u series dating by the tims technique of land snails from paleosols in the canary islands a sequence of seven superimposed paleosols developed on eolian calcarenites and alluvium was sampled on the island of lanzarote in order to examine the possibility of dating land snail shells by the u series method using a tims technique allowing measurement of u and th isotopes in very small samples in the lower six units the fossil shells yielded d allo l isoleucine a i ratios of about 0 5 and apparent ams 14c ages ranging from 41 000 to 34 000 yr b p indicating that most paleosols formed during a relatively short mid wurm humid episode the upper unit paleosol 7 yielded more variable a i ratios ranging from 0 6 to 0 2 and a younger 14c age 27 000 yr b p most samples contained enough u to allow the calculation of u series ages after correction for the presence of a detrital component in samples containing a few tens of ppb of u paleosols 1 2 3 6 and 7 the ages are strongly dependent upon the model used for the correction in samples containing more than 300 ppb of u paleosols 4 and 5 concordant ages of 31 000 1000 yr were obtained regardless of the correction model used u uptake in these shells occurred during one single early diagenetic phase soon after burial since shells of modern snails do not contain any significant amount of u the arid conditions subsequent to the mid wurm humid episode have likely ensured since then a fair closure of the radioactive system

new 230th u and 14c ages from lake lahontan carbonates nevada usa and a discussion of the origin of initial thorium five sets of coeval lacustrine carbonate samples from pleistocene lake lahontan in western nevada were dated by both the ams 14c and 230th u isochron methods all five groups of samples were analyzed for u th isotopes by alpha spectrometry and one of the groups was additionally measured by thermal and secondary ionization mass spectrometry tims and sims for comparison the 14c ages were corrected to calendar years using the calibration curve recommended by bard et al 1992 without local reservoir correction on the 14c ages mean 230th u isochron ages of some sets are apparently older than their calendar corrected 14c ages by up to 2300 years modern carbon contamination of these carbonate samples through recrystallization or deposition of secondary calcite is likely to be responsible for part of the age discrepancies we explored additional biases associated with the isochron ages maybe produced by the presence of initial th coprecipitated from the lake water it can be shown that if dissolved hydrogenous th is directly incorporated into the pure carbonates then the three component mixing among 1 detrital th 2 hydrogenous th adsorbed on detritus and 3 hydrogenous th incorporated by the carbonate can introduce a positive age bias we have developed an approach to estimate the magnitude of this bias of the lake lahontan carbonates the preliminary estimates suggest a positive age bias of 1000 to 2000 years for two sets of the samples

comment on permanent human occupation of the central tibetan plateau in the early holocene meyer et al reports 6 january 2017 p 64 suggest a new chronology for permanent human occupation of tibet based on their dating of the travertine and colluvium deposits that contain or are deposited near fossil human handprints and footprints however misinterpretations in both stratigraphic reconstruction and dating data may have caused the newly proposed age of these human imprints to have been seriously underestimated

a calcrete based u th chronology for landform evolution in the sorbas basin southeast spain nodular and massive calcretes from alluvial terraces in the sorbas area of southeast spain have been dated by the u th isochron method to between 8 350 ka alternative approaches have been used to generate the multiple samples for the isochron method for several of the calcretes dates agreed closely when obtained by total dissolution of a number of nodules or by the sequential dissolution in different acids of a single nodule however fractional dissolution of a single nodule in the same acid revealed two stages of precipitation of different age it is considered that the calcretes may not be strictly pedogenic in origin and that groundwater processes linked to the alluvial environment may have been involved also nevertheless their dates provide minimum ages for a chronostratigraphic framework for terrace evolution

u series isochron dating a generalized method employing total sample dissolution the age of a sedimentary deposit can be determined by measuring 238u 234u 232th and 230th in two or more samples of the deposit with the requirement that these samples have different u th ratios but an identical 230th232th ratio when deposited this paper presents the theoretical and experimental aspects of the dating method the latter involves techniques for securing samples of varied u th ratios from the same deposit plotting isochrons of the radiochemical data obtained on totally dissolved samples for age analysis and estimating the errors involved although applications to impure carbonates and lacustrine evaporite sediments are given as examples in this paper the method potentially can be generalized for dating a variety of authigenic deposits such as rock varnish peat opal sulfate minerals and marine phosphorites

