oceanic plateau formation by seafloor spreading implied by tamu massif magnetic anomalies tamu massif is an immense mesozoic submarine volcano the main edifice of the shatsky rise oceanic plateau it is located at a spreading ridge triple junction but considered to be a shield volcano formed by effusive volcanism from an emerging mantle plume however it is unclear how tamu massif eruptions interacted with the spreading ridges which are enormous linear volcanoes themselves here we create a magnetic anomaly map for tamu massif which can provide clues about crustal formation for tamu massif we find dominantly linear magnetic field anomalies caused by crustal blocks of opposite magnetic polarity this pattern suggests that tamu massif is not a shield volcano but was emplaced by voluminous focused ridge volcanism if the magma source at the shatsky rise was a plume it was closely connected to and controlled by seafloor spreading by implication even the largest oceanic plateau edifices can be formed by seafloor spreading we suggest that the widely accepted analogy between continental flood basalts and oceanic plateaus requires reconsideration the shatsky rise oceanic plateau formed by spreading ridge volcanism according to analyses of linear magnetic anomalies over the tamu massif submarine volcano

groundwater composition hydrochemical evolution and mass transfer in a regional detrital aquifer baza basin southern spain abstract the physical chemical characteristics of the groundwater in the baza caniles detrital aquifer system indicate that a wide diversity of hydrochemical conditions exists in this semiarid region defining geochemical zones with distinct groundwater types the least mineralized water is found closest to the main recharge zones and the salinity of the water increases significantly with depth towards the center of the basin geochemical reaction models have been constructed using water chemistry data along flow paths that characterize the different sectors of the aquifer system namely quaternary aquifer unconfined sector and shallow and deep confined sectors of the mio pliocene aquifer geochemical mass balance calculations indicate that the dominant groundwater reaction throughout the detrital system is dedolomitisation dolomite dissolution and calcite precipitation driven by gypsum dissolution this process is highly developed in the central part of the basin due to the abundance of evaporites apart from this process there are others which influence the geochemical zoning of the system in the quaternary aquifer which behaves as a system open to gases and which receives inputs of co 2 gas derived from the intensive farming in the area the interaction of the co 2 with the carbonate matrix of the aquifer produces an increase in the alkalinity of the water in the shallow confined sector of the mio pliocene aquifer the process of dedolomitisation evolves in a system closed to co 2 gas ca 2 na cation exchange and halite dissolution processes are locally important which gives rise to a relatively saline water finally in the deep confined sector a strongly reducing environment exists in which the presence of h 2 s and nh 4 in the highly mineralized groundwater can be detected in this geochemical zone the groundwater system is considered to be closed to co 2 gas proceeding from external sources but open to co 2 from oxidation of organic matter the geochemical modeling indicates that the chemical characteristics of this saline water are mainly due to so 4 dissolution dedolomitisation and so 4 reduction coupled with microbial degradation of lignite

northwest pacific tectonic evolution in the middle mesozoic abstract two magnetic bights implying pacific izanagi farallon p i f ridge ridge ridge rrr triple junctions occur in the northwest pacific basin one is formed in anomalies m26 m21 southeast of shatsky plateau while the second is formed in anomalies m10 cl1 the latter a new anomaly between m1 and m0 northeast of shatsky plateau the offset between these two bights is in a n40 e direction parallel to the p f relative motion during m20 to m10n time half spreading rates and directions and the assumption of symmetric orthogonal spreading permit the solution of the p i f velocity triangle and reveal an isosceles triangle with the i f rate equal to the p f rate this is a necessary condition for a stable fault fault ridge ffr triple junction which would migrate in the n40 e direction of the offset between magnetic bights the growth of the p f transform fault explains the increasing offset across the ends of anomalies m20 m10n projected onto the trace of shatsky plateau subsequent evolution led to the formation of a system of long transform faults offset by short spreading centers which we call the shatsky fracture zones and which extend eastward across the pacific from the plateau the original trace of the transform fault fracture zone that formed at the triple junction underlies the main mass of the shatsky plateau and would suggest that the formation of the plateau is at least coincidental with the existence of the ffr triple junction at 21 ma

a new cretaceous tertiary boundary site at flaxbourne river new zealand biostratigraphy and geochemistry abstract an exceptionally complete rock sequence across the cretaceous tertiary k t boundary has been discovered near the flaxboume river marlborough province south island new zealand the boundary is marked by a large ir anomaly 21 ng g on a decalcified basis with an integrated abundance of 134 ng cm2 after correction for background above the boundary there is a 30 cm transition zone in which a few cretaceous foraminiferal taxa such as hedbergella monmouthensis and guembelitria cretacea survived though with reduced abundance and size only ca 1 2 to 1 4 of normal apparently reflecting environmental stresses inaa and icp analyses show that in addition to ir the boundary clay is also enriched in cr and ni mainly from meteoritic material and as co cu sb and zn from terrestrial sources volcanic sources even when scaled to the 10 7 km 3 volume of the deccan basalts fail by three orders of magnitude to account for the ir and as at the k t boundary and by even larger factors for sb zn cu etc comparison of our data with those from six other k t boundary sites shows that the zn sb as sb and zn as ratios generally fall between crustal and oceanic values suggesting contributions from both sources mass balance calculations show that as and sb could be derived from only 300 500 m of ocean water or also from modest amounts 20 36 g cm 2 of average crustal rock copper and zn on the other hand can only be derived from crustal or mantle rock 5 15 g cm 2 presumably impact ejecta such an amount of ejecta is fairly close to the global fallout of boundary clay 2 5 g cm 2

the use of hydrochemical sections to identify recharge areas and saline intrusions in alluvial aquifers southeast queensland australia abstract alluvial ground waters used for irrigation in the southwest lockyer valley queensland experience varying salinity problems samples were taken on three occasions along transects in the valleys of the tenthill and ma ma creeks during the dry season august 1984 the wet season february 1985 and during a period of drought january 1987 the waters are categorized by chemical type and hydrochemical sections are plotted for major cation data magnesium calcium and sodium the interpretation of the sections permits the identification of recharge areas for the alluvial aquifers and the location of saline intrusions into the alluvium from underlying sandstone aquifers it is also possible with the assistance of chemical type to correlate some saline intrusions in adjacent valleys as issuing from the same sandstone aquifers a diagram to summarize the locations and magnitudes of salinity changes along transects is given

microplate tectonics along a superfast seafloor spreading system near easter island there is a growing easter microplate between the pacific and nazca plates the eastern and western boundaries of the microplate are actively spreading and propagating north and south respectively the northern and southern boundaries are broad transform zones most of the interior of the microplate contains the sort of oblique sheared structures characteristic of zones of transferred lithosphere between propagating and failing spreading centres

