on the linkage between antarctic surface water stratification and global deep water temperature the suggestion is advanced that the remarkably low static stability of antarctic surface waters may arise from a feedback loop involving global deep water temperatures if deep water temperatures are too warm this promotes antarctic convection thereby strengthening the inflow of antarctic bottom water into the ocean interior and cooling the deep ocean if deep waters are too cold this promotes antarctic stratification allowing the deep ocean to warm because of the input of north atlantic deep water a steady state deep water temperature is achieved such that the antarctic surface can barely undergo convection a two box model is used to illustrate this feedback loop in its simplest expression and to develop basic concepts such as the bounds on the operation of this loop the model illustrates the possible dominating influence of antarctic upwelling rate and antarctic freshwater balance on global deep water temperatures

alleghanian reactivation of the acadian fold belt meguma zone southwest nova scotia shear zones and northwest verging folds define a 30 km wide belt of deformation that overprints the acadian fold belt and telescopes isograds in the meguma zone in southwest nova scotia the shear zones appear to form a linked system that accommodated convergence dominated transpression of the meguma zone against an irregular avalon boundary available geochronological data indicate a mid carboniferous alleghanian variscan age for the overprinting deformation the mid carboniferous basement reactivation in southwest nova scotia is likely coeval with deformation of carboniferous strata and reactivation of basement structures meguma group in the northern meguma zone together these mid carboniferous structures may define a wide belt of alleghanian variscan deformation across the northwest cratonward margin of the meguma zone

dating ice shelf edge marine sediments a new approach using single grain quartz luminescence 1 to develop an alternative dating tool for the antarctic peninsula where the 14c method requires large spatially variable reservoir corrections we tested the clock zeroing assumption of photon stimulated luminescence psl dating using a variety of psl procedures at ice shelf edges around the antarctic peninsula sediment water interface zero age analogs silt rich short cores were collected in 2001 2003 originally only for fine silt dating tests later access to suitable instrumentation also permitted testing the potential of single grain quartz sgq dating of sand grains from these cores for the fine silt grains we employed multiple aliquot and single aliquot methods to obtain last daylight exposure age estimates from near core top material with the sand fraction we employed automated sgq psl methods to isolate the youngest grains five of six fine silt samples gave unreasonably large age estimates 20 ka with the sixth sample yielding a multiple aliquot short bleach age estimate of 1 1 0 6 ka in contrast five of seven sand samples yielded geologically reasonable last daylight exposure ages of 0 2 0 6 ka these sgq results are also remarkable when compared to published 14c ages of 1 ka to 9 7 ka from core top living calcite and acid insoluble organic matter these sgq results establish the likely utility of this single grain dating approach in such settings to provide chronologies for calving line histories of ice shelves to take advantage of this utility core collection should employ large diameter coring devices e g kasten and multicorers a caveat is that large numbers e g 10 000 of quartz grains may need analysis to provide acceptable statistics for useful age calculations

application of osl single aliquot and single grain dating to quartz from anthropogenic soil profiles in the se united states abstract reduced luminescence intensity in soils is a widespread phenomenon but is not understood we examine the distributions of equivalent dose d e values of quartz from surface and buried soils from earthen mounds in louisiana where pedoturbation is the likely cause of luminescence resetting we first investigated the effectiveness of optical bleaching in surface samples similar in composition and environment to those at depth resulting d e distributions from the top 10 cm of the topsoils approach 0 gy the concentration of zero age grains decreases rapidly with depth paleosol ages based on low values of the d e distribution demonstrate good agreement with previously published ages we propose a three zone model for the explanation of surface bleaching in which the soil surface is continuously stressed and loaded by subsurface turbation mechanisms creating surface relief surface relief is reduced by subaerial impact and transport forces causing disaggregation and single grain dispersion this results in a surface bleaching zone where soil particles and topography are in constant transition suggesting that many soils may offer frequent and thorough presentation of mineral grains to surface insolation and thus high concentrations of zero dose grains

on the abruptness of bolling allerod warming abstractprevious observations and simulations suggest that an approximate 3 5 c warming occurred at intermediate depths in the north atlantic over several millennia during heinrich stadial 1 hs1 which induces warm salty water wsw lying beneath surface cold freshwater this arrangement eventually generates ocean convective available potential energy ocape the maximum potential energy releasable by adiabatic vertical parcel rearrangements in an ocean column the authors find that basin scale ocape starts to appear in the north atlantic 67 5 73 5 n and builds up over decades at the end of hs1 with a magnitude of about 0 05 j kg 1 ocape provides a key kinetic energy source for thermobaric cabbeling convection tcc using a high resolution tcc resolved regional model it is found that this decadal scale accumulation of ocape ultimately overshoots its intrinsic threshold and is released abruptly 1 month into kinetic energy of tcc with further intensification from cabbeling tcc has convective

fast advective recovery of the atlantic meridional overturning circulation after a heinrich event ice core reconstructions and ocean sediment analysis have revealed that the climate of the last glacial period was highly variable with rapid stadial interstadial transitions and glacial meltwater pulses heinrich events modulating the climate evolution in the northern and southern hemispheres heinrich events had the potential to weaken the atlantic meridional overturning circulation amoc substantially mechanisms that led to the resumption of the amoc after such events have not been fully disentangled yet here a coupled atmosphere ocean sea ice model of intermediate complexity is employed to identify important negative climate feedbacks that contribute to a fast recovery of the glacial amoc shortly after the amoc collapse thermal processes weaken the stratification in the northern north atlantic making it more vulnerable to perturbations eventually 300 400 years after the main collapse of the amoc the mean advection of salinity anomalies within the horizontal gyres generates an unstable stratification that will be homogenized through the resumption of convective activity eventually isopycnal slopes in the north atlantic are readjusted thereby reinitiating the large scale meridional overturning flow

origin and evolution of lunettes on the high plains of texas and new mexico lunettes isolated dunes on the lee side of playa basins are common landforms on the southern high plains of northwest texas and eastern new mexico the dunes contain calcareous 15 40 caco3 sandy loam or loamy sand with minor amounts of sepiolite deposited 25 000 800014c yr b p and derived by deflation of lacustrine carbonate in the basins the dunes also contain low carbonate 0 15 caco3 sand or loamy sand that was deposited 25 000 15 000 yr b p and 8000 5000 yr b p and was derived by deflation that created the basins or deflated from sand deposited in the basins buried soils are common in the lunettes a bk profiles characterize soils formed in the calcareous sandy loam the sandy low carbonate sediments contain a bt profiles in the oldest sand of some dunes and a bw a bt or a btk in the early and middle holocene sand the dune stratigraphy combined with carbon isotope data derived from dated a horizons in lunettes suggests the following scenario for the southern high plains the lunettes began forming as low carbonate sand dunes in the late pleistocene as playa basins were formed or deepened by wind erosion the erosion repeatedly alternated with stability the environment probably was cool and dry but one or more cool and wet intervals 25 000 15 000 yr b p resulted in a rise in the water table and deposition of lacustrine carbonate in the deepest basins there may have been short departures toward warmer and probably toward drier conditions throughout this time episodically dry conditions 15 000 8 000 yr b p resulted in deflation of the carbonate and further dune construction by repeated accretion of calcareous sandy loam or loamy sand the low carbonate sand was deposited during widespread drought and deflation 8000 5000 yr b p the dunes have been largely stable in the late holocene

optical dating of geoarchaeologically significant sites from the southern high plains and south texas usa the southern high plains of the united states is a key region for the investigation of early human occupation of north america this area including selected archaeological sites located in southern texas contains a range of aeolian fluvial and playa deposits such deposits may be suited to luminescence based methods of age assessment in this paper we describe a combination of multiple and single aliquot optical dating results for a selection of sites with some independent age control we find generally good agreement between multiple and single aliquot equivalent dose de estimates for the 20 samples compared but note that the single aliquot approach results in errors in mean de s to be less by an order of magnitude our results suggest that at least some of the single aliquot methods are of great utility for low latitude geoarchaeological sites

the effect of cold climate upon north atlantic deep water formation in a simple ocean atmosphere model the sensitivity of north atlantic deep water formation to variations in mean surface temperature is explored with a meridional vertical plane ocean model coupled to an energy balance atmosphere it is found that north atlantic deep water formation is favored by a warm climate while cold climates are more likely to produce southern ocean deep water or deep decoupling oscillations when the southern sinking region is halocline covered this behavior is traced to a cooling induced convective instability near the north atlantic sinking region that is to unstable horizontal spreading of a halocline that stratifies part of the region under the convective instability it is found that climate cooling is generally equivalent to increased freshwater forcing this is because in a cold climate high latitude water masses approach the temperature of maximum density and the convectiondriving upward thermal buoyancy flux induced by surface cooling becomes insufficient to overcome the stratifying effect of surface freshening a downward buoyancy flux an extensive halocline is then formed and this halocline interferes with the heat loss necessary for the steady production of north atlantic deep water

