molybdenum behavior during early diagenesis insights from mo isotopes 1 we present molybdenum isotope and concentration data from 14 sites in the eastern pacific from the central california to the peru margin the environments studied have been chosen to represent a broad range in oxidation reduction redox potential which provide a framework for the behavior of this redox sensitive element manganese rich hemipelagic sediments from the eastern tropical pacific have a mean characteristic mo isotope signature 98 95mo 0 49 0 04 two times the standard deviation of the mean 2 sdom with n 14 that reflects fractionation between ocean water and authigenic mo associated with mn oxides authigenic fe mo s deposits from reducing continental margin settings also have a characteristic mo isotopic signature 98 95mo 1 64 0 04 2 sdom with n 136 both of these values are in contrast to highly sulfidic 11 m h2saq restricted basin environments which contain mo isotope values analytically indistinguishable from seawater in terms of the mo isotope composition the modern oceanic mo sink is dominated by continental margin type environments where 98 95mo 1 6 and mn rich sediments where 98 95mo approximately 0 5 with a minor contribution from euxinic settings where h2saq 11 m

the sea level fingerprints of ice sheet collapse during interglacial periods abstract studies of sea level during previous interglacials provide insight into the stability of polar ice sheets in the face of global climate change commonly these studies correct ancient sea level highstands for the contaminating effect of isostatic adjustment associated with past ice age cycles and interpret the residuals as being equivalent to the peak eustatic sea level associated with excess melting relative to present day of ancient polar ice sheets however the collapse of polar ice sheets produces a distinct geometry or fingerprint of sea level change which must be accounted for to accurately infer peak eustatic sea level from site specific residual highstands to explore this issue we compute fingerprints associated with the collapse of the greenland ice sheet west antarctic ice sheet and marine sectors of the east antarctic ice sheet in order to isolate regions that would have been subject to greater than eustatic sea level change for all three cases these fingerprints are more robust than those associated with modern melting events when applied to infer eustatic sea level because 1 a significant collapse of polar ice sheets reduces the sensitivity of the computed fingerprints to uncertainties in the geometry of the melt regions and 2 the sea level signal associated with the collapse will dominate the signal from steric effects we evaluate these fingerprints at a suite of sites where sea level records from interglacial marine isotopes stages mis 5e and 11 have been obtained using these results we demonstrate that previously discrepant estimates of peak eustatic sea level during mis5e based on sea level markers in australia and the seychelles are brought into closer accord

natural mass dependent variations in the isotopic composition of molybdenum we present the first observations of natural mass dependent fractionation of the isotopic composition of molybdenum mo using multi collector inductively coupled plasma mass spectrometry variations in the isotopic composition of mo are reported as 97 95mo 97mo 95mo sample 97mo 95mo standard 1 1000 external analytical precision of 97 95mo is 1 between sediments deposited under anoxic conditions 97 95mo 1 02 to 1 52 relative to our in house standard and ferromanganese nodules 97 95mo 0 63 to 0 42 97 95mo of pacific ocean seawater 97 95mo 1 48 lies within the range of values for anoxic sediments closest to modern black sea anoxic sediments molybdenites from continental ore deposits have intermediate 97 95mo ranging from 0 26 to 0 09 variations in the abundances of 92mo 95mo 96mo 97mo and 98mo are consistent with mass dependent fractionation a sporadic unidentified interference occurs at mass 94 and 100mo is not measured we hypothesize that the 97 95mo offset between anoxic sediments and ferromanganese nodules results from mo isotope fractionation during inefficient scavenging of mo from seawater by mn oxides under oxic conditions the similarity in 97 95mo of anoxic sediments and seawater is consistent with the very efficient removal of mo from seawater under anoxic conditions in the presence of h2s the data can be interpreted in terms of a steady state mass balance between the mo flux into the oceans from the continents and the mo flux out of the oceans into oxic and anoxic sediments such an interpretation is quantitatively consistent with existing estimates of the removal fluxes of mo to anoxic and oxic sediments these findings suggest that 97 95mo in seawater may co vary with changes in the relative proportions of anoxic and oxic sedimentation in the oceans and that this variation may be recorded in 97 95mo of anoxic sediments hence the mo isotope system may be useful in paleoredox investigations

