the stable isotopic composition of modern soil carbonate and its relationship to climate the oxygen isotopic composition of modern soil carbonate is well correlated with the isotopic composition of local meteoric water the carbon isotopic cycle for co 2 in soils can be described in terms of the proportion of biomass using the c 4 photosynthetic pathway and the co 2 respiration rate of the soil at low soil respiration rates significant atmospheric co 2 mixing can occur in general the carbon isotopic composition of soil carbonate is related to the proportion of c 4 biomass present in soil but soils that freeze to the depth of carbonate formation often have a significant atmospheric component this suggests that freezing of the soil solution should be considered as another important mechanism for soil carbonate formation because of these relationships the isotopic composition of soil carbonate may be a paleoclimatic and paleoecologic indicator in cases in which diagenetic alteration has not occurred

isotopic reconstructions of habitat change surrounding the extinction of sivapithecus a miocene hominoid in the siwalik group of pakistan abstract this research presents an isotopic study of a wide range of mammalian taxa from the miocene siwalik group of pakistan with a focus on two time intervals 9 3 9 2 ma when the hominoid sivapithecus was present and 8 1 8 0 ma shortly after sivapithecus became extinct the purpose of this investigation is to reconstruct the vegetation mosaic at both time levels in order to determine what sivapithecus habitat was like and what changes in habitat and climate may have taken place by the time sivapithecus went extinct both carbon and oxygen stable isotopes were sampled from inorganic carbonate in tooth enamel results indicate a vegetation mosaic of both closed and open habitat at both time intervals but a decrease in forest accompanied by an increase in open habitat including c 4 grasses during the younger interval individuals from many species cluster with each other with respect to both carbon and oxygen isotope values indicating a spectrum of feeding adaptations exploiting different parts of the habitat isotopic values for sivapithecus suggest that it fed in the forest upper canopy taxa feeding in the most closed wet habitat become extinct by 8 1 ma furthermore higher 18 o values over time suggest a change in climate with a reduction in annual rainfall perhaps accompanied by changes in precipitation sources or rainfall regime as well these results suggest that forests became fragmented over time while still present in the younger level much of the forest was replaced by open habitat including patches of c 4 grass forest loss and fragmentation is a likely cause of the extinction of sivapithecus

systematic variations in the carbon and oxygen isotopic composition of pedogenic carbonate along elevation transects in the southern great basin united states stable carbon and oxygen isotope variations in holocene soil carbonates that formed in the unsaturated zone were examined along several elevation transects in the southern great basin united states a region with a semi arid climate our intent was to study the relationship between the stable isotopic composition of pedogenic carbonates and climate ecological variations differences in parent material and soil depth 13 c of pedogenic carbonate in three different soil profiles from different elevations decreases with soil depth indicating a decrease in the ratio of atmospheric to plantderived co 2 downprofile pedogenic carbonate at the soil air interface approaches a 13 c value in equilibrium with atmospheric co 2 in all three soils observed 13 c profiles for pedogenic carbonate can be described using a one dimensional model for 12 co 2 and 13 co 2 assuming isotopic equilibrium between soil co 2 and soil carbonate the modeled best fit to observed isotopic profiles suggests that the profile differences in part result from differing soil respiration rates at each site 13 c in deep pedogenic carbonate 50 cm varies by about 12 per mil over a 2 440 m elevation range being enriched in 13 c at the lowest elevations the slope of 13 c for these carbonates versus elevation is very similar for soils developed on carbonate and on noncarbonate parent materials depletion by 4 6 to 4 7 per mil per 1 000 m increase in altitude between 300 to 2 740 m above mean sea level for the localities studied this concordance makes it likely that there has been complete isotopic exchange between hco 3 in solution and soil co 2 prior to carbonate precipitation soil co 2 and soil respiration rates increase systematically with elevation the plantderived component of soil co 2 indicates that c 3 plants dominate the biomass at most measured sites in agreement with plant surveys calculated equilibrium fractionation factors between soil co 2 and soil carbonate are very similar to those observed again indicating complete isotopic exchange between carbon species in all the soil co 2 and soil carbonate data suggest that the 13 c variation with elevation observed in the soil carbonates results from differing soil respiration rates at each site as well as from variations in the proportion of c 3 to c 4 and cam plants in each site9s surface biomass 18 o values in pedogenic carbonates are higher at lower elevations due in part to the more positive 18 o values for meteoric waters at lower elevations the average 18 o value of deep 50 cm pedogenic carbonate at all sites however is enriched 2 4 to 3 7 per mil with respect to values we predict should be in equilibrium with the isotopic composition of local meteoric waters this suggests that evaporative isotopic enrichment of soil waters may have occurred at all elevations prior to precipitation of carbonate or that seasonal differences in the isotopic composition of meteoric waters coupled with differential infiltration may be taking place one or both of these processes also may explain the 18 o decrease in soil carbonate with depth observed in two of three soil profiles

mineralogy sulphur isotope geochemistry and the development of sulphide structures at the broken spur hydrothermal vent site 29 10 n mid atlantic ridge a large collection of hydrothermal sulphides from the broken spur hydrothermal vent site including representative samples of mound sulphide materials has been characterized using optical mineralogy and sulphur isotope analysis young mound sulphides from broken spur have a pyrrhotite dominated mineralogy unusual for bare ridge vent systems however pyrrhotite is metastable and is ultimately converted to iron disulphides mature sulphides are indurated recrystallized and contain abundant quartz sulphide mound materials are developed by three major processes i coalescing of chimney structures ii accumulation of talus from mass wasting and iii precipitation and growth in response to hydrothermal flow progressive maturation of mound materials is by modification of primary textures development of mineralogical zoning and replacement of metastable phases sulphur isotope analysis of 35 mineral separates returned 34 s values of 0 5 to 3 2 these values are similar to those previously measured for broken spur and snakepit but are distinctly 32 s enriched compared to the tag active mound and some pacific sites seawater entrainment and sulphate reduction within the subsurface feeder zone below broken spur mounds do not appear to be important processes at broken spur in contrast to the tag active mound

the structure and rate of late miocene expansion of c4 plants evidence from lateral variation in stable isotopes in paleosols of the siwalik group northern pakistan this study uses stable isotope variation within individual mio pliocene paleosols to investigate subkilometer scale phytogeography of late miocene vegetation change in southeast asia between ca 8 1 and 5 ma a time interval that coincides with dramatic global vegetation change we examine trends through time in the distribution of low latitude grasses c4 plants and forest c3 plants on indo gangetic floodplains using carbon 13c and oxygen isotopic 18o values in buried soil carbonates in siwalik series sediments exposed in the rohtas anticline north central pakistan revised high resolution magnetostratigraphy and a new 40ar 39ar date provide improved age control for the 2020 m rohtas section carbon isotope results capture lateral variability of c3 versus c4 plants at five stratigraphic levels r11 8 0 ma r15 6 74 6 78 ma r23 5 78 ma r29 4 8 4 9 ma and upper boundary tuff ubt 2 4 ma using detailed sampling of paleosols traceable laterally over hundreds of meters paleosols and the contained isotopic results can be assigned to three different depositional contexts within the fluvial sediments channel fill crevassesplay and floodplain environments 13c results show that near the beginning 8 0 ma and after 4 0 ma the period of major ecological change vegetation was homogeneously c3 or c4 respectively regardless of paleo landscape position in the intervening period there is a wide range of values overall with c4 grasses first invading the drier portions of the system floodplain surfaces and c3 plants persisting in moister settings such as topographically lower channel swales although abrupt on a geologic timescale changes in abundance of c4 plants are modest 2 per 100 000 yr compared to rates of vegetation turnover in response to glacial and interglacial climate changes in the quaternary earlier research documented a sharply defined c3 to c4 transition in pakistan between 8 1 and 5 0 ma based on vertical sampling but this higher resolution study reveals a more gradual transition between 8 0 and 4 5 ma in which c3 and c4 plants occupied different subenvironments of the siwalik alluvial plain 18o values as well as 13c values of soil carbonate increase up section at rohtas similar to isotope trends in other paleosol records from the region spatially however there is no correlation between 13c and 18o values at most stratigraphic levels this implies that the changes in soil hydrology brought about by the shift from forest to grassland i e an increase in average soil evaporation did not produce the shift through time in 18o values we interpret the trend toward heavier soil carbonate 18o values as a response to changes in external climatic factors such as a net decrease in rainfall over the past 9 ma

