did a katian large igneous province trigger the late ordovician glaciation a hypothesis tested with a carbon cycle model abstract the ordovician is known as a period with high temperatures and high sea levels and a cooling event at the end of the period leading to the hirnantian glaciation and the first of the big five mass extinctions of marine life the cause of this cooling event remains unclear several authors correlate it with a drop in atmospheric p co 2 to a threshold permitting the ice accumulation on the gondwana supercontinent however what are the reasons of the atmospheric p co 2 decrease here we follow the hypothesis initiated by barnes 2004 that an ordovician superplume event occurred during the middle to late ordovician such an event would not only have a large impact on the ordovician biodiversification barnes 2004 but it would also be responsible for the climatic upheaval during the late ordovician by the emplacement of a low latitudinal continental basaltic province that had an impact on the atmospheric p co 2 there is no direct evidence of a superplume event or of basaltic traps and the present study is therefore a hypothetical modelling approach where we demonstrate with a numerical box model including carbon alkalinity and phosphorus cycles coupled with a 1d climate model energy balance model ebm francois and walker 1992 that such a scenario allows to explain both the mid ashgill katian global warming event known as the boda event fortey and cocks 2005 and the subsequent late ordovician hirnantian glaciation because silicate weathering is enhanced upon warm and wet climate we try to constrain the size and the latitudinal emplacement of the basaltic province leading to a sufficient consumption of atmospheric p co 2 to the threshold proposed by herrmann et al 2004 a b to initiate a glaciation on gondwana

structural and chemical variations in phlogopite from lamproitic rocks of the central mediterranean region abstract micas from mafic ultrapotassic rocks with lamproitic affinity from several localities of the central mediterranean region were studied through single crystal x ray diffraction sc xrd electron microprobe analysis empa and secondary ion mass spectrometry sims mossbauer spectroscopy mos when feasible was also applied to minimise the number of unknown variables and uncertainties analysed lamproitic samples cover the most important central mediterranean type localities from plan d albard western alps to sisco corsica montecatini val di cecina and orciatico tuscany italy and torre alfina northern latium italy the studied crystals show distinctive chemical and structural features all of them belong to the phlogopite annite join and crystallise in the 1 m polytype except for micas from torre alfina where both 1 m and 2 m 1 polytypes were found studied micas have variable but generally high f and ti contents with mg mg fe ranging from 0 5 to 0 9 2 m 1 crystals from torre alfina radically differ in chemical composition showing high contents of ti and fe as well as of al in both tetrahedra and octahedra leading to distinctive structural distortions especially in tetrahedral sites sims data indicate that studied micas are generally dehydrogenated with oh contents ranging from 0 2 apfu atoms per formula unit for orciatico and torre alfina to 1 4 for plan d albard crystals this feature is also testified by the length of the c parameter which decreases with the loss of hydrogen and or the increase of the f oh substitution chemical and structural data suggest that the entry of high charge octahedral cations is mainly balanced by an oxy mechanism and to a lesser extent by a m 3 4 tschermak substitution our data confirm that ti preferentially partitions into the m2 site and that different ti and f contents as well as different k al values are both dependant upon f h2o and the composition of magma rather than controlled by p and t crystallisation conditions the obtained data help to discriminate among lamproite like rocks formed within a complex geodynamic framework but still related to a destructive tectonic margin and evidence different trends for micas from the youngest torre alfina northern latium lamproites referred to the apennine orogeny and those of the older lamproites from orciatico montecatini val di cecina tuscany western alps and corsica the latter referred to the alpine orogeny phlogopite crystals from the older lamproites fall within the compositional and structural field of worldwide phlogopites from both within plate and subduction related settings phlogopite from the plio pleistocene lamproite like occurrence in tuscany and northern latium despite crystals with low mg of the torre alfina rock plot well within the general field of the other crystals in less evolved samples follows a different evolution trend similar to that of shoshonites from tuscany and northern latium on this basis we argue that the observed differences are inherited by slight differences in the magma compositions that are related to different genetic and evolution pathways

structure and composition of organic reefs and carbonate mud mounds concepts and categories defined here as essentially in place calcareous deposits created by sessile organisms organic reefs are diverse and complex structures with a long geological history their classification has been the subject of fierce debate often characterized by reliance on subjective features such as wave resistance and qualitative attempts to discriminate between first and second class reefs in contrast emphasis is here placed on the objective characteristic of the type of sedimentary support which largely determines the sedimentary composition of the deposit constructional and depositional processes result in three principal sedimentary components matrix m essentially in place skeletons s and cavity cement c whose proportions can be represented on msc triangular plots separately or together these components also provide the structural support for the reef on these compositional and structural bases three main categories of organic reef are recognized 1 matrix supported reefs agglutinated microbial reefs cluster reefs segment reefs 2 skeleton supported reefs frame reefs 3 cement supported reefs cement reefs agglutinated microbial reefs possess laminated clotted or aphanitic fabrics created by microbial trapping of particulate sediment in place skeletons and large primary cavities are rare early cementation may provide added support topographic relief is limited by the need for currents to provide sediment to accreting surfaces cluster reefs skeletal reefs in which essentially in place skeletons are adjacent but not in contact resulting in matrix support characterized by relatively high matrix skeleton ratios and low volumes of extra skeletal early cement sediment trapping is an important corollary of skeletal growth and cluster reef organisms are tolerant of loose sediment absence of framework limits the topographic relief that cluster reefs can attain relative to spatial extent and may permit bedding to develop within the reef close cluster reefs have skeletons up to 1 unit distance apart spaced cluster reefs have skeletons more than 1 and up to 2 unit distances apart with increasing separation of skeletons they grade to level bottom communities segment reefs matrix supported reefs in which skeletons are adjacent and may be in contact but are mostly disarticulated and mainly parauthochtonous matrix abundance is high and early cement relatively low moderate relief can develop in response to intense on reef sediment production frame reefs skeletal reefs in which essentially in place skeletons including calcified microbes are in contact characterized by relatively high skeleton matrix ratio skeletal support enables them to raise themselves above the substrate independently of cementation and particulate sedimentation simultaneously by creating partly open shelter cavities skeletal support may facilitate early cementation both relief and early lithification promote marginal talus formation skeletal shape and orientation distinguish conical stick like dendritic domical and laminar frames each of these may be open or filled open frame reefs

an oscillating trend in phanerozoic non skeletal carbonate mineralogy textural and compositional data on ancient ooids and cements suggest the existence of an oscillating phanerozoic temporal trend in non skeletal carbonate mineralogy the trend shows good correlation with eustatic climatic oscillations proposed by others the most likely mechanism appears to be plate tectonically influenced oscillations in pco2 the vapour pressure of co2 rather than change in oceanic mg ca ratio