u pb geochronology of devonian granites in the meguma terrane of nova scotia canada evidence for hotspot melting of a neoproterozoic source abstract u pb isotopic analyses of monazite and zircon from six granitic plutons in the meguma terrane yield nearly concordant ages of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 373 pm 3 end document ma interpreted as the time of intrusion u pb analyses of euhedral zircons with thick rims overgrowing cores which were abraded to remove all or most of the rim plot on chords between documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage p

weak oceanic heat transport as a cause of the instability of glacial climates the stability of the thermohaline circulation of modern and glacial climates is compared with the help of a two dimensional ocean atmosphere sea ice coupled model it turns out to be more unstable as less freshwater forcing is required to induce a polar halocline catastrophy in glacial climates the large insulation of the ocean by the extensive sea ice cover changes the temperature boundary condition and the deepwater formation regions moves much further south the nature of the instability is of oceanic origin identical to that found in ocean models under mixed boundary conditions with similar strengths of the oceanic circulation and rates of deep water formation for warm and cold climates the loss of stability of the cold climate is due to the weak thermal stratification caused by the cooling of surface waters the deep water temperatures being regulated by the temperature of freezing weaker stratification with similar overturning leads to a weakening of the meridional oceanic heat transport which is the major negative feedback stabilizing the oceanic circulation within the unstable regime periodic millennial oscillations occur spontaneously the climate oscillates between a strong convective thermally driven oceanic state and a weak one driven by large salinity gradients both states are unstable the atmosphere of low thermal inertia is carried along by the oceanic overturning while the variation of sea ice is out of phase with the oceanic heat content during the abrupt warming events that punctuate the course of a millennial oscillation sea ice variations are shown respectively to damp amplify the amplitude of the oceanic atmospheric response this sensitivity of the oceanic circulation to a reduced concentration of greenhouse gases and to freshwater forcing adds support to the hypothesis that the millennial oscillations of the last glacial period the so called dansgaard oeschger events may be internal instabilities of the climate system

optimal excitation of interannual atlantic meridional overturning circulation variability the optimal excitation of atlantic meridional overturning circulation moc anomalies is investigated in an ocean general circulation model with an idealized configuration the optimal three dimensional spatial structure of temperature and salinity perturbations defined as the leading singular vector and generating the maximum amplification of moc anomalies is evaluated by solving a generalized eigenvalue problem using tangent linear and adjoint models despite the stable linearized dynamics a large amplification of moc anomalies mostly due to the interference of nonnormal modes is initiated by the optimal perturbations the largest amplification of moc anomalies found to be excited by high latitude deep density perturbations in the northern part of the basin is achieved after about 7 5 years the anomalies grow as a result of a conversion of mean available potential energy into potential and kinetic energy of the perturbations reminiscent of baroclinic instability the time scale of growth of moc anomalies can be understood by examining the time evolution of deep zonal density gradients which are related to the moc via the thermal wind relation the velocity of propagation of the density anomalies found to depend on the horizontal component of the mean flow velocity and the mean density gradient determines the growth time scale of the moc anomalies and therefore provides an upper bound on the moc predictability time the results suggest that the nonnormal linearized ocean dynamics can give rise to enhanced moc variability if for instance overflows eddies and or deep convection can excite high latitude density anomalies in the ocean interior with a structure resembling that of the optimal perturbations found in this study the findings also indicate that errors in ocean initial conditions or in model parameterizations or processes particularly at depth may significantly reduce the atlantic moc predictability time to less than a decade

atlantic thermohaline circulation in a coupled general circulation model unforced variations versus forced changes a 1200 yr unforced control run and future climate change simulations using the parallel climate model pcm a coupled atmosphere ocean land sea ice global model with no flux adjustments and relatively high resolution 2 8 for the atmosphere and 2 3 for the oceans are analyzed for changes in atlantic ocean circulations for the forced simulations historical greenhouse gas and sulfate forcing of the twentieth century and projected forcing for the next two centuries are used the atlantic thermohaline circulation thc shows large multidecadal 15 40 yr variations with mean peak amplitudes of 1 5 3 0 sv 1 sv 10 6 m 3 s 1 and a sharp peak of power around a 24 yr period in the control run associated with the thc oscillations there are large variations in north atlantic ocean heat transport sea surface temperature sst and salinity sss sea ice fraction and net surface water and energy fluxes which all lag the variations in thc strength by 2 3 yr however the net effect of the sst and sss variations on upper ocean density in the midlatitude north atlantic leads the thc variations by about 6 yr which results in the 24 yr period the simulated sst and sea ice spatial patterns associated with the thc oscillations resemble those in observed sst and sea ice concentrations that are associated with the north atlantic oscillation nao the results suggest a dominant role of the advective mechanism and strong coupling between the thc and the nao whose index also shows a sharp peak around the 24 yr time scale in the control run in the forced simulations the thc weakens by 12 in the twenty first century and continues to weaken by an additional 10 in the twenty second century if co2 keeps rising but the thc stabilizes if co2 levels off the thc weakening results from stabilizing temperature increases that are larger in the upper and northern atlantic ocean than in the deep and southern parts of the basin in both the control and forced simulations as the thc gains loses strength and depth the separated gulf stream gs moves southward northward while the subpolar gyre centered at the labrador sea contracts from expands to the east with the north atlantic current nac being shifted westward eastward these horizontal circulation changes which are dynamically linked to the thc changes induce large temperature and salinity variations around the gs and nac paths

early carboniferous deformation and mineralization in the cobequid shear zone nova scotia an 40ar 39ar geochronology study latest devonian to early carboniferous igneous rocks along the cobequid shear zone of central nova scotia record a complex history of intrusion volcanism deformation and hydrothermal alteration twenty new 40ar 39ar age determinations have been made on biotite and hornblende separates from rocks that constrain these events the mafic plutons of the cobequid shear zone are synchronous with the previously dated granitoid plutons 363 355 4 ma with the exception of the younger folly lake gabbro 355 350 4 ma high temperature shear deformation continued along the cobequid shear zone during this entire period of pluton emplacement several samples from mylonitic and hydrothermally altered rocks with dates ca 340 ma indicate renewed movement along the cobequid shear zone at this time accompanied by minor magmatism younger 330 320 ma ages reflect final movement along the rockland brook fault resulting in mid namurian uplift of the cobequid highlands following this regional alleghenian event ther

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

sediment mixing at nonda rock investigations of stratigraphic integrity at an early archaeological site in northern australia and implications for the human colonisation of the continent archaeological excavations in sediments dating to between 60 000 and 40 000 years ago are rare in australia yet this is precisely the period in which most archaeologists consider that aboriginal people arrived on the continent in the few cases where such early sites have been investigated questions have invariably been raised as to the reliability of stratigraphic associations between cultural items and the surrounding sediments this paper describes a method for examining sediment mixing in a stratigraphic sequence using the optically stimulated luminescence osl signals from individual sand sized grains of quartz we apply this method to the archaeological site of nonda rock north queensland in combination with radiocarbon dating of charcoal fragments to construct chronologies for human occupation and for the preceding culturally sterile deposits our age estimates have implications for the timing of first human arrival in australia

osl chronology of middle to late holocene aeolian activity in the st anthony dune field southeastern idaho usa abstract late quaternary aeolian dune systems in the great plains and western u s are valuable archives of past climate variability this study investigates the st anthony dune field located in the eastern portion of the snake river plain in southern idaho the current semi arid climate of the region allows for a central core of active dunes that is approximately 450 km 2 in area and includes compound crescentic blowout falling and climbing dunes that range in height from 15 to 121 m the surrounding region is characterized by 20 000 km 2 of relic parabolic linear and complex linear dunes that are currently fixed by vegetation this study examined nine samples collected from the fixed dunes that surround the active core of the st anthony dune field optically stimulated luminescence osl dating results indicate that these fixed dunes were mobile and actively accreting 6 6 ka middle holocene and 1 0 2 0 ka late holocene evidence for activation of a large region currently occupied by stable dunes suggests these periods were characterized by hydrologic deficit and or warmer climate conditions than currently exists this research provides new information regarding the depositional timing of the st anthony dunes and assists in providing evidence of palaeo climatic conditions for the eastern snake river plain during the middle to late holocene

optically stimulated luminescence dating of southern high plains archaeological sites the southern high plains of north america is rich in archaeological sites but many are not well constrained chronologically owing to a lack of material for radiocarbon dating a program of optically stimulated luminescence osl dating applying mainly single grain analyses was therefore initiated many samples have independent age estimates from radiocarbon to check the osl results but osl age estimates are also provided for those sites that otherwise lack secure chronological control sediment samples for osl were obtained primarily from paleoindian and archaic localities though include deposits of more recent age through the analysis of single grains equivalent dosedthe numerator of the age equationdis evaluated independently on numerous grains the distribution of these values is relatively broad for some samples and this is attributed to post depositional mixing mixing is also evident in some samples with more narrow distributions selecting portions of the mixed distributions for age determination allows more accurate dating for some samples but the nature of the distributions limits the resolution on others conclusions that cannot be as easily drawn from multi grain analysis where independent age control is available most osl results broadly conform however some samples show discrepancies that are not readily explained but may relate to association or dose rate problems this underscores the desirability of obtaining where possible suites of chronological evidence 2006 elsevier ltd all rights reserved