mass mortality and its environmental and evolutionary consequences the latest mesozoic and earliest tertiary sediments at deep sea drilling project site 524 provide an amplified record of environmental and biostratographic changes at the end of cretaceous closely spaced samples representing time intervals as short as 102 or 103 years were analyzed for their bulk carbonate and trace metal compositions and for oxygen and carbon isotopic compositions the data indicate that at the end of cretaceous when a high proportion of the ocean9s planktic organisms were eliminated an associated reduction in productivity led to a partial transfer of dissolved carbon dioxide from the oceans to the atmosphere this resulted in a large increase of the atmospheric carbon dioxide during the next 50 000 years which is believed to have caused a temperature rise revealed by the oxygen isotope data the lowermost tertiary sediments at site 524 include fossils with cretaceous affinities which may include both reworked individuals and some forms that survived for a while after the catastrophe our data indicate that many of the cretaceous pelagic organisms became extinct over a period of a few tens of thousands of years and do not contradict the scenario of cometary impact as a cause of mass mortality in the oceans as suggested by an iridium anomaly at the cretaceous tertiary boundary

siderophile interelement variations in the cretaceous tertiary boundary sediments from caravaca spain a detailed profile of the siderophiles fe co ni pd re os ir pt and au from the cretaceous tertiary boundary at caravaca spain indicates that the major fractionation mechanisms are local redistribution reworking and diagenetic mobilization large fractions of the fe co and re and perhaps a small amount of pd and au appear to be terrestrially derived but nearly all of the ni os ir and pt must be derived from an extraterrestrial source only the basal layers at caravaca and stevns klint denmark are representative of the initial fallout and integrated ir fluences across entire sections can be misleading these suggest a concentration oft 120mg g projectile in the fallout and a global fluence of chondritic matter oft 140mg cm 2

a new mesozoic isochron chart of the northwestern pacific ocean paleomagnetic and tectonic implications a new mesozoic isochron chart of the northwestern pacific ocean has been completed by utilizing all the currently available data from 357 cruises our chart provides the first whole view of the mesozoic magnetic anomaly lineations in the northwestern pacific ocean and yields essential information for understanding the problems of the mesozoic magnetostratigraphic time scale and the evolutional history of the pacific plate the mesozoic magnetostratigraphic time scale must be recalibrated using not only the hawaiian lineation set but also other mesozoic lineation sets the lack of lineation m0 in the lineation sets other than the hawaiian lineation set indicates that a reorganization of the ridges except the pacific farallon ridge occurred between chrons m1 and m0 nearly 120 ma the eruption rate of the shatsky hotspot seems to be comparable to that of the reunion hotspot at their initial stage the growth of the pacific plate began in a small triangular region at about 190 ma

a sequence of events across the cretaceous tertiary boundary the lithological and biological sequence of events across the cretaceous tertiary k t as developed in thick and complete landbased sections and termed the standard k t event sequence is also found in many i sdp cores from all over the globe microtektite like spherules have been found in almost every core or outcrop with an iridium anomaly forming a worldwide strewnfield these microtektites are an excellent indicator of the k t boundary level the basal paleocene globigerina eugubma zone is nov established in all major ocean basins the difference in faunas between the pacific and tethys atlantic oceans may point to different recovery of the oldest paleocene planktonic foraminiferal assemblages in different ocean basins planktonic foraminifera and nannoplankton apparently show a different extinction recovery pattern at the k t boundary which suggests that the nannoplankton underwent final extinction later and recover later than planktonic foraminifera these patterns may be important for the evaluation of the type and range of environmental stress resulting from the hypothesized impact s at the k t boundary blocking of sunlight by the dust cloud still can account for the clear mass mortality at the k t boundary but seems less likely as explanation for the final nannoplankton extinction as photosynthetic nannoplankton appear to survive 1000 10 000 years into the tertiary

the cretaceous tertiary boundary problem an assessment from lead isotope systematics abstract the presence of high concentrations of siderophile elements particularly of ir in clay rich layers corresponding to the end of the cretaceous and thus correlated with many mass extinctions has given rise to various studies and theories to provide new isotopic arguments we have used u pb systematics in four famous ir rich locations where the cretaceous tertiary k t boundary is exposed three were marine sites stevns klint denmark caravaca spain and a site just near woodside creek new zealand and one was a continental site raton basin colorado new mexico u s a in all sites the pb isotopic ratio is characteristic of local conditions without any change at the k t boundary these pb isotopic ratios are different from those of carbonaceous chondrites iron meteorites and deccan basalts in stevns klint as in caravaca the amount of pb increases very suddenly at the k t boundary because this pb enrichment occurred only in two boundary beds which contain pyrites or iron hydroxides we suggest that pb and ir might have been concentrated in the clay level by precipitation of sulfides produced by anaerobic bacterial activity the enrichment remained even after alteration processes destroyed the sulfides leaving iron hydroxides in the boundary levels these pb data imply also a possible terrestrial explanation for other volatile element enrichments in k t boundary layers

the early cenozoic tectonic history of the southeast pacific abstract new marine geophysical data collected in the southeast pacific enable us to define better the early cenozoic tectonic history of the area and to interpret previously unexplained bathymetric features in terms of plate tectonic processes we have identified a major re organization in plate boundaries at anomaly 21 time when a large fragment was broken off the pacific plate and joined to the antarctic plate this interpretation is based on three pieces of evidence 1 a major ese trending feature the humboldt fracture zone marks the southern boundary of the farallon antarctic anomalies identified by weissel et al 1 2 a north south striking trough the hudson trough which represents the scar left by the pacific antarctic ridge when it rifted across old pacific crust 3 a sequence of north south striking magnetic lineations south of the humboldt fracture zone and east of the hudson trough is correlated to anomalies 27 to 22 and identified with pacific aluk spreading

iridium emissions from kilauea volcano during may 1983 gas and particulate samples were collected at the cooling vents of kilauea volcano in hawaii three vents on the southeast rift zone were sampled using base treated filter packs to absorb acidic gases and teflon filters for particles the samples were analyzed for 40 elements by nondestructive neutron activation analysis as with other volcanoes the chalcophilic and volatile elements were enriched by up to seven orders of magnitude relative to the erupted basalt unlike the case of other volcanoes iridium was observed to be highly enriched 105 fold at the two higher temperature vents and the degree of enrichment appears to be related to both high temperatures and high fluorine content of the gases estimates of the emission rates of ir during eruptions of kilauea based on the measurements reported here and those of the hawaiian volcano observatory yield an emission rate of about 3 g ir per 106 m3 of magma this rate amounts to only about 0 3 of the ir present in the magma that is being released by the volcano this previously unidentified ir source to the atmosphere and ocean may have a significant influence on the geochemical cycle of ir on the earth the uniqueness of kilauea in its emissions of ir is probably related to the deep source of magma which contains significant levels of ir 0 32 ppb and high levels of the halogens f and cl

the mesozoic tectonic history of the magellan microplate in the western central pacific magnetic lineation mapping in the western central pacific has revealed a pair of opposite sensed fanned lineation patterns that define the accretionary boundaries of the fossil magellan microplate this tectonic synthesis results from extensive magnetic mapping of two new lineation patterns over a large area and extended mapping of previously identified lineations the entire evolutionary history of the magellan microplate is well constrained to a 9 m y period in the early cretaceous by synchronous spreading patterns and associated geologic data during this period the microplate grew and evolved as a generally rectangular structure to a final size of 700 km 600 km with spreading centers on two opposing sides and transform faults on the other two sides the lifetime and size of the magellan microplate are somewhat longer and larger respectively than presently active microplates on the east pacific rise however these modern structures are still evolving and growing and the tectonic behavior of the modern and cretaceous systems appears to be similar study of both active and fossilized microplates should provide additional insights on their common tectonic histories in particular we show that the magellan trough spreading center behaved as an asymmetric accretionary plate boundary that can be described with two separate poles of motion very close to this spreading center during much of its history the magellan trough spreading center then failed as a result of a larger ridge reorganization at the triple junction of the pacific farallon and phoenix plates at ml0n time microplate activity ceased when the microplate became welded to the pacific plate at m9 time