maar and crater lakes of the long gang volcanic field n e china overview laminated sediments and vegetation history of the last 900 years abstract maar and crater lakes have due to their specific formation and characteristic morphology a high potential for the development and preservation of seasonally laminated sediments varves within the long gang volcanic field lgvf in n e china eight maar or crater lakes with water depths between 15 and 127 m are located the seasonal climate with strong influence of the east asian summer and winter monsoon and the position of the lgvf at the northern margin of the east asian dust storm tracks make these lakes key archives for palaeoclimatic studies in the course of a site survey for a deep lake coring water samples and short sediment cores were investigated from all of these lakes detailed pollen investigations of the last 900 years from lake sihailongwan an ideally bowl shaped maar lake with 50 m water depth revealed a very stable vegetation cover except for the last 150 years which shows increasing anthropogenic influence no evidence could be found in the pollen record for a climatic deterioration comparable to the european little ice age however climatic variations can be revealed from high resolution geochemical and sedimentological investigations on a decadal to annual scale

magmatic affinity of modern and ancient subalkaline volcanic rocks determined from trace element discriminant diagrams when dealing with ancient subalkaline volcanic rocks the alkali total iron magnesium afm diagram is of limited use in assigning a tholeiitic versus calc alkaline affinity because these eleme

the geochemistry of boron and its isotopes in groundwaters from marine and non marine sandstone aquifers abstract the geological evolution of b in two uk sandstone aquifers is followed from precipitation chemistry through to groundwaters in both the unconfined and confined zones measurements have been made of major element geochemistry b concentrations and b isotopic ratios the isotopic measurements were carried out using icp ms following a simple preconcentration step isotopic measurements of rainfall show a bimodal distribution and it is suggested that enriched signatures are characteristic of atlantic air over britain and depleted signatures representative of continental air in the marine lower greensand aquifer dissolution of glauconite results in the mobilisation of b and a correlation with so 4 suggests that this dissolution is related to the oxidation of pyrite which appears to be the so 4 forming reaction in the aquifer in the non marine hastings beds isotopic ratios and a correlation with hco 3 suggest that b is associated with the dissolution of ferroan carbonates in both aquifers the geochemical evolution of b is complex and more information is needed on the behaviour of b isotopes during evapotranspiration and groundwater recharge

the behavior of molybdenum and its isotopes across the chemocline and in the sediments of sulfidic lake cadagno switzerland abstract molybdenum mo isotope studies in black shales can provide information about the redox evolution of the earth s oceans provided the isotopic consequences of mo burial into its major sinks are well understood previous applications of the mo isotope paleo ocean redox proxy assumed quantitative scavenging of mo when buried into sulfidic sediments this paper contains the first complete suite of mo isotope fractionation observations in a sulfidic water column and sediment system the meromictic lake cadagno switzerland a small alpine lake with a pronounced oxygen sulfide transition reaching up to h 2 s 200 m in the bottom waters or about 300 m total sulfide s 2 h 2 s hs s 2 we find that mo behaves conservatively in the oxic zone and non conservatively in the sulfidic zone where dissolved mo concentrations decrease from 14 nm to 2 8 nm across this transition dissolved mo in the upper oxic waters has a 98 mo oxic 0 9 0 1 which matches that of the riverine input 98 mo river 0 9 0 1 in the deeper sulfidic waters a subaquatic source delivers mo at 1 55 0 1 but the dissolved mo is even heavier at 98 mo sulfidic 1 8 sediment traps in the sulfidic zone of the lake collect particles increasingly enriched in mo with depth with 98 mo values significantly fractionated at 0 8 to 1 2 both near the chemocline and in the deepest trap suspended particulates in the sulfidic waters carry lighter mo than the ambient dissolved mo pool by 0 3 1 5 sedimentary mo concentrations correlate with total organic carbon and yield mo levels which are two orders of magnitude higher than typical crustal values found in rocks from the catchment area solid phase mo in the sediment shows a slightly positive 98 mo trend with depth from 98 mo 1 2 to 1 4 while the pore waters show dramatic enrichments of mo 2000 nm with a relatively light isotope signature of 98 mo 0 9 1 0 these data are explained if mo is converted to particle reactive oxythiomolybdates in the sulfidic waters and is fractionated during removal from solution onto particles isotope fractionation is expressed in the water column despite the high sulfide concentrations because the rate of mo removal is fast compared to the slow reaction kinetics of thiomolybdate formation however elemental and isotopic mass balances show that mo is indeed quantitatively removed to the lake sediments and thus the isotopic composition of the sediments reflects sources to the sulfidic water this efficient mo drawdown is expected to occur in settings where h 2 s is very much in excess over mo or in a restricted setting where the water renewal rate is slow compared to the mo burial rate we present a model for the mo isotope fractionation in sulfidic systems associated with the slow reaction kinetics and conclude that quantitative removal will occur in highly sulfidic and restricted marine systems