response of the inter tropical convergence zone to southern hemisphere cooling during upper ordovician glaciation stable isotope ratios of whole rock carbonates and faunas from three low latitude upper ordovician sections demonstrate a coherent pattern of shifting subtropical and tropical water masses and associated climate belts we suggest that tropical water beneath the inter tropical convergence zone 18o 10 to 15 was isotopically light reflecting high sea surface temperatures sst and reduced sea surface salinity sss subtropical water in both the northern and southern hemispheres is characterised by 18o values from 4 to 6 our interpretation is consistent with the published results from physical chemical modeling of ordovician seawater composition and climate models deriving absolute temperature and salinity values from ordovician 18ocarb data is problematic as they could reflect sst sss ice volume and potentially a variable diagenetic overprint and currently no independent constraints exist for any of these variables the itcz was positioned at 10 n of the equator during the early katian consistent with the likely position of the region of peak sea surface temperature sst during the late katian boda event the itcz moved across the equator to 10 s by analogy with the pliocene this resulted in a symmetrical distribution of sea surface currents about the equator this led to a net increase in heat flow towards the poles and lower latitudinal temperature gradients potentially explaining the migration of warm water faunas to higher southern latitudes during the boda event during the hirnantian the itcz shifted north of the equator resulting in a reduced poleward heat transport itcz position has been proposed as a likely forcing mechanism of neogene glaciation and the evidence from the ordovician suggests a discrete palaeo itcz whose position was controlled by solar insolation and latitudinal temperature gradients linked to ice volume changes and hadley cell dynamics we hypothesize that the position of the itcz was sensitive to the growing hirnantian ice sheet during the late ordovician we propose that latitudinal movement of the itcz during the late ordovician resulted in a major atmosphere ocean circulation re organization higher pco2 estimates for the ordovician caused more extreme changes in this circulation that were the likely primary cause of biotic mass extinction if the monsoon is considered an off equator itcz then we provide direct evidence of an ordovician monsoonal climate

late ordovician global warming the boda event there is substantial evidence for mid ashgillian global warming before the latest ordovician hirnantian glaciation as shown by the movement of previously lower latitude benthic faunas such as trilobites and brachiopods to progressively higher latitudes and by an increase in endemic faunas at low latitudes this is shown by faunal dynamics on several paleocontinents and endorsed by published evidence of warmer water sediments from gondwana and elsewhere we term this global warming the boda event

reconstruction of currents in the mid ordovician early silurian central baltic basin using geochemical and mineralogical indicators in the central part of the mid ordovician early silurian baltic basin two different transport pathways of terrigenous material can be recognized kaolinite indicates the south to north sediment influx and cr the western and northern sources a pathway from the ukrainian shield in the south was active in the floian darriwilian ordovician and in the llandovery silurian the influx from the western source was active in the sandbyan and influx from the northern side occurred in the katyan the sediment transport was carried out by water flows which turned into rapid currents from time to time these currents coming from the ocean surface waters were oxygenated facilitating red facies formation in the central baltic basin currents on the shelf were linked to the adjacent oceanic currents which changed in course with the drift of baltica from temperate to subtropical latitudes

shock barometry of the siljan impact structure sweden the siljan impact structure in sweden is the largest confirmed impact structure in western europe despite this the structure has been poorly studied in the past and detailed studies of shock metamorphic features in the target lithologies are missing here we present the results of a detailed systematic search for shock metamorphic features in quartz grains from 73 sampled localities at siljan at 21 localities from an area approximately 20 km in diameter located centrally in the structure the orientations of 2851 planar deformation feature sets in 1179 quartz grains were measured observations of shatter cones outside of the zone with shocked quartz extend the total shocked area to approximately 30 km in diameter the most strongly shocked samples recording pressures of up to 20 gpa occur at the very central part of the structure and locally in these samples higher pressures causing melting conditions in the affected rocks were reached pressures recorded in the studied samples decrease outwards from the center of the structure forming roughly circular envelopes around the proposed shock center based on the distribution pattern of shocked quartz at siljan the original transient cavity can be estimated at approximately 3238 km in diameter after correcting for erosion we conclude that the original rim to rim diameter of the siljan crater was somewhere in the size range 5090 km less

submarine hydrothermal weathering global eustasy and carbonate polymorphism in phanerozoic marine oolites abstract the age and fabric of 347 phanerozoic marine oolites have been compiled in order to evaluate secular variations in the original mineralogy of these physicochemically precipitated sedimentary carbonates variation in the abundance of oolite correlates with positions of global sea level ooids were common during transgressions and regressions but were rare during lowstands and highstands the rarity of oolite during continental emergence early cambrian permo triassic late tertiary quaternary probably relates to a diminished areal extent of shallow carbonate environments causes for a similar paucity during continental submergence silurian devonian late cretaceous are more problematic higher atmospheric co2 and lower hydrospheric co3 concentration during highstands may have inhibited a biotic carbonate precipitation ooids coeval with highstands display a preponderance of fabrics evincing a primary calcite mineralogy ooids formed during lowstands are more diverse but record dominance of aragonite variations in cortical mineralogy relate more closely to flooded continental areas than to sea level elevations supporting the suggestion of mackenzie and pigott 1981 that variations in hydrospheric carbonate concentrations were more important in determining phanerozoic ooid mineralogy than were magnesium calcium concentrations these data on oolite abundance and mineralogy demonstrate that abiotic phanerozoic marine carbonates have been responsive to secular changes in atmospheric hydrospheric chemistry and global eustasy and suggest that these changes are directly related to past rates of submarine weathering and sea floor hydrothermal activity

internal structure of segment reefs halimeda algal mounds in the mediterranean miocene halimeda reefs in the upper miocene strata 6 ma of the sorbas basin southeastern spain shed light on the internal structure of more extensive but less accessible holocene counterparts and challenge conventional reef concepts coarse discoid segments released by halimeda during life or immediately after death dominate the lenslike mounds their chaotic loose appearance disguises the reefal nature of the mounds segments accumulating at or very close to sites of growth were quickly stabilized by microbial and cement crusts that bound them into distinctive rigid gravel fabrics this early lithification generated relief but inhibited off mound export of sediment although large blocks detached locally and moved downslope encrustation of parautochthonous halimeda gravel created a unique reef type segment reefs

subaerial speleothems and deep karst in central sweden linked to hirnantian glaciations the limestones of the upper katian boda mud mounds ordovician of the siljan district in central sweden are deeply fractured the fissures were partly synsedimentary and are often lined with stromatolite like crusts these crusts thus far are the only known subaerial ordovician speleothems they reach depths of up to 30 m below the former mound top macroscopically the crusts form decimetre sized cone shaped domal aggregates stalactites and stalagmites microfabric and morphology identify them as microbially mediated speleothems in a dark environment combined sr and c isotope values indicate a formation of the speleothems from meteoric waters without influence of a significant soil horizon for the first time the age of the speleothems can be precisely constrained by 13 c whole rock and brachiopod shell isotope data to the mid hirnantian repeated and or prolonged subaerial exposure of the boda mud mounds during the hirnantian is evident from karst surfaces and early cements in the mound capping carbonates the speleothems and the karst surfaces record an estimated sea level fall in the range of 80 130 m within the time window of the hirnantian isotopic carbon excursion this massive regression coincides with maximum ice sheet extent inferred from sections in west gondwana supplementary material 87 sr 86 sr isotope ratio of selected brachiopod shells and results of energy dispersive x ray spectroscopy are available from http www geolsoc org uk sup18809

