate during the late pleistocene to the middle holocene with substantial reactivation of eolian systems during decadal scale drought variability in the past 2000 yr

deglaciation and postglacial timberline in the san juan mountains colorado abstract lake emma which no longer exists because of a mining accident was a tarn in a south facing cirque near the headwaters of the animas river in the san juan mountains of southwestern colorado during the pinedale glaciation this area was covered by a large transection glacier centered over the lake emma region three radiocarbon dates on basal organic sediment from lake emma indicate that by ca 15 000 yr b p this glacier one of the largest in the southern rocky mountains no longer existed twenty two radiocarbon dates on picea and abies krummholz fragments in the lake emma deposits indicate that from ca 9600 to 7800 yr b p from 6700 to 5600 yr b p and at 3100 yr b p the krummholz limit was at least 70 m higher than present these data in conjunction with picea pinus pollen ratios from both the lake emma site and the hurricane basin site of j t andrews p e carrara f b king and r struckenrath 1975 quaternary research 5 173 197 suggest than from ca 9600 to 3000 yr b p timberline in the san juan mountains was higher than present cooling apparently began ca 3000 yr b p as indicated by decreases in both the percentage of picea pollen and picea pinus pollen ratios at the hurricane basin site andrews et al 1975 cooling is also suggested by the lack of picea or abies fragments younger than 3000 yr b p at either the lake emma or the hurricane basin site

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

how precisely can the temperature of a fluid event be constrained using fluid inclusions u s national science foundation ear 1624589 chemical sciences geosciences and biosciences division office of basic energy sciences office of science u s department of energy de fg02 03er15430 bureau of economic geology jackson school of geosciences university of texas austin

origin and palaeoenvironmental significance of c25 and c27 n alk 1 enes in a 25 000 year lake sedimentary record from equatorial east africa we studied the distribution of long chain alkenes n c23 to n c31 in well dated sediments from lake challa a deep crater lake near mt kilimanjaro in equatorial east africa to reveal signatures of palaeo environmental and palaeo climatic changes affecting the production of these compounds during the last 25 kyr the apolar fractions of organic sediment extracts dated to the last 16 kyr showed an unusual dominance of d13c depleted n c25 1 and n c27 1 alk 1 enes these alkenes were not detected in soil and litter from near the shoreline and from the inner rim of the crater pointing to an autochthonous aquatic source analysis of suspended particulate matter indicated that the n alk 1 enes are produced in the well oxygenated upper 30 m of the water column indicating a phytoplanktonic origin sedimenting particles collected monthly from december 2006 to november 2007 showed increased fluxes of n alk 1 enes following the locally prominent short rain season in november december green algae and or cyanobacteria were identified as candidate sources of these alkenes production of the n c25 1 and n c27 1 alkenes in lake challa was much reduced during the last glacial maximum and early late glacial period suggesting a temperature or co2 effect on habitat suitability we explored the potential of n alk 1 ene accumulation rates and of a derived alkene index n c27 1 n c25 1 n c27 1 to record longer term climatic changes the alkene index record of lake challa over the past 25 kyr shows clear periodicity with a dominant frequency of 2 3 kyr potentially indicative of monsoon variability directly or indirectly forced by variation in solar radiation

holocene forcing of the indian monsoon recorded in a stalagmite from southern oman a high resolution oxygen isotope record from a thorium uranium dated stalagmite from southern oman reflects variations in the amount of monsoon precipitation for the periods from 10 3 to 2 7 and 1 4 to 0 4 thousand years before the present ky b p between 10 3 and 8 ky b p decadal to centennial variations in monsoon precipitation are in phase with temperature fluctuations recorded in greenland ice cores indicating that early holocene monsoon intensity is largely controlled by glacial boundary conditions after approximately 8 ky b p monsoon precipitation decreases gradually in response to changing northern hemisphere summer solar insolation with decadal to multidecadal variations in monsoon precipitation being linked to solar activity

