meteoric like fabrics forming in marine waters implications for the use of petrography to identify diagenetic environments petrographic fabrics have long been used to identify meteoric diagenesis of carbonate sediments however on the basis of oxygen isotopic data we document similar fabrics forming in marine pore fluids in the shallow subsurface of great bahama bank therefore petrographic fabrics alone are not reliable indicators of diagenetic environments even for shallow water sediments in our study skeletal grainstones show two distinct diagenetic assemblages either dissolution of aragonitic grains and minimal cementation high permeability intervals or abundant blocky spar cement and neomorphism of aragonitic skeletal grains low permeability intervals these marine burial fabrics are present as shallow as 110 m below sea level well above the aragonite compensation depth a feature that must be considered for models of diagenesis in ancient carbonate sediments marine burial diagenesis may be important in ancient carbonate sediments deposited in moderate water depths or in shallow water during rising sea level where meteoric diagenesis is suppressed

a middle pleistocene shallow marine interglacial sequence inner silver pit southern north sea pollen and dinoflagellate cyst stratigraphy and sea level history abstract the sand hole formation is a pleistocene seismic stratigraphic unit of restricted distribution in the inner silver pit southern north sea pollen and dinoflagellate cyst analyses of this formation penetrated by british geological survey borehole 81 52a and three juxtaposed vibrocores enables division of the formation into two sequences of contrasting palaeoclimatic affinity the lower sequence contains poorly preserved pollen assemblages of low concentration dominated by residual reworked elements and is coincident with cold shallow marine dinoflagellate cyst and foraminiferal assemblages this sequence overlies till and glacimarine sediments of the anglian elsterian swarte bank formation and is also interpreted as late anglian elsterian in age the upper sequence contains well preserved high concentration pollen spectra recording the latter half of an interglacial stage with hoxnian affinities the strong terrigenous signal preserved in this sequence suggests a subtidal depositional setting within a region of freshwater influence correlation of the vibrocores with the longer record preserved in borehole 81 52a indicates that interglacial sedimentation rates increase southwards towards a suggested terrigenous source in the wash area the first half of the interglacial cycle is missing indicating that a hiatus is present between the two sequences preserved within the sand hole formation but the termination of the interglacial is clearly preserved the spatial and temporal distribution of the sediments and bounding unconformities in the region are consistent in recording the transgressive ravinement surface highstand systems tract and subsequent regression within an interglacial eustatic cycle the character age and elevation of the preserved facies is typical of a glacio isostatically controlled emergence cycle and lends support to the notion that this temperate stage oxygen isotope stage 9 immediately followed extensive glaciation during the late anglian elsterian

contribution of calcareous nannoplankton to carbonate deposition a new approach applied to the lower jurassic of central italy abstract the upper pliensbachian lower toarcian somma section central italy which displays alternation of limestone marl lithotypes was investigated for its calcareous nannofossil and caco3 contents in order to evaluate the contribution of calcareous phytoplankton to the carbonate fraction of rocks a new method was applied combining a recently developed technique for nannofossil absolute quantification and measurements of nannofossil size our study suggests that nannofossils did not produce the bulk of carbonate mud in the studied section although they contribute to different extents to the pelagic carbonate fraction the contribution of both coccoliths and mainly the incertae sedis schizosphaerella to the carbonate fraction was important only in periods of reduced accumulation rates and or during clay deposition the highest estimated carbonate production by nannofossils in the studied section is around 83 of the total carbonate content but in a sample containing only 12 8 wt caco3 the observed limestone marl alternations therefore result from cyclical export of carbonate mud from the shallow latium abruzzi carbonate platform to the umbria marche basin and only subordinately from changes in nannoplankton productivity

geochemical and petrographical characterization of fine grained carbonate particles along proximal to distal transects the origin of carbonate ooze particles is often poorly understood this is due to their polygenic origin and potential post depositional alteration here the outcome of a physical separation study with regard to different component classes of micritic carbonates is shown the focus is on grain size and morphology mineralogy and isotope signatures two contrasting proximal to distal transects were investigated 1 the miocene leeward margin of great bahama bank odp leg 166 and 2 the transition between the maiella platform and the umbria marche basin in central italy near the cenomanian turonian boundary in both case settings carbonate particles of biogenic origin include at least three groups of organisms i planktonic foraminifera ii calcareous nannofossils and iii fragments of unspecified neritic skeletal material two further particle types lack diagnostic structures and based on particle size and mineralogy are here referred to as iv macroparticles 5 20 m mainly xenomorphic and v microparticles 12 m mainly automorphic to sub automorphic macro and microparticles represent 50 to 80 of the carbonate phase in slope and toe of slope domains and share characteristic carbon and oxygen isotope signatures macro and microparticles are considered shallow water precipitation products subsequently exported into the slope and toe of slope domains macroparticles are probably related to the fragmentation of neritic skeletal components while microparticles point to inorganic and or bioinduced precipitation in the water column in some cases macro and microparticles may have an early diagenetic origin the identification of the origin of fine grained particles allows for a quantitative assessment of exported in situ and diagenetic carbonate materials in periplatform environments the data shown here represent an important step towards a more complete characterization of carbonate ooze and micrite

geochemistry of proterozoic volcanic and granitic rocks from the gold hill wheeler peak area northern new mexico abstract early proterozoic supracrustal and plutonic rocks from the gold hill wheeler peak area in northern new mexico define three populations amphibolite diorite tonalite hornblendite cumulus amphibolite and felsic volcanics and porphyries also present are mid proterozoic granites amphibolites are similar in ti zr cr ni and ree contents to young calc alkaline and arc basalts and diorites and tonalites are similar in composition to young andesites and to high al2o3 tonalites respectively felsic volcanics resemble young felsic volcanics from mature arc systems in their immobile element contents geochemical model studies suggest that the amphibolites hornblendites diorites and tonalites are related by progressive fractional crystallization of a hydrous parent tholeiite magma produced from partial melting of undepleted lherzolite amphibolites represent parent tholeiites modified by olivine removal hornblendite is an early solid residue comprised chiefly of hornblende clinopyroxene and olivine diorite and cumulus amphibolite represent respectively residual solid clinopyroxene plagioclase hornblende and liquid after 50 crystallization tonalite represents a residual liquid after 80 crystallization felsic volcanic rocks are produced by partial melting of a tonalite or diorite source with granulite facies mineralogy in the lower crust granites have a similar origin to felsic volcanics although requiring an inhomogeneous source with the presence of residual hornblende or garnet the calc alkaline igneous rocks in the gold hill wheeler peak area suggest the presence of an arc system in northern new mexico during the early proterozoic the fact that these rocks interfinger with and are overlain by mature clastic sediments favors a model in which a continental arc system is uplited eroded and buried by cratonic sediments from the north

lithophile elements in huronian low ti continental tholeiites from canada and evolution of the precambrian mantle abstract huronian basalts from central ontario canada dated at about 2450 ma and associated with an early rifting episode are classified as siliceous low ti nb tholeiites they display strong enrichment in large ion lithophile lile and light rare earth lree elements compared to modern oceanic lavas the tectonic setting and geochemistry resemble mesozoic rift related low ti flood basalts including the ferrar group of antarctica and the parana and equivalent etendeka volcanics of south brazil and namibia respectively high lile lree ratios are also similar to subduction related island arc tholeiites and it is suggested that enrichment of the huronian lithospheric mantle source occurred through ancient subduction of crustal material probably during formation and consolidation of the archean continental crust melting models suggest that huronian subcontinental mantle source compositions derived from least contaminated aphyric mafic end members had already undergone a complex evolution including withdrawal of archean basalts and hydrous enrichment in incompatible components despite several subsequent melting episodes and a second probably magmatic enrichment event however many aspects of the huronian source signature were preserved and appeared in later basaltic products of this mantle mass keweenawan volcanics for example dated at about 1100 ma preserve low p zr ti and hree abundances

carbon and oxygen isotope analysis of hydrate associated oligocene authigenic carbonates using nanosims and irms abstract petrography and stable c and o isotope compositions of methane derived carbonates from the polish outer carpathians were analyzed two methods were employed in isotopic determinations conventional irms measurements on powdered bulk samples were performed with high precision errors 0 06 for 13c and 0 08 for 18o 1 whilst in situ measurements were made using nanosims with a high spatial resolution 5 m but with low precision errors 3 0 for 13c and 2 3 for 18o 2 the nanosims study proved very useful in identifying the sources of the fluids precipitating the carbonates they were formed predominantly by the anaerobic oxidation of biogenic methane aom the very fine spatial resolution of the analyses revealed highly variable 13c values even in individual crystals from extremely negative related to aom to positive related to methanogenesis me other methods including laser ablation and standard sims would fail to detect this variability and would not indicate the true sources of the fluids because they would show average values therefore any straightforward interpretation of the sources of authigenic carbonates based on these methods may be attempted only if the 13c values are extreme i e resulting either from aom or me unusual clast like druses from the carbonates were analyzed in detail with nanosims they are filled with strongly 13c depleted down to 54 and 18o enriched up to 6 fringe calcite which precipitated on the walls of the druses from heavy oxygen containing water released from the dissociation of gas hydrates the rocks can thus be termed clathrites the bulk rock isotope analyses of powdered samples of these druses show no evidence of hydrates and alone would not point to the true origin of these rocks precipitation of the carbonate cements started when hydrates were still present so they are a type of melt and seal fabric the recognition of such features may be crucial evidence of the former occurrence of hydrates in fossil methane derived carbonates the isotopic results have demonstrated that nanosims can be very useful in reconstructing the diagenetic evolution of pore water by analyzing the stable isotope compositions of even microcrystalline cements this provides a basis for the application of nanosims to tracing diagenetic pathways from various even complexly formed authigenic precipitates

temperature dependence of oxygen isotope fractionation in coccolith calcite a culture and core top calibration of the genus calcidiscus abstract reconstructions of seawater temperature based on measurement of oxygen isotopes in carbonates mostly derive from analyses of bulk sediment samples or manually picked foraminifera the temperature dependence of 18 o fractionation in biogenic calcite was first established in the 1950s and the objective of the present study is to re evaluate this temperature dependence in coccolith calcite with a view to developing a robust proxy for reconstructing vital effect free 18 o values coccoliths the micron sized calcite scales produced by haptophyte algae that inhabit surface mixed layer waters are a dominant component of pelagic sediments despite their small size recent methodological developments allow species specific separation and thus isotopic analysis of coccoliths from bulk sediments this is especially the case for calcidiscus spp coccoliths that are relatively easy to separate out from other sedimentary carbonate grains including other coccolith taxa three strains of coccolithophores belonging to the genus calcidiscus and characterised by distinct cell and coccolith diameters were grown in the laboratory under controlled temperature conditions over a range from 15 to 26 c the linear relationship that relates 18 o fractionation to the temperature of calcification is here calibrated by the equation t c 5 83 18 o calcidiscus 18 o medium 4 83 r 0 98 the slope of the regression is offset of 1 1 from that of equilibrium calcite this offset corresponds to the physiologically induced isotopic effect or vital effect the direction of fractionation towards light isotopic values is coherent with previous reports but the intensity of fractionation in our dilute batch cultures was significantly closer to equilibrium compared to previously reported offset values no significant isotopic difference was found between the three calcidiscus coccolithophores ruling out a control of the cell geometry on oxygen isotope fractionation within species of this genus this also indicates that our culture calibration may be applied to all calcidiscus coccoliths found in the sediment we compared the culture calibration to 18 o measured from near monogeneric calcidiscus fractions separated out from core top sediments we found concordant 18 o fractionation factors for the core top calibration with a good linear coefficient r 0 94 the near monogeneric calcidiscus assemblages seem however to record slightly heaviest 18 o values compared to the data of culture study this discrepancy may be due to a possible seasonality effect on the production of calcidiscus coccoliths the uncertainty of the calibration is of similar magnitude to those of other proxies used for sst reconstruction such as foraminiferal 18 o or the alkenone undersaturation index this confirms that coccoliths can be used as a complementary or alternative substrate to foraminiferal shells for isotopic analyses comparing 18 o of coccoliths to these other sst proxies or developing an interspecific comparison of coccolith geochemistry may give insights into the carbonate chemistry of seawater through key periods of the geological record