introduction to special section on measurements and monitoring at the tag hydrothermal field mid atlantic ridge 26 n 45 w the international oceanographic community carried out a coordinated series of cooperative cruises and dives with japanese russian and u s submersibles between 1993 and 1995 to perform baseline measurements and to establish a seafloor observatory to monitor the active sulfide mound in the tag hydrothermal field before during and after drilling by ocean drilling program leg 158 in october november 1994 the results of that effort and related work are reported herein as group of 23 papers comprising a special section in this issue of geophysical research letters

active and relict sea floor hydrothermal mineralization at the tag hydrothermal field mid atlantic ridge the tag hydrothermal field is a site of major active and inactive volcanic hosted hydrothermal mineralization in the rift valley of the slow spreading mid atlantic ridge at 26 degree n the axial high is the principal locus of present magmatic intrusions the tag field contains three main areas of present and past hydrothermal activity 1 an actively venting high temperature sulfide mound 2 two former high temperature vent areas 3 a zone of low temperature venting and precipitation of fe and mn oxide deposits the volcanic centers occur at the intersections between ridge axis parallel normal faults and projected axis transverse transfer faults the intersections of these active fault systems may act as conduits both for magmatic intrusions from sources beneath the axial high that build the volcanic centers and for hydrothermal upwelling that taps the heat sources radiometric dating of sulfide samples and manganese crusts in the hydrothermal zones and dating of sediments intercalated with pillow lava flows in the volcanic center adjacent to the active sulfide mound indicate multiple episodes of hydrothermal activity throughout the field driven by heat supplied by episodic intrusions over a period of at least 140 times 10 sup 3 yr the sulfide deposits are built by juxtaposition and superposition during relatively long residence times near episodic axial heat sources counterbalanced by mass wasting in the tectonically active rift valley of the slow spreading oceanic ridge hydrothermal reworking of a relict hydrothermal zone by high temperature hydrothermal episodes has recrystallized sulfides and concentrated the first visible primary gold reported in a deposit at an oceanic ridge

the effects of late quaternary climate and pco2 change on c4 plant abundance in the south central united states the late quaternary was a time of substantial environmental change with the past 70 000 years exhibiting global changes in climate atmospheric composition and terrestrial floral and faunal assemblages we use isotopic data and couple climate and vegetation models to assess the balance between c3 and c4 vegetation in texas during this period the carbon isotope composition of fossil bison mammoth and horse tooth enamel is used as a proxy for c3 versus c4 plant consumption and indicates that c4 plant biomass remained above 55 through most of texas from prior to the last glacial maximum lgm into the holocene these data also reveal that horses did not feed exclusively on herbaceous plants consequently isotopic data from horses are not reliable indicators of the c3 c4 balance in grassland biomes estimates of c4 percentages from coupled climate vegetation models illuminate the relative roles of climate and atmospheric carbon dioxide co2 concentrations in shaping the regional c4 signal c4 percentages estimated using observed modern climate vegetation relationships and late quaternary climate variables simulated by a global climate model are much lower than those indicated by carbon isotope values from fossils when the effect of atmospheric co2 concentration on the competitive balance between c3 and c4 plants is included in the numerical experiment however estimated c4 percentages show better agreement with isotopic estimates from late quaternary mammals and soils this result suggests that low atmospheric co2 levels played a role in the observed persistence of c4 plants throughout the late quaternary

neogene mammalian faunal change in southern asia correlations with climatic tectonic and eustatic events the fluvial neogene siwalik formations of northern pakistan span long time intervals with only minor hiatuses and being highly fossiliferous are uniquely suited for studies of change in mammalian faunas magnetostratigraphic correlations of a critical stratigraphic section give dates for 45 middle and late miocene biostratigraphic events these mark either first appearances or extinctions in the mammal fauna and show that in the siwaliks there were major fauna turnovers at between 20 and 16 ma and at 9 5 and 7 4 ma two minor faunal events are dated at 13 2 and about 12 ma many species making their first appearance were immigrants from europe or africa and indicate when connections to those regions existed immigration and extinction were the dominant modes of faunal change in situ evolution was much less important the siwalik biostratigraphic record correlates closely to climatic oceanographic and tectonic events which probably controlled immigration into southern asia abiotic events were therefore important factors affecting evolution of the mammal communities

comparative mineralogy and geochemistry of gold bearing sulfide deposits on the mid ocean ridges a comparative study of the mineralogy and geochemistry of sulfide deposits on mid ocean ridges in the northeast pacific and the mid atlantic reveals common characteristics associated with primary gold enrichment average gold contents of 0 8 to 5 ppm au occur in sulfides from southern explorer ridge and axial seamount northeast pacific and from the tag hydrothermal field and snakepit vent field mid atlantic ridge the enrichment of gold in these deposits is consistently related to a phase of late stage low temperature 1 ppm au occur exclusively in pyritic assemblages and commonly with abundant fe poor sphalerite and a suite of complex pb sb as sulfosalts amorphous silica and locally barite or carbonate are important constituents of the gold rich precipitates but do not contain gold themselves high temperature 350 c black smoker assemblages consisting dominantly of pyrite chalcopyrite pyrrhotite isocubanite and abundant anhydrite are uniformly gold poor 0 2 ppm au to the extent that individual sulfides can be mechanically separated chemical analyses by neutron activation indicate that gold is most abundant in sphalerite up to 5 7 ppm au but also occurs in pyrite and marcasite samples of sphalerite with abundant inclusions of fine grained sulfosalts locally contain up to 18 ppm au suggesting that sulfosalts may be repositories for gold no free gold has been observed at 4000 magnification of polished specimens indicating that the gold is present only as submicroscopic inclusions or as a chemical constituent within the sulfides samples from gold rich deposits in the northeast pacific and mid atlantic are compared with similar but relatively gold poor sulfides from the galapagos rift and 13 n on the east pacific rise epr and with barren sulfides from 11 n epr 21 n epr the endeavour ridge and the southern juan de fuca ridge trace element analyses of more than 170 samples show that gold enrichment in almost all of the deposits is associated with high concentrations of ag as sb pb and zn and locally with high cd hg tl and ga in contrast gold is typically depleted in samples with high co se and mo the close association of au with ag as sb and pb may reflect the common behavior of these metals as aqueous sulfur complexes e g au hs 2 at low temperatures similar mineralogical and geochemical associations are observed in sulfide deposits from modern back arc settings and in the ancient geologic record

