modern lacustrine microbialites towards a synthesis of aqueous and carbonate geochemistry and mineralogy abstract lacustrine microbialites record evidence of their depositional environments in their morphology mineralogy and geochemistry this synthesis reviews geochemical data both for microbialites and lake water for 21 modern lacustrine microbialite occurrences younger than 15 000 years although these data are limited several trends and associations are identified that provide useful criteria to aid interpretation of the environmental settings of ancient analogues these microbialites are either thrombolites or thrombolitic stromatolites they form in diverse settings including karst volcanic coastal and inland athalassic lakes surveyed lakes are mostly closed basins subject to evaporative processes lakes vary from stratified to totally mixed examples the major hydrochemical types include na cl ca so4 ca hco3 and soda lakes the salinity and alkalinity of the lakes correlates as expected with the mode of mineralization which in turn is reflected in the microstructure of the microbialites a correlation between the 13ccarbonate and the ca2 alkalinity ratio ca alk is observed generally lakes with ca alk 1 are subject to carbonate precipitation driven by either uptake of co2 that results in a broad range of positive and negative 13ccarbonate or mineralization mediated by sulphate reducing bacteria in saline to hypersaline environments that results in negative 13ccarbonate ca alk 39 are associated with hydromagnesite dolomite high mg calcite hmc and monohydrocalcite are associated with the total aqueous concentration of mg cmg occurring only in lakes with cmg 75 meq l evaporites e g gypsum are related to high lake water salinities the absolute concentration of aqueous si commonly associated with dissolution of diatom tests influences precipitation of crystalline and or amorphous silicates silica to form microbialites with si concentrations above 0 54 mmol l either as amorphous phases or as minerals such as stevensite and kerolite combined the trends observed in this survey demonstrate that modern lacustrine microbialites can preserve information symptomatic of the environments in which they formed however in a limited number of examples this is not the case as subsequent diagenesis may alter the original mineralogy to a point where the geochemical evidence conserved from the depositional environment is lost additionally some geochemical information may be the product of isolated microenvironments within living biofilms and thus not necessarily directly related to the chemistry of ambient lake water further misleading conclusions would result if the hydrochemistry at the time of study had changed from those prevailing when the microbialites were mineralized thus all the correlations developed in this work represent hypotheses to be tested rather than interpretations to be accepted this compilation demonstrates how microbialites respond to different lacustrine hydrochemical environments and is intended to provide a guide for future field studies and laboratory simulations this review also highlights the general scarcity of trace element data for lake waters and microbialites however some recent studies suggest that lacustrine microbialites may fractionate rare earth elements ree during uptake from water whereas marine microbialites appear to record the ree distribution from seawater without modification this possible difference in ree behaviour could have implications for the interpretation of palaeoenvironmental proxies

controls on reservoir quality of an upper jurassic reef mound in the palmers wood field area weald basin southern england an upper jurassic oxfordian reef mound in the palmers wood field area of the weald basin southern england clearly shows the relationship of facies diagenesis and porosity development to relative sea level changes the coral microbial reef mound was initiated over a drowned oolite shoal during a third order marine transgression and exhibits changes in coral morphology from base to top as the reef mound caught up with sea level during the subsequent highstand during the highstand extensive encrustation of the reef mound took place by microbial stromatolitic cements with subsequent porosity loss during the following lowstand of relative sea level extensive leaching of the aragonitic corals took place and a lowstand wedge accumulated down ramp porosity was lost during burial related cementation and compaction coral microbial reef mounds of this type have moderate potential for porosity formation unlike tighter deeper ramp reefs in the jurassic which are less prone to subaerial leaching and have less potential for secondary porosity development because they are less rich in aragonitic components although the upper jurassic reef mound is only a minor contributor to reservoirs in the palmers wood field it may provide a model for other upper jurassic reef plays

