gradient array profiles over thin resistive veins f urness p 1993 gradient array profiles over thin resistive veins geophysical prospecting 41 113 130 gradient array geoelectric surveys are commonly used to explore for buried sheet like targets it is demonstrated that under certain circumstances the responses of such features can become practically equivalent for a range of models differing considerably in resistivity and thickness this is the dual of the familiar equivalence phenomenon associated with thin beds in geoelectric sounding operations hence the geoelectric response profiles over thin dipping resistive veins are conveniently studied by considering equivalent forms with vanishing thickness and infinite resistivity but with the same resistivity thickness product the effects of varying the dip depth of burial depth extent and resistivity thickness product of such features on the gradient array response profiles are investigated by means of a series of numerical experiments these demonstrate several effects of significance to survey design and data interpretation in particular it is found that while the resistivity thickness product influences the amplitude of the response profiles it has little effect on their form a similar behaviour is found to be exhibited by veins with extended depth extent when the dip is varied a complete quantitative interpretation of gradient array profiles over thin dipping resistive veins is thus only possible for veins of limited depth extent for veins with an extended depth dimension the data is ambiguous in a practical sense in terms of the dip and the resistivity thickness product here the interpretation can only be expected to deduce the horizontal and vertical locations of the vein apex

age limits on the late quaternary evolution of the upper loire river abstract the loire river is one of the largest rivers in europe draining a basin area of southern france which exceeds 100 000 km 2 despite its significance the loire remains poorly studied and has a very limited amount of chronological control on the deposited sedimentary record of fluvial activity these factors coupled with the availability of a set of 10 radiocarbon dates on younger c

reconstructing past seawater mg ca and sr ca from mid ocean ridge flank calcium carbonate veins proxies for past seawater chemistry such as mg ca and sr ca ratios provide a record of the dynamic exchanges of elements between the solid earth the atmosphere and the hydrosphere and the evolving influence of life we estimated past oceanic mg ca and sr ca ratios from suites of 1 6 to 170 million year old calcium carbonate veins that had precipitated from seawater derived fluids in ocean ridge flank basalts our data indicate that before the neogene oceanic mg ca and sr ca ratios were lower than in the modern ocean decreased ocean spreading since the cretaceous and the resulting slow reduction in ocean crustal hydrothermal exchange throughout the early tertiary may explain the recent rise in these ratios

late eocene to early miocene ice sheet dynamics and the global carbon cycle paired benthic foraminiferal trace metal and stable isotope records have been constructed from equatorial pacific ocean drilling program site 1218 the records include the two largest abrupt 1 myr increases in the cenozoic benthic oxygen isotope record oi 1 in the earliest oligocene 34 ma and mi 1 in the earliest miocene 23 ma the paired mg ca and oxygen isotope records are used to calculate seawater 18o w calculated w suggests that a large antarctic ice sheet formed during oi 1 and subsequently fluctuated throughout the oligocene on both short 0 5 myr and long 2 3 myr timescales between about 50 and 100 of its maximum earliest oligocene size the magnitudes of these fluctuations are consistent with estimates of sea level derived from sequence stratigraphy the transient expansion of the antarctic ice sheet at mi 1 is marked in the benthic 18o record by two positive excursions between 23 7 and 22 9 ma each with a duration of 200 300 kyr bottom water temperatures decreased by 2 c over the 150 kyr immediately prior to both rapid 18o excursions however the onset of each of these phases of ice growth is synchronous within the resolution of the records with the onset of a 2 c warming over 150 kyr we suggest that the warming during these glacial expansions reflect increased greenhouse forcing prompted by a sudden decrease in global chemical weathering rates as antarctic basement silicate rocks became blanketed by an ice sheet this represents a negative feedback process that might have operated during major abrupt growth phases of the antarctic ice sheet

evidence of cyclic dust deposition in the us great plains during the last deglaciation from the high resolution analysis of the peoria loess in the eustis sequence nebraska usa the peoria loess unit is a well defined stratigraphical unit in the upper pleistocene of the north american quaternary deposited between 30 25 ka and about 12 ka ago it has been indicated that this unit shows the highest known worldwide depositional rate for eolian deposits as its thickness varies near the source area between 19 m and 46 m extreme values that are not even recorded in the chinese sequences the results of our present investigation indicate that this particular unit is not homogenous its shows different subunits where lithological variation can be observed through the occurrence of embryonic gley horizons alternating with laminated loess furthermore the grain size analysis shows cycles corresponding to variations in the eolian dynamics responsible for dust transportation and deposition a grain size index interpreted as characterizing the eolian dynamics higher values corresponding to stronger wind conditions shows higher values than those observed in europe a comparison of this index is proposed with the greenland dust and 18 o records it shows that the main climatic history as corresponding to events occurring mainly in north atlantic domain is recorded in the peoria loess deposits however the variation in the magnitude of the eolian events indicates differences from the european loess sequences the strong north atlantic coolings expressed by heinrich events as recorded in europe by coarser deposits are not differentiated in the studied sequence hence they follow more closely observations obtained off california for the north east pacific domain 2007 elsevier b v all rights reserved

floodplain construction of the rio grande at el paso texas usa response to holocene climate change abstract the rio grande is one of the larger rivers in north america and the development of its floodplain is related to holocene climate and climate change the late pleistocene through early holocene channel is characterized by a meander or braided system with lateral cutting and backfilling resulting in the valley wide deposition of massive to cross bedded fine to medium textured sand the late pleistocene early holocene floodplain is also the sand source for the adjacent bolson sand sheet the sand sheet stopped accumulating new sand 5000 yrs ago an event directly related to the shutting off of the sand supply caused by the deposition of overbank muds that covered and sealed the floodplain surface during the middle holocene the river may have dried intermittently with the floodplain becoming deflated and local sand dunes forming on the floodplain after 5000 yrs the climate was less arid and the river shifted to a regime of increased flooding and overbank deposition of silt and clay by 2500 yrs a late holocene period of wet climate resulted in further overbank deposition and the formation of a cumulic mollisol across the floodplain the socorro paleosol the period of wet climate corresponds to the audubon neoglacial and active rock glaciers in the southern rocky mountains speleothem growth in nearby caves and other evidence for wet cool conditions in the region after 1000 yrs the climate became drier and the deposition and accumulation of overbank muds by the flooding rio grande came to a halt even though the river has flooded often in historic times and presumably during late prehistoric times as well there is little evidence for deposition of overbank sediments on the floodplain since a d 1000 accordingly the present day surface of the lower valley is ten centuries old three channels occur on the us side of the lower valley floodplain and during the past 2500 yrs stream flow has shifted from one to the other by the avulsion process of channel reoccupation although most flow has been in the rio grande channel the largest of the three

reevaluation of the oxygen isotopic composition of planktonic foraminifera experimental results and revised paleotemperature equations cultured planktonic foraminifera orbulina universa symbiotic and globigerina bulloides nonsymbiotic are used to reexamine temperature 18o relationships at 15 25 c relationships for both species can be described by linear equations equations for o universa grown under low light ll and high light hl share a slope of 4 80 0 21 c 1 with a hl ll offset of 0 33 due to symbiont photosynthetic activity the effect of co32 on o universa is 0 002 mol 1 kg 1 and is insensitive to temperature for g bulloides ontogenetic effects produce size related trends in temperature 18o whereby larger shells are enriched in 18o relative to smaller specimens the o universa temperature 18o relationships are more accurate than previously published equations for describing plankton tow data our equations do not explain planktonic core top data with the same precision but provide a good fit to benthic cibicidoides data below 10 c temperature 18o relationships for g bulloides provide good agreement with field data for this species from the northeast pacific