migration of the sulphate methane reaction zone in marine sediments of the sea of marmara can this mechanism be tectonically induced abstract sulphate concentration profiles with non constant gradients are reported from porewater samples in continental margin sediments recent results on a sediment core from the ganos fault in the sea of marmara revealed a porewater pattern with two gradients comparisons with the respective methane concentrations show the position of a sulphate methane reaction zone smrz within the sediment column methane emanations from the seafloor as observed at other locations along the deeper ganos fault indicate the smrz as a transient system a computer model based on an unsteady process estimates the starting time of the methane rise between 1096 and 907 years before present within this time period one remarkable earthquake occurred in the direct vicinity of our study area in the year 1063 a d we suggest this could have opened the pathways for the ascending methane front

three proterozoic orogenic episodes and an intervening exhumation event in the black canyon of the gunnison region colorado abstract u pb zircon titanite geochronology in situ monazite geochronology and 40ar 39ar thermochronology provide an unusually complete data set for reconstructing the tectonic history of proterozoic rocks exposed in the black canyon gunnison colorado these new geochronologic data record three protracted orogenic episodes and an exhumation event between orogenic pulses 1 yavapai orogeny 1741 1689 ma 2 exhumation marked by an angular unconformity beneath post yavapai pre mazatzal quartzites 3 mazatzal orogeny postquartzite deposition and 4 1434 1403 ma intracratonic tectonism supracrustal rocks of the black canyon succession were deposited or crystallized at or prior to documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wn

transient global cooling at the onset of early aptian oceanic anoxic event oae 1a the onset of the early aptian oceanic anoxic event oae 1a ca 120 ma coincided with a major perturbation of the carbon cycle which is reflected in the sedimentary carbon isotope record triggering mechanisms duration and climatic repercussions of this episode of accelerated organic matter burial remain poorly constrained here we present millennial scale bulk rock carbon and oxygen isotope data from a marly subtropical intrashelf basin la bedoule southeast france with unusually high sedimentation rates which track the onset of oae1a in unprecedented resolution our record reveals that the negative low amplitude delta 13 c excursion preceding oae1a lasted 100 k y implying that enhanced volcanic co 2 emission and or pulsed methane dissociation over a prolonged time span were instrumental in triggering oae1a the main positive carbon isotope shift at the onset of oae1a previously regarded as continuous occurred stepwise over an extended period of 300 k y transient climate cooling during the initial delta 13 c increase probably reflects ephemeral high latitude glaciation triggered by changes in radiative forcing and drawdown of atmospheric co 2

an iridium rich layer at the cretaceous tertiary boundary in the bidart section southern france we have measured an anomalous iridium concentration in the clays of the cretaceous tertiary boundary in the bidart section thus confirming the planetary character of this anomaly the iridium anomaly is synchronous with a very sharp minimum of the 13c 12c ratio and there is no evidence of any other iridium enrichment in marly concentrations away from the boundary the integrated iridium content is of the same order of magnitude as that calculated from deep sea cores in the pacific ocean in spite of a careful search we have found no evidence of any exotic material

recognition of diagenetic contribution to the formation of limestone marl alternations a case study from permian of south china abstract limestone marl alternations lmas have been extensively used in studies of sedimentology cyclostratigraphy and paleoclimatology however it has long been debated whether lmas are derived from primary sedimentation or from diagenetic alteration and the markers to distinguish one source from the other remain ambiguous in this study we investigate widespread permian lmas from various depositional settings in south china using traditional petrographic mineralogical and major element al2o3 vs tio2 methods as well as emerging trace and rare earth element ree analyses performed using solution based and in situ techniques the original depositional differences of the limestone and its coupled marl beds can be identified based on 1 the high volumes of siliciclastic minerals in the marls 2 unremarkable differential compaction 3 the recognizable signatures of the detrital admixture i e the al zr and th characteristics and 4 flattened shale normalized ree patterns diagenetic lmas are characterized by the compaction of fossils rare aragonite skeletons and abundant diagenetic clays sepiolite and talc preserved in the marl beds we also identified fully diagenetic lmas originating from primary carbonate sedimentation based on two almost completely consistent seawater like ree patterns with diagnostic positive la and negative ce anomalies la la sn 1 64 0 17 ce ce sn 0 73 0 05 superchondritic y ho 47 4 and depleted light rees relative to high rees pr yb sn 0 39 0 09 diagenesis also dominates the formation of lmas with limited terrigenous input in shallow water carbonate depositional environments it is likely that both the active metabolic processes in early diagenesis especially aerobic sulfide oxidation and the temporal palaeoceanographic conditions e g high levels of dissolved oxygen and sulfate play crucial roles in the genesis of the diagenetic lmas in the permian strata of south china

depositional sequences in deep shelf environments a response to sea level changes and shallow platform carbonate productivity oxfordian germany and spain abstract oxfordian deep shelf deposits of southern germany and southern spain are characterized by marl limestone alternations that are stacked into small scale medium scale and large scale depositional sequences the german sections contain autochthonous sponge reefs and associated fragments of microbialites whereas the sections studied in spain display tempestites composed of autochthonous deeper water and allochthonous shallow water particles the facies of the german limestones and marl layers has been analyzed in detail condensed intervals implied by glauconitization of particles abundant cephalopods intense bioturbation and generally more marls are also enriched in sponge reefs and associated particles and in nannofossils limestone rich intervals however contain fewer sponges and fewer nannofossils neither bioerosion of sponge reefs nor nannofossil blooms can thus explain the abundance of carbonate mud that forms the limestones consequently it is suggested that most of the carbonate mud is exported from shallow platform areas where carbonate productivity is high the clay fraction was derived from weathering of massifs in the hinterland the observed depositional sequences can be correlated between the studied sections in germany and spain situated in different paleotectonic and paleoclimatic domains and also between deeper water sections and platform sections this suggests that they formed through allocyclical processes comparision with published time scales implies that the small scale and medium scale sequences formed in tune with the 100 kyr and 400 kyr orbital eccentricity cycles respectively however the number of marl limestone alternations is not always consistent with the expected number of 20 kyr precessional cycles 5 per 100 kyr cycle the large scale sequences reflect long term third order sea level changes the observed marl limestone alternations are interpreted to have formed through cyclically varying export of carbonate mud from the platform towards the deep shelf the variations being controlled by climatically induced high frequency sea level fluctuations enhanced marl deposition on the deep shelf can be related to sea level fall causing exposure of the shallow platform and reducing the area of carbonate production or by rapid sea level rise maximum flooding leading to partial or total drowning of the platform and or to retrogradation of facies belts enhanced carbonate deposition occurs during transgression when large production areas are created on the platform or during late highstands when progradation is forced and carbonate mud export enhanced depending on the long term trend of sea level change on which the high frequency fluctuations are superimposed one high frequency 20 kyr sea level cycle can thus create one or two marl limestone alternations or only marly deposits consequently one 100 kyr eccentricity cycle may be formed of a variable number of marl limestone alternations commonly 2 to 8 the studied deeper water depositional sequences thus are strongly linked to the history of the adjacent shallow carbonate platforms

fluid flow methane fluxes carbonate precipitation and biogeochemical turnover in gas hydrate bearing sediments at hydrate ridge cascadia margin numerical modeling and mass balances a numerical model was applied to investigate and to quantify biogeochemical processes and methane turnover in gas hydrate bearing surface sediments from a cold vent site situated at hydrate ridge an accretionary structure located in the cascadia margin subduction zone steady state simulations were carried out to obtain a comprehensive overview on the activity in these sediments which are covered with bacterial mats and are affected by strong fluid flow from below the model results underline the dominance of advective fluid flow that forces a large inflow of methane from below 869 mumol cm 2 a 1 inducing high oxidation rates in the surface layers anaerobic methane oxidation is the major process proceeding at a depth integrated rate of 870 mumol cm 2 a 1 a significant fraction 14 of bicarbonate produced by anaerobic methane oxidation is removed from the fluids by precipitation of authigenic aragonite and calcite the total rate of carbonate precipitation 120 mumol cm 2 a 1 allows for the build up of a massive carbonate layer with a thickness of i m over a period of 20 000 years aragonite is the major carbonate mineral formed by anaerobic methane oxidation if the flow velocity of methane charge fluids is high enough greater than or equal to10 cm a 1 to maintain super saturation with respect to this highly soluble carbonate phase it precipitates much faster within the studied surface sediments than previously observed in abiotic laboratory experiments suggesting microbial catalysis the investigated station is characterized by high carbon and oxygen turnover rates approximate to1000 mumol cm 2 a 1 that are well beyond the rates observed at other continental slope sites not affected by fluid venting this underlines the strong impact of fluid venting on the benthic system even though the flow velocity of 10 cm a 1 derived by the model is relative low compared to fluid flow rates found at other cold vent sites non steady state simulations using measured fluid flow velocities as forcing demonstrate a rapid respond of the sediments within a few days to changes in advective flow moreover they reveal that efficient methane oxidation in these sediments prevents methane outflow into the bottom water over a wide range of fluid flow velocities 80 cm a 1 only at flow rates exceeding approximately 100 cm a 1 does dissolved methane break through the sediment surface to induce large fluxes of up to 5000 mumol ch4 cm 2 a 1 into the overlying bottom water

caco3 precipitation kinetics in waters from the great bahama bank implications for the relationship between bank hydrochemistry and whitings the source of whitings on the great bahama bank and their relationship to major changes in the chemistry of bank waters have been among the longest and most hotly debated topics in carbonate geochemistry in this paper we demonstrate that the reaction kinetics of calcite with bank waters for a given saturation state are similar to but somewhat slower 2 to 3 times than with gulf stream water the interpretation of the reaction kinetics of suspended bank sediment with bank water requires that the precipitating phase be about twice as soluble as aragonite good agreement at equivalent saturation states was found between experimental precipitation rates and those calculated for the rate of change of bank water chemistry in the region of whitings these results indicate that the dominant mode of carbonate removal is via precipitation on resuspended sediments rather than the rapid pseudo homogeneous precipitation of calcium carbonate in the water column resulting in the formation of a whiting estimates indicate that single aragonite needles may be resuspended many times over a period of decades during which they experience repeated overgrowth a major portion 98 of suspended calcium carbonate is outside the visually dramatic whitings thus as visually spectacular as they are whitings do not represent a short term locally massive precipitation of carbonate on the great bahama bank nor are they even likely to be the dominant sites of carbonate removal in this region although future refinements are needed that include seafloor processes we have at this point arrived at a mechanistic kinetic model that provides a reasonably quantitative explanation for the hydrochemistry of the carbonate system on the northern great bahama bank

