deep sea carbonates reading the carbon isotope signal the carbon isotope signal in deep sea sediments reflects a mix of 1 global changes in the rates of exchange of the ocean s carbon reservoir with biosphere soil and sediments 2 global and regional changes in surface water productivity 3 internal shifts in water mass structure and circulation basin basin fractionation oxygen minimum development and 4 organism specific fractionation effects due to changes in micro habitat and or ontogenic fractionation vital effects additional complications arise from differential preservation it is impossible to entirely isolate these various factors as a rule of thumb long period signals that are parallel for planktonic and benthic data reflect external global fractionation patterns whilst short period signals are more likely tied to internal patterns water mass fractionation the various approaches to interpretation are illustrated with three case studies the glacial holocene transition the messinian carbon shift and the miocene monterey excursion

middle holocene aeolian activity on the high plains of west central kansas the uppermost surficial stratigraphic unit south of the arkansas river in the kansas high plains is a previously unidentified middle holocene or younger loess the unit fits both a loess thickness and a particle size model for loess distribution with increasing distance from source a soil immediately below this unit is radiocarbon dated 6000 to 6700 years bp the radiocarbon ages indicate that the loess unit is younger than the commonly reported ranges for bignell loess in the great plains and demonstrate a need for re examining holocene loess stratigraphy of the great plains in some locations closest to the arkansas valley source this middle holocene unit is overlain by or interfingers with dune sands the presence of these aeolian sands indicates that following a period of relative landscape stability and soil development a shift toward a middle holocene climate in which aeolian processes dominated occurred about 6000 years ago on the high plains of west central kansas

continental shelf upwelling and benthic ostracoda in the benguela system southeastern atlantic ocean abstract the distribution of benthic ostracoda micro crustacea on the continental shelf off southwestern africa is controlled by sea floor physical and chemical parameters which in turn can be correlated with the positions of quasi permanent upwelling cells of the benguela system the linkage between benthic and surface physico chemical environments and consequently between benthos and surface parameters is not direct however being modulated by equatorward motion in the upper water column benthic poleward water motion fluvial input and cross shelf advection of off shelf water these phenomena and in particular the intensity of surface upwelling show a sharp distinction about 27 s so that the ostracod faunas in the northern benguela region are relatively sparse and dominated by palmoconcha walvisbaiensis hartmann and cytherella namibensis dingle while in the southern benguela region the faunas are more diverse and dominated by pseudokeijella lepralioides brady and ruggieria cytheropteroides brady when minor taxa are also considered each cell in the benguela upwelling system is shown to be associated with a unique ostracod fauna which can be related to its in particular environmental characteristics

stable isotope fractionation in benthic foraminifera from the peruvian continental margin abstract stable isotope ratios of the carbonate tests of benthic foraminifera are now widely used to reconstruct past changes in ocean circulation and chemistry such studies depend on the accuracy with which changing environmental conditions are recorded and preserved in the tests of foraminifera found in deep sea deposits assessment of this aspect of the method is best accomplished through rigorous study of modern core tops collected from a wide range of depths and bottom water temperatures stable isotope analyses of eighteen taxa of benthic foraminifera from surface sediments of the peru continental margin indicate that most foraminifera are slightly depleted in 18o relative to the benthic foram uvigerina peregrina for at least five taxa we observed subparallel 18 o temperature relationships with slopes nearly equal to that predicted by shackleton 1974 for uvigerina for seven taxa isotopic analysis of six size fractions in the range 210 590 mm reveals that with the exception of u peregrina variation between different size classes for each species is small for 18 o and for 13 c a slight inverse correlation between foraminiferal 18o and 13c content implies a complex interplay of biological and environmental factors which control isotopic composition

