magnetic links among lava flows tuffs and the underground plumbing system in a monogenetic volcano derived from magnetics and paleomagnetic studies abstract a combined study using magnetics and paleomagnetism of the toluquilla monogenetic volcano and associated lavas and tuffs from valsequillo basin in central mexico provides evidence on a magnetic link between the lavas ash tuffs and the underground volcanic conduit system paleomagnetic analyses show that the lava and ash tuff carry reverse polarity magnetizations which correlate with the inversely polarized dipolar magnetic anomaly over the volcano the magnetizations in the lava and tuff show similar southward declinations and upward inclinations supporting petrological inferences that the tuff was emplaced while still hot and indicating a temporal correlation for lava and tuff emplacement modeling of the dipolar anomaly gives a reverse polarity source magnetization associated with a vertical prismatic body with southward declination and upward inclination which correlates with the reverse polarity magnetizations in the lava and tuff the study documents a direct correlation of the paleomagnetic records with the underground magmatic conduit system of the monogenetic volcano time scale for cooling of the volcanic plumbing system involves a longer period than the one for the tuff and lava suggesting that magnetization for the source of dipolar anomaly may represent a long time average as compared to the spot readings in the lava and tuff the reverse polarity magnetizations in lava and tuff and in the underground source body for the magnetic anomaly are interpreted in terms of eruptive activity of toluquilla volcano at about 1 3 ma during the matuyama reverse polarity c1r 2r chron

the benguela current and associated upwelling on the southwest african margin a synthesis of the neogene quaternary sedimentary record at dsdp sites 362 and 532

geology and quaternary environments of the first preglacial palaeolithic sites found in alberta canada abstract a pebble tool industry including two chronologically different stone artifact assemblages reminiscent of the eurasian palaeolithic has been recorded in late pleistocene sections at two locations in the bow river valley southwestern alberta authenticity and provenance of the deeply buried archaeological record is evidenced by culture diagnostic percussion flaked artifacts incorporated in preglacial fluvial gravels and overlying glacial diamictons and by identical textural patterns on stone tools found in and eroded from the exposures geological context suggests a fluctuating braided river setting during the earlier occupation discarded lower series quartzite and hard carbonate rock artifacts subglacially entrained into the cordilleran bow valley till document distortion of the earlier site silver springs by a valley glacier emerging from the rocky mountain ice lobe following the valley deglaciation a later occupation episode is manifested by a formally analogous flaked lithic assemblage excavated in situ on top of the till at a nearby site varsity estates this more recent occupation surface was subsequently buried under 24 m of glaciolacustrine sediments after submergence of the river valley by a proglacial lake glacial lake calgary dammed by the laurentide ice advance into the eastern calgary area implying a minimum early late wisconsinan age ca 21 000 bp for the lithic industry the presence of the later upper series artifact assemblage and the associated palynological data do not support the view that envisages an extremely cold inhospitable glacial environment on the eastern slopes of the canadian rocky mountains throughout the late wisconsinan their stratigraphic position also indicates temporal asynchroneity between cordilleran and laurentide ice during the last glacial maximum in the bow river valley the area of presumed coalescence of the two ice masses although a more rapid response of the western mountain glacier to climatic change is evident the apparent absence of pedogenic alteration of the till surface and the fresh appearance of the excavated stone artifacts suggest that a short time span separated the two glacial events the archaeological record provides evidence of an earlier palaeo american peopling of western interior canada long before the emergence of the final pleistocene palaeoindian cultures characterized by elaborate bifacial stone projectile point flaking technologies traditionally regarded as the earliest cultural manifestations in north america silver springs is the first early site on the continent found below glacial deposits realization that other american palaeolithic sites potentially of considerable antiquity should be recognized in similar geological settings and introduction of adequate geoarchaeological site survey techniques have crucial relevance for elucidation of the earliest new world prehistory