late mesozoic stratigraphy in western liaoning china a review abstract based on progress in stratigraphic and paleontological studies we discuss the subdivision of late mesozoic strata in western liaoning focusing on the controversies surrounding the relationship of the jiufotang and shahai formations and whether or not the zhanglaogongtun and banlashan formations exist as distinct lithologic units we describe the late mesozoic strata at representative sections associated with the new discoveries we also discuss the correlation of late mesozoic strata between western liaoning and eastern heilongjiang and their implications for the ages of the strata in liaoning

evidence for mo isotope fractionation in the solar nebula and during planetary differentiation abstract mass dependent mo isotope fractionation has been investigated for a wide range of meteorites including chondrites enstatite ordinary and carbonaceous chondrites iron meteorites and achondrites eucrites angrites and martian meteorites as well as for lunar and terrestrial samples magmatic iron meteorites together with enstatite ordinary and most carbonaceous chondrites define a common 98 95 mo value of 0 16 0 02 relative to the nist srm 3134 mo standard which is interpreted to reflect the mo isotope composition of bulk planetary bodies in the inner solar system heavy mo isotope compositions for iab iron meteorites most likely reflect impact induced evaporative losses of mo from these meteorites carbonaceous chondrites define an inverse correlation between 98 95 mo and metal content and a positive correlation between 98 95 mo and matrix abundance these correlations are mainly defined by cm and ck chondrites and may reflect the heterogeneous distribution of an isotopically light metal and or an isotopically heavy matrix component in the formation region of carbonaceous chondrites alternatively the elevated 98 95 mo of the cm and ck chondrites could result from the loss of volatile isotopically light mo oxides that formed under oxidized conditions typical for the formation of these chondrites the mo isotope compositions of samples derived from the silicate portion of differentiated planetary bodies are heavy compared to the mean composition of chondrites and iron meteorites this difference is qualitatively consistent with experimental evidence for mo isotope fractionation between metal and silicate the common 98 95 mo values of 0 05 0 03 of lunar samples derived from different geochemical reservoirs indicate the absence of significant mo isotope fractionation by silicate differentiation or impact metamorphism volatilization on the moon the most straightforward interpretation of the mo isotope composition of the lunar mantle corresponds to the formation of a lunar core at a metal silicate equilibration temperature of 1800 200 c the investigated martian meteorites angrites and eucrites exhibit more variable mo isotope compositions which for several samples extend to values above the maximum 98 95 mo 0 14 that can be associated with core formation for these samples post core formation processes such as partial melting metamorphism and in the case of meteorite finds terrestrial weathering must have resulted in mo isotope fractionation estimates of the metal silicate equilibration temperatures for mars 2490 770 c and the angrite parent body 1790 230 c are thus more uncertain than that derived for the moon although the mo isotope composition of the bulk silicate earth has not been determined as part of this study a value of 0 16 98 95 mo 0 can be predicted based on the chondrite and iron meteorite data and by assuming a reasonable temperature range for core formation in the earth this estimate is in agreement with four analyzed basalt standards 0 10 0 10 improved application of mass dependent mo isotope fractionation to investigate core formation most of all requires an improved understanding of potential mo isotope fractionation during processes not related to metal silicate differentiation

a 9000 year oxygen and carbon isotope record of hydrological change in a small ethiopian crater lake the stable isotope composition of authigenic calcites in a core from lake tilo a small crater lake in the ethiopian rift valley provides a subcentury scale record of lake response to climatic change over the last 8850 14c yrs 9850 cal yrs an unusually high range of 18o and 13c values 15 is attributed in part to major changes in hydrothermal groundwater flux although hydrothermal groundwater influx to the lake was high during the early holocene its flow rate was relatively stable and thus climatically induced changes to the water budget can be inferred from variations in 18o and 13c ratios a major decline in hydrothermal groundwater input from 5500 14c yrs bp increased lakewater residence time and led to substantial increases in 18o and 13c before the mid holocene transition to more arid conditions during the last 2700 14c years diagenetic processes have resulted in extremely varied 18o and 13c values the holocene isotope record from lake tilo challenges the widely held assumpti