paleolatitudes of the tibetan himalaya from primary and secondary magnetizations of jurassic to lower cretaceous sedimentary rocks the tibetan himalaya represents the northernmost continental unit of the indian plate that collided with asia in the cenozoic paleomagnetic studies on the tibetan himalaya can help constrain the dimension and paleogeography of greater india the indian plate lithosphere that subducted and underthrusted below asia after initial collision here we present a paleomagnetic investigation of a jurassic limestones and lower cretaceous volcaniclastic sandstones section of the tibetan himalaya the limestones yielded positive fold test showing a prefolding origin of the isolated remanent magnetizations detailed paleomagnetic analyses rock magnetic tests end member modeling of acquisition curves of isothermal remanent magnetization and petrographic investigation reveal that the magnetic carrier of the jurassic limestones is authigenic mag netite whereas the dominant magnetic carrier of the lower cretaceous volcaniclastic sandstones is detrital magnetite our observations lead us to conclude that the jurassic limestones record a prefolding remagnetiza tion whereas the lower cretaceous volcaniclastic sandstones retain a primary remanence the volcaniclastic sandstones yield an early cretaceous paleolatitude of 55 5 s 52 5 s 58 6 s for the tibetan himalaya suggesting it was part of the indian continent at that time the size of greater india during jurassic time cannot be estimated from these limestones instead a paleolatitude of the tibetan himalaya of 23 8 s 21 8 s 26 1 s during the remagnetization process is suggested it is likely that the remagnetization caused by the oxidation of early diagenetic pyrite to magnetite was induced during 103 83 or 77 67 ma the inferred paleolatitudes at these two time intervals imply very different tectonic consequences for the tibetan himalaya

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

thermo tectonic history of the tethyan himalayas deduced from the palaeomagnetic record of metacarbonates from shiar khola central nepal abstract palaeomagnetic measurements were carried out on low grade metamorphic carbonates of mesozoic age from the shiar area 85 1 e 28 6 n of the tethyan himalaya th in north central nepal two characteristic remanence components carried by pyrrhotite chrm 1 and magnetite chrm 2 could be identified by their unblocking temperature spectra of 270 340 and 430 580 c respectively fold tests are not significant due to the uniform bedding of all sites however according to results from other areas of the th the pyrrhotite component has been probably acquired as a secondary p trm during exhumation and cooling thus the age of remanence acquisition can be related to the last cooling event 25 17 ma in the surrounding areas the inclination of the magnetite component matches the value expected from the indian apwp this may the primary origin of the chrm 2 pyrrhotite site mean directions show a small circle distribution with a best fit parallel to the n s direction backtilting to the expected inclination i exp by intersection of the remanence small circle with the small circle of constant i exp yields a clockwise block rotation of 30 35 with respect to the indian plate characteristics of the pyrrhotite component small circle distribution of site means secondary origin p trm with unblocking temperatures below about 300 c allow the interpretation of the chronologic order of the thermo tectonic history i an earlier main folding phase at elevated temperatures ii a later event of cooling through about 300 c coinciding with the acquisition of chrm 1 iii clockwise block rotations with respect to the indian plate and iv long wave folding as the youngest tectonic event

magnetic properties of sedimentary greigite fe3s4 an update greigite fe3s4 is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction it can also grow at any time when dissolved iron and sulfide are available during diagenesis greigite is important in paleomagnetic environmental biological biogeochemical tectonic and industrial processes much recent progress has been made in understanding its magnetic properties greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites the crystallographic c axis is the easy axis of magnetization with magnetic properties dominated by magnetocrystalline anisotropy robust empirical estimates of the saturation magnetization anisotropy constant and exchange constant for greigite have been obtained recently for the first time and the first robust estimate of the low field magnetic susceptibility is reported here the curie temperature of greigite remains unknown but must exceed 350 c greigite lacks a low temperature magnetic transition on the basis of preliminary micromagnetic modeling the size range for stable single domain behavior is 17 200 nm for cubic crystals and 17 500 nm for octahedral crystals gradual variation in magnetic properties is observed through the pseudo single domain size range we systematically document the known magnetic properties of greigite at high ambient and low temperatures and with alternating and direct fields and illustrate how grain size variations affect magnetic properties recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records including widespread remagnetizations