uranium series dating of marine and lacustrine carbonates of the possible uranium series dating schemes the most important and most widely applied to marine carbonates is 230th dating with 231pa dating playing an increasingly important role for this reason this review will focus on these two methods 230th dating also referred to as u th dating or 238u 234u 230th dating involves calculating ages from radioactive decay and ingrowth relationships among 238u 234u and 230th 232th is also typically measured as a long lived essentially stable index isotope over the time scales relevant to 230th dating at present 230th dating can in principle be used to date materials as young as 3 years and in excess of 600 000 years edwards et al 1987a 1993 edwards 1988 see stirling et al 2001 for an example of dating corals in excess of 600 000 years old 231pa dating also referred to as u pa dating involves calculating ages from the ingrowth of 231pa from its grandparent 235u at present 231pa dating can be used to date materials as young as 10 years and as old 250 000 years edwards et al 1997 230th dating covers all of the 231pa time range and more with somewhat higher precision and is therefore the method of choice if a single method is applied however the combination of 231pa and 230th dating is of great importance in assessing possible diagenetic mobilization of the pertinent nuclides and thereby the accuracy of the ages allegre 1964 ku 1968 even if the primary age exceeds the 250 000 year limit of 231pa dating the combined methods can be used to assess the degree to which the samples have remained closed over the past 250 000 years e g edwards et al 1997 thus

u th dating and post depositional alteration of a cold seep carbonate chimney from the campos basin offshore brazil abstract dating authigenic cold seep carbonates allows determining the timing of active seepage of hydrocarbon rich fluids at continental margins and ultimately the detection of the geological factors affecting it u th dating has been selected in this study the method having been successfully used before however incorporation of initial thorium in substantial amounts of detrital material due to the precipitation of carbonates within the seabed and post depositional alteration potentially pose a major problem violating the basic assumptions of the method here we report results from several samples from the campos basin off the coast of brazil they are unusual in their high 13 c values mean of 10 0 vpdb high detrital contents and being dominantly dolomite elevated 230 th 238 u activity ratios indicate that most of the carbonates lost uranium during diagenesis isochrones are obtained from the unaffected samples yielding an age of 140 4 3 1 ka vertical growth of the chimney must have been short and rapid since results from the top and bottom parts of the chimney concur within analytical uncertainties furthermore profiles of u th radioisotope specific activity 234 u and 232 th specific activity on two quarter slices of a carbonate chimney reveal the occurrence of three distinct diagenetic processes u loss u exchange with seawater and th gain

interpreting the response of a dryland river system to late quaternary climate change abstract a u series calcrete chronology has been constructed for three late quaternary terrace units termed the d1 d2 and d3 terraces in age descending order from the rio aguas river system of the sorbas basin southeast spain the d1 terrace formed between 30 300 4400 year bp and 12 140 360 year bp correlating well with the last glacial maximum when rates of sediment supply would have increased greatly because of higher rates of weathering reduced vegetation cover and weak soil development the d2 terrace formed between 12 800 1100 year bp and 9 600 530 year bp correlating well with the younger dryas event the d3 terrace could only be poorly constrained to the early holocene and no unequivocal cause could be assigned to this period of aggradation the sedimentology and geomorphology of the d2 terrace suggests however that the aggradation of this unit was a response to diapirism karstic processes occurring within the underlying messinian gypsum strata and the subsequent damming of the aguas system therefore despite its coincident occurrence with the younger dryas aggradation of the d2 terrace is unrelated to climate change the style of this response controlled predominantly by the characteristics of the underlying bedrock makes correlating the terrace record of the aguas with other systems in the mediterranean unreliable this study therefore highlights the problems of correlating fluvial sequences in regions of variable tectonics climatic history and bedrock geology and emphasises the need to properly understand the main controls on individual fluvial systems before any attempt is made to correlate their depositional histories

the 1994 sefidabeh eastern iran earthquakes revisited new evidence from satellite radar interferometry and carbonate dating about the growth of an active fold above a blind thrust fault in 1994 three shallow earthquakes of mw 6 occurred close together on blind thrusts near sefidabeh in eastern iran in an earlier study of the teleseismic waveforms the geomorphology and the faulting in the epicentral region it was suggested that these earthquakes were associated with the growth of a ridge above a blind thrust fault system whose activity could be detected by its effect on the surface drainage in this study we present a sar interferogram that precisely determines the location and amount of coseismic surface displacements showing that the earthquakes in the sefidabeh sequence probably occurred on en echelon fault segments associated with three stepping ridges we also present u th dates of 100 ka for lake deposits uplifted by the growing ridge from the cumulative dated uplift and knowledge of the surface displacements due to an earthquake sequence we estimate that 120 such events have occurred in the past 100 ka with an average recurrence interval of 830 yr and an average convergence rate of 1 5 mm yr 1 on the sefidabeh thrust each estimate has an uncertainty of a factor of two either way we argue that the sefidabeh fault originally formed by coalescence of many small fault segments and has grown in length at about 2 cm yr 1 in the past 100 ka though the coseismic surface deformation observed in the sar interferogram closely resembles folding the overall topography does not because of inherited topography associated with earlier geological deformation in spite of this the activity of the buried thrust fault can easily be detected by its effect on the surface drainage a significant lesson when interpreting landscapes that are not entirely due to the present day deformation