radiocarbon dating of soil organic matter radiocarbon ages of soil organic matter are evaluated with a model which incorporates the dynamics of the14c content of soil organic matter measured14c ages of soil organic matter or its fractions are always younger than the true ages of soils due to continuous input of organic matter into soils differences in soil c dynamics due to climate or soil depth will result in significantly different14c signatures of soil organic matter for soils of the same age as a result the deviation of the measured14c age from the true age of soil formation could differ significantly among different soils or soil horizons our model calculations also suggest that14c ages of soil organic matter will eventually reach a steady state provided that no climatic or ecological perturbations occur once a soil or a soil horizon has reached a steady state 14c dating of soil organic matter will provide no useful information regarding the age of the soil however for soils in which steady state has not been reached it is possible to estimate the age of soil formation by modeling the measured14c contents of soil organic matter radiocarbon dating of buried soils could in general overestimate the true age of the burial by as much as the steady state age of the soil or soil horizon

holocene dune formation at ash meadows national wildlife area nevada usa abstract small isolated dune fields in the northern mojave desert are important centers of biodiversity and archaeological occupation sites currently dunes at ash meadows nevada are stabilized by vegetation and are experiencing erosion of their upwind margins indicating a negative sediment budget new osl ages from dunes at ash meadows indicate continuous eolian accumulation from 1 5 to 0 8 ka with further accumulation around 0 2 ka prior studies e g mehringer and warren 1976 indicate periods of dune accumulation prior to 3 3 ka 1 9 1 ka and after 0 9 ka these periods of eolian accumulation are largely synchronous with those identified elsewhere in the mojave desert the composition of the ash meadows dunes indicates their derivation from regional fluvial sources most likely during periods when axial washes were active as a result of enhanced winter precipitation

biogeochemistry of lipids in surface sediments of the peru upwelling area at 15 s abstract organic carbon lycopane a monoene of 2 6 10 trimethyl 7 3 methyl butyl dodecane c37 alkenones and 20 5 and 22 6 polyunsaturated fatty acid concentrations in two box cores from the peru continental margin area at 15 s in 90 and 268 m water depth have depth profiles that are synchronous and appear to indicate an historical record of fluctuating higher productivity lower productivity enso periods the u37k ratio calculated using c37 2 and c37 3 alkenones in the 268 m water depth core in the center of the upwelling zone varies with depth in the core in a manner consistent with the u37k functioning as an historical recorder of euphotic zone temperature concentrations and cpi for n alkanes c25 c31 are reported for both cores a c27 anthrasteroid has been identified and quantified in the 268 m water depth core

loess record of dry climate and aeolian activity in the early to mid holocene central great plains north america thick holocene bignell loess sections provide new evidence for the timing of early to middle holocene aridity in the central great plains the immediate source of loess in these sections was dune fields just upwind based on thickness trends and grain size data thus periods of rapid loess accumulation indicate episodes of extensive dune activity under drier than present climate in typical bignell loess sections a thick zone of coarse textured loess with minimal pedogenic alteration is interpreted as a record of the most rapid sustained holocene loess accumulation optical ages indicate that loess in this zone was deposited beginning about 9000 10000 years ago at five study sites accumulation ended shortly after 6500 years ago at three sites but possibly earlier at the other two in two well dated sections the average rate of loess accumulation between 9000 10000 and 6500 years ago was about 1 6 3 5 times greater than in the late holocene thus we infer that the most extensive sustained holocene

star and linear dunes on mars abstract a field containing 11 star and incipient star dunes occurs on mars at 8 8 s 270 9 w examples of linear dunes are found in a crater at 59 4 s 343 w while rare dune varieties that form in bi and multidirectional wind regimes are not absent from the surface of mars the occurrence of both of these dune fields offers new insight into the nature of martian wind conditions and sand supply the linear dunes appear to have formed through modification of a formerly transverse aeolian deposit suggesting a relatively recent change in local wind direction the 11 dunes in the star dune locality show a progressive change from barchan to star form as each successive dune has traveled up into a valley into a more complex wind regime the star dunes corroborate the model of n lancaster 1989 progr physical geogr 13 67 91 1989 sedimentology 36 273 289 for the formation of star dunes by projection of transverse dunes into a complex topographically influenced wind regime the star dunes have dark streaks emanating from them providing evidence that the dunes were active at or near the time the relevant image was obtained by the viking 1 orbiter in 1978 the star and linear dunes described here are located in different regions on the martian surface unlike most star and linear dunes on earth both martian examples are isolated occurrences neither is part of a major sand sea previously published mars general circulation model results suggest that the region in which the linear done field occurs should be a bimodal wind regime while the region in which the star dunes occur should be unimodal the star dunes are probably the result of localized complication of the wind regime owing to topographic confinement of the dunes local topographic influence on wind regime is also evident in the linear dune field as there are transverse dunes in close proximity to the linear dunes and their occurrence is best explained by funneling of wind through a topographic gap in the upwind crater wall