post glacial microbialite formation in coral reefs of the pacific atlantic and indian oceans abstract the occurrence of microbialites in post glacial coral reefs has been interpreted to reflect an ecosystem response to environmental change the greater thickness of microbialites in reefs with a volcanic hinterland compared to thinner microbial crusts in reefs with a non volcanic hinterland led to the suggestion that fertilization of the reefal environment by chemical weathering of volcanic rocks stimulated primary productivity and microbialite formation using a molecular and isotopic approach on reef microbialites from tahiti pacific ocean it was recently shown that sulfate reducing bacteria favored the formation of microbial carbonates to test if similar mechanisms induced microbialite formation in other reefs as well the tahitian microbialites are compared with similar microbialites from coral reefs off vanuatu pacific ocean belize caribbean sea atlantic ocean and the maldives indian ocean in this study the selected study sites cover a wide range of geological settings reflecting variable input and composition of detritus the new lipid biomarker data and stable sulfur isotope results confirm that sulfate reducing bacteria played an intrinsic role in the precipitation of microbial carbonate at all study sites irrespective of the geological setting abundant biomarkers indicative of sulfate reducers include a variety of terminally branched and mid chain branched fatty acids as well as mono o alkyl glycerol ethers isotope evidence for bacterial sulfate reduction is represented by low 34s values of pyrite 43 to 42 enclosed in the microbialites and compared to seawater sulfate slightly elevated 34s and 18o values of carbonate associated sulfate 21 9 to 22 2 and 11 3 to 12 4 respectively microbialite formation took place in anoxic micro environments which presumably developed through the fertilization of the reef environment and the resultant accumulation of organic matter including bacterial extracellular polymeric substances eps coral mucus and marine snow in cavities within the coral framework tof sims analysis reveals that the dark layers of laminated microbialites are enriched in carbohydrates which are common constituents of eps and coral mucus these results support the hypothesis that bacterial degradation of eps and coral mucus within microbial mats favored carbonate precipitation because reefal microbialites formed by similar processes in very different geological settings this comparative study suggests that a volcanic hinterland is not required for microbialite growth yet detrital input derived from the weathering of volcanic rocks appears to be a natural fertilizer being conductive for the growth of microbial mats which fosters the development of particularly abundant and thick microbial crusts

ams carbon 14 dating of microbial carbonates in holocene coral reefs western luzon philippines abstract microbial carbonates microbialites occurred in the holocene coral reefs at paraoir the philippines these microbialites appeal grayish to buff in color with a stromatolitic to thrombolitic structures and encrust on corals directly or on a thin layer of calcareous red algae or encrusting foraminifera that covered the corals first the microbialites only occur in the lower earlier parts of the holocene reef sequence where heliopora coerulea dominated no microbial carbonates were observed in the upper younger part of the holocene reefs that is interpreted to grow in water depth shallower than 6 m on the basis of coral community samples of microbial carbonate and associated coral were dated by ams 14 c the results show that the microbialites formed about 10 to 7 5 kyr bp nearly coeval to or a few hundred years younger than the corals on which the microbialites encrusted ams 14 c ages of samples recovered at reef margins are consistent to 230 th ages of corals associated with the microbialites but those recovered in the back reef are much older that the previously published 230 th ages of corals and interpreted to be reworked into the backreef it is proposed that the microbialites at paraoir formed in early holocene on reef slope when the deglacial sea level was rising rapidly after 7 5 kyr bp when the sea level rise slowed down and coral reefs caught up with sea level the condition became favorable to corals prospered in shallow water and no longer suitable for microbialite formation

microbial carbonates the geological record of the calcified bacterial algal mats and biofilms summary deposits produced by microbial growth and metabolism have been important components of carbonate sediments since the archaean geologically best known in seas and lakes microbial carbonates are also important at the present day in fluviatile spring cave and soil environments the principal organisms involved are bacteria particularly cyanobacteria small algae and fungi that participate in the growth of microbial biofilms and mats grain trapping is locally important but the key process is precipitation producing reefal accumulations of calcified microbes and enhancing mat accretion and preservation various metabolic processes such as photosynthetic uptake of co2 and or hco3 by cyanobacteria and ammonification denitrification and sulphate reduction by other bacteria can increase alkalinity and stimulate carbonate precipitation extracellular polymeric substances widely produced by microbes for attachment and protection are important in providing nucleation sites and facilitating sediment trapping microbial carbonate microfabrics are heterogeneous they commonly incorporate trapped particles and in situ algae and invertebrates and crystals form around bacterial cells but the main component is dense clotted or peloidal micrite resulting from calcification of bacterial cells sheaths and biofilm and from phytoplankton stimulated whiting nucleation interpretation of these texturally convergent and often inscrutable fabrics is a challenge conspicuous accumulations are large domes and columns with laminated stromatolite clotted thrombolite and other macrofabrics which may be either agglutinated or mainly composed of calcified or spar encrusted microbes stromatolite lamination appears to be primary but clotted thrombolite fabrics can be primary or secondary microbial precipitation also contributes to hot spring travertine cold spring mound calcrete cave crust and coated grain deposits as well as influencing carbonate cementation recrystallization and replacement microbial carbonate is biologically stimulated but also requires favourable saturation state in ambient water and thus relies uniquely on a combination of biotic and abiotic factors this overriding environmental control is seen at the present day by the localization of microbial carbonates in calcareous streams and springs and in shallow tropical seas and in the past by temporal variation in abundance of marine microbial carbonates patterns of cyanobacterial calcification and microbial dome formation through time appear to reflect fluctuations in seawater chemistry stromatolites appeared at 3450 ma and were generally diverse and abundant from 2800 to 1000 ma inception of a proterozoic decline variously identified at 2000 1000 and 675 ma has been attributed to eukaryote competition and or reduced lithification thrombolites and dendrolites mainly formed by calcified cyanobacteria became important early in the palaeozoic and reappeared in the late devonian microbial carbonates retained importance through much of the mesozoic became scarcer in marine environments in the cenozoic but locally re emerged as large agglutinated domes possibly reflecting increased algal involvement and thick micritic reef crusts in the late neogene famous modern examples at shark bay and lee stocking island are composite coarse agglutinated domes and columns with complex bacterial algal mats occurring in environments that are both stressed and current swept products of mat evolution ecological refugia sites of enhanced early lithification or all three