use of radiometric cs 137 pb 210 geomorphic and stratigraphic techniques to date recent oxbow sediments in the rio puerco drainage grants uranium region new mexico in the absence of historic geochemical baseline data for the grants uranium region environmental changes resulting from uranium mine mill activities can be determined only by indirect methods a methodology for determining the age of recent sediments in streams draining the region has been established based on combined geomorphic stratigraphic and radiometric dating techniques because clayrich sediments retain possible radionuclides and heavy metals derived from mineralization and mined sources sample sites which contain fine grained deposits that both predate and postdate mine mill activity were located in abandoned channel segments oxbows of major streams draining the eastern grants uranium region aerial photographs and derivative maps taken between 1935 and 1971 provided the historical and geomorphic documentation of approximate dates of oxbow formation and ages of alluvial fills in the abandoned channel segments pits were dug at these oxbow sites to determine stratigraphy and composition of the deposits samples collected from pit walls and auger holes below the pits were subjected to radiometric analysis by gamma ray spectrometry for the artificial radionuclide cs 137 and the natural radionuclide pb 210 as well as other u 238 and th 232 daughters because of the dynamic nature of the system absolute dating with cs 137 was not possible but samples could be dated as either pre or post 1950 the 1950 date is important because it marked the beginning of the uranium exploitation in the region the pb 210 dating was not possible because background pb 210 was very high relative to fallout pb 210

microstructural and geochemical perspectives on planktic foraminiferal preservation glassy versus frosty in recent years it has become apparent that the cool tropic paradox of paleogene and cretaceous greenhouse climates arises because of the diagenetic alteration of tropical planktic foraminiferal calcite near the seafloor yielding artificially high 18o values because the mg ca compositions of foraminiferal and inorganic calcite are thought to be quite different mg ca measurements should be a sensitive way of monitoring diagenetic alteration here we examine the extent of diagenetic alteration of eocene planktic foraminiferal calcite using scanning electron microscope imaging of foraminiferal test microstructures and geochemical 18o and mg ca analyses we compare microstructural and geochemical characteristics between given species exhibiting two contrasting states of preservation those that appear frosty under reflected light and those that appear glassy microstructural evidence reveals extensive diagenetic alteration of frosty foraminiferal tests at the micron scale while 18o analyses document consistently higher 18o and therefore lower paleotemperatures in this material yet we find that 18o offsets between species in these frosty foraminiferal assemblages appear to be generally preserved suggesting that frosty foraminifera remain valuable for generating relatively short approximately 1 myr paleoceanographic time series that do not demand absolute estimates of paleotemperature we also find that the observed increase in mg ca for planktic foraminifera exhibiting diagenetic alteration compared to glassy taphonomies is far smaller than would be expected from the addition of inorganic calcite based on laboratory derived mg2 partition coefficients our findings imply that a much lower mg2 partition coefficient controls inorganic calcite formation in deep sea sedimentary sections in accordance with the findings of baker et al 1982

effect of ocean gateways on the global ocean circulation in the late oligocene and early miocene 1 we investigate the effect of changes in the tectonic boundary conditions on global ocean circulation patterns using a fully coupled climate model in an idealized setup we compare situations corresponding to the late oligocene the early miocene and present day the model results show the existence of a flow reversal through the panama seaway between the oligocene and miocene this flow reversal is induced by global tectonic changes related to the widening of the southern ocean passages and the closing of the tethys seaway it mainly involves the wind driven ocean circulation in agreement with previous model studies the global thermohaline circulation in the oligocene and miocene simulations is significantly different from the present day conveyor circulation as there is deepwater formation in both the north atlantic and the north pacific oceans in particular in the oligocene simulation the salinity contrast between the atlantic and pacific oceans is reduced because of water mass exchange through the low latitude connections between the two oceans

geochronology and stratigraphy of the bolson sand sheet reply the geology of the bolson sand sheet has been studied extensively especially its relationship to prehistoric sites on the fort bliss military reservation an early systematic evaluation of the stratigraphy and paleosols of the eolian sand was conducted by blair et al 1990 1 who named the q1

oxygen isotope ranking of late eocene and oligocene planktonic foraminifers implications for oligocene sea surface temperatures and global ice volume abstract oxygen isotope analyses of late eocene and oligocene planktonic foraminifers from low and middle latitude sites in the atlantic basin show that different species from the same samples can yield significantly different isotopic values the range of isotopic values observed between species is greatest at low latitudes and declines poleward many planktonic foraminifers exhibit a systematic isotopic ranking with respect to each other and can therefore be grouped on the basis of their isotopic ranking the isotopic ranking of some taxa however appears to vary geographically and or through time isotopic and paleontologic data from dsdp site 522 indicate that commonly used isotopic temperature scales underestimate oligocene sea surface temperatures we suggest these temperature scales require revision to reflect the presence of oligocene glaciation comparison of isotopic and paleontologic data from sites 522 511 and 277 suggests cold low salinity surface waters were present in high southern latitudes during the early oligocene lowsalinity high latitude surface waters could be caused by eocene oligocene paleogeography or by the production of warm saline bottom water

cool wet conditions late in the younger dryas in semi arid new mexico abstract a thick alluvial sequence in central new mexico contains the scholle wet meadow deposit that traces upstream to a paleospring the wet meadow sediments contain an abundant fauna of twenty one species of freshwater and terrestrial mollusks and ten species of ostracodes the mollusks and ostracodes are indicative of a local high alluvial water table with spring supported perennial flow but without standing water pollen analysis documents shrub grassland vegetation with sedges willow and alder in a riparian community stable carbon isotopes from the wet meadow sediments have 13c values ranging from 22 8 to 23 3 indicating that 80 of the organic carbon in the sediment is derived from c3 species the wet meadow deposit is ams dated 10 400 to 9700 14c yr bp corresponding to 12 300 to 11 100 cal yr bp and overlapping in time with the younger dryas event yd the wet meadow became active about 500 yr after the beginning of the yd and persisted 400 yr after the yd ended the scholle wet meadow is the only record of perennial flow and high water table conditions in the abo arroyo drainage basin during the past 13 ka

quaternary continental stratigraphy landscape evolution and application to archeology jarilla piedmont and tularosa graben floor white sands missile range new mexico the upper pleistocene holocene sequence in southeastern white sands missile range was deposited in an eolian sand sheet environment that interacted with gravel depositing arroyos near the jarilla mountain piedmont and with mud depositing arroyo mouth ponds and more extensive lakes on the basin floor the sand sheet contains ridge and swale coppice dune and deflation depression subenvironments three distinctive and locally exhumed buried soils occur within these eolian alluvial and lacustrine deposits the youngest buried soil is characterized by an a to bw to bk stage i profile the second by a btk stage ii to bk stage i profile and the third by a btk stage ii to k stage iii or iv to bk stage ii profile the tops of these buried soils which in most cases are truncated represent easily identifiable regional bounding surfaces they separate the stratigraphic sequence into four units with approximate time spans of 0 to 100 yr b p unit q4 100 to 7300 yr b p unit q3 9400 to 15 000 yr b p unit q2 and about 50 000 to 250 000 yr b p unit q1 lateral variations in unit thickness delineate eolian depocenters and areas of maximal erosion during these time spans the stratigraphic sequence records three cyclic alternations between landscape instability during which erosion and sedimentation took place and landscape stability during which the soil profiles became well developed the initiation of the earliest two episodes of landscape instability represented by erosion of or deposition upon the lower two buried soils is attributed to climatic change toward drier conditions which caused the breakup of the sediment binding vegetative cover the most recent landscape destabilization episode was initiated about 100 yr b p by the destruction of the desert grassland cover by cattle overgrazing prehistoric holocene cultural remains largely are restricted to unit q3 and the surface of unit q2 or to the present surface at localities where units q2 or q1 are exhumed remains found within distinct eolian lag accumulations or within bedded arroyo sediments likely are removed from their original context the degree to which can be estimated by the extent of incision into the soil profile horizons prehistoric habitants occupied a landscape containing the ridge and swale and deflation depression eolian topography but without the current hummocky mesquite coppice dunes