stratigraphic evidence for late quaternary dune activity near hudson on the piedmont of northern colorado stabilized dune fields are common features near hudson on the piedmont of northern colorado exposures in dune and interdune sites expose a sequence of eolian sediments and paleosols that record a complex history of eolian activity during the late quaternary radiocarbon and thermoluminescence age estimates on a horizons buried by eolian sand indicate that dunes were reactivated sometime between 7 and 9 ka on the basis of morphology of surface soils the dunes were most recently stabilized 3 ka at present there are no data to indicate if there were multiple periods of dune movement and stabilization during the holocene the penultimate pre holocene dune forming episode probably terminated ca 13 ka and may be coincident with the pinedale glaciation the stratigraphy at interdune sites shows at least two eolian depositional events prior to the penultimate event they were separated by periods of pedogenesis one of which may have exceeded 40 ka this analysis indicates that dunes in northern colorado were active during both cold arid and warm arid periods in the late quaternary

dinoflagellate cysts paleoproductivity and upwelling systems a late cretaceous example from israel abstract diverse assemblages of dinoflagellate cysts occur in coniacian maastrichtian sedimentary sequences in israel which represent the inner shallower and outer deeper seaward belt of a late cretaceous upwelling system along the margin of the southern tethys the assemblages are composed mainly of peridinioid p and gonyaulacoid g cysts in varying concentrations it is suggested that the observed fluctuations in the numerical peridinioid gonyaulacoid p g cyst ratio reflect mainly changes in upwelling intensity and productivity in the studied area high p g values represent a higher productivity and vice versa thep g curve indicates a generally high productivity during the campanian followed by a decrease during the maastrichtian the inner part of the upwelling region is characterized by higherp g values representing a higher productivity the similarity between thep g ratio curve and the foraminiferal based paleoproductivity curve from the same sections supports the utility of thep g ratio as a reliable productivity indicator applied here for the first time to upper cretaceous sediments

global variations of surface ocean productivity in low and mid latitudes influence on co2 reservoirs of the deep ocean and atmosphere during the last 21 000 years based on detailed reconstructions of global distribution patterns both paleoproductivity and the benthic 13c record of co2 which is dissolved in the deep ocean strongly differed between the last glacial maximum and the holocene with the onset of termination i about 15 000 years ago the new export production of low and mid latitude upwelling cells started to decline by more than 2 4 gt carbon year this reduction is regarded as a main factor leading to both the simultaneous rise in atmospheric co2 as recorded in ice cores and with a slight delay of more than 1000 years to a large scale gradual co2 depletion of the deep ocean by about 650 gt c this estimate is based on an average increase in benthic 13c by 0 4 0 5 the decrease in new production also matches a clear 13c depletion of organic matter possibly recording an end of extreme nutrient utilization in upwelling cells as shown by sarnthein et al 1987 the productivity reversal appears to be triggered by a rapid reduction in the strength of meridional trades which in turn was linked via a shrinking extent of sea ice to a massive increase in high latitude insolation i e to orbital forcing as primary cause