uranium series dating of fen peat horizons from pit nachtigall in northern germany abstract peat and other terrestrial sediments are important archives of climate forcing this study discusses the numerical age determination of stratigraphically important quaternary sequences from clay pit nachtigall near holzminden in northern germany which have been the subject of debate concerning their chronological classification since they were first described by grupe in 1929 uranium series dating 230 th u of different organic horizons as well as sedimentology including loss on ignition loi and palynology was carried out to set up a more reliable chronological framework for the weser river area 230 th u ages were calculated using an isochron approach with the leachate leachate method for chemical preparation open system requirements and specification of sample selection criteria for organic samples are investigated by determination of uranium and thorium composition of the rim layers different organic sediments are investigated the organic deposits from nachtigall show middle pleistocene 230 th u ages ranging from 227 9 8 ka to 201 15 13 ka and appear to represent a warm temperate phase preceding the eemian interglacial mis 5e and the drenthe stadial mis 6 indicating a correlation with marine isotope substage mis 7c these results are in good agreement with the investigation of the pollen spectra indicating a correlation with a warm stage within the lower saalian complex

on the principles of the u trend method for dating quaternary sediments i model experimental procedures and data abstract the u trend method for dating a wide range of quaternary sediments was first presented by john rosholt in a series of u s geological survey open file reports the thinking was that upon fluviation uranium filtering down through the unit left behind a trail of daughter products in a systematic fashion a trend the exponential decrease of this u flux was defined by a flux factor the significance of which was not known exactly equivalently the flux of u could be characterized by a half period which appeared to act analogously to a radioactive decay half life by using leaching techniques the loosely bound isotopes would define a trend when plotted on a u trend isochron diagram this trend was characterized in the plot by a gradient and intercept which could be referred to calibration diagrams in order to recover the half period and the age of the unit such age estimates have now appeared in the more fully refereed literature though without the fuller descriptions found in the usgs publications the assumptions of the method are examined here together with the details of the method from the earlier publications various problems with the method stand out among which are i it is not clear how recoil effects are included in the empirical equations ii the use of independently dated sediments as calibration units has not been fully justified iii the leaching techniques appear to be too strong to select only the loosely bound isotopes and no others and iv several of the key data sets do not show a sufficiently strong variation of isotope ratios with depth to warrant being described as a trend despite these and other difficulties and because of its potential importance to quaternary chronology it is felt that the continuing study of u trend type methods is worth the effort

u series dating of impure carbonates an isochron technique using total sample dissolution abstract u series dating is a well established technique for age determination of late quaternary carbonates materials of sufficient purity for nominal dating however are not as common as materials with mechanically inseparable aluminosilicate detritus detritus contaminates the sample with extraneous th we propose that correction for contamination is best accomplished with the isochron technique using total sample dissolution tsd experiments were conducted on artificial mixtures of natural detritus and carbonate and on an impure carbonate of known age results show that significant and unpredictable transfer of radionuclides occur from the detritus to the leachate in commonly used selective leaching procedures the effects of correcting via leachate residue pairs and isochron plots were assessed isochrons using tsd gave best results followed by isochron plots of leachates only

holocene environmental changes in lake bangong basin western tibet part 1 chronology and stable isotopes of carbonates of a holocene lacustrine core abstract a 12 4 m core collected from the eastern part of lake bangong is analyzed for mineralogy radiometric chronology and stable isotope contents of authigenic inorganic carbonate isotopic compositions of the modern lake waters and of its major tributaries are first presented radiocarbon activity of these waters show that equilibrium with the atmospheric co2 is not reached because of admixture of dead carbon in solution in the system a model is thus developed to correct the radiocarbon ages of fossil material of aquatic origin for this ageing the core provides a continuous holocene record changes in mineralogy of bulk sediments and in 13c and 18o contents of authigenic carbonates reveal extremely large changes in environmental conditions induced by climate changes and or local hydrological factors e g closure opening of the lake system controlled by the altitude of the outflow sill the eastern basin of lake bangong was closed before 9 6 and after 1 2 ka b p in response to aridity on the catchment wet warm conditions attributed to the monsoon influence were established suddenly at 9 6 ka b p and maintained until 6 2 ka b p although minor reversal events occurred between 8 6 and 7 7 ka b p the return towards aridity is non linear dry spells at 6 2 3 9 3 2 and 1 3 ka b p are alternated with short term periods with positive precipitation evaporation balance