the holocene history of the white sands dune field and influences on eolian deflation and playa lakes abstract white sands national monument is the world s largest gypsum dune field the dunes are downwind of a 20 m deep 19 km wide deflation basin containing large playa lakes today the gypsum sand is derived from the edge of the deflation basin next to the dune field rather than the alkali flat and playa lakes where gypsum crystals are forming three erosional shorelines mark wetter episodes when playa lakes formed in the deflation basin successively younger shorelines formed within older ones as the basin deepened but was not widened by deflation the modern shoreline is forming around lake lucero playa the oldest shoreline termed l1 is degraded and formed near the pleistocene holocene transition deflation from the l1 to the l2 shoreline cut through pleistocene bedded evaporites and probably marks initiation of the dune field this event was before 5840 yr bp based on radiocarbon dates this reinforces an evolving consensus that episodes of deflation have characterized desert basins in the southwestern united states regional deflation events have been dated at 7000 and 4000 yr bp the shorelines imply the white sands dune field was created in short episodes and the modern dune field may not represent conditions during expansion of the dune sea

extended orbitally forced palaeoclimatic records from the equatorial atlantic ceara rise abstract we extend existing high resolution oligocene miocene proxy records from ocean drilling program odp leg 154 the extended record spans the time interval from 17 86 to 26 5 ma the data are age calibrated against a new astronomical solution that affords a re evaluation of the intricate interaction between orbital milankovitch forcing of the climate and ocean system and the fidelity with which this forcing is recorded in oxygen and carbon stable isotope measurements from benthic foraminifera and associated lithological proxy records of magnetic susceptibility colour reflectance and the measured sand fraction our records show a very strong continual imprint of the earth s obliquity cycle modulate in amplitude every 41 ka a very strong eccentricity signal in the carbon isotope records and a strong but probably local imprint of climatic precession on the coarse fraction and magnetic susceptibility records our data allowed us to evaluate how the interaction of long multi million year beats in the earth s eccentricity and obliquity are implicated in the waxing and waning of ice sheets presumably on antarctica our refined age model confirms the revised age of the oligocene miocene boundary previously established by analysis of the lithological data and allows a strong correlation with the geomagnetic time scale by comparison with data from odp site 1090 southern ocean

development of an osl chronology for late pleistocene channel belts in the lower mississippi valley usa abstract the broad alluvial valley of the lower mississippi river contains large channel belts for which age control has been limited by the lack of organic material for radiocarbon dating and deposit ages greater than the limit of radiocarbon in order to produce a detailed chronology of fluvial evolution samples for optical dating were collected from late pleistocene channel belt deposits quartz sand was analyzed using the coarse grained single aliquot regenerative dose sar technique samples showed little evidence of partial bleaching and optical ages are for the most part consistent with the available radiocarbon and relative age constraints optical ages range from 85 to 11 ka and along with geomorphic and stratigraphic relationships indicate that the mississippi river was meandering during the end of the last interglacial 85 83 ka and switched to a braided regime by 64 50 ka during initial marine oxygen isotope stage ois 4 3 glaciation of the upper mississippi drainage basin the mississippi river switched back to a meandering regime after formation of the youngest braid belt at 11 ka the 69 optical ages presented here provide the first detailed chronology of lower mississippi valley braid belt and pleistocene meander belt formation and provide an example of the utility of optical dating in fluvial settings where material for radiocarbon dating is scarce

identifying and characterizing dust point sources in the southwestern united states using remote sensing and gis abstract documenting geomorphology and land use land cover associated with the aeolian dust sources is needed to build more robust models simulating wind erosion and dust emission it is also important to set the base for future studies aiming to quantify the effect of anthropogenic land use change on dust emission in this research dust point sources were detected in the southern great plains and chihuahuan desert regions of the united states u s for 2001 2016 encompassing a period of extreme drought dust points were detected using overlay analysis of brightness temperature difference images of modis and modis true color images for the days where dust plumes were visible this study aims to identify low emission surfaces as well as high emission surfaces in terms of geomorphology and land cover a total of 1508 dust points were detected where 1258 points are located in the southern great plains and 187 points are located in the chihuahuan desert point pattern analysis showed a significant cluster of these points in west texas nearest neighbor ratio 0 33

geophysical signatures of uranium mineralization and its subsurface validation at beldih purulia district west bengal india a case study the beldih open cast mine of the south purulia shear zone in eastern india is well known for apatite deposits associated with nb rare earth element uranium mineralization within steeply dipping altered ferruginous kaolinite and quartz magnetite apatite rocks with e w strikes at the contact of altered mafic ultramafic and granite quartzite rocks a detailed geophysical study using gravity magnetic and gradient resistivity profiling surveys has been carried out over 1 km2 area surrounding the beldih mine to investigate further the dip depth lateral extension and associated geophysical signatures of the uranium mineralization in the environs of south purulia shear zone the high to low transition zone on the northern part and high to low anomaly patches on the southeastern and southwestern parts of the bouguer reducedto pole magnetic and trend surface separated residual gravity magnetic anomaly maps indicate the possibility of highly altered zone s on the northern southeastern and southwestern parts of the beldih mine the gradient resistivity survey on either side of the mine has also revealed the correlation of low resistivity anomalies with low gravity and moderately high magnetic anomalies in particular the anomalies and modeled subsurface features along profile p6 perfectly match with subsurface geology and uranium mineralization at depth two dimensional and three dimensional residual gravity models along p6 depict the presence of highly altered vertical sheet of low density material up to a depth of 200 m the drilling results along the same profile confirm the continuation of uranium mineralization zone for the low density material this not only validates the findings of the gravity model but also establishes the geophysical signatures for uranium mineralization as low gravity moderate tohigh magnetic and low resistivity values in this region this study enhances the scope of further integrated geophysical investigations along the south purulia shear zone to delineate suitable target areas for uranium exploration

eolian landscape evolution and soil formation in the chaco dune field southern colorado plateau new mexico abstract eolian deposits of the chaco dune field cover a broad expanse of the southeastern colorado plateau in new mexico and comprise a large part of the late quaternary sedimentary record in this region the primary eolian landforms include sand sheets parabolic and modified parabolic dunes barchan barchanoid dune complexes and ridge dunes many of the eolian landforms in the chaco dune field have remained stable over a sufficiently long time to permit soil development weakly to moderately developed calcic soils which formed in eolian deposits of mid holocene and latest pleistocene age possess clay enriched weak argillic b horizons these soils have been buried progressively by younger eolian sediments forming regionally correlative bounding surfaces over the chaco dune field and recording relict latest pleistocene and middle holocene eolian landscapes soil geomorphic relations and radiocarbon dated material within the eolian deposits have been useful in 1 delineating three regionally correlative soil stratigraphic units and their associated landforms and 2 studying the types and rates of landscape evolution and climatic changes within this portion of the colorado plateau the most significant period of eolian landform construction is inferred to have occurred during the latest pleistocene glacial to interglacial climatic change forming widespread sand sheets and modified parabolic dunes eolian deposition during the middle and late holocene were dramatically influenced by the presence of previously developed latest pleistocene to early holocene buried soils which may have enhanced sand sheet and parabolic dune development due to the binding effect of the soils middle holocene eolian deposition may be related primarily to neoglacial climatic changes which enhanced ground surface moisture and promoted sand sheet and parabolic dune formation the youngest period of wolian deposition occurred after 1 5 ka and resulted in the formation of the most diverse suite of eolian landforms and deposition of volumetrically more eolian sediments than the mid holocene event in contrast to earlier interpretations the classic mid holocene or altithermal event apparently did not play a major role in the formation of eolian landscapes in the chaco dune field