holocene landscape response to seasonality of storms in the mojave desert abstract new optically stimulated and radiocarbon ages for alluvial fan and lake deposits in the mojave desert are presented which greatly improves the temporal resolution of surface processes the new mojave desert climate landscape record is particularly detailed for the late holocene evidence from ephemeral lake deposits and landforms indicates times of sustained stream flow during a wet interval of the latter part of the medieval warm period at ca ad 1290 and during the little ice age at ca ad 1650 the former lakes postdate megadroughts of the medieval warm period whereas the latter match the maunder minimum of the little ice age periods of alluvial fan aggradation across the mojave desert are 14 9 cal ka and 6 3 cal ka this timing largely correlates to times of increased sea surface temperatures in the gulf of california and enhanced warm season monsoons this correlation suggests that sustained alluvial fan aggradation may be driven by intense summer season storms these data suggest that the close proximity of the mojave desert to the pacific ocean and the gulf of california promotes a partitioning of landscape process responses to climate forcings that vary with seasonality of the dominant storms cool season pacific frontal storms cause river flow ephemeral lakes and fan incision whereas periods of intense warm season storms cause hillslope erosion and alluvial fan aggradation the proposed landscape process partitioning has important implications for hazard mitigation given that climate change may increase sea surface temperatures in the gulf of california which indirectly could increase future alluvial fan aggradation

eolian dynamics and sediment mixing in the gran desierto mexico determined from thermal infrared spectroscopy and remote sensing data the gran desierto dune field is only partially composed of quartz rich sands from the ancestral colorado river local sources have been previously underestimated as a major source of sand because previous remote sensing studies were limited in their capability to detect silicate minerals compositions of sands were evaluated in this study using a combination of laboratory thermal emission spectroscopy and thermal remote sensing data acquired from the advanced spaceborne thermal emission and reflection radiometer aster instrument the spatial interpolation of sample compositions allowed visualization of the sand transport pathways from feldspar rich local sources by revealing gradients of composition between the dune field and surrounding local sources the laboratory data were comparable to the remote sensing retrievals of quartz and feldspar abundance the mineralogical maturity of the desierto dunes could be determined by the quartz feldspar ratio therefore providing a context for understanding the provenance of the gran desierto in relation to other mojave and sonoran dune fields the composition of a previously undescribed group of dunes east of the pinacate the sonoyta dunes was measured as higher in potassium feldspar relative to the rest of the dune field the composition of sonoyta dunes is characteristic of other mojave dune fields which are more isolated near local feldspar rich sources south of the pinacate quartz rich sand from the west admixes with feldspar rich sand from the sonoyta dunes to the east the northern margin of the gran desierto is similarly enriched in feldspar from alluvial fans and the coastal sand is influenced by carbonate sand that does not appear to survive transport to the inland dunes

alluvial sediment or playas what is the dominant source of sand and silt in desert soil vesicular a horizons southwest usa 1 vesicular a av soil horizons form beneath desert pavements from the accretion of aeolian sediment dust commonly thought to be derived primarily from desiccating pluvial lakes and playas with contributions from ephemeral washes and alluvial fans particle size distributions of av horizons are typically bimodal with primary modes of very fine silt and fine sand suggesting that the horizon matrix is derived from multiple sources here we conduct detailed chemical and physical analysis of both av horizon soil samples and potential sources of aeolian sediment to better constrain the relative contributions of dust associated with the development of av horizons geochemical data from both sand 125 250 m and silt 2 32 m fractions in av horizons and potential dust sources in the eastern mojave desert and western sonora desert usa point to large contributions from nearby sources including distal alluvial fans and washes and comparably lower contributions from regional sources such as playas the silt mode is derived from suspension transport of dust and the fine sand mode is derived from saltating sand the desiccation of pluvial lakes in the mojave desert is commonly believed to have driven episodes of aeolian activity contributing to sand dunes and av horizon formation we propose that alluvial fans and washes are underappreciated as desert dust sources and that pulses of dust from late pleistocene and holocene alluvial fans dwarfed pulses of dust from desiccating pluvial lakes in the eastern mojave desert