the development of antarctic glaciation and the neogene paleoenvironment of the maurice ewing bank abstract a micropaleontologic magnetostratigraphic and sedimentologic analysis of 56 piston cores provides the basis of a geologic study of the late miocene to holocene depositional and erosional history of the intermediate depth maurice ewing bank meb located at the eastern extremity of the falkland malvinas plateau southwest atlantic ocean sedimentation on the meb is controlled presently by the position of the polar front and strong bottom currents beneath the axis of the antarctic circumpolar current acc fluctuations in the position of the polar front and in the intensity of the acc are inferred to have largely controlled deposition on the meb since the initiation of acc flow over the bank in the miocene the sedimentary record of the meb is used to infer the late miocene to holocene oceanographic and climatic conditions in the south atlantic sector of the southern ocean the meb suffered major erosion by intense acc flow during late miocene time which exposed cretaceous miocene sediment near the surface and shaped the present configuration of the bank this erosional event is inferred to have occurred during late miocene to early pliocene time 7 2 4 7 m y b p with the major phase of erosion occurring between middle magnetostratigraphic chron 7 and late chron 6 7 2 6 2 m y b p we suggest that the miocene sedimentary record of the meb and other paleoenvironmental evidence from the circum antarctic region precludes the existence of a west antarctic ice sheet or extensive ice shelves along the antarctic margin prior to the late miocene during the late miocene a west antarctic ice shelf formed in the former west antarctic sea and rapidly thickened until it grounded below sea level to form the west antarctic ice sheet formation of this ice sheet with floating and partially grounded extensions ice shelves in the ross and weddell embayments led to the first major production of true antarctic bottom water formed in a manner similar to today which permanently altered global abyssal circulation late miocene changes in oceanic ccd levels and a permanent shift in the oceanic 13 c 12 c composition may be attributed to this major change in abyssal circulation a decrease in acc velocity during an early pliocene amelioration of climate led to resumption of deposition on the bank from 4 7 to 3 9 m y b p intensification of the acc during the late gilbert and early gauss chrons again resulted in limited deposition and widespread erosion and or non deposition over most of the meb from 4 0 to 3 2 m y b p acc velocity apparently began to wane during the middle of the gauss chron and allowed widespread deposition on the bank during late gauss time 2 9 2 48 m y b p and over more limited areas during earliest matuyama time 2 48 to about 2 2 m y b p during middle to late matuyama time bottom current intensity reached its greatest post miocene level forming a regional disconformity between sediments of about 2 0 and 1 0 m y in age this increased circumpolar circulation and associated erosion is inferred to have peaked during the late matuyama chron 1 2 1 0 m y b p and is approximately the same age as the tentatively dated greatest patagonian glaciation in nearby southern argentina between 1 0 and 0 7 m y b p the bank was blanketed with a coarse erosion resistant layer of ice rafted detritus which armored the older sediment thereby protecting it from major subsequent erosion sedimentation on the meb during the bruhnes chron 720 000 yr b p present was intermittent and finally culminated with the deposition of a veneer of carbonate ooze during or since latest pleistocene time

early mid pleistocene environments in the valsequillo basin central mexico a reassessment the valsequillo basin in central mexico has been of interest due to the presence of megafauna and evidence for early human occupation but research has been controversial it has been suggested that extensive and deep lakes characterised the early pleistocene environment but sediment exposure is highly fragmentary and reliable dating has been difficult here we report for the first time early pleistocene palaeoenvironmental reconstructions using stable isotopes diatoms tephra and pollen we studied several stratigraphic sections of mainly non volcanic rocks containing the 1 3 ma xalnene ash as a stratigraphic marker the isotope and other proxy data show that topographically low points in the basin were occupied by spring fed shallow water lakes during the early mid pleistocene with a trend to drier conditions the basin was a dynamic volcaniclastic environment during this period with the production of the toluquilla volcano sequence and other rhyolitic dacitic volcanic ashes interbedded with the lake sediments at the sections studied there is no evidence from the sections for extensive and deep lakes before or after the xalnene ash deposition the presence of lakes in the basin during the early pleistocene would have made it attractive for megafauna

the 23 500 y 14 c bp white pumice plinian eruption and associated debris avalanche and tochimilco lava flow of popocatepetl volcano mexico abstract the white pumice wp is one of the thickest and most voluminous plinian fallouts produced by popocatepetl volcano in central mexico during the late pleistocene holocene its eruption 23 500 14 c y bp 27 800 cal bp was triggered by the catastrophic failure of the sw flank of the volcano the resulting debris avalanche was highly mobile reaching 72 km from the cone with an apparent coefficient of friction l h of 0 06 the deposit covers an area of 1200 km 2 and has a volume of 10 4 km 3 this gigantic landslide characterized by exceptionally large proximal hummocks 400 m provoked the sudden decompression of the hydrothermal and magmatic systems which produced an initial blast followed by the rise of a plinian column that reached an altitude of 33 km the isopach map allows the recognition of a dispersal axis pointing toward the south where an area of 2490 km 2 was covered by 10 cm of pumice and ash the total volume of the pumice fallout was estimated at 1 9 km 3 dre dense rock equivalent pumice clasts are dacitic 62 66 wt sio 2 anhydrous basis highly vesicular 55 88 vol and display a seriate texture with phenocrysts of plagioclase hornblende augite hypersthene oxides ti magnetite and ilmenite apatite as the eruption advanced discharge rates became more intermittent and the height of the column fluctuated and finally collapsed generating pumice and ash flows that were emplaced around the volcano this short but intense activity was followed during subsequent years by rain induced lahars that reached great distances from the volcano at the same time more degassed andesitic dacitic 61 65 wt sio 2 magma was erupted effusively 4 4 km 3 dre in the new horseshoe shaped 5 km wide crater from which the tochimilco lava flow descended toward the sse where it inundated an area of 68 km 2 and reached as far as 22 km from its source since then multiple eruptions have reconstructed the summit cone almost completely obliterating the horseshoe shaped crater during the course of this catastrophic eruption vei 5 a total volume of 6 3 km 3 dre of juvenile magma pumice and lava were emitted and at least an additional 10 km 3 of pre existing rocks debris avalanche mobilized it surpasses in magnitude most other known plinian eruptions from popocatepetl and can be envisaged as an example of a worst case scenario for hazard evaluation purposes it dramatically changed the morphology of the volcano and had profound and far reaching effects beyond its immediate vicinity along rivers draining surrounding plains as well as on the lacustrine basins e g chalco to the ne a repeat of such an eruption in this densely populated area would certainly cause a major calamity of unprecedented dimensions