ferric ferrous ratios h 2 o contents and d h ratios of phlogopite and biotite from lavas of different tectonic regimes complete chemical analyses including ferric and ferrous iron h2o contents and dd values for 16 phlogopite and biotite and 2 hornblende separates are presented samples were obtained from volcanic rocks from four localities 1 phlogopite phenocrysts from minette lavas from the western mexico continental arc 2 biotite and hornblende phenocrysts from andesite lavas from mono basin california 3 phlogopite and biotite from clinopyroxenite nodules entrained in potas sic lavas from the east african rift uganda and 4 phlogopite phenocrysts from a wyomingite lava in the leucite hills wyoming the fe 2 o 3 contents in the mi cas range from 0 8 to 10 5 wt corresponding to 0 09 to 1 15 fe 31 per formula unit pfu water contents vary from 1 6 to 3 0 wt corresponding to 1 58 to 3 04 oh pfu significantly less than would be expected for a site fully occupied by hydroxyl cation and anion based normalization procedures provide accurate mineral for mulae with respect to most cations and anions but are unable to generate accurate estimates of fe 31 yfe t a nd overestimate oh at the expense of o on the hydroxyl site these inaccuracies are present despite acceptable adjust ed totals and stoichiometric calculated site occupancies the phlogopite and biotite phenocrysts in arc related lavas from western mexico and eastern california have the highest fe 31 yfe t ratios 56 87 reflecting high magmatic oxygen fugacities dnno 51 2t o 15 in contrast to those from uganda 25 40 and the leucite hills 23 there is no correlation between the oh content and the fe 31 yfe t ratio in the micas values of k mgyfe 21 d 2s errors were calculated for three phlogo pite olivine pairs 0 12 0 12 0 26 0 14 0 09 0 12 two biotite hornblende pairs 0 73 0 08 and 1 22 0 10 and a single phlogopite augite pair 1 15 0 12 values of k fyoh d for two biotite and hornblende pairs could not be determined without significant error because of the extremely low f contents 0 2 wt of the four phases the dd values obtained in this study encompass a large range 2137 to 243 the phlogo pite and biotite separates from uganda have dd values of 270 to 249 which overlap those believed to represent primary mantle there is a larger range in dd values 2137 to 243 for phlogopite phenocrysts from west ern mexico minette lavas although their range in d 18 o values 5 2 6 2 is consistent with normal mantle it is unlikely therefore that the variable dd values reflect heterogeneity in the mantle source region of the minette magmas nor can the extremely low dd values reflect degassing of h2 or h2o since almost 100 loss of dis solved water in the magma is required an unrealistic scenario given the stability of the hydrous phenocrysts the very low dd values of the mascota minette phlogo pites require that the hydrogen be introduced from an external source e g meteoric water whatever the pro cess responsible for the observed hydrogen isotope com position it had no effect on the d 18 o value fo2 ah2o or bulk composition of the host magmas

the ti saturation surface for low to medium pressure metapelitic biotites implications for geothermometry and ti substitution mechanisms the ti content of biotite can serve as a geothermometer for graphitic peraluminous metapelites that contain ilmenite or rutile and have equilibrated at roughly 4 6 kbar the relationship between ti content temperature and mg mg fe value was calibrated empirically using an extensive natural biotite data set 529 samples from western maine and south central massachusetts in combination with the petrogenetic grid of spear et al 1999 the calculated ti saturation surface is curved such that for a given mg mg fe value ti concentration increases as a function of temperature in a nonlinear fashion and for a given temperature ti concentrations decrease with an increase in mg mg fe the fit to the ti saturation surface can be reformulated as the geothermometric expression t ln ti a c x mg 3 b 0 333 in which t is temperature in degrees celsius ti is the number of atoms per formula unit apfu normalized on the basis of 22 o atoms x mg is mg mg fe a 2 3594 b 4 6482 10 9 and c 1 7283 the calibration range for this expression is x mg 0 275 1 000 ti 0 04 0 60 apfu and t 480 800 c precision of the ti in biotite geothermometer is estimated to be 24 c at the lower temperature range and improves to 12 c at higher temperatures application of the ti in biotite geothermometer to ilmenite or rutile bearing graphitic peraluminous metapelites equilibrated at 3 6 kbar is generally consistent with independent temperature determinations but with some deviations that represent local reequilibration consequently the ti systematics in biotite can also serve as the basis of a very sensitive indicator of chemical equilibrium or lack thereof application of the geothermometer to metapelites not containing the requisite mineral assemblages can lead to minor to significant errors in estimated temperatures biotite ti substitution mechanisms are controlled by several factors based on the biotite calibration data set magnesian biotites x mg 0 65 incorporate ti in accordance with the exchange vector tial2r 1si 2 where r is the sum of the divalent cations mg fe mn this substitution mechanism is primarily a response to misfit of the octahedral and tetrahedral layers in magnesian biotites intermediate biotites x mg 0 65 particularly at higher temperatures exhibit enhanced ti concentrations most consistent with the ti deprotonation tio2r 1 oh 2 exchange vector dominance of ti deprotonation substitution is largely a function of reduction of h2o activity at higher metamorphic grades supplementary biotite data from metaluminous amphibolites and mafic granulites metamorphosed isothermally with variable h2o activities reveal that low al biotite incorporates significantly higher concentrations of ti relative to peraluminous biotite as a result of a combination of the exchange vectors tio2r 1 oh 2 and rsial 2 substituting in roughly an 8 1 ratio

towards a better understanding of the fibrolite problem the effect of reaction overstepping and surface energy anisotropy trachytes from the euganean hills district italy contain metapelitic xenoliths that have been pyrometamorphosed during incorporation in the melt in xenoliths containing sillimanite crystallized during a previous regional ht lp metamorphism fibrolite systematically nucleates at the grain boundaries of sillimanite prisms and within plagioclase crystals ternary feldspar thermometry shows that plagioclase in contact with sillimanite plots along the 750 c solvus that reflects near equilibrium conditions of regional metamorphism plagioclase containing fibrolite plots closer towards the 950 c solvus reflecting the tendency of plagioclase to re equilibrate at high temperature during pyrometamorphism by a fibrolite forming reaction

diagenetic versus biotic accretionary mechanisms of bryozoan sponge buildups lower silurian anticosti island canada abstract lower silurian mud rich fenestrate bryozoan sponge buildups of anticosti island quebec canada display facies ranging from fenestrate bryozoan cementstone to crinoid and fenestrate bryozoan hash mudstone wackestone with distinctive polymud fabric sponge spicules and stromatactis these buildups consist of a massive core with thinner stratiform counterparts that show no significant compositional variation raising the question as to how such a deposit could have accreted these features are similar to those mentioned for many paleozoic carbonate mounds in particular the lower carboniferous waulsortian or waulsortian type mounds here the combination of petrographic fluorescence spectrometric and geochemical evidence carbon and oxygen stable isotopes rare earth elements yttrium shows that the accretionary mechanisms of the anticosti buildups were the results of marine cement precipitation near the sea floor for the fenestrate cementstone facies and of early calcification of sponge derived organic matter organogenic lithification within the sediment for the crinoid and fenestrate bryozoan hash mudstone wackestone facies thus the accretionary mechanisms of anticosti fenestrate bryozoan sponge buildups are essentially diagenetic and conventional mechanisms invoked for reef growth such as frame building or sediment trapping or binding based on the preserved fauna or flora do not constitute an appropriate explanation for such paleozoic mud rich buildups these conclusions may well shed light on the more general ongoing debate as to the origin and accretionary mechanisms of carbonate mudmounds