anomalous sudden commencement on march 24 1991 an anomalous geomagnetic sudden commencement sc occurred on march 24 1991 it is characterized by an exceptionally large and sharp impulse observed in its initial part along the noon meridian in middle and low latitudes the analysis of the sc was made by using high time resolution digital data from the 210 meridian magnetometer chain in the west pacific sub auroral magnetometer network samnet in the united kingdom and southern scandinavia the eiscat magnetometer cross in northern scandinavia and svalbard and canopus in canada together with other ground and satellite goes 6 goes 7 crres and gms data the results of the analysis suggest that the pulse observed at lower latitude ground stations was caused by the propagation of a strong magnetospheric compression of short duration less than l min which has never been observed before this event the hf doppler observation in kyoto near local noon seems to be consistent with existence of the bipolar electric field associated with the propagating compressional magnetic pulse the samnet stations and crres in the early morning also detected positive pulses which delays 30 50 s from the pulses in noon sector although the delay in the peak time of the pulse observed on the ground is consistent with ionospheric hydromagnetic wave propagation from the dayside to the nightside with finite speed the initial onset time of the pulse on the ground was almost simultaneous everywhere suggesting the existence of an almost instantaneous propagation mode below the ionosphere

heat flow and mineralogy of tag relict high temperature hydrothermal zones mid atlantic ridge 26 n 45 w submersible investigations employing heat flow measurements 12 stations sampling and imagery of the two relict high temperature hydrothermal zones of the tag field the alvin and mir sulfide zones elucidate relations between heat sources and mineralization including an active sulfide mound that has been the focus of prior studies values of heat flow in the mir zone and at the margin of the active mound are inversely proportional to distance from adjacent volcanic centers this observation supports the hypothesis that intrusions at volcanic centers adjacent to the high temperature hydrothermal zones supply the heat to drive hydrothermal activity the chronology of hydrothermal deposits in the different zones indicates that the intrusions are episodic with field wide high temperature hydrothermal events recurring at intervals of tens of thousands of years while activity at individual zones may recur at intervals of hundreds to thousands of years a sequence of hydrothermal deposits ranges to at least 140 000 years ago from massive sulfides forming at the active mound to recrystallization of sulfides in the active and relict zones to pyritization of an inactive mound in the alvin zone low temperature mineral phases precipitate before during and after the sulfides

calcium carbonate nodules in soils 13o 16o and 13c 12c ratios and 14c contents abstract 18 o values 13 c values and 14 c ages were determined in ten caco 3 nodule populations collected from soil and paleosols in the israeli coastal plain the selected soils were carbonate free when formed and the nodules in them represent either reprecipitation of carbonate illuviated from overlying horizons descending mode or precipitated from a raised brackish water table ascending mode the 14 c ages represent the times of migration illuviation or eustatic movement both climate related and the stable isotopic conditions reflect the environmental conditions at such times the 13 c values of most populations were found to have a wide range and cannot be used to reconstruct former climates this wide range is attributed to microvariations of pco 2 in the soil because the 18 o ranges in the same samples are usually comparable to analytical uncertainties and because the 18 o mean values follow an age correlated pattern they permit us to place the soils in one of three categories in two cases where nodules were soft both turned out to be young 1000 years and to have wide ranges in 18 o values the latter are apparently due to continual re equilibration with new soil solutions in three cases of the descending mode of nodule formation 12 000 14 000 year old hard nodules had very narrow 18 o whose means reflect the rainfall value at that time the period between 12 000 14 000 years b p is independently known to be one of higher precipitation evaporation ratio and of more intensive soil formation these ascending mode populations of hard nodules had narrow ranges in 18 o reflecting a mixture of sea water and rainfall and all turned out to be 3 000 4 000 years old at this period the sea reached its highest elevation resulting in raising the nearcoast water table

isochronous fluvial systems in miocene deposits of northern pakistan a palacomagnetic isochron dated at about 8 1 myr bp and detailed lithostratigraph of a 40 m interval exposed along strike for 40 km establish the depositional patterns of two contemporaneous interfingering fluvial systems in the upper part of the meddle siwalik sequence the two systems referred to as the buff and blue grey differ in unit shape lithofacies bedding sequence palaeocurrent direction and sand composition interfingering occurs along the south west north east strike of the outcrops with the palaeodrainage directions of the two systems generally perpendicular to this line the axis of the blue grey system which deposited widespread sheet sands and silts lay toward the south west end of the study area the more complex axis of the buff system which deposited shoe string sand bodies and lage volumes of silt and clay lay toward the north east the source area for both systems was the rising himalyan belt to the north and noth east of the study area at maximum extent the blue grey system occupied a channel belt at least 25 km wide channel belt widths and depths for the buff system are 1 3 km and 3 7 m respectively current directions averge 94 for blue grey sands and 136 for buff sands blur grey sands contain 20 more rock fragments and are otherwise less mature than buff sands the buff system shows a verticla pattern of avulsion palaeosol formation and floodplain aggradation which we attribute to autocyclic processes of parallel rivers the blue grey system shows phases of erosion accompaniced laterally by plaeosol formation folowed by valley fill and overfowing of interfluve surfaces theis pattern may be caused by allocyclic presses affecting the source area we interpret the blue grey system as a major drainage from the interior himalayas perhaps the ancestral indus and the buff system as a complex of smaller drinages along the mountain front which were probably ributaries to the bluegrey syste vertebrate fossils including hominoid primates from the area are almost exclusively associated with lithofacies of the buff system and this probably refects both taphonomic and palaeoecological differences between the two systems

a very large scale flow burst observed by the superdarn radars we examined the dynamics of the ionospheric plasma in the dayside sector by using the hf radar data at iceland west and at finland from 1100 to 1230 ut on september 5 1995 during that period the solar wind density was high and the imf was strongly southward the dayside magnetopause was highly compressed nearly to the geosynchronous orbit the two radars simultaneously detected a poleward flow burst in the noon sector which assuming uniformity of flow in the region of the data gap 1 5 mlt between the two radars showed a magnetic local time extent of 5 hours this local time extent is 2 to 3 hours wider than previous results the maximum poleward plasma velocity of the flow burst is 750 m s and the latitudinal size of the flow burst region is 100 to 200 km this flow burst region initially expanded in longitude up to 5 hours and then shifted poleward with a phase speed of 400 to 670 m s the flow burst has a duration of 20 min this large scale poleward flow burst is likely to be due to large scale reconnection occurring at the dayside magnetopause and subsequent convection as the magnetic field lines are transported across the polar cap

volcanism and metallogenesis of axial and off axial structures on the east pacific rise near 13 degrees n a fast spreading segment 12 cm yr super 1 of the east pacific rise near 12 degrees 50 n explored in detail by surface ship and manned submersible displays intense hydrothermal activity the rise crest about 1 500 m in width with a regional depth of 2 600 m is occupied at its center by an axial graben 1 3 titania alkali enriched olivine ti k basalts with high tio 2 1 4 and high alkali content na 2 o 3 0 k 2 o 0 3 occur on the southeastern seamount the fractionated tholeiites are believed to have been derived from primitive melts during differentiation which took place within a magma reservoir asymmetrical with respect to the axis of the graben which supplied both axial and off axial lava flows the ti k enriched basalts are believed to have been produced from an alkali enriched magmatic source