denisovan dna in late pleistocene sediments from baishiya karst cave on the tibetan plateau a late middle pleistocene mandible from baishiya karst cave bkc on the tibetan plateau has been inferred to be from a denisovan an asian hominin related to neanderthals on the basis of an amino acid substitution in its collagen here we describe the stratigraphy chronology and mitochondrial dna extracted from the sediments in bkc we recover denisovan mitochondrial dna from sediments deposited 100 thousand and 60 thousand years ago ka and possibly as recently as 45 ka the long term occupation of bkc by denisovans suggests that they may have adapted to life at high altitudes and may have contributed such adaptations to modern humans on the tibetan plateau

the role of the north atlantic drift in the millennial timescale glacial climate fluctuations abstract the greenland ice core records show that the overall cold climate of the last glacial period was repeatedly interrupted by short rapid warmings the events known as dansgaard oeschger d o events are strongly imprinted in the north atlantic marine records suggesting that they were linked to north atlantic circulation changes however the causes of the d o events are poorly understood and they represent one of the most intriguing puzzles of the last glacial period in order to investigate a possible mechanism we have studied variations in the distribution of benthic and planktonic foraminifera oxygen isotopes and ice rafted debris during the last glacial period in eight cores from the north atlantic and the nordic seas taken at mid depth from 853 to 1760 m the parameters indicate in agreement with previous studies that the circulation system during the interstadials resembled the present system atlantic surface water flowed north into the nordic seas where most of it sank through convection and as cold deep water flowed back into the north atlantic during the stadials a halocline was established in the nordic seas and in the northernmost part of the north atlantic and the outflow from the nordic seas stopped however below the cold light surface layer the relatively warm water of the north atlantic drift continued to flow across the north atlantic and into the nordic seas here warming up the deep water masses we suggest that the warm water gradually made the water column unstable the pivotal abrupt climate warmings were caused by sudden upwelling of the deep warm water masses causing overturning of the entire water column and onset of convection

stratigraphy and geochronology of upper quaternary eolian sand on the southern high plains of texas and new mexico united states eolian sand in dune fields and sand sheets cover 10 000 km 2 10 of the southern high plains of northwestern texas and eastern new mexico these deposits are concentrated in three west east trending belts of dunes the muleshoe lea yoakum and andrews dune fields from north to south that appear to be eastern extensions of the mescalero monahans dune system in the pecos river valley and in the seminole sand sheet a discontinuous accumulation of sheet sands between the lea yoakum and monahans andrews dunes the most common landforms are parabolic dunes associated with blowouts coppice dunes and sand sheets all typical of sandy vegetated semiarid landscapes barchan dunes in keeping with a relatively limited sand supply and an underlying surface that is relatively hard blackwater draw formation pleistocene and fence row dunes historic dunes formed along field boundaries these eolian deposits accumulated episodically in the late pleistocene and holocene and provide clues to the history of regional drought and aridity the earliest phase of sedimentation occurred when sheet sands were deposited between 11 000 and 8000 14 c yr b p probably in several phases based on archaeological data as a result of episodic drought beginning between 11 000 and 10 000 yr b p eolian deposits dating between 8000 and 3000 yr b p are rare although eolian sediment 8000 4500 14 c yr b p is ubiquitous in the draws that cross the region and paleoenvironmental indicators show that the region was subjected to aridity throughout middle holocene time the middle holocene deposits most likely were remobilized in late holocene time most of the deposits and landforms of the dune fields and sand sheets are late holocene dating before 4000 14 c yr b p and mostly before 1500 14 c yr b p the muleshoe and lea yoakum dunes and the seminole sand sheet underwent substantial eastward expansion in late holocene time buried soils and radiocarbon ages show that the eolian sand accumulated in several stages probably in response to cyclic drought the muleshoe dunes accumulated after ca 1300 cal yr b p ca 1400 14 c yr b p after ca 750 670 cal yr b p ca 850 750 14 c yr b p just after ca 500 cal yr b p ca 450 14 c yr b p and during the nineteenth and twentieth centuries the lea yoakum dunes were active after ca 3400 cal yr b p ca 3200 14 c yr b p and historically the seminole sand sheet was active ca 430 330 cal yr b p ca 360 14 c yr b p and in the twentieth century the andrews dunes were subjected to at least two phases of eolian sedimentation after ca 2320 cal yr b p ca 2320 14 c yr b p comparisons with eolian chronologies from other regions on the great plains suggest that dune mobilization was a regional phenomenon after ca 2300 cal yr b p ca 2300 14 c yr b p after ca 1500 1400 cal yr b p ca 1650 1550 14 c yr b p after ca 700 cal yr b p ca 800 14 c yr b p between 500 and 300 cal yr b p ca 450 300 14 c yr b p and in the nineteenth century the climatic fluctuations responsible for mobilizing the dunes probably were relatively minor yet the landscape impacts were substantial resulting in widespread wind erosion and dune construction

derivation of the early carboniferous wedgeport pluton by crystal fractionation of a mafic parental magma a rare case of an a type granite within the meguma terrane nova scotia canada the cambrian ordovician metasedimentary rocks of the meguma terrane canadian appalachians were extensively intruded by silicic plutons during middle devonian to early carboniferous times syn plutonic but volumetrically minor mafic ultramafic intrusions were also emplaced in most localities the silicic plutons and mafic ultramafic intrusions do not appear to be petrogenetically related and are likely derived from different sources the attwoods brook gabbronorite of sw nova scotia yielded an in situ zircon weighted mean 206pb 238u age of 357 9 3 3 ma that is within the uncertainty of the age of the neighbouring wedgeport pluton 357 1 ma the wedgeport pluton is a rare example of a mantle derived peraluminous a type granite within the meguma terrane the similar ages and nd isotopes of the attwoods brook gabbronorite end t 1 1 to 4 0 and wedgeport pluton end t 2 1 to 3 3 suggest the two intrusions are petrogenetically related fractional crystallization modelling demonstrates that a parental magma similar to the attwoods brook gabbronorite can produce residual silicic liquids that resemble the granites of the wedgeport pluton indicating that they could be members of the same intrusive complex the emplacement of the gabbronorite and wedgeport pluton occurred during a period of tensional plate stress that was contemporaneous with rifting of the maritimes basin that produced the fountain lake continental flood basalts and a type granites of the cobequid highlands within the avalon terrane it is possible that the early carboniferous rocks of sw nova scotia are related to the rifted related magmatism within the maritimes basin

mechanisms of abrupt climate change of the last glacial period 1 more than a decade ago ice core records from greenland revealed that the last glacial period was characterized by abrupt climate changes that recurred on millennial time scales since their discovery there has been a large effort to determine whether these climate events were a global phenomenon or were just confined to the north atlantic region and also to reveal the mechanisms that were responsible for them in this paper we review the available paleoclimate observations of abrupt change during the last glacial period in order to place constraints on possible mechanisms three different mechanisms are then reviewed ocean thermohaline circulation sea ice feedbacks and tropical processes each mechanism is tested for its ability to explain the key features of the observations particularly with regard to the abruptness millennial recurrence and geographical extent of the observed changes it is found that each of these mechanisms has explanatory strengths and weaknesses and key areas in which progress could be made in improving the understanding of their long term behavior both from observational and modeling approaches are suggested finally it is proposed that a complete understanding of the mechanisms of abrupt change requires inclusion of processes at both low and high latitudes as well as the potential for feedbacks between them some suggestions for experimental approaches to test for such feedbacks with coupled climate models are given

present day and last glacial maximum ocean thermohaline circulation in a zonally averaged coupled ocean sea ice atmosphere model abstract a simple ocean atmosphere model suitable for long term paleoclimate studies is presented it consists of a three basin zonally averaged ocean model coupled to an energy moisture balance atmospheric model and a thermodynamic sea ice model the model is used to simulate the present day climate conditions and the conditions resembling those of the last glacial maximum lgm in the first case the model has an ocean thermohaline thc circulation in which the overturning in the north atlantic is slightly more than 15 sv two methods are used to spin up the model to the lgm climate first a cool state is obtained by running the coupled model with a reduced model greenhouse effect and increased albedo but without making any explicit changes to the hydrological cycle in the second approach the model is directly set up for lgm conditions in a manner similar to the method used to simulate the present day climate in both cases the computed lgm sea surface temperatures are reasonable but only in t