geologic framework of nonmarine cretaceous tertiary boundary sites raton basin new mexico and colorado iridium concentrations are anomalously high at the palynological cretaceous tertiary boundary in fluvial sedimentary rocks of the lower part of the raton formation at several localities in the raton basin of new mexico and colorado the iridium anomaly is associated with a thin bed of kaolinitic claystone in a discontinuous carbonaceous shale and coal sequence

structural variation of the oceanic moho in the pacific plate revealed by active source seismic data abstract the characteristics of the oceanic moho are known to depend on various factors such as seafloor spreading rate crustal age and accretionary processes at a ridge however the effect of local magmatic activities on the seismic signature of the moho is poorly understood here an active source reflection and refraction survey is used to investigate crustal structure and moho characteristics along a 1000 km long profile southeast of the shatsky rise in a pacific ocean basin formed from the late jurassic to early cretaceous and spanning the onset of shatsky rise volcanism although the seismic velocity structure estimated from the refraction data showed typical characteristics of the oceanic crust of the old pacific plate the appearance of the moho reflections was spatially variable we observed clear moho reflections such as those to be expected where the spreading rate is fast to intermediate only at the southwestern end of the profile whereas moho reflections were diffuse weak or absent along other parts of the profile the poor moho reflections can be explained by the presence of a thick crust mantle transition layer which is temporally coincident with the formation of the shatsky rise we inferred that the crust mantle transition layer was formed by changes in on axis accretion process or modification of the primary moho by off axis magmatism induced by magmatic activity of the shatsky rise

iridium sulfur isotopes and rare earth elements in the cretaceous tertiary boundary clay at stevns klint denmark microbial activity and redox controlled precipitation have been of major importance in the process of metal accumulation in the strongly ir enriched cretaceous tertiary k t boundary clay the fish clay at stevns klint in denmark two important findings support this view 1 1 kerogen recovered by leaching the fish clay in hcl and hf shows an ir concentration of 1100 ppb this represents about 50 of the ir present in the bulk sample fish clay strong organometallic complexes is the most probable carrier phase for this fraction of ir kerogen separated from the k t boundary clay at caravaca spain similarly exhibits enhanced ir concentrations 2 2 sulfur isotope analyses of metal rich pyrite spherules which occur in extreme abundance about 10 by weight in the basal fish clay give a 34s value of 32 this very low value shows that sulfide formation by anaerobic bacteria was intensive in the fish clay during early diagenesis since the pyrite spherules are major carriers of elements such as ni co as sb and zn microbial activity may have played an important role for concentrating these elements in the fish clay large amounts of rare earth elements have precipitated from sea water on fish scales analyses reveal that compared with sea water the fish clay is only about four times less enriched in sea water derived lanthanides than in ir this shows that a sea water origin is plausible for elements that are strongly enriched in the clay but whose origin cannot be accounted for by a lithogenic precursor

palaeo digital elevation models for use as boundary conditions in coupled ocean atmosphere gcm experiments a maastrichtian late cretaceous example abstract palaeogeography specifically palaeotopography and palaeobathymetry provides an essential boundary condition for computer based atmosphere and ocean modelling it also provides the geographic context for understanding surface processes palaeodrainage palaeoweathering and biotic interactions palaeoecology palaeobiogeography with increased model resolution coupled ocean atmosphere general circulation models gcms and the addition of vegetation soil weathering ice and chemical modules there is now a need for more robust detailed palaeotopographies and palaeobathymetries that are fully integrated with the processes being modelled especially the hydrological system here we present a new geographic information system gis based hydrologically correct palaeo digital elevation model dem for the maastrichtian late cretaceous we describe the methods and concepts used to construct the map and draw attention to the limits imposed by scale and uncertainty and how these factors must be considered as part of the error analysis of derived model results the underlying palaeogeography is one of a series of 27 global maps that represent the stages of the cretaceous and sub epochs of the cenozoic each map is generated at a scale of 1 30 million in arcview gis and arcinfo using data from the lead author s own databases of lithologic tectonic and fossil information the lithologic databases of the paleogeographic atlas project the university of chicago a survey of published literature and dsdp odp data interpretations of elevation are derived following the methods outlined in ziegler et al ziegler a m rowley d b lottes a l sahagian d l hulver m l gierlowski t c 1985 paleogeographic interpretation with an example from the mid cretaceous annual review of earth and planetary sciences 13 385 425 an understanding of the tectonic regime and evolution of each feature and the age depth relationship for the ocean the global palaeo dem was derived using the elevation contours from the paleogeography and the suite of hydrological tools available in arcinfo grid the palaeo dem has been constrained by defining areas of internal palaeodrainage palaeoriver mouths and known palaeoriver courses the maastrichtian has been completed first to provide the boundary conditions for a series of coupled atmosphere ocean experiments when integrated with the results of the coupled ocean atmosphere model the result is a powerful tool for understanding surface processes and an important step towards the development of a continuous series representing a fully evolving palaeolandscape

trace element patterns at a non marine cretaceous tertiary boundary at the fossil pollen defined cretaceous tertiary boundary in the raton basin of new mexico and colorado an iridium abundance anomaly and excess scandium titanium and chromium are associated with a thin ash or dust fallout bed now kaolinitic clay that was preserved in freshwater coal swamps

late cretaceous and paleogene tectonic evolution of the north pacific ocean the late cretaceous history of the northern pacific ocean has not been adequately deciphered largely because a major plate reorganization occurred during the cretaceous magnetic quiet interval using primary data to reconstruct plate motions from fracture zone trends and late cretaceous seafloor spreading magnetic anomalies allows formulation of a reasonable sequence of events that accounts for all the geologic features of that region especially the emperor and chinook troughs the primary event in our reconstruction is the subduction of the old northwest pacific triple junction new relative plate motions imposed by formation of convergent boundaries along both the northern pacific and farallon plates caused the farallon plate to crack this subdivision occurred 82 m y ago and resulted in the formation of the kula and chinook plates the chinook plate was bounded on the north by the chinook kula ridge the western arm of the great magnetic bight on the west by the southern emperor trough a slowly spreading rift valley on the south by the mendocino transform and on the east by the chinook farallon ridge the northern emperor trough formed the initial western boundary of the kula plate from 82 m y ago to 50 m y ago this configuration was stable the chinook plate expanded the chinook kula ridge migrated north and the kula plate was subducted with the subduction of this ridge the chinook plate adhered to the pacific plate and the pacific farallon ridge became the north south trending feature reflected by anomalies 22 and younger this reconstruction obviates the awkward late cretaceous shifting of triple junctions found in other proposed histories and provides reasonable explanations for the several structural features of the region to achieve closure of vector triangles representing the relative late cretaceous plate motions some extension must occur presumably across the eastern mendocino fracture zone between the northern and southern portions of the farallon plate these two plates apparently were independent units between 105 and 50 m y ago the initiation and cessation of rapid north farallon north american convergence 82 and 50 m y ago may have determined the nature and duration of the laramide orogeny