geochemical assessment of the palaeoecology ontogeny morphotypic variability and palaeoceanographic utility of dentoglobigerina venezuelana to better understand the links between the carbon cycle and changes in past climate over tectonic timescales we need new geochemical proxy records of secular change in silicate weathering rates a number of proxies are under development but some of the most promising e g palaeoseawater records of li and nd isotope change can only be employed on such large samples of mono specific foraminifera that application to the deep sea sediment core archive becomes highly problematic dentoglobigerina venezuelana presents a potentially attractive target for circumventing this problem because it is a typically large 355 m diameter abundant and cosmopolitan planktic foraminifer that ranges from the early oligocene to early pliocene yet considerable taxonomic and ecological uncertainties associated with this taxon must first be addressed here we assess the taxonomy palaeoecology and ontogeny of d venezuelana using stable isotope oxygen and carbon and mg ca data measured in tests of late oligocene to early miocene age from ocean drilling program odp site 925 on ceara rise in the western equatorial atlantic to help constrain the depth habitat of d venezuelana relative to other species we report the stable isotope composition of selected planktic foraminifera species within globigerina globigerinoides paragloborotalia and catapsydrax we define three morphotypes of d venezuelana based on the morphology of the final chamber and aperture architecture we determine the trace element and stable isotope composition of each morphotype for different size fractions to test the validity of pooling these morphotypes for the purposes of generating geochemical proxy datasets and to assess any ontogenetic variations in depth habitat our data indicate that d venezuelana maintains a lower thermocline depth habitat at ceara rise between 24 and 21 ma comparing our results to published datasets we conclude that this lower thermocline depth ecology for the oligo miocene is part of an eocene to pliocene evolution of depth habitat from surface to sub thermocline for d venezuelana our size fraction data advocate the absence of photosymbionts in d venezuelana and suggest that juveniles calcify higher in the water column descending into slightly deeper water during the later stages of its life cycle our morphotype data show that 18o and 13c variation between morphotypes is no greater than within morphotype variability this finding will permit future pooling of morphotypes in the generation of the sample hungry palaeoceanographic records

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

neodymium isotopes in planktonic foraminifera a record of the response of continental weathering and ocean circulation rates to climate change the neodymium nd isotope composition of ancient seawater is a potentially useful tracer of changes in continental inputs and ocean circulation on timescales of a few ka here we present the first nd isotope record for seawater using sedimentary foraminifera cleaned using standard oxidative reductive techniques the data along with mn ca ratios suggest that cleaned foraminifera provide a reliable record of nd in seawater and hold out the prospect of using nd in foraminifera to examine changes in seawater that accompany glacial interglacial climatic cycles the principal potential problem to be overcome with the use of forams as records of trace elements in ancient seawater is their diagenetic fe mn coatings these contain large amounts of nd and other trace elements but can be cleaned off using highly reducing reagents mn ca ratios for the majority of the cleaned sedimentary foraminifera analysed here lie within the range 10 100 mol mol that has yielded success in studies of transition elements in forams mass balance modelling suggests that for residual mn ca ratios 100 mol mol nd added to the foram in the coating will never shift the measured nd isotope composition significantly away from the seawater value acquired by the foram test in the water column additionally nd concentrations measured in cleaned sedimentary foraminifera are comparable with those for a modern sample that has never encountered diagenetic fluids finally core top planktonic foraminifera for two sites have nd isotope compositions that are identical to local surface seawater the data we present here for labrador sea forams over the past 2 5 m y are interpreted in terms of changes in the seawater isotopic composition the data show a pronounced shift from end values of 12 to 19 in the period 2 5 1 5 ma this change is interpreted to result from the initiation of northern hemisphere glaciation and the increased derivation of labrador sea nd via ice rafting from archaean terranes in central canada in combination with stable isotope and foraminiferal relative species abundance data the new nd data are consistent with the surface hydrography of the labrador sea being dominated by a fluctuating balance between cold polar waters containing unradiogenic nd and warm subtropical waters containing more radiogenic nd the major change in labrador sea nd that is observed in the past 2 5 ma can on its own account for the change in the nd isotope composition of north atlantic deep water over the same time period

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

optical and radiocarbon ages of stacked paleosols and dune sands in the nebraska sand hills usa abstract optical ages for eolian sands from the nebraska sand hills indicate periods of extensive eolian activity at ca 115 25 840 70 2300 240 and 3560 340 a activity was also noted at single sampling locations at ca 6180 370 8430 510 and 13110 800 a many of these ages are similar to those noted by earlier authors optical ages from samples collected within paleosols indicate shorter and possibly less extensive periods of eolian activity at approximately 1220 150 1590 110 and possibly 1950 150 a during which the paleosol sands accumulated what was originally interpreted as a single 1 2 m thick paleosol is shown by optical dating to consist of three or more welded soils developed within eolian sands with optical ages of ca 3800 240 2740 240 1560 110 and possibly 1930 140 a each of which match eolian pulses recognized elsewhere scatter in some optical ages is attributable to intersection of sand filled rodent burrows extending in outcrop 1 5 m below the contact between paleosol and overlying topset beds a 5310 360 a optical age for one probable intersected burrow provides evidence for upward or lateral transport of older sands

interpretations and common challenges of aeolian records from north american dune fields abstract geomorphic and chronologic data for dune fields are evaluated for three contrasting areas of north america 1 the prairie parkland and boreal ecozones of the northern great plains in canada 2 the central great plains of the usa and 3 the deserts of southwestern usa and northern mexico luminescence and radiocarbon ages for periods of dune accumulation and stability are compared with palaeoenvironment proxies to provide an assessment of the boundary conditions of dune system response to changes in sediment supply availability and mobility dune fields in the northern great plains were formed from sediment originating from glaciofluvial or glaciolacustrine sediments deposited during deglaciation 16 11 ka subsequent aeolian deposition occurred in parkland and prairie dune fields as a result of mid holocene 8 5 ka and late holocene despite spatial and temporal gaps in chronometric data as a result of sampling biases the record from north american dune fields indicates the strong influence of sediment supply on dune construction with changes in sediment availability as a result of drought episodes resulting in dune field reactivation and reworking of pre existing sediment

glacial interglacial variations in the neodymium isotope composition of seawater in the bay of bengal recorded by planktonic foraminifera this study presents neodymium isotope and elemental data for cleaned planktonic foraminifera from odp site 758 in the southernmost reaches of the bay of bengal in the north east indian ocean cleaning experiments using oxidative reductive techniques suggest that diagenetic fe mn oxyhydroxide coatings can be effectively removed and that the measured nd isotope composition reflects the composition of seawater from which the foraminiferal calcium carbonate was precipitated modern core top pulleniatina obliquiloculata and globorotalia menardii give end values of 10 12 0 16 and 10 28 0 16 respectively indistinguishable from recent direct measurements of surface seawater in this area a high resolution nd isotope record obtained from g menardii for the past 150 kyr shows systematic variations end 3 on glacial interglacial timescales the timing of those variations shows a remarkable correspondence with the global oxygen isotope record which suggests a process controlling the nd isotope composition that responds in phase with global climate cycles palaeoclimate reconstruction indicates that during the last glacial maximum changes in monsoon circulation resulted in a reduction in rainfall over the indian subcontinent and a decrease in the flux of river water delivered to the bay of bengal thus changes in the riverine input of nd a change in either flux or composition most likely caused the isotope variations although changes in dust source or local ocean circulation may have also played a role these results clearly establish a link between climate change and variations in radiogenic isotopes in the oceans and illustrate the potential of nd isotopes in foraminifera for high resolution palaeoceanographic reconstruction