hf radar signatures of the cusp and low latitude boundary layer continuous ground based observations of ionospheric and magnetospheric regions are critical to the geospace environment modeling gem program it is therefore important to establish clear intercalibrations between different ground based instruments and satellites in order to clearly place the ground based observations in context with the corresponding in situ satellite measurements hf radars operating at high latitudes are capable of observing very large spatial regions of the ionosphere on a nearly continuous basis in this paper we report on an intercalibration study made using the polar anglo american conjugate radar experiment radars located at goose bay labrador and halley station antarctica and the defense meteorological satellite program dmsp satellites the dmsp satellite data are used to provide clear identifications of the ionospheric cusp and the low latitude boundary layer llbl the radar data for eight cusp events and eight llbl events have been examined in order to determine a radar signature of these ionospheric regions this intercalibration indicates that the cusp is always characterized by wide complex doppler power spectra whereas the llbl is usually found to have spectra dominated by a single component the distribution of spectral widths in the cusp is of a generally gaussian form with a peak at about 220 m s the distribution of spectral widths in the llbl is more like an exponential distribution with the peak of the distribution occurring at about 50 m s there are a few cases in the llbl where the doppler power spectra are strikingly similar to those observed in the cusp

the influence of spreading rate basement composition fluid chemistry and chimney morphology on the formation of gold rich sms deposits at slow and ultraslow mid ocean ridges seafloor massive sulphide sms deposits are variably enriched in precious metals including gold however the processes invoked to explain the formation of auriferous deposits do not typically apply to mid ocean ridge settings here we show a statistically significant negative correlation between the average gold concentration of sms deposits with spreading rate at non sedimented mid ocean ridges deposits located at slow spreading ridges 20 40 mm a have average gold concentrations of between 850 and 1600 ppb however with increasing spreading rate up to 140 mm a gold concentrations gradually decrease to between 50 and 150 ppb this correlation of gold content with spreading rate may be controlled by the degree and duration of fluid rock interaction which is a function of the heat flux crustal structure faulting and the permeability of the source rocks deposits at ultraslow ridges including ultramafic hosted deposits are particularly enriched in gold this is attributed to the higher permeability of the ultramafic source rocks achieved by serpentinisation and the inherent porosity of serpentine minerals combined with relatively high gold concentrations in peridotite compared with mid ocean ridge basalt variations in fluid chemistry such as reducing conditions and the potential for increased sulphur availability at ultramafic hosted sites may also contribute to the high concentrations observed beehive chimneys which offer more favourable conditions for gold precipitation may be more prevalent at ultramafic hosted sites due to diffuse low velocity venting compared with more focussed venting at basalt hosted sites

the 14c dates and 13c 12c ratios of soil organic matter as a means of studying the past vegetation in intertropical regions examples from colombia south america abstract the 14 c ages and 13 c 12 c values were obtained on soil organic matter of various soils sampled in the guasca valley adjacent to the high plain of the sabana de bogota in the colombian cordillera oriental a soil distribution is observed as a function of climatic and altitudinal changes in the lowest parts of the valley planosols are found from the bottom of the valley to 2700 2800 m at altitudes up to 2900 m brunizems and ferrisols occur above 3000 m andic soils are almost everywhere present in more humid mountain zones at present the landscape is widely open without any natural forest nevertheless it was proved by pollen analytical data that at altitudes up to 3000 m the andean forest replaced a paramo formation 13 000 yr ago to conciliate the facts with the known evolution of the vegetal population a first hypothesis consists of admitting that open landscapes were due to a recent anthropogenic destruction of the forest the forest degradation hypothesis was tested by determination of the 13 c 12 c ratio because the trees have a 13 c of 25 to 28 influence of plants with c 3 photosynthesis pathway whereas herbs characteristic of open vegetation such as tropical grasses may have a 13 c of 23 to 16 due to a c 4 photosynthetic cycle the organic matter dated at 13 000 30 000 yr has 13 c close to 25 in the andic soil zone this indicates that at elevations of around 3000 m a cold paramo vegetation was probable for organic matter dated at 12 000 6000 yr 13 c shows a variation from 23 to 21 that does not indicate forest conditions it is uncertain that a forest stage occurred in the past for the planosols brunizem and ferrisol organic matter dated at 13 000 20 000 yr has a 13 c 23 compatible with the paramo vegetational phase occurring at that time for organic matter dated at 9500 yr and less 13 c varies from 20 to 17 these values do not confirm the evidence of forest conditions but rather the existence of open vegetation typified by a mixture of c 3 plants shrubs and rare trees and tropical graminaea thus the 13 c values prove that the valley has known drier and warmer influences that prevented the forest from developing during the holocene to explain the origin of the open landscapes of the valley the hypothesis of recent human deforestation must be discarded moreover the valley appears as a persistent dry enclave in the oriental cordillera

convection surrounding mesoscale ionospheric flow channels 1 we evaluate data from the european incoherent scatter eiscat svalbard radar esr and defense meteorological satellite program dmsp spacecraft coupled with data from the super dual auroral radar network superdarn polar cap convection patterns in order to study how the ionospheric convection evolves around a sequence of transient mesoscale flow channel events in the duskside of the cusp inflow region on a northwestward convection background for the interplanetary magnetic field imf by positive and bz negative a sequence of three eastward flow channels formed over the course of 1 hour in response to three sharp imf rotations to imf by negative and imf bz positive the first and third channels due to imf by negative periods of 13 min and 30 min respectively develop in a similar manner they span the entire esr field of view and widen poleward with increasing time elapsed since their first appearance until the imf rotates back the convection patterns are consistent with the line of sight data from the esr and dmsp within a 10 min adaption time the flow lines form a twin vortex flow with the observed channel being the twin vortices center flow the fitting algorithm was pushed to its limits in terms of spatial resolution in this study during portions of the channel events the suggested twin cell flow is not in agreement with our physical interpretation of the flow channels being reconnection events because cell closure is suggested across an anticipated nonreconnecting open closed boundary for these segments we present simulated patterns which have been arrived at by a combination of looking at the raw data and examining the fitted convection patterns

growth of large sulfide structures on the endeavour segment of the juan de fuca ridge abstract mapping and sampling with dsrv alvin has established that sulfide blocks 0 5 m across dredged from the axial valley of the endeavour segment at 47 57 n are samples of unusually large sulfide structures the steep sided structures up to 30 m in length 20 m in height and 10 15 m across are localized by venting along normal faults at the base of the western axial valley wall and are distributed for about 200 m along strike paralleling the 020 trend of the ridge crest high temperature fluids 350 to more than 400 c pass through the massive sulfide structures and enter seawater through small concentric nozzle like features projecting from the top or the sides of the larger vent structures diffuse low temperature flow is pervasive in the vicinity of the active sulfide structures exiting from basalt and sulfide surfaces alike evidence of recent volcanic activity is sparse the two largest samples taken with the dredge would not have been recoverable using the submersible these samples represent massive complex portions of the sulfide structures which were not closely associated with rapid high temperature fluid flow at the time of sampling they contain textural evidence of sealed hydrothermal fluid exit channels mineralogy is dominated by fe sulfides nnd amorphous silica pyrite marcasite wurtzite chalcopyrite and iss are the most common sulfide phases pyrrhotite galena and sphalerite are present in trace amounts barite amorphous silica and chalcedony are the only non sulfide phases anhydrite is not observed in any of the dredge samples although it is common in the chimney like samples recovered by alvin specific mineralogical textural zones within the dredge samples are anaoogous to individual layers in east pacific rise at 21 n and southern juan de fuca ridge samples with two exceptions a coarse grained highly porous fe sulfide rich interior containing sulfidized tubeworm casts and a 2 5 cm thick zone near the outer margin of the samples dominated by late stage amorphous silica the porous interior may have formed by dendritic crystal growth from a slowly circulating fluid within a large enclosed chamber the amorphous silica deposited from a seawater hydrothermal fluid mixture percolating slowly through the walls of the enclosed chamber conductive cooling of the fluid as it traversed the walls allowed amorphous silica to precipitate these silica rich zones are the densest most durable portions of the structures and may be responsible for the lasting stability of the large sulfide features observations in these samples are consistent with two distinct phases of development phase 1 is analogous to chimney growth and construction at 21 n and ends when flow channels become sealed to rapid flow of through going fluid the flow is evidently redirected within the structure phase 2 includes dissolution of anhydrite and precipitation of amorphous silica during conductive cooling of sluggishly circulating hydrothermal fluid or seawater hydrothermal fluid mixtures evolution of vent structures through phase 2 allows lateral and vertical growth of unusually large structures

