strong ground motions observed at the upsar during the 2003 m 6 5 san simeon and 2004 m 6 0 parkfield california earthquakes the u s geological survey parkfield dense seismograph array up sar recorded successfully strong ground motions during the 2004 parkfield earth quake m 6 0 and its aftershock series after waiting for 15 years for an anticipated event like this the array also recorded the 2003 san simeon earthquake m 6 5 because the array covers a very small area 0 45 km 2 these data offer some inter esting fresh insights into intrasite variations of seismic ground motions in this article we study strong motion data recorded at the upsar from the san simeon event the parkfield event and its seven aftershocks we find that the variations of high frequency ground motions e g 3 hz are very considerable the largest horizontal peak ground acceleration pga p11 408 cm sec 2 from the parkfield event is close to three times of the smallest one p01 157 cm sec 2 the largest peak response spectrum is even over three times of the smallest one the shortest station to station distance between p06 and p07 in the array is only 25 m but three component pgas of the two stations differ from a factor of 1 5 for the parkfield event the coefficient of variation cv r mean of fourier acceleration is about 50 at frequencies higher than about 3 hz we find that cv depends strongly on frequency while it is nearly stable for different earthquakes the significant variation of the high frequency ground motions seems to be brought about mostly by the local and neighboring topographic effects which have a larger effect in horizontal than vertical directions we also calculate the ratio of vertical to horizontal response spectrum our plotting shows that the ratio is not sensitive to earthquake magnitude we compare the ob served motions pga 5 damped pseudoacceleration response spectrum psa from 0 02 to 5 sec with estimations from four commonly used prediction equations abra hamson and silva 1997 boore et al 1997 campbell and bozorgnia 2003 and sadigh et al 1997 the comparisons indicate that the significant station to station variation reduces largely the accuracy with which a site specific estimation can be predicted however the mean of the observations at the upsar compares reasonably well with these estimations

the physicochemical habitat of sclerolinum sp at hook ridge hydrothermal vent bransfield strait antarctica at hook ridge hydrothermal vent a new species of sclerolinum monilifera siboglinidae was found at a water depth of 1 045 m on the basis of investigations of multicores and gravity cores the species habitat is characterized sclerolinum does not occur in sediments that are most strongly influenced by hydrothermal fluids probably because of high temperature up to 49 c and precipitation of siliceous crusts about 800 individuals m 2 occur in sediments that are only weakly exposed to hydrothermal flow and have the following characteristics 20 c 15 cm sediment depth to 21 5 c bottom water 18 40 cm yr 1 advection rates ph 5 5 25 mol l 1 methane 170 mol l 1 sulfide and 0 0054 mol m 2 yr 1 sulfide flux comparison with geochemical data from other reducing sediments indicates that the two groups of siboglinidae monilifera and frenulata occur in sediments with low sulfide concentration and flux in contrast sulfurbased chemosynthetic organisms that typically occur at hydrothermal vents and cold seeps e g vestimentifera vesicomyid clams and bacterial mats occur in sediments with higher sulfide availability threshold values are around 500 mol l 1 sulfide and 0 1 mol m 2 yr 1 sulfide fluxes we did not find typical hydrothermal vent species at hook ridge hydrothermal vent which might be explained by the unfavorable physicochemical habitat at sites inhabited by sclerolinum sulfide availability appears to be too low whereas at sites with higher sulfide availability the temperatures might be too high siliceous crust precipitation could preclude their occurrence or both

review of fossil chemosynthetic assemblages in japan abstract seventy five fossil chemosynthetic assemblages of cretaceous to holocene age have been discovered in japan many are interpreted to have been cold seep dependent and one is associated with a whale fall fossil hydrothermal vent assemblages have not been found in japan non seep assemblages of solemyids lucinids and or thyasirids are not included in this study various combinations of five diagnostic features characterize cold seep assemblages 1 the abundant and often exclusive occurrence of large invertebrate taxa suspected of harboring chemosynthetic bacteria 2 associated authigenic carbonates greatly depleted in 13c 3 a three dimensional distribution of the assemblage that is laterally restricted 4 associated pockmark or diapir related sedimentary structures and 5 the presence of conduit related tectonic or sedimentological structures such as faults slumps or debris flows excluding the whale fall assemblage we can divide cenozoic fossil chemosynthetic assemblages in japan into three types regardless of geochronologic age type i is dominated by vesicomyid bivalves and or tube worms characterized by its allochthonous or parautochthonous mode of occurrence in deep water greater than 1000 m settings along with tectonically or sedimentologically induced structures such as faults slumps diapirs or debris flows likely dependent on seepage controlled by subduction related tectonism and or on methane coming from the drastic decomposition of gas hydrate type ii is dominated by vesicomyid bivalves in most cases the species differ from those in type i and is characterized by autochthonous occurrences in muddy sediments from less than 1000 m water depth type iii is dominated by lucinoma and or conchocele and characterised by autochthonous occurrences in muddy sediments from depths of less than 300 m a living community similar to the type ii and iii assemblages has not yet been observed in the seas around japan it is noteworthy that no tectonically or sedimentologically induced structures have been observed in and around the type ii and iii fossil localities although the muddy sediments sometimes show brecciated facies that suggest an explosive effusion of methane

an authoritative global database for active submarine hydrothermal vent fields 1 the interridge vents database is available online as the authoritative reference for locations of active submarine hydrothermal vent fields here we describe the revision of the database to an open source content management system and conduct a meta analysis of the global distribution of known active vent fields the number of known active vent fields has almost doubled in the past decade 521 as of year 2009 with about half visually confirmed and others inferred active from physical and chemical clues although previously known mainly from mid ocean ridges mors active vent fields at mors now comprise only half of the total known with about a quarter each now known at volcanic arcs and back arc spreading centers discoveries in arc and back arc settings resulted in an increase in known vent fields within exclusive economic zones consequently reducing the proportion known in high seas to one third the increase in known vent fields reflects a number of factors including increased national and commercial interests in seafloor hydrothermal deposits as mineral resources the purpose of the database now extends beyond academic research and education and into marine policy and management with at least 18 of known vent fields in areas granted or pending applications for mineral prospecting and 8 in marine protected areas