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

paleoenvironmental and geoarchaeologic implications of late quaternary sediments and paleosols north central to southwestern san juan basin new mexico abstract soil sediment relationships from the west fork of gallegos canyon near farmington nm to the manuelito plateau northwest of gallup nm provide insight into the late pleistocene and holocene history of the north central to southwestern san juan basin soil formation suggests six widespread episodes of land surface stability the oldest and strongest paleosol is in eolian alluvial and playa sediments that were buried between 9560 and 7800 14 c yrs b p the second oldest paleosol developed within eolian and alluvial sediments and was buried at ca 4500 14 c yrs b p subsequent eolian alluvial and playa deposits contain a third paleosol that was buried ca 3100 14 c yrs b p in turn these sediments are mantled by eolian alluvial and playa sediments that exhibit remnants of a fourth paleosol that was buried by ca 2100 14 c yrs b p a fifth paleosol within eolian and alluvial sediments is associated with basketmaker iii pueblo iii cultural materials dating from 1300 to 660 14 c yrs b p the sixth soil sediment unit consists of eolian and alluvial deposits that date from 660 14 c yrs b p to the present and exhibits the modern soil in addition a very weak paleosol that was buried at ca 300 14 c yrs b p was encountered at a few locales because of discontinuous burial and preservation not all of these units are present at every locale within the study area several locales across the basin however exhibit five to six of these units in stratigraphic succession the widespread occurrence of these paleosols suggests that six episodes of regional stability and soil formation occurred in the western san juan basin during the holocene

the zero velocity layer migration from irregular surfaces seismic data acquired in areas with irregular topography are usually corrected to a flat datum before migration a time honored technique for handling elevation changes is to time shift the data before application of migration this simple time shift or elevation static correction cannot properly represent wide angle or dipping reflections as they would have been recorded at the datum as a result when elevation varies significantly accuracy in event positioning may be compromised for migration and other wave equation processes such as dip moveout processing dmo traditionally such over and under migration artifacts have been dealt with by increasing or decreasing the migration velocity however simple adjustment of the migration velocity cannot undo the wave field distortions induced in seismic data acquired over varying elevations more sophisticated and accurate solutions such as wave equation datuming are too computationally demanding for routine use here we propose an efficient and accura

sources and paleoclimatic significance of holocene bignell loess central great plains usa abstract this paper reexamines the stratigraphy sources and paleoclimatic significance of holocene bignell loess in the central great plains a broadly similar sequence of loess depositional units and paleosols was observed in thick bignell loess sections up to 300 km apart suggesting that these sections record major regional changes in the balance between dust deposition and pedogenesis new optical ages together with previously reported radiocarbon ages indicate bignell loess deposition began 9000 11 000 yr ago and continued into the late holocene some bignell loess is

comparison of quaternary coastal upwelling proxies off central peru abstract three apc advanced hydraulic piston core holes of odp leg 112 from the shelf sites 681 and 680 and upper slope site 686 off the central peruvian coast contain a productivity and coastal upwelling record of the last 400 000 yrs marine planktonic diatom sediment assemblages are used as primary productivity proxies their downcore record is compared to variations in other proxies weight percent organic carbon percent large diatoms in the coarse fraction 13c of the endobenthic foraminiferbolivina seminuda and the geochemical parameter uk37 unsaturation index of c37 alkenonones low values of calculated primary paleo productivity are matched by low values of organic carbon but the percentages of diatoms in the coarse fraction do not follow this trend levels of 13c correspond to neither the diatom productivity proxy nor the organic carbon proxy the geochemical index uk37 seems to indicate high surface water temperatures whereas the calculated primary productivity indicates increased upwelling and thus lower temperatures

common shot fresnel beam migration based on wave field approximation in effective vicinity under complex topographic conditions gaussian beam migration is a versatile imaging method for geologically complex land areas which overcomes the limitation of kirchhoff migration in imaging multiple arrivals and has no steep dip limits of one way wave equation migration however its imaging accuracy depends on the geometry of gaussian beam that is determined by the initial parameter of dynamic ray tracing as a result its applications in exploration areaswith strong variations in topography and near surface velocity are limited combined with the concept of fresnel zone and the theory of wave field approximation in effective vicinity we present a more robust common shot fresnel beam imaging method for complex topographic land areas in this paper compared with the conventional gaussian beam migration for irregular topography our method improves the beam geometry by limiting its effective half width with fresnel zone radius moreover through a quadratic travel time correction and an amplitude correction that is based on the wave field approximation in effective vicinity it gives an accurate method for plane wave decomposition at complex topography which produces good imaging results in both shallow and deep zones trials of two typical models and its application in field data demonstrated the validity and robustness of our method