the impact of hydrogeological settings on geochemical evolution of groundwater in karstified limestone aquifer basin in northwest sri lanka the geochemical and isotope analysis of groundwaters from the murunkan basin in north western sri lanka was carried out to examine their evolution and recharge sources in order to shed light to enhance the current knowledge of the hydro geochemical processes in a karst geological setting a total of 40 water samples from ground and surface water bodies were collected from the miocene limestone terrain nearby metamorphic and from unconsolidated quaternary terrains for major anions cations and environmental isotopes ratios 18ovsmow and 2hvsmow distinct geochemical differences were noted between waters from limestone terrain and nearby metamorphic terrain indicating the modification of groundwater flow paths bicarbonate chloride rich water is dominated in the limestone terrain in which water flows through a less mineralized aquifer system and is modified by the sea water intrusion groundwater in the metamorphic terrain is modified by dissolving of ca mg rich mineral phases and subsequent ion exchange processes the environmental isotopes of groundwater from both limestone and metamorphic terrains vary from 0 38 to 6 68 and 2 41 to 42 3 for 18ovsmow and 2hvsmow respectively however slightly enriched isotope signatures and low d excess values from limestone terrain indicate an excessive evaporation compared to that of the metamorphic terrain where rapid infiltration occurs through the overlying highly permeable grumusols soil layers

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

the nature of the kt impactor a 54cr reappraisal according to the variations observed in the various meteorite classes 54cr represents an isotopic tool for planetary body discrimination in the search for the nature of the cretaceous tertiary kt impactor cr isotopic measurements were already performed at kt boundary the current work is aimed to modernize cr isotopic data with high precision measurements to confirm a carbonaceous chondrite type infall the contribution of which is also estimated isotopic signatures of two marine clays caravaca stevens klint exhibit an isotopic ratio which would represent a mixing of a carbonaceous chondrite of cm2 type with terrestrial material in a ratio 6 to 19 a single impactor may account for both marine and continental cr isotopic signatures

geochemical and isotope characterization of geothermal spring waters in sri lanka evidence for steeper than expected geothermal gradients summary seven geothermal springs from the precambrian high grade metamorphic terrain of sri lanka were investigated to assess their formation processes and to determine reservoir temperatures based on their chemical compositions silica based geothermometric calculations for the marangala and nelumwewa springs showed the highest average reservoir temperatures of 122 c and 121 c respectively samples of low temperature

terminal cretaceous environmental events the geologic record of terminal cretaceous environmental events indicates that iridium and other associated elements were not deposited instantaneously but during a time interval spanning some 10 000 to 100 000 years the available geologic evidence favors a mantle rather than meteoritic origin for these elements these results are in accord with the scenario of a series of intense eruptive volcanic events occurring during a relatively short geologic time interval and not with the scenario of a single large asteroid impact event

pre cretaceous tectonic evolution of the pacific plate and extension of the geomagnetic polarity reversal time scale with implications for the origin of the jurassic quiet zone abstract linear magnetic anomalies resulting from seafloor spreading were mapped in the vicinity of the magnetic bight in the western pacific ocean new aeromagnetic data allowed the magnetic bight to be more accurately mapped from m21 to m28 and enabled the identification of low amplitude magnetic lineations in the jurassic quiet zone these lineations were formed by magnetic field reversals prior to m29 a revised jurassic geomagnetic polarity reversal time scale was constructed with nineteen reversals older than m29 numbered m30 m38 these reversals extend the record of geomagnetic polarity back in time by approximately 8 ma and are important constraints on the origin of the quiet zone in particular they imply that the jurassic was not a period of constant normal polarity an explanation offered by some authors further they cast doubt on a model of systematically decreasing geomagnetic field strength with increasing age during this period the early history of the northern pacific plate and pacific farallon izanagi p f i triple junction was traced by mapped magnetic isochrons the pacific plate seems to have evolved from a small plate that formed near the phoenix farallon izanagi triple junction about 180 188 m y ago at approximately 17 n 160 e in present coordinates until m21 time the evolution of the northern pacific plate was relatively simple and the p f i triple junction migrated north northwest with respect to the pacific

isotopic evidence for the cretaceous tertiary impactor and its type high precision mass spectrometric analysis of chromium in sediment samples from the cretaceous tertiary k t boundary coincident with the extinction of numerous organisms on earth confirms the cosmic origin of the k t phenomenon the isotopic composition of chromium in k t boundary samples from stevns klint denmark and caravaca spain is different from that of earth and indicates its extraterrestrial source the chromium isotopic signature is consistent with a carbonaceous chondrite type impactor the observed differences in the chromium isotopic composition among various meteorite classes can serve as a diagnostic tool for deciphering the nature of impactors that have collided with earth during its history

origin of groundwater salinity current seawater vs saline deep water in a coastal karst aquifer based on sr and cl isotopes case study of the la clape massif southern france origin of groundwater salinity current seawater vs saline deep water in a coastal karst aquifer based on sr and cl isotopes case study of the la clape massif southern france a b s t r a c t in this study a typical coastal karst aquifer developed in lower cretaceous limestones on the western med iterranean seashore la clape massif southern france was investigated a combination of geochemical and isotopic approaches was used to investigate the origin of salinity in the aquifer water samples were collected between 2009 and 2011 three groundwater groups a b and c were identified based on the hydrogeological setting and on the cl a concentrations average and maximum cl a concentrations in the recharge waters were calculated cl ref and cl ref max to be 0 51 and 2 85 mmol l respectively group a includes spring waters with cl a concentrations that are within the same order of magnitude as the cl ref concentration group b includes groundwater with cl a concentrations that range between the cl ref and cl ref max concentrations group c includes brackish groundwater with cl a concentrations that are significantly greater than the cl ref max concentration overall the chemistry of the la clape groundwater evolves from dominantly ca hco 3 to nacl type on binary diagrams of the major ions vs cl most of the la clape waters plot along mixing lines the mixing end members include spring waters and a saline component current seawater or fossil saline water based on the br cl molar ratio the hypothesis of halite dissolution from triassic evaporites is rejected to explain the origin of salinity in the brackish groundwater groundwaters display 87 sr 86 sr ratios intermediate between those of the limestone aquifer matrix and current mediterranean seawater on a sr mixing diagram most of the la clape waters plot on a mixing line the end members include the la clape spring waters and saline waters which are similar to the deep geo thermal waters that were identified at the nearby balaruc site the 36 cl cl ratios of a few groundwater samples from group c are in agreement with the mixing hypothesis of local recharge water with deep saline water at secular equilibrium within a carbonate matrix finally phreeqc modelling was run based on cal cite dissolution in an open system prior to mixing with the balaruc type saline waters modelled data are consistent with the observed data that were obtained from the group c groundwater based on several trac ers i e concentrations and isotopic compositions of cl and sr calculated ratios of deep saline water in the mixture are coherent and range from 3 to 16 and 0 to 3 for groundwater of groups c and b respectively with regard to the la clape karst aquifer the extension of a lithospheric fault in the study area may favour the rise of deep saline water such rises occur at the nearby geothermal balaruc site along another lithospheric fault at the regional scale several coastal karst aquifers are located along the gulf of lion and occur in mezosoic limestones of similar ages the 87 sr 86 sr ratios of these aquifers tend toward values of 0 708557 which suggests a general mixing process of shallow karst waters with deep saline fossil waters the occurrence of these fossil saline waters may be related to the introduction of seawater during and after the flandrian transgression when the highly karstified massifs invaded by seawater formed islands and peninsulas along the mediterranean coast