quasi quadrennial and quasi biennial variability in the equatorial pacific evaluation of competing el nino southern oscillation enso theories requires one to identify separate spectral peaks in equatorial wind and sea surface temperature sst time series to sharpen this identification we examine the seasonal to interannual variability of these fields by the data adaptive method of multi channel singular spectrum analysis m ssa m ssa is applied to the equatorial band 4 n 4 s using 1950 1990 data from the comprehensive ocean and atmosphere data set two major interannual oscillations are found in the equatorial sst and surface zonal wind fields u the main peak is centered at about 52 months we refer to it as the quasi quadrennial qq mode quasi biennial qb variability is split between two modes with periods near 28 months and 24 months a faster 15 month oscillation has smaller amplitude the qq mode dominates the variance and has the most distinct spectral peak in time longitude reconstructions of this mode the sst has the form of a standing oscillation in the eastern equatorial pacific while the u field is dominated by a standing oscillation pattern in the western pacific and exhibits also slight eastward propagation in the central and western pacific the locations of maximum anomalies in both qb modes are similar to those of the qq mode slight westward migration in sst across the eastern and central and eastward propagation of u across the western and central pacific are found the significant wind anomaly covers a smaller region than for the qq the qq and qb modes together represent the enso variability well and interfere constructively during major events the sharper definition of the qq spectral peak and its dominance are consistent with the devil s staircase interaction mechanism between the annual cycle and enso

new isotopic data solving an old biostratigraphic problem the age of the upper ordovician brachiopod holorhynchus giganteus recent work has shown that there is a pronounced positive late ordovician excursion in both 13 c and 18 o which appears to be global and identifies a clear chemostratigraphic interval that is confined to part of the hirnantian stage pre hirnantian rawtheyan brachiopod carbonate values of 13 c are typically in the range 1 to 1 pdb and lower hirnantian values are typically in the range of 4 to 7 o pdb this contrast in isotopic values has been used to assess the age of the distinctive late ordovician brachiopod holorhynchus giganteus the age of the holorhynchus association is important in terms of its relationship to the major late ordovician extinction that was initiated at the start of the hirnantian the species which traditionally was regarded as uppermost ordovician hirnantian in age has been shown on stratigraphic evidence to be pre hirnantian at many locations if this is so it predates the first phase of extinction and is not associated with the hirnantia fauna which characterizes the interval between the two main episodes of late ordovician extinction however at its type locality in the asker district of norway the stratigraphic evidence has pointed to it being hirnantian in age to resolve this ambiguity analyses have been made on holorhynchus at three locations 1 in the east baltic region where holorhynchus occurs below demonstrable hirnantian rocks 2 in the boda limestone of central sweden where holorhynchus occurs high on a carbonate mud mound where its age is unclear and 3 in the type area of the species in norway at all three locations the 13 c values are low indicating a pre hirnantian pre extinction age for holorhynchus the re assessment of the age of the asker sequence in norway implies the presence of a cryptic unconformity there and suggests uplift rather than subsidence during the latest ordovician this study emphasizes the value of chemostratigraphy in high resolution stratigraphy

the modern calcifying sponge spheciospongia vesparium lamarck 1815 great bahama bank implications for ancient sponge mud mounds abstract modern calcified siliceous sponges from the great bahama bank living at water depth ranges of 2 to 5 m have been proposed as likely analogues for calcified sponges in upper jurassic sponge reefs e g southern germany or lower jurassic bioherms that consist of reddish spiculiferous limestones e g broccatello formation of the southern alps indeed sponge related calcification or siliceous sponge diagenesis in general is widely considered a key feature for the mechanisms of accretion and textural maturation in phanerozoic sponge mounds or spiculiferous carbonate mud mounds based on a revisit of the original sites on the great bahama bank nw of andros island the biostratonomy of the calcifying sponge spheciospongia vesparium lamarck 1815 was explored using the patterns of fluorescent dissolved organic matter fdom as revealed by the application of three dimensional excitation emission matrix eem fluorescence spectroscopy geochemical sampling distinguished between fdom that was extracted from sponge tissue and fdom that was intimately associated with caco 3 from particles due to sediment agglutination and authigenic caco 3 both obtained from the living sponge at the sediment surface and from the calcified sponge at the shallow subsurface from 5 to 10 cm of depth as expected the sponge tissue shows highest intensities for protein like fluorescence however from the surface to the subsurface there is a loss of such relatively pristine fluorescent material in the range of 70 humic like fluorescence that occurs associated with sponge tissue is relatively mature or aged thus it most probably represents seawater fdom taken up through active filter feeding relative to the sponge tissue material the fdom patterns associated with ca carbonates show much lower total fluorescence intensities by up to two orders of magnitude the agglutinated sedimentary carbonate particles from the surface pellets ooids grapestones exclusively show a relatively mature humic like fluorescence the deeper calcified parts of spheciospongia which represent a mixture of particles and authigenic caco 3 provided a fdom pattern that obviously combines the mature fdom pattern of particles with diagenetically fresh protein like and fulvic acid like fluorescence we conclude that shallow subsurface calcification of s vesparium correlates with the initial stage of sponge biomass humification such a mechanism of biomass transformation i e from biopolymers to geopolymers via degradation and condensation has also been suggested for the large scale development of carbonate sponge mud mounds therefore we consider the modern calcifying siliceous sponge s vesparium lamarck 1815 a potential pardigm to decipher in more detail the geologically important process of biomass induced calcification or organomineralisation with its subsequent effect of pore water fdom preservation and sediment lithification

a pre hirnantian late ordovician interval of global cooling the boda event re assessed facies analysis suggests that the early mid ashgill late ordovician boda event of fortey and cocks fortey r a cocks l r m 2005 late ordovician global warming the boda event geology 33 5 405 408 was an episode of global cooling not of warming as they proposed limestones are widely developed at this level from mid latitude to high latitude commonly interrupting thick ordovician successions of fine clastic sediments bryozoan carbonate mud mounds in a laterally extensive cool water carbonate belt along the high palaeolatitude margin of w gondwana are associated with glacial diamictites and grew during glacial lowstand with sea ice prior to the hirnantian glaciation from high to mid palaeolatitudes contemporaneous cool water carbonates with mud mounds including stromatactis both in gondwana and in baltica developed in mid outer ramp settings during regression in the dominantly clastic welsh basin of avalonia phosphatic facies on an iapetus facing carbonate ramp indicate upwelling cold waters down ramp yet up ramp coralliferous algal limestones suggest warm surface waters we propose that the late ordovician cool water carbonates developed in response to global cooling and expansion of gondwanan terrestrial and sea ice that lowered eustatic sea level and on the w gondwanan shelf reduced or shut down fluvial clastic input previously poorly circulated salinity stratified oceans that had restricted benthic dispersal in mid low latitudes were replaced by thermohaline ocean circulation that oxygenated bottom waters and dispersed benthic faunas an earlier caradoc carbonate interval in low mid palaeolatitudes that shares many facies characteristics with the ashgill cool water limestones and correlates with a widely recognised isotope excursion is interpreted as indicating an earlier global cooling episode

influences of temperature and mg ca ratio on caco3 precipitates from seawater the mg ca ratio at which the calcium carbonate mineral that has pseudohomogeneously precipitated from seawater changes from calcite to aragonite was experimentally determined as a function of temperature results indicate a dramatic change in the critical mg ca ratio over a relatively small temperature range for example in normal seawater with a mg ca of 5 1 the critical temperature is 6 3 c at a temperature of 25 c however mg ca must be less than 1 4 0 1 for calcite to form these data indicate that paleoclimatic conditions at the time of carbonate deposition must be considered when attempting to make inferences about variation in other factors such as paleo p co 2 p pressure and paleo mg ca ratios that may control carbonate mineral formation by abiotic processes however the inverse relation between inferred global paleoclimatic conditions and the dominant type of caco 3 polymorph being abiotically deposited i e warm conditions and low mg calcite versus cool conditions and aragonite suggest that substantial changes must have taken place in ocean chemistry over phanerozoic time