crustal deformation in taiwan results from finite source inversions of six mw 5 8 chi chi aftershocks 1 we inverted strong motion data for the finite source parameters of six large aftershocks of the 1999 chi chi taiwan earthquake to investigate seismogenic structure in taiwan for each event we derived a preferred model by testing different focal mechanisms hypocenters rupture velocities and dislocation risetimes as well as different combinations of stations in more than 1000 inversions we documented how the fits between the waveforms and the corresponding synthetics deteriorated as the hypocenter and focal mechanism deviate from those of the preferred model if the deviation in focal mechanisms and hypocenters is less than 20 and 5 km respectively we generally recovered 80 of the preferred model s synthetic waveform fit unlike the dislocation risetime the rupture velocity used in the inversion had a strong influence on the waveform fits in this study we also used the slip models to study fault geometry two of the aftershocks ruptured on the southern extension of the main shock slip area one strike slip aftershock nucleated within the basement but ruptured mainly within the overlying sedimentary strata suggesting that seismogenic deformation in the basement can influence shallow structures p axes of the derived models have azimuths consistent with current plate motion finally gps displacement derived from the six slip models can explain 80 of the postseismic deformation observed in the aftershock regions indicating that studies of postseismic deformation must take into account the cumulative effects of large shallow aftershocks

comparisons of ground motions from the 1999 chi chi earthquake with empirical predictions largely based on data from california this article has the modest goal of comparing the ground motions recorded during the 1999 chi chi taiwan mainshock with predictions from four empirical based equations commonly used for western north america these empirical predictions are largely based on data from california comparisons are made for peak acceleration and 5 damped response spectra at periods between 0 1 and 4 sec the general finding is that the chi chi ground motions are smaller than those predicted from the empirically based equations for periods less than about 1 sec by factors averaging about 0 4 but as small as 0 26 depending on period on which equation is used and on whether the sites are assumed to be rock or soil there is a trend for the observed motions to approach or even exceed the predicted motions for longer periods motions at similar distances 30 60 km to the east and to the west of the fault differ dramatically at periods between about 2 and 20 sec long duration wave trains are present on the motions to the west and when normalized to similar amplitudes at short periods the response spectra of the motions at the western stations are as much as five times larger than those of motions from eastern stations the explanation for the difference is probably related to site and propagation effects the western stations are on the coastal plain whereas the eastern stations are at the foot of young and steep mountains either in the relatively narrow longitudinal valley or along the eastern coast the sediments underlying the eastern stations are probably shallower and have higher velocity than those under the western stations manuscript received 1 december 2000

hydrothermalism in the mediterranean sea abstract hydrothermalism in the mediterranean sea results from the collision of the african and european plates with the subduction of the oceanic part of the african plate below europe high heat flows in the resulting volcanic arcs and back arc extensional areas have set up hydrothermal convection systems most of the known hydrothermal sites are in shallow coastal waters

why did ancient chemosynthetic seep and vent assemblages occur in shallower water than they do today cold seep communities have relatively low diversity are dominated by one or two taxa present in high density and high biomass in comparison with the surrounding fauna and are restricted to aphotic habitats their associated heterotrophic fauna are usually distinctive from the fauna of their surroundings in contrast a more commonplace chemoautotrophically based community occurs in shallow photic habitats the associated heterotrophic fauna includes many of the species typical of the surrounding communities and typically dominates abundance whereas the species with chemoautotrophic symbionts dominate biomass all modern seep assemblages are deeper than 550 m many fossil seep assemblages occurred in water as shallow as the mid shelf 200 m in contrast communities where species with chemoautotrophic symbionts are biomass dominants but not numerical dominants are common in shallow waters at present but rarely reported in the geological record we suggest that the absence of cold seep communities on the continental shelf presently is due to a combination of predation and competitive exclusion by primary consumers limiting the presence of species dependent on chemoautotrophic symbionts we suggest that cold seep assemblages are more common at shelf depths in the fossil record for two reasons a the biases of preservation have accentuated their distribution by transforming communities where species with chemoautotrophic symbionts dominate by biomass but not numerically into cold seep appearing assemblages b the importance of predation pressure and oligotrophy has varied with decreased predation pressure accompanying increased oligotrophy favoring cold seep communities we suggest that the paucity of shallow water assemblages with species harboring chemoautotrophic symbionts as biomass dominants in the fossil record is based on the reliance of paleoecological analysis on numerical abundance data when energy flow analyses are required to identify these assemblages the distinctiveness of the fossil seep assemblage is intensified by taphonomic processes that bias the assemblage against small individuals and epifaunal species so that diversity declines the small heterotrophic component of the assemblage is significantly reduced and the epifaunal component is minimized the final assemblage is usually dominated by the better preserved large infaunal clams which perchance are also the species with chemoautotrophic symbionts in contrast preservation does not enhance the distinctiveness of these chemoautotroph harboring species in shallow water

abrupt deep sea warming palaeoceanographic changes and benthic extinctions at the end of the palaeocene a remarkable oxygen and carbon isotope excursion occurred in antarctic waters near the end of the palaeocene 57 33 myr ago indicating rapid global warming and oceanographic changes that caused one of the largest deep sea benthic extinctions of the past 90 million years in contrast the oceanic plankton were largely unaffected implying a decoupling of the deep and shallow ecosystems the data suggest that for a few thousand years ocean circulation underwent fundamental changes producing a transient state that although brief had long term effects on environmental and biotic evolution

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

trace elements in a north sea drill core abstract chalk samples systematically taken along a drill core from one of the hydrocarbon producing fields of the north sea tyra field were analysed by a neutron activation technique involving measurement of radioisotopes with relatively short half lives elements determined include na al cl ca mn v sr dy mg ti ba and eu the major elements ca and al could be related to petrophysical parameters particularly al which shows a correlation with the silicification argillaceous content na and cl has a low content in the hydrocarbon bearing section of the drill core suggesting that the pore space is filled mainly by hydrocarbons a significant decrease of mn with depth probably suggests diagenesis of chalk prior to or with hydrocarbon emplacement investigations of drilling fluids and cuttings reveal a strong contamination of the latter mainly by ba chalk data from comparable onshore localities danish subbasin show a different geochemistry with generally less variation of elements with depth