crustal structure interpreted from magnetic anomalies this review discusses publications during the last quadrennium 1987 1990 that used aeromagnetic data marine magnetic data satellite magnetic data and rock magnetic and petrologic data to provide information on the sources of magnetic anomalies the publications reviewed reflect increased integration of rock magnetic property and petrologic studies with magnetic anomaly interpretation studies particularly in deep crustal magnetization exploration for hydrocarbons and inversion of marine magnetic anomalies interpretations of aeromagnetic data featuring image display techniques and using the horizontal gradient method for locating magnetization boundaries became standard

identification and evolution of hydrogeochemical processes in the groundwater environment in an area of the palar and cheyyar river basins southern india the palar and cheyyar river basins in tamil nadu state of southern india are characterised by different geological formations and groundwater is the major source for domestic agricultural and other water related activities hydrogeochemical studies were carried out in this area with the objective of identifying the geochemical processes and their relation to groundwater quality groundwater samples were collected once a month from 43 groundwater wells in this area from january 1998 to july 1999 sampling procedures and chemical analysis were carried out as per the standard methods chemical data are used for mathematical calculations and graphical plots to understand the chemical process and its relation to the groundwater quality the chemical composition of groundwater in the central part of the study area mainly depends on the recharge from lakes and the river which is explained by a mixing mechanism in addition weathering of silicate minerals controls the concentration of major ions such as sodium calcium magnesium and potassium in the groundwater of this area further the activity ratios indicate that the groundwater is in equilibrium with kaolinite smectite and montmorrillonite the reverse ion exchange process controls the concentration of calcium magnesium and sodium in hard rock formations and dissolution of carbonate minerals and accessory minerals is the source of ca and mg in addition to cation exchange in the sedimentary formations in general the chemical composition of the groundwater in this area is influenced by rock water interaction dissolution and deposition of carbonate and silicate minerals ion exchange and surface water interactions

a statistical correlation between ridge crest offsets and spreading rate ridge crest offsets follow the statistical distribution n a exp b ln l lo where n is the cumulative number of ridge crest offsets with lengths greater than l lo is the cutoff length and a and b are constants we measured a and b for the slow spreading mid atlantic ridge the intermediate spreading juan de fuca ridge and the fast spreading east pacific rise the predicted mean distances between offsets greater than 4 km long are 30 32 and 74 km on the slow intermediate and fast spreading ridge crests these distances are in agreement with observations of other workers and calculations of modal ridge lengths

the cretaceous tertiary transition the fossil sequences from cores across the cretaceous tertiary boundary show a range of transition times and transition time intervals depending on the fossil indicators and the location of the site these variations together with the pattern of iridium distribution with depth at some sites differences in total amounts of iridium variations in noble metal abundances normalized to extraterrestrial concentrations the depositional effects that might be expected in a reducing environment and the clay mineralogy of the boundary layer clays put into question the interpretation that an extraterrestrial event was the cause of the faunal changes and the iridium anomaly in the vicinity of the cretaceous tertiary transition it seems more likely that an explanation for the changes during the transition will come from continued examination of the great variety of terrestrial events that took place at that time including extensive volcanism major regression of the sea from the land geochemical changes and paleoclimatic and paleoceanographic changes

seismic stratigraphy of the shatsky rise sediment cap northwest pacific and implications for pelagic sedimentation atop submarine plateaus abstract pelagic sediments accumulated atop shatsky rise since early cretaceous time provide a valuable record of pacific paleoceanography the accepted sedimentation model invokes relatively continuous pelagic sediment accumulation punctuated by four regional hiatuses caused by changes in oceanic chemistry that framework was proposed more than two decades in the past and based on low quality single channel seismic profiles it was developed at the center of tamu massif the main topographic high of shatsky rise and extended to other highs with uncertainty owing to sparse data the availability of high quality seismic data and additional coring by the ocean drilling program and integrated ocean drilling program allows a re examination seismic and core data indicate that sedimentation atop shatsky rise has been complex with regional and local variability four major hiatuses were previously defined and tentatively correlated to nearby topographic highs these breaks occurred approximately during the aptian barremian and cenomanian coniacian ages of the cretaceous and during the paleocene and oligocene epochs of the cenozoic they are of variable length and tend to be of shorter duration at the summit of tamu massif but of longer durations elsewhere all produced erosional unconformities that may be related to mass wasting of the sediment cap edges each hiatus is approximately correlated with global falls of sea level suggesting a connection perhaps to intensified bottom currents at those times local variability is observed in the form of discontinuous layering non uniform sedimentation thinning of the sediment caps at their edges and local erosion near topographic highs these variations are likely the result of shifting bottom currents possibly resulting from interactions of internal tides with topography shatsky rise sedimentation is inherently punctuated and non uniform and the sedimentary record includes significant lateral variability other large oceanic plateaus are likely to exhibit similar sedimentary cap complexities

metal precipitation in the cretaceous tertiary boundary clay at stevns klint denmark abstract alvarez et al 1980 explained anomalous ir concentrations in the cretaceous tertiary k t boundary informal type sediment the fish clay in denmark in terms of a global fall out of extraterrestrial matter however geochemical and mineralogical analyses of the various sublayers in the clay reveal that not only ir but also elements such as ni co zn mo as se and sb are substantially enriched in the clay the enrichments may be due to precipitation of elements as sulfides at a redoxcline the facts supporting this view are 1 1 the over all chalcophile element abundance pattern in the clay 2 2 the occurrence of a redoxcline deduced from the distribution of organic matter and oxidized iron in the clay 3 3 the large amounts of small pyrite spheroids present at this redoxcline 4 4 the high concentration of trace elements in these spheroids also ni co and ir elements that have previously been presumed to have a meteoritic origin are substantially enriched in the spheroids a model is suggested in which metal bearing pore solutions lose their metals by precipitation upon passing the boundary between the oxic cretaceous chalk and the overlying anoxic and hydrogen sulnde rich fish clay the great similarity in geochemistry and lithological appearance between several strongly ir enriched clays suggest that the model may also be applied to other k t boundary clays