no evidence for extreme long term warming in early pliocene sediments of the southern ocean abstract during the past 10 years there has been considerable debate with respect to the response of the antarctic ice sheets to early pliocene warmth interpretations generally fall into one of three categories 1 there was a major drawdown of the east antarctic ice sheet eais 2 drawdown of eais was relatively small 3 eais was largely unaffected by early pliocene warmth here this question is approached by using biogenic opal and carbonate in deep sea sediments as proxy for paleoceanographic change during the present day these measures are largely dictated by the position of the antarctic polar front apf with biogenic carbonate exhibiting an increase north of the apf and biogenic opal increasing to the south that the carbonate opal transition zone is a reliable feature in deep sea sediments is evidenced by the fact that it is recorded north of its present position during the last glacial maximum lgm and to the south of it during oxygen isotope stage 11 a warmer than present interglacial when these proxies are applied to southern ocean sediments high frequency glacial interglacial changes are seen in the pliocene and early pleistocene what appears to be a major glacial is recorded at about 4 5 myrs b p three short duration carbonate highs up to 18 suggesting a southward movement of apf occur at discrete intervals between about 4 6 4 8 myrs b p however these disappear or become reduced in amplitude toward the continent such data do not support any appreciable warming of surface waters near the antarctic continent neither does it support any claims of high circa 20 c sea level air temperatures over the eais it is concluded that there is no evidence for significant drawdown of eais during early pliocene warmth

on the late pleistocene ocean geochemistry and circulation a box model of the atmosphere and ocean was developed to investigate how geochemical distributions extant during the late pleistocene may have come about the model simulates the regional distribution of calcium carbonate dissolution as well as the chemical oceanography and atmospheric co2 13c and radiocarbon if the downward biological flux of particulate carbon increases by a factor of 2 to 3 in the antarctic and if this increase is combined with a relative increase of the atlantic sector antarctic bottom water aabw versus north atlantic deep water nadw source ratio from 1 3 to about 2 1 then the model predicts several changes that seem to be recorded in the sedimentary record as follows 1 a global redistribution of nutrients and 12c from the intermediate to deep water takes place with the atlantic intermediate water phosphate decreasing 0 6 mole kg 1 and the 13c increasing 0 3 to 0 5 2 the dissolved oxygen level of the deep sea is reduced from an average of about 180 to 70 mole kg 1 but the intermediate water oxygen declines only a small amount 3 the decrease in intermediate water nutrient concentration results in lower average organic carbon and calcium carbonate production in the warm surface ocean 4 the atmospheric co2 decreases by 90 to 110 ppm 5 initially a global increase in calcium carbonate dissolution occurs which is followed by a relaxation toward better preservation than exists for the present ocean in the model in this paper the reduction of nadw by itself does not produce these effects rather the nutrient decrease that does occur is found mostly in north pacific intermediate water and the model atmospheric co2 decrease is only 10 to 30 ppm it is observed that 92 of the atmospheric co2 change takes place according to a 200 year time constant in the model this corresponds to the response time of the upper ocean and atmosphere to a change in the stationary state atmospheric pco2 thus according to this model the time lag between the nutrient based cause and the atmospheric co2 response is not expected to be particularly large

terrestrial ocean environmental change in the northwestern pacific from the glacial times to holocene abstract the mixed water region mwr between the oyashio current and the kuroshio extension and surrounding terrestrial region have been affected by the global climatic change the sedimentary core kr02 15 pc6 collected at 40 23 8871 n 143 29 8663 e in the mwr provides detailed records of pollen and spores and alkenone sea surface temperature sst during the last 15 kyr showing that both ocean and terrestrial environments have been appreciably linked abundance of picea a proxy for terrestrial cool climate fluctuated coinciding with the alkenone sst change during the interstadial period 15 0 7 7 cal kyr b p alkenone sst increased to the modern level at 8 0 cal kyr b p and remained almost constant until 2 0 kyr b p while the terrestrial warming still continued until 4 0 cal kyr b p the rainfall under terrestrial warm condition increased gradually from 7 9 cal kyr b p and rapidly from 2 0 cal kyr b p both terrestrial and marine environments cooled down during the last 2 kyr possibly due to the intensified cold oyashio current

a preliminary view of the coexistence of mammoth and early peoples in mexico abstract a progress report about human mammoth interactions in mexico is provided based primarily on a literature search more than 270 mammoth localities are known in mexico but only 17 of them have shown some evidence of an association between early peoples and mammoth however that number is even less when each locality is assessed in detail due to the lack of a supportable association or the loss of the actual specimens that precludes their analysis using current techniques only six localities have modified mammoth bone among them the greatest potential for demonstrating such a relationship is at santa isabel iztapa valsequillo villa de guadalupe and tocuila establishing an analysis methodology for the materials from those as well as new localities is warranted to provide the basis for interpreting the human mammoth relationship in mexico future research calls for detailed stratigraphic and radiometric control and an analytical protocol for bone analysis focused on taphonomy and biotechnology