some observations on colocated and closely spaced strong ground motion records of the 1999 chi chi taiwan earthquake the digital accelerograph network installed in taiwan produced a rich set of records from the 20 september 1999 chi chi taiwan earthquake mw 7 6 teledyne geotech model a 800 and a 900a digital accelerographs were colocated at 22 stations that recorded this event comparisons of the amplitudes frequency content and baseline offsets show that records from several of the a 800 accelero graphs are considerably different than those from the colocated a 900a accelero graphs on this basis and in view of the more thorough predeployment testing of the newer a 900a instruments we recommend that the records from the a 800 instruments be used with caution in analyses of the mainshock and aftershocks at the hualien seismic station two a 900a and one a 800 instruments were colocated along with a global positioning system instrument although the records from the two a 900a instruments are much more similar than those from a colocated a 800 instrument both three component records contain unpredictable baseline offsets which produced completely unrealistic ground displacements derived from the ac celerations by double integration as do many of the strong motion data from this event the details of the baseline offsets differ considerably on the two three component records there are probably numerous sources of the baseline offsets including sources external to the instruments such as tilting or rotation of the ground and sources internal to the instruments such as electrical or mechanical hysteresis in the sensors for the two colocated a 900a records at the hualien seismic station however the differences in the baseline offsets suggest that the principal source is some transient disturbance within the instrument the baseline offsets generally man ifest themselves in the acceleration time series as pulses or steps either singly or in combination we find a 0 015 hz low cut filter can almost completely eliminate the effects of the baseline offsets but then information regarding the permanent dis placements is lost the causative mechanisms of the baseline offsets are unknown presently hence it is very difficult to recover the permanent displacements from the modern digital records although for records close to large earthquakes the signal to noise ratio should theoretically be adequate to obtain ground motions with periods of hundreds of seconds this study reinforces our conclusion from previous studies that the sources of baseline offsets occurring in digital strong motion records are very complex and often unpredictable and that therefore it is difficult to remove the baseline effects to maximize the information content of the record the baseline offsets only affect very long period motions e g 20 sec however and therefore are of little or no engineering concern

first evidence of widespread active methane seepage in the southern ocean off the sub antarctic island of south georgia an extensive submarine cold seep area was discovered on the northern shelf of south georgia during r v polarstern cruise ant xxix 4 in spring 2013 hydroacoustic surveys documented the presence of 133 gas bubble emissions which were restricted to glacially formed fjords and troughs video based sea floor observations confirmed the sea floor origin of the gas emissions and spatially related microbial mats effective methane transport from these emissions into the hydrosphere was proven by relative enrichments of dissolved methane in near bottom waters stable carbon isotopic signatures pointed to a predominant microbial methane formation presumably based on high organic matter sedimentation in this region although known from many continental margins in the world s oceans this is the first report of an active area of methane seepage in the southern ocean our finding of substantial methane emission related to a trough and fjord system a topographical setting that exists commonly in glacially affected areas opens up the possibility that methane seepage is a more widespread phenomenon in polar and sub polar regions than previously thought

hydrothermal gases offshore milos island greece hydrothermal fluids emerge from the seafloor of paleohori bay on milos the gases in these fluids contain mostly co2 but ch4 concentrations up to 2 are present the stable carbon isotopic composition of the co2 near 0 indicates an inorganic carbon source dissociation of underlying marine carbonates the carbon and hydrogen isotopes of most ch4 samples are enriched in the heavy species delta 13c 9 4 to 17 8 per mille sign delta d 102 to 189 per mille sign which is believed to be characteristic for an abiogenic production of ch4 by co2 reduction fischer tropsch reactions depletions in the deuterium content of three ch4 samples to 377 are probably caused by unknown subsurface rock alteration processes secondary hydrogen isotope exchange processes between methane hydrogen and water are most likely responsible for calculated unrealistic methane formation temperatures we show that excess helium slightly enriched in 3he is present in the hydrothermal fluids emerging the seafloor of paleohori bay when the isotopic ratio of the excess component is calculated a 3he 4heexcess of 3 6 10 6 is obtained this indicates that the excess component consists of about one third of mantle helium and two thirds of radiogenic helium we infer that the mantle derived component has been strongly diluted by radiogenic helium during the ascent of the fluids to the surface

neotethyan late cretaceous volcanic arc hydrothermal vent fauna the barite polymetallic ore zone of the late cretaceous madneuli volcanogenic massive sulfide vms deposit in the republic of georgia contains large tube worm fossils that represent the first ancient hydrothermal vent fauna to be identified from a volcanic arc setting and very likely the shallowest ancient example yet found the fossil tube walls are formed of an unusual quartz barite pyrite and galena mineral assemblage rather than only pyrite as in other vent fossils this probably reflects the low temperature high salinity vent chemistry at the time of preservation as deduced from fluid inclusion analyses the madneuli vent fauna is one of three examples that occurred over a time span of 6 m y in an area of the neotethyan ocean that was as tectonically complex as the present day west pacific the ancient and modern vent faunas appear to occur in a similar range of tectonic settings and bathymetry

vent fluid chemistry in bahia concepcion coastal submarine hydrothermal system baja california sur mexico abstract shallow submarine hydrothermal activity has been observed in the bahia concepcion bay located at the gulf coast of the baja california peninsula along faults probably related to the extensional tectonics of the gulf of california region diffuse and focused venting of hydrothermal water and gas occurs in the intertidal and shallow subtidal areas down to 15 m along a nw se trending onshore offshore fault temperatures in the fluid discharge area vary from 50 c at the sea bottom up to 87 c at a depth of 10 cm in the sediments chemical analyses revealed that thermal water is enriched in ca as hg mn ba hco3 li sr b i cs fe and si and it has lower concentrations of cl na so4 and br than seawater the chemical characteristics of the water samples indicate the occurrence of mixing between seawater and a thermal end member stable isotopic oxygen and hydrogen composition of thermal samples plot close to the local meteoric water line on a mixing trend between a thermal end member and seawater the composition of the thermal end member was calculated from the chemistry of the submarine samples data by assuming a negligible amount of mg for the thermal end member the results of the mixing model based on the chemical and isotopic composition indicate a maximum of 40 of the thermal end member in the submarine vent fluid chemical geothermometers na li na k ca and si were applied to the thermal end member concentration and indicate a reservoir temperature of approximately 200 c the application of k mg and na li geothermometers for vent fluids points to a shallow equilibrium temperature of about 120 c results were integrated in a hydrogeological conceptual model that describes formation of thermal fluids by infiltration and subsequent heating of meteoric water vent fluid is generated by further mixing with seawater

fossil clams from a serpentinite hosted sedimented vent field near the active smoker complex rainbow mar 36 13 n insight into the biogeography of vent fauna hydrothermal circulation at ultramafic hosted sites supports a large variety of high and low temperature hydrothermal vents and associated ecosystems the discovery of abundant fossil vesicomyid and thyasirid shell accumulations at the ridge crest approximately 2 5 km east of the active rainbow vent field on the mid atlantic ridge mar 36 degrees 13 n increased our knowledge regarding the diversity of vent communities at slow spreading ridges bivalve molluscs of the family vesicomyidae were represented by the genus phreagena here we present the first record of this genus in the atlantic ocean this second vesicomyid species known from the mar phreagena sp was found to be associated with a thyasira species that is affiliated with t southwardae at the logatchev vent field on the mar and with t vulcolutre in the gulf of cadiz these two clams have close relationships with seep taxa along the continental margin and were likely associated with sedimented vent fields delta o 18 and delta c 13 analyses of the shells suggested that the burrowing bivalve thyasira could incorporate isotopically light carbon derived from the oxidation of methane in the sediment while the signature of phreagena sp shells denoted a different carbonate source c 14 dating of the shells denoted that the hydrothermal activity in the rainbow area began at least similar to 25 5 kyr bp which is similar to the model of the hydrothermal vent field distribution that was proposed for the logatchev hydrothermal site the results provide new insight regarding the diversity of chemosynthetic fauna on the mar over geologic time ultramafic hosted on axis sedimented vent fields extend the range of habitats for chemosynthetic communities underlying the need to further explore the geology of these types of environments on slow spreading ridges and to determine their role in the ecology of deep sea vent communities