late oligocene to early miocene geochronology and paleoceanography from the subantarctic south atlantic mi2 as well as several other distinctive isotope events our data suggest that the d 18 o maximum commonly associated with the oligocene miocene o m boundary falls within c6cn 2r 23 86 ma the d 13 c maximum coincides within the temporal resolution of our record with c6cn 2n r boundary and hence to the o m boundary comparison of the stable isotope record from site 1090 to the orbitally tuned stable isotope record from odp site 929 across the o m boundary shows that variability in the two records is very similar and can be correlated at and below the o m boundary site 1090 stable isotope records also provide the first deep southern ocean end member for reconstructions of circulation patterns and late oligocene to early miocene climate change comparison to previously published records suggests that basin to basin carbon isotope gradients were small or nonexistent and are inconclusive with respect to the direction of deep water flow oxygen isotope gradients between sites suggest that the deep southern ocean was cold in comparison to the north atlantic indian and the pacific oceans dominance of cold southern component deep water at site 1090 at least until 17 ma suggests that relatively cold circumpolar climatic conditions prevailed during the late oligocene and early miocene we believe that a relatively cold southern ocean reflects unrestricted circumpolar flow through the drake passage in agreement with bathymetric reconstructions index terms 4267 oceanography general paleoceanography 1035 geochemistry geochronology 4870 oceanography biological and chemical stable isotopes keywords paleoceanography 1035 geochronology stable isotopes

lithium isotopes as a probe of weathering processes orinoco river abstract lithium isotopes have the potential to be effective tracers of weathering processes due to their large relative mass difference and therefore fractionation in this study an attempt is made to fill a major gap in the knowledge of li isotope fractionation during continental weathering and of the mechanisms involved finally the relationship between the suspended and dissolved material is made on a basin wide scale the orinoco basin provides a clear contrast in reaction limited and transport limited weathering regimes that has already been documented by a comprehensive study on its fluvial geochemistry edmond et al geochim cosmochim acta 60 1996 2949 2976 edmond et al geochim cosmochim acta 59 1995 3301 3325 conspicuous in our new results is the difference in 6li of the dissolved load between the andean 30 to 22 and shield 22 to 7 tributaries while the 6li of the suspended load is similar between the two to a first approximation during superficial weathering in high relief tectonically active terrains the dissolved load is high in li and isotopically heavy more negative 6li whereas in stable shield regions the concentrations are low and isotopically light in proportion to the increasing degree of weathering

size related stable isotope changes in late cretaceous planktic foraminifera implications for paleoecology and photosymbiosis abstract the paleoecology of cretaceous planktic foraminifera during the late cenomanian to coniacian period 95 86 ma remains controversial since much of the tropical marine record is preserved as chalk and limestone with uncertain geochemical overprints here we present 13c and 18o data from sieve size fractions of monospecific samples of exceptionally well preserved planktic foraminifera recovered during ocean drilling program leg 207 demerara rise western tropical atlantic our results suggest that all species studied hedbergella delrioensis heterohelix globulosa marginotruncana sinuosa whiteinella baltica grew primarily in surface waters and did not change their depth habitat substantially during their life cycle comparison of size related ontogenetic trends in 13c in cretaceous and modern foraminifera further suggests that detection of dinoflagellate photosymbiosis using 13c is confounded by physiological effects during the early stages of foraminifer growth raising doubts about previous interpretations of photosymbiosis in small foraminifera species we propose that obligate photosymbiosis involving dinoflagellates may not have evolved until the campanian or maastrichtian since our survey of cenomanian coniacian species does not find the 18o and 13c size related trends observed in modern foraminifer dinoflagellate symbioses

loess record of the pleistocene holocene transition on the northern and central great plains usa various lines of evidence support conflicting interpretations of the timing abruptness and nature of climate change in the great plains during the pleistocene holocene transition loess deposits and paleosols on both the central and northern great plains provide a valuable record that can help address these issues a synthesis of new and previously reported optical and radiocarbon ages indicates that the brady soil which marks the boundary between late pleistocene peoria loess and holocene bignell loess began forming after a reduction in the rate of peoria loess accumulation that most likely occurred between 13 5 and 15 cal ka brady soil formation spanned all or part of the bolling allerod episode approximately 14 7 12 9 cal ka and all of the younger dryas episode 12 9 11 5 cal ka and extended at least 1000 years beyond the end of the younger dryas the brady soil was buried by bignell loess sedimentation beginning around 10 5 9 cal ka and continuing episodically through the holocene evidence for a brief increase in loess influx during the younger dryas is noteworthy but very limited most late quaternary loess accumulation in the central great plains was nonglacigenic and was under relatively direct climatic control thus brady soil formation records climatic conditions that minimized eolian activity and allowed effective pedogenesis probably through relatively high effective moisture optical dating of loess in north dakota supports correlation of the leonard paleosol on the northern great plains with the brady soil thick loess in north dakota was primarily derived from the missouri river floodplain thus its stratig raphy may in part reflect glacial influence on the missouri river nonetheless the persistence of minimal loess accumu lation and soil formation until 10 cal ka at our north dakota study site is best explained by a prolonged interval of high effective moisture correlative with the conditions that favored brady soil formation burial of both the brady soil and the leonard paleosol by renewed loess influx probably represents eolian system response that occurred when gradual change toward a drier climate eventually crossed the threshold for eolian activity overall the loess paleosol sequences of the central and northern great plains record a broad peak of high effective moisture across the late pleistocene to holocene boundary rather than well defined climatic episodes corresponding to the bolling allerod and younger dryas episodes in the north atlantic region

opening the carbon isotope vital effect black box 1 seasonal temperatures in the euphotic zone recent laboratory experiments with orbulina universa show that the carbon and oxygen isotope signals from this symbiont bearing planktonic foraminifera record distinct environmental parameters irradiance and temperature since light levels are correlated with depth in the marine environment the carbon isotopic composition of o universa can be utilized to differentiate individuals from different depth habitats by combining 18o and 13c data from individuals in an assemblage estimates of seasonal temperature ranges are possible for the upper and lower euphotic zone of the ocean reconstruction of seasonal temperatures using individual holocene o universa from western equatorial pacific ocean boxcore erdc 83 yields ranges of 23 5 to 29 9 c and 20 1 to 24 5 c for the shallow and deep euphotic zone respectively these values are in excellent agreement with recent temperature measurements from this region analyses of individual late glacial 12 000 13 000 years b p o universa from this site suggest the seasonal upper euphotic zone temperature range was approximately 2 5 c lower than it is today a function of cooler summers

the enigma of early miocene biserial planktic foraminifera small biserial foraminifera were abundant in the early miocene ca 18 9 17 2 ma in the eastern atlantic and western indian oceans but absent in the western equatorial atlantic ocean weddell sea eastern indian ocean and equatorial pacific ocean they have been assigned to the benthic genus bolivina but their high abundances in sediments without evidence for dysoxia could not be explained apertural morphology accumulation rates and isotopic composition show that they were planktic genus streptochilus living streptochilus are common in productive waters with intermittent upwelling the widespread early miocene high streptochilus abundances may reflect vigorous but intermittent upwelling inducing high phytoplankton growth rates however export production estimated from benthic foraminiferal accumulation rates was low possibly due to high regeneration rates in a deep thermocline the upwelled waters may have been an analog to subantarctic mode waters carrying nutrients into the eastern atlantic and western indian oceans as the result of the initiation of a deep reaching antarctic circumpolar current active agulhas leakage and vigorous vertical mixing in the southern oceans