intensification of the asian monsoon and a chemical weathering event in the late miocene early pliocene implications for late neogene climate change several sedimentary geochemical and isotopic records indicate that the hypothesized intensification of the asian monsoon at about 8 ma triggered a physical and chemical weathering event in the himalayan tibetan plateau records of sediment input and clay composition from the northern indian ocean reveal clear weathering changes in the plateau at this time and ge si ratios of opaline silica as well as biogenic sedimentation rates indicate that increased dissolved element fluxes from this weathering event had an oceanwide effect it is likely that weathering intensity also increased in the andes and amazon basin in this interval as well perhaps the most important impact of this weathering event was to increase the net flux of the biolimiting nutrient phosphorus to the ocean as evidenced by a peak in phosphorus accumulation rates at this time the temporary increase in nutrient inputs to the ocean triggered increased oceanic productivity and organic carbon burial recorded in carbon isotopic records and paleooxygen concentrations the net result of this weathering event may have been an increase in the rate of drawdown of atmospheric co2 through the late miocene early pliocene via direct silicate weathering reactions and increased burial of organic carbon in the ocean this increased rate of atmospheric co2 drawdown may have destabilized the climate system by the late pliocene thus initiating a period of intense cooling and ice buildup leading to the present

the evidence for cainozoic aridification in southern africa abstract the namib and kalahari deserts preserve evidence of a long history of aridification in southern africa the earliest sedimentation within the kalahari basin dates to the late cretaceous but the first onset of arid conditions probably occurred in the early tertiary other arid episodes recurred throughout the tertiary but the development of the kalahari sand sea with its linear dune system is unlikely to predate the late pliocene this the largest single body of aeolian sand in the world is now largely stabilized by vegetation but evidence exists for its periodic reactivation throughout the quaternary although episodes of dune building are evident within the namib region as early as the eocene the namib sand sea represented by the now consolidated tsondab sandstone formation and the overlying active dunes of the sossus sand formation does not predate the upper miocene evidence from the upper pleistocene suggests however that there were significant fluctuations in the degree of aridity which prevailed during and after the last glacial period

sedimentation and volcanism in a permian arc related basin southern new zealand an exceptionally well exposed ancient intra arc basin in the permian takitimu group of new zealand contains 14 km of interbedded primary volcanic and marine volcaniclastic rocks of basaltic to rhyodacitic composition these are the products of subaerial and submarine arc volcanism and closely associated turbidite sedimentation the takitimu oceanic arc basin setting formed a dynamic closed sedimentary system in which large volumes of volcaniclastic material generated at the arc was rapidly redeposited in marine basins flanking the eruptive centres volcanism probably included 1 moderate to deep water extrusion of lava and deposition of hyaloclastite 2 extrusive and explosive eruptions from shallow marine to marginally emergent volcanoes in or on the margin of the basin and 3 plinian and phreato plinian eruptions from more distant subaerial vents along the arc much of the newly erupted material was rapidly transported to the adjacent marine basin by debris flows slumping and sliding hemipelagic sedimentation predominated on the outer margin of the basin infrequently interrupted by deposition of ash from the most explosive arc volcanism and the arrival of extremely dilute turbidites turbidite sedimentation prevailed in the remainder of the basin producing a thick prograding volcaniclastic apron adjacent to the arc the volcaniclastic strata closely resemble classic turbidite deposits and show similar lateral facies variations to submarine fan deposits study of such sequences provides insight into poorly understood processes in modern arc related basins

the latest miocene early pliocene biogenic bloom a revised indian ocean perspective abstract numerous investigations of neogene sediment sequences from the indian and pacific oceans have led to a paleoceanographic hypothesis that recently has been coined the biogenic bloom this hypothesis suggests that primary productivity increased substantially at indo pacific upwelling zones during the latest miocene early pliocene although it is clear that recognition of a widespread biogenic bloom provides a fundamental framework for understanding late neogene global change it also is apparent that investigations of this phenomena require syntheses of a variety of information from time correlated sediment sequences that span large oceanic regions late neogene polarity chron boundaries and nannofossil events are placed on a common and current time scale for 12 ocean drilling program odp sediment sequences in order to construct a coherent description of the biogenic bloom in the indian ocean carbonate mass accumulation rates mars were significantly higher than present day in the north and west indian ocean between 9 0 and 3 5 ma sediment deposited during this time interval also is dominated by planktic faunal assemblages that are indicative of high productivity and redox conditions that are explained by an increase in the supply of organic carbon further sediment of this age contains benthic faunal assemblages and chemical characteristics that are consistent with low dissolved o 2 in intermediate waters these observations collectively suggest that between 9 0 and 3 5 ma productivity in the north and west indian ocean was significantly higher than present day and the intermediate water oxygen minimum zone omz was more extensive than present day this paleoceanographic condition was most intense between 6 0 and 5 0 ma in the indian ocean comparison of the indian ocean sites with two locations in the pacific ocean suggests that the biogenic bloom was time coincident in the indian and pacific oceans the biogenic bloom therefore implies an important change in global nutrient cycling occurred during the latest miocene early pliocene although a satisfactory explanation for the change remains elusive