syn depositional alteration of coral reef framework through bioerosion encrustation and cementation taphonomic signatures of reef accretion and reef depositional events the development of coral reef framework and the preservational character of both in situ and rubble coral is strongly influenced by a range of physical chemical and biologically mediated taphonomic processes these operate at or just below the reef framework water interface and can be defined as having either a constructive or destructive effect upon primary reef framework i e coral constituents constructional activities add additional calcium carbonate to the primary framework structure via secondary framework growth and early cementation destructive processes which remove or degrade primary and secondary framework carbonate are associated with the effects of either physical mainly storm disturbance or biological erosion termed bioerosion key bioeroding groups include the grazing fish and echinoid groups as well as the activities of an array of infaunal borers these include specific groups of sponges bivalves and worms termed macroborers as well as cyanobacteria chlorophytes rhodophytes and fungi termed microborers the relative importance of each process and the rates at which they operate vary spatially across individual reef systems in addition many of these processes leave distinctive signatures on or in the coral framework in some cases e g calcareous encrusters these are the skeletons of the organisms themselves whilst in other cases the organism may leave behind a trace of their activity e g macro and microborers these represent useful palaeoenvironmental tools firstly because they often have good preservation potential and secondly because the range and extent of many of the individual species groups and processes involved exhibit reasonably well constrained environment and or depth related distributions as a result these taphonomically important organisms or processes can be used to delineate between reef environments in core or outcrop and to aid the interpretation of reef depositional processes and events this review summarises current understanding regarding the distribution of these species processes within contemporary reef settings and considers the suites of taphonomic signatures that may aid in the recognition and interpretation of depositional environments and events

persistently low asian paleolatitudes implications for the india asia collision history 1 paleomagnetism provides independent paleolatitude constraints on the india asia convergence however implied cenozoic latitudinal convergence within asia thousands of km largely exceeds geologic estimates of tectonic shortening hundreds of km this discrepancy may result from a notoriously low bias in paleomagnetically determined cenozoic paleolatitudes in asia we provide here new paleomagnetic data from cenozoic mongolian volcanic rocks and from chinese paleogene sediments corrected from the depositional bias of inclination shallowing these results combined with similar asian data sets confirm that paleolatitudes are still 5 10 lower than predicted by the paleomagnetic apparent polar wander path apwp for asia between 50 and 20 ma inclination shallowing being excluded from the selected data sets we investigate the likeliness of other proposed mechanisms for this discrepancy 1 more southerly positions of asia than expected by the apwp due to apwp inaccuracies rather than eurasian non rigidity or 2 non dipolar geomagnetic field contributions fully explaining this discrepancy by only one of these mechanisms would imply either unrealistically large 10 apwp inaccuracies or unrealistically large octupolar field contributions up to 16 a combination of these mechanisms is found more likely to have produced the observed latitudinal discrepancy but their respective contributions cannot be quantified given the still relatively low amount and poor quality of cenozoic paleomagnetic data from stable cratons of asia india and europe by allowing for reasonable time dependant non dipolar contributions and a slight 5 apwp bias the latitudinal discrepancy can be resolved and the excessive amounts of intra asian shortening decrease to values in line with tectonic shortening from structural studies