chemostratigraphy of upper cretaceous chalk in the danish subbasin the application of well cuttings to geochemical analyses was studied on the basis of the elemental distribution of mg sr mn and zn and the carbonate contents in upper cretaceous chalk sequences from six wells in the central part of the danish subbasin the cuttings were washed and dried at the well site with no other cleaning in the laboratory the analytical results from well cuttings were tested by comparison with data from corresponding cored intervals the results indicate that contamination by drilling mud is negligible in the material studied whereas caving occasionally may cause some veiling of the elemental distribution patterns the elemental data are therefore considered representative for carbonate sediments and the sequences studied a chemostratigraphic correlation of the sections is carried out on the basis of several chemostratigraphic markers each defined by one or more elements the chemostratigraphy hereby established for the central part of the danish subbasin is in accordance with the existing log stratigraphy and biostratigraphy in the area furthermore the geochemical profiles show remarkable similarity to corresponding profiles from the upper cretaceous in the north sea central graben the results emphasize the applicability of cuttings for geochemical analysis and of geochemistry as a useful tool for chemostratigraphy and basin analyses in the subsurface exploration for chalk as hydrocarbon reservoirs

genetic implications of trace element distributions in carbonate and non carbonate phases of limestones and dolostones from western cantabria spain abstract fe zn sr ba and pb contents are reported for carbonate and non carbonate clay fractions of lower cretaceous limestones and dolostones from western cantabria spain increases in the zn contents of both carbonate and non carbonate fractions are observed in association with the calcite dolomite transformation suggesting that the dolomitising fluids were probably enriched in this element relationships between regional geochemical trends for some elements zn in the carbonate fraction and zn ba and pb in non carbonate fraction the paleogeography of the area during the aptian and albian and the geographical positions of the localities studied lead to the conclusion that the dolomitising fluids originated from adjacent basinal mud rocks stable isotope and fluid inclusion data support this hypothesis

magnitude scaling of the near fault rupture directivity pulse abstract current ground motion models all assume monotonically increasing spectral amplitude at all periods with increasing magnitude however near fault recordings from recent earthquakes confirm that the near fault fault normal forward rupture directivity velocity pulse is a narrow band pulse whose period increases with magnitude this magnitude dependence of the period of the near fault pulse is expected from theory because the period of the pulse is related to source parameters such as the rise time duration of slip at a point on the fault and the fault dimensions which generally increase with magnitude this magnitude dependence of the pulse period causes the response spectrum to have a peak whose period increases with magnitude such that the near fault ground motions from smaller earthquakes may exceed those of larger earthquakes at intermediate periods around 1 s this paper presents preliminary equations relating the period of the fault normal component of the forward rupture directivity velocity pulse to the earthquake magnitude and a preliminary model for the acceleration response spectra of near fault fault normal ground motions that includes the magnitude dependence of the period of the response spectral peak

deep sea and shallow water hydrothermal vent communities two different phenomena hydrothermal vent communities from 76 areas lying at depths from 0 to 4100 m were split into two groups bdeepq and bshallow waterq based on the occurrence of vent obligate restricted to hydrothermal vents taxa of a high rank genus and family the two groups were then compared based on a large set of environmental faunistic and community parameters the degree of obligacy of fauna sharply changes at the depth of approximately 200 m in 21 shallow water areas located at depths b200 m vent obligate taxa are absent or their taxonomic rank does not exceed a species the total number of vent obligate species in shallow water is 8 and their obligacy is uncertain data from 55 areas lying at depths from 200 to 4100 m indicate there are ca 400 obligate species in 47 of these areas obligate vent taxa of the genus or the family rank are present obligate taxa of the genus rank appear at 200 m depth and of the family rank at 386 m the depth ca 200 m corresponds well with differences in community structure and composition and environmental parameters deep sea n200 m hydrothermal communities differ from shallow water ones b200 m in a much higher ratio of vent obligate taxa deep sea vent communities are also distinguished by the lack of diatom and algae bacterial mats phytoplankton lower species richness the development of large sulphide structures higher role of active vents in structuring communities in space replication of a set of dominant taxa in communities within large regions reproduced through the succession at newly formed vent fields more pronounced concentric and vertical zonality higher biomass of macrofauna the important role of symbiotrophic forms dominating the biomass the ratio of species with a bpointq species range endemic to small areas is higher in the deep sea there are no significant differences between the vent and non vent biogeography both in shallow waters and the deep ocean modern deep sea hydrothermal vent communities have a longer evolutionary history than their shallow water counterparts to explain major differences between deep and shallow water hot vent communities evolutionary processes need to be analysed over long geological periods and within global vertical zones it is suggested that the proportion of organic matter derived from chemosynthesis and photosynthesis plays a crucial role in evolution when calculated for global zones this proportion increases with depth to a certain critical level that separates deep and shallow water hot vent communities and fauna vertical zonation in the distribution of obligate taxa in hydrothermal vent communities indicates that this level is reached at approximately 200 m depth

deep sea hydrothermal communities in southwestern pacific back arc basins the north fiji and lau basins composition microdistribution and food web abstract during the year 1989 two diving cruises of the french deep sea submersible nautile were devoted to the study of hydrothermal vent biology in spreading centers of two southwestern pacific back arc basins lau basin and north fiji basin in both cases two major active sites were visited white lady and mussel valley in the north fiji basin and hine hina and vai lili in lau basin the faunal associations clustered around active vents are dominated by two species of snails ifremeria nautilei and alviniconcha hessleri and one or two species of mytilids belonging to bathymodiolus these species are associated with chemoautolithotrophic bacteria in intracellular symbiosis as detected by the activity of the calvin benson cycle diagnostic enzyme rubpcase pedunculate and sessile barnacles dominated the outer rim of the site and are analogs of the fiter feeding serpulids living in the epr sites the hot extremes of the sites are poorly or not colonized by alvinellids or other taxa in the lau basin cold seep sites are found at the periphery of active hot or warm ventsand are dominated by vestimentiferans and pogonophorans no major differences were seen between associations of the two back arc basins at the generic level with the exeption of the abundance of synaptid holothurians associated with bathymodiolus in side the mussel valley site