u series chronology of lacustrine deposits in death valley california uranium series dating on a 186 m core dv93 1 drilled from badwater basin in death valley california and on calcareous tufas from nearby strandlines shows that lake manly the lake that periodically flooded death valley during the late pleistocene experienced large fluctuations in depth and chemistry over the last 200 000 yr death valley has been occupied by a long standing deep lake perennial shallow saline lakes and a desiccated salt pan similar to the modern valley floor the average sedimentation rate of about 1 mm yr for core dv93 1 was punctuated by episodes of more rapid accumulation of halite arid conditions similar to the modern conditions prevailed during the entire holocene and between 120 000 and 60 000 yr b p from 35 000 yr b p to the beginning of the holocene a perennial saline lake existed over 70 m at its deepest a much deeper and longer lasting perennial lake manly existed from about 185 000 to 128 000 yr b p with water depths reaching about 175 m if not 330 m this lake had two significant dry excursions of 102 103yr duration about 166 000 and 146 000 yr b p and it began to shrink to the point of halite precipitation between 128 000 and 120 000 yr b p the two perennial lake periods correspond to marine oxygen isotopic stages ois 2 and 6 based on the shoreline tufa ages we do not rule out the possible existence 200 000 yr ago of yet a third perennial lake comparable in size to the ois 6 lake the234u 238u data suggest that u in tufa owes its origin mainly to ca rich springs fed by groundwater that emanated along lake shorelines in southern death valley and that an increase of this spring water input relative to the river water input apparently occurred during ois 6

precise timing of the last interglacial period from mass spectrometric determination of thorium 230 in corals the development of mass spectrometric techniques for determination of 230 th abundance has made it possible to reduce analytical errors in 238 u 234 u 230 th dating of corals even with very small samples samples of 6 x 10 8 atoms of 230 th can be measured to an accuracy of 3 percent 2 and 3 x 10 10 atoms of 230 th can be measured to an accuracy of 0 2 percent the time range over which useful age data on corals can be obtained now ranges from about 50 to about 500 000 years for young corals this approach may be preferable to 14 c dating the precision with which the age of a coral can now be determined should make it possible to critically test the milankovitch hypothesis concerning pleistocene climate fluctuations analyses of a number of corals that grew during the last interglacial period yield ages of 122 000 to 130 000 years the ages coincide with or slightly postdate the summer solar insolation high at 65 n latitude which occurred 128 000 years ago this supports the idea that changes in pleistocene climate can be the result of variations in the distribution of solar insolation caused by changes in the geometry of the earth s orbit and rotation axis

calibration of the 14c time scale to 40 ka by 234u 230th dating of lake lisan sediments last glacial dead sea a new comparison of 14c dates with 234u 230th ages is presented of aragonites from lake lisan the last glacial dead sea between 20 52 cal ka bp the lisan data are coincident with the coral based 14c calendar age calibration through the continuous portion of the curve to 23 5 cal ka bp and with the additional checkpoints at 30 and 40 cal ka bp the agreement with the corals provides evidence for the accuracy of the u th and 14c ages and indicates that lisan aragonites can potentially be used to generate a nearly continuous record of the atmospheric 14c variations through this crucial time interval the lisan data are compared with other records older than 25 cal ka bp from lake suigetsu japan north atlantic foraminifera south african cave deposits and tufa from spain over some age intervals the records show broad agreement over other intervals they diverge all agree that 14c ages were 2 5 0 5 ka younger than calendar ages between 20 32 cal ka bp for ages 32 cal ka bp the lake suigetsu data indicate small differences between 14c ages and calendar ages of less than 1 5 ka the other records broadly agree that 14c ages are too young by 3 1 ka from 32 39 cal ka bp at 40 cal ka bp the foraminifera show equal 14c and calendar ages while the corals lisan aragonites and the spanish tufa indicate a large age difference of 4 5 ka a recent paleomagnetic study of the lisan formation indicates that the high calendar 14c age difference at 40 cal ka bp may be associated with the laschamp paleomagnetic excursion

use of uranium thorium dating to determine past 14c reservoir effects in lakes examples from antarctica the chronologies of many lacustrine records suffer from radiocarbon reservoir effects due to the presence of dissolved dead carbon or to slow air water exchange here we use the tims uranium thorium disequilibrium method in conjunction with ams radiocarbon dating to determine the age of lacustrine carbonates and to quantify the past radiocarbon reservoir effect in two antarctic lakes with differing characteristics by correcting a single sample u th age for detrital contamination a 14 c offset of v18 000 yr was obtained for carbonates from the former grounding line of the ross sea ice sheet in glacial lake trowbridge this large reservoir effect is believed to result from the direct input of old co2 from glacial meltwater in the second example an isochron approach on coeval samples formed at the bottom of lake vida now exposed due to lower lake level yielded an age of 9550 340 yr b p and a radiocarbon reservoir age of 3600 yr this offset was probably the result of lack of aeration due to perennial ice cover and or strong density stratification this evidence for long term isolation of the lake bottom indicates another level of hardship for life in the dry valley lacustrine environment an environment studied as an analogue for extreme periods of earth history as well as for exobiological implications the success of the u th technique on these two examples indicates that tims u th dating will be of widespread use in dating the important climate information recorded in the dry valleys both within and beyond the 14 c age range fl 2001 elsevier science b v all rights reserved