an isotopic study of paleosol carbonates from olduvai gorge the isotopic composition of paleosol carbonates from olduvai gorge can be used to give estimates of the paleoclimatic history of east africa during the plio pleistocene oxygen isotopes are indicative of major changes in the isotopic composition of meteoric water while carbon isotopes are indicative of major changes in the fraction of biomass using the hatch slack c4 photosynthetic pathway three major periods of climatic change in this terrestrial setting occurred at about 1 67 1 3 and 0 6 myr these involved major changes in flora as well as major changes in the isotopic composition of meteoric water overall the climate has become warmer and more arid over the past 2 2 myr

effects of changing environments on siwalik rodent faunas of northern pakistan abstract fossil assemblages from the siwalik group collected over the past six years reveal the development of the modern rodent fauna of the potwar plateau pakistan the history of the potwar rodent fauna proceeds through four stages 1 extinction of archaic elements by 9 ma 2 faunal turnover around 7 ma by phyletic evolution immigration and local extinction 3 attainment of modern grade at about the pliocene pleistocene boundary and 4 increase in abundance of desert forms and agricultural pests probably associated with man and overgrazing

an isotopic study of a fluvial lacustrine sequence the plio pleistocene koobi fora sequence east africa stable isotopic analyses of plio pleistocene and modern sediments in the fluvial lacustrine system occupying the turkana basin east africa provide constraints on the paleoenvironmental and diagenetic histories of the pliocene through the recent sediments in the basin the 13c values for carbonates in lacustrine sediments range from 15 to 22 relative to pdb depending on the varying proportions of co2 from the atmospheric reservoir and from various metabolic sources the 18o values of carbonates in lacustrine sediments indicate that the isotopic composition of paleolake water varied by over 10 from the pliocene to the present the 13c values for pedogenic carbonates record paleoccologic variations and suggest that c4 plants did not become well established in the preserved depositional parts of the basin until about 1 8 myr ago the 18o values pedogenic carbonates suggest a range of over 10 for the isotopic composition of soil water during this interval they also suggest a period of major climatic instability from about 3 4 to 3 1 myr and at about 1 8 myr together the 13c and 18o values of pedogenic carbonates indicate that the present conditions are as arid and hot as any that had prevailed during deposition of these plio pleistocene sediments

initial development of hf radar polar patch caused by azimuthal flow burst in the cusp 1 in this study we report evidence proving the existence of a close relationship between the initial development of a poleward propagating feature of the hf radar backscatter that is hf radar polar patch and a fast azimuthal flow in the cusp we focus on two events in which the hf radar polar patch was formed and was identified by the super dual auroral radar network radars at saskatoon canada and prince george canada on 27 march 2001 we examine their spatial and temporal development the two events occurred at the interface between the interplanetary magnetic field imf by controlled azimuthal flow and the antisunward flow typical of a negative imf bz they developed in a longitudinally asymmetric manner the increase in by bz when bz was negative triggered the fast azimuthal flow we interpret the temporal and spatial development of the hf radar polar patch in terms of the plasma density gradient created by the azimuthal flow and in terms of the subsequent development of the gradient in the direction of both the azimuthal flow and the antisunward flow

stratification of east west plasma flow channels observed in the ionospheric cusp in response to imf by polarity changes 1 during a period of predominantly north westward flow for imf bz negative and imf by positive a sequence of three distinct negative excursions of imf by resulted in a train of three eastward directed flow channels interleaved by westward flow enhancements propagating into the polar cap the high resolution of the eiscat svalbard radar data enables us to track formation and movement of the flow channels which are interpreted as a sequence of intermittent reconnection alternating between different reconnection sites our observations are consistent with the view that a new region of reconnected flux manifests as development of a distinct flow channel near the polar cap boundary and that successive events stay separated while pushing each other into the polar cap each flow channel will remain separated from neighboring channels mapping to different reconnection sites as long as the magnetic tension force with its associated field aligned current systems is maintained

the trans atlantic geotraverse hydrothermal field a hydrothermal system on an active detachment fault abstract over the last ten years geophysical studies have revealed that the trans atlantic geotraverse tag hydrothermal field 26 08 n on the mid atlantic ridge is located on the hanging wall of an active detachment fault this is particularly important in light of the recognition that detachment faulting accounts for crustal accretion extension along a significant portion of the mid atlantic ridge and that the majority of confirmed vent sites on this slow spreading ridge are hosted on detachment faults the tag hydrothermal field is one of the largest sites of high temperature hydrothermal activity and mineralization found to date on the seafloor and is comprised of active and relict deposits in different stages of evolution the episodic nature of hydrothermal activity over the last 140 ka provides strong evidence that the complex shape and geological structure of the active detachment fault system exerts first order but poorly understood influences on the hydrothermal circulation patterns fluid chemistry and mineral deposition while hydrothermal circulation extracts heat from a deep source region the location of the source region at tag is unknown hydrothermal upflow is likely focused along the relatively permeable detachment fault interface at depth and then the high temperature fluids leave the low angle portion of the detachment fault and rise vertically through the highly fissured hanging wall to the seafloor the presence of abundant anhydrite in the cone on the summit of the tag active mound and in veins in the crust beneath provides evidence for a fluid circulation system that entrains significant amounts of seawater into the shallow parts of the mound and stockwork given the importance of detachment faulting for crustal extension at slow spreading ridges the fundamental question that still needs to be addressed is how do detachment fault systems and the structure at depth associated with these systems e g presence of plutons and or high permeability zones influence the pattern of hydrothermal circulation mineral deposition and fluid chemistry both in space and time within slowly accreted ocean crust