paleoindians paleolakes and paleoplayas landscape geoarchaeology of the tularosa basin new mexico abstract karl butzer was one of the first prehistorians to think about and write about people ancient and modern on the landscape moreover as a geomorphologist he knew that past landscapes were as dynamic and varied as they are today not static entities that formed the backdrop for human activity that viewpoint is critical in understanding the physical environment faced by humans during the early peopling of the americas 13 500 to 10 000 cal ybp north america was undergoing rapid environmental changes as the continent was coming out of the last ice age in the southwestern part of the continent large paleolake levels were falling and smaller playa basins were aggrading as the area was occupied by paleoindian groups such as clovis folsom and later first americans the paleolakes would have provided some resources but modern gypsum deposits indicate that lake waters were unpotable numerous small depressions playas scattered around the floor of the tularosa basin contain muddy fills that span the holocene and terminal pleistocene indicative of fresher water at least seasonally paleoindian sites tend to be clustered around these small basins and along drainages off of the mountains that flank the basin as well as settings with good viewsheds rather than the main paleolake margins paleolakes were dramatic components of the late pleistocene landscape in the southwest but likely were not the primary attractions to the first americans

last glacial maximum paleochemistry and deepwater circulation in the southern ocean evidence from foraminiferal cadmium south atlantic benthic foraminiferal cd ca shows no glacial interglacial variation suggesting that the glacial contribution of north atlantic deep water to the southern ocean was not much different than at present in contrast cd ca in southeast indian ridge cores show lower glacial bottom water cd comparable to levels in intermediate depths of the north atlantic and significantly lower than in the deep south atlantic low glacial cd ca was also recorded in planktonic foraminifera suggesting a substantial decrease in the nutrient concentration of subantarctic surface water during the glacial maximum which most likely was caused by increased biological productivity the cd data are inconsistent with low glacial benthic foraminiferal 13c which suggest higher nutrient concentration we propose that the low cd ca in the southeast indian ridge records reflects a local source of nutrient depleted deepwater formed during the last glacial maximum by open ocean convection near the antarctic polar front downstream of the kerguelene plateau if this source was limited to the southeast indian basin then its impact on the overall chemistry of glacial circumpolar deepwater was rather small however if during glaciations open ocean convection became the dominant mode of bottom water formation it might have had a greater impact on cpdw chemistry

history of carbonate ion concentration over the last 100 million years abstract instead of having been more or less constant as once assumed it is now apparent that the major ion chemistry of the oceans has varied substantially over time for instance independent lines of evidence suggest that calcium concentration ca2 has approximately halved and magnesium concentration mg2 approximately doubled over the last 100 million years on the other hand the calcite compensation depth and hence the caco3 saturation has varied little over the last 100 my as documented in deep sea sediments we combine these pieces of evidence to develop a proxy for seawater carbonate ion concentration co32 over this period of time from the calcite saturation state which is proportional to the product of ca2 times co32 but also affected by mg2 we can calculate seawater co32 our results show that co32 has nearly quadrupled since the cretaceous furthermore by combining our co32 proxy with other carbonate system proxies we provide calculations of the entire seawater carbonate system and atmospheric co2 based on this reconstructed atmospheric co2 is relatively low in the miocene but high in the eocene finally we make a strong case that seawater ph has increased over the last 100 my

a weathering hypothesis for glaciation at high atmospheric pco2 during the late ordovician new paired carbonate and organic carbon isotope analyses from nevada usa together with a consideration of the effects of mountain building and ice sheet coverage of the continents on atmospheric pco2 lead to a new hypothesis for the cause of the late ordovician glaciation we suggest that the taconic orogeny which commenced in the late middle ordovician caused a long term decline in atmospheric pco2 through increased weatherability of silicate rocks ice sheet growth was triggered when pco2 decreased to a threshold of 10 present atmospheric level and proceeded by positive ice albedo feedback in the midst of glaciation atmospheric pco2 began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets at first this enhanced greenhouse effect was overcompensated for by ice albedo effects ultimately however atmospheric pco2 reached a level which overwhelmed the cooling effects of ice albedo and the glaciation ended the isotope results can be interpreted to indicate that atmospheric pco2 rose during the glaciation consistent with other proxy information although alternative interpretations are possible the large positive carbonate isotope excursion observed in late ordovician rocks around the world is explained as the expected response to increased carbonate platform weathering during glacioeustatic sea level lowstand rather than as a response to increased organic carbon burial

calibration of mg ca thermometry in planktonic foraminifera from a sediment trap time series 1 paired mg ca and 18o measurements on planktonic foraminiferal species g ruber white g ruber pink g sacculifer g conglobatus g aequilateralis o universa n dutertrei p obliquiloculata g inflata g truncatulinoides g hirsuta and g crassaformis from a 6 year sediment trap time series in the sargasso sea were used to define the sensitivity of foraminiferal mg ca to calcification temperature habitat depths and calcification temperatures were estimated from comparison of 18o of foraminifera with equilibrium calcite based on historical temperature and salinity data when considered together mg ca mmol mol of all species except two show a significant r 0 93 relationship with temperature t c of the form mg ca 0 38 0 02 exp 0 090 0 003 t equivalent to a 9 0 0 3 change in mg ca for a 1 c change in temperature small differences exist in calibrations between species and between different size fractions of the same species o universa and g aequilateralis have higher mg ca than other species and in general data can be best described with the same temperature sensitivity for all species and pre exponential constants in the sequence o universa g aequilateralis g bulloides g ruber g sacculifer other species this approach gives an accuracy of 1 2 c in the estimation of calcification temperature the 9 sensitivity to temperature is similar to published studies from culture and core top calibrations but differences exist from some literature values of pre exponential constants different cleaning methodologies and artefacts of core top dissolution are probably implicated and perhaps environmental factors yet understood planktonic foraminiferal mg ca temperature estimates can be used for reconstructing surface temperatures and mixed and thermocline temperatures using g ruber pink g ruber white g sacculifer n dutertrei p obliquiloculata etc the existence of a single mg thermometry equation is valuable for extinct species although use of species specific equations will where statistically significant provide more accurate evaluation of mg ca paleotemperature

geochronology of the bolson sand sheet new mexico and texas and its archaeological significance discussion as part of an archaeology study on the fort bliss military reservation in west texas hall et al 2010 1 reported optically stimulated luminescence osl dates from two backhoe trenches referred to as localities 100 and 200 in both trenches a sedimentary unit termed q3 was designated

climate driven glacial interglacial variations in the osmium isotope composition of seawater recorded by planktic foraminifera abstract this study presents osmium os isotope and elemental data for cleaned planktic foraminifera authigenic fe mn oxyhydroxides and pelagic carbonate host sediments from odp site 758 in the southernmost reaches of the bay of bengal the os in the bulk sediments appears to be dominantly hydrogeneous sourced by carbonate and fe mn oxyhydroxide but variations in this particular core are controlled by the presence of volcanic ash fe mn oxyhydroxide leachates of the bulk sediments from holocene samples also yield an os isotope composition close to that of seawater but the record diverges from that of foraminifera at a depth corresponding to the oxic post oxic boundary suggesting diagenetic mobilization of os at depths below this holocene planktic foraminifera cleaned using oxidative reductive techniques also give os isotope compositions indistinguishable from modern seawater but the record obtained for the past 150 kyr shows strong covaraitions of 187 os 188 os with both the local and global oxygen isotope record with less radiogenic os isotope compositions during glacial intervals these results indicate that foraminifera provide a robust record of seawater os isotope compositions and comparison of the data obtained here with records from the other major oceans demonstrate global changes in 187 os 188 os over this time interval while the covariation with oxygen isotopes suggest a process controlling the os isotope composition that is in phase with global climate cycles global excursions to relatively unradiogenic 187 os 188 os during glacial intervals are consistent with decreased input of radiogenic continental material reflecting cooler temperatures and reduced continental runoff modelling indicates that the shift to unradiogenic values during glacial intervals could be caused by an 30 decrease in the global river flux with an 5 change in river composition if the residence time of os in the oceans is 5 ka then the post glacial recovery to present day seawater values is consistent with a corresponding increase in the river flux of around 30 however if the residence time of os is closer to 40 ka as is suggested by the global river flux then this demands either significant changes in both the riverine os flux and composition of around 40 and 30 respectively that closely follow the oxygen isotope record or else a short lived post glacial pulse of weathering some 75 greater than the steady state flux in either case these results clearly indicate that climatic changes affect both the flux and composition of weathered material delivered to the oceans on glacial interglacial timescales