century long record of mo isotopic composition in sediments of a seasonally anoxic estuary chesapeake bay abstract a double spike method was used to obtain mo isotope data for sediments and waters of the seasonally anoxic chesapeake bay and its primary tributary the susquehanna river the dissolved mo distribution in the estuary is non conservative reflecting minor mo loss to the sediments although removal of mo to the sediments does not have a large influence on the isotopic composition of the water column the 98mo of dissolved mo in most of the estuary is dominated by seawater six samples with salinity 15 have an average 98mo 2 17 0 12 which agrees well with a 98mo value for the cass 4 seawater standard of 2 23 a single sample of susquehanna river water has a 98mo of 1 02 consistent with recent findings of positive 98mo in rivers worldwide susquehanna river sediments in contrast have 98mo 0 1 the difference between the river water and sediment values implies that isotopic fractionation occurs within the river basin the 98mo values for estuarine sediments are offset from values in the overlying water most samples deposited before 1925 have 98mo less than 0 similar to the susquehanna sediments subsequently there is an increase in the variability of 98mo with values ranging up to 0 8 the transition to increased variability coincides with the onset of authigenic mo deposition which was previously attributed to escalating summertime anoxia authigenic mo concentrations correlate poorly with 98mo in core samples suggesting that independent mechanisms influence the two parameters authigenic mo concentrations may be controlled by shifting pore water h2s levels while 98mo may be primarily affected by annual variations in mn refluxing

relationship of pyroclastic volcanism and lake water acidification to jehol biota mass mortality events early cretaceous northeastern china abstract geochemical analysis of the 14 4 m thick lacustrine succession of the lower cretaceous yixian formation jehol group has yielded new insights concerning vertebrate mass mortality events in the lake sihetun volcanic caldera in western liaoning province northeastern china that produced the jehol biota fossil lagerstatten the long term evolution of the caldera system resulted in a shift from felsic to mafic magma chemistry accompanied by a reduced frequency of pyroclastic eruptions declining hydrothermal activity and lower lacustrine productivity the basal tetrapod beds exhibit strong hydrothermal influence as indicated by enrichments of boron b certain alkalis rb cs rare earth elements rees yttrium y and many metals e g co cr cu ge mo ni sb u v and w and by strongly negative molybdenum isotope compositions 98mo to 2 50 that may record large fractionations between molybdate and thiomolybdate species in the sihetun caldera hydrothermal system in contrast the overlying fish beds and non fossiliferous beds have an elemental and mo isotopic composition similar to calc alkaline basalts 98mo 0 29 0 04 in the surrounding watershed suggesting weathering of yixian formation volcanic rocks as the major source of sediment during its

determination of molybdenum isotope fractionation by double spike multicollector inductively coupled plasma mass spectrometry 1 molybdenum isotopic compositions are precisely determined by mc icp ms measurements using a mo double spike the double spike is added prior to chemical purification so that laboratory and instrumental mass fractionations are separated from natural mass dependent fractionation fractionation is determined on four mo mass ratios providing an internal consistency check the external standard reproducibility is at 0 06 per mil on the 98mo 95mo ratio 2 standard deviation using a normal microconcentric nebuliser with a cyclonic spray chamber the minimum quantity of mo is 1 g for high precision results a hydrothermal molybdenite shows fractionation of 0 3 per mil on the 98mo 95mo ratio relative to our standard johnson matthey 1000 g ml 0 3 icp standard solution lot 602332b fine grained sediments show fractionation of 0 1 and 0 3 per mil on the 98mo 95mo ratio the observed mo isotope fractionation is small but resolvable with the presented high resolution technique

climatic and volcanic forcing revealed in a 50 000 year diatom record from lake massoko tanzania the interactions between climatic and volcanic forcing on diatom communities contained in a 50 000 year sedimentary sequence from lake massoko tanzania were examined at the century scale 19 discrete tephra inputs to the lake isolated the sedimentary nutrient supply and shifted the diatom communities to those tolerant of low phosphorus levels whereas at the millennial scale diatom inferred shifts in precipitation evaporation based on conductivity optima and diatom life form ratios were broadly similar to lake level reconstructions from lake rukwa lake malawi and others in the region some fluctuations of lake massoko are consistent with the precession driven changes in insolation but the major climate shifts do not relate directly to orbital forcing of summer insolation south of the equator and show more consistency with records from the equatorial and northern tropics that receive rainfall from the passing of the intertropical convergence zone sea surface temperatures are strongly correlated to multimillennial scale climate patterns over this region of africa