late quaternary loess in northeastern colorado part ii pb isotopic evidence for the variability of loess sources a new application of the pb isotopic tracer technique has been used to determine the relative importance of different silt sources for late wisconsin loess in the central great plains of eastern colorado samples of the peoria loess collected throughout the study area contain k feldspar derived from two isotopically and genetically distinct sources 1 glaciogenic material from early and middle proterozoic crystalline rocks of the colorado province and 2 volcaniclastic material from the tertiary white river group exposed on the northern great plains pb isotopic compositions of k feldspar in loess from two dated vertical sections at beecher island and last chance colorado vary systematically implying climatic control of source availability we propose a model whereby relatively cold conditions promoted the advance of front range valley glaciers discharging relatively little glaciogenic silt but strong winds caused eolian erosion of white river group silt due to a decrease in vegetation cover during warmer periods valley glaciers receded and discharged abundant glaciogenic silt while surfaces underlain by the white river group were stabilized by vegetation isotopic data from eastern colorado loess sections record two warm cold warm cycles during late wisconsin time between about 21 000 and 11 000 radiocarbon yr b p similar to results from other studies in the united states and greenland

holocene alluvial stratigraphy and response to climate change in the roaring river valley front range colorado usa abstract stratigraphic analyses and radiocarbon geochronology of alluvial deposits exposed along the roaring river colorado lead to three principal conclusions 1 the opinion that stream channels in the higher parts of the front range are relics of the pleistocene and nonalluvial under the present climate as argued in a water rights trial usa v colorado is untenable 2 beds of clast supported gravel alternate in vertical succession with beds of fine grained sediment sand mud and peat in response to centennial scale changes in snowmelt driven peak discharges and 3 alluvial strata provide information about holocene climate history that complements the history provided by cirque moraines periglacial deposits and paleontological data most alluvial strata are of late holocene age and record among other things that 1 the largest peak flows since the end of the pleistocene occurred during the late holocene 2 the occurrence of a mid to late holocene interval 2450 1630 cal yr bp of warmer climate which is not clearly identified in palynological records and 3 the little ice age climate seems to have had little impact on stream channels except perhaps for minor 1 m incision

new evidence for a reduced water balance in east africa during the last glacial maximum implication for model data comparison some syntheses of lake level data for the last glacial maximum lgm in east africa 10 n and 30 s east of 25 e show apparently wetter conditions than present for some basins whereas palaeovegetation reconstruction indicates a generally dry climate pmip gcm simulations for the lgm support both scenarios for this region when run under different boundary conditions here we compare three new records from lakes in the data poor southern part of east africa lake malawi lake massoko and lake rukwa we also re assess previously published lake level data and apply a salinity transfer function to the diatom record from lake manyara our results show that in contrast to previous interpretations these lakes were at least as low as today at the lgm and are thus in agreement with the palaeovegetation data relative drought across east africa is best simulated by gcms that use computed ssts rather than the higher climap values lower ssts and the presence of the northern hemisphere ice sheets must have been dominant over any monsoon precipitation rise caused by astronomically induced summer insolation enhancement in the southern african tropics

integrating soils and geomorphology in mountains an example from the front range of colorado soil distribution in high mountains reflects the impact of several soil forming factors soil geomorphologists use key pedological properties to estimate ages of quaternary deposits of various depositional environments estimate long term stability and instability of landscapes and make inferences on past climatic change once the influence of the soil forming factors is known soils can be used to help interpret some aspects of landscape evolution that otherwise might go undetected the front range of colorado rises from the plains of the colorado piedmont at about 1700 m past a widespread dissected tertiary erosion surface between 2300 and 2800 m up to an alpine continental divide at 3600 to over 4000 m pleistocene valley glaciers reached the western edge of the erosion surface parent rocks are broadly uniform granitic and gneissic climate varies from 46 cm mean annual precipitation map and 11 jc mean annual temperature mat in the plains to 102 cm and 4 jc respectively near the range crest vegetation follows climate with grassland in the plains forest in the mountains and tundra above 3450 m soils reflect the bioclimatic transect from plains to divide a bw or bt bk or k grassland to a e bw or bt c forest to a bw c tundra corresponding soil ph values decrease from 8 to less than 5 with increasing elevation the pedogenic clay minerals dominant in each major vegetation zone are smectite grassland vermiculite forest and 1 0 1 8 nm mixed layer clays tundra within the lower forested zone the topographic factor aspect results in more leached colder soils with relatively thin o horizons well expressed e horizons and bt horizons alfisols on n facing slopes whereas soils with thicker a horizons less developed or no e horizons and bw or bt horizons mollisols are more common on s facing slopes the topographic factor in the tundra results in soil patterns as a consequence of wind redistributed snow and the amount of time it lingers on the landscape an important parent material factor is airborne dust which results in fine grained surface horizons and if infiltrated contributes to clay accumulation in some bt horizons the time factor is evaluated by soil chronosequence studies of quaternary deposits in tundra upper forest and plains grassland few soils in the study area are 10 000 years old in the tundra 100 000 years old in the forest and 2 million years old in the grassland stages of granite weathering vary with distance from the continental divide and the best developed is grus near the sedimentary granitic rock contact just west of the mountain front grus takes a minimum of 100 000 years to form