uranium series age estimates and paleoclimatic significance of pleistocene tufas from the lahontan basin california and nevada abstract an extended chronology of lahontan basin lake levels based on uranium series age estimates correlates with the global ice volume record lake highstands occur at or shortly after times of maximum ice sheet size moderate size lakes occur when the global ice volume is about 80 of its maximum the data indicate that lake levels rise and fall relative to the proximity of the mean position of the jetstream when the continental ice sheet is above some threshold size or shape it appears that the large scale circulation and climatic conditions are right for producing lakes of moderate to large size within the lahontan basin

atacamite formation by deep saline waters in copper deposits from the atacama desert chile evidence from fluid inclusions groundwater geochemistry tem and 36 cl data the presence of large amounts of atacamite in oxide zones from ore deposits in the atacama desert of northern chile requires saline solutions for its formation and hyperarid climate conditions for its preservation we investigated the nature and origin of atacamite forming solutions by means of coupling groundwater geochemical analyses with fluid inclusion data high resolution mineralogical observations and chlorine 36 36cl data in atacamite from the mantos blancos and spence cu deposits in both deposits the salinities of fluid inclusions in atacamite are comparable to those measured in saline groundwaters sampled from drill holes the average salinity of fluid inclusions in atacamite for the mantos blancos and spence deposits 7 9 and 2 3 wt nacleq respectively are strongly correlated to the salinities at which gypsum supersaturates from groundwaters in both deposits total dissolved solids 5 9 and 1 3 wt nacleq respectively this correlation is confirmed by transmission electron microscopy observations of atacamite bearing samples revealing an intimate association between atacamite and gypsum that can be traced down to the nanometer scale 36cl data in atacamite provide new lines of evidence concerning the origin and age of the saline waters that formed atacamite in various stratabound and porphyry cu deposits from the atacama desert all atacamite samples show very low 36cl to cl ratios 11 10 15 to 28 10 15 at at 1 comparable to previously reported 36cl to cl ratios of deep formation waters and old groundwaters in addition 36cl to cl ratios in atacamite correlate with u and th concentration in the host rocks but are independent from distance to the ocean this trend supports an interpretation of the low 36cl to cl ratios in atacamite as representing subsurface production of fissiogenic 36cl in secular equilibrium with the solutions involved in atacamite formation therefore 36cl in atacamite strongly suggest that the chlorine in saline waters related to atacamite formation is old 1 5 ma but that atacamite formation occurred more recently 1 5 ma than suggested in previous interpretations our data provide new constraints on the origin of atacamite in cu deposits from the atacama desert and support the recent notion that the formation of atacamite in hyperarid climates such as the atacama desert is an ongoing process that has occurred intermittently since the onset of hyperaridity

oceanic 232th a reconnaissance and implications of global distribution from manganese nodules abstract 232 th concentrations in seawater reported in the literature since 1961 are compiled and evaluated and new data are presented in general recent data are lower than old ones by 1 to 2 orders of magnitude with nearly all post 1980 data lower than 0 1 dpm 10 3 1 and pre 1980 values higher than 0 1 dpm 10 3 1 we believe this is caused by improvements in sampling and analytical techniques reliable profiles of 232 th in coastal and open ocean waters have been measured in recent years some by the newly developed neutron activation analysis and mass spectrometry techniques the data indicate external sources of 232 th to the surface and its scavenging throughout the water column we suggest that aeolian input dominates in the open ocean but fluvial input may be more important in coastal regions if mechanisms exist for recycling th from estuarine sediments some information on the global distribution pattern and marine geochemistry of 232 th in seawater can be inferred from initial 230 th 232 th activity ratios observed on surfaces of manganese nodules and crusts the rather distinct interocean variation of the 230 th 230 th activity ratio in the central gyre region averaging 160 in the south pacific 60 in the north pacific and 17 in the atlantic may reflect higher aeolian input of materials including 232 th to the atlantic and to the northern hemisphere superimposed on the large scale global trend are some small scale variations which are indicative of the following effects 1 abrupt change of water depth and hence 230 th concentration at regions of irregular seafloor topography 2 higher 232 th input at ocean boundaries and 3 enhanced supply of 232 th from hydrothermal vents it is still unclear whether seafloor sediments act as a net source or sink of dissolved 232 th and 230 th in bottom waters pore water study should be undertaken to address this important issue this task can be accomplished by the mass spectrometric technique