where is the zero velocity layer the zero velocity layer was introduced in higginbotham et al 1985 to increase the maximum dip imaging capability of finite difference depth migration beasley and lynn 1992 adapted the idea to improve the imaging again using finite difference depth migration of seismic data acquired in areas of irregular topography beasley and lynn s application improves upon the conventional method of processing which is to time shift the data from the acquisition surface to a horizontal datum and then migrate using the near surface velocity above the surface and the best estimate of seismic velocity below the surface the conventional procedure typically produces artifacts in the shallow part of the section that are characteristic of overmigration to reduce these artifacts velocities are often reduced for the migration step the use of the zero velocity layer overcomes the need to adjust the migration velocities here a component of the migration velocity is set to zero in the layer between the datum and th

holocene eolian activation as a proxy for broad scale landscape change on the gila river indian community arizona abstract eolian sediments are common within the middle gila river valley southern arizona and reflect variability in eolian and fluvial processes during the late holocene this study focuses on deciphering the stratigraphic record of eolian deposition and associated luminescence dating of quartz extracts by single aliquot regeneration sar protocols stratigraphic assessment coupled with luminescence ages indicates that there are four broad eolian depositional events at ca 3145 220 yr 1950 1360 yr 800 100 yr and 690 315 yr this nascent chronology correlated with regional archeological evidence and paleoclimate proxy datasets leads to two general conclusions 1 loess deposits transverse dune formation and sand sheet deposition in the late holocene are probably linked to flow variability of the gila river though the last two events are concordant with regional megadroughts and 2 the stability of eolian landforms since the 19th century reflects the lack of eolian sediment supply during a period of fluvial incision resulting in entisol formation on dunes the prime catalyst of eolian activity during the late holocene is inferred to be sediment supply driven by climate periodicity and variable flow within the gila river catchment

late quaternary stratigraphy and luminescence geochronology of the northeastern mojave desert abstract the chronology of the holocene and late pleistocene deposits of the northeastern mojave desert have been largely obtained using radiocarbon ages our study refines and extends this framework using optically stimulated luminescence osl to date deposits from valjean valley silurian lake playa red pass and california valley of particular interest are eolian fine silts incorporated in ground water discharge gwd deposits bracketed at 185 140 and 20 50 ka alluvial fan deposits proved amenable for osl by dating both eolian sand lenses and reworked eolian sand in a matrix of gravel that occurs within the fan stratigraphy lacustrine sand in spits and bars also yielded acceptable osl ages these osl ages fill gaps in the geochronology of desert deposits which can provide data relevant to understanding the responses of several depositional systems to regional changes in climate this study identifies the most promising deposits for future luminescence dating and suggests that for several regions of the mojave desert sediments from previously undated landforms can be more accurately placed within correct geologic map units