u pb and rb sr geochronology of the wedgeport granitoid pluton southwestern nova scotia zircons from biotite monzogranite of the wedgeport pluton intrusive into deformed metasediments of the cambrian goldenville formation in the southwestern meguma terrane of nova scotia yield concordant u pb ages of 316 5 ma this is interpreted as the time of intrusion and crystallization within the error limits the 323 12 ma rb sr whole rock isochron age is identical and gives an initial 87 sr 86 sr ratio of 0 7137 0 0056 rb sr analyses of mineral separates of biotite potassium feldspar and quartz plagioclase from several samples yield subparallel internal isochrons with an average age of 257 8 ma initial ratios of the internal isochrons range from 0 716 to 0 759 a slow cooling model for these latter data is discarded because the mineral data fall on straight lines instead a reheating event related to plutonism ca 257 ma ago which was sufficient to cause local grain to grain migration and reequilibration of strontium and rubidium but not large scale redistribution is invoked this reheating is also inferred to be responsible for the hydrothermal alteration and sn u mineralization concentrated along the northwestern margin of the pluton a dextral northeast southeast shear zone cutting the pluton is also inferred to be ca 257 ma old it may be related to the last stages of westward obduction of the meguma terrane these results provide a clear example of permo carboniferous plutonism in the southwestern meguma terrane and suggest a similar interpretation may apply to other anomalously young ages recorded in this area in light of these results the permo carboniferous age of the large east kemptville tin deposit and its location in a dextral shear zone suggest that the association of younger plutonism and shear zones may be a significant factor for economic mineralization

dating mineralization using several isotopic methods an example from the south mountain batholith nova scotia canada abstract granitic plutons in the meguma terrane may be broadly divided into two suites 1 an areally predominant cogenetic suite of peraluminous granitoid rocks tonalite leucogranite and 2 an areally subordinate paraintrusive suite specialized in sn w mo cu f u p zn as which appears to have been produced by fluid interaction with either residual magma or crystallized rock granite related mineralization is generally considered to have developed during terminal stages of magmatic crystallization that lead to metasomatism and hydrothermal alteration previous geochronological data have suggested that the main cogenetic suite was emplaced at 350 375 ma whereas some of the paraintrusive suite is markedly younger 250 350 ma the apparently large time interval between the intrusion of the cogenetic suite and some of the mineralization suggests that they are not coeval and has led to the conclusions that the younger mineralization may be related to later magmatism and or tectonothermal events rocks associated with two mineral occurrences in the south mountain batholith that had previously yielded anomalously young isotopic ages millet brook and westfield have been analyzed for u pb and ar isotopes in an attempt to resolve this dilemma new geochronological data from the millet brook area indicate that intrusion of granodiorite at 377 5 3 ma nearly concordant monazite age was closely followed by intrusion of most other phases all of which had cooled through 375 c closure temperature for ar in muscovite by 377 373 6 ma 40 ar 39 ar plateau ages new data from westfield indicate that a paraintrusive porphyry was intruded at 385 2 ma upper intercept of a chord through four nearly concordant zircon fractions and that greisen muscovite cooled through the closure temperature by 368 2 6 ma 40 ar 39 ar plateau age these data indicate that a previously reported rb sr whole rock isochron age of 270 ma at westfield reflect isotopic resetting that occurred below the blocking temperature for 40 ar 39 ar in muscovite 375 c clearly further rb sr analysis is required to document this process other 250 300 ma ages in the meguma terrane are recorded along the tobeatic fault zone away from the fault ages increase upto 370 ma suggesting that the fault may have acted as an important pathway for fluids and heat that facilitated isotopic resetting rb sr whole rock and mineral systems may be affected after formation of an alteration assemblage associated with mineralization by the migration of rb and its concentration in secondary minerals

optical dating of holocene sediments from a variety of geomorphic settings using single grains of quartz this paper presents an improved method for the optical dating of holocene sediments from a variety of geomorphic settings we have measured the equivalent dose de in individual grains of quartz using green laser light for optical stimulation and have simulated the de distributions for multiple grain synthetic aliquots using the single grain data for 12 samples of known independent age we show that application of a minimum age model to the single grain and small 10 grain aliquot de data provides the most accurate estimate of the burial dose for nine of the samples examined 3 aeolian 5 fluvial and 1 marine the weighted mean de as obtained using the central age model gives rise to burial age overestimates of up to a factor of 10 for these nine samples whether single grains small aliquots or large 100 grain aliquots are used for the other three samples two aeolian and one fluvial application of either the minimum age model or the central age model to the single grain small aliquot and large aliquot de data yields burial ages in accord with the independent age control we infer that these three samples were well bleached at the time of deposition these results show that heterogeneous bleaching of the optical dating signal is commonplace in nature and that aeolian transport offers no guarantee that the sample will be well bleached at the time of deposition we also show that grains sensitive to infrared ir stimulation can give rise to low de values which will result in significant underestimation of the burial dose and hence of the age of deposition we demonstrate that use of a modified single aliquot regenerative dose protocol incorporating ir stimulation prior to green light stimulation deals effectively with contamination by ir sensitive grains we conclude that application of the modified protocol to single grains or small aliquots of quartz using the lowest de population to estimate the burial dose is the best means of obtaining reliable ages for holocene sediments from a wide range of depositional environments d 2003 elsevier b v all rights reserved

single grain osl dating at la grotte des contrebandiers smugglers cave morocco improved age constraints for the middle paleolithic levels abstract optically stimulated luminescence osl measurements of individual quartz grains are reported for middle paleolithic mp or middle stone age msa deposits in la grotte des contrebandiers morocco single grain measurements enable rejection of grains that may lead to under or over estimation of age due to malign luminescence properties or remobilisation of grains after burial we identified the former using a range of experimental procedures and objective rejection criteria three post depositional factors influenced the distribution pattern of equivalent dose d e values for the remaining single grains the disintegration of roof spall liberating unbleached older grains into otherwise well bleached sediments the intrusion of a small number of younger grains from the overlying upper paleolithic deposits and small scale differences in the beta dose received by individual grains roof spall contamination was a significant problem so we characterised the osl signals and obtained d e values for the calcareous sandstone in which the cave is formed which allowed bedrock derived grains in the mp msa deposits to be identified and rejected prior to age determination we collected 31 samples from the mp msa deposits and were able to calculate ages for 30 of them these ages suggest that mp msa occupation of contrebandiers was restricted to the interval between about 120 and 90 ka ago marine isotope stages 5d to 5b weighted mean osl ages were obtained for samples associated with archaeologically sterile deposits at the base of the excavations 126 9 ka followed by two phases of the moroccan mousterian 122 5 and 115 3 ka interspersed with archaeologically sterile deposits 112 4 ka the base and top of the aterian were dated to 107 4 and 96 4 ka respectively based on the grand weighted mean ages of 116 3 and 103 3 ka for the moroccan mousterian and aterian at contrebandiers we suggest that these two industries may be separated by an occupation hiatus of up to 13 3 ka at this site our single grain osl chronology confirms previous age estimates obtained for contrebandiers and other sites in the vicinity but are generally more precise and stratigraphically more coherent due to the inherent advantages of measuring individual quartz grains and rejecting contaminant and other unsuitable grains before age determination

glacial abrupt climate changes and dansgaard oeschger oscillations in a coupled climate model 1 there are three fundamental features which characterize large glacial millennial dansgaard oeschger oscillations 1 the climatic transitions were abrupt and large 2 the lengths of both interstadials and stadials and the period of dansgaard oeschger oscillations were not uniform and 3 there were no large millennial oscillations during an early stage of a glacial period and a peak glacial period in this modeling study we offer a consistent explanation for these three features by employing an earth system model of intermediate complexity we demonstrate that a moderate global cooling forces the atlantic meridional overturning circulation moc into an unstable state and hence causes the flip flop of the atlantic moc between a strong mode and a weak mode the durations of both interstadials and stadials associated with these millennial oscillations are modulated by the changing background climate in qualitative agreement with the observations in a warm climate the atlantic moc is strong and stable with the deep water formed mainly by intense heat loss to the atmosphere in a cold climate the atlantic moc is weak and stable and this mode is largely maintained by the process of sea ice brine rejection since the dansgaard oeschger oscillations result from an alternation between these two moc states during an intermediate phase climate we conclude that brine rejection plays a necessary role in the oscillations confirming a hypothesis suggested in some proxy data studies

ocean subsurface warming as a mechanism for coupling dansgaard oeschger climate cycles and ice rafting events 1 heinrich events have been attributed to surging of the laurentide ice sheet every 7 thousand years or so during glacial time these massive ice rafting events only occurred during cold phases of millennial scale dansgaard oeschger climate cycles other observed ice rafting events sourced from ice streams at various locations around the northern north atlantic occurred during all dansgaard oeschger cold phases and led heinrich events when the latter took place here it is suggested that ocean subsurface warming in the northern north atlantic during the cold phases may provide the key to explaining these climate ice rafting phasings such warming would lead to ice shelf melting and breakup without buttressing by ice shelves ice streams may surge leading to increased iceberg production this interpretation is supported by results of a simplified coupled climate model and by available sediment and ice sheet data