crater lake and post eruption hydrothermal activity el chichon volcano mexico abstract explosive eruptions of volcan el chichon in chiapas mexico on march 28 and april 3 4 1982 removed 0 2 km3 of rock to form a 1 km wide 300 m deep summit crater by late april 1982 a lake had begun to form on the crater floor and by november 1982 it attained a maximum surface area of 1 4 105 m2 and a volume of 5 106 m3 accumulation of 4 5 m of rainfall between july and october 1982 largely formed the lake in january 1983 temperatures of fumaroles on the crater floor and lower crater walls ranged from 98 to 115 c by october 1983 the maximum temperature of fumarole emissions was 99 c in january 1983 fumarole gas emissions were greater than 99 vol h2o with traces of co2 so2 and h2s the water of the lake was a hot t 52 58 c acidic ph 0 5 dilute solution 34 046 mg l 1 dissolved solids cl s 20 5 sediment from the lake contains the same silicate minerals as the rocks of the 1982 pyroclastic deposits together with less than 1 of elemental sulfur the composition and temperature of the lake water is attributed to 1 solution of fumarole emissions 2 reaction of lake water with hot rocks beneath the lake level 3 sediments washed into the lake from the crater walls 4 hydrothermal fluids leaching sediments and formational waters in sedimentary rocks of the basement 5 evaporation and 6 precipitation

the molybdenum isotopic composition in river water constraints from small catchments we report molybdenum isotope compositions and concentrations in water samples from a variety of river catchment profiles in order to investigate the influence of anthropogenic contamination catchment geology within river precipitation and seasonal river flow variations on riverine molybdenum our results show that the observed variations in 98 95mo from 0 to 1 9 are primarily controlled by catchment lithology particularly by weathering of sulfates and sulfides erosion in catchments dominated by wet based glaciers leads to very high dissolved molybdenum concentrations in contrast anthropogenic inputs affect neither the concentration nor the isotopic composition of dissolved molybdenum in the rivers studied here seasonal variations are also quite muted the finding that catchment geology exerts the primary control on the delivery of molybdenum to seawater indicates that the flux and isotope composition of molybdenum to seawater has likely varied in the geologic past

diagenetic uptake of rare earth elements by bioapatite with an example from lower triassic conodonts of south china abstract the distribution patterns of rare earth elements rees are frequently used as proxies for ancient seawater chemistry or paleomarine environmental conditions however recent work has shown that diagenesis can lead to remobilization and inter elemental fractionation of rees and that these effects often occur in conjunction with redox reactions in sediment porewaters here we review existing literature on the diagenetic fluxes of rees in marine sediments and porewaters in order to systematize existing knowledge on this subject rees undergo significant redistribution among sediment phases during both early and late diagenesis as a consequence of adsorption and desorption processes remobilization of rees commonly leads to inter elemental fractionation variously leading to enrichment or depletion of the light middle or heavy ree fractions further ree remobilization can be facilitated by redox changes e g through reductive dissolution of host phases in suboxic and anoxic porewaters characteristic ree distribution patterns develop through these processes 1 a flat distribution signifying predominantly terrigenous siliciclastic influence 2 a middle ree bulge probably due to adsorption of light and heavy rees to mn and fe oxyhydroxides respectively and 3 heavy ree enrichment indicative of hydrogenous seawater influence note all patterns in this paper are normalized to the ree composition of average upper continental crust or ucc in the second part of this study we undertake an analysis of the ree distributions in conodonts and whole rock samples from west pingdingshan a permian triassic boundary section in south china using ree th and y ho ratios we show that almost all of the conodont samples have a strong diagenetic overprint and that the hydrogenous ree fraction is small and not isolatable furthermore the conodonts contain two diagenetic ree components one characterized by low ree 100 300 ppm high ree th ratios 1000 strong middle ree enrichment and eu eu ratios of 1 5 2 0 and the second by high ree 300 2000 ppm low ree th ratios 20 30 little or no middle ree enrichment and eu eu ratios of 1 0 the first component exhibits a pronounced middle ree bulge that represents an early diagenetic signature associated with suboxic conditions possibly related to adsorption of rees onto fe mn oxyhydroxides in the shallow subsurface environment the second component shows a flat ree distribution that is similar to both our whole rock samples and average ucc indicating derivation from rees released from detrital siliciclastics e g clay minerals probably at a range of burial depths from shallow to deep failure of the conodont samples to yield an isolatable hydrogenous component demonstrates that bioapatite does not always preserve a primary marine ree signature given that bioapatite rees have been widely used for this purpose often on the assumption of minimal or no diagenetic influence our findings are likely to necessitate a re evaluation of the results of many earlier studies in general we counsel caution in inferring a hydrogenous origin for rees in bioapatite owing to frequent diagenetic alteration of ree distributions