ordovician carbonate mud mounds of the baltoscandian basin in time and space a geophysical approach abstract carbonate mud mounds developed within the baltoscandian basin an epicratonic basin on the baltica palaeocontinent during the ordovician in the upper ordovician succession of the baltoscandian basin a large number of mud mounds are present at three stratigraphic levels namely the kullsbergs mounds late sandbian early katian nabala and rakvere mounds middle katian and the boda mounds late katian these formed in a subtropical tropical carbonate platform environment covered by a shallow epicontinental sea the mud mounds at these stratigraphic levels beneath and around gotland have been characterized using a comprehensive seismic and well dataset acquired during a period of hydrocarbon exploration and exploitation which began more than 30 years ago interpretation of the largely unpublished seismic data in this study provides details on the distribution of mound complexes in the basin and constraints on the geometry of the mounds detailed structure contour maps of the top and the base of the ordovician succession beneath gotland based on the seismic interpretation are presented the results give a comprehensive characterization of carbonate mud mound generation on gotland which may help in understanding the distribution patterns of similar mound complexes in other parts of the ordovician world formed in similar environments

multiple titanium substitutions in biotites from high grade metapelitic xenoliths euganean hills italy complete crystal chemistry and appraisal of petrologic control biotites from metapelitic xenoliths included within trachytes from the euganean hills italy were analyzed by single crystal x ray diffraction xrd electron microprobe emp scanning electron microscope sem secondary ion mass spectrometry sims and mossbauer spectroscopy these biotites are ti rich and occur in gneissic xenoliths that underwent regional high t low p metamorphism at about 750 c followed by pyrometamorphism during incorporation in the melt at temperatures close to 950 c biotites are zoned with tio2 content ranging from 6 79 cores to 8 14 wt rims sims measurements show that the h2o content is in the range 2 88 4 08 wt the simultaneous occurrence of high ti and high h2o contents and the main cation substitutions based on emp analyses suggest that the role of ti oxy in these biotites is less important than ti vacancy and ti tschermak substitutions single crystal xrd confirms that the ti oxy exchange was indeed effective but not the dominant substitution mechanism based on our data and those taken from literature on petrologically well constrained systems we propose that there is a petrologic control on the type of ti substitution mechanisms we consider two types of petrologic groupings for biotites 1 group a consisting of biotites from h2o free or h2o poor petrologic environments e g volcanic rocks ultrabasic xenoliths and crustal xenoliths in which biotite underwent incongruent melting ti substitution in these biotites occurs via ti oxy predominantly or more specifically fe3 ti oxy and 2 group b consisting of biotites from h2o rich petrologic environments e g metamorphic rocks and crustal granitoids ti vacancy or more specifically fe3 ti vacancy is the dominant mechanism in them it is concluded that during high grade metamorphism the dominating type of ti substitution in biotite is controlled by h2o activity

accretionary mechanisms and temporal sequence of formation of the boda limestone mud mounds upper ordovician siljan district sweden abstract late ordovician mud mounds in the boda limestone siljan area central sweden consist mainly of massive spiculitic stromatactis limestone similar to other well known paleozoic mud mounds the mound cores are predominantly formed by spiculite and ostracod rich mudstone and or wackestone and networks of spar filled syndepositional cavities the mounds of the boda limestone are specific in two ways 1 large parts of the lower mound cores are formed by a palaeoporella framestone and or bafflestone with signs of early recrystallization and cementation and 2 the stromatactis facies is accompanied by masses of limestone with non stromatactis cryptocrystalline patches cryptocrystalline patches are filled with massive fibrous low mg calcite have cloudy transitional margins contain randomly spaced skeletal grains and dolomitic micro inclusions and are interpreted as recrystallized patches of the original host lime mud models of formation of paleozoic mud mounds generally hypothesize accretionary processes that are based on sponge biomass degradation and associated diagenetic processes in the boda limestone palaeoporella skeleton disintegration early recrystallization and additionally cementation were essential for the mound formation the boda mounds exemplify that formation of paleozoic mud mounds reflects a spectrum of different essentially diagenetic accretionary mechanisms this diversity should be taken into account in future discussions about paleozoic mud mound formation the new name osmundsberget formation is given for a hirnantian succession of brachiopod rich limestone above the boda limestone the osmundsberget formation occurs only in direct geographical vicinity to the mud mounds

substitution mechanisms and implications for the estimate of water fugacity for ti rich phlogopite from mt vulture potenza italy the crystal chemistry of ti rich phlogopite from deposits of the oldest activity 740 ka of mt vulture potenza italy was investigated to identify the substitution mechanisms in ti rich phlogopite and to determine its potential as a geohygrometer substitution mechanisms were determined by electron probe microanalysis epma single crystal x ray diffraction scxrd mossbauer spectroscopy and micro fourier transform infrared ftir spectroscopy magnetite and sanidine from the mica rich host rocks were also analyzed use of the biotite magnetite sanidine geohygrometer in these volcanics essentially trachytic phonolitic ignimbrites is exploited

significance of oh f and cl content in biotite during metamorphism of the western adamello contact aureole the hydroxyl o 4 site composition of biotite can in principle be used to retrieve information about fluid composition during fluid rock interaction however due to low f and cl content as well as difficulties involved with analyzing the h2o content using in situ techniques measuring these species in biotite has remained an elusive goal here we present high precision secondary ion mass spectrometry sims oh f cl measurements from biotite within metapelites from the western adamello tonalite wat contact aureole northern italy fluorine chlorine and hydrogen are analyzed on the sims sequentially by peak hopping at the same biotite spot h2o f and cl content were measured with a precision 1 0 06 50 5 e e e the compositions of isolated biotite crystals in andalusite are compared with that of biotite in the matrix documenting that halogens and h2o behave refractory in biotite during the time scale of contact metamorphism the h2o and halogen contents of biotite are mostly locked in during the prograde to peak formation of biotite and are not reset during further heating or cooling unless significant biotite recrystallization occurs it also appears that both ti content and xmg of the biotite from the western adamello contact aureole were not significantly reset during cooling the concentration of f and cl does not vary systematically with metamorphic grade which indicates that these species reflect initial compositions no significant rayleigh fractionation behavior was observed for these elements h2o variations in the biotite from samples throughout the western adamello contact aureole suggest that al oxy substitution partially controls the variations in oh content through charge balance of the type r2 vi oh al3 vi o2 h2 while the ti oxy substitution does not seem to influence the o 4 site occupation the main titanium substitution appears to be the ti vacancy text 2 text r text 2 text t text i text 4 square text vi exchange variations in h2o and halogen concentrations in biotite define sub mm scale areas of localized equilibration even for biotite recrystallized during dehydration reactions that produced large amounts of fluid chlorite or muscovite breakdown similar systematics were observed for ti4 and al3 these findings further support the increasing number of observations that kinetics control much of the mineralogical reactions occurring in contact aureoles and hence care is advised in using equilibrium thermodynamics in this environment