subsidence history of the tethys himalaya abstract this article reconstructs the evolution of the passive northern margin of peninsular india facing the neotethys from the earliest rifting stages in the carboniferous to final collision with the active southern margin of asia in the early eocene classical techniques of basin analysis were applied to an extensive and coherent stratigraphic data base facies analysis biostratigraphic dating and palaeoenvironmental interpretation of an over 10 km thick sedimentary succession from eight main composite sections allowed us to draw accurate sedimentation and backstripped subsidence curves and to place quantitative constraints on the palaeotectonic scenario inferred from the sedimentary record in each of the investigated composite sections three major stratigraphic gaps are consistently recognised in the rift sequence the rift unconformity mississippian the carboniferous permian hiatus largely kasimovian to asselian and the break up unconformity mid sakmarian two or even all three gaps may merge into a single hiatus spanning up to 70 ma overall gaps are associated with rift shoulder uplift which after calculation of backstripped subsidence sums up to 600 m at least in three of the investigated sections thermal subsidence and low sedimentation rates are documented by the mid permian to middle triassic drift sequence deposited while the neotethys was expanding between northern gondwana and the detached peri gondwanan blocks a marked increase in accumulation rates is recorded in the carnian norian but calculations of uniform lithospheric stretching suggest that extension was minor and associated with tectonic processes affecting distant parts of gondwana the tethys himalayan margin was uplifted around the jurassic cretaceous boundary at the onset of a major volcanic episode finally uplift exceeding in magnitude all previous events is documented by the uppermost units of the stratigraphic column deposited during the earliest stages of collision between india and asia in the latest paleocene to early eocene

sem stem observations of magnetite in carbonates of eastern north america evidence for chemical remagnettzation during the alleghenian orogeny early to middle paleozoic carbonates of eastern north america have been pervasively remagnetized in order to determine the process of remagnetization scanning and scanning transmission electron microscopy have been used to characterize magnetite in thin sections and in concentrated separates samples included ordovician knox carbonates from east tennessee ordovician trenton limestone and devonian onondaga and helderberg limestones from new york and ordovician trenton carbonates from michigan inclusions of authigenic minerals within magnetite grains lack of cations other than iron and a variety of textural relations all imply that the magnetite is authigenic these data are consistent with estimates that paleotemperatures never exceeded values that would reset magnetic directions the remagnetization is thus a chemical remanent magnetization crm rather than viscous remanent magnetization vrm as the timing of remagnetization corresponds to the alleghenian orogeny the observed relations imply stress induced crystallization of magnetite that was mediated by fluids consistent with but not requiring fluid flow on a regional basis

diagenetic sources of stable remanence in remagnetized paleozoic cratonic carbonates a rock magnetic study hysteresis measurements lowrie fuller tests cisowski tests and low temperature demagnetization experiments on samples of the knox dolomite trenton limestone and onondaga limestone indicate that the remanence in these remagnetized carbonate units resides principally in single domain magnetite in contrast to previous interpretations volumetrically however a significant fraction of the magnetite is coarser grained as previous scanning electron microscopic work on magnetic extracts has shown these conclusions are based mainly on the characteristic wasp waisted shape of the hysteresis loops and on the anomalously high ratios of remanent coercivity to bulk coercivity both of which clearly indicate a bimodal distribution of coercivities grain sizes the magnetization is controlled mainly by cubic magnetocrystalline anisotropy consistent with the spheroidal particles commonly observed in extracts hysteresis trends show that especially the finest particles lack shape anisotropy in view of these conclusions the two components of natural remanence in these rocks are interpreted as 1 a late paleozoic kiaman chemical remanence carried by single domain diagenetic magnetite and 2 a cenozoic thermoviscous overprint residing principally in coarser pseudosingle domain and multidomain magnetite also of late paleozoic diagenetic origin

cryptic biostalactites in a submerged karst cave of the belize barrier reef revisited pendant bioconstructions cemented by microbial micrite abstract microbial carbonates are important components of postglacial tropical coral reef frameworks curious calcium carbonate protrusions in a submarine karst cave in the belize barrier reef platform were initially interpreted as being largely the product of physicochemical precipitation around a meshwork of serpulid tubes new petrographical mineralogical geochronological and inorganic and organic geochemical data suggest that sulfate reducing bacteria played a substantial role in carbonate formation just as in cryptic settings of other modern tropical coralgal reefs the bacterial involvement in carbonate authigenesis is indicated by the content and isotopic composition of carbonate associated sulfate the presence of non isoprenoidal sn 1 mono o alkyl glycerol monoethers mages and terminally branched fatty acids and the observation that the majority of molecular fossils assigned to sulfate reducing bacteria are tightly associated with the carbonate mineral lattice the carbonate protrusions that were formerly named pseudostalactites are termed biostalactites here to underline the significance of biological activity during carbonate formation the belize biostalactites are composed of four carbonate phases including serpulid tubes lithified sediment and microbial carbonate as well as unconsolidated sediment usually found in macroborings of bivalve molluscs lithified sediment and microbial carbonate are fine grained with clotted mottled and peloidal textures while serpulid tubes are aragonitic lithified sediment and microbial carbonates are mixtures of high magnesium calcite and aragonite magnesium calcite content increases with increasing consolidation lithified sediment and microbial carbonate are interpreted as fine biodetrital sediment that was subsequently cemented by authigenic micrite scalenohedral high magnesium calcite and acicular aragonite cements played a subordinate role in cementation carbon and oxygen stable isotope ratios of lithified sediment and microbial carbonate exhibit similar ranges as modern reefal microbialites our study exemplifies that heterotrophic bacteria can be a crucial agent that contributes to the formation of carbonate build ups by cementing skeletal frameworks in cryptic environments