a biostratigraphic zonation of the middle and upper siwaliks of the potwar plateau of northern pakistan abstract two biostratigraphic reference sections are designated for the middle and upper siwalik formations of northern pakistan the stratigraphic ranges of 41 mammalian taxa are established in the reference sections and the two sections are correlated to each other and to the standard magnetic polarity time scale by the magnetic polarity stratigraphy the resulting stratigraphic and magnetostratigraphic framework is used to define four contiguous biostratigraphic interval zones the proposed interval zones are bounded by five important faunal events which are identified in the reference sections the four proposed interval zones and the approximate ages in the reference sections of their lower and upper boundaries are 1 the hipparion s l interval zone 7 4 9 5 m y b p 2 the selenoportax lydekkeri interval zone 5 3 7 4 m y b p 3 the hexaprotodon sivalensis interval zone 5 3 2 9 m y b p and 4 the elephas planifrons interval zone 2 9 1 5 m y b p the paleomagnetic and biostratigraphic evidence indicates that the tatrot beds from which pilgrim developed his tatrot faunal zone are between 2 5 and 3 4 m y b p with a preferred maximum age of 2 9 m y b p the biostratigraphic evidence indicates the sethi nagri locality y311 the primary source of pilgrim s nagri faunal zone is in the hipparion s l interval zone the paleomagnetic evidence suggests an approximate age of 9 m y b p a major faunal event at 9 5 m y b p introduced equids suids and large giraffes from eurasia into the indain subcontinent a second more diffuse faunal event between 5 3 and 2 9 m y b p introduced several african taxa as well as equus and cervids from eurasia

northwestward decline of magnetic susceptibility for the red clay deposit in the chinese loess plateau 1 the loess paleosol red clay sequence in the chinese loess plateau has attracted attention for its potential in recording late cenozoic climatic changes over northern china although the details of the climatic implications of the proxies for example magnetic susceptibility and the reconstructed summer monsoon changes remain unclear here we report on a new section from baishui in gansu province and compare it with the lingtai and jingchuan sections by correlating the magnetic susceptibility from these sections we show that there is a clear systematic decrease of the magnetic susceptibility in the red clay deposits from the southeast lingtai section to the northwest baishui section this result suggests that the climatic parameters probably precipitation had a decreasing northwestward gradient during the red clay deposition this climatic pattern is similar to the present condition implying that the influence of the east asian summer monsoon on the chinese loess plateau has persisted from at least 6 ma

gold in sea floor polymetallic sulfide deposits bulk chemical analyses of 48 samples of hydrothermal precipitates from seven polymstallic sulfide deposits at sea floor spreading centers in the eastern pacific reveal distinct patterns of gold enrichment high gold contents are found in samples from two sulfide deposits the axial seamount 45057 n 130002 w and the southern explorer ridge 49045 6 n 130016 2 w a large 160 kg sample composed of silica barite and sphalerite from the top of the axial seamount deposit gives analyses up to 6 700 ppb au averaging 4 900 ppb au similar material from the southern explorer ridge gives analyses up to 1 500 ppb au averaging 660 ppb au detailed mineralogical studies together with bulk chemical analyses of a wide range of samples from these two sites reveal strong elemental associations and paragenetic controls on gold deposition gold at concentrations of about 200 ppb is associated with high cu 1 wt and mo up to 470 ppm at concentrations 800 ppb with high zn 10 wt ba 3 wt and sio 20 wt and at concentrations 1 200 ppb with high pb 0 1 wt ag 100 ppm as 300 ppm and sb 50 100 ppm samples with the highest gold values from both locations contain late sinterlike low temperature sulfosalts of pb as sb ag and s in a matrix of amorphous silica these sulfosalts are the probable repositories of the gold published analyses of au poor 300oc cu fe rich sulfides and subsequently remobilized by late sustained low temperature 250oc fluids and concentrated in sio ba zn rich precipitates near the surface suitable fluid chemistry is required to mobilize the gold and a favorable precipitation mechanism is needed to concentrate it these observations could explain the occurrence of high gold concentrations in selected ore types of some ancient massive sulfide deposits on land

lateral trends in carbon isotope ratios reveal a miocene vegetation gradient in the siwaliks of pakistan isotopic analyses of mammalian tooth enamel from a well defined laterally extensive 150 k y interval 9 15 9 30 ma reveal an ecological gradient in vegetation on the late miocene sub himalayan alluvial plain two contemporaneous river systems deposited the sediments of this interval with a mountain sourced system herein blue gray to the southwest interfingering with a foothill sourced system buff to the northeast fossil mammal teeth collected from a 32 km transect across this fluvial gradient are significantly more depleted in 13c from northeastern localities than from southwestern localities this trend occurs in equids giraffids suids sivapithecine hominoids and anthracotheres we propose that the buff fluvial system provided more equably moist substrate conditions and supported more closed canopy vegetation than the blue gray fluvial system herbivores living along the paleovegetation gradient thus acquired different carbon isotopic signatures during the period of tooth enamel formation resulting from higher 13c values in the forage supported by the blue gray fluvial system compared with forage associated with the buff system the data also imply that many siwalik mammalian herbivores displayed marked fidelity in juvenile home ranges and habitats

development of asian monsoon revealed by marked ecological shift during the latest miocene in northern pakistan carbon isotopes from soil carbonate1 4 and soil organic matter5 6 yield palaeoecological information because the carbon in the soil carbonate forms in isotopic equilibrium with local soil co2 refs 1 7 the isotopic composition of which is in turn determined by local plant cover siwalik group sediments in northern pakistan contain a well exposed palaeosol record spanning the past 18 myr here we report on stable carbon isotope results from associated pedogenic carbonate which indicate a dramatic ecological shift from to dominated floodplain biomass beginning 7 4 7 0 myr ago the earlier c3 floodplain biomasses were probably mainly composed of trees and shrubs whereas c4 grasslands dominated in the plio pleistocene oxygen isotopes also exhibit a shift in the latest miocene probably corresponding to a major climate change which may have induced the forest to grassland transition this dramatic ecological shift in the latest miocene may mark the inception or a marked strengthening of the asian monsoon system

black smokers massive sulphides and vent biota at the mid atlantic ridge the discovery of black smokers massive sulphides and vent biota in the rift valley of the mid atlantic ridge demonstrates that this assemblage of hydrothermal phenomena is not limited to intermediate to fast spreading oceanic ridges hydrothermal exchange processes may thus be important at the ridges which extend through the atlantic ocean and western indian ocean comprising more than half of the 55 000 km global length of seafloor spreading centres

ostrich expansion into india during the late pleistocene implications for continental dispersal corridors abstract new evidence is presented for the earliest occurrence of ostrich struthio sp in india during the late pleistocene along with a synthesis on the evidence for ostrich populations in the subcontinent direct dating of ostrich eggshell using accelerator mass spectrometry ams radiocarbon methods on excavated samples from katoati rajasthan india is supported by optically stimulated luminescence osl dating of associated sediments to demonstrate the arrival of ostrich in india before 60 thousand years ago ka in addition the first stable isotope studies on ostrich eggshell from india have been conducted yielding a new form of palaeoenvironmental proxy data for the late pleistocene the geographic expansion of ostrich into india corresponds with the distribution of sahel like environments bordering but not substantially colonising endemic indian vegetation zones the dispersal of ostrich into india marks a rare introduction of megafauna into the subcontinent during the late pleistocene the longevity of which spans a period more than 40 ka the timespan and range of this colonisation indicate the availability and exploitation of suitable habitats in india the continental dispersal of ostrich into india during the late pleistocene offers useful insights into the debate surrounding the dispersal of modern humans and contrasts with the hypothesized coastal movement of homo sapiens into india