late quaternary environments of the waco mammoth site texas usa abstract the waco mammoth site wms in central texas contains the remains of the largest mammoth herd mammuthus columbi in north america that died in a single catastrophic event most mammoths at the site died on a gravel bar of the ancient bosque river adjacent to a collapsing tributary wall however the timing and cause of death of the 26 mammoths documented to date are controversial the objectives of this research are to describe and interpret the alluvial stratigraphy and infer the cause of death employ optically stimulated luminescence osl dating to determine the timing of death and analyze stable c isotopes of pedogenic carbonate to infer local plant communities dietary habits and summer temperatures dating of quartz from seven sediment samples by osl places the death event to a weighted mean of 66 8 5 0 ka the site is coeval with marine oxygen isotope stage 4 consistent with our reconstructed mean july temperatures 4 c cooler than today based on a buried soil isotopic transfer function our buried soil isotopic interpretation of a dominance of c3 plants is contrary to previous studies of mammoth tooth enamel at the site suggesting a dietary preference for warm season grasses c4

investigations into the potential effects of pedoturbation on luminescence dating much effort has been focussed on understanding the luminescence properties of natural minerals to achieve a reliable accurate and precise dating technique however some field related aspects such as the influence or effect of post depositional disturbance on luminescence dates are as yet underexplored in the case of pedoturbation depending on its intensity the rate of sedimentation and unit thicknesses potentially the whole sedimentary record at a site can be affected this may lead to distorted osl chronologies and erroneous sediment burial ages pedoturbation can result in sediment mixing and or exhumation that affect luminescence both at the bulk and single grain level effects of these two principle processes on luminescence ages are examined using standard multigrain and single grain protocols high resolution sampling of surface gopher mounds was used to determine the efficiency of bio exhumation in resetting luminescence signal results show this is an inefficient mechanism for onsite sediment bleaching the effects on luminescence signal of bio mixing were explored by comparing a sample collected from within a krotovina infilled burrow to an adjacent undisturbed sample results show the difficulties in identifying pedoturbated samples at the single aliquot level and the possible inaccuracies in using the lowest palaeodose values to calculate osl ages where pedoturbation of samples is suspected use of probability plots of palaeodoses data is recommended from these plots it is proposed that only data falling within a normal distribution centred on the peak probability be used to calculated osl ages and to mitigate problems arising from pedoturbation

late holocene dune accretion and episodes of persistent drought in the great plains of northeastern colorado abstract the great plains of the mid west usa contain dunefields which display stratigraphic evidence of episodic sand drift throughout the holocene and in some cases the late pleistocene widespread aeolian activity has been linked to persistent megadroughts caused by a weakening of the tropical moisture laden circulation from the gulf of mexico and equatorial pacific infrared stimulated luminescence irsl dating has been applied to two exposures in the fort morgan dunefield of northeastern colorado where radiocarbon dated buried soils provide ages for land surface stability irsl ages show that sand drift occurred episodically during the late holocene at 4 85 2 37 1 06 0 80 0 6 0 53 and 0 37 ka

investigation of late and post fremont alluvial stratigraphy of range creek east central utah use of osl when radiocarbon fails abstract range creek in east central utah hosts a unique abundance of fremont aged archaeological sites although good age control is available for cultural artifacts and sites radiocarbon ages from alluvial sequences along the valley floor have generally produced age overestimates in order to provide an alternative dating technique and test the reliability of the radiocarbon results samples for single grain quartz optically stimulated luminescence osl dating were collected from three alluvial profiles where 18 radiocarbon samples of charcoal and aggregated pollen had been analyzed while ams radiocarbon age results were more precise the single grain osl ages were stratigraphically consistent and suggest radiocarbon age overestimation of up to 2 6 ka with the greatest divergence observed in the results from aggregated pollen samples osl results and geochemical analyses suggest that hydrocarbon contamination may have affected the accuracy of the radiocarbon results age overestimates are likely due to contribution from oil shale deposits within the catchment osl and stratigraphic results suggest fluvial deposition began prior to 0 88 ka ad 1130 at the site floodplain aggradation continued by conformable deposition of tabular flood packages commonly capped with charcoal and ash rich sediments until interrupted by channel arroyo entrenchment prior to 0 34 ka ad 1670 sandy flood packages filled this channel and re aggradation continued until historic arroyo entrenchment of range creek in the late ad 1800 s osl age constraints indicate that the alluvial sediments at the study sites were deposited during and immediately following fremont occupation of range creek at ad 1000 1200 and may contain valuable archives of environmental conditions leading up to and following the abandonment of fremont peoples from the region moreover osl results point to the importance of the use of multiple chronometers to date alluvial deposits as the radiocarbon chronology alone would suggest that the study sites contained pre fremont aged deposits and may lead to the misinterpretation of associated cultural and environmental records

millennial variability in an idealized ocean model predicting the amoc regime shifts abstracta salient feature of paleorecords of the last glacial interval in the north atlantic is pronounced millennial variability commonly known as dansgaard oeschger events it is believed that these events are related to variations in the atlantic meridional overturning circulation and heat transport here the authors formulate a new low order model based on the howard malkus loop representation of ocean circulation capable of reproducing millennial variability and its chaotic dynamics realistically it is shown that even in this chaotic model changes in the state of the meridional overturning circulation are predictable accordingly the authors define two predictive indices which give accurate predictions for the time the circulation should remain in the on phase and then stay in the subsequent off phase these indices depend mainly on ocean stratification and describe the linear growth of small perturbations in the system thus monitoring particular indices of the ocean state could help predict

petrology age and tectonic setting of the seal island pluton offshore southwestern nova scotia the seal island pluton outcrops only on small islands located on the continental shelf 45 km south of nova scotia although geophysical data indicate that the pluton is part of large granitoid units that cover thousands of square kilometres farther offshore based on the island outcrops the seal island pluton consists of biotite monzogranite and muscovite biotite monzogranite of uncertain relative age metasedimentary xenoliths combined with characteristic magnetic patterns indicate that the pluton intruded the cambrian ordovician meguma group compared with the biotite monzogranite the muscovite biotite monzogranite is higher in sio2 more peraluminous and more depleted in heavy rare earth elements and also has lower end 1 39 versus 0 82 possibly the result of more contamination by meguma group sedimentary rocks the biotite monzogranite yielded a late devonian u pb zircon age of 362 8 0 7 ma although the relatively minor petrological differences between the two units do not preclude a co m

late carboniferous and early permian drainage patterns in atlantic canada paleoflow data have been compiled for late carboniferous late westphalian a to early permian alluvial deposits over a large area of atlantic canada the data which include more than 36 000 measurements of large scale trough cross strata indicate a predominantly northeasterly paleoflow and suggest that a major source area lay to the southwest of the region throughout the 30 ma period represented uplands within the basin deflected paleoflow and probably formed important local drainage and sediment sources tectonostratigraphic analysis suggests that the drainage originated in the fold and thrust belt of the central appalachians and parts of the northern appalachians rivers probably followed northeast oriented structural lineaments through the older acadian mountains of the northern appalachians a considerable proportion of the rising orogen s drainage and probably detritus may have traversed basins along the strike of the mountain belt a situation analogous to that of the modern himalayas

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

middle and late holocene dune reactivation in the nebraska sand hills usa a combination of optical dating of aeolian sand and radiocarbon dating of palaeosols and interdune sediments has allowed some refinements to the chronostratigraphy of the nebraska sand hills our results directly confirm previous inferences that multiple phases of aeolian deposition including megabarchan dune reactivation occurred within the sand hills during the middle and late holocene optical dates on stratified aeolian barchan dune sediments from two localities provide ages ranging from c 6 ka to 300 years ago weakly developed palaeosols intercalated within the dune sediments and other forms of post depositional modification indicate periods of relative stability and reduced dune mobilization optical dates on the upper 12 15 m of two dune exposures indicate repeated possibly regionally correlative reactivations which occurred between c 400 500 and 200 300 years ago comparison of late holocene aeolian activity with local and regional palaeoclimatic proxy records suggests that dune reacti

the east kemptville tin deposit yarmouth county nova scotia a pb isotope study of the leucogranite and mineralized greisens evidence for a 366 ma metallogenic event pb isotopic data for whole rock leucogranite and mineral separates variably mineralized greisen and galena from the east kemptville sn cu zn ag deposit yarmouth county southwestern nova scoti