titanium substitution in biotite an empirical model with applications to thermometry o2 and h2o barometries and consequences for biotite stability abstract an empirical model for solution of ti in biotite is developed and calibrated on the basis of various equilibria among biotite and ti saturating assemblages sources of data used for the calibration include both experimental products and published analyses of natural assemblages ti solution in biotite can be satisfactorily modelled by means of the exchange component tife 2 whose non ideal behavior can be approximated empirically by means of a linear function of the relative concentrations of ti and octahedral al in biotite within assemblages which fix the chemical potential of the exchange component tife 2 the concentration of this component in biotite is very sensitive to temperature and oxygen fugacity making it possible to use biotite oxide assemblages to obtain reliable t and f o 2 estimates in igneous and metamorphic rocks in assemblages which also contain alkali feldspar it is possible to use breakdown equilibria of ti additive components in biotite to estimate h 2 o fugacity such uses are likely to find important applications in rocks in which other assemblages sensitive to temperature f o 2 and or f h 2 o are absent in general rocks containing a single oxide phase plus biotite as the only mafic silicate phase alkali feldspar the empirical model for solution of ti in biotite also allows to calculate the loci of various terminal reactions for the assemblage titanian biotite quartz which by comparison with equivalent equilibria in ti free systems provide insight into the effect of ti on the stability of biotite solubility of ti in biotite increases with temperature in a strongly non linear fashion as a consequence calculations predict that the effect of ti on the stability of biotite is very small up to conditions typical of the beginning of the granulite facies of metamorphism but beyond those conditions ti concentration increases markedly and biotite is rendered very refractory at mid to deep crustal pressures 10 kbar the vapor absent solidus of biotite quartz in rocks containing ti saturating phases is likely to exceed 900 c even in bulk compositions with subequal concentrations of fe and mg

a model for the origin and petrogenesis of the red stromatactis limestone of paleozoic carbonate mounds abstract from study of the devonian recifs rouges of belgium and the silurian mounds of the quebec appalachians we propose that deposition of the red stromatactis limestone facies of cambrian devonian carbonate mounds was controlled by sponges and that the red color and stromatactis result from early diagenesis within a few meters below the substrate water interface in the deep marine environment the common presence of sponge body fossils as well as various stages of sponge preservation ranging from easily delineated to indiscernible bodies in the belgian recifs rouges support the conclusion that a large part of the red finely crystalline limestone originated from early cementation of sponge communities or spicule rich organic mats derived from degradation of sponge communities the role of bacteria and other microbes although widely advocated as the main primary builders of carbonate mounds is difficult to assess microbial communities may have contributed to mound accretion as primary builders and or mud producers but their presence cannot be documented we suggest that they may have acted as agents for concurrent sponge putrefaction and early cementation biodiagene is during the sulfate reduction phase under alkaline ph and anoxic conditions we interpret stromatactis as a spar body that resulted from early marine cementation of a cavity network created by excavation of uncemented material in partly indurated decaying sponges and spicule rich organic mats derived from degradation of sponge communities through circulation of interstitial water in the uppermost few meters of the sediment this cavity network was connected to sea floor and flushed by oxic waters that changed the diagenetic environment from anoxic to oxic converting amorphous iron sulfide to hematite and giving the facies its red color recognition of the dominant role of sponge communities in the construction of red stromatactis limestone mounds of the cambrian devonian time interval has important implications for the geological record this community was the main deep water mound building community during this time but is difficult to recognize because of its variable but often poor preservation

crystal chemistry of al rich biotites coexisting with muscovites in peraluminous granites a comparison was made between single crystal structure refinements electron microprobe analyses and octahedral site populations of seven biotite crystals and data obtained previously for coexisting muscovite in peraluminous granites using the same methods both micas from several plutons of northern victoria land antarctica and sardinia italy show significant octahedral substitutions biotite has a composition characterized by relatively high 6 al content 0 32 6 al 0 59 apfu whereas muscovite is characterized by phengite like substitutions 0 12 6 mg fe ti mn 0 35 mean bond lengths and electron count data for six biotites 1 m space group c 2 m agreement factor 2 7 r obs 3 6 and a biotite 2 m 1 space group c 2 c r obs 2 8 show that al substitutes for divalent cations in the octahedral m2 site and that the fe and mg distribution is disordered the mean tetrahedral bond lengths determined for biotite 2 m 1 reflect al si disorder in coexisting muscovite 2 m 1 crystals small positive electron density residuals close to m1 site position as well as the increase in m2 mean atomic number is in agreement with the presence of a significant phengitic component reduction in biotite unit cell dimensions with the increase of al follows a pattern similar to that of associated muscovites and the octahedral site volumes of both micas are influenced by the al saturation index asi of the rock these results and the calculated partition coefficients between biotite and muscovite for elements in m sites are consistent with continuous reaction and re equilibration of biotite and muscovite during crystallization of peraluminous granitic melts

titanium aluminum and interlayer cation substitutions in biotite from high grade gneisses west greenland detailed electron microprobe study pelitic to mafic metamorphic rocks from two localities higher ti k k na and possible k na similar al iv and lower al vi in granulite vs amphibolite grade samples modified journal abstract

model of seawater composition for the phanerozoic we present an inverse model of phanerozoic seawater com position calibrated against updated paleoseawater compositions from fluid inclusions in marine halites the model considers step wise alteration of seawater composition via 1 variable input of river water 2 variable rates of alteration of seawater through reactions at mid ocean ridges and 3 variable rates of alteration of seawater through reactions on ridge flanks and across the ocean floor in general the model achieves agreement with paleoseawater fluid inclusion data for na 1 c a 2 1 s o 4 2 2 and k 1 particularly when variable runoff is considered variable rates of basalt seawater interactions at both ridges and ridge flanks are required to understand the evolution of seawater particularly the observed near constant concentration of k 1 through time

middle and upper ordovician carbon isotope chemostratigraphy in baltoscandia a correlation standard and clues to environmental history abstract the baltoscandian epeiric sea was characterised by relatively slow but continuous carbonate deposition from the middle ordovician up to the silurian relatively good preservation of ordovician and silurian carbonate rocks in baltoscandia makes this region unique for chemostratigraphical and palaeoenvironmental studies previous palaeozoic carbon isotope studies have revealed isotopic events in different lithologies over the baltoscandian palaeobasin and some of them on different palaeocontinents we propose a composite carbon isotope curve for the baltoscandian region a new subdivision of the middle and upper ordovician succession into seventeen chemostratigraphic zones and an improved nomenclature of isotopic excursions based on analyses of 13 c carb data from 25 drillcore and outcrop sections of estonia latvia and sweden the composite carbon isotope curve is compared with biostratigraphical data on conodont and chitinozoan distribution the katian hirnantian interval with highly fluctuating isotopic values shows climatic cyclicity of about two million years in baltoscandia no abrupt changes occur in the dapingian sandbian part of the baltoscandian isotope curve but the mid darriwilian excursion nearly coincides with the onset of cooler temperatures similar to modern like equatorial sea surface temperatures and related global biodiversification increase suggesting a climatic background for this event