late quaternary loess in northeastern colorado part i age and paleoclimatic significance loess in eastern colorado covers an estimated 14 000 km 2 and is the westernmost part of the north american midcontinent loess province stratigraphic studies indicate there were two periods of loess deposition in eastern colorado during late quaternary time the first period spanned ca 20 000 to 12 000 14 c yr b p ca 20 14 ka and correlates reasonably well with the culmination and retreat of pinedale glaciers in the colorado front range during the last glacial maximum the second period of loess deposition occurred between ca 11000 and 9000 14 c yr b p this interval may be holocene or may correlate with a hypothesized younger dryas glacial advance in the colorado front range sedimentologic mineralogic and geochemical data indicate that as many as three sources could have supplied loess in eastern colorado these sources include glaciogenic silt derived from the colorado front range and two bedrock sources volcaniclastic silt from the white river group and clays from the pierre shale the sediment sources imply a generally westerly paleowind during the last glacial maximum new carbon isotope data combined with published faunal data indicate that the loess was probably deposited on a cool steppe implying a last glacial maximum july temperature depression relative to the present of at least 5 6 c overall loess deposition in eastern colorado occurred mostly toward the end of the last glacial maximum under cooler and drier conditions with generally westerly winds from more than one source

the epica ice core from dronning maud land first results from stable isotope measurements the european project for ice coring in antarctica epica focuses on the drilling of two deep ice cores the first at dome c and the second at kohnen station 75 00 s 0 04 e in dronning maud land dml this paper deals with stable isotope records from ice cores drilled in dml in the first season the deep epica dml core reached a depth of 450 m recovering ice approximately 7000 years old generally the d 18 o record indicates a stable holocene climate and shows low variability however during the last 4000 years based on a preliminary time scale the d 18 o values decrease continuously by about 0 6 and the deuterium excess values increase by about 0 5 t he correlation between d 18 o and the deuterium excess d is investigated for a 50 m long core section and the near surface snow high pass filtered profiles are positively correlated whereas the correlation between low pass filtered profiles is negative a post depositional effect due to diffusion processes can be seen in a sub annually resolved profile from snow pit samples changes in the seasonality of the evolution of the snow cover and the consequences for stable isotope content are demonstrated with data from ice core b31

timing of millennial scale climate change in antarctica and greenland during the last glacial period a precise relative chronology for greenland and west antarctic paleotemperature is extended to 90 000 years ago based on correlation of atmospheric methane records from the greenland ice sheet project 2 and byrd ice cores over this period the onset of seven major millennial scale warmings in antarctica preceded the onset of greenland warmings by 1500 to 3000 years in general antarctic temperatures increased gradually while greenland temperatures were decreasing or constant and the termination of antarctic warming was apparently coincident with the onset of rapid warming in greenland this pattern provides further evidence for the operation of a bipolar see saw in air temperatures and an oceanic teleconnection between the hemispheres on millennial time scales

oxygen isotopes of marine diatoms and relations to opal a maturation in order to develop the potential tool of diatom oxygen isotopes for paleoenvironmental studies we compared oxygen isotopes of natural marine diatoms sampled from ocean surface water sediment traps and surface sediments with oxygen isotopic fractionations determined for laboratory diatom cultures freshly grown natural diatoms phytoplankton samples and sediment trap material and cultured diatoms reveal similar oxygen isotope fractionation factors the fresh diatoms have 3 to 10 parts per thousand lower isotope fractionation factors than fossil sedimentary diatoms a temperature related oxygen isotope fractionation could not be established for the laboratory cultures and the natural phytoplankton samples and there is evidence that diatom growth rate until reaching the stationary growth state also controls the measured silica water oxygen isotope fractionation factor it is possible however that slow diatom growth in sea surface water may well lead to a temperature dependent silica water oxygen isotope fractionation which is the prerequisite for a use of diatom oxygen isotopes in palco surface water studies ftir spectroscopic analyses of various diatomaceous materials revealed that the ratio of integrated peak intensities for si o si si oh correlates with the 3 to 10 parts per thousand delta o 18 silica increase from fresh to fossil diatoms open system flow through silica dissolution experiments suggest that the diatom frustules are isotopically homogenous and that the increase in o 18 is therefore not due to dissolution of isotopically light surficial si oh groups it is concluded that slow internal condensation reactions during silica maturation in surface sediments cause both an increase in the intensity ratio of si o si si oh and the o 18 content of framework oxygen these findings also indicate that the oxygen isotope compositions of marine sediment diatoms do not indicate sea surface water temperature but rather reflect variable o 18 contents of surface sediments copyright c 2001 elsevier science ltd