stable isotopic and carbonate cyclicity in lower cretaceous deep sea sediments dominance of diagenetic effects abstract oxygen and carbon isotopic variability of the dominant m carbonate fraction within bedded organic carbon rich lower cretaceous sediment intervals from various dsdp sites are closely correlated with preservational changes in the carbonates isotopic fluctuations are absent where carbonate contents vary little and where the carbonate fraction is dominated by biogenic phytoplankton remains within each of the studied intervals oxygen and carbon isotopic ratios become increasingly more negative in samples with carbonate contents higher than about 60 in which the proportion of diagenetic microcarbonate increases rapidly carbon isotopic ratios show a trend towards positive values in samples with carbonate contents of less than 40 and strong signs of dissolution the taxonomic composition of nannofossil assemblages varies little within single intervals despite significant differential diagenesis among individual beds this points towards ecological stability of oceanic surface waters during the deposition of alternating beds bedding is however closely related to changing bioturbation intensity indicating repeated fluctuations of the deep water renewal rates and oxygen supply various microbial decomposition processes of organic matter leading to bed specific differential carbonate diagenesis resulted in an amplification of primary bedding features and are considered responsible for most of the observed fluctuations in the stable isotopic ratios and carbonate contents

palaeoclimatic cyclicity in central mediterranean pliocene sediments the mineralogical signal abstract a high resolution x ray mineralogical study of four pliocene sediment sections from the central mediterranean punta piccola sicily monte singa and vrica calabria odp hole 964 a ionian sea shows cyclic opposite abundance variations of two groups of clay minerals the first is dominated by palygorskite and kaolinite whose abundance maxima can be interpreted as resulting from periods of aridity on the african continent allowing their transport as wind blown dust the second is dominated by smectite whose abundance maxima can be interpreted as indicative of periods of abundant rainfall on the mediterranean borderlands these variations in abundance show a 22 ky cyclicity implying a control by the precession of the earth s orbit sapropels and sapropelic layers always occur in the smectite rich beds and therefore appear to have been deposited during the wet periods

basinal and shelf sedimentation in relation to the archaean proterozoic boundary abstract in most countries the archaean proterozoic boundary has been designated as an unconformity between the oldest shelf type quartz arenites and a basement consisting of greenstone belts intruded by granitic rocks or high grade gneisses the quartz arenites have been characterized as shallow water shelf deposits and sediments in the greenstone belts as resedimented basinal wackes of volcanogenic origin radiometric ages for rocks of the canadian shield led to initial designation of the archaean proterozoic boundary as thus defined at about 2 5 ga subsequent age determinations have clearly established that a number of quartz arenites originally assigned to the proterozoic are 2 7 2 94 ga old e g witwatersrand and pongola supergroups of south africa to accommodate these old quartz arenites some have suggested that the arch aean proterozoic boundary should be regarded as diachronous embracing a span of time about equivalent to that of the phanerozoic however quartz arenites also are abundant in 3 2 3 5 ga old greenstone belts and high grade metamorphic terranes of south africa western australia and the u s a likewise large regions of basinal volcanogenic sediments occur in post 2 5 ga terranes this greatly expands the diachroneity of geological units that were initially regarded as either uniquely archaean or proterozoic we therefore recommend that chronometric and chronostratigraphic sense be retained for the archaean proterozoic boundary divested of the requirement for all quartz arenites to be proterozoic according to this analysis of the precambrian record shelf sediments have increased in abundance relative to basinal sediments through time but at different rates in different areas this change is attributable to progressive cratonization of shield areas a process that was strongly diachronous on a worldwide scale

an early proterozoic volcanic arc succession in southeastern wyoming the green mountain formation of early proterozoic age in the sierra madre range of southeastern wyoming comprises a bimodal mafic and felsic volcanic assemblage the rocks which are chiefly breccias agglomerates flows and volcaniclastic sediments represent both subaerial and submarine eruptions and in part were redeposited in fluvial and nearshore marine environments volcanic rocks are clearly calc alkaline in character and share a large number of geochemical features in common with continental margin arcs or evolved oceanic arc systems the low mg numbers and ni contents of the basalts require 30 40 olivine fractional crystallization and the high contents of the most incompatible elements high la sm n ratios and low zr nb ratios require an undepleted or enriched mantle source geochemical data are consistent with an origin for the felsic volcanics and associated encampment river granodiorite by shallow fractional crystallization of calc alkaline basalt in a tectonic setting similar to modern ar

an authigenic calcite layer in the sediments of the sea of marmara a geochemical marker horizon with paleoceanographic significance abstract three sediment cores taken from the central sea of marmara were sedimentologically and geochemically investigated all the cores show a continuous sedimentation record according to isotopic age dating measurements two of the cores cover an age range of more than 20 000 years since the sea of marmara was an isolated lacustrine system during the last glacial maximum about 20 ka bp the two cores should include the transition from lacustrine to marine environmental conditions the third core has a very high sedimentation rate 70 cm ka and therefore does not reach the transitional section in two cores however a very calcite rich layer unit c was observed carbon isotope investigations show that this calcite is not of diagenetic or biogenic origin but may represent a direct inorganic precipitate out of a deepwater layer in order to prove this hypothesis we used a geochemical model based on thermodynamic calculations this model shows that the calcite precipitation can only be explained by mixing of anoxic deep lacustrine water with oxic marine mediterranean water the necessary aqueous conditions are also obtained the model presented thus provides an important contribution to the evolution of the sea of marmara during the transition from lacustrine to marine conditions the thickness of the unit c layer indicates that this environmental change lasted approximately 1 5 ka

early proterozoic oceanic crust in the northern colorado front range implications for crustal growth and initiation of basement faults 1 early proterozoic oceanic crust exposed in a shear zone in the northern colorado front range provides insight into the tectonic evolution of the colorado province tectonically intermingled mylonitic amphibolite locally pillowed metagabbro clinopyroxenite metachert and melange are preserved within the buckhorn creek shear zone west of fort collins whole rock geochemical data support formation of the igneous protoliths in an oceanic environment major and trace element data overlap samples from modern oceanic ridges and are distinct from modern arc trends metagabbros and clinopyroxenites preserve a subhorizontal lineation defined by igneous pyroxenes core and mantle microstructures indicate initial deformation at temperatures 900 c secondary amphiboles define both subhorizontal and subvertical lineations kinematic analysis indicates that the rocks experienced e w strike slip deformation prior to and during amphibolite facies metamorphism followed by n s shortening 40ar 39ar data from secondary hornblende yield age steps as old as 1 9 ga based on the chemical data and kinematic evidence for strike slip deformation we propose that the buckhorn creek rocks originated at an oceanic ridge transform intersection the high t pyroxene fabrics formed during ocean floor deformation prior to 1 7 ga regional deformation of the colorado province the buckhorn creek fragment likely represents original oceanic basement for 1 8 1 7 ga arc magmatism and sedimentation that ultimately formed the colorado province continental crust reactivation of original oceanic fracture zones may be responsible for the network of regularly spaced northeast trending shear zones in colorado once formed subvertical transform faults are likely to survive the transition from oceanic to continental crust and persist as long lived crustal discontinuities

changes in sea surface conditions in the equatorial pacific during the middle miocene pliocene as inferred from coccolith geochemistry abstract this study presents a new alkenone derived sea surface temperature sst record and 18onoelaerhabdaceae data of the 2 5 m carbonate fractions from the iodp site u1338 located in the eastern equatorial pacific eep over the miocene pliocene our data and those available from other sites of the same area show the establishment of a cold tongue during the early pliocene 4 4 3 6 ma sst and 18onoelaerhabdaceae time series indicate periods of significant salinity variations comparison with the 18obenthic curve from sediment cores of the equatorial pacific ocean allows us to distinguish between global changes and local surface salinity variations in the eep ice sheet growth and evaporation precipitation are then discussed as possible drivers of such changes along with the role of central american and the indonesian seaway restriction our data suggest a shallowing of the thermocline in the eep between 6 8 and 6 ma and its shoaling between 4 8 and 4 0 ma suggesting the appearance of the cold tongue steph et al 2010 the pliocene climate transition would therefore not be primarily driven by northern hemisphere glaciation

a proterozoic volcano plutonic terrane gunnison and salida areas colorado early proterozoic supracrustal rocks near gunnison and salida colorado include tholeiitic basalt dacite to rhyolite and intercalated sedimentary rocks these were intruded by essentially synchronous gabbroic sheets folded and intruded by major plutons ranging from quartz diorite to granite preservation of primary volcanic and sedimentary features is good to excellent sedimentary rocks in the gunnison area are mostly turbidite wackes whereas near salida they are mostly near source volcaniclastic rocks u pb age measurements on zircons indicate that the earliest bimodal volcanic rocks erupted between 1770 and 1760 m y ago and were intruded by granitic plutons between 1760 and 1750 m y ago renewed bimodal volcanism occurred between 1740 and 1730 m y ago and was followed by a period of deformation producing two fold sets before 1725 m y ago when the first of a series of younger plutons was emplaced the compositions and features of the sedimentary rocks and their association with volcanics are con

multiproxy approach for determination of nature and origin of carbonate micro particles so called micarb in pelagic sediments abstract carbonate fractions of pelagic and hemipelagic sediments may contain significant proportions of microcrystalline 6 m calcitic particles with no specific structure so called micarb depending on the authors this term is related to different descriptions and yet there is no clear consensus on their origin although micarb can dominate the sediment compositions in the pelagic realm their isolation from the other carbonate particles is not easy due to their small size the use of a granulometric separation method of the main carbonate particles allows the characterization and quantification of the compositions of the carbonate phases of the sediments and gives access to the geochemical signatures oxygen and carbon stable isotopes and mineralogical characteristics of each type of carbonate particles in the sediments this approach was applied to three sections the mid pliocene punta piccola section south sicily the upper campanian bidart section sw france and the aptian sediments from la marcouline quarry se france the quantifications showed quite important proportions up to 70 of the carbonated phases of micarb in the sections studied excepted in the mid pliocene punta piccola samples however depending on the sedimentary context it appears that those fine calcitic particles may result from fine biogenic fragmentation aptian la marcouline quarry authigenic precipitation in the water column mid pliocene punta piccola section or they may have an allochthonous origin upper campanian bidart section based on these interpretations it is then possible to estimate precisely the composition of the carbonate fractions in each studied section this work shows that except for the bidart sediments which are dominated by fine exported carbonate mud the other sections mainly consist in biogenic carbonates

early proterozoic bimodal volcanic rocks in central colorado u s a part i petrography stratigraphy and depositional history abstract early proterozoic metamorphic rocks of the salida area include two exceptionally well preserved sections of undeformed volcanogenic rocks a northern section nearly 4000 m thick and a southern section 1700 m thick the stratigraphic and lithologic character of the sections are the same indicating that they were formed contemporaneously in the same subaqueous environment the suite is strongly bimodal consisting of felsic volcaniclastic rocks 50 mafic volcanic and volcaniclastic rocks 22 and gabbro diabase sheets 23 the remaining rocks are mostly pelitic metasediments the felsic rocks which dominate the upper portions of both sections comprise four distinct facies crystal lithic tuff breccia crystal lithic lapilli breccia tuff bedded tuff and massive tuff lithic fragments are typically foreign dominated by pelitic material crystal clasts are feldspar mainly plagioclase and quartz sequences of these facies representing cycles of eruptive activity are uniform in thickness and extend along strike for the entire width of precambrian exposure up to 4 km but individual facies units especially the coarser ones are limited in their lateral extent individual sequences average about 100 m thick but continuous exposures of felsic rocks locally exceed 600 m primary sedimentary structures and vertical and lateral variations indicate deposition typically following remobilization as debris flows and turbidites in a shallow marine environment of low relief mafic volcanic and volcaniclastic rocks form accumulations up to 600 m thick and extend uniformly for several kilometers along strike although individual units are more restricted laterally the principal rock types are basalt flows basaltic lithic breccia pillow lava pillow breccia bedded basaltic lapillistone and laminated basaltic tuff all deposited in a submarine setting accumulations typically including several types are interbedded with felsic rocks but mechanical mixing of mafic and felsic volcaniclastics is rare suggesting separate eruptive centers for the mafic and felsic volcanics the northern section may be more proximal than the southern section as indicated by the greater abundance of flows the thicker sheets of gabbro and the greater abundance of mafic rocks relative to felsic rocks gabbro diabase sheets up to 350 m thick and dominantly concordant intrude throughout both sections they are characterized by lack of internal differentiation simple mineralogy consisting of plagioclase and hornblende pseudomorphous after pyroxene and sub ophitic texture the minor pelitic material indicates that the area was accessible to continental sediment sources although their location is not known the salida bimodal volcanic suite is part of a more extensive accumulation of similar 1700 1800 ma supracrustal rocks that extends west to the gunnison area and perhaps south into northern new mexico