thermal rheological and kinematic conditions for channelized lower crustal flow in a threshold example abstract lower crustal flow zones occur in large and hot orogens and rifts where they occur in association with large areas possessing high gravitational potential energy gpe and low lower crust viscosity lower crustal flow zones are also known from regions where neither the rheological nor gpe conditions are sufficiently well developed for widespread flow to occur for these examples the conditions required to form crustal flow zones are less well defined one such case occurs within the ca 600 530 ma petermann orogeny in central australia this work investigates the conditions under which this flow zone developed considering differences with the adjacent regions where flow is not observed a transect of crustal structure in the central petermann orogen is developed that takes into account geological and geophysical data including time constrained pressure and temperature estimates for the petermann orogeny the deformation of this region is reconstructed showing that lower crustal flow was driven by high amplitude crustal scale folding the folding extruded the lower crust towards the foreland causing crustal thickening and buoyant uplift of the orogenic core channelized lower crustal flow is not known from either the eastern or western parts of the orogen and a comparison with these regions suggests three main conditions are required firstly the lower crust must be sufficiently hot and volatile rich to allow melt weakening secondly crustal rheology must allow ductile deformation of the mid to lower crust without high strength layers finally a high degree of lateral confinement is required to form pressure gradients capable of driving flow these three conditions were met only in the central petermann orogen and not in the east or the west this example of a lower crustal flow zone illustrates the combined influence of thermal rheological and kinematic threshold conditions on the initiation and development of channelized lower crustal flow in orogens

molybdenum isotope fractionation during adsorption by manganese oxides the isotopic composition of mo 97 95mo in seawater is 2 heavier than mo in marine ferromanganese crusts and nodules barling et al earth planet sci lett 193 2001 447 457 siebert et al earth planet sci lett 211 2003 159 171 to explore this phenomenon we have conducted an experimental investigation into the mass dependent fractionation of mo isotopes during adsorption onto mn oxyhydroxide two series of experiments were carried out a time series in which adsorption proceeded for 2 96 h and a ph series in which ph varied from 6 5 to 8 5 the extent of mo adsorption by mn oxyhydroxides decreases with increasing ph a trend typical of anion adsorption and takes 48 h to reach steady state lighter mo isotopes are preferentially adsorbed experimentally determined fractionation factors soln mnox exhibit no systematic variation with either time or experimental ph the mean soln mnox for all experiments is 1 0018 0 0005 2 s d comparison of the mo isotopic data for experimental solutions and mo adsorbed to mn oxyhydroxide with predictions for closed system equilibrium and rayleigh fractionation models indicates that isotope fractionation occurs as a result of closed system equilibrium exchange between dissolved and adsorbed mo the isotopic offset between dissolved and adsorbed mo is comparable to that observed between mo in seawater and mo in ferromanganese nodules and crusts it is therefore likely that adsorption of mo to mn oxyhydroxides is a significant factor in the fractionation of mo isotopes in the oceans