microbialite development patterns in the last deglacial reefs from tahiti french polynesia iodp expedition 310 implications on reef framework architecture abstract the widespread occurrence of microbialites in the last deglacial reef frameworks 16 6 ka bp implies that the accurate study of their development patterns is of prime importance to unravel the evolution of reef architecture through time and to reconstruct the reef response to sea level variations and environmental changes the present study is based on the sedimentological and chronological analysis 14c ams dating of drill cores obtained during the iodp expedition 310 tahiti sea level on the successive terraces which typify the modern reef slopes from tahiti it provides a comprehensive data base to investigate the microbialite growth patterns i e growth rates and habitats to analyze their roles in reef frameworks and to reconstruct the evolution of the reef framework architecture during sea level rise the last deglacial reefs from tahiti are composed of two distinctive biological communities 1 the coralgal communities including seven assemblages characterized by various growth forms branching robust branching massive tabular and encrusting that form the initial frameworks and 2 the microbial communities developed in the primary cavities of those frameworks a few meters 1 5 to 6 m below the living coral reef surface where they heavily encrusted the coralgal assemblages to form microbialite crusts the dating results demonstrate the occurrence of two distinctive generations of microbialites the reefal microbialites which developed a few hundred years after coralgal communities in shallow water environments whereas the slope microbialites grew a few thousands of years later in significantly deeper water conditions after the demise of coralgal communities the development of microbialites was controlled by the volume and the shape of the primary cavities of the initial reef frameworks determined by the morphology and the packing of coral colonies the most widespread microbialite development occurred in frameworks dominated by branching thin encrusting tabular and robust branching coral colonies which built loose and open frameworks typified by a high porosity 50 in contrast their growth was minimal in compact coral frameworks formed by massive and thick encrusting corals where primary cavities yielded a low porosity 30 and could not host a significant microbialite expansion

a new type of messinian composite microbialitic build up salemi sicily italy abstract a new type of metric scale composite patch reef is described in the messinian sedimentary rocks from the vicinity of salemi western sicily these build ups are composed of two distinctive parts 1 a steep sided dome shaped microbial mound with thrombolitic fabric and encrusting organisms 2 a coral microbial build up covering the microbialitic mound in which coral colonies are coated with microbialites these are associated with various encrusting organisms the accretion of the microbialitic mound is characterized by the development of botryoidal micritic aggregates and skeletons of melobesians encrusting foraminifers bryozoans serpulids and cemented bivalves the accretionary process involving the microbialite development appears to have been homogeneous within different parts of the composite microbialitic build up as in other similar messinian build ups of the mediterranean consequently it is considered that microbial growth continued throughout messinian times in mediterranean regions being recorded in a wide range of habitats and paleoenvironments

the tethys himalayan passive margin from late triassic to early cretaceous south tibet the mesozoic succession of south tibet both in lithologies and stratigraphic thicknesses compares much more closely with that of central nepal than has been reported in the literature facies distribution from relatively proximal environments in the south to more distal settings in the north reflects the paleogeography of the tethys himalaya passive margin representing the southern edge of neotethys as in central nepal and nw himalaya accumulation rates increased greatly in the latest triassic when very thick shelfal siliciclastics and locally volcaniclastics tarap formation were followed by coastal sandstones zhamure formation contrary to reports in the literature this latter unit reaches well into the liassic at least in proximal southern sectors the base of the overlying kioto group yielding rich faunal associations characterized by benthic foraminifers and lithiotis similar to those of western tethys carbonate platforms is dated as middle liassic most probably late pliensbachian the monotonous growth of carbonate ramps during the middle liassic to early dogger was interrupted by two siliciclastic episodes related to major paleogeographic changes in the toarcian middle part of the kioto group and in the bajocian bathonian laptal formation an early callovian flooding event ferruginous oolite formation widespread all along the tethys himalaya was locally followed by deposition of a markedly lenticular pelagic unit previously reported only from the thakkhola graben dangar formation the overlying spiti shale is reduced to only 20 m in southern sectors where it is not younger than the oxfordian whereas in northern sectors the unit is several tens of m thick and mainly ascribed to the tithonian volcaniclastic deposition wolong formation reported herein for the first time probably began as early as the kimmeridgian early tithonian and lasted until aptian times when it was replaced by sedimentation of relatively deep water marlstones this magmatic episode related to tectonic extension associated with the detachment of india from gondwanaland took place earlier in south tibet and progressively later in central nepal and northern india as in nepal volcanic products changed with time from predominantly basaltic composition to intermediate and bimodal character the early cretaceous sedimentary evolution of south tibet matches that of offshore nw australia