whiting related sediment export along the middle miocene carbonate ramp of great bahama bank modern aragonite needles are present all along the modern leeward margin of great bahama bank odp leg 166 while middle miocene sediments contain needles only in more distal areas sites 1006 and 1007 in contrast to the rimmed flat topped platform topography during the plio pleistocene the miocene great bahama bank morphology is a carbonate ramp profile this might imply a different location and precipitation type for miocene aragonite needles in this study aragonite needles in miocene sediments were isolated using a granulometric separation method furthermore the isolation of the various carbonate components enables the identification of primary versus diagenetic components the miocene aragonite needles are concentrated in the finest granulometric sediment fractions 12 m the fraction specific geochemical analyses 13c 18o and sr elemental abundance represent useful tools to assess the possible sources of the aragonite mud the geochemical variation of the fractions rich in pristine aragonite needles and the characteristics of the needle morphology point to whiting phenomena as the main sediment source and algal fragmentation as a minor component both components indicate shallow water environments as the main sediment source area ramp top related fine grained particles now present at distal sites were likely exported as suspended material similar to present day transport mechanisms the scarcity of needles at proximal sites is probably linked to hydrodynamic processes but dissolution and recrystallization processes cannot be excluded the granulometric separation approach applied here enables a better characterization of the finest carbonate particles representing an important step towards the discrimination between primary and diagenetic fine grained components

1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle 1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle

new chronostratigraphy of the cretaceous paleogene boundary interval at bidart france abstract the cretaceous paleogene boundary section at bidart french basque coast was prevented for a long time to be considered as a reference section for the k t boundary due to a suspected hiatus comprising most of chron c29r in our new magneto and biostratigraphic study of this section we discovered that the underlying primary magnetic polarity is not revealed in the vector directions but in a small but consistent intensity excursion during laboratory demagnetization trends magnetite and maghemite are the main carriers of remanence with contribution of hematite to the magnetic mineralogy of the red brown lowermost danian and maastrichtian marly limestones magnetite is the main magnetic mineral in the white to pink danian limestones the nrm of marls and marly limestones is characterized by 1 a present day field overprint 2 a medium temperature 300 400 c reversed component interpreted as a relic of the characteristic remanent magnetization chrm carried by magnetite grains which are not completely oxidized into maghemite and 3 a high temperature up to 590 c normal component due to late diagenesis hematite growth the nrm of danian limestones is characterized by 1 a present day field overprint and 2 a chrm component of normal or reversed polarity the chrm directions allow a new correlation of the magnetic polarity sequence with the geomagnetic polarity time scale gpts chron c29r corresponds to the maastrichtian unit and extends up to 3 5 m above the k t boundary and chron c29n occurs at the top of the section there is no evidence for a hiatus at or below the k t boundary and the section does not extend up to chron c28r as previously suggested these new chronostratigraphic assignments are in good agreement with the previous biostratigraphic data and from the new biostratigraphic analyses based on calcareous nannofossils presented in this paper our study demonstrates that the section of bidart can be considered as a key section to study the k t boundary event s

coccolith δ18o and alkenone records in middle pliocene orbitally controlled deposits high frequency temperature and salinity variations of sea surface water 1 the calcareous nannoplankton flora is strongly controlled by water masses and is sensitive to ambient nutrient levels in consequence fossil remains of calcareous nannoplankton are useful tools for paleoceanography although they are the major producers of carbonates in the pelagic realm their use as a seawater 18o recorder is not easy due to their small size and the difficulty in isolating them from the bulk carbonate using a granulometric separation technique minoletti et al 2001 it is now possible to concentrate coccoliths in monospecific fractions we applied this method on three precession cycles insolation cycles i 288 to i 282 from the mid pliocene punta piccola section south sicily which led to an almost pure pseudoemiliana lacunosa fraction it was then possible to measure the isotopic signatures 18o and 13c and alkenone unsaturation ratio on the same recorder p lacunosa this permits a precise reconstruction of environmental conditions temperature and salinity that prevailed in the upper photic zone during the cyclic deposition of these sediments our data provide a clear confirmation that sapropelic marl formation occurred during a wetter climate compared to the deposition of the marly limestones moreover these results point out that salinity variations are not as previously thought always the primary factor that controlled the deposition of these cyclic events this study is a step forward in the utilization of calcareous nannoplankton isotopic records in paleoenvironmental reconstructions

extension of the astronomically calibrated polarity time scale to the miocene pliocene boundary the early pleistocene to late pliocene astronomically calibrated time scale of shackleton et al 1 and hilgen 2 is extended to the miocene pliocene boundary this is done by correlating the detailed record of caco3 cycles in the trubi and the lower part of the overlying narbone formation rossello composite section sicily to the astronomical record using 1 inferred phase relations between these caco3 cycles and the corresponding orbital cycles of precession and eccentricity and 2 calibration points provided by the previously established astronomical calibration of sapropelitic layers which occur in the topmost part of the caco3 record 2 this correlation allows all small scale caco3 cycles in the rossello composite section to be coded after the correlative precession peak and to be dated astronomically with an accuracy of 1 ky in combination with the available high resolution magnetostratigraphy it further allows the construction of an astronomically calibrated geomagnetic polarity time scale for the major part of the gilbert and gauss chrons this new time scale gives ages for the reversal boundaries of the kaena 3 04 3 11 ma and mammoth 3 22 3 33 ma the gilbert gauss chron boundary 3 58 ma and the reversal boundaries of the cochiti 4 18 4 29 ma nunivak 4 48 4 62 ma sidufjall 4 80 4 89 ma and thvera 4 98 5 23 ma these ages are significantly older than the ages given by conventional time scales age discrepancies gradually increase with increasing age the age of 5 32 ma for the miocene pliocene boundary as defined by the base of the trubi on sicily is almost 0 5 m y 9 5 older than the presently accepted age of 4 86 ma 3 these considerable discrepancies with the conventional time scale can either be explained by an consistent error in the decay constants used in k ar radiometric dating or more likely by diffusional loss of radiogenic argon

middle miocene pliocene stable isotope stratigraphy and paleoceanography of the mediterranean abstract stable isotope analyses of the planktonic foraminifers globigerinoides obliquus and globigerinoides trilobus from a number of sections from sicily italy yielded a rather detailed middle miocene to upper pliocene record a comparison between this record and previously published stable isotope records from the pacific and atlantic oceans suggests that at least during the time span considered the mediterranean had a paleoclimatic and paleoceanographic history that bears a close resemblance to that of the world s oceans four different local mediterranean overprints on the general isotope signal are distinguished 1 the disruption of the tethys sea way and the subsequent installation of a strong salinity stratification 2 short term eustatic sea level changes during the miocene resulting in salinity stratification and sapropel deposition 3 periodic increase of run off and subsequent salinity stratification during the pliocene also resulting in sapropel deposition 4 pliocene trends in the carbon isotope pattern deviating from the oceanic one the reason for which remained obscure

origin of thick first cycle quartz arenite successions evidence from the 1 7 ga ortega group northern new mexico abstract the 1700 ma old ortega group in northern new mexico represents a 1000 m thick succession of metamorphosed quartz arenites and subordinate pelites published ages of detrital zircon grains from the ortega group indicate that the sediments were derived mainly from 1760 1700 ma old volcano plutonic basement rocks and not from the archean wyoming province to the north evidence for recycling within the ortega group also is lacking consequently the ortega group quartz arenites are considered to be first cycle sediments the compositional and textural maturity of these sandstones is attributed to intense chemical and mechanical weathering abnormally high al contents of pelites within the ortega group imply intense chemical weathering in the provenance similar to present day low relief tropical regions quartz arenites in the ortega group were deposited exclusively within a shallow marine shelf environment influenced by tide storm and wave processes prolonged sediment transport especially on the tide dominated inner shelf in conjunction with inferred gradual subsidence optimized mechanical destruction of labile grains and rounding of quartz grains although the maturity of sediments in the ortega group can be accounted for the mode of origin of 850 m of quartz arenites remains enigmatic thick quartz arenite sequences of global distribution in the phanerozoic coincide with first order eustatic sea level rises world wide occurrences of 1700 ma old thick quartz arenite shelf successions suggest that the ortega group may have accumulated in response to a similar major eustatic sea level rise

paleoproterozoic rocks of central colorado accreted arcs or extended older crust paleoproterozoic 1770 1735 ma bimodal volcanic rocks in central colorado have been considered to represent southward growth of laurentia by arc accretion although the bimodality of these rocks suggests an extensional continental setting rather than continental or oceanic arcs there has been little evidence for pre 1800 ma crust south of the wyoming craton other than the 1840 1 ma elves chasm pluton in the upper granite gorge of the grand canyon arizona we report shrimp u pb ages of inherited zircons from metarhyolites and plutons in central colorado that are latest archean earliest proterozoic 2520 2000 ma and trans hudson penokean 1878 1814 ma associated quartzites contain detrital zircons with mean ages of 1735 ma indicating only local derivation a meta arkose however contains detrital zircons of trans hudson penokean and archean ages we believe it likely that the 1900 1800 ma trans hudson penokean orogens including archean enclaves extended farther south and west than is currently thought and were the source of the bimodal volcanic rocks and associated plutons during the period 1770 1700 ma

refining the early history of the mojave yavapai boundary zone rifting versus arc accretion as mechanisms for paleoproterozoic crustal growth in southwestern laurentia abstractthree outstanding questions regarding paleoproterozoic crustal growth and assembly in southwestern laurentia are 1 the mechanisms of crustal growth back arc rifting and associated juvenile magmatism vs accretion of juvenile arcs 2 the nature and significance of pre yavapai orogeny 1 70 ga deformation and 3 how the isotopically mixed boundary zone between the mojave and yavapai paleoproterozoic crustal provinces formed supracrustal successions associated with the mojave yavapai boundary zone in northwestern arizona are dominated by 1730 ma bimodal metavolcanic sequences and compositionally siliciclastic metasedimentary rocks with minor ultramafic rocks chert and carbonate this lithologic assemblage requires juvenile mafic volcanism in a basin that also received abundant quartz rich i e continental detritus a pre d1 first deformational event back arc rift setting for northwestern arizona best explains the observed supracrustal assemblage plutonic rocks that intrude the s