genesis of stromatactis cavities between submarine crusts in palaeozoic carbonate mud buildups it is concluded that the sedimentary structure stromatactis is a calcite cement fill of a swarm of labyrinthine cavities developed in marine carbonate muds of palaeozoic bioherms commonly with geopetal sediment field and petrographic evidence is examined from the ordovician of dalarna sweden the devonian of the canning basin australia and of belgium and the carboniferous of lancashire u k in all four occurrences the host microspar encloses masses of radiaxial fibrous calcite with sub horizontal floors following other workers radiaxial fibrous calcite is regarded as an altered marine cement a conclusion supported by the presence of overlying geopetal sediments with marine fossils widespread evidence of early submarine cementation in palaeozoic mud mounds isotopic analyses and the presence of microdolomites the stromatactis masses are separated from each other by laminae of host biomicrospar which show discontinuities resembling fractures which are healed with the early marine cement or filled with marine internal sediments the fractures are therefore of submarine origin the evidence points to episodic submarine lithification of the carbonate muds yielding successive crusts between which cavities were eroded and then filled the crusts were broken and rotated during bioturbation differential compaction or shearing during burial

titanium in biotite from metapelitic rocks temperature effects crystal chemical controls and petrologic applications an extensive natural biotite data set from western maine constrains the temperature and crystal chemical controls on the saturation ti levels in biotites from metapelites the geologically and petrologically well characterized metamorphic terrain associated with the m3 metamorphism of the acadian orogeny of western maine is ideal for this approach in that metamorphism occurred at roughly isobaric conditions of 3 3 kbar and chemical equilibrium was closely approached the data set from these metapelites exhibits systematic variations in ti contents over a continuum of metamorphic grades garnet through sillimanite k feldspar zones mineral assemblages and bulk compositional ranges samples were selected so that competing substitutions are restricted to those in metapelites with quartz aluminous phases chlorite staurolite or sillimanite ti phases ilmenite or rutile and graphite due to crystal chemical factors in any given metamorphic zone an inverse linear relationship exists between ti and mg contents decreasing octahedral ti and increasing tetrahedral si in mg rich biotite helps alleviate size disparity between octahedral and tetrahedral sheets for a biotite with a given mg content ti most dramatically increases above staurolite zone conditions our constrained data set allows us to calculate a ti saturation surface for natural biotite as a function of temperature and mg content at 3 3 kbar the ti saturation surface can be used to establish several important metamorphic features in similar metamorphic settings these include a general approach to equilibrium local and or subtle departures from equilibrium due to minor alteration to chlorite and relative and absolute geothermometry based on ti in biotite inclusions in refractory minerals and in matrix biotite

the sphene centered ocellar texture an effect of grain supported flow and melt migration in a hyperdense magma mush abstractthe sphene centered ocellar texture consists of leucocratic ocelli with sphene titanite crystals at the center enclosed in a biotite rich matrix this texture has been recognized worldwide in hybrid intermediate rocks on the basis of structural petrological and geochronological data from selected outcrops of the variscan ribadelago pluton nw iberian massif we propose that the ocelli were formed by migration and accumulation of a residual melt through a plagioclase and biotite dominated crystalline framework at the late stage of crystallization the magma acted as a hyperdense suspension and reacted to the pressure gradient caused by the regional stress field entering the domain of grain supported flow microstructures reveal that aligned crystal domains arose in the crystal framework from the shearing and compaction of the crystal mush and behaved as magmatic microshears relative displacement of adjacent crystal clusters along these microshears corresponded to the onset of reynolds di

why is stromatactis so rare in mesozoic carbonate mud mounds the sedimentary diagenetic structure stromatactis is widespread in palaeozoic spiculitic carbonate mud mounds but occurs only sporadically in mesozoic sponge carbonate mud mounds comparative analysis of palaeozoic and mesozoic stromatactis limestones suggests that this variation results from the degree of siliceous sponge skeletal rigidity and the amount of internal sediment accumulation in the original cavity network partial to entire filling by internal sediment resulted in a continuum from a small amount of internal sediment and large amount of cement stromatactis common in the palaeozoic to only internal sediments aborted stromatactis common in the mesozoic these observations match independent lines of evidence concerning the siliceous sponge evolution and sediment recycling e g bioerosion across the palaeozoic to mesozoic biotic revolution

incongruent melting of biotite to spinel in a quartz free restite at el joyazo se spain textures and reaction characterization in the grt bt sil restitic xenoliths of el joyazo cerro de hoyazo hercynitic spinel is a minor phase commonly associated with biotite the possible reaction relationships among biotite and spinel are studied in reaction textures developed around biotites at their contact with patches of fibrolitic sillimanite and rhyolitic melt in these textures resorbed biotite crystals about 1 mm long are rimmed by a layer of glass 5 mm away from the reaction texture biotite is chemically zoned cores bt 1 have xmg 0 35 0 02 and ti 0 58 0 01 atoms whereas the outer rims bt 2 have xmg 0 45 0 01 and ti up to 0 68 atoms the hercynite rich spinel spl has low zno content 0 80 wt and xmg 0 26 0 04 the chemical compositions of the mix aggregate represent linear combinations between sillimanite and a silica rich melt this melt melt 1 is different from that of the layer around biotite melt 2 which is also richer in ca and alkalis garnet rims grt have low ca and mn and xmg 0 14 plagioclase is characterized by large homogeneous cores pl 1 an31 2 and more calcic rims pl 2 an49 6 matrix analysis in the 9 component al ca fe k mg mn na si ti 9 phase bt1 bt2 grt spl ilm melt2 mix pl1 pl2 system provides the mass balance in mole units

palaeoecological evidence for early aragonite dissolution in ancient calcite seas it has been proposed that shallow carbonate seas of upper ordovician and jurassic ages precipitated calcite as the primary calcium carbonate phase1 3 unlike such settings today it is expected that such seas were undersaturated with respect to the more soluble aragonite so that aragonitic invertebrate skeletons would have tended to dissolve here we describe examples in which this seems to have been the case our evidence consists of submarine cemented horizons in which aragonite fossils have been dissolved out to leave open moulds the walls of the moulds are encrusted by contemporaneous organisms confirming the early timing of the aragonite dissolution

upper ordovician sequences of western estonia abstract the upper ordovician uppermost caradoc ashgill section of western estonia consists of a series of seven open shelf carbonate sequences depositional facies grade laterally through a series of shelf to basin facies belts grain supported facies shallow shelf mixed facies middle shelf mud supported facies deep shelf and slope and black shale facies basin locally a stromatactis mud mound occurs in a middle to deep shelf position shallow to deep shelf facies occur widely across the estonian shelf and grade laterally through a transitional slope belt into the basinal deposits of the livonian basin each sequence consists of a shallowing upward prograding facies succession sequences 1 upper nabala stage and 2 vormsi stage record step wise drowning of underlying shelf units lower nabala that culminated in the deposition of the most basinal facies fjacka shale in the livonian basin sequences 3 6 comprise the overlying pirgu stage and record the gradual expansion of shallow and middle shelf facies across the estonian shelf the porkuni stage sequence 7 is bracketed by erosional surfaces and contains the shallowest water facies of the preserved strata the uppermost part of the section normalograptus persculptus biozone is restricted to the livonian basin and includes redeposited carbonate and siliciclastic grains it is the lowstand systems tract of the lowest silurian sequence 8 sequence 7 and the overlying basinal redeposited material i e the lowstand of sequence 8 correspond to the latest ordovician hirnantian glacial interval and the bracketing unconformities are interpreted as the widely recognized early and late hirnantian glacial maximums the sequences appear correlative to upper ordovician sequences in laurentia graptolite biozones indicated that the estonian sequences are equivalent to carbonate ramp sequences in the western united states great basin and mixed carbonate siliciclastic sequences in the eastern united states appalachian basin cincinnati arch region these correlations indicate a strong eustatic control over sequence development despite the contrasting tectonic settings of these basins