paleocene eocene foreland basin evolution in the himalaya of southern tibet and nepal implications for the age of initial india asia collision siliciclastic sedimentary rocks derived from the southern lhasa terrane sitting depositionally upon rocks of the northern indian passive continental margin provide an estimate of the age of initial contact between the continental parts of the indian and asian plates we report sedimentological sedimentary petrological and geochronological data from upper cretaceous paleocene strata in the sangdanlin section located along the southern flank of the indus yarlung suture zone in southern tibet this is probably the most proximal and therefore the oldest record of the india asia collision these strata were deposited by high density turbidity currents or concentrated density flows and suspension settling of pelagic biogenic debris in a deep marine setting an abrupt change from quartz arenitic to feldspatholithic sandstone compositions marks the transition from indian to asian sediment provenance the abrupt compositional change is accompanied by changes in u pb ages of detrital zircons diagnostic of a sediment provenance reversal from indian to asian sources the timing of the transition is bracketed between 60 ma and 58 5 0 6 ma by detrital zircon u pb ages and zircon u pb ages from a tuffaceous bed in the upper part of the section in the context of a palinspastically restored regional paleogeographic framework data from the sangdanlin section combined with previously published data from the northern tethyan himalaya and the frontal nepalese lesser himalaya and subhimalaya suggest that a flexural wave migrated 1300 km southward across what is now the himalayan thrust belt from paleocene time to the present

quaternary development history of coral reefs from west indian islands a review the western indian ocean results from a complex geological history implying both plate motion since about 180 million years and volcanic hot spot activity since about 68 million years this region exhibits a myriad of islands different in nature and origin including continental remnants madagascar the granitic seychelles high volcanic islands reunion mauritius rodrigues the comoros high and low lying carbonate reef platforms the carbonate seychelles the french scattered island group agalega islands and a few submerged reef banks geyser nazareth saya de malha this region contains all known types of reefs fringing barrier atoll platform and bank the data about reef development history over the quaternary were gained mainly from two distinct sources boreholes and emerged outcrops reef drilling indicates that modern reefs started regionally to grow from about 10 000 9600 years bp rates of vertical reef accretion ranges from 0 2 to about 10 mm year according to exposure and shape of coral builders the sea level curves reconstructed from vertical reef accretion and lagoon deposition curves express a rapid rise in sea level at rates averaging 6 mm year between 10 000 and 7500 7000 years bp with a stabilization at the present position around 2500 3000 years bp there are significant variations in the composition and distribution of dominant reef building coral assemblages and facies framework versus detritus according to exposure to wave energy frameworks composed of robust coral branching assemblages mainly acropora robusta a humilis groups isopora palifera dominate in higher energy settings while those with domal and tabular branching forms mainly porites various faviids acropora hyacinthus group are best represented in lower energy to protected environments detritus reefs are best developed in sheltered sites the recent contribution of some dominant coral species in modern wio reefs seem to have resulted from their delayed arrival in the region as controlled by the regional prevailing surface circulation south equatorial current and agulhas current and or by the competency period of their larvae coral based geochemical records suggest that during the mid holocene the seasonal variability of sea surface temperatures was lower than today the reconstruction of the wio pleistocene reef history is chiefly based on a limited number of exposed reef remains deposited during the last interglacial mis 5 5 stage 129 000 125 000 years bp at present culminating at elevations ranging from 20 to 9 m with respect to present mean sea level such differences in elevation can be explained by a differential tectonic behaviour of reef bearing substrates from stable continental areas to subsiding volcanic islands the existence of older reef generations plio pleistocene was identified from coral rich inclusions trapped into pyroclastites of comoro volcanoes