u series disequilibria in early diagenetic minerals from lake magadi sediments kenya dating potential abstract at the southern end of the gregory rift valley lake magadi occupies the bottom of a relatively large drainage basin it is presently covered by a thick trona crust which overlies two silty clay units deposited during late pleistocene high lake stands these units consist of a mixture of detrital grains anorthoclase amphiboles quartz clays illite authigenic zeolites phosphates and sedimentary sodium silicates and cherts a late diagenetic calcite is occasionally observed the authigenic and or diagenetic mineralogical phases were extracted and analyzed for their uranium and thorium isotope contents all yielded highly correlated 234 u 232 th vs 238 u 232 th and 230 th 232 th vs 234 u 232 th isochrons defining two component mixing systems detrital and authigenic phases the detrital component is characterized by large excesses of 230th over 234u and by 230 th 232 th ratios carrying an imprint of the source rocks e g precambrian basement vs volcanics and indicating efficient uranium leaching processes during the previous pedologic cycle the slope of the isochrons defines the 234 u 238 u and 230 th 234 u ratios of the authigenic component i e the age of the uranium uptake episode zeolites yield an age of 10 4 0 6 ka this age is in agreement with the 14c chronology already established for the most recent high lake level episode in the basin 10 000 12 000 yr bp sodium silicates and cherts yield distinct ages of 98 5 20 and 6 3 ka respectively for the lower and upper lacustrine units these ages allow the conclusion that 1 sedimentary silicates are of late diagenetic origin and 2 that the lower lacustrine unit was deposited during a former high lake magadi level possibly during the lacustrine episode dated at 135 10 ka from uranium series measurements on littoral stromatolites finally the late diagenetic calcite which yields an age of about 5 ka indicates a significant change in the sediment interstitial water chemistry this is also shown by the occurrence of calcitic oncolites in flat deposits assigned to a mid holocene minor oscillation of lake magadi level on the basis of their 14c ages it is concluded that uranium series measurements in diagenetic minerals may give access to the age of their formation

uranium series isochron dating at el castillo cave cantabria spain the acheulean mousterian question abstract a massive flowstone layer level 23 near the base of the stratigraphic sequence in el castillo cave cantabria spain is dated by the uranium series isochron method to 89 11 10 ka bp the flowstone separates cultural layers traditionally labelled mousterian above and acheulean below the date reported here in association with other recent age determinations for mousterian and acheulean artifact assemblages in western europe calls into question the temporal significance of these archaeological designations in addition it further underlies the importance of the castillo cultural sequence and allows tentative discussion of changing rates of sedimentation in the vestibule of the cave where some 18 m of cultural and geological deposits were laid down during the course of the upper pleistocene

uranium series and radiocarbon geochronology of deep sea corals implications for southern ocean ventilation rates and the oceanic carbon cycle we present new uranium series and radiocarbon measurements for deep sea corals from the southern ocean these data are used to reconstruct ventilation ages both at present and at the end of the last glacial period approximately 16 500 years ago we apply an improved two component mixing approach to correct uranium series dates for contaminant thorium and protactinium present in oxide coatings calculated seawater radiocarbon values for contemporary samples decrease with depth in the water column and agree with direct seawater radiocarbon measurements for this area this indicates that deep sea corals can accurately record seawater radiocarbon distributions two of three glacial samples experienced open system uranium series systematics however a third sample from the drake passage yields concordant thorium and protactinium dates as well as seawater values for initial 234u 238u this coral yields a ventilation age that is approximately 20 40 greater than modern values for its location this increase is consistent with published deep sea coral and calibrated planktonic benthic foraminifera radiocarbon data suggesting that the glacial oceans as a whole may have been substantially less ventilated presumably due to decreased formation of north atlantic deep water an overall decrease in oceanic mixing rates could have contributed to lower dissolved carbon in surface ocean water and lower atmospheric pco2 during the past glacial period

uranium series dating of sediments from searles lake differences between continental and marine climate records one of the major unresolved questions in pleistocene paleoclimatology has been whether continental climatic transitions are consistent with the glacial 18 o marine record searles lake in california now a dry salt pan is underlain by sediment layers deposited in a succession of lakes whose levels and salinities have fluctuated in response to changes in climate over the last 3 x 10 6 years uraniumseries dates on the salt beds range from 35 x 10 3 to 231x 10 3 years this range of dates allows identification of lake sediment horizons that are time correlatives of the boundaries of marine isotope stages from the recent 3 4 boundary back to the 8 9 boundary the 5 6 boundary coincided with a deepening of the lake but the analogous 1 2 boundary coincided with desiccation the 3 4 4 5 6 7 7 8 and 8 9 boundaries correspond in age to horizons that record little or no change in sedimentation or climate these hydrologic results demonstrate that the continental paleoclimate record at this mid latitude site does not mimic the marine record