the end ordovician mass extinction a single pulse event abstract the end ordovician mass extinction eome is widely interpreted as consisting of two pulses associated with the onset and demise of the gondwana glaciation respectively with the second pulse eradicating the distinctive glacially related hirnantian benthic biota hbb a global review of occurrence data of latest ordovician benthic marine organisms reveals that virtually all warm water benthic assemblages previously assigned to the hbb comprise two distinct and clearly postglacial faunas both younger middle and late hirnantian respectively than the cool water hirnantia fauna latest katian to early hirnantian the newly recognised three transitional benthic faunas i e tbfs 1 3 can be closely tied to graptolite conodont and chitinozoan biozonations the hirnantian isotope carbon excursion hice and the glaciation thereby providing an integrated much higher resolution timescale for understanding the tempo and nature of the eome at this finer resolution we postulate a more profound impact of the first pulse of the eome than hitherto envisaged as evidenced by opportunistic expansion of the hirnantia fauna globally and the complete absence of metazoan reefs in its immediate aftermath we also argue based on high quality data from well documented benthic groups in south china i e brachiopods tabulate and rugose corals trilobites and sponges that the magnitude of the second pulse of the eome caused by the deglaciation has been overestimated because the two postglacial faunas i e tbfs 2 3 were part of a subsequent recovery phase of marine ecosystems rather than contributing to biodiversity decline thus it is more plausible to reinterpret the eome as a single pulse rapid event that was followed by a prolonged initial recovery intermittently impeded by climatic shocks through the hirnantian prior to the onset of a progressive reestablishment of marine ecosystems during the early silurian rhuddanian and aeronian associated with an overall amelioration of climatic conditions

tertiary extension in the southern trento platform southern alps italy the southern trento platform southern alps ne italy was the locus of abundant mafic volcanism associated with paleogene extensional tectonics during the pal eocene and throughout the eocene a nnw sse trending graben the alpone agno graben aag developed in the eastern lessini mountains the graben was filled with basaltic volcaniclastics calcarenites and lava flows the aag is a half graben system probably bounded by rotational listric normal faults most faults in the graben dip towards the wsw paleostress analysis within the aag shows that during the eocene the direction of horizontal extension was ene wsw the volcanic activity continued into the oligocene however the relation between oligocene tectonics and volcanism remains unclear gravimetric and magnetic anomalies in conjunction with the crustal thickness variations suggest a locus of lithospheric thinning slightly off axis from the aag this can be explained by an overall asymmetric extensional geometry dominated by a wsw dipping detachment fault the paleogene geodynamic scenario of the trento platform suggests that the aag developed as a foreland extensional response to the active collisional convergence between the european and adriatic plates neogene compressional tectonics reactivated some of the paleogene normal faults as strike sup faults

on oriented ilmenite needles in garnet porphyroblasts from deep crustal granulites implications for fluid evolution and cooling history abstract garnet porphyroblasts from a litho assemblage containing aluminous granulite and quartzofeldspathic gneisses of the eastern ghats granulite belt india contain nanometer to micrometer thick ilmenite needles oriented crystallographically petrographic and chemical analyses reveal that garnet was formed by dehydration melting reaction s of titaniferous biotite in an oxidized condition elevated oxygen fugacity might have promoted enrichment of ti bearing andradite component of garnet porphyroblasts formed during pre to peak metamorphic condition in appropriate bulk chemistry during the post peak cooling history ti bearing components in garnet decomposed to rhombohedral oxide solid solution ilmenite hematite detailed transmission electron microscopic study of the host garnet and ilmenite solid solution indicates that though there is an overall parallelism of 011 plane of host garnet and 011 plane of ilmenite structural coherence between the two phases was progressively lost during growth from thin to thick needles appropriate cooling rate from high temperature peak metamorphic condition arguably promoted growth of ilmenite solid solution through reaction exsolution process within garnet porphyroblasts

increased production of calcite and slower growth for the major sediment producing alga halimeda as the mg ca ratio of seawater is lowered to a calcite sea level abstract laboratory experiments were conducted to subject the calcareous alga halimeda a prolific producer of aragonite in the modern ocean to a variety of ambient mg ca ratios these experiments were based on three predictions 1 that halimeda would calcify at a lower rate when the ambient mg ca ratio was reduced from its high level in modern seawater 5 2 2 that this alga would grow more slowly under these conditions because its calcification yields as a byproduct co2 which is available for photosynthesis and 3 that the calcite content of halimeda s skeleton would increase as a percentage of total caco3 as predicted reduction of the mg ca ratio of ambient artificial seawater from its modern level to estimated early oligocene and early eocene levels 2 5 and 1 5 respectively resulted in progressively lower rates of calcification production of organic matter and linear growth for halimeda incrassata colonies the ambient mg ca ratio exerted a stronger influence over the rate of calcification than ca2 which in the geologic past has risen in seawater whenever the ambient mg ca ratio has declined thus although elevation of ca2 at such times would have promoted the calcification and growth of halimeda the concomitant decline of the mg ca ratio would have had a stronger negative effect in addition the percentage of skeletal material consisting of calcite rose from 8 in modern day seawater to 16 in artificial early oligocene seawater and 46 in artificial early eocene seawater magnesium was incorporated into this calcite as a function of the ambient mg ca ratio with a fractionation pattern differing only slightly from that of nonskeletal calcite halimeda exerts less biological control over its biomineralization than the related genera penicillus and udotea which in previous experiments produced only 22 25 of skeletal caco3 as calcite in seawater having a very low mg ca ratio 1 0 nonetheless it is likely that many taxa of green algae that produce little or no calcite today would have produced a substantial proportion of skeletal low mg caco3 as calcite in ancient seas such as those of the cretaceous whose mg ca ratios were close to unity

end ordovician extinctions a coincidence of causes the end ordovician hirnantian extinction was the first of the five big phanerozoic extinction events and the first that involved metazoan based communities it comprised two discrete pulses both linked in different ways to an intense but short lived glaciation at the south pole the first occurring at or just below the normalograptus extraordinarius graptolite biozone mainly affected nektonic and planktonic species together with those living on the shallow shelf and in deeper water whereas the second within the n persculptus graptolite biozone was less focused eradicating faunas across a range of water depths in all about 85 of marine species were removed proposed kill mechanisms for the first phase have included glacially induced cooling falling sea level and chemical recycling in the oceans but a general consensus is lacking the second phase is more clearly linked to near global anoxia associated with a marked transgression during the late hirnantian most recently however new drivers for the extinctions have been proposed including widespread euxinia together with habitat destruction caused by plate tectonic movements suggesting that the end ordovician mass extinctions were a product of the coincidence of a number of contributing factors moreover when the deteriorating climate intensified causing widespread glaciation a tipping point was reached resulting in catastrophe

reflection seismic investigations of the siljan impact structure sweden reflection seismic measurements were carried out during 1985 in the area of the siljan impact structure of sweden a total profile length of 80 km was shot to get a clearer picture of the subsurface geometry of impact structures and the processes that formed them than could be obtained from surface observations or theoretical modeling studies standard stacked sections have been interpreted in terms of current impact models the transient crater diameter is found to be approximately 22 km considerably smaller than current estimates based on interpretation of surface geology the structural uplift is estimated to be 3 km at the 5 to 8 km level and the material flow necessary to support the uplift is mainly horizontal rather than vertical