multiproxy approach to understanding the origin of cretaceous pelagic limestone marl alternations dsdp site 391 blake bahama basin limestone marl alternations are usually directly interpreted to reflect cyclic palaeoenvironmental signals however uncertainty in such interpretations stems from the differential diagenesis that most limestone marl alternations have undergone differential diagenesis results in markedly different alterations between limestones and marls and in the loss of comparability of many measurable parameters for an unequivocal interpretation of the origin of rhythmic alternations diagenetically robust parameters or parameters that clearly indicate the degree of diagenetic bias are required the present study uses a multiproxy approach independent biotic sedimentary and geochemical parameters in order to unravel the palaeoenvironmental signal recorded in valanginian early cretaceous limestone marl alternations from the blake bahama basin dsdp site 391 using this approach terrestrial and marine influences can be distinguished changes in nutrient levels estimated and prediagenetic differences in the non carbonate fraction constrained surprisingly no systematic variations in any of these parameters were observed between limestone and marl layers implying that none of these was directly responsible for the formation of the rhythmic alternation hence none of the current models of sedimentary formation of limestone marl rhythmites is applicable here calcareous nannofossils are equally well preserved in limestone and marl layers ruling out their dissolution in marl layers as a source of the calcite cement in the limestone beds sr values of 700 900 p p m indicate that aragonite may have been present in the original pelagic sediment the assumption of fine grained sedimentary aragonite imported from nearby carbonate platforms as the source of the cement would explain a number of otherwise enigmatic features in these rhythmites including the source of the calcite cement observed in the limestones the equally good preservation of calcareous nannofossils in limestones and marls and the higher concentration of calcareous nannofossils in marl layers the study demonstrates that examination of diagenetically inert parameters or parameters in which diagenetic effects can be filtered can yield unexpected results clearly careful analysis of such parameters needs to be undertaken in order to make reliable palaeoenvironmental interpretations from rhythmite successions

petrological evolution of the crust and mantle in its simplest form crust mantle evolution can be thought of as a study of the processes by which material is lost from the mantle and forms new crust the overall goal is achievement of mass balance between the average composition of material known to have been added to the crust and the estimated average composition of the entire crust attainment of this goal for the earth has been elusive primarily because the materials available for study have generally been involved in several stages of evolution including fractionation within both the crust and mantle and recycling of material from crust to mantle thereby complicating the interpretation of available data although the overall composition of the continental crust generally has been modelled as andesitic taylor and mclennan 1981a the present upper continental crust is granodioritic and requires a more mafic lower crust it has been proposed taylor and mclennan 1981a that originally the archean crust had an andesitic composition throughout its volume and that partial melting in the lower crust developed more felsic melts that rose to enrich the upper crust leaving a more mafic residuum of lower crust this concept has been challenged by weaver and tarney 1981a who argue that the granulites of presumed lower crustal origin do not have the correct compositions for this model if one considers more recent additions to the crust from the mantle over the past several hundred million years it appears that these latest additions are basaltic with very little if any granitic additions in fact barker 1981 in his introduction to a special issue of jgr on granites and rhyolites states to date no one has demonstrated that such magmas may form in the mantle indeed much experimental and geochemical work indicates that such siliceous liquids cannot originate there therefore such magmas must be generated in the crust in island arc environments where the average composition of added material normally is thought to be andesitic a anderson 1982 has shown that the parent melt is basaltic not andesitic in addition kay 1980 has shown that the k content of the newly added material is less than the average k content of the previously existing crust in the oceanic crust the additions from the mantle are basaltic and according to o hara 1982 probably are derived from very mafic picritic melts as parents continental flood basalts which have also added material from mantle to crust also are clearly derived from basaltic parental melts basaltic volcanism study project 1981 thus the evidence for the nature of material subtracted from the mantle and added to the crust for the past several hundred million or perhaps a billion years points to very mafic melts this contrasts significantly with the overall andesitic composition of continental crust it implies furthermore that if the overall archean crustal composition was andesitic as estimated by taylor and mclennan 1981a then additions to the crust have become more mafic with time a concept in direct conflict with most models in which secular trends are from mafic to felsic with time veizer and jansen 1979 and mclennan et al 198o a question which is beginning to appear either explicitly or implicitly is whether crustal volcanic rocks or the crust as a whole really represents the parent melt that formed in the mantle a anderson 1982 o hara 1982 and nisbet and walker 1982

vegetation markers of palaeoclimate cyclical changes in the pliocene of punta piccola sicily italy abstract astronomically driven climate changes are recorded in mineralogical composition of marine sediments in order to view the implications of precession cycles on continental vegetation high resolution records of pollen and fresh water algae are presented in five precession cycles no 104 108 age 3 00 2 92 ma from the central mediterranean punta piccola section pliocene each cycle has been subdivided into four phases based on the pollen composition three phases represent the brown laminated part of the cycle formed during precession minima their pollen associations suggest relatively heavy precipitation and high temperatures the fourth phase represents the light carbonate rich part of the precession cycles and its pollen associations express arid conditions this phase also experiences maxima in percentages of palygorskite and higher pollen input from the south

early proterozoic bimodal volcanic rocks in central colorado u s a part ii geochemistry petrogenesis and tectonic setting abstract the early proterozoic supracrustal rocks of the salida area of central colorado consist of strongly bimodal sequences of volcanogenic rocks the mafic rocks basalts basaltic volcaniclastics and related gabbro sheets are distinctly tholeiitic display a strong iron enrichment trend and typically contain less than 50 sio 2 the felsites are rhyolites to dacites and contain more than 70 sio 2 major and trace element modeling show that the mafic rocks underwent two stages of crystal fractionation the first involving olivine and plagioclase the second involving plagioclase and clinopyroxene fractionation occurred within individual injections as they rose toward the surface rather than in a single magma chamber at depth field relations and major element data support the derivation of the felsic rocks from a magma generated by anatexis of sialic crust however the low sr and high heavy ree concentrations in these rocks are not compatible with a partial melting model and suggest that the felsic volcanic rocks could have been derived by extensive fractional crystallization of the mafic magma normalized trace element abundances and trace element ratios of the mafic rocks are most like continental flood basalts such as the columbia river basalts they also display some similarity to immature back arc basin tholeiites developed on continental crust such as those of the sarmiento complex the felsic rocks have strong chemical affinities to within plate rhyolites rather than calc alkaline rhyolites from orogenic areas the chemical data as well as the petrographic stratigraphic and regional field data all indicate that the early proterozoic supracrustal rocks of the salida area developed along a continental margin probably within an immature back arc basin underlain by sialic crust remnants of the arc system of similar age may lie to the south in northern new mexico and southwestern colorado

the influence of sediment recycling and basement composition on evolution of mudrock chemistry in the southwestern united states this paper reports systematic changes in mudrock composition through time on a single con tinental cmstal block the changes reflect both sediment recycling processes and changes through time in the composition of crystalline material being added to the sedimentary system and are related to tectonic evolution as the block matures from a series of accreted arc terranes to a stable craton the major and trace element distributions reflect different aspects of the provenance of the mudrocks in this study major elements record sediment recycling processes as well as changing proportions of sedi mentary and first cycle source rocks with the exceptions of kzo which tends to increase and sioz and a o3 which show no trend most major oxides tend to decline in relative abundance in younger mud rocks patterns shown by the index of compositional variability fe03 kro naao cao mgo mno tiojialro and by k20 a1203 indicate that the major oxide trends are due to decreasing proportions of nonclay silicate minerals and a concomitant increase in the proportion of clay minerals probably due to decreasing input of first cycle detritus coupled with recycling of sedimentary material excursions from progressive trends marked by increases in mgo k20 and cao reflect episodes of large scale input of nonclay first cycle minerals from crystalline source rocks due to large scale basement uplift the chemistry of low solubility trace elements in contrast is not sensitive to recycling effects and reflects the composition of first cycle input incompatible elements are progressively enriched relative to compatible elements in younger mudrocks and values for chondrite normalised rare earth elements also increase in addition the eu anomaly becomes systematically more negative in younger samples these trends cmnot be explained by diagenetic or weathering processes and therefore indicate that the proportion of fnc tionated granitic first cycle detritus being added to the sedimentary system becomes greater with time these results confirm the importance of tectonic setting in controlling mudrock chemistry and also demonstrate that there is a dynamic relationship between the tectonic evolution of a continental block and the composition of its sedimentary mantle

long chain alkenones in the early pliocene sicilian sediments trubi formation punta di maiata section implications for the alkenone paleothermometry abstract article history received 6 september 2010received in revised form 15 march 2011accepted 16 march 2011available online 23 march 2011keywords alkenoneearly pliocenereticulofenestridsmediterranean seasea surface temperature the u k 37 record and coccolithophorid remains in sediments from the early pliocene sicilian trubi formation punta di maiata have been estimated to assess the possible in uence of changes in the alkenone producingcoccolithophoridsontheu k 37 temperaturerelationship thesamplesweredominatedbysmallreticulofenestra r minutaandr minutula andthec 37 andc 38 methylandethylunsaturationratios u k 37 u k38me andu k38et weresimilar to those found in modern populations e huxleyi and g oceanica those results suggest that the earlypliocene small reticulofenetrids had a similar temperature sensitivity of u k 37 to thatfound in present day theseresults support the application of the u k 37 modern calibrations to reconstruct sst at least back in the pliocene 2011 published by elsevier b v

orbitally forced glacio eustatic origin of third order sequences and parasequences in the middle permian maokou formation south china abstract astronomical forcing on the development of depositional sequences has been of particular interest to researchers in recent years to investigate the forming mechanism of depositional sequences in the middle permian maokou formation outcrop sequence stratigraphic analysis and cyclostratigraphic analysis were conducted on one outcrop section and one drilling well in south china up to ninety two meter scale 6th order parasequences thirty one 5th order parasequences eight 4th order parasequences and two 3rd order sequences were recognized based on vertical facies changes and stratal stacking patterns astronomical cycles i e eccentricity obliquity and precession were detected by spectral analysis an 3 1 myr temporal framework and a high resolution sedimentation rate curve were constructed by cyclostratigraphic analysis the similar ratios of average thickness for the high frequency sequences and the spectral periodicities suggest an astronomical forcing origin for the parasequences the 4th 5th and 6th order high frequency parasequences correlate well with long eccentricity short eccentricity and obliquity cycles respectively the durations of local and global 3rd order sea level sequences match the s4 s3 obliquity modulation cycles our results suggest that tectonic and autocyclic processes did not exert a major influence on the development of depositional sequences and that in the middle permian maokou formation the development of 3rd order sequences and 4th 5th and 6th order parasequences was controlled by orbitally driven glacio eustatic sea level changes the coevolution trends in obliquity power sea level change and sedimentation rate hold important implications for stratigraphic correlation and sequence stratigraphic subdivision on a regional scale

geology and geochemistry of huronian rhyolites and low ti continental tholeiites from the thessalon region central ontario bimodal volcanism associated with early phases of huronian rifting in central ontario dated about 2450 ma produced low ti tholeiitic basalts and two varieties of crustally derived calc alkaline rhyolite early tholeiites are characteristically highly evolved have mg values from 30 to 50 and display pronounced enrichment in large ion lithophile elements lile and light rare earth element lree in comparison with modern oceanic basalts fractionated heavy rare earth element hree patterns and low ti zr p nb ba and k abundances ti zr ratios rise progressively in early basalts and associated basaltic andesite fractionates from about 35 in early flows to 55 in central units late basalts also carry enriched lile and lree but in contrast to early types have average mg values greater than 50 and lower rare earth element ree abundances with flat hree patterns they also display negative ba nb and p anomalies on chondrite normalized distribution diagrams but lack low k zr and ti contents

component specific petrographic and geochemical characterization of fine grained carbonates along carboniferous and jurassic platform to basin transects abstract fine grained carbonates are present throughout much of the geological record and are widely used as geochemical archives even though their origin and diagenetic pathways remain poorly understood here petrographical and geochemical properties of granulometrically separated component spectra of marine mudstones sampled along two proximal to distal transects carboniferous of spain and jurassic of morocco are documented these settings represent end members in terms of platform geometry steep flanked versus gentle ramp and the aragonite versus calcite sea mode the data from spain reveal a bimodal organization of microcrystalline carbonate isotope values from platform top and slope and toe of slope settings the data from morocco lack a clear spatial and bathymetrical pattern the significance of the complex site specific biological and physico chemical parameters is emphasized mudstones have been separated in granulometric fractions of 8 5 5 3 and