optical dating in archaeology thirty years in retrospect and grand challenges for the future in 1982 when richard klein first became one of the editors of this journal the luminescence dating community was embarking on a new phase of exploratory research attention was turning from the use of thermoluminescence tl dating to estimate the time of last heating of archaeological objects such as pottery and burnt flint to the tl dating of unheated sediments that had been transported by wind and then deposited on the landscape this revolutionary development enabled the extension of tl dating to sedimentary deposits in a variety of environmental settings and to the multitude of archaeological sites that lack suitably heated artefacts in sediment dating the age of most interest is usually the time elapsed since grains of quartz or feldspar were last exposed to sunlight as the energy of the sun s rays is sufficient to evict electrons from their light sensitive traps these traps are steadily refilled after sediment deposition and the longer the grains remain buried the more tl they will emit when measured in 1985 huntley and colleagues proposed optical dating as a simpler and superior means of stimulating the light sensitive traps in quaternary sediments and this is now the principal luminescence based method of dating geological and archaeological deposits optical dating is an umbrella term for an armada of acronyms the most common in archaeological contexts being osl optically stimulated luminescence tt osl thermally transferred osl irsl infrared stimulated luminescence and pirir post infrared irsl all of these variants are founded on the same basic tenet measurement of a light sensitive signal to determine typically the last time that sediment grains were sun bleached but each approach has its virtues and vices in this paper we review this family of luminescence dating techniques and look back on 30 years of optical dating in archaeology some of the more interesting and important achievements are highlighted including the critical insights gained in the last two decades from osl measurements of individual grains of quartz we also look to the future of optical dating in archaeological contexts efforts to extend the age limits of optical dating to older hominin and archaeological sites will remain a key goal and understanding how archaeological sites of all ages form and evolve over time could be improved greatly by combining micromorphology analysis with optical dating of undisturbed intact sediments the latter poses a series of particularly formidable technical challenges but if the past is any guide to the future then we can expect optical dating to illuminate much more of human history before celebrating its golden jubilee

testing of a single grain osl chronology across the middle to upper palaeolithic transition at les cottes france abstract the timing of the middle palaeolithic to upper palaeolithic transition in france is important to help understand when where and how neanderthals have been replaced by homo sapiens radiocarbon dating has been the dating workhorse in constructing the chronological framework pertinent to these questions in this study we are testing whether single grain osl dating has the accuracy and precision to be useful as a complementary dating method the site of les cottes provides an ideal testing ground because of its stratigraphic integrity and reliable radiocarbon chronology we applied single grain osl dating of quartz to 19 samples and multi aliquot met pirir dating of potassium rich feldspar grains to 5 samples to explicitly test assumptions of pre depositional resetting of the osl signal and post depositional exposure to variable beta dose rates the good agreement between the single grain osl and the multi aliquot met pirir ages suggest that the optical signals of both quartz and feldspar grains were reset prior to deposition and that much of the extra scatter observed in the equivalent dose distribution of quartz grains are likely due to the small scale differences in beta dose delivered to individual grains both the quartz osl and feldspar met pirir ages show great consistency with the 14 c ages on bone collected from the same units this gives confidence in the measurement and analytical approaches used to derive both the equivalent dose and dose rate the numerator and denominator respectively of the luminescence age equation these results suggest that a systematic and detailed single grain osl dating study can have the accuracy and precision that is necessary to play a powerful role in the dating of the middle and upper palaeolithic transition and other questions of importance in this time range and geographical area

quasi decadal variability in paleoclimate records sunspot cycles or intrinsic oscillations 1 paleoclimatic proxies for interannual variability reveal power within the 7 15 year quasi decadal qd band in earth s history despite the fact that the best established sources of qd variability in modern climate are internal this qd variability in paleoclimatic records is commonly attributed to the 11 year sunspot cycle of solar variability intrinsic qd variability has received little attention in paleoclimatology we apply two different modeling frameworks to assess the degree to which qd variability might have occurred in the past and question whether existing paleoclimate records no matter how well resolved distinguish between internally driven and solar driven variability we show that even a simple atmospheric general circulation model gcm driven by constant solar radiation produces climate oscillations with a period of 11 years then motivated by numerous studies arguing for the presence of sunspot cycles in eocene 50 ma paleoclimate records we demonstrate that a fully coupled gcm simulation with constant solar forcing reproduces the full spectrum of reconstructed eocene variability in the examples we consider there is no compelling reason to invoke solar cycles as a cause of interpreted variability we stress the importance of identifying internal modes of variability from paleoclimate records to increase understanding of the climate system

frequency domain multimodel analysis of the response of atlantic meridional overturning circulation to surface forcing the dynamics of the atlantic meridional overturning circulation amoc vary considerably among different climate models for example some models show clear peaks in their power spectra while others do not to elucidate these model differences transfer functions are used to estimate the frequency domain relationship between surface forcing fields including sea surface temperature salinity and wind stress and the resulting amoc response these are estimated from the outputs of the coupled model intercomparison project phase 5 cmip5 and phase 3 cmip3 control runs for eightdifferent models with a specificfocus on geophysical fluid dynamics laboratory climate model version 2 1 gfdl cm2 1 and the community climate system model version 4 ccsm4 which exhibit rather different spectral behavior the transfer functionsshowvery littleagreement amongmodelsforany ofthe pairsofvariablesconsidered suggestingthe existence of systematic model errors and that considerable uncertainty in the simulation of amoc in current climate models remains however a robust feature of the frequency domain analysis is that models with spectral peaks in their amoc correspond to those in which amoc variability is more strongly excited by high latitude surface perturbations that have periods corresponding to the frequency of the spectral peaks this explains why different models exhibit such different amoc variability these differences would not be evident without using a method that explicitly computes the frequency dependence rather than a priori assuming a particular functional form finally transfer functions are used to evaluate two proposed physical mechanisms for model differences in amoc variability differences in labrador sea stratification and excitation by westward propagating subsurface rossby waves

single grain osl dating of sediments from the southern high plains usa single grain osl dating is applied to sediments from different depositional settings on the southern high plains of western texas and eastern new mexico criteria of acceptance are used to screen equivalent doses from individual grains and resulting distributions are evaluated in terms of normality wide variation is found in proportion of acceptable grains and in the distributions while some of the latter are normal many show broadening that may be the result of mixing of different aged grains and skewness that may result from variant depositional and post depositional modes geological modeling will be required to understand better these distributions although for most samples means produced ages that agree with independent evidence the resolution possible with single grains is necessary for best estimates of equivalent dose for other samples a few samples do not agree with independent evidence even with a normal distribution of equivalent dose sand dunes seem the most difficult to date accurately probably because of mixing

lithostratigraphy and geochronology of fills in small playa basins on the southern high plains united states playa basins are small depressions typically 1 5 km 2 on the southern high plains of northwestern texas and eastern new mexico there are about 25 000 playas in the region they lie on the blackwater draw formation pleistocene a widespread eolian deposit and locally on the ogallala formation miocene pliocene understanding the lithostratigraphy and chronostratigraphy of the fill in the basins is important because it should 1 provide clues to the origin and evolution of playas which have been long debated 2 yield a paleoenvironmental record for the region and 3 aid in understanding the history and future of the regional aquifer because playas are the principal source of recharge data from 19 playa basins combined with published data from 4 other basins show that the basin fill is composed of six distinctive facies 1 lacustrine mud 2 lacustrine carbonate 3 lacustrine delta deposits 4 eolian sand and silt 5 eolian loam and 6 accretionary eolian deposits blackwater draw formation mud deposited under ponded conditions is the most common facies and is the surficial deposit on the floors of most playas often producing vertisols the carbonate was precipitated under lacustrine conditions and is another common facies and surface deposit delta deposits are common near the basin margins well sorted layers of eolian sand and silt and poorly sorted eolian loam occur locally above within or below the lacustrine deposits the modern basins in all study areas are locally or completely inset into the blackwater draw formation supporting the interpretation that the basins are at least in part erosional features in larger basins with thicker fills generally coincident with thicker blackwater draw formation the formation interfingers with the lacustrine fill dating is based on radiocarbon ages from the fill in 12 basins and from lunettes adjacent to 5 basins all dated basins were present at the end of the pleistocene and some were present in some form throughout the pleistocene lacustrine mud and other clastic deposits accumulated in the late quaternary and locally much earlier showing that at least some basins contained water throughout the time of human occupation of the region dating of eolian sediments supports other data indicative of aridity and wind deflation in the early and late holocene the lacustrine carbonate is late pleistocene or older and its paleoenvironmental significance is unknown these lithostratigraphic and chronostratigraphic relationships show that some basins have a prolonged history as depressions persisting in more or less the same location as the high plains surface aggraded by eolian addition blackwater draw formation throughout the pleistocene sizes of the basins varied through time as they were encroached upon by the blackwater draw formation enlarged by fluvial lake margin and eolian erosion were filled and reexposed or were buried some basins are newly formed on the high plains surface and have no apparent predecessors the only evidence for subsidence beneath the basins is gently warped fill in the basins on the northern margin of the region known to be affected by salt dissolution in paleozoic bedrock pedogenic carbonates typically are absent from or beneath the basin fill due to focused recharge through the basins playa basins probably have been a ubiquitous component of the high plains landscape through much of the quaternary