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

berino paleosol late pleistocene argillic soil development on the mescalero sand sheet in new mexico abstractthe berino paleosol is the first record of a directly dated aridisol in the american southwest where paleoclimatic conditions during the time of pedogenesis can be estimated the noncalcic argillic paleosol formed in eolian sand during the cool wet climate of the mid and late wisconsin marine isotope stages 3 and 2 in presently semiarid southeastern new mexico optically stimulated luminescence dating of the mescalero sand sheet and the berino indicates that soil formation occurred during the period 50 18 ka the paleosol is a red 2 5yr hue bt horizon 120 cm thick with 25 clay 0 36 fe and an absence of visible carbonate it is buried by younger eolian sand although at the edges of the sand sheet it is unburied and a relict soil red argillic paleosols in other sand sheets in the region may correlate with the berino the berino paleosol is formally named as a pedostratigraphic unit

white sands dune field new mexico age dune dynamics and recent accumulations abstract the white sands dune field situated within the tularosa basin in southern new mexico is thought to have been largely derived by a stepwise progressive deflation of pleistocene lake otero strata with the onset of regional aridity optically stimulated luminescence osl dating of samples from a core that penetrated the gypsum accumulation of the dune field confirm a time of origin at 7000 yr dune sediment is characterized as lagged influx from previously stored lake otero sediment and contemporaneous influx derived from subsequent playas sediment became available for aeolian transport and dune field construction because of a falling water table driven by the regional aridity the current dune field is primarily a wet aeolian system in which the behavior of the accumulation surface over time is a function of the water table although surface cementation by gypsum also imparts aspects of a stabilizing system dune crests are oriented borderline transverse to the annual transport resultant and a monitored dune and its cross strata show that transverse winds from the sw during the late winter and spring account for most of the crest normal migration but a significant along crest component of migration of dune sinuosity occurs with fall and winter winds from the nw and n that strike the crests obliquely trenches across three interdune areas show dune sets and interdune strata climbing at about 0 1 depth of interdune scour increases with interdune streamwise length which acts to enhance the probability of dune cross strata deflation during arid years but also increases the probability of interdune accumulation because the water table shows a net rise over time within the trenches dune sets show a bundling of foresets between reactivation surfaces interpreted as annual cycles that reveal lee face reworking by the oblique nw and n winds and slipface progradation fostered by transverse winds from the sw a progression from grainflow dominated to wind ripple dominated cross strata within a single set revealed in the longest trench is consistent with along crest migration of dune sinuosity interdune laminations show light dark couplets interpreted as annual varves reflecting dry wet portions of the year the number of laminations preserved in vertical sections across an interdune area shows that the record is incomplete and endorses the interpretation of periods of interdune deflation a calculation of the current accumulation rate as based upon the climbing strata within the trenches is significantly greater than the long term accumulation rate determined by osl dates taken from the core in addition to compaction and possible dissolution of gypsum as partial explanations the long term accumulation rate may reflect significant variation in the accumulation rate over time short term deflation of accumulations on a regular basis and or the presence of unconformities within the gypsum body

what do we know about the evolution of mg to ca ratios in seawater 1 although most reconstructions of the evolution of the mg to ca ratio in seawater conclude that it has increased during the course of the cenozoic they disagree widely regarding the magnitude of this change on the basis of fluid inclusion and caco3 mineralogy observations the increase was at least threefold on the basis of mg content of foraminifera shells it was only a factor of 1 7 a recently published reconstruction based on the mg content of calcite fillings of voids in ridge flank basalts lends support to the conclusion that the change was severalfold but as it is very difficult to come up with a plausible geologic scenario which could account for such a large change we lean toward the smaller estimate based on the magnesium content of foraminifera shells

low latitude sea surface temperatures for the mid cretaceous and the evolution of planktic foraminifera isotopic data from exceptionally well preserved foraminifera show that the subtropical north atlantic had sea surface temperatures ssts between 30 and 31 c during the late albian and early cenomanian these temperatures were warmer than average modern temperatures in the tropical western atlantic and support previous inferences based on diagenetically suspect material of albian tropical ssts of 30 c this finding is consistent with basic theories of oceanic thermal response in the tropics to the high atmospheric carbon dioxide levels during the middle cretaceous greenhouse however our data do not support the concept of a super tethyan region of hypersaline waters in the western north atlantic during late albian and early cenomanian time because seawater salinities substantially higher than modern ones would require significantly higher ssts than those reported here isotopic analysis of mid cretaceous planktic foraminifera shows that planomalina ticinella and biticinella grew in surface waters and provide isotopic proxies for sst whereas deeper growing species include hedbergella rotalipora and praeglobotruncana none of these taxa display isotopic indications of photosymbiosis suggesting that this ecology did not evolve in planktic foraminifera before the late cretaceous contrary to previous hypotheses none of these species increased their depth habitat during their life cycles unlike many extant planktic foraminifera the rotaliporids may have been preferentially susceptible to extinction during oceanic anoxic events because they lived within or below the thermocline for most of their life cycles

osl age and stratigraphy of the strauss sand sheet in new mexico usa abstract the strauss sand sheet occurs in south central new mexico usa and northern chihuahua mexico covering an area of about 4740 km2 its chronology is determined by 19 osl ages the sand sheet formed primarily during three phases of eolian deflation and deposition each phase with a separate sand source and under different climatic and environmental circumstances the first phase of eolian sedimentation occurred 45 to 15 ka with the deposition of unit 1 the sand source for the first phase was beach related features along the eastern shoreline of pluvial lake palomas in mexico the glacial age climate was cool wet and windy because of the southern path of the jet stream at that time after 15 ka with the onset of warmer conditions of the bolling allerod the shutting down of the palomas sand source and wet conditions of the younger dryas the sand sheet stabilized with weak soil development in unit 1 by 11 ka the climate shifted to holocene drying conditions and the second phase of sand accumulation began forming unit 2 the sand source was the local deflation of the previously deposited unit 1 sand the sand sheet stabilized again by 1 9 ka with slightly wetter late holocene climate a weak soil formed in unit 2 sand about a d 1500 and extending to about a d 1850 or later an a horizon formed on the sand sheet probably in response to a desert grassland vegetation during the period of wet climate of the little ice age in an anthropogenic third phase of eolian activity after a d 1850 the vegetation was likely disturbed by overgrazing and the unit 2 and a horizon unit 3 sands were deflated resulting in the deposition of a thin layer of massive eolian sand unit 4 across the sand sheet by about a d 1900 mesquite shrubs had increased in abundance and deflated sand largely from unit 2 began to accumulate around the shrubs forming coppice dunes unit 5 mesquite coppice dunes continued to increase in number and volume during the twentieth century and at present dominate most of the sand sheet this third phase of eolian deflation deposition is ongoing today