late quaternary variability of ocean circulation in the southeastern south atlantic inferred from the terrigenous sediment record of a drift deposit in the southern cape basin odp site 1089 during leg 177 of the ocean drilling program an expanded sequence of pliocene to holocene calcareous muds was recovered at site 1089 on a drift deposit in the southern cape basin se south atlantic the reconstruction of detrital sources and modes of sediment transport gives insight into the operational modes of regional current systems in response to climate variability over the last 590 kyr as inferred from sedimentological and mineralogical parameters of the terrigenous sediment fraction terrigenous sediments mainly originate from african sources with minor contributions from distant southern sources south america and antarctica and are supplied by circumpolar water masses north atlantic deep water nadw and surface currents of the agulhas current changes in clay mineralogy as tracers of deep and shallow ocean circulation best displayed by variations in quartz feldspar ratios and kaolinite chlorite ratios of clay reflect both the northward displacement of nadw injection into the antarctic circumpolar current and a weakening of agulhas current leakage from the indian ocean around south africa to the south atlantic during glacial stages sub stages and stadials modifications of these regional current patterns are consistent with perturbations in global conveyor circulation and climate variability on milankovitch and sub milankovitch time scales elevated mass accumulation rates of terrigenous matter generally document high particle fluxes and focusing effects by bottom current action throughout the late quaternary current sorting and coarsening of terrigenous mud independently of its source signals prevails during interglacial periods and is linked to a stronger flow of antarctic bottom water and the invigoration of deep contour currents in response to long term changes 100 kyr cyclicity in antarctic ice sheet dynamics high amplitude fluctuations in global sea level and increased bottom water formation

late miocene through early pliocene deep water circulation and climate change viewed from the sub antarctic south atlantic benthic foraminiferal stable isotope records for the past 11 myr from a recently drilled site in the sub antarctic south atlantic site 1088 ocean drilling program leg 177 41 s 15 e 2082 m water depth provide for the first time a continuous long term perspective on deep water distribution patterns and southern ocean climate change from the late miocene through the early pliocene i have compiled published late miocene through pliocene stable isotope records to place the new south atlantic record in a global framework carbon isotope gradients between the north atlantic south atlantic and pacific indicate that a nutrient depleted watermass probably of north atlantic origin reached the sub antarctic south atlantic after 6 6 ma by 6 0 ma the relative proportion of the northernprovenance watermass was similar to today and by the early pliocene it had increased to greater than the modern proportion suggesting that thermohaline overturn in the atlantic was relatively strong prior to the early pliocene interval of inferred climatic warmth site 1088 oxygen isotope values display a two step increase between v7 4 ma and 6 9 ma a trend that parallels a published n 18 o record of a site on the atlantic coast of morocco this is perhaps best explained by a gradual cooling of watermasses that were sinking in the southern ocean i speculate that relatively strong thermohaline overturn at rates comparable to the present day interglacial interval during the latest miocene may have provided the initial conditions for early pliocene climatic warmth the impact of an emerging central american seaway on atlantic pacific ocean upper water exchange may have been felt in the north atlantic beginning in the latest miocene between 6 6 and 6 0 ma which would be v1 5 myr earlier than previously thought 9 2002 elsevier science b v all rights reserved

improved chronostratigraphic reference curve of late neogene seawater 87sr 86sr we present a reference curve of seawater 87 sr 86 sr variation through the past 7 m y based on 455 samples of planktonic foraminifera from ocean drilling program odp site 758 in the indian ocean this single site curve is superior to extant curves because of several features 1 a continuous sedimentary section free of disturbances 2 a well determined and precise chronostratigraphy that is calibrated to numeric age on the basis of continuous magnetostratigraphy and a refined astronomical time scale 3 a high temporal resolution sample interval averages 15 ka 4 an improved analytical reproducibility and 5 a uniformly high sample quality the gross structure of the site 758 curve is generally similar to that observed in previous work but the increased temporal resolution and decreased analytical scatter of the curve provide improved chronostratigraphic resolution at the 95 confidence level uncertainties on age estimates range from 0 60 m y in the lower pleistocene and upper miocene to 2 03 m y in the middle pliocene moreover this detailed record identifies two periods of steplike increases in seawater 87 sr 86 sr 6 10 to 5 80 ma and 1 46 to 1 13 ma that are characterized by rates of change of 150 10 6 m y these features if verified at other sites indicate abrupt changes in the sr mass balance of the ocean and may be used as precise event markers in late neogene marine sections

human footprints in central mexico older than 40 000 years abstract the timing route and origin of the first colonization to the americas remains one of the most contentious topics in human evolution a number of migration routes have been suggested and there are different views as to the antiquity of the earliest human occupation some believe that settlement happened as early as 30 ka bp but most of the currently accepted early sites in north america date to the latest pleistocene related to the expansion of the clovis culture while the oldest directly radiocarbon dated human remains are 11 5 ka bp in this context new evidence is presented in this paper in the form of human footprints preserved in indurated volcanic ash to suggest that central mexico was inhabited as early as over 40 ka bp human and animal footprints have been found within the upper bedding surfaces of the xalnene volcanic ash layer that outcrops in the valsequillo basin south of puebla mexico this ash layer was produced by a subaqueous monogenetic volcano erupting within a palaeo lake dammed by lava within the valsequillo basin during the pleistocene the footprints were formed during low stands in lake level along the former shorelines and indicate the presence of humans deer canids big felids and probably camels and bovids the footprints were buried by ash and lake sediments as lake levels rose and transgressed across the site the ash has been dated to at least 40 ka bp by osl dating of incorporated baked lake sediments