a thermal oscillation under a restorative forcing summary the authors report an interdecadal oscillation in a wind and thermally driven ocean general circulation model ogcm the oscillation is tantalizing in that it occursunder a relatively strong thermal damping 26 3 w m 2k examinations involving a two dimensional ogcm a simple thermal flip flop model and a three dimensional ogcm with and without the nonlinear effect of temperature in the state equation of sea water demonstrate that the oscillation is not driven by mechanisms such as the so called convective oscillator or the advective overshooting oscillator instead the oscillation is associated with the propagation of modelled long rossby waves with westward phase velocity it is found that the north south basin extent of the ogcm is an important factor a larger northsouth basin extent is conducive to the generation of thermal wind flows crucial for initiating the propagation of disturbances the geophysical relevance of these results is discussed

the blackwater draw formation quaternary a 1 4 plus m y record of eolian sedimentation and soil formation on the southern high plains the southern high plains of northwestern texas and eastern new mexico are mantled by a vast 100 000 km 2 sheet of quaternary eolian sediment locally as much as 27 m thick and termed the blackwater draw formation these sediments generally fine from southwest to northeast exhibiting a linear relationship of decreasing sand content and sand size and increasing silt content with increasing distance indicating the source area to be the pecos river valley as many as six well developed buried soils 2 5yr to 5yr hues bt horizons 1 2 m thick stage ii iii calcic horizons similar to the regional paleustalf and paleustoll surface soils occur in the formation the buried soils indicate episodic sedimentation separated by long periods of landscape stability eolian sedimentation probably occurred during prolonged aridity and stability and pedogenesis likely obtained during subhumid to semiarid conditions similar to those of the late quaternary the presence of the 0 62 m y lava creek b ash and the 1 4 m y guaje ash in the formation shows that the deposit accumulated throughout most of the quaternary data from paleomagnetic thermoluminescence and radiocarbon studies suggest that each cycle of sedimentation stability lasted for several hundred thousand years and that the last depositional event occurred at least several tens of thousands of years ago

the different nature of the interdecadal variability of the thermohaline circulation under mixed and flux boundary conditions the differences between the interdecadal variability under mixed and constant flux boundary conditions are investigated using a coarse resolution ocean model in an idealized flat bottom single hemisphere basin objective features are determined that allow one type of oscillation to be distinguished versus the other first by performing a linear stability analysis of the steady state obtained under restoring boundary conditions it is shown that the interdecadal variability under constant flux and mixed boundary conditions arises respectively from the instability of a linear mode around the mean stratification and circulation and from departure from the initial state based on the budgets of density variance it is shown next that the two types of oscillations have different energy sources under the constant flux boundary condition the thermal mode the downgradient meridional eddy heat flux in the western boundary current regions sustains interdecadal variability whereas under mixed boundary conditions the salinity mode a positive feedback between convective adjustment and restoring surface heat flux is at the heart of the existence of the decadal oscillation furthermore the positive correlations between temperature and salinity anomalies in the forcing layer are shown to dominate the forcing of density variance in addition the vertical structure of perturbations reveals vertical phase lags at different depths in all tracer fields under constant flux while under mixed boundary conditions only the temperature anomalies show a strong dipolar structure the authors propose that these differences will allow one to identify which type of oscillation if any is at play in the more exhaustive climate models

offshore continuation of meguma terrane southwestern nova scotia eight short drill cores have been examined from the continental shelf southwest of nova scotia four cores recovered granitoid rocks of two types ilmenite bearing granitoid rocks petrographically and geochemically resemble granodiorites of the south mountain batholith and granites of the seal island pluton magnetite bearing granitoid rocks are also peraluminous but have no exact analogues onshore in nova scotia two cores recovered metamorphic rocks in a small area 50 km south of seal island one consits of chlorite muscovite quartz schist geochemically similar to rocks of the halifax formation the second sampled epidote chlorite quartz schist similar to metavolcanic rocks of the white rock formation one further core sampled quartzite and another sampled a metavolcanic rock possibly erratic the regional extent of these lithotypes can be inferred from gravity and aeromagnetic data regional gravity data suggest the presence of a large granite body off southwestern nova scotia in this area magneti

a heuristic model of dansgaard oeschger cycles part i description results and sensitivity studies abstracta simple model for studying the dansgaard oeschger d o cycles of the last glacial period is presented based on the t dokken et al hypothesis for d o cycles the model is a column model representing the nordic seas and is composed of ocean boxes stacked below a one layer sea ice model with an energy balance atmosphere no changes in the large scale ocean overturning circulation are invoked parameterizations are included for latent heat polynyas and sea ice export from the column the resulting heuristic model was found to cycle between stadial and interstadial states at times scales similar to those seen in the proxy observational data with the presence or absence of perennial sea ice in the nordic seas being the defining characteristic for each of these states the major discrepancy between the modeled oscillations and the proxy record is in the length of the interstadial phase which is shorter than that observed the modeled oscillations were found to be robust to parameter changes inclu

tropical air sea interactions accelerate the recovery of the atlantic meridional overturning circulation after a major shutdown abstract using a coupled ocean sea ice atmosphere model of intermediate complexity the authors study the influence of air sea interactions on the stability of the atlantic meridional overturning circulation amoc mimicking glacial heinrich events a complete shutdown of the amoc is triggered by the delivery of anomalous freshwater forcing to the northern north atlantic analysis of fully and partially coupled freshwater perturbation experiments under glacial conditions shows that associated changes of the heat transport in the north atlantic lead to a cooling north of the thermal equator and an associated strengthening of the northeasterly trade winds because of advection of cold air and an intensification of the trade winds the intertropical convergence zone itcz is shifted southward changes of the accumulated precipitation lead to the generation of a positive salinity anomaly in the northern tropical atlantic and a negative anomaly in the southern tropical atlantic during the shutdown phase of

a triple sea ice state mechanism for the abrupt warming and synchronous ice sheet collapses during heinrich events 1 abrupt switch like changes in sea ice cover are proposed as a mechanism for the large amplitude abrupt warming that seemed to have occurred after each heinrich event sea ice changes are also used to explain the colder than ambient glacial conditions around the time of the glacier discharge the abrupt warming events occur in this mechanism owing to rapid sea ice melting which warmed the atmosphere via the strong sea ice albedo and insulating feedbacks such abrupt sea ice changes can also account for the warming observed during dansgaard oeschger events the sea ice changes are caused by a weak order of 5 sv response of the thermohaline circulation thc to glacier discharges the main point of this work is therefore that sea ice may be thought of as a very effective amplifier of a weak thc variability explaining the abrupt temperature changes over greenland synchronous ice sheet collapses from different ice sheets around the north atlantic indicated by some proxy records are shown to be possible via the weak coupling between the different ice sheets by the atmospheric temperature changes caused by the sea ice changes this weak coupling can lead to a nonlinear phase locking of the different ice sheets which therefore discharge synchronously it is shown that the phase locking may also lead to precursor glacier discharge events from smaller ice sheets before the laurentide ice sheet discharges the precursor events in this mechanism are the result rather than the cause of the major glacier discharges from the laurentide ice sheet

variation in radiocarbon ages of soil organic matter fractions from late quaternary buried soils radiocarbon dating of three organic matter fractions total humic acid and residue isolated from late quaternary buried soils of the central great plains reveals that there often are considerable differences among but no consistent order to the ages of fractions for late holocene soils the residue fraction or the total fraction generally produces the oldest age for late pleistocene soils however no fraction was consistently the oldest the absence of a consistent sequence of fraction ages is attributed to postburial contamination of soils when bulk samples from the same soil were split and sent to two laboratories different radiocarbon ages were usually obtained the variability in radiocarbon ages of soil organic matter confirms that caution should be taken when using radiocarbon ages obtained from different laboratories to make regional stratigraphic correlations

late paleozoic collision delamination short lived magmatism and rapid denudation in the meguma terrane nova scotia canada constraints from 40ar 39ar isotopic data the meguma terrane consists of 8 15 km of early paleozoic stratified rocks deposited on a continental basement that were deformed and metamorphosed during the late paleozoic as a result of lithospheric plate collision the oldest cleavage previously published whole rock 40ar 39ar plateau ages of 415 395 ma dates the onset of crustal thickening which was followed by voluminous but short lived late devonian granitic and minor mafic magmatism 380 370 ma this magmatism may have been the product of delamination of the lower lithosphere and upwelling of asthenosphere which effected melting above the new moho and resulted in intrusion at depths of 5 12 km 40ar 39ar plateau ages of hornblende muscovite and biotite indicate that at the present erosion level most of the meguma terrane cooled through 300 c by 368 360 ma slightly earlier than for the southwestern meguma terrane ca 345 ma the present erosion level was exhumed by the latest devonian early carboniferous visean the age of the

optical dating of holocene dune sands in the ferris dune field wyoming abstract optical dating of late quaternary quartz dune sands from the clear creek portion of ferris dune field wyoming demonstrates the considerable potential of the technique as a chronostratigraphic tool a sequence of radiocarbon dated holocene interdune strata permit optical dating of the intercalated dune sand to be tested the concordance is good the optical dates for the aeolian deposits not datable by radiocarbon suggest that aeolian sedimentation at clear creek peaked during two relatively short phases at ca 8500 and 4000 yr b p the dates indicate that aeolian accumulation maxima at least in the clear creek area may not be synchronous with previously defined phases of marked aridity