peruvian coastal upwelling late quaternary productivity changes revealed by diatoms a transfer function pdu 1 relating modern marine diatom sediment surface assemblages to a 10 year averaged surface water primary productivity map spanning a range of 0 1 1 0 g c m2 day is used to interpret downcore variation in productivity at three sites site 680 681 and 686 of odp leg 112 off peru variation in coastal upwelling determined by the transfer function method and by qualitative assessment using smear slides from each site were staked into one composite record for the last 400 000 years this record revealed the following productivity at 11 and 13 5 s was highly variable during the last 400 000 years with the strongest coastal upwelling phases occurring during the following oxygen isotope stages middle of 1 upper part of 3 lower part of 5 the 67 and 78 transition lower part of 8 upper and lower part of 9 and middle part of 11 significant decreases in upwelling occurred during the following oxygen isotope stages lower part of 1 4 upper part of 5 upper part of 7 upper part of 9 the 1011 transition and the upper part of 11 the overall decreases and increases in paleoproductivity are not in phase with the general glacial interglacial cycles the variability of coastal upwelling is more complex and largest swings in the magnitude of paleoproductivity seem to occur across oxygen isotope stage boundaries the periods of highest paleoproductivity are characterized by excellent diatom assemblage preservation high abundance of skeletonema and delphineis and varved or finely laminated sediments periods when paleoproductivity was weaker on the other hand are characterized by bioturbated sediments poor diatom preservation and high abundance of azpeitzia coscinodiscus nodulifer and pseudoeunotia doliolus

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

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

molecular stratigraphy a new tool for climatic assessment variations in sea surface temperatures over the past 500 000 years are inferred from the relative abundance behaviour of two organic compounds c37 alkenones over the upper 8 metres of a sediment core from the eastern equatorial atlantic this molecular record ascribed to contributions from prymnesiophyte algae correlates well with the variations in the 18 signal for the calcareous skeletons of certain planktonic foraminifera thus providing the first demonstration of a new stratigraphical technique which may be especially valuable where methods based on carbonate 18 fail

efficient migration through complex water bottom topography an efficient means of imaging structures beneath complex water bottom topography is obtained using a conventional time migration algorithm with a simple modification to the migration velocity field the process consists of two migration steps one with the migration velocity set to zero below the water bottom and the other with the migration velocity set to zero above the water bottom between the two steps the data are vertically time shifted to account for the lateral velocity variations between the water sediment interface the time shifts used are equivalent to the so called thin lens term used in depth migration algorithms efficiency is obtained by applying the thin lens term only once and by using computationally optimized time migration algorithms results obtained from this technique are nearly identical to more costly wave equation layer replacement and depth migration techniques

large scale reorganization of ocean currents offshore western australia during the late quaternary abstract the history of coastal surface currents in the eastern indian ocean over the last 130 kyr has been studied through the application of the fi 2 transfer function to planktic foraminifera in deep sea cores in a n s transect offshore western australia the concept of sea surface temperature sst gradient residuals is the amount by which the sea surface temperature gradient of a given time differs from that in the modern sea the sst gradient residuals for several episodes of climate extremes in the late quaternary the last glacial maximum 20 kyr b p the last interglacial maximum 122 120 kyr b p and at the end of the penultimate glaciation 130 kyr b p indicate that areas north of 18 s off western australia have changed very little during the extremes in climate over the last 130 kyr while south of 18 s there have been significant changes in surface water temperatures large areas of anomalously cool surface water were established off north west cape 22 s 24 s during both glacial and interglacial extremes these reflect the increased influence of the west australian current on the surface water circulation during the last glacial maximum and at the end of the penultimate glaciation cold sea surface temperature anomalies were established near to the coast indicating that the leeuwin current did not warm the coastal zone as it does today during the last interglacial maximum the leeuwin current flowed as an intensified current in a narrow zone close to the coast but did not significantly warm surface waters offshore as far as 20 25 s where a zone of cold water was maintained

coastal upwelling and atmospheric co2 changes over the last 400 000 years peru abstract a transfer function relating diatom assemblages in surface sediments and primary production in the photic zone was used to calculate variations in primary production in hole odp leg 112 site 681a over the last 400 kyr primary production off central peru was enhanced during peak glaciations and it decreased during peak interglacials but low and high production periods also occurred in both glacials and interglacials the close resemblance of the primary production curve off peru to the atmospheric co2 vostok record suggests a relationship between the peruvian neritic biological pump and atmospheric pco2