the archean proterozoic transition evidence from the geochemistry of metasedimentary rocks of guyana and montana abstract chemical and isotopic compositions of ancient sedimentary rocks have in some cases provided evidence of first order changes yin the composition and extent of continental crust from the archean to the proterozoic we reversed the typical sampling pattern comparing metapelites and carbonates from proterozoic greenstone belts with those from an archean continental margin sequence and found the reverse of the typical age composition relations metapelites and volcaniclastic metagreywackes from the early proterozoic greenstone belts of northern guyana resemble those of many archean greenstone belts in ree trace and major element compositions mean ratios la sm 5 0 la th 5 1 eu eu 0 80 th u 3 4 median k 2 o na 2 o 0 8 schists from an archean continental marginal sequence in montana have compositions like many post archean metapelites derived from continental sources mean ratios la sm 6 8 la th 2 7 eu eu 0 67 th u 5 2 median k 2 o na 2 o 1 7 other archean continental basin sediments also resemble their post archean counterparts implying that upper continental crusts of these eras may not have differed significantly in these respects however temporal change may have occurred in upper crustal contents of ni and cr many archean clastic metasedimentary rocks of both continental and greenstone belt provenance have contents of these compatible elements higher than in their post archean equivalents dolomitic limestone from a greenstone belt of northern guyana has a very low 87 sr 86 sr ratio of 0 7007 such low values may reflect the local influence of mantle derived volcanic rocks during carbonate deposition diagenesis or metamorphism and they may not be reliable indicators of the evolution of sr in seawater archean dolomitic marbles from the continental marginal sequence of montana have 87 sr 86 sr ratios of 0 7065 and higher significantly more radiogenic than many accepted values for carbonates of archean greenstone belts

stable c and o isotopic study of the campanian chalk from the mielnik section eastern poland signals from bulk rock belemnites benthic foraminifera nannofossils and microcrystalline cements abstract exceptionally well preserved chalk from the mielnik section in eastern poland has been analyzed for sr c and o isotope composition recorded in bulk rock samples belemnite rostra hand picked benthic foraminifera and separated fine fractions incl calcareous nannoplankton and calcite cements the stratigraphic 13 c and 18 o trends are consistent with the global seawater variations during the campanian and confirm strong climatic perturbation accompanying the global sea level fall during the uppermost campanian chalk samples contain disseminated early diagenetic microcrystalline calcite cement having 13 c and 18 o values only slightly different by 13 c and 18 o values between different constituents measured from the same levels are discernible they are related with thermal gradients in the water column and with the habitat and vital effects of biogenic components benthic foraminifera reflect strong vital effects in 13 c and 18 o with a tendency to lower values than the other sample types the minimum strength of the disequilibrium fractionation effect for 18 o during foraminiferal shell secretion is estimated up to 2 belemnites show offsets in 13 c values which are difficult to estimate but their 18 o values appear to have recorded composition of seawater the large difference in 18 o values between belemnites and nannoplankton or bulk material suggests that belemnites were nectobenthic organisms living close to the seafloor the 87 sr 86 sr values and biostratigraphic position of the belemnites correspond well to those in the lagerdorf mcarthur et al 1993 the reference section for the european upper cretaceous epicontinental marine deposits but are not in agreement with the campanian stratigraphy defined in the geological time scale 2012 gradstein et al 2012 where positions of some biostratigraphic divisions are not well calibrated against the absolute age

microspar development during early marine burial diagenesis a comparison of pliocene carbonates from the bahamas with silurian limestones from gotland sweden comparison of ultrastructures in pliocene periplatform carbonates from the bahamas with silurian limestones from gotland sweden reveals that despite the differences in primary sediment composition and age they reflect a similar mechanism of lithification in both sequences calcite microspar was formed as a primary cement at an early stage of marine burial diagenesis neither significant compression nor meteoric influence are necessary for the formation of calcite microspar a model is proposed for the process of microsparitic cementation of fine grained aragonite needle muds comprising four stages 1 unconsolidated aragonite dominated carbonate mud 2 precipitation of microspar that engulfs aragonite needles 3 dissolution of aragonite resulting in pitted surfaces of the microspar crystals and 4 slight recrystallization our results contradict the widespread opinion that microspar necessarily is a product of secondary recrystallization of a previously lithified micrite

exploring the potential of clumped isotope thermometry on coccolith rich sediments as a sea surface temperature proxy understanding past changes in sea surface temperatures ssts is crucial however existing proxies for reconstructing past ssts are hindered by unknown ancient seawater composition foraminiferal mg ca and 18o or reflect subsurface temperatures tex86 or have a limited applicable temperature range uk 37 we examine clumped isotope 47 thermometry to fossil coccolith rich material as an sst proxy as clumped isotopes are independent of original seawater composition and applicable to a wide temperature range and coccolithophores are widespread and dissolution resistant the 47 derived temperatures from 63 m fraction removes most non mixed layer components however the 47 derived temperatures display an unexpected slight decreasing trend with decreasing size fraction this unexpected trend could partly arise because larger coccoliths 5 12 m are removed during the size fraction separation process the c1 and 63 m c2 47 derived temperatures are comparable to concurrent uk 37 ssts the 20 10 and 2 5 m c2 47 derived temperatures are consistently cooler than expected the 47 uk 37 temperature offset is probably caused by abiotic diagenetic calcite present in the c2 2 5 m fraction 53 by area which potentially precipitated at bottom water temperatures of 6 c our results indicate that clumped isotopes on coccolith rich sediment fractions have potential as an sst proxy particularly in tropical regions providing that careful investigation of the appropriate size fraction for the region and timescale is undertaken this article is protected by copyright all rights reserved

orbitally induced limestone marlstone rhythms in the albian cenomanian cismon section venetian region northern italy sedimentology calcareous and siliceous plankton distribution elemental and isotope geochemistry abstract a multidisciplinary study of the upper albian cenomanian portion of the cismon section venetian region northern italy was undertaken in order to characterize the cyclic alternations of carbonate rich and carbonate poor layers and to investigate their possible origin and cyclic patterns limestone semicouplets are characterized by abundant radiolarians and micarbs micron sized calcitic fragments common planktonic foraminifera strong bioturbation good oxygenation as expressed by the mn and v v ni parameters high si al ratio low k al in the absence of pyrite and organic matter the marlstone semicouplets are on the contrary frequently laminated rich in pyrite and organic matter with common fish remains in the absence of radiolarians the mn and v v ni parameters indicate that the marlstone deposition occurred in dysoxic to anoxic conditions which allowed the preservation of organic matter the low si al ratio and high k al ratio the latter related to abundant feldspars indicate that marlstones received a substantial contribution from a terrigenous source the abundance of biogenic silica suggests that during limestone deposition surface waters were rather fertile due to an efficient recycling of nutrients from deeper waters the enhanced fertility was coupled with an efficient current system at the welloxygenated sea floor which prevented the organic matter from accumulating during marlstone deposition surface waters were less fertile than in the limestone semicouplets but still within the mesotrophic spectrum as indicated by the presence of fertility index species among calcareous nannofossils whereas the bottom waters were oxygen depleted to accommodate fertility at the surface with stagnation at the bottom the scenario implies an enhanced stratification caused by an increase in freshwater runoff monitored by 18o depleted values in the meantime the water discharge supplied the nutrients to the surface waters necessary for the calcareous nannofossils to proliferate the cyclic organization of limestone marlstone couplets as revealed by power spectral analysis seems to represent climatic cycles the alternating conditions are 1 efficient mixing highly fertile surface and near surface waters and a well oxygenated seafloor to 2 enhanced water runoff or decreased evaporation or both and oxygen depleted bottom waters we suggest that this deterministic sedimentary cyclicity could be explained by periodic orbital climatic cycles in tune with the milankovitch cycles within this scenario the latest cenomanian bonarelli level is an exceptional event marked by a large positive 13c spike it is characterized by high to very high fertility conditions in surface waters caused by increased upwelling that engendered high nutrient levels in near surface waters the enhanced production of organic matter was in excess for the oxygen available at the bottom floor thus allowing the organic matter to accumulate in the sediments

crustal evolution of southern laurentia during the paleoproterozoic insights from zircon hf isotopic studies of ca 1 75 ga rocks in central colorado lu hf depleted mantle model tdm ages obtained by analysis of zircons previously dated by u pb methods demonstrate that ca 1 75 ga bimodal paleoproterozoic rocks in the gunnison salida region of central colorado and by extension in much of the southwestern u s were formed by partial melting of preexisting crustal rocks the ages of which mostly ages greater than 2 0 ga indicate that range from 1 83 to 1 87 ga some calculated lu hf tdm even older crust was probably involved consistent with the limited presence of ca 2 5 ga xenocrystic zircons in some rocks these results suggest that rocks related to the trans hudson and penokean orogens are cryptically present much farther to the south than previously believed coupled with the bimodality of the volcanic suite in central colorado and indeed in much of the southwestern u s these results indicate that in contrast to current juvenile arc accretion models melting of older crust related to extensional tectonics played an important role in the genesis of many magmatic rocks between 1 6 and 1 8 ga

new models for paleoproterozoic orogenesis in the cheyenne belt region evidence from the geology and u pb geochronology of the big creek gneiss southeastern wyoming the disputed age of the deep crust of the colorado province is central to hypotheses for paleoproterozoic crustal growth in the region we studied the high grade big creek gneiss southeastern wyoming as a potential exposure of pre 1780 ma basement rocks new geologic mapping and u pb geochronological data indicate that the big creek gneiss exposes a deeper but coeval level of the green mountain arc relative to the predominantly supracrustal section to the west the big creek gneiss is composed of supracrustal rocks a ca 1780 ma green mountain arc correlative bimodal intrusive suite a ca 1763 ma extensional bimodal intrusive suite and widespread ca 1630 ma pegmatitic leucogranite the mafic member of the younger bimodal suite is documented here for the first time u pb zircon ages from migmatite leucosomes indicate penetrative deformation of the big creek gneiss at ca 1750 ma we find that the postarc intrusive suite is mantle involved implying a second period of crustal growth shortening postdates arc magmatism by 20 m y implying that termination of arc magmatism and accretion were separate events finally criteria previously used to constrain the polarity of subduction for the green mountain arc are not reliable we propose two competing models 1 southward dipping green mountain arc subduction present coordinates with slab breakoff related magmatism following arc accretion or 2 northward dipping subduction with extensional postarc magmatism in both models high temperature deformation coincides with accretion along the cheyenne belt and extensional magmatism is an important component of crustal growth we prefer the northward dipping subduction model because it can be better integrated with regional tectonic events and published isotopic compositions of the igneous rocks

shallow platform palaeoenvironmental conditions recorded in deep shelf sediments c and o stable isotopes in upper jurassic sections of southern germany oxfordian kimmeridgian a high resolution carbon and oxygen isotope analysis of late oxfordian early kimmeridgian deep shelf sediments of southern germany is combined with investigation of nannofossil assemblage composition and sedimentological interpretations in order to evaluate the impact of regional palaeoenvironmental conditions on isotopic composition of carbonates this study suggests that carbonate mud was essentially derived from the jura shallow platform environments and also that the isotopic signature of carbonates deposited in the swabian alb deep shelf indirectly expresses the palaeoenvironmental evolution of the platform short term fluctuations in d 13 c and d 18 o are probably controlled by changes in salinity fresh water input versus evaporation in platform environments long term fluctuations in carbon and oxygen isotope record throughout the late oxfordian early kimmeridgian result from the interplay of increasing temperature and decreasing humidity which both control the trophic level changes from mesotrophic to oligotrophic conditions in platform environments and in the deep shelf surface waters are inferred during the late oxfordian bimammatum subzone to planula zone the d 13 c curve displays a positive shift of about 1x which is comparable in intensity to global perturbations of the carbon cycle this evident isotopic shift has not been documented yet in other basinal settings it can be reasonably explained by local palaeoenvironmental changes on the jura platform salinity temperature and nutrient availability that controlled platform carbonate production and the geochemistry of overlying waters however increasing carbonate production on the jura platform and related positive d 13 c shifts recorded in the