geochronological evidence for multiple tectono thermal overprinting events in the east kemptville muscovite topaz leucogranite yarmouth county nova scotia canada the east kemptville muscovite topaz leucogranite located in yarmouth county nova scotia canada is host rock to the only producing primary tin deposit in north america 56 mt 0 165 sn previous geochronological studies include i rb sr whole rock analyses of the quartz topaz greisens which indicated a date of 337 5 ma and ii 40ar 39ar analyses of greisen muscovite which indicated apparent plateau dates of ca 300 ma however the pervasive development of deformational fabrics at east kemptville suggests that both the rb sr whole rock and 40ar 39ar muscovite ages are at best minimum estimates for the inferred time of mineralization in the present study rb sr whole rock and mineral muscovite plagioclase k feldspar analyses and 40ar 39ar incremental step heating of a muscovite separate indicate the following i diffusion of sr on the whole rock scale terminated at 344 5 ma 11 point isochron date coincident with closure of muscovite to intracrystalline diffusion of ar apparent plat

how much deep water is formed in the southern ocean three tracers are used to place constraints on the production rate of ventilated deep water in the southern ocean the distribution of the water mass tracer po4 phosphate star in the deep sea suggests that the amount of ventilated deep water produced in the southern ocean is equal to or greater than the outflow of north atlantic deep water from the atlantic radiocarbon distributions yield an export flux of water from the north atlantic which has averaged about 15 sv over the last several hundred years cfc inventories are used as a direct indicator of the current production rate of ventilated deep water in the southern ocean although coverage is as yet sparse it appears that the cfc inventory is not inconsistent with the deep water production rate required by the distributions of po4 and radiocarbon it has been widely accepted that the major part of the deep water production in the southern ocean takes place in the weddell sea however our estimate of the southern ocean ventilated deep water flux is in conflict with previous estimates of the flux of ventilated deep water from the weddell sea which lie in the range 1 5 sv possible reasons for this difference are discussed

synchronization of polar climate variability over the last ice age in search of simple rules at the heart of climate s complexity evidence is presented supporting the hypothesis of polar synchronization which states that during the last ice age and likely in earlier times millennial scale temperature changes of the north and south polar regions were coupled and synchronized the term synchronization as used here describes how two or more coupled nonlinear oscillators adjust their initially different natural rhythms to a common frequency and constant relative phase in the case of the polar regions heat and mass transfer through the intervening ocean and atmosphere provided the coupling as a working hypothesis polar synchronization brings new insights into the dynamic processes that link greenland9s dansgaard oeschger do abrupt temperature fluctuations to antarctic temperature variability it is shown that consistent with the presence of polar synchronization the time series of the most representative abrupt climate events of the last glaciation recorded in greenland and antarctica can be transformed into one another by a 2 phase shift with antarctica temperature variations leading greenland9s this plus the fact that remarkable close simulations of the time series are obtained with a model consisting of a few nonlinear differential equations suggest the intriguing possibility that there are simple rules governing the complex behavior of global paleoclimate

an improved osl chronology for the still bay layers at blombos cave south africa further tests of single grain dating procedures and a re evaluation of the timing of the still bay industry across southern africa this paper presents a series of new single grain optically stimulated luminescence osl ages for the still bay at blombos cave south africa and compares them to previously published osl thermoluminescence tl and electron spin resonance esr ages for this site details are provided about the measurement and analytical procedures including a discussion of the characteristics of the osl signals of individual quartz grains this forms the basis for further investigations into the sensitivity of the equivalent dose de estimates to a range of different analytical approaches including changes in the size of the test dose the choice of signal integration interval the subtraction of an appropriate background and isolation of the most light sensitive fast component of quartz osl we also report the results of an inter operator test of de determination using seven new samples from blombos cave and demonstrate the reproducibility of results obtained for two samples that had been dated previously at another laboratory and were measured and analysed again in this study together these tests validate the robustness of the blombos cave single grain osl age estimates to a variety of alternative osl dating procedures we have incorporated for the first time these ages for blombos cave into a data set of all single grain osl ages for still bay and howieson s poort sites across southern africa and have used a statistical model to re evaluate the timing and duration of the still bay industry we calculate the most plausible start and end dates of the still bay as 72 2 ka and 71 3 ka respectively amounting to a duration of 0 9 ka and estimate with conventional 95 confidence that this industry began no earlier than 75 5 ka ended no later than 67 8 ka and lasted no longer than 6 6 ka

human responses to middle holocene altithermal climates on the north american great plains abstract the climate of the great plains during the middle holocene varied considerably but overall it was marked by a north south gradient of increasingly warmer and drier conditions with a reduction in effective moisture surface water and resource abundance and an increase in resource patchiness sediment weathering erosion and aeolian activity pronounced drought conditions were most evident on the southern high plains understanding the human responses to middle holocene climates is complicated by a lack of archaeological data which is partly a result of geomorphic processes that removed or deeply buried sites of this age and by the varying adaptive responses of hunter gatherers during this period on the southern high plains where drought was most severe surface and groundwater sources dried and bison populations were diminished prompting substantial adaptive changes including local abandonment well digging to tap underground water and a widening of the diet breadth to incorporate higher cost lower return seed and plant resources sites of this age on the central and northern plains also show a possible increase in diet breadth with the incorporation of plant foods in the diet and perhaps changes in settlement mobility including possible shift into higher elevation areas or mapping on to extant rivers and springs but linking those changes to middle holocene drought is less straightforward

origin of late quaternary dune fields on the southern high plains of texas and new mexico mostly stabilized late holocene eolian sands on the southern high plains of the united states were studied to determine their origins and to assess whether present dune stability depends more strongly on sediment supply sediment availability or transport limitations geomorphic sedimentological and geochemical trends indicate that late holocene dunes formed under westerly paleowinds broadly similar to those of today mineralogical and geochemical data indicate that the most likely source for the sands is not the pecos river valley but the pleistocene blackwater draw formation an older extensive eolian deposit in the region these observations suggest that new sand is supplied whenever vegetation cover is diminished to the extent that the blackwater draw formation can be eroded in agreement with modern observations of wind erosion in the region we conclude therefore that southern high plains dunes are stabilized primarily due to a vegetation cover the dunes are thus sediment availability limited this conclusion is consistent with the observation that in the warmest driest part of the region where vegetation cover is minimal dunes are currently active over a large area geochemical data indicate that southern high plains dunes are the most mineralogically mature quartz rich sands yet studied in the great plains which suggests a long history of eolian activity either in the dune fields or during deposition of the blackwater draw formation

age of detrital zircon and titanite in the meguma group southern nova scotia canada clues to the origin of the meguma terrane abstract precise u pb isotopic analysis of seventeen selected high quality laboratory abraded zircon single grains seven multigrain fractions of similar material and four paramagnetic titanite fractions record the ages of the various detrital grain types from two turbidite samples collected from the top and about 5 km below the top of the goldenville formation of the meguma group the results show that the source of both sediments contained 600 ma and 2000 ma old volcanic plutonic components whereas a third 3000 ma old component occurs exclusively in the upper sample and the youngest zircons and titanite are found in the lower sample lead lead ages of bulk fractions at 2300 ma and two nearly concordant single grains at 2040 ma and 2140 ma upper sample together with 2157 ma bulk fraction and 2060 ma and 2080 ma single grain fractions lower sample indicate that the 2000 ma component dominates the pink purple euhedral grain types detrital titanite gives ages of 605 ma upper sample and 552 ma lower sample with the latter sample containing a minor 1975 ma component colourless zircons record this same age difference the lower sample contained the youngest concordant single zircon grain 566 ma and 580 605 ma bulk grain fraction even though two concordant single grains have ages of 632 and 643 ma estimated bulk fraction ages for certain zircon grain types from the upper sample fall in the range of 605 to 675 ma these data suggest that while the youngest rocks in the source region were removed by erosion during the early stages of deposition of the goldenville formation the oldest source rocks were not unroofed until later in the depositional history the 566 8 ma age of the youngest concordant zircon from the stratigraphically lower sample provides a maximum age for the deposition of this grain with the base of the cambrian placed between 545 and 530 ma the goldenville formation may straddle the precambrian cambrian boundary graptolites from the immediately overlying halifax formation are of early ordovician tremadocian age the lower sample contains spherical zircons that may have been derived from mafic granulite with an estimated age of 2000 2200 ma with this one possible exception all of the zircons analyzed appear to be igneous in origin and all are relatively euhedral indicating that rocks of the ages referred to above probably outcrop in the source area anomalous discordance patterns imply that part of the 2960 ma area was metamorphosed by the 2000 ma event and that both the 2960 and 2000 ma regions were metamorphosed during the 600 ma event the complete lack of detritus with ages between 680 and 2040 ma virtually eliminates a northern north american or baltic shield provenance however the available data are not sufficient to distinguish between a west african or guyana shield source