tectonic evolution of the north pacific during the cretaceous quiet period the cretaceous quiet period ended with a significant reorganization of the plate tectonics of the north pacific the farallon plate broke into three primary pieces 1 the kula plate 2 a large fragment transferred to the pacific plate and 3 the continued farallon plate the time of the rearrangement is constrained to just before magnetic anomaly 34 by its presence contrasts in tectonic spreading fabric tsf the emperor trough and the chinook trough our model is based on 1 a complete reevaluation of primary bathymetric and magnetic data 2 the distribution of major troughs emperor chinook hokkaido and krusenstern 3 mapping and analysis of tsf orientation and amplitude on quiet zone crust and 4 available deep sea drilling project drill hole data east west trending magnetic reversal lineations and tsf south of the aleutian trench are bounded on the west by the buldir fracture zone at 175 e the major troughs mark the abandonment and initiation of spreading in the reorganization their absence elsewhere precludes other long distance boundary relocations emperor trough is the abandoned pacific farallon spreading center rift initiation and early slow spreading have created the high amplitude topography of the chinook trough we believe that the hokkaido trough is the initiation site of a failed oceanic rift the incorporation of large amounts of farallon plate into the pacific plate obviates the need for the very rapid pacific farallon spreading rates of previous models and significantly alters the interpretation of mid pacific geology

tectonic evolution of the northern pacific plate and pacific farallon izanagi triple junction in the late jurassic and early cretaceous m21 m10 abstract magnetic lineations in the northwestern pacific near the shatsky plateau were interpreted to trace the tectonic history of the northern pacific plate from the time of anomalies m21 to m10 at m22 time the pacific farallon izanagi p f i triple junction was located southwest of the plateau and migrating north northwest with respect to the pacific plate shortly thereafter the stress field acting on the three plates changed significantly and the pacific lzanagi ridge reacted by pivoting 24 clockwise from m21 to m19 time several ridge propagation events occurred while the ridge adjusted to the new spreading regime to the east of the m22 location of the junction it appears that this reorganization triggered a series of at least three ridge jumps along the pacific farallon ridge the geometry of magnetic isochron m21 suggests that several triple junctions existed simultaneously during the reorganization bordering one or more microplates differences in the spreading rate of the japanese magnetic lineations north of the plateau imply that one microplate which we call the shatsky microplate existed until about m15 m13 time it was probably captured by either the farallon or the izanagi plate and subsequently destroyed by subduction after m15 the p f i triple junction was reformed on seafloor that is now beneath the shatsky plateau the junction drifted generally northeast either in a ridge ridge ridge or in a ridge fault fault configuration perhaps alternating between the two from m11 m10 time the stress field on the three plates changed again and the pacific farallon ridge southeast of the plateau reoriented 15 clockwise at this time the p f i junction assumed a ridge ridge ridge geometry at the northeast end of the plateau

multiple factors in the origin of the cretaceous tertiary boundary the role of environmental stress and deccan trap volcanism a review of the scenarios for the cretaceous tertiary k t boundary event is presented and a coherent hypothesis for the origin of the event is formulated many scientists now accept that the event was caused by a meteorite impact at chicxulub in the yucatan peninsula mexico our investigations show that the oceans were already stressed by the end of the late cretaceous as a result of the long term drop in atmospheric co2 the long term drop in sea level and the frequent development of oceanic anoxia extinction of some marine species was already occurring several million years prior to the k t boundary the biota were therefore susceptible to change the eruption of the deccan traps which began at 66 2 ma coincides with the k t boundary events it erupted huge quantities of h2so4 hcl co2 dust and soot into the atmosphere and led to a significant drop in sea level and marked changes in ocean temperature the result was a major reduction in oceanic productivity and the creation of an almost dead ocean the volcanism lasted almost 0 7 m y extinction of biological species was graded and appeared to correlate with the main eruptive events elements such as ir were incorporated into the volcanic ash possibly on soot particles this horizon accumulated under anoxic conditions in local depressions and became the marker horizon for the k t boundary an oxidation front penetrated this horizon leading to the redistribution of elements the eruption of the deccan traps is the largest volcanic event since the permian triassic event at 245 ma it followed a period of 36 m y in which the earth s magnetic field failed to reverse instabilities in the mantle are thought to be responsible for this eruption and therefore for the k t event we therefore believe that the k t event can be explained in terms of the effects of the deccan volcanism on an already stressed biosphere the meteorite impact at chicxulub took place after the onset of deccan volcanism it probably played a regional rather than a global role in the k t extinctions

structural evolution of preexisting oceanic crust through intraplate igneous activities in the marcus wake seamount chain multichannel seismic reflection studies and seismic refraction surveys with ocean bottom seismographs in the marcus wake seamount chain in the northwestern pacific ocean reveal p wave velocity structures of hot spot origin seamounts and adjacent oceanic crust inside the seamounts are central high velocity 6 5 km s structures extending nearly to the top that may indicate intrusive cores thick sediment layers up to 4 km with p wave velocities of 4 5 km s have accumulated on seafloor that predates seamount formation downward crustal thickening of up to 2 km was documented beneath a large seamount cluster but thickening was not confirmed below a small seamount cluster volume ratios of an intrusive core to a seamount body are 15 20 indicating that most of the supplied magma was consumed in forming the thick sedimentary and volcaniclastic layer constituting the seamount flanks underplating and downward crustal thickening may tend to occur when second or later intrusive cores are formed in a seamount p wave velocities in the lowest crust and in the uppermost mantle below the seamount chain are 0 1 0 2 km s higher and 0 3 0 5 km s lower respectively than velocities below oceanic crust we explain this difference as a result of sill like intrusion of magma into the lower crust and uppermost mantle reflected waves observed at offsets 200 km are from mantle reflectors at depths of 30 45 km and 55 70 km the shallower reflectors may indicate structures formed by intraplate igneous activities and the deeper reflectors may correspond to the lithosphere asthenosphere boundary

intense acidic volcanism at the cretaceous tertiary boundary abstract a 87 sr 86 sr spike in seawater strontium with amplitude 2 10 4 and duration of order 2 ma is superimposed on longer term variations at the cretaceous tertiary boundary the anomaly has been attributed to increased continental runoff due either to meteorite impact related acid rain or sea level regression we speculate here that the spike could have resulted from intense explosive acid volcanism preceding the development of the deccan traps a good model both in tectonic position and geochemical characteristics for these as yet elusive acidic products is provided by granites in the seychelles islands and particularly mahe

geochemistry water balance and stable isotopes of a clean pit lake at an abandoned tungsten mine montana usa abstract the calvert mine is a small tungsten rich scheelite skarn deposit in a remote mountainous region of southwest montana usa the open pit mine closed in the 1970s and subsequently flooded to form a pit lake that is roughly conical in shape 30 m deep and 120 m in diameter with no surface inlet or outlet the lake is holomictic with a groundwater flow through hydrology and an estimated residence time of 2 5 5 y water isotopes show that the lake is at an approximate steady state with respect to water balance and has experienced 30 evaporation the lake has a near neutral ph exceptional clarity and extremely low concentrations of nutrients sulfate and most metals including tungsten manganese concentrations are slightly elevated and increase with depth towards the sediment water interface despite seasonally anoxic conditions in the deep water dissolved fe concentrations are orders of magnitude lower than mn suggesting that insufficient organic carbon is present in the sediment of this oligotrophic lake to drive bacterial fe reduction based on stable isotope fingerprinting diffuse seepage that enters a nearby headwater stream at the base of a large waste rock pile can be directly linked to the partially evaporated pit lake however this seepage has neutral ph and low metal concentrations and poses no threats to the environment stable isotopes of dissolved inorganic carbon dic and dissolved oxygen do are used to track the relative importance of photosynthesis and respiration with depth in summer a zone of high productivity exists near the base of the chemocline releasing do that is isotopically light respiration exceeds photosynthesis below the secchi depth which causes do concentrations to approach zero towards the bottom of the lake in winter thick ice and snow cover prohibits photosynthesis however because of the low nutrient content most of the volume of the lake remains oxic through the winter overall the calvert lake is a good example of a pit lake formed from metal mining that has excellent water quality which is a result of its favorable geology paucity of sulfide minerals and hydrology flow through lake with short residence time in a temperate climate