statistical patterns of high latitude convection obtained from goose bay hf radar observations we have derived patterns that describe the statistical interplanetary magnetic field imf dependencies of ionospheric convection in the high latitude region of the northern hemisphere the observations of plasma motion were made with the hf coherent backscatter radar located at goose bay labrador over the period september 1987 to june 1993 the area covered by the measurements extended poleward of 65 to a working limit of about 85 distributions of electrostatic potential have been derived and expressed as series expansions in spherical harmonics the patterns are the first derived from direct ground based observations of ionospheric convection that approach in completeness and level of detail the patterns derived in recent satellite studies rich and hairston 1994 weimer 1995 we show the dependence of the convection on imf angle in the gsm y z plane for three intervals of imf magnitude in this plane except for predominantly northward imf the convection is primarily two cell the dusk cell is larger in terms of both spatial extent and potential variation the effect of imf by is apparent in the global shaping of the cells and the orientation of the overall pattern in mlt for by by the dusk dawn cell is more round crescent shaped and the pattern more rotated toward earlier mlts the by effect on the nightside convection is pronounced and is hemispherically antisymmetric like the well known day side by effect for imf increasingly northward the convection trajectories on the dayside become increasingly distorted evolving through a three cell to a four cell circulation the additional cells appear on either side of the noon meridian and result in sunward flow the overall agreement with the results of the satellite studies is good and extends to quite fine detail in the case of the comparison with weimer 1995 there are significant differences with the statistical patterns derived from magnetometer measurements which tend to show domination by the dawn rather than the dusk cell

activation of the nyainqentanghla shear zone implications for uplift of the southern tibetan plateau neogene extension of the tibetan plateau is manifested as a series of north south trending graben the most prominent of which is the yadong gulu rift the nyainqen tanghla shan a ne sw trending mountain range 100 km nw of lhasa bounds the western margin of the yangbajian graben the central segment of the yadong gulu rift the eastern edge of the nyainqentanghla massif is marked by a low angle 25 o detachment fault shear zone of amphibolite grade mylonites the noar 3 ar thermal history results from samples collected along two deeply incised valleys within the massif reveal that a rapid cooling event propagated from 8 ma in the core of the range to 4 ma within the high strain zone at the eastern boundary assuming that faulting initiated at high angle 60o thermal histories were fit to a numerical simulation of slip on a normal fault to yield estimates of both the age of fault initiation and the slip rate history the form of the isotopically derived thermal histories are similar to general form predicted by the thermal model and suggests that significant movement began at 8 1 ma in the southern valley goring la and proceeded at an average slip rate of 3 mm yr between 8 and 3 ma a more complex history is required to fit the data from the northern valley balum chun but the timing of initiation and average slip rate are similar to the goring la result numerical simulations in which the fault angle is varied indicate that the isotopically derived temperature histories are inconsistent with slip occurring at low angle 40o because the extension direction of the yangbajian graben is representative of most rifts on the southern tibetan plateau our data suggest that crustal thickness and elevation reached close to their present values by 8 1 ma a carbon isotopic shift in pedogenic carbonates from the siwalik formation at about 7 5 ma appears to reflect intensification of the asian monsoon and by inference that the plateau had attained an important threshold elevation by that time formation of a diffuse plate boundary in the indian oceanic lithosphere beginning at 7 5 8 0 ma is also consistent with this history we suggest that the plateau had attained a threshold area and elevation by 8 1 ma sufficient to trigger these three independent manifestations

chemical and mineralogical influences on concentrations of trace metals in hydrothermal fluids abstract concentrations of trace metals in hydrothermal solutions from the southern juan de fuca ridge sjfr and the tag hydrothermal field on the mid atlantic ridge are influenced by the combined effects of source rock composition brine seawater mixing and complex interplay among various metals and minerals deposited and remineralized in vent mounds and chimneys copper mo and co show predicted sharp decreases in concentration for vent fluids as temperatures decrease below 350 c however concentrations of cu in 363 c fluids at the tag site are well above values determined from seawater basalt interactions due to subseafloor refining processes and remineralization of chalcopyrite concentrations of zn cd pb as and ga do not show any temperature dependence between 214 to 363 c however zn values do correlate well with cl on an area by area basis due to subseafloor mixing of brine with altered seawater zinc levels also correlate well with concentrations of cd pb and as for all vent fluid samples as well as with basalt and zn rich sulfides implying some continuity in behavior from source rock to vent fluid to sulfide minerals gallium values correlate with zn and cl levels for the sjfr but are high relative to zn at the tag site due to subseafloor refining processes concentrations of tl follow cl k and rb consistent with the behavior predicted for tl overall the cl and zn rich fluids from the sjfr are characteristic of a brine dominated system formed during recent volcanism in contrast high temperature fluids from the tag area are representative of a system that has evolved over the past 100 000 yr with high concentrations of cu co and ga that result from higher temperatures and complex refining processes within a large vent mound

transition stratigraphy and the problem of remanence lock in times in the siwalik red beds the record of paleomagnetic polarity transitions can provide relative time information in the stratigraphic record on scales of hundreds to thousands of years transition stratigraphy is here used to determine the relative lock in times of several components of magnetization in the siwalik red beds

paleomagnetic chronology fluvial processes and tectonic implications of the siwalik deposits near chinji village pakistan a 2800 m section of siwalik strata containing the stratotypes for both the chinji and nagri formations has been dated by magnetic polarity stratigraphy and the observed polarity zonation has securely been correlated with the chron 17 7 segment of the time scale the base of the section is the base of the kamlial formation which occurs near the top of chron 17 18 3 m y the kamlial chinji formation boundary occurs in the middle of chron 15 14 3 m y the chinji nagri boundary near the bottom of chron 10 10 8 m y and the nagri dhok pathan boundary at the chron 8 9 boundary 8 5 m y the siwalik deposits near chinji village consist of four distinct classes of fluvial cycles each with a characteristic periodicity a first order of 10 7 years a second order of 10 6 years a third order of 10 4 10 5 years and a fourth order of 10 0 10 4 years the river system responsible for siwalik sedimentation flowed from west to east and had properties much like the modern ganges river syste

magnetic polarity stratigraphy and ages of siwalik group rocks of the potwar plateau pakistan abstract six magnetic polarity sections have been established over the potwar plateau region of pakistan including the major stratotypes of the siwalik group in all six sections the dominant feature of the magnetic polarity stratigraphy is a long normal polarity zone which is contained within the nagri formation this conspicuous normal polarity zone has been radiometrically dated at 9 5 0 6 m y which identifies it as magnetic chron 9 radiometric dates from the upper siwalik formation have also been used to identify the chron 2 3 boundary in two of the sections the magnetic polarity stratigraphy of three of the sections has been correlated securely with the accepted magnetic polarity time scale so that the ages of the local stratigraphy are indexed accordingly based exclusively on data from stratotype sections the chinji nagri and dhok pathan formations have nominal age ranges of 10 1 13 1 7 9 10 1 and 5 1 7 9 m y age fluctuations on the order of 105 years may be anticipated for these formational boundaries within radius of some 20 km of the designated stratotype mean sediment accumulation rates during the chinji nagri and dhok pathan interval range from 13 to 52 cm 103 yr essentially linear sediment accumulations are locally maintained over time intervals of several million years the chinji nagri lithofacies boundary marks a transition from slow to faster sediment accumulation over much of the potwar plateau indicating a fundamental sedimentary tectonic change at this time