lower maastrichtian cyclostratigraphy of the bidart section basque country sw france a remarkable record of precessional forcing abstract cyclostratigraphic analysis of the maastrichtian limestone marl alternations of bidart sw france allows the hypothesis of orbital control on lithological cycles to be evaluated magnetic susceptibility ms oxygen and carbon isotope measurements sampled at a high resolution are analyzed using various cyclostratigraphic tools a statistically significant orbital signal is detected with a remarkable record of the precession corresponding to the limestone marl couplets this well expressed orbital forcing allows the building of a relative cyclostratigraphic time scale for the ms and 13 c records based on the 100 kyr eccentricity cycle the total duration of the section is estimated at 1 44 0 22 myr correlation based on calcareous nannofossil biostratigraphy and comparison of the scaled bidart 13 c record to the astronomically calibrated 13 c signal of odp hole 762c shows that the studied section extends from 71 5 to 70 ma covering the upper part of chron c32n 1n and 2 3 of chron c31r oxygen isotope data suggest a 2 c cooling of sea surface temperatures during the studied interval when placed on the long term 18 o trend of the bidart section this interval is here recognized as the onset of the early maastrichtian cooling event with its excellent record of the precessional forcing the bidart section along with other sections of the basque country is a useful tool for the refinement of the maastrichtian timescale

subaqueous pyroclastic flows and ignimbrites an assessment an assessment of the literature on subaqueous pyroclastic flows and their deposits shows that the term pyroclastic flow is frequently used loosely to describe primary hot gas rich pyroclastic flows mass flows which resulted from the transformation of gassupported flows into water supported ones and secondary mass flows carrying redeposited pyroclastic debris based on subaerial pyroclastic flows the term pyroclastic flow should be restricted to demonstrably hot gas rich mass flows of pyroclastic debris using this definition very few examples of subaqueous pyroclastic deposits with evidence for hot emplacement and of having been wholly submerged have been described in the majority of these cases the evidence for a hot state of emplacement and for the subaqueous nature of the host depositional environment is inadequate the only unequivocal cases of hot pyroclastic flow deposits with adequate supporting evidence are the ordovician nearshore shallow marine ignimbrites of ireland and wales and miocene ignimbrites of southwest japan resulting from the passage of subaerially erupted pyroclastic flows into shallow water other possible examples are near vent dense clast deposits in the donzurobo formation of japan possible submarine intra caldera ponded ignimbrite successions in california and wales and near vent pumiceous deposits of ramsay island wales all other purported cases are either clearly the result of water supported mass flow transportation and deposition debris avalanches debris flows turbidity currents or lack adequate supporting evidence regarding the heat state or the palaeoenvironment only the shallow marine ignimbrites of ireland and wales show adequate evidence of welding but even these could have been nearly wholly exposed above sea level when welding occurred we conclude that when pyroclastic flows enter water they are generally disrupted explosively and or ingest water and transform into water supported mass flows and we suggest the various scenarios in which this occurs there is no evidence to suggest that welding in wholly subaqueous environments is common

cold seep deposits of beauvoisin oxfordian southeastern france and marmorito miocene northern italy microbially induced authigenic carbonates the relation of two well known ancient carbonate deposits to hydrocarbon seepage was confirmed by this study archaea are found to be associated with the formation of oxfordian seep carbonates from beauvoisin and with a miocene limestone from marmorito tube worm limestone carbonates formed due to a mediation by archaea exhibit extremely positive or extremely negative 13ccarbonate values respectively highly positive values 15 reflect the use of 13c enriched co2 produced by methanogenesis low 13c values of the marmorito carbonates 30 indicate the oxidation of seepage derived hydrocarbons likewise the 13c content of specific tail to tail linked isoprenoids biomarkers for archaea was found to be strikingly depleted in these samples as low as 115 the isotopic signatures corroborate that archaea were involved in the cycling of seepage derived organic carbon at the ancient localities another miocene limestone marmorito limestone shows a strong imprint of methanotrophic bacteria as indicated by 13c values of carbonate as low as 40 and biomarker evidence epifluorescence microscopy and field emission scanning electron microscopy revealed that bacterial biofilms were involved in carbonate aggregation in addition to lucinid bivalves previously reported from both localities we infer that sponges from beauvoisin and tube worms from marmorito depended on chemosynthesis as well low 13c values of nodules related to sponge taphonomy 27 indicate that sponges might have been linked to an enhanced hydrocarbon oxidation tube worm fossils from marmorito closely resemble chemosynthetic pogonophoran tube worms from recent cold seeps and are embedded in isotopically light carbonate 13c 30

authigenic carbonates derived from oxidized methane vented from the makran accretionary prism off pakistan we report the first discovery and sampling of a methane hydrogen sulfide rich cold seep from the makran accretionary prism off pakistan arabian sea a variety of cm to m scale pockmarks and gas seepage structures were identified from underwater tv photo sled profiles crossing the oxygen minimum zone omz related to highresolution seismic parasound records from the seeps isotopically light bacterially formed methane is emanating that is partly oxidized to hco3 in the bacterial sulfate reduction zone this results in the precipitation of irregular dark gray to black crusts of indurated authigenic carbonates mainly cryptocrystalline magnesian calcite and ca rich dolomite near the sediment seawater interface within the omz downcore the crusts grade into hemipelagic carbonate poor silty clays with transitional lithologies in between similar to the pockmark carbonates from the oregon margin and in the north sea the authigenic carbonates are extremely depleted in 13c 13ccarbonate 40 this suggests that they were derived from bacterial methane 13cmethane 77 8 that was oxidized under anaerobic conditions the authigenic carbonates are associated with white fluffy mats of chemoautotrophic h2s oxidizing bacteria small pockmarks appear to be concentrated at small scale escarpments suggesting focussed fault controlled pore fluid expulsion due to the tectonic dewatering and degassing of the accretionary prism whereas diffuse discharge of pore fluids is inferred from the widespread occurrence of tiny gas bubble tubes

biochemical and ultrastructural evidence for the origin of whitings a biologically induced calcium carbonate precipitation mechanism we propose that bahamian whitings floating patches of lime mud are in part the result of biological precipitation of calcium carbonate induced by picoplankton and cellular components the cells may act as nucleation sites for crystallization from seawater supersaturated with caco3 our model represents a new hypothesis concerning bahamian whiting formation and is based on several lines of evidence biochemical data suggest that the macromolecules from carbonate suspended in bahama banks whitings are distinct from those found in the lime mud sediment producer penicillus and from bottom sediment direct ultrastructural evidence indicates that mineralization occurs on the surfaces of picoplankton cells and degrading organic cellular components the organic constituents include but are not restricted to whole and fragmented algal cells and composite structures 20 30 m in diameter the observed calcium carbonate crystals are distinct from skeletal debris because of the magnitude of the whiting phenomenon calculations of carbonate sediment production budgets should include the potential contribution of calcium carbonate produced by epicefular precipitation knowledge of carbonate mud genesis is critical to interpretation of ancient occurrences of lime mud as well as to understanding ancient and global carbon cycles

the rossello composite a mediterranean and global reference section for the early to early late pliocene a high resolution magnetostratigraphy planktonic foraminiferal biostratigraphy and cyclostratigraphy is presented for the pliocene trubi marls in the punta di maiata section on sicily the integrated stratigraphy of the rossello composite section of hilgen 1 newslett stratigr 17 1987 is thereby completed this composite section provides an unprecedented high quality reference section for the early to early late pliocene containing a continuous sequence ranging from below the thvera subchron into the matuyama chron 4 86 2 45 ma the punta di maiata section extends from the sidufjall subchron of the gilbert into the gauss chron 4 50 3 30 ma linear interpolation between paleomagnetic datum planes in this section yields first order age estimates of 3 72 0 01 and 3 59 0 01 ma for the last common occurrence lco and the actual last occurrence lo of globorotalia margaritae in addition this age of 3 59 ma provides an accurate age for the zanclean piacenzian z p boundary provided the lo of g margaritae is maintained as a criterion to define this boundary in the mediterranean irrespective however of the criterion used the punta di maiata and punta piccola subsections of the rossello composite are at present the most suitable sections to be designated as stratotypes for the z p boundary the global significance of the rossello composite is further strongly enhanced by the establishment of an astronomically calibrated geomagnetic polarity time scale based on the correlation of the trubi sedimentary cycles with the astronomical record

geochemistry of the neoproterozoic tilemsi belt of iforas mali sahara a crustal section of an oceanic island arc abstract the pre pan african neoproterozoic tilemsi belt of the western iforas mali sahara comprises a predominantly bimodal mafic and felsic volcanic assemblage associated with numerous intrusions mainly of mafic composition the volcanics include island arc tholeiites and the calc alkali rocks rhyolites dacites and andesites the volcanic domain shows similarities to recent oceanic island arc systems as well as to archaean greenstone belts the tilemsi magmatic sequence represents an example of a section through the middle and upper levels of a neoproterozoic intra oceanic island arc crust that is composed nearly exclusively of igneous material the middle crustal section is mainly of mafic composition while the upper crustal level contains more silica rich plutonic volcanic and volcaniclastic assemblages the mafic intrusions include a large proportion of cumulates indicating that fractional crystallization played an important role during the evolution of the oceanic arc suites compositionally the arc changed through time from an immature to a relatively developed system the rocks became more silica and incompatible element rich with higher sr isotopic ratios this trend reflects the increasing growth and maturity of the arc

geochemistry of the dubois greenstone succession an early proterozoic bimodal volcanic association in west central colorado abstract the dubois greenstone succession an early proterozoic supracrustal succession in west central colorado is composed of fine grained felsic volcaniclastic sediments ash flow tuffs and tholeiitic flows sills and dikes the volcanic rocks comprise a bimodal suite of tholeiite and rhyolite the tholeiites are divided into two groups group i exhibiting nearly flat ree patterns 20 30 chondrites and group ii with light ree enriched patterns 40 70 chondrites these groups can be produced by two stage melting of an undepleted or enriched garnet lherzolite source in which garnet remains in the residue of only the first melting stage which produces group ii magmas the felsic magmas can be produced by partial melting of a high grade gneiss in the lower crust in which zircon is entrained in the magma the dubois greenstone succession appears to have formed by felsic and mafic subaqueous eruptions in which some ash flows upon entering water underwent phreatomagmatic explosions giving rise to detritus which formed the volcaniclastic sediments although tectonic setting cannot be assigned to the succession with a high degree of confidence an immature back arc basin developed on sialic crust in consistent with most data