late pleistocene holocene stratigraphy and radiocarbon dating of la malinche volcano central mexico abstract previous studies of la malinche identified and radiocarbon dated several volcanic layers the youngest of which yielded an age of ca 7 5 ka an additional ash fallout layer that crops out at high altitudes was considered the most recent deposit with an estimated age of 6 ka in the present work 38 new radiocarbon ages are presented from these several date the young ash fallout layer and lie around 3 1 ka with the aid of these dates a new and comprehensive stratigraphy documenting the late pleistocene holocene eruptive history of la malinche is presented the stratigraphy indicates two main stages of volcanic activity pre malinche and malinche the first undoubtedly comprises the major part of the eruptive history but its deposits are largely covered by the products of the latter stage on which this study is focused the malinche stage was subdivided into three eruptive periods period 1 started with the emplacement of the huamantla pumice more than 45 ka ago this deposit consists of a thick pumice fallout overlain by pyroclastic flow deposits subsequently several episodes of construction and collapse of summit domes occurred the oldest dome was dated at ca 45 ka period 2 started 21 5 ka ago with the malinche pumice i a widespread pumice fallout covering the entire slopes of the volcano pyroclastic flows and lahars related to this eruption were channeled along deep barrancas and reached considerable distances deposits produced by partial sector collapse and dated at ca 20 9 ka and a pumice and ash flow deposit dated at 15 9 ka were also generated during this period the last period started with the eruption of the malinche pumice ii a distinctive fallout deposit overlain by ash flow deposits on the ne slope of the volcano the age of this pumice layer is estimated between 12 and 9 ka formation of block and ash flows lahars and pumice and ash flows followed during this period and peaked in a most intensive episode that was dated at 7 5 ka after this a long interval of quiescence was interrupted 3 1 ka ago with the eruption of an ash fallout accompanied by small pyroclastic flows whose deposits are the youngest volcanic products recognized at la malinche the new stratigraphic findings identify la malinche as a potentially active and dangerous volcano that could severely affect densely populated areas at its lower slopes in addition the results of this work can shed some light on archaeological research that is being carried out in the puebla tlaxcala region

environmental history of the south eastern south atlantic since the middle miocene evidence from the sedimentological records of odp sites 1088 and 1092 during leg 177 of the ocean drilling program odp well preserved middle miocene to pleistocene carbonate rich sediment records were recovered on a north south transect through the south eastern atlantic sector of the southern ocean at site 1088 on the agulhas ridge and site 1092 on meteor rise both sites were dominated by the deposition of calcareous nannofossil oozes through the miocene indicating low biological productivity in warm to temperate surface waters a continuous increase in the proportions of foraminifera since the latest miocene 6 5 ma points to enhanced nutrient supply possibly related to the global biogenic bloom event across the miocene pliocene boundary since the late pliocene different styles of biological productivity developed between the sites enhanced deposition of biosiliceous constituents at the southern site 1092 particularly in the early pleistocene is consistent with the formation of the circum antarctic opal belt since 2 5 ma in a setting near the polar front whereas carbonate deposition still prevailed at the northern site 1088 situated near the subtropical front clay mineral tracers of water mass advection together with the pattern of sedimentation rates and hiatuses reflect distinct pulses in the development of regional ocean circulation between 14 and 12 ma around 8 ma and since 2 8 ma these pulses can be related to antarctic ice sheet extension that mediates the production and flow of southern source water and stepwise increases in north atlantic deep water production that drives global conveyor circulation at site 1088 illite chemistry and silt clay ratios of the terrigenous sediment fraction reflect the history of terrestrial climate in southern africa with humid conditions prior to the early late miocene 9 7 ma followed by a dry episode until 7 7 ma the latest miocene and early pliocene were characterized by a humid episode until modern aridity was established in the late pliocene between 4 0 and 2 8 ma these climate changes were related to the latitudinal migration of climate belts in response to tectonically caused reorganizations in atmospheric and ocean circulation

clay mineral distribution in surface sediments of the south atlantic sources transport and relation to oceanography abstract surface samples mostly from abyssal sediments of the south atlantic from parts of the equatorial atlantic and of the antarctic ocean were investigated for clay content and clay mineral composition maps of relative clay mineral content were compiled which improve previous maps by showing more details especially at high latitudes large scale relations regarding the origin and transport paths of detrital clay are revealed high smectite concentrations are observed in abyssal regions primarily derived from southernmost south america and from minor sources in southwest africa near submarine volcanoes of the antarctic ocean south sandwich bouvet island smectite contents exhibit distinct maxima which is ascribed to the weathering of altered basalts and volcanic glasses the illite distribution can be subdivided into five major zones including two maxima revealing both south african and antarctic sources a particularly high amount of mg and fe rich illites are observed close to east antarctica they are derived from biotite bearing crystalline rocks and transported to the west by the east antarctic coastal current chiorite and well crystallized dioctaedral illite are typical minerals enriched within the subantarctic and polarfrontal zone but of minor importance off east antarctica kaolinite dominates the clay mineral assemblage at low latitudes where the continental source rocks west africa brazil are mainly affected by intensive chemical weathering surprisingly a slight increase of kaolinite is observed in the enderby basin and near the filchner ronne ice shelf the investigated area can be subdivided into ten large scale clay facies zones with characteristic possible source regions and transport paths clay mineral assemblages of the largest part of the south atlantic especially of the western basins are dominated by chlorite and illite derived from the antarctic peninsula and southernmost south america and supported by advection within the circumantarctic deep water flow in contrast the east antarctic provinces are relatively small assemblages of the eastern basins north of 30 s are strongly influenced by african sources controlled by weathering regimes on land and by a complex interaction of wind river and deep ocean transport the strong gradient in clay mineral composition at the brazilian slope indicate a relatively low contribution of tropically derived assemblages to the western basins