latest cretaceous himalayan tectonics obduction collision or deccan related uplift abstract diverging interpretations and incompatible scenarios have been proposed for the early stages of himalayan history numerous researchers have postulated that northern india was involved in ophiolite obduction arc continent or continent continent collision during the late cretaceous or early paleocene but firm geological evidence was never produced in this article we argue against orogenic events predating the late paleocene when the neotethys ocean was still open the tethys himalayan sedimentary record testifies to anorogenic evolution primarily controlled by dynamic uplift of the passive margin prior to the massive outburst of deccan lavas and eventually followed by thermal subsidence major stratigraphic gaps in pelagic sediments suggest that such tectono magmatic episode started to affect the base of the indian plate in the campanian or possibly even in the santonian 10 to 20 ma before the climax of deccan flood basalt eruptions the abrupt increase in siliciclastic supply and accumulation rates recorded in sedimentary basins all around the indian subcontinent during the maastrichtian was followed by progradation of coastal quartzarenites along the northern indian margin in the early paleocene sandstones derived from the rejuvenated craton and uplifted inner continental margin in the south are dominantly but not exclusively quartzose felsitic volcanic rock fragments and cr spinels many of which with the same geochemical fingerprint as deccan spinels and newly found throughout the maastrichtian to danian succession resisted the combined effect of subequatorial weathering and subsequent diagenesis and testify that detritus from deccan basalts reached the indian passive margin as far as south tibet at the close of the early paleocene india drifted away from the seychelles block and thermal subsidence led to widespread carbonate deposition along the tethys himalaya this article illustrates how mega events of magmatic upwelling followed by lithospheric cooling may control passive margin sedimentation and stratigraphic patterns as occurred in northern india first in the early cretaceous and next in the latest cretaceous paleocene

scanning and transmission electron microscope observations of magnetite and other iron phases in ordovician carbonates from east tennessee previous paleomagnetic observations for the carbonates of the lower ordovician knox group have indicated that ancient magnetizations in these rocks are of the same age as the late paleozoic alleghenian orogeny rock magnetic properties strongly suggest magnetite as the carrier of the magnetization but the textural and crystalline characteristics sizes morphologies and mineral associations of these magnetites are poorly known we have examined magnetic extracts and iron oxides in thin sections with scanning sem and scanning transmission stem electron microscope techniques to determine whether the observed iron oxide grain textures match the rock magnetic properties and paleomagnetic inferences about the mode of formation of the magnetic carriers several different forms of magnetite in limestones and dolomites which in places are host to mississippi valley type deposits are documented by imaging and energy dispersive analysis using sem and stem by x ray diffraction and electron diffraction patterns using stem the magnetite is either spherical with a dimpled surface or nonspherical in the form of void filling single grains or grain aggregates most of the iron oxides have the composition of pure end member magnetite but occasional titanomagnetite and hematite including rare zincian hematite have been observed only in limestone wherever found in thin section nonspherical magnetites occur in association with secondary dolomite potassium feldspar illite and quartz some iron oxides have in fact inclusions of k feldspar and quartz some of the magnetite spherical and nonspherical is polycrystalline this implies that the larger observed grains may consist of single domains or pseudo single domains this provides an explanation of the observed rock magnetic properties that apparently reflect the presence of single domain but interacting subgrains on the basis of remanent coercivities and blocking temperatures we interpret the pure end member magnetite to be authigenic having formed at approximately the same time as the k feldspars which in nearby areas have yielded late paleozoic radioisotopic ages 278 322 ma the knox carbonates therefore are inferred to carry a chemical remanent magnetization iron rich clays or original iron titanium oxides in the carbonates may have been the source materials for at least some of the secondary magnetite as it formed through complete dissolution precipitation processes these processes require rock fluid interactions which are thought to be related to migrating connate brines during the alleghenian orogeny

the iron isotope composition of microbial carbonate abstract in a comparative study of fossil and recent stromatolites and microbialites we have investigated whether reefal stromatolitic limestones are able to preserve fe isotope compositions that potentially serve as proxies of seawater chemistry at the time of their formation it was found that 56 fe values of archean stromatolites vary between 2 1 and 0 5 devonian and carboniferous microbiallites vary between 0 and 1 and modern microbialites from the great barrier reef show a range from 0 12 to 0 15 five lines of evidence support the possibility that these compositions are potentially pristine 1 fe concentrations and fe isotope ratios are not correlated 2 the concentrations of other elements that are potentially mobile during carbonate diagenesis mg sr do not correlate with 56 fe 3 mn concentrations are inversely correlated with 56 fe which might hint at meteoric alteration however if mn concentrations and fe isotope signatures were pristine features they might serve as