the influence of organic matter on the boron isotope geochemistry of the gulf coast sedimentary basin usa large variations in the boron isotopic composition of sedimentary environments make boron an attractive monitor of fluid rock interactions during diagenesis studies of b in marine sediments have shown that preferential adsorption of 10b on clay minerals leaves pore waters enriched in 11b during diagenesis clay minerals recrystallize and incorporate 10b into the mineral structure spivack a j palmer m r edmond j m 1987 the sedimentary cycle of the boron isotopes geochim cosmochim acta 51 1939 1949 this process should cause a depletion of b in the pore water with an increase in the 11b in the gulf coast sedimentary basin usa however there is a general increase in b content of formation waters land l s macpherson g l 1992 origin of saline formation waters cenozoic section gulf of mexico sedimentary basin geochim cosmochim acta 76 1344 1362 moldovanyi e p walter l m 1992 regional trends in water chemistry smackover formation southwest arkansas geochemical and physical controls aapg bull 76 864 894 and a decrease in 11b with depth this suggests that another source of 10b exists in deep basinal environments we know that oil reservoir brines are commonly enriched in boron collins a g 1975 geochemistry of oilfield waters elsevier new york p 496 therefore this study examines organic matter as a possible source of boron during thermal maturation samples of water oil and cored sediments were collected from three stacked hydrocarbon reservoirs in the gulf of mexico sedimentary basin at a depth of 3500 4350 m extraction of boron from organic matter oil and kerogen was done by parr bomb volatilization with mannitol used as a b complexing agent the isotope ratios were measured using negative thermal ionization and compared to in situ analyses using secondary ion mass spectrometry the 11b values of pore filling clays in sandstone reservoirs is 2 2 the b content of the clay averages 144 ppm oil field waters show a range in b content from 8 85 ppm and 11b values from 28 to 37 increasing from the lowest to the uppermost reservoir there is an apparent 11b enrichment of fluids with progressive migration through clay rich sediments very little b ppb was found in the oil but kerogen extracted from the oil source rock sassen r 1990 lower tertiary and upper cretaceous source rocks in lousiana and mississippi implications to gulf of mexico crude oil aapg bull 74 857 878 contains significant b 140 ppm with a 11b of 2 2 similar to the pore filling clay minerals in the sandstones while kerogen comprises only 2 of the sedimentary basin its influence can be significant if b with distinctly low 11b is released over a specific temperature interval during thermal maturation the release of b from organic compounds could cause the observed regional 10b enrichment in waters deep in the gulf coast basin

depositional evolution of the early cretaceous sihetun lake and implications for regional climatic and volcanic history in western liaoning ne china numerous well known fossils of the jehol biota including early birds feathered theropods primitive mammals and putative early angiosperms have been discovered in lacustrine deposits lake sihetun of the lower cretaceous yixian formation of western liaoning province ne china based on extensive field investigations and four high resolution excavations we document in detail the spatio temporal changes of sedimentary facies facies associations and limnic community relicts reconstruct the depositional history and discuss the significance of these deposits regarding the regional climatic and volcanic history four phases are recognized in the history of lake sihetun they are 1 a phase of fluctuating but gradually rising water level indicated by subaerial and shallow water lacustrine deposits 2 a lake with beach nearshore facies along the marginal areas and suspension dominated lake floor facies in central areas 3 a lake phase with a lake floor dominated by hyperpycnal flows and 4 progradation of a fan delta two distinct depositional conditions are observed stratified and unstratified lake intervals the former occurred mainly during the second phase and produced clay poor and clay rich laminae whereas the latter developed during the other three phases and were dominated by deposition of subaqueous sedimentary density flows these alternations of depositional conditions in offshore lacustrine deposits suggest that the yixian formation may have formed under a transitional climate regime fluctuating gradually from warm and dry to warm and humid conditions resurgent volcanism is inferred to have become stronger during the late phase of lake development based on distribution of deposits characterizing the unstratified lake interval and volcanic rocks of the overlying upper lava unit a higher content of primary pyroclastic fragments and evidence of strong syndepositional disturbance

lacustrine responses to tephra deposition examples from mexico one of the major problems with palaeoclimate investigations in volcanic regions is that tephra inputs to lakes can cause changes in proxies analogous to those of climate forcing we review the range of impacts thought to be associated with tephra deposition distinguishing between direct effects on lake ecosystems and indirect changes to catchment nutrient cycles to achieve better understanding of these complex responses we have used high resolution diatom analysis from around 17 tephra layers in three mexican lakes a positive response to the tephra inputs has been identified for over half of these layers the most common response was for diatom concentrations to increase after tephra deposition also in plankton dominated systems fragilaria spp replaced stephanodiscus spp and aulacoseira spp an increase in the supply of silica to the lakes through their catchments is probably the cause of the diatom changes in the lakes studied direct effects of the tephra inputs to these lakes can be excluded as they would not generate changes of sufficient longevity the impact of these tephras which lasted for several decades was insufficient to perturb the underlying long term climate forcing of these lake systems c 2004 elsevier ltd all rights reserved