faunal burrows as indicators of paleo vegetation in eastern washington usa abstract the palouse loess deposits in the pacific northwestern united states contain buried paleosols with distinctive biological fabrics the washtucna soil formed 40 000 15 000 yr ago during the last full glacial contains up to 90 by volume distinctive cylindrical pedotubules that are 1 2 cm in diameter formed by burrowing fauna the overlying post glacial loess and modern surface soils that support perennial grasslands lack these cylindrical burrows we investigated the paleoecological significance of the burrows observations of several types of burrowing fauna under native vegetation indicated that nymphs of cicadas homoptera cicadidae are responsible for cylindrical back filled burrows that match those in the washtucna soil we measured the abundance of active cicada burrows in soils in four native vegetation zones soil sites establishing that cicada burrows comprise 19 of the rooting zone volume in sagebrush steppe but just 1 3 of the rooting zone volume in bunchgrass steppe meadow steppe and coniferous forest this suggests strongly that abundant cicada burrows in paleosols are a proxy for the geographic extent of plant communities that contained sagebrush to assay the activity of cicada nymphs through time we measured burrowed volume by depth from the soil surface down through the washtucna soil i e from the present to ca 40 000 yr bp at five stratigraphic research sites paleosol sites cicada burrowed volume is generally less than 5 in the rooting zone of the surface soils and throughout holocene loess but is up to 94 in the rooting zone of the washtucna soil at paleosol sites that currently support bunchgrass steppe outside the present day sagebrush steppe zone cicada host preferences in vegetation zones today suggest that sagebrush was the dominant shrub as the washtucna soil formed from this reconstruction combined with information from studies of pollen and plant opal we infer that periglacial steppe plant communities dominated by sagebrush spread from 40 000 to 15 000 yr bp into areas of the columbia plateau that support bunchgrass steppe meadow steppe and even some coniferous forest today trace fossils of soil fauna in paleosols are indicators of fluctuations in biological activity in response to climatic changes of the quaternary period in palouse loess deposits

interpretation and depth of investigation of gradient measurements in direct current geoelectrics gradient measurements in a homogeneous electrical primary field can easily be interpreted for simple models the simplified solution conducting or resistant body in a homogeneous space in a homogeneous electrical field is often sufficiently accurate as comparisons with the exact solution body of finite resistivity in a homogeneous half space in a quasihomogeneous electrical field show the exact geometry of the body cannot be determined by gradient measurements the same anomaly of apparent resistivity can be caused by different bodies in particular the similarity between a sphere and a cube of the same volume is very high there is a distinct influence of the resistivity of the overburden the higher this resistivity is the stronger is the effect caused by a buried body if a deviation of 10 of the apparent resistivity is assumed as the lower boundary at which a buried body can be detected by gradient measurements the depth of investigation for a three dimensional body is approximately equal to its width in the two dimensional case the thickness of the overburden can be twice the width if the overburden has a resistivity which is higher than the resistivity of the substratum these depths are greater the greatest possible depth is approximately three times the width of the body

geochronology of the bolson sand sheet new mexico and texas and its archaeological significance the bolson sand sheet occurs in the tularosa valley new mexico and the hueco bolson texas and it consists of two principal eolian sand units optically stimulated luminescence osl dating provides a new chronology of the sand sheet that relates as well to the formation preservation and visibility of the local archaeological record the lower sand unit q2 44 8 2 9 ka and the upper sand unit q3 22 2 1 6 5 2 0 3 ka have a combined thickness of less than 2 m the q2 sand is characterized by a red bt paleosol and the q3 sand has a weak calcic paleosol with stage i carbonate morphology elevated amounts of airborne silt were incorporated in the q3 sand during the period 24 14 5 ka representing higher amounts of dust in the atmosphere during glacial and late glacial time multiple osl ages from the q3 sand indicate a slow net sedimentation rate of 0 06 0 09 mm yr similar to other osl dated sand sheets in the region sand deposits in dune fields have higher accumulation rates the mcgregor a horizon soil radiocarbon dated to younger than 0 5 ka occurs at the top of the sand sheet and likely formed with desert grassland vegetation thousands of recent mesquite coppice dunes unit q4d mantle the sand sheet and two are dated to the twentieth century by osl and 137 cs archaeological sites that postdate 3000 b c are concentrated sometimes together on the surface of the sand sheet while sites that predate 3000 b c although rare may be buried within the q3 eolian sand the q2 sand is too old to contain archaeological sites although site features may intrude into the sand previous chronologies of the sand sheet are based on radiocarbon dates of charcoal from archaeological sites on radiocarbon dates of soil carbonate and on soil geomorphology correlations with rio grande valley alluvium the optical chronology does not support these various correlations we recommend that the alluvial names isaacks9 ranch fillmore and organ no longer be applied to the bolson sand sheet

the relationship between shell size and mg ca sr ca δ18o and δ13c of species of planktonic foraminifera the relationship between shell size and mg ca sr ca 18o and 13c of planktonic foraminiferal calcite has been investigated using seventeen species in six different size fractions mg ca increases and sr ca decreases with increasing size except for two globorotaliid species which show the opposite trend the changes in mg ca broadly follow 18o calcification temperatures except that surface and near surface dwelling species show larger changes in mg ca than can be accounted for by differences in calcification temperature derived from 18o the increases in mg ca and decreases in sr ca vary linearly with the well established increase in 13c with size this is consistent with smaller individuals calcifying faster than larger individuals and larger individuals forming calcite that more closely reflects seawater temperature and composition it appears that variations in calcification rate affect mg and sr shell chemistry the observations may account for part of the temporal variability in foraminiferal sr ca within a single size fraction that has been attributed to changes in seawater sr ca

orbitally induced climate and geochemical variability across the oligocene miocene boundary to assess the influence of orbital scale variations on late oligocene to early miocene climate and ocean chemistry high resolution 5 kyr benthic foraminiferal carbon and oxygen isotope and percent coarse fraction time series were constructed for ocean drilling program site 929 on ceara rise in the western equatorial atlantic these time series exhibit pervasive low to high frequency variability across a 5 myr interval 20 5 25 4 ma the records also reveal several large scale secular variations including two positive 1 6 oxygen isotope excursions at 22 95 and 21 1 ma suggestive of large but brief glacial maxima mi 1 and mi 1a events of miller et al 1991 and a long term cyclical increase in the carbon isotopic composition of seawater shift of 1 52 that reaches a maximum coincident with peak 18o values at 22 95 ma lower resolution 25 kyr records constructed from benthic and planktonic foraminifera as well as bulk carbonate at a shallower site on ceara rise site 926 for the period 21 7 24 9 ma covary with site 929 18o values reflecting changes in antarctic ice volume likewise covariance among carbon isotopic records of bulk sediment benthic and planktonic foraminifera suggest that the low frequency cycles 400 kyr and long term increase in 13c values represent changes in the mean carbon composition of seawater co2 the time series presented here constitute the longest most continuous and highest resolution records of pre pliocene climate and oceanography to date the site 929 carbon and oxygen isotope power spectra show significant concentrations of variance at 400 100 and 41 kyr demonstrating that orbitally induced oscillations have been a normal characteristic of the global climate system since at least the oligocene including periods of equable climate and times with no apparent northern component water production

geochemical and mineralogical evidence from eolian sediments for northwesterly mid holocene paleowinds central kansas usa a prominent 4500 km2 dune eld in the great plains is the great bend sand prairie of south central kansas dunes here overlie late quaternary alluvium and were reactivated extensively in the late holocene geomorphic and soil evidence suggests that the most likely eolian sand source is the arkansas river valley to the northwest nevertheless orientations of stabilized dunes indicate that the most recent dune forming winds came from the south or southwest in agreement with modern wind data mineralogy and trace element concentrations in eolian sands of the great bend sand prairie are similar to those of the arkansas river which permits the arkansas river as a sediment source ca and sr abundances which re ect small amounts of carbonate minerals are higher in arkansas river sand compared to eolian sands and show a systematic depletion away from the arkansas river to the southeast these trends are likely due to carbonate mineral depletion downwind from abrasion and size reduction thus paleowinds probably were northwesterly during initial deposition northwesterly winds occur today when dry paci c derived air is dominant we hypothesize that the residence time of this air mass was much greater while dunes initially formed possibly during a warmer and drier mid holocene period 2000 elsevier science ltd and inqua all rights reserved