on the potential generality of depth related ecologic structure in cold seep communities evidence from cenozoic and mesozoic examples faunal communities inhabiting hydrocarbon seeps in the deep sea today show a depth related change in their ecologic structure the proportion of obligate endemic taxa increases with depth while the abundance of predators and other background taxa decreases here the fossil record of seep communities is investigated for similar gradients the middle eocene to middle miocene deep water cold seep deposits in western washington state usa are from different depths those of the middle to late eocene humptulips bear river and whiskey creek sites lived in depths of 500 to 2000 m those of the late oligocene to middle miocene seep communities of the lincoln creek and astoria formations in depths of 16 to 900 m these seeps were inhabited by the same taxa that inhabit cold seeps today and like their modern analogs they show an increasing proportion of obligate taxa and a decreasing abundance of predators and other background taxa with depth also among late mesozoic seep sites a depth related change in the proportion of obligate taxa is apparent seep communities in shallow shelf settings harbor virtually no obligates while seep communities in outer shelf to slope settings show a high proportion of obligates although the constituting species have different taxonomic affinities than their cenozoic and extant counterparts unlike in modern examples differences between seep communities from outer shelf and slope depths were not seen it is hypothesized that due to the higher mesozoic sea levels many seep communities on the continental shelves were located below the photic zone where an obligate seep fauna could develop because an increase in the proportion of obligate taxa with depth can be seen both in the cenozoic and in the late mesozoic despite the different taxonomic affinities of the obligate taxa it is suggested here that this depth related structure is a general feature of seep faunas independent of the taxa that express this pattern

observations on regional variability in ground motion amplitudes for small to moderate earthquakes in north america abstract the regional variability in earthquake ground motion amplitudes for a given magnitude and distance in western north american environments was examined using shakemap data from small to moderate events the abundance of data for small to moderate events in california allows average ground motion levels as a function of magnitude and distance to be resolved with a high level of confidence ground motion amplitudes in northern california are lower on average than those for southern california for events of the same magnitude at distances in the range from 120 to 250 km over all frequencies the observed regional variations could be indicative of regional differences in attenuation effects or site effects an unexpected result of the study is the finding that ground motions for events of m m m m

surface waves in the western taiwan coastal plain from an aftershock of the 1999 chi chi taiwan earthquake significant surface waves were recorded in the western coastal plain wcp of taiwan during the 1999 chi chi taiwan earthquake and its series of aftershocks we study in detail the surface waves produced by one aftershock 20 september 1999 18hr 03m 41 16sec m 6 2 in this paper we take the chelungpu chukou fault to be the eastern edge of the wcp because it marks a distinct lateral contrast in seismic wave velocities in the upper few kilometers of the surface for many records from stations within the wcp body waves and surface waves separate well in both the time domain and the period domain long period e g 2 sec ground motions in the plain are dominated by surface waves significant prograde rayleigh wave particle motions were observed in the wcp the observed peak ground velocities are about 3 5 times larger than standard predictions in the central and western part of the plain observed response spectra at 3 sec 4 sec and 5 sec at the center of the plain can be 15 times larger than standard predictions and 10 times larger than the predictions of joyner 2000 based on surface wave data from the los angeles basin the strong surface waves were probably generated at the boundary of the wcp and then propagated toward the west largely along radial directions relative to the epicenter the geometry of the boundary may have had a slight effect on propagation directions of surface waves group velocities of funda mental mode rayleigh and love waves are estimated using the multiple filter analysis mfa technique and are refined with phase matched filtering pmf group velocities of fundamental mode surface waves range from about 0 7 km sec to 1 5 km sec for the phases at periods from 3 sec to 10 sec one important observation from this study is that the strongest surface waves were recorded in the center of the plain the specific location of the strongest motions depends largely on the period of surface waves rather than on specific site conditions or plain structures accordingly we conjecture that surface waves could be generated in a wide area close to boundaries of low velocity sedimentary wave guides in the case studied in this article the area can be as wide as 30 km from the chelungpu fault to the center of the plain surface waves converted by p and s waves at different locations would overlap each other and add constructively along their propagation paths as a result the surface waves would get stronger and stronger beyond a certain distance to the boundary no more surface waves would be generated consequently no more local surface waves would be superimposed into the invasive surface waves and the surface waves would tend to decay in amplitude with distance

time series studies of vent fluids from the tag and mark sites 1986 1990 mid atlantic ridge a new solution chemistry model and a mechanism for cu zn zonation in massive sulphide orebodies

mg and sr distribution in carbonate rocks from the maastrichtian danian boundary of the danish subbasin and the north sea central graben abstract examination of the mg and sr concentrations in carbonate rocks in a number of sections provides the basis for the establishment of two geochemical facies within the region studied 1 a basinal facies including the axial part of the danish subbasin and the north sea central graben indicating relatively stable geochemical conditions and 2 an eastern marginal facies in the danish subbasin reflecting unstable geochemical conditions and distinguished by partial cementation and increasing mg and sr concentrations towards the northeastern limits of the basin a slight microtextural variation in the chalk between the basinal and marginal facies indicates that the origin and composition of the biogenic carbonate components strongly influence the bulk rock geochemistry the elemental distribution reveals a negative correlation between the mg and sr concentrations in the topmost maastrichtian chalk expressed by increasing mg and decreasing sr upwards towards the maastrichtian danian boundary this relation occurs in both geochemical facies and in all sections studied and is believed to have been governed by changing physicochemical conditions in the late maastrichtian sea

mediterranean fossil whale falls and the adaptation of mollusks to extreme habitats the hypothesis that sunken carcasses of mesozoic marine reptiles and cenozoic whales acted as evolutionary stepping stones to deep sea reducing habitats is underlain by the question of whether vent like chemosymbiotic specialization fi rst evolved at shelf depths fossil skeletons of large whales have long been known from ancient shallow water strata but they have never been considered as a source of information on ecosystem development we present a study on a 3 ma old fossil whale fall and a survey of other pliocene fossil skeletons to show that the associated biota is dominated by heterotrophs with subsidiary chemoautotrophs the taphonomy of the mediterranean shelf whale falls shows some differences with respect to deep water studies quantitative analyses of abundance data within a large data set on fossil and modern mollusk families confi rm that deep and shallow water communities at reducing habitats are composed of a different set of taxa i e specialists occurring only below the shelf break mediterranean carcasses sunken in coastal settings do not seem to be favorable for the evolution of whale fall specialists among the mollusks the situation reverses as the shelf break is approached