seawater sr isotopes at the cretaceous tertiary boundary abstract seawater 87 sr 86 sr values increase abruptly by 28 10 6 across the cretaceous tertiary boundary ktb this small but rapid shift is superimposed on the larger scale structure of the seawater sr isotope curve the time scale of radiogenic sr addition appears to be too rapid to reconcile with sources associated with volcanism and we show that the amount of sr required to produce even this small increase is too large to be derived from 1 a kt bolide of the size constrained by the ir anomaly 2 continental crust ejecta from the impact of such a bolide 3 soot from global wildfires initiated by an impact or 4 any combination of these sources the probable source of the radiogenic sr is enhanced continental weathering but the high rate of increase appears to rule out processes such as sea level regression glaciation or tectonism a plausible mechanism for rapid addition of radiogenic sr to the oceans is enhanced weathering associated with globally distributed acid rain ph 1 which is a proposed by product of a bolide impact 51 epsl vol 83

saline groundwater in israel its bearing on the water crisis in the country abstract one of the major causes for the deterioration of water quality bearing heavily on the water crisis in israel is the ongoing contamination of its water resources by saline water bodies the present paper reviews the geochemical processes forming saline water lists and explains certain chemical and isotopic parameters which enable understanding these processes and describes the saline groundwater bodies and various salinization phenomena occurring in the country s various aquifers deterioration of groundwater in israel is caused by numerous natural processes such as encroachment of sea water migration of connate highly pressurized brines penetrating into fresh groundwater by subsurface dissolution of soluble salts originating in surrounding country rocks and by water rock interaction in addition to sea water two saline water bodies were identified as the main factors causing salinization of fresh groundwater a ca chloride brines encountered in the jordan dead sea rift valley in various parts of the negev and of the coastal plain and b na chloride saline water identified in the subsurface of the negev and in the southern part of the coastal plain intensive exploitation of groundwater in israel has disturbed the natural equilibrium which prevailed between fresh and saline water the newly established groundwater flow regimes have facilitated the migration of saline water bodies their participation in the active hydrological cycle and the progressive contamination of fresh groundwater these processes which were not anticipated by planners and water resources managers emphasize that large scale groundwater exploitation was undertaken without giving sufficient consideration to the occurrence and subsurface migration of saline water and brines

the hydrochemical identification of groundwater flowing to the bet she an harod multiaquifer system lower jordan valley by rare earth elements yttrium stable isotopes h o and tritium abstract the bet she an and harod valleys in israel are regional recipients and mixing zones for groundwater draining from a multiple aquifer system which includes carbonate and basalt aquifers and deep seated pressurized brines the aquifers drain through two types of outlets distinct and mixed the latter type is mainly conditioned by the occurrence of fault blocks related to the jordan rift system which act as connecting media between the aquifers and facilitate interaquifer flow conjoint application of rare earth element distribution and water isotopes enables detection of the local areas replenishment by rainfall infiltration and in connection with the position of wells or springs the identification of groundwater flow paths once stationary equilibria are established changes of rey composition between rey in groundwater and their surface adsorption are negligible in areas with little soil coverage and vegetation even recharge over young tertiary and diagenetic cretaceous limestones is distinguishable by their rey distribution patterns groundwater recharged over tertiary limestones show higher rey abundance and more significant ce anomalies than those derived from the cretaceous limestones weathering of alkali olivine basalts leads to rey patterns in groundwater depleted in the middle ree the improved knowledge of the hydrological systems is thought to be useful for regional hydrogeological modeling and for designing rational water management schemes

late cretaceous genesis of the kula plate abstract we propose that the pacific kula ridge began spreading approximately 85 m y b p during late cretaceous time extrapolation of the great magnetic bight backwards in time results in an implausible ridge configuration this implies that plate velocity vectors for the pacific kula and farallon plates were not constant during this interval evidence for splitting of the kula plate from the pacific plate along the chinook trough is the relationship between the north striking amlia and adak fracture zones the chinook and emperor troughs and the magnetic lineations south of the aleutian trench if this hypothesis is correct it will require that mesozoic reconstructions of the pacific basin and their relation to cenozoic reconstructions be re examined a previously unrecognized mesozoic plate may be required we propose calling this the izanagi plate

rb sr sm nd k ca o and h isotopic study of cretaceous tertiary boundary sediments caravaca spain evidence for an oceanic impact site isotopic ratios and trace element abundances were measured on samples of ir enriched clay at the cretaceous tertiary boundary and in carbonate and marl from 5 cm below and 3 cm above the boundary samples were leached with acetic acid to remove carbonate and with hydrochloric acid leachates and residues were measured the sr nd o and h isotopic compositions of the boundary clay residues are distinct from those of the stratigraphically neighboring materials the data indicate that most of the clay material was derived from a terrestrial source with relatively low87sr 86sr and high143nd 144nd ratios the 18o data suggest that the detritus has been modified by submarine weathering k ca and rb sr systematics as well as o isotope ratios of k feldspar spherules within the boundary clay suggest that they are predominantly authigenic and may have formed after the time of deposition however sm nd and rb sr isotopic data indicate that the spherules contain relict material that provides information on the nature of the original detritus the isotopic evidence for foreign terrestrial detritus in the boundary clay the low rare earth element concentrations and high ni concentration support the hypothesis of a terminal cretaceous asteroidal impact that produced a global layer of fallout the data are most easily explained if the impact site was on oceanic crust rather than continental crust and if a substantial fraction of the fallout was derived from relatively deep within the lithosphere 3 km this would probably require a single large impactor