uranium series chronology and environmental applications of speleothems an increasing number of scientists recognize the value of speleothems1 as often extremely well preserved archives of information about past climate vegetation hydrology sea level nuclide migration water rock interaction landscape evolution tectonics and human action well constrained data are required to document past changes reconstruct past patterns and predict future responses of the earth system at a wide range of spatial and temporal scales speleothems are particularly useful in this regard because they can be found in many locations of the globe sampled at high resolution and reliably dated using high precision uranium series techniques speleothems are bodies of mineral material formed in caves as the result of chemical precipitation from groundwater flowing or dripping in a cave most speleothems are composed of calcite formed by slow degassing of co2 from supersaturated groundwater but aragonite and gypsum forms are also common particularly near cave entrances where evaporative effects are important a host of different speleothem types decorate the walls ceilings and floors of caves and their mineralogy and morphology is a function of fluid flow and chemistry of waters feeding a particular location as well as the ambient conditions temperature chemistry light in the air or water filled void subaerial forms include the familiar stalagmites stalactites draperies flowstones subaqueous forms include rimstone pools rafts mammillary calcite wall coatings and dog tooth spar for an extensive review of the types of speleothem that have been observed see hill and forti 1997 speleothems are used in a multitude of ways to explore past environmental conditions perhaps the most fundamental of which is their very presence or absence deposition of speleothems relies on sufficient water supply and soil co2 to enable dissolution and transport of reactants in the vadose zone to underlying caves in arid or glacial times conditions may not have been favorable for speleothem formation

200 k y paleoclimate record from death valley salt core a 186 m long core dv93 1 from death valley california composed of interbedded salts and muds contains a 200 k y record of closed basin environments and paleoclimates interpreted on the basis of sedimentology ostracodes homogenization temperatures of fluid inclusions in halite and correlation with shoreline tufa the 200 k y paleoclimate record is dominated by two dry and or warm and wet and cold cycles that occurred on a 100 k y time scale these cycles begin with mud flat deposits 192 ka to bottom of core and 60 ka to 120 ka wetter and or colder conditions produced greater effective moisture saline pan and shallow saline lake evaporites overlie mud flat sediments 186 ka to 192 ka and 35 ka to 60 ka eventually enough water entered death valley to sustain perennial lakes that had fluctuating water levels and salinities 120 ka to 186 ka and 10 ka to 35 ka when more arid conditions returned mud flat deposits accumulated on top of the perennial lake sediments completing the cycle 120 ka and 10 ka of particular significance are the major lacustrine phases 10 ka to 35 ka and 120 ka to 186 ka oxygen isotope stages 2 and 5e 6 which represent markedly colder and wetter conditions than those of modern death valley of the two major lake periods the penultimate glacial lakes were deeper and far longer lasting than those of the last glacial

multi isotopic age assessment of dirty speleothem calcite an example from altamira cave spain the dating of dirty speleothem calcite of late pleistocene or holocene age can be problematic this is notably the case with the 14 c method due to uncertainties in the initial 14 c activity of the cave co2 and withth e 230 th 234 u method due to either low u contents in the authigenic calcite and or to the presence of heterogeneous detrital particles containing high concentrations of u and thseries isotopes frequently out of secular equilibrium in this study of a b60 cm thick flowstone sealing archaeological remains of the magdalenian period in the altamira cave nw spain we used a combination of isotopic approaches and analytical techniques to confront dating methods and finally tried to validate chronological interpretations based on stable isotope paleoclimate constraints u series data indicate the presence of a relatively important and heterogeneous detrital fraction resulting in 232 th 230 th mass ratios generally too high 0 1 0 25 10 6 to allow precise tims measurements mc icp ms measurements provided more reliable results and allowed the calculation of 230 th 234 u isochron ages of 11 870 8 ka in the upper part of the flowstone and of b22 ka in the lower part of it in view of the calibrated 14 c ages of these layers b13 and b15 ka bp respectively the isochron age of the lower unit of the flowstone seems erroneous suggesting the presence of an isotopically heterogeneous contaminating fraction in this layer 226 ra 230 thratio measurements indicate near secular equilibrium conditions seriate stable isotope measurements indicate calcite precipitation in isotopic equilibrium with ambient waters they suggest that flowstone deposition ended at the very onset of the younger dryas this transition seems to have occurred here at 11 870 8 72s ka based on th u isochron and 10 770 08 71s uncalibrated 14 c kiloyears the 14 c activity for the cave co2 seems to have been near that of atmospheric co2 during the precipitation interval it is concluded that the 14 c data provide here relatively reliable ages in spite of the fact that they partly fall into a period characterized by 14 c plateau effects but that other isotopic data revealed essential in validating them