geochemistry of submarine warm springs in the limestone cavern of grotta azzurra capo palinuro italy evidence for mixing zone dolomitisation abstract subtidal springs in and around the submarine limestone cavern of grotta azzurra at capo palinuro italy discharge fluids which are warm 10 excess si alkalinity ca2 sr2 and mn relative to seawater are likely derived from the groundwater component or dissolution hydrothermal alteration of the host rocks magnesium has been removed from the seawater component in exchange for ca2 due to dolomitisation of the limestone and or hydrothermal alteration reactions saturation state calculations suggest that the vented fluids are near saturation with respect to calcite and supersaturated with respect to dolomite this and the presence of dolomite in the host rocks and cave floor sediments suggest that mixing zone dolomitisation of the limestones is occurring perhaps kinetically assisted by elevated temperature and or bacterial mediation in the reducing subseafloor zone one possible end member condition is considered for the thermal fluid zero mg which suggests an end member temperature of 50 5 c and a fluid composition derived from 38 seawater and 62 groundwater the heat source for the circulating fluids is uncertain but may involve warm underlying igneous rocks or heating via the geothermal gradient a continuous in situ record of vent fluid temperature salinity ph and o2 concentration collected within the cavern is consistent with our interpretation of the fluid origin and suggests that tidal forcing affects circulation and venting of the warm fluids

hydrothermal venting within a coral reef ecosystem ambitle island papua new guinea shallow water 5 10 m hydrothermal venting in a nearshore coral reef environment at ambitle island in the tabar feni island arc east of papua new guinea occurs as focused discharge of boiling fluid from discrete ports 10 15 cm in diameter and as dispersed discharge of diffuse bubble streams that issue through the sandy mixed carbonate volcaniclastic sea floor abiotic aragonite and microcrystalline ferroan low mg calcite interlaminated with fe oxyhydroxides are the prominent hydrothermal precipitates geochemical attributes of aragonite 18o 13c and fluid inclusions suggest that cements formed from a solution with salinities 5 at temperatures of 100 c with probable contribution of hydrothermal co2 sr isotope ratios in abiotic hydrothermal aragonite 0 704 15 are similar to those in island arc basalt and denote considerable subsurface water rock interaction of meteoric water derived from the adjacent volcanic island the sr isotope ratio of a coral sample 0 707 46 collected adjacent to a vent portal suggests coral growth within a mixed seawater hydrothermal environment

fe sulfide formation due to seawater gas sediment interaction in a shallow water hydrothermal system at lihir island papua new guinea fe sulfide minerals are forming in the shallow water hydrothermal system in luise harbor lihir island papua new guinea due to the interaction of hydrothermal gas or liquid seawater and fe rich sediments upon contact with oxygenated seawater hydrothermal h 2 s is oxidized and forms h 2 so 4 leading to the simultaneous dissolution of primary fe rich sediment grains and the neoformation of mainly marcasite and pyrite they are present as alternating colloform layers as framboidal aggregates in vugs or they replace organic fragments primary olivine and primary magnetite deposition of either marcasite or pyrite appears to reflect variations in acidity due to varying admixture of air saturated seawater the gas phase is composed of mainly co 2 n 2 and h 2 s assuming equilibration of the gases dissolved in a liquid phase and preservation of equilibrium ch 4 co 2 the ch 4 co 2 equilibrium temperature is approximately 310 degrees or 20 degrees c

hydrothermal vent alvinellid polychaete dispersal in the eastern pacific 1 influence of vent site distribution bottom currents and biological patterns deep sea hydrothermal vent habitats are typically linear discontinuous and short lived some of the vent fauna such as the endemic polychaete family alvinellidae are thought to lack a planktotrophic larval stage and therefore not to broadcast release their offspring the genetic evidence points to exchanges on a scale that seems to contradict this type of reproductive pattern however the rift valley may topographically rectify the bottom currents thereby facilitating the dispersal of propagules between active vent sites separated in some cases by 10s of kilometers or more along the ridge axis a propagule flux model based on a matrix of intersite distances long term current mete data and information on the biology and ecology of alvinellidae was developed to test this hypothesis calculations of the number of migrants exchanged between two populations per generation n allowed comparisons with estimates obtained from genetic studies n displays a logarithmic decrease with increasing dispersal duration and reaches the critical value of 1 after 8 d when the propagule flux model was run in standard conditions at most propagule traveling time cannot reasonably exceed 15 30 d according to the model whereas reported distances between sites would require longer lasting dispersal abilities two nonexclusive explanations are proposed first some aspects of the biology of alvinellidae have been overlooked and long distance dispersal does occur second such dispersal never occurs in alvinellidae but the spatial temporal dynamics of vent sites over geological timescales allows short range dispersal processes to maintain gene flow deep sea hydrothermal vent habitats are characterized by temperatures higher than ambient for the deep sea lower oxygen concentration higher levels of toxic compounds metals sulfide and extreme spatial and temporal variability tunnicliffe 1991 the biology and life history of the vent fauna are closely linked with these features which are stable over geological time on a large spatial scale in contrast vent sites seem to be ephemeral on short temporal scales lalou 199 1 hydrothermal activity indeed shifts along the ridges generating numerous short lived spots and consequently forcing the endemic fauna to colonize new vents as they appear in the eastern pacific hydrothermally active venting areas are restricted to a few portions of oceanic ridges the continuity between the sediment covered ridge in the gulf of california guaymas basin the east pacific rise epr and the galapagos rift is of particular interest to biologists segmentation of the ridge by transform faults and the great distance separating the main hydrothermal areas raise the question of dispersal processes this