a multi isotopic and trace element investigation of the cretaceous tertiary boundary layer at stevns klint denmark inferences for the origin and nature of siderophile and lithophile element geochemical anomalies abstract os sr nd and pb isotope data were collected from a profile across the cretaceous tertiary k t boundary layer at stevns klint denmark nd t 65 ma values from within the boundary layer fish clay are lower by 1 unit than those of the underlying maastrichtian limestone and the overlying danian chalk sequences systematic profile upward changes of pb sr and os isotopic compositions and concentrations in the boundary layer cannot be accounted for by in situ growth of daughter products since the sedimentation of the fish clay while os nd and pb isotopes indicate the admixing of less radiogenic components to the fish clay sr isotopes show elevated radiogenic values in the boundary layer relative to the carbonate sequences beneath and above it the sudden change in lithophile e g sr pb and nd isotope compositions at the base of the fish clay and profile upward trends of 87sr 86sr and 206pb 204pb ratios towards those of the overlying danian chalk are interpreted to reflect recovery from enhanced acid rain induced continental local weathering input to the seawater however a continental crustal source is invalid for the siderophile element os in the light of evidence from chromium isotopes for a cosmic origin of the platinum group elements pges and certain moderately siderophile elements cr ni co v in k t boundary sediments including stevns klint shukolyukov and lugmair science 282 1998 927 929 and supported by the finding of projectile debris bauluz et al earth planet sci lett 182 2000 127 136 and the occurrence of abundant ni rich spinel at many k t sites robin et al nature 363 1993 615 617 kyte nature 396 1998 237 239 we favor to explain the sudden drop of 187os 188os ratios from 0 210 to 0 160 at the k t boundary to derive from global fall out of extraterrestrial matter the present 186os 188os ratio of 0 119836 0 000004 measured in the basal layer of the fish clay is within the uncertainty a chondritic value we therefore exclude the possibility of a major contribution of pges to the sediment from iron meteorites chondrite normalized ru ir n ratios of 0 95 0 14 and os ir n ratios of 0 93 0 14 in the fish clay cannot distinguish between abundance ratios of different types of chondrites and strongly sub chondritic pt ir n ratios of 0 62 0 09 2 suggest differential pge remobilization through the sedimentary column and consequently the alteration of inter element ratios pges and the moderately siderophile elements cr ni v and co form an elemental association with systematically upward decreasing concentrations in the fish clay low co ni ratios of 0 12 in the fish clay relative to values of 0 35 in the over and underlying carbonate sequences support mixing of meteorite derived co ni 0 05 and terrestrial upper mantle crustal co ni 0 3 sources while lithophile element isotope data indicate an increased continental crustal input to the fish clay at the k t transition the uncertainty with respect to possible post depositional alteration of abundance patterns of siderophile and moderately siderophile elements though not affecting the chondritic isotopic composition of os does not allow confirmation of indications from chromium isotopes for a carbonaceous cv type meteorite as the preferred k t impactor type by shukolyukov and lugmair science 282 1998 927 929

quantification of groundwater seawater interaction in a coastal sandy aquifer system a study from panama sri lanka the panama coastal aquifer system is an important water resource in the southeast coast of sri lanka that provides adequate supplies of water for agriculture and domestic uses one of the biggest threats to these fragile aquifers is the sea water intrusion in this study recharging mechanism and geochemical evaluation of groundwater in the coastal sandy aquifer of panama were evaluated using chemical and stable isotope techniques thirty groundwater samples were collected and analyzed for their major ion concentrations and stable isotope ratios of oxygen 18o 16o and hydrogen d h all studied samples showed a ranking of major anions in the order cl hco 3 so 4 2 n no3 while cations showed a decreasing order of abundance with na ca2 mg2 k dominant groundwater hydrogeochemical types were na cl and mixed ca mg cl results of saturation index calculations indicate that the investigated groundwater body was mostly saturated with respect to calcite dolomite and gypsum in addition stable isotope and geochemical data suggest that fresh groundwater in the aquifer is recharged mainly by local precipitation with slight modification from evaporation and saline water intrusions isotope data suggest that mixing of salt water with freshwater occurs in aquifers which are located towards the lagoon since the communities in the study area depend entirely on groundwater an understanding of the hydrogeochemical characteristics of the aquifer system is extremely important for the better water resource management in the region

pb isotopic tracers of the cretaceous tertiary extinction event the global excess of ir in sediments at the cretaceous tertiary k t boundary has been attributed to either a meteorite impact or enhanced volcanism e g deccan traps the isotopic composition of pb associated with this ir provides a test of these hypotheses because meteoritic pb is isotopically quite distinct from pb of volcanic or continental origin pb abundances and isotopic compositions of pelagic oozes from dsdp site 577a shatsky rise nw pacific measured in this study show little change over the deposition interval of ir rich sediment 10 cm however a pb enriched layer with a more radiogenic isotopic composition is found in the basal 0 5 cm of the tertiary these observations appear to preclude the ir excess in these sediments originating as impact fallout from a stony meteorite although an iron meteorite impactor cannot be excluded any pb flux accompanying ir in volcanogenic aerosols from the deccan traps should be considerable however the boundary pb spike does not isotopically resemble known deccan trap lavas there is isotopic overlap with coal ash although the lack of correlation between soot and pb abundance precludes any simple relationship between the two overall any wind pattern changes following the k t event were apparently short lived as neither airborne sources of pb nor their magnitude to the then central pacific were affected while the ir enriched sediments were being laid down

magnetic anomaly lineations from late jurassic to early cretaceous in the west central pacific ocean summary late jurassic to early cretaceous mesozoic magnetic anomaly lineations the phoenix magellan mid pacific mountains hawaiian and japanese lineation sets with fracture zones in the west central pacific ocean were identified more comprehensively than in any previous studies we fixed 2100 positions of identified magnetic anomalies based on magnetic data of 283 cruise tracks two remarkable fracture zones the phoenix and central pacific fracture zones were mapped and newly named our newly identified lineations from mlon to mo around the mid pacific mountains which belong to the hawaiian lineation set illustrated that the sea floor south of the mid pacific mountains has the same age as that of the north 132 118 ma our analysis of skewness parameters revealed that the older part of the phoenix set m17 m29 has skewness different from that of the younger part ml m14 implying an effect of magnetic overprints by the cretaceous volcanism it was confirmed that the spreading rate of the mesozoic pacific spreading system was the fastest in the world in the mesozoic a drastic change in spreading rates occurred simultaneously at the period between chrons m21 and m20 149 5 148 5 ma in all the mesozoic pacific spreading systems the event appears to be synchronous with events in other oceans such as the mesozoic atlantic and indian oceans

tectonics and evolution of the juan fernandez microplate at the pacific nazca antarctic triple junction in this study magnetic and bathymetric data from an r v endeavor survey are combined with earthquake mechanism studies to produce a tectonic model for the juan fernandez microplate at the pacific nazca antarctic triple junction magnetic lineations along the east ridge of the microplate fan to the south indicating that the nazca juan fernandez pole is near the northern end of this feature relative motions of the juan fernandez pacific nazca antarctic four plate system are determined using relative plate motion inversions the resulting tectonic model predicts that the northern boundary of the juan fernandez microplate is a zone of compression and that the west ridge and southwestern boundary are spreading obliquely the southeastern boundary is predicted to be obliquely divergent but present data fail to constrain its geometry we also present a schematic evolutionary model which relates the formation of the juan fernandez microplate to differential spreading rates at the triple junction and discuss related complexities the future evolution of the fourplate system depends on whether one or both ridges remain active

osmium tungsten and chromium isotopes in sediments and in ni rich spinel at the k t boundary signature of a chondritic impactor it is now established that a large extraterrestrial object hit the earth at the end of the cretaceous period about 65 ma ago we have investigated re os hf w and mn cr isotope systems in sediments from the cretaceous and the paleogene in order to characterize the type of impactor within the cretaceous tertiary k t boundary layer extraterrestrial material is mixed with terrestrial material causing a dilution of the extraterrestrial isotope signature that is difficult to quantify a phase essentially composed of ni rich spinel formed in the atmosphere mainly from melted projectile material is likely to contain the extraterrestrial isotopic signature of the impactor we show that the analysis of spinel is indeed the best approach to determine the initial isotope composition of the impactor and that w and cr isotopes confirm that the projectile was a carbonaceous chondrite