large scale imaging of high latitude convection with super dual auroral radar network hf radar observations the hf radars of the super dual auroral radar network superdarn provide measurements of the e b drift of ionospheric plasma over extended regions of the high latitude ionosphere with the recent augmentation of the northern hemisphere component to six radars a sizable fraction of the entire convection zone approximately one third can be imaged nearly instantaneously 2 min to date the two dimensional convection velocity has been mapped by combining line of sight velocity measurements obtained from pairs of radars within common volume areas we describe a new method of deriving large scale convection maps based on all the available velocity data the measurements are used to determine a solution for the distribution of electrostatic potential expressed as a series expansion in spherical harmonics the addition of data from a statistical model constrains the solution in regions of no data coverage for low order expansions the results provide a gross characterization of the global convection we discuss the processing of the radar velocity data the factors that condition the fitting and the reliability of the results we present examples of imaging that demonstrate the response of the global convection to variations in the interplanetary magnetic field imf in the case of a sudden polarity change from northward to southward imf the convection is seen to reconfigure globally on very short 6 min timescales

a time framework based on magnetostratigraphy for the siwalik sediments of the khaur area northern pakistan the magnetostratigraphy of eleven new sections in the khaur area of northern pakistan is presented all specimens have been subjected to thermal demagnetization the sections taken in adjacent ravines incising the siwalik formations nearly penpendicular to strike form a temporal framework in which to interpret the biological and lithological information a composite section is constructed from several long sections and is correlated to the magnetic polarity time scale chron 6 to chron 15 providing absolute age information for the biostratigraphic zonations of barry et al 1982 detailed sections along the strike of major stratigraphic marker beds document an isochronous horizon and form the basis for paleoenvironmental reconstructions the preservation of a polarity transition in fluvial sediments suggests continuity of sedimentation on a time scale of 103 104 years

hf radar polar patch and its relation with the cusp during by dominated imf simultaneous observations at two altitudes 1 recent studies have shown that the motion of the cusp can be deduced from the energetic neutral atom signals detected in the magnetosphere by the low energy neutral atom lena imager on the image spacecraft we use this approach to understand the characteristics of the formation of a polar patch seen in the dayside ionosphere during a period of the 28 march 2001 lena cusp signal event the superdarn radars at syowa east syowa south and kerguelen island identified large scale features of a polar patch a region of high backscatter power observed by the radars separates into two parts around 77 and its high latitude part moves in the poleward and duskward direction the separation latitude is about 5 higher than the equatorward boundary of the cusp which is deduced from the lena cusp signal we interpret these observations including features obtained with other superdarn radars in the northern hemisphere as being due to the imf by controlled zonal jet flow that occurred during a period of increase in by bz without requiring the change in by polarity that has been often invoked in previous studies the sharp equatorward boundary of the radar signatures of the polar patch would be an interface between the preexisting flow generally in the antisunward direction and the enhanced zonal flow the flow enhancement appears to be a fundamental process that forms the large scale polar patch at latitudes several degrees higher than the cusp

small mammal carbon isotope ecology across the miocene pliocene boundary northwestern argentina abstract the late miocene expansion of plants using the c 4 photosynthetic pathway in south america has been documented by tooth enamel carbon isotope ratios 13 c en however a more detailed understanding of this ecological event is hampered by poor chronological control on the widespread fossil localities from which isotopic data are derived this study develops a 13 c en record from a single 2500 m thick stratigraphic section in subtropical south america strata at puerta de corral quemado pcq northwestern argentina span 9 to 3 5 ma in age and existing paleosol carbonate data 13 c pc document c 4 expansion across the miocene pliocene boundary comparison of 13 c en data with 13 c pc data at high stratigraphic resolution refines understanding of this ecological event in south america small mammal 13 c en data in particular are complementary to that of large mammal and paleosol 13 c data small mammal teeth integrate isotopic data over much shorter temporal and spatial scales than large mammal teeth providing a sensitive measure of local vegetation and placing constraints on the landscape distribution of c 3 and c 4 plants explicit consideration of the distinctive carbon isotope enrichment factor between enamel and diet for rodents e en diet 11 as opposed to 14 for large mammals allows for unequivocal inference of c 4 vegetation 1 ma prior to that inferred from large mammal 13 c en data and 2 ma prior to 13 c pc data this multiproxy record demonstrates that c 4 plants were a stable component of the ecosystem hundreds of thousands of years prior to their major ecological expansion and that the expansion of c 4 plants was pulsed at pcq two periods of ecological change are demonstrated by 13 c and 18 o data at 7 ma and 5 3 ma coincident with the miocene pliocene boundary development of small mammal 13 c en records on other continents may provide similar insight into the early stages of the global c 4 event

mineralogy chemical composition and structure of the mir mound tag hydrothermal field the study of samples collected from the surface of the mir mound tag hydrothermal field by video controlled hydraulic grab allowed identification of a number of mineralogical types these include pyrite chalcopyrite py cp bornite chalcopyrite opaline bn cp op and sphalerite opaline sp op sulfide chimneys massive sulfides composed of pyrite py chalcopyrite pyrite cp py marcasite pyrite opaline mc py op sphalerite pyrite opaline sp py op and sphalerite chalcopyrite pyrite opaline sp cp py op as well as siliceous and fe mn oxide hydrothermal deposits most of the minor elements ag au cd ga hg sb and pb are associated with zn rich massive sulfides co bi pb and as with ferich ones while cu rich sulfides are depleted of trace metals cu enriched assemblages are concentrated in the northern part zn enriched in the center and siliceous rocks in the south of the mir mound according to paragenetic relations the development of the mound started with the formation of quartz originally opaline rocks and dendritic assemblages of melnikovite pyrite followed by deposition of chalcopyrite and recrystallization of primary pyrite subsequent generation of sphalerite rich assemblages and final deposition of opaline rocks the late renewal of hydrothermal activity led to local formation of cu rich chimneys enriched in au ag hg and pb probably due to their remobilization from inner parts of the deposit

pakistan mammal tooth stable isotopes show paleoclimatic and paleoenvironmental changes since the early oligocene abstract this research presents an isotopic study of three large mammal groups elephantoid proboscideans hyracodontid and rhinocerotid rhinocerotoids equids of the hipparion group from the bugti hills sequence eastern balochistan pakistan combined with published data from the miocene siwalik group northern central pakistan these two exceptional fossil records from pakistan provide an unambiguous source of biotic information from the early oligocene up to the late miocene the 13 c c and 18 o c results from the same herbivore groups show several significant variations in environmental and climatic conditions throughout the 30 to 7 5 myr period in this region first from 30 to 22 myr the environment is characterized by a relatively dry but dense forest existing under a temperate to subtropical climate with a more closed tropical environment at 23 myr oligocene 18 o of the terrestrial herbivore teeth agrees with the marine foraminifera 18 o and mg ca trends suggesting that the pakistan region was affected by global climate rather than regional conditions after 22 myr the 18 o c mammal enamel results show a long term decreasing trend until 15 myr toward a wetter environment with the appearance of tropical forests in response to the probable onset of the asian monsoon this is a period of high tectonic activity with the india asia collision leading to uplift of the tibetan plateau and shrinkage of the epicontinental paratethys sea this activity modified the land sea distribution and the thermal contrast between ocean and continent the driving force of the monsoon climate the middle miocene environment in pakistan experienced moist conditions under an intense monsoon system with relatively closed rainforest after 12 myr the 18 o c enamel values record a positive shift until 9 3 myr toward a drier environment with an increase of seasonality as suggested by higher 18 o c variability after 9 3 myr these late miocene drier climatic conditions and the asian monsoon intensification could potentially favour the c 4 photosynthetic pathway and hence the development of grassland to the detriment of subtropical forests resulting in the higher enamel 13 c values recorded after 9 3 myr