temporal and spatial patterns of biological community development at nascent deep sea hydrothermal vents 9 50 n east pacific rise the april 1991 discovery of newly formed hydrothermal vents in areas of recent volcanic eruption between 9 45 n and 9 52 n on the east pacific rise provided a unique opportunity to follow temporal changes in biological community structure from the birth of numerous deep sea hydrothermal vents in march l992 dsv alvin was used to deploy an on bottom observatory the biologic geologic transect to monitor faunal succession along a 1 37 km segment of the axial summit caldera between 9 49 61 n and 9 50 36 n depth 2520 m photo and videographic documentation of megafaunal colonization and chemical analyses of diffuse hydrothermal fluids associated with many of these developing communities within the transect were performed in march 1992 december 1993 october 1994 and november 1995 photographic and chemical time series analyses revealed the following sequence of events in low temperature venting areas 1 immediately following the 1991 eruption hydrogen sulfide and iron concentrations in diffuse fluids were extremely high 1 mmol kg 1 and microbially derived material blanketed active areas of venting in the form of thick microbial mats 2 mobile vent fauna e g amphipods copepods octopods and galatheid and brachyuran crabs and non vent fauna e g nematocarcinid shrimp proliferated in response to this increased biological production 3 within 1 yr of the eruption areal coverage of microbial mat was reduced by 60 and individuals of the vestimentiferan tube worm tevnia jerichonana settled gregariously in areas where diffuse flow was most intense 4 two years after the eruption maximum levels of h2s decreased by almost half from 1 90 to 0 97 mmol kg 1 and dense thickets of the vestimentiferan riftia pachyptila dominated vent openings previously inhabited by tevnia jerichonana 5 three years after the eruption maximum hydrogen sulfide levels declined further to 0 88 mmol kg 1 and mussels bathymodiolus thermophilus were observed on basaltic substrates 6 four years after the eruption galatheid crabs and serpulid polychaetes increased in abundance and were observed close to active vent openings as maximum hydrogen levels decreased to 0 72 mmol kg 1 also by this time mussels had colonized on to tubes of riftia pachyptila 7 between 3 and 5 yr after the eruption there was a 2 to 3 fold increase in the number of species in the faunal assemblages in the absence of additional volcanic tectonic disturbance we predict that mytilid and vesicomyid bivalves will gradually replace vestimentiferans as the dominant megafauna 5 10 yr following the eruption we also anticipate that the abundance of suspension feeders will decline during this period while the abundance of carnivores will increase we hypothesize that the above series of events 1 7 represents a general sequence of biological successional changes that will occur at newly formed low temperature deep sea hydrothermal vents along the northern east pacific rise and contiguous ridge axes megafaunal colonization at deep sea hydrothermal vents is considered to be the consequence of an intimate interaction of the life history strategies of individual species physical oceanographic processes and the dynamic hydrothermal environment our observations indicate that the successful sequential colonization of dominant megafaunal vent species from tevnia jerichonana to riftia pachyptila to bathymodiolus thermophilus also may be strongly influenced by temporal changes in geochemical conditions additional evidence demonstrating the close link between diffuse vent flux fluid geochemistry and faunal succession included the rapid death of several newly formed biological assemblages coincident with abrupt changes in the geochemical composition of the venting fluid and the local refocusing or cessation of vent flow these correlations suggest that future models of faunal succession at hydrothermal vents along intermediate to fast spreading mid ocean ridges should consider not only the interplay of species specific life history strategies community productivity and physical oceanographic processes but also the influence of changing geochemical conditions on the sequential colonization of megafaunal species

comparisons of ground motions from five aftershocks of the 1999 chi chi taiwan earthquake with empirical predictions largely based on data from california the observed ground motions from five large aftershocks of the 1999 chi chi taiwan earthquake are compared with predictions from four equations based primarily on data from california the four equations for active tectonic re gions are those developed by abrahamson and silva 1997 boore et al 1997 campbell 1997 2001 and sadigh et al 1997 comparisons are made for horizontal component peak ground accelerations and 5 damped pseudoaccelera tion response spectra at periods between 0 02 sec and 5 sec the observed motions are in reasonable agreement with the predictions particularly for distances from 10 to 30 km this is in marked contrast to the motions from the chi chi mainshock which are much lower than the predicted motions for periods less than about 1 sec the results indicate that the low motions in the mainshock are not due to unusual localized absorption of seismic energy because waves from the mainshock and the aftershocks generally traverse the same section of the crust and are recorded at the same stations the aftershock motions at distances of 30 60 km are somewhat lower than the predictions but not nearly by as small a factor as those for the mainshock suggesting that the ground motion attenuates more rapidly in this region of taiwan than it does in the areas we compare with it we provide equations for the regional attenuation of response spectra which show increasing decay of motion with distance for decreasing oscillator periods this observational study also demonstrates that ground motions have large earthquake location dependent variability for a specific site this variability reduces the accuracy with which an earthquake specific predic tion of site response can be predicted online material pgas and psas from the 1999 chi chi earthquake and five aftershocks

geochemistry of hydrothermal fluids from axial seamount hydrothermal emissions study vent field juan de fuca ridge subseafloor boiling and subsequent fluid rock interaction hydrothermal fluids collected from the ashes vent field in 1986 1987 and 1988 exhibit a very wide range of chemical composition over a small area 60 m in diameter compositions range from a 300 c gas enriched 285 mmol kg co2 low chlorinity 33 of seawater fluid to a 328 c relatively gas depleted 50 mmol kg co2 high chlorinity 116 of seawater fluid the entire range of measured compositions at ashes is best explained by a single hydrothermal fluid undergoing phase separation while rising through the ocean crust followed by partial segregation of the vapor and brine phases other mechanisms proposed to produce chlorinity variations in hydrothermal fluids precipitation dissolution of a chloride bearing mineral or crustal hydration cannot produce the covariation of chlorinity and gas content observed at ashes there is good agreement of the measured fluid compositions with compositions generated by a simple model of phase separation in which gases are partitioned according to henry s law and all salt remains in the liquid phase significant enrichments in silica lithium and boron in the low chlorinity fluids over levels predicted by the model are attributed to fluid rock interaction in the upflow zone depletions in iron and calcium suggest that these elements have been removed by iron sulfide and anhydrite precipitation at some time in the history of the low chlorinity fluids the distribution of low and high chlorinity venting is consistent with mechanisms of phase segregation based on differential buoyancy or relative permeability the relatively shallow depth of the seafloor 1540 m and the observed chemistry of ashes fluids are consistent with phase separation in the sub critical or near critical region

influence of sea floor spreading on the global hydrothermal vent fauna one remarkable discovery of recent decades is the presence of hundreds of unusual species including fourteen new families at hydrothermal vents these animals unknown from other habitats live in extreme chemical and thermal conditions around vents on spreading centres of the mid ocean ridges and back arc basins chemosynthesis provides an in situ energy source for the thriving vent fauna this habitat has existed through the phanerozoic1 2 and probably since the archaean thus providing sites for long term adaptation we now test the hypothesis that animal distribution among hydrothermal vents is related to tectonic plate history3 4 the predominant migration pathway is most likely to occur along mid ocean ridges rather than by shortest oceanic routes similarity analyses suggest that the distribution patterns of today s vent fauna display the strong imprint of the timing and geometry of ancient plate boundaries study of past ridge geometry provides a method to predict relationships among vent communities yet to be discovered

dynamic character of the hydrothermal vent habitat and the nature of sulphide chimney fauna abstract a major substratum for vent organisms on juan de fuca and explorer ridges of the northeast pacific is the polymetallic sulphide chimney the deposition processes and subsequent growth of such chimneys provide a dynamic and extreme habitat that changes rapidly during initial venting stages colonization must await stabilization of the chimney structure as the porous anhydrite shell accumulates sulphide minerals protection from direct hot water contact increases for organisms colonizing the outer walls while sufficient vent water still diffuses for the support of chemosynthesis vestimentiferan worms are found on small sulphide mounds but the active growth and collapse of anhydrite spires is a major source of disruption and mortality high temperature venting and spire growth appear to attract alvinellid polychaetes which may be implicated in the process of strengthening and sealing the anhydrite spires fauna on the growing chimney is subject to changes in fluid flow patterns that can make the structure uninhabitable large chimneys are inhabited by more species indicating that a diversification of habitat results from variations in mineralization and fluid availability chimney fauna has become specialized in exploiting an unpredictable and unstable habitat in an extreme example of the control of biotic development by the physical environment