geologic evidence for age of deposits at hueyatlaco archeological site vasequillo mexico direct tracing of beds during excavation in may 1973 confirmed that the artifact bearing layers at hueyatlaco underlie 10 m of fine grained water laid deposits that constitute part of the wide spread valsequillo gravels dissection of these deposits by the adjacent rio atoyac has reached a depth of 50 m the stratigraphic section at hueyatlaco includes four distinctive tephra units the oldest one occupies a small channel in a series of cut and fill stream deposits that have yielded bifacial tools it lies more than a meter above flat lying fine grained beds from which edge retouched tools have been recovered the three other tephra units occur higher in the section fission track ages on zircon phenocrysts from two of the younger tephra layers 370 000 200 000 and 600 000 340 000 yr 2 agree with concordant uranium series dates for a camel pelvis that was found associated with bifacial tools at hueyatlaco 245 000 40 000 yr by 230th and 180 000 yr by 231pa these dates are compatible with the depth of burial and subsequent dissection of the hueyatlaco deposits as well as with the degree of hydration of volcanic glass shards and with the extent of etching of heavy mineral phenocrysts from within the tephra layers these findings suggest to us that further search for archaeological remains in deposits as old as those at hueyatlaco would be warranted

miocene benthic foraminiferal isotope records a synthesis abstract 18 o 16 o and 13 c 12 c ratios of miocene benthic foraminifera from a number of atlantic pacific and indian ocean dsdp sites 71 77b 206 208 238 279 289 296 329 357 and 366a have been compiled these provide a rather detailed history of miocene deep water especially in the pacific ocean bottom water temperatures rose during the early miocene and then declined rapidly during the middle miocene this decline was accompanied by an increase in antarctic glaciation late miocene bottom temperatures and antarctic ice volumes are inferred to be similar to today s but exhibited some fluctuation the early miocene ocean was less thermally stratified at intermediate and abyssal depths while the late miocene deep ocean had a thermal structure generally similar to the modern ocean foraminiferal carbon isotope ratios at most of the sites varied quasi sympathetically throughout the miocene these variations must reflect comparable variations in the mean 13 c 12 c of marine hco 3 however the causes of such variations are not yet clear

miocene to pliocene calcareous nannofossil biostratigraphy at odp leg 177 sites 1088 and 1090 the calcareous nannofossil biostratigraphy of odp leg 177 sites 1088 and 1090 subantarctic sector from the atlantic ocean is discussed most nannofossil zonal boundaries of martini 1971 and okada and bukry 1980 were recognized for the studied mid high latitude sediments conventional low latitude marker species such as amaurolithus spp discoaster spp triquetrorhabdulus spp ceratolithus spp were recorded as rare and scattered which impeded the development of a detailed nannofossil biostratigraphic zonation of some miocene and pliocene intervals because of the absence of some primary biostratigraphic marker species additional second order bioevents such as the first occurrence of calcidiscus macintyrei and the last occurrence of coccolithus miopelagicus have been used to approximate the base of zones nn7 and nn8 respectively several disconformities disturbing the pliocene and miocene intervals of site 1090 could be determined based on nannofossil distribution although the occurrence of intervals with dissolved nannofloras and low species diversity prevented a reliable age assignment an acme of small gephyrocapsa was recognized near the lower middle pliocene boundary close to the nn15 nn16 zonal boundary presenting an event for further improvement of the calcareous nannofossil biostratigraphy of this interval time the first occurrence of pseudoemiliania lacunosa 4 m occurs close to this interval representing a fairly reliable event to approximate the base of nn15 zone when other biozonal events are absent a paracme of r pseudoumbilicus 7 m was detected in the lower pliocene nn12 and in the upper miocene nn11 these temporary absences of the species seem to be isochronous between high latitude and low middle latitude areas

past hydrographic and climatic changes in the subantarctic zone of the south atlantic the pleistocene record from odp site 1090 abstract pleistocene summer sea surface temperatures ssst have been reconstructed on a composite core section recovered in the subantarctic zone of the southern ocean from planktonic foraminifers applying the modern analog technique the composite consists of core ps2489 2 and the sections recovered at odp site 1090 and documents the last 1 83 ma three distinct climatic periods can be identified that mirror the pleistocene development of the southern ocean hydrography cold climatic conditions prevailed at 43 s during glacial as well as during interglacial periods during the early pleistocene 1 83 0 87 ma indicating a northward shift of isotherms that characterize the present day polar front zone by about 7 of latitude evidence shows a strong linkage between southern ocean and low latitude climate during that interval time between the mid pleistocene revolution ca 0 9 ma and the mid brunhes event ca 0 4 ma we observe higher amplitude fluctuations in the ssst between glacial and interglacial periods corresponding to the temperature range between the present polar front and subantarctic front these climatic variations have been related to changes in the northern hemisphere ice sheets the past 0 4 ma are characterized by strong ssst variations of up to 8 c between glacials and interglacials only during the climatic optima stages 11 3 9 3 7 5 7 1 5 5 and the early holocene ssst exceeded present ssst at the core locality 10 2 c although the carbonate dissolution record exhibits high variability during the pleistocene it can be shown that ssst estimates were not significantly biased the mid brunhes dissolution cycle as well as the mid pleistocene enhanced carbonate preservation appear to belong to a global long term variability in carbonate preservation