large temperature variability in the southern african tropics since the last glacial maximum 1 the role of the tropics in global climate change is actively debated particularly in regard to the timing and magnitude of thermal and hydrological response continuous high resolution temperature records through the last glacial maximum lgm from tropical oceans have provided much insight but surface temperature reconstructions do not exist from tropical continental environments here we used the tex86 paleotemperature proxy to reconstruct mean annual lake surface temperatures through the last glacial maximum lgm in lake malawi east africa 9 14 s we find a 3 5 c overall warming since the lgm with temperature reversals of 2 c during the younger dryas 12 5 ka bp and at 8 2 ka bp maximum holocene temperatures of 29 c were found at 5 ka bp a period preceding severe drought in africa these results suggest a substantial thermal response of southeastern tropical africa to deglaciation and to varying conditions during the holocene

origin of the late quaternary dune fields of northeastern colorado stabilized eolian deposits mostly parabolic dunes and sand sheets cover much of the landscape of northeastern colorado and adjacent parts of southwestern nebraska in four geographically distinct dune fields stratigraphic and soil geomorphic relations and accelerator radiocarbon dating indicate that at least three episodes of eolian sand movement occurred between 27 ka and 11 ka possibly between 11 ka and 4 ka and within the past 1 5 ka thus eolian sand deposition took place under both glacial and interglacial climatic conditions in the youngest episodes of eolian sand movement holocene parabolic dunes partially buried pleistocene sand sheet deposits late holocene sands in the fort morgan and wray dune fields to the south of the south platte river have trace element ratios that are indistinguishable from modern south platte river sands but different from ogallala formation bedrock which has previously been cited as the main source of dune sand on the great plains sands in the greeley dune field to the north of the south platte river have trace element concentrations that indicate a probable laramie formation source measurements of parabolic dunes indicate paleowinds from the northwest in all dune fields in good agreement with resultant drift directions calculated for nearby weather stations thus paleowinds were probably not significantly different from present day winds and are consistent with a south platte river source for the fort morgan and wray dune fields and a laramie formation source for the greeley dune field sand accumulated downwind of the south platte river to form the fort morgan duen field in addition sand was also transported farther downwind over the upland formed by the calcrete caprock of the ogallala formation and deposited in the lee of the upland on the southeast side because of high wind energy the upland itself served as a zone of sand transport but little or no sand accumulation took place on this surface these studies which demonstrate the importance of fluvial source sediments for dune fields in colorado may be applicable to other dune fields in north america because modern drift potentials in northeastern colorado are among the highest in the world the present stability of dunes in the region may be in part a function of the dunes being supply limited rather than solely transport limited extensive 7700 km2 late holocene dunes document that eolian sand in northeastern colorado is very sensitive to small changes in climate or fluvial source conditions

26al and10be dating of late pleistocene and holocene fill terraces a record of fluvial deposition and incision colorado front range cosmogenic 26al 10be and 14c dating of fluvial fill terraces in steep canyons of the colorado front range provides a temporal framework for analysing episodic aggradation and incision results from boulder canyon show that terrace heights above the modern channel grade can be divided into 1 bull lake 100 ka 20 15 m above grade 2 pinedale 32 10 ka 15 4 m above grade and 3 holocene age 15 m above grade record bull lake deposition well preserved terraces of pinedale age suggest that the range of terrace height above grade reflects short term fluctuations in the river profile during periods of rapidly changing stream load and power net river incision apparently occurred during transitions to interglacial periods soil development and stratigraphic position along with limited cosmogenic and 14c dating suggest that 130 ka terraces in boulder canyon correlate with the louviers alluvium and that 32 to 10 ka fills in the canyon correlate with the broadway alluvium on the adjacent high plains late pleistocene incision rates 0 15 m ka 1 along boulder canyon exceed pre late pleistocene incision rates and are higher than middle to late pleistocene incision rates 0 04 m ka 1 on the high plains this study provides an example of how modern geochronologic techniques allow us to understand better rivers that drain glaciated catchments copyright 2002 john wiley sons ltd