changes in the pelagic fine fraction carbonate sedimentation during the cretaceous paleocene transition contribution of the separation technique to the study of bidart section abstract the bidart section southwestern france displays an almost complete late maastrichtian early danian transition as the cretaceous paleocene k p crisis mainly affects carbonate producers the aim of this study is to precisely evaluate the evolution of pelagic sediments composition across the boundary a new protocol for separation of fine carbonates into discrete size fractions was used 1 to precisely evaluate in weight of carbonate and not only number of particles per gram of rock the contribution of each carbonate component type foraminifera calcareous nannofossils and nonbiogenic carbonates calcite microparticles and calcareous macrocrystals and 2 to measure their isotopic signature and thus constrain the origin of these carbonate particles the maastrichtian pelagic carbonates are mostly biogenic 70 of the bulk carbonate but also contain nonbiogenic carbonates calcite microparticles 4 m due to their isotopic signature calcite microparticles cannot result from fragmentation and partial dissolution of biogenic remains and the most likely origin appears to be early diagenetic processes and or export from shallow water platforms the calcareous macrocrystals derive from an early diagenetic process the lowermost danian sediments are dominated by nonbiogenic carbonates between 50 and 80 of the bulk carbonate nevertheless two kinds of biogenic components are present thoracosphaera operculata and maastrichtian like calcareous nannofossils thoracosphaera operculata is an opportunistic calcareous dinoflagellate which frequently occurs at high abundances in basal danian sediments and is certainly in situ this is confirmed by it showing a danian isotopic signature the maastrichtian like calcareous nannofossils are nannofossils of taxa that indisputably occur in the maastrichtian but which do not occur consistently in later danian strata they show a maastrichtian type isotopic signature which strongly supports the inference that they are mainly reworked from the maastrichtian this study shows that nonbiogenic carbonates are important components of the pelagic carbonate even during periods of normal sedimentation in the crisis interval the biogenic carbonate production drops sharply while calcite microparticles and carbonate macrocrystals remain almost unaffected and represent the major part of the carbonate in the earliest danian sediments finally the sediments return to a composition similar to that of the maastrichtian in m pseudobulloides foraminifera zone

internal precipitation of microcrystalline carbonate a fundamental problem for sedimentologists abstract recent studies of submarine lithification have shown that microcrystalline carbonate or carbonate mud traditionally thought to represent ooze deposited in quiet water commonly forms as an internal precipitate below the sediment water interface this internally precipitated micrite alters the mud grain ratio of pre existing sedimentary deposits and invalidates classifications that relate the amount of micrite matrix in limestones to the degree of water energy associated with the environment of deposition

numerical modeling of carbonate crust formation at cold vent sites significance for fluid and methane budgets and chemosynthetic biological communities at many cold vent sites authigenic carbonates precipitate due to the release of carbonate alkalinity during the anaerobic oxidation of methane carbonate precipitation often induces the formation of massive crusts at the sediment surface or within surface sediments the range of physical and biogeochemical conditions allowing for the formation of carbonate crusts is largely unknown so that the significance of these widespread manifestations of fluid flow is unclear here we use numerical modeling to investigate the conditions that induce carbonate crust formation in the sediment and the effect of crust formation on sediment porosity and fluid flow rate starting with the conditions prevailing at a previously investigated reference site located on hydrate ridge off oregon several parameters are systematically varied in a number of numerical experiments these parameters include coefficients of bioturbation and bioirrigation sedimentation rate fluid flow velocity methane concentration in the ascending vent fluids and ph and saturation state at the sediment water interface the simulations show that carbonate crusts in the sediments only form if the fluids contain sufficient dissolved methane 50 mm and if bioturbation coefficients are low 50 cm ka 1 inhibit crust formation bioirrigation induces a downward displacement of the precipitation zone and accelerates the formation of a solid crust crusts only form over a rather narrow range of upward fluid flow velocities 20 60 cm a 1 which is somewhat enlarged up to 90 cm a 1 if the overlying bottom waters are supersaturated with respect to calcite at higher flow rates methane is rapidly exported into the water column so that methane oxidation and carbonate precipitation cannot proceed within the surface sediment the formation of a several centimeters thick carbonate crust in surface sediments is typically completed after a few hundred years 100 500 a crust formation reduces the supply of methane to surface sediments which imposes a strong resistance against diffusive and advective methane transport therefore rates of anaerobic methane oxidation and sulfide production are diminished and thus the density and metabolism of chemosynthetic biological communities is limited by crust formation due to the moderate flow rates and the slow diffusive transport only very little methane escapes into the bottom water overlying carbonate encrusted vent areas

methane derived carbonates and authigenic pyrite from the northwestern black sea abstract methane seeps in the northwestern black sea are accompanied by carbonate and pyrite precipitates sediments were sampled at locations on the romanian 120 m depth and ukrainian 180 200 m depth shelf and slope layered carbonate crusts are formed of i carbonate cemented siliciclastic sediment containing dreissenoid bivalves ii microcrystalline high mg calcite or aragonite and iii aragonitic cement the dreissena sediment is subrecent and was deposited during the freshwater phase of the black sea it has been affected by seepage of methane rich fluids which induced intergranular precipitation of authigenic carbonates the microcrystalline carbonates exhibit an intense autofluorescence high mg calcite contains 11 14 mol mgco3 the aragonitic cement 8300 9500 ppm sr forms either isopachous layers or botryoids microbial filaments about 10 20 m in diameter and up to 900 m in length are preserved within and on carbonate crusts the carbonates are depleted in 13c microcrystalline carbonate ranges from 27 to 41 pdb and botryoidal aragonite ranges from 26 to 38 pdb the 13c depletion indicates that the carbonates predominantly derive from the microbial oxidation of methane carbonate deposits do not project up into the oxic water column they are restricted to the anoxic water column and to anoxic sediments revealing the crucial role of anaerobic methane oxidation for carbonate precipitation 14c contents give apparent radiocarbon ages of 20 640 180 a bp for a sample of botryoidal aragonite and 19 110 180 a bp for a sample of microcrystalline carbonate compatible with minimum ages of carbon derived from a fossil hydrocarbon source the 18o values of methane derived carbonates show a narrow range from 1 2 to 0 2 pdb 87sr 86sr ratios of microcrystalline carbonate mean 0 70927 and aragonitic cement mean 0 70918 are indistinguishable from ambient seawater mean 0 70917 and thus indicate a shallow sr source higher 87sr 86sr ratios of the dreissena sediment mean 0 71005 are probably caused by sr derived from detrital mica carbonates are accompanied by blocks and crusts composed of pyrite the framboidal sulphide exhibits a palisade like fabric with framboids arranged to parallel pillars sulphur isotopic ratios 34s ranging from 16 8 to 19 7 cdt indicate that the sulphur derives not from the 34s depleted h2s of the water column or the uppermost sediment layers most likely pyrite formed in the lacustrine sediments after the first incursion of mediterranean seawater

mediterranean quaternary sapropels an immediate response of the african monsoon to variation of insolation abstract in the subsurface of the east mediterranean sea eleven discrete basin wide organic rich black sapropels were deposited during the last 465 000 years they are dated by oxygen isotopic chronostratigraphy the most recent sapropel has been shown to be the consequence of heavy nile river summer floods due to increased monsoonal summer precipitation over ethiopia and not of scandinavian ice sheet meltwater influx into the east mediterranean sea as was previously believed while most sapropels were deposited during interglacials two thick sapropels formed during glacial periods as shown by their position in the oxygen isotopic stratigraphy and by their pollen record in core rc9 181 the new evidence presented here is that the sapropel sequence in this core including the two glacial sapropels correlates on a one to one basis with the highest values of an index of insolation on top of the atmosphere the concept of this index derives from an analysis of the african monsoon process the index variation is computed for the last 465 000 years from the caloric northern summer insolation in the tropical latitudes controlled by variations of the earth s orbital parameters it is conjectured that this index might be linked through the climate system to the monsoonal precipitation which in africa produces the nile river summer flood

sheeting and dyking emplacement of the gunnison annular complex sw colorado abstract emplacement of the proterozoic gunnison annular complex colorado involved brittle failure and both subhorizontal sheeting and steeply dipping dyking the annular complex consists of a central diorite body 1730 ma surrounded by a ring of metamorphosed supracrustal rocks in turn ringed by tonalite and granodiorite 1721 ma the older central diorite was emplaced as sills parallel to bedding prior to regional deformation of the gunnison volcanic arc 1730 1710 m y ago this central body was deformed during regional shortening into an upright bowl with inward dipping walls the surrounding country rocks were folded and locally transposed against the central body forming an arcuate foliation conforming roughly to the shape of the body this foliation and bedding acted as mechanical planes of weakness which localized the syn deformational emplacement of the outer ring intrusions as multiple magma sheets parallel to steeply dipping bedding and foliation portions of the outer rings were also injected along concentric inward dipping shear fractures generated under high magma pressure a process similar to cone sheet emplacement in volcanic ring dyke complexes multiple mechanisms were therefore involved in the emplacement of this sheeted annular plutonic complex which otherwise superficially resembles a subvolcanic ring dyke complex

whitings a sedimentologic dilemma whitings drifting clouds of water milky because of suspended carbonate have been claimed to originate from either the action of bottom feeding fish or direct precipitation of calcium carbonate five cruises during different seasons were made to the great bahama bank to collect data pertinent to the controversy measurements of particulate concentrations average 10 mg liter with a maximum of 20 mg liter of carbonate sediment suspended in whiting water compared with an average of 1 5 mg liter for clear water outside the whitings the particles are dominantly acicular aragonite but mg calcite composes as much as 20 percent of some whitings sedimentation rates measured with fixed and drifting sediment traps were as great as 34 g m 2 hr sediment suspended in whitings aggregated into silt and sand size fioccules and settled to the bottom of settling tanks within six hours even on a rocking ship sediment in artificial whitings created by stirring sediment from the bottom with a shrimp trawl settled to the bottom in about the same time natural whitings on the other hand were never observed to dissipate because sedimentation from whitings occurs at rates sufficient to cause dissipation of the whitings within six hours we conclude that the natural whitings are continually replenished by direct precipitation the search for fish in whitings utilized sidescan sonar and fathometer imaging shrimp trawls rotenone remote video and direct scuba observation these methods and 25 years of casual observations leading to this study indicate that fish are not involved in the formation of most bahamian whitings several whitings were found over rocky or sandy bottoms where there was no mud available for fish to suspend the distance of these whitings from areas of muddy bottom precluded their having been made elsewhere by fish stable carbon and oxygen isotopic analyses and delta 14 c activity are interpreted to indicate that the suspended sediment in whitings contains some precipitated calcium carbonate and is not merely bottom sediment stirred into suspension estimates indicate that the amount of new carbonate produced in whitings on the great bahama bank is substantially higher than that arising from algal production consequently the amount of sediment transported to deep water may be much greater than previously thought

geochemistry and petrogenesis of early proterozoic amphibolites west central colorado u s a abstract early proterozoic supracrustal rocks exposed in west central colorado include amphibolites intimately associated with felsic volcanics and associated volcaniclastic sediments three successions are recognized the dubois succession 1770 1760 ma is comprised chiefly of amphibolites and felsic volcanics the amphibolites exhibit low concentrations of light ree and hfse and are broadly similar in composition to tholeiites from immature island arcs and in some respects to nmorb the cochetopa succession 1745 1730 ma includes large amounts of felsic volcanics and volcaniclastic sediments together with amphibolites that exhibit variable but slight incompatible element enrichment similar to basalts from evolved island arc systems and associated back arc basins the black canyon succession is coeval with the cochetopa succession and is comprised chiefly of volcaniclastic sediments with minor amphibolite these amphibolites exhibit significant incompatible element enrichment similar to basalts from continental margin arc systems geochemical modeling indicates that the amphibolites evolved by closed system fractional crystallization of depleted for the dubois succession to variably enriched parental basalts for the cochetopa and black canyon successions chiefly at shallow depths incompatible element distributions in the amphibolites indicate that mantle sources changed with time from relatively depleted 1770 1760 ma to variable but slightly enriched 1745 1730 ma a provisional model calls upon the dubois succession forming in an island arc system followed by collision with the archean wyoming province at 1750 ma the cochetopa succession was deposited in an intra arc basin developed within a new continental margin arc and the black canyon succession formed in a back arc basin associated with this arc