pliocene uplift of the northern tibetan plateau neogene redbeds passing upward into upward coarsening conglomerate and debris flow deposits at the foot of the kunlun mountains record the change in paleoslope related to uplift of the surface of the northern tibetan plateau detailed magnetostratigraphy of a 4 5 km section near yecheng in the western kunlun mountains shows that the change from deposition on distal alluvial plains to proximal alluvial fans occurred during the gilbert reversed chron 4 5 3 5 ma the change in depositional facies was accompanied by an increase in sedimentation rate from an average 0 15 mm yr between the earliest oligocene and the earliest pliocene to 1 4 mm yr in the gauss normal chron 3 6 2 6 ma we interpret the change in depositional facies and increase in sedimentation rate as indicating that the main uplift of the northwestern tibetan plateau began ca 4 5 ma

effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation the late cenozoic closure of the seaway between the north andsouth american continents is thought to have caused extensive changes in ocean circulation and northern hemisphere climate1 2 but the timing and consequences of the emergence of the isthmus of panama which closed the seaway remain controversial1 2 3 4 5 here we present stable isotope and carbonate sand fraction records from caribbean sediments which when compared to atlantic and pacific palaeoceanographic records indicate that the closure caused a marked reorganization of ocean circulation starting 4 6 million years ago shallowing of the seaway intensified the gulf stream and introduced warm and saline water masses to high northern latitudes these changes strengthened deep water formation in the labrador sea over the next million years as indicated by an increased deep water ventilation and carbonate preservation in the caribbean sea and favoured early pliocene warming of the northern hemisphere the evaporative cooling of surface waters during north atlantic deep water formation would have introduced moisture to the northern hemisphere although the pronounced intensification of northern hemisphere glaciation between 3 1 and 2 5 million years ago substantially lagged the full development of north atlantic deep water formation we propose that the increased atmospheric moisture content was a necessary precondition for ice sheet growth which was then triggered by the incremental changes in the earth s orbital obliquity

holocene climatic optimum in southwest africa evidence from the marine clay mineral record abstract sediment composition grain size and clay mineral record of a high resolution sediment core from the continental slope off namibia was investigated to gain information on the deposition of terrigenous matter in this part of the southwest african continental margin during the last 18 k y the depositional processes involved are fluvial input by the kunene river and eolian input from the namib and kalahari deserts each supplying characteristic mineral suites during low sea level erosion of the exposed shelf yields additional material the amount of eolian or fluvial matter depends on the strength of the transport process which is a function of aridity or humidity of the source area thus allowing paleoclimatic interpretations arid conditions prevailed during a low sea level from 18 to 15 ka and unconsolidated shelf sediment was mobilized and supplied to the slope by short distance transport by southerly winds a dramatic increase in the accumulation of terrigenous sediment is recorded from 15 to 10 ka without major changes in sediment composition which is attributed to increased runoff of the kunene river and fluvial erosion of shelf sediments this period coincides with a strengthening of the monsoonal system during a precessional minimum which is observed in numerous sites in central africa and indicates an intensified influence of the monsoon at the kunene headwaters a distinct shift in clay mineralogy towards river derived material marks a second period of increased river runoff during high sea level from 9 to 5 ka indicating maximum humidity in the source area from 6 to 5 ka this corresponds to the holocene climatic optimum observed in the arid belts of northern africa the present balance between fluvial and eolian input was reached approximately at 4 ka

post miocene disconformities and paleoceanography in the atlantic sector of the southern ocean abstract the magnetostratigraphy and biostratigraphy of 166 islas orcadas piston cores in the atlantic sector of the southern ocean reveal disconformities in the sedimentary record which are attributed to bottom current erosion the hiatus frequency as a function of water depth reveals both shallow erosion by circumpolar deep water cdw and deep erosion by antarctic bottom water aabw the post miocene history of erosion by both bottom currents is delineated by analysis of areal and temporal fluctuations of hiatus frequency erosion by cdw caused disconformities on the maud rise astrid ridge indian antarctic ridge scotia sea and islas orcadas rise the shallow disconformities increased in frequency and size from 4 0 to 1 0 ma with two periods of distinctly widespread disconformities from 5 5 to 4 5 ma and 2 5 to 1 5 ma after 1 0 ma the frequency and extent of the hiatuses decreased erosion by aabw caused disconformities in the weddell abyssal plain argentine basin and through a narrow conduit in the south sandwich basin downstream from the bullard fracture zone the deep disconformities increased in frequency and size from 4 5 to 5 0 ma 3 5 to 4 0 ma and from 1 0 to 2 5 ma after 1 5 ma the frequency and extent of hiatuses decreased fluctuations in hiatus frequency are consistent with results of a similar study in the southeast indian ocean and may be correlated with paleoceanographic events southern ocean cooling and possible re establishment of the west antarctic ice sheet and fringing ice shelves at 4 3 3 5 ma was accompanied by increased aabw and cdw activity the formation expansion of the northern hemisphere ice sheets at 2 5 3 0 ma resulted in a decrease in aabw velocity and was accompanied by increased cdw activity both deep and shallow bottom currents increased in velocity during the late matuyama chron 2 0 1 5 ma as a result of antarctic cooling after 1 0 ma however the frequency and extent of shallow and deep hiatuses was reduced and the modern extent of hiatuses is the lowest within the post miocene