cenozoic marine sedimentation in the sechura and pisco basins peru abstract the central and northern peruvian margin consists of a series of 8 paired forearc basins which may be separated into an inner set of shelf basins and a seaward set of slope basins we have examined the cenozoic stratigraphy of the onshore portions of the sechura basin 5 7 s and pisco basin 13 16 s two shelf basins which have accumulated marine sediment discontinuously since the mid to late eocene cenozoic sediments in the pisco basin were deposited during at least three major transgressive cycles each sequence is preserved as a similar vertical progression of facies including coarse nearshore bioclastic conglomerates and sandstones grading upwards into sandy siltstones and mudstones and capped by biogenic deposits including diatomites diatomaceous mudstones dolomitic horizons and phosphate deposits stratigraphic nomenclature for the pisco basin has recently evolved a stratigraphy presented here includes the eocene caballas fm upper eocene los choros fm upper eocene to lowermost oligocene yumaque fm uppermost oligocene to middle miocene chilcatay fm and upper miocene to pliocene pisco fm major hiatuses in the pisco basin span the late cretaceous to middle eocene early to late oligocene middle miocene and late pliocene pleistocene to recent cenozoic sediments of the sechura basin were deposited within at least 4 major transgressive cycles with hiatuses during the paleocene to middle eocene oligocene early to middle miocene and late miocene based on recent biostratigraphic studies sediments enriched in biogenic components accumulated between about 40 36 ma 24 16 ma and 11 3 ma in the pisco basin and between 40 37 ma and 8 5 4 5 ma in the sechura basin in both basins the most diatomaceous sediments are restricted to the late eocene and late miocene through pliocene the temporal distribution of biogenic sediments suggests that high productivity conditions linked to coastal upwelling have occurred episodically since at least the late eocene the occurrence of diatomites and phosphorites is diachronous between the pisco and sechura basins and between the peruvian forearc and other circum pacific monterey formation analogs a reflection of the strong influence of local tectonism on sedimentation patterns the volume of neogene sediments along the peruvian forearc is nearly twice that of the monterey fm despite basin to basin facies diachroneity these deposits very likely contributed to fluctuations of the late miocene carbon co 2 system by acting as large carbon sinks

benthic foraminiferal δ13c and accumulation rates of organic carbon uvigerina peregrina group and cibicidoides wuellerstorfi the glacial to interglacial 13c records of the benthic foraminifera cibicidoides wuellerstorfi and the uvigerina peregrina group from deep sea cores cannot be adjusted by a generally valid constant the 13c values of the u peregrina group largely correlate with the accumulation rates of organic carbon suggesting a local habitat effect those of c wuellerstorfi vary independently with respect to the carbon flux and record fluctuations in the 13c of the ambient bottom water isotopic composition

major neogene events of the southeastern pacific the chilean and peruvian record abstract according to the biochronostratigraphic model proposed the neogene marine deposits in chile and peru represent two independent transgressive sequences alloformations separated by a major regional hiatus the earlier sequence nseps i would stretch from the top of the early miocene up to the base of the late miocene ca 19 10 m y b p the upper sequence nseps ii would correspond in turn to the late pliocene ca 3 5 1 6 m y b p the regional hiatus separating both sequences would encompass the late miocene and the early pliocene ca 10 3 5 m y b p the planktonic foraminiferal events described suggest an age of terminal early miocene middle miocene and late pliocene respectively the paleoclimatic events recorded are three a warming event lasting from the terminal early miocene to the end of the middle miocene mmwe 18 11 5 m y b p a cooling event of the middle late pliocene lpce 2 5 0 2 m y b p and warming event spanning most of the late pliocene lpwe 3 5 2 2 m y b p restricted to the surface and intermediate sea waters of the middle and low latitudes the last paleoclimatic event not recorded as such so far is both synchronous and mutually exclusive in latitudinal terms with the second one three tectonic regional events are analyzed the quechuan 1 phase 19 1 5 m y b p the quechuan 2 phase 10 1 m y b p and the geographic rhodanian phase 3 5 0 5 m y b p in the studied area there is no late miocene marine sedimentary record to confirm the terminal tortonian or quechuan 3 tectonic phase ca 7 m y b p admitted to occur in the volcanoclastic continental sequences of northern chile and southern peru