a review of the geodynamic setting of large scale late mesozoic gold mineralization in the north china craton an association with lithospheric thinning abstract abundant gold deposits are distributed along the margins of the north china craton ncc occurring throughout the precambrian basement and located in or proximal to mesozoic granitoids these deposits show a consistent spatial temporal association with late jurassic early cretaceous magmatism and are characterized by quartz lode or disseminated styles of mineralization with extensive alteration of wall rock their ages are mainly early cretaceous 130 110 ma and constrain a very short period of metallogenesis sr nd pb isotopic tracers of ores minerals and associated rocks indicate that gold and associated metals mainly were derived from multi sources i e the wall rocks precambrian basement and mesozoic granites and associated mafic rocks previous studies including high surface heat flow uplift and later basin development slow seismic wave speeds in the upper mantle and a change in the character of mantle xenoliths sampled by paleozoic to cenozoic magmas have been used to suggest that ancient cratonic mantle lithosphere was removed from the base of the ncc some time after the ordovician and replaced by younger less refractory lithospheric mantle the geochemistry and isotopic compositions of the mafic rocks associated with gold mineralization 130 110 ma indicate that they were derived from an ancient enriched lithospheric mantle source whereas the mafic dikes and volcanic rocks younger than 110 ma were derived from a relatively depleted mantle source i e asthenospheric mantle according to their age and sources relation to magmatism and geodynamic framework the gold deposits were formed during lithospheric thinning the removal of lithospheric mantle and the upwelling of new asthenospheric mantle induced partial melting and dehydration of the lithospheric mantle and lower crust due to an increase of temperature the fluids derived from the lower crust were mixed with magmatic and meteoric waters and resulted in the deposition of gold and associated metals

strategies for successful u th dating of paleolake carbonates an example from the bolivian altiplano we report over 90 u th dates from carbonates deposited around paleolakes on the bolivian altiplano petrographic and chemical data for tufas and the siliciclastic detritus contained within them allow 1 assessment of possible diagenetic effects 2 a development of a strategy for selection of carbonate samples with low initial th contents and 3 assessment of the uncertainty due to initial th this strategy allows us to produce precise u th dates from lacustrine carbonates the principal consideration in dating such carbonates is the composition and quantity of initial th incorporated into the carbonate and additional uncertainty is introduced because this initial th may have two sources in lacustrine deposits isochron plots measured 230th 232th and x ray diffraction and trace element chemistry of silicic residues all favor regional soil and dust as the sole source of initial th in carbonates from the bolivian altiplano a situation that simplifies single sample dating of altiplano carbonates

the water column distributions of thorium isotopes in the western north pacific abstract four vertical profiles of 230 th and 228 th were determined using large volume water samples in the western north pacific an almost linear increase of 230 th with depth was observed for all of the profiles for which the unidirectional first order scavenging model was difficult to explain we developed a model which included a dissolved particulate transformation as well as parameters of the scavenging model application of the model to the vertical distributions of total and the geosecs particulate th isotopes 230 th and 234 th yielded the residence time of dissolved th with respect to adsorption to particles and the turnover time of particulate th to be 235 days and 57 days respectively the th isotopes appeared to be carried down the water column by fine particles with a mean settling velocity of 1 m day which continually release th into sea water as well as pick up th from the water along their journey to the bottom for 228 th a large excess over 232 th was observed throughout the water column with pronounced high concentrations in surface and bottom waters suggesting that the 228 th was derived from 228 ra diffused out of sediments the vertical distributions of 228 th seemed to be significantly influenced by lateral mixing along isopycnals

hillslope gradient particle size relations evidence for the formation of debris slopes by hydraulic processes in the mojave desert this study is concerned with debris slopes free of superincumbent cliffs or basal stream channels and developed on closely jointed and or mechanically weak rocks in the mojave desert analyses of the relation between hillslope gradient s and mean particle size bar d reveal that the slope coefficient of the relation varies with debris slope plan form and where plan form is constant the intercept varies with rock type a theoretical model is developed which shows that the observed pattern of variation in the slope of the s bar d relation with plan form is consistent with the debris slopes being formed by and adjusted to hydraulic processes debris slopes in the mojave desert appear to lie along a continuum at one end are debris slopes underlain by closely jointed and or mechanically weak rocks such as those examined in this study that are transport limited well adjusted to present hydraulic processes and exhibit a strong relationship between s and bar d at the other end are debris