effect of baseline corrections on displacements and response spectra for several recordings of the 1999 chi chi taiwan earthquake displacements derived from many of the accelerogram recordings of the 1999 chi chi taiwan earthquake show drifts when only a simple baseline de rived from the pre event portion of the record is removed from the records the appearance of the velocity and displacement records suggests that changes in the zero level of the acceleration are responsible for these drifts the source of the shifts in zero level are unknown but in at least one case it is almost certainly due to tilting of the ground this article illustrates the effect on the ground velocity ground dis placement and response spectra of several schemes for accounting for these baseline shifts a wide range of final displacements can be obtained for various choices of baseline correction and comparison with nearby gps stations none of which are colocated with the accelerograph stations do not help in choosing the appropriate baseline correction the results suggest that final displacements estimated from the records should be used with caution the most important conclusion for earthquake engineering purposes however is that the response spectra for periods less than about 20 sec are usually unaffected by the baseline correction although limited to the analysis of only a small number of recordings the results may have more general significance both for the many other recordings of this earthquake and for data that will be obtained in the future from similar high quality accelerograph networks now being installed or soon to be installed in many parts of the world

oxygen and carbon isotope compositions of upper cretaceous chalk from the danish sub basin and the north sea central graben the oxygen and carbon isotopic compositions of upper cretaceous chalk have been studied from a large number of outcrops and boreholes in the danish sub basin and the north sea central graben the carbon isotopic compositions vary from 0 50 o to 3 00 o 13c which correspond to the expected carbon isotopic composition of carbonate precipitated from normal sea water the oxygen isotopic compositions of samples from outcrops and near surface drilled sections fall in the range from 0 50 o to 2 00 o 18o which is close to the expected values for carbonate produced by coccoliths in the late cretaceous sea a geographic distribution exists in the danish sub basin showing relatively heavy oxygen isotope values in the centre of the basin and slightly more negative values in the eastern part of the basin primarily the phenomenon is thought to reflect minor variations in the oceanographic parameters in the late cretaceous sea the deep subsurface sections represent a depth interval of 0 3100 m mechanical compaction in the uppermost part of the sequences apparently has no influence on the isotopic composition chemical compaction dominates at greater depth leading to temperature induced isotopic re equilibration in the water rock system in these sequences the oxygen isotope values range from 1 50 o to 7 50 o 18o and are significantly correlated with depth of burial cementation and porosity the data indicate that pressure dissolution recrystallization reprecipitation and ion exchange between solid carbonate phase and pore water are all actively involved in the oxygen isotope re equilibration process this process is believed to take place in a diagenetically closed system and is tentatively divided into two phases 1 an early diagenetic phase which takes place at porosities down to approximately 20 in which the chalk possesses a pore water controlled isotopic re equilibration system and 2 a late diagenetic phase at porosities below 20 in which the re equilibration process increasingly becomes influenced by the rock introduced isotopic change in the composition of the formation water

exceptional crinoid occurrences and associated carbonates of the keasey formation early oligocene at mist oregon usa abstract an unusual concentrated zone of well preserved fossil crinoids corals and other organisms exists with carbonates in an otherwise barren outcrop of fine grained sandstone and siltstone of the early oligocene keasey formation at mist oregon the origin of this lagerstatte has long puzzled paleontologists taxonomically similar modern isocrinids have a strong preference for indurated substrates the mist carbonates may have provided seafloor hardgrounds for the isocrinids a physical limiting factor on the formation and distribution of the crinoid lagerstatte other paleoenvironmental factors such as currents sedimentation rates and food flux also may have affected crinoid community development stable carbon and oxygen isotopic analyses of micritic slabs concretions and sparry veins in several lithologic units at mist indicate that most of the carbonates formed in the zone of archaeal methanogenesis enrichment in 13ccarbonate with values as high as 9 5 vpdb the carbonates may be a late diagenetic feature entombing the faunal material following burial depletion in 18ocarbonate to 7 8 vpdb and therefore have no temporal relation to the crinoids alternatively the carbonates could have formed in the methanogenic zone deep in the sediment pile later exhumed by erosion and colonized again with no temporal affiliation to the epifauna finally the carbonates at mist could have precipitated in an unusually shallow zone of archaeal methanogenesis induced by seepage from the subsurface resulting in availability of reduced organic compounds and bacterial consumption of sulfate near the sediment water interface in this case the sulfate reduction zone must have been thin and the upward advection of deep sourced isotopically evolved fluids sufficient to impart negative 18o and positive 13c signatures to the carbonates forming possibly at only a few centimeters depth further evidence that supports a shallow methanogenic origin for the mist carbonates includes 1 presence of an infaunal low oxygen and or chemosymbiotic fauna 2 pristine preservation of molluscan shell secreted in isotopic equilibrium with seawater bicarbonate 3 lack of major erosional surfaces associated with the carbonates and 4 the occurrence of reservoir rocks beneath mist that were charged with natural gas during the early oligocene locally mapped faults and or clastic dikes of this age may have conveyed methane to the near surface with crinoid growth occurring upon indurated calcareous sediments that were intermittently exposed by minor current winnowing of the draping bottom silts

geochemistry diagenesis and nannofacies of chalk in the north sea central graben abstract core material including late cretaceous and early tertiary chalk is studied from eight wells in the danish sector of the north sea central graben quantitative and qualitative evaluation of the carbonate constituents diagenetic features and nannofacies was carried out using scanning electron microscopy geochemical analyses include the distribution of mg sr mn sio2 and al2o3 and carbonate contents and determination of the major constituents of the non carbonate matter it is demonstrated that the diagenetic impact observed generally is related to pressure dissolution processes and reprecipitation and according to nannofacies the most important trend in rock quality is the increasing formation of secondary microspar as a function of depth of burial a profound silica cementation is recorded at a stratigraphic restricted horizon in contrast the elemental distributions show no correlation to depth despite abundant petrographic evidence of deep burial diagenesis thus the geochemical data indicate that the north sea chalk possesses a rock dominated diagenetic equilibrium system with respect to trace and minor elements and that the elemental distribution to a large extent is dependent on the biogenic composition of the sediment the depositional environments and the geochemical realm in the sediment water interface this suggests that geochemistry is a conceivable tool in basin analysis in the lithologically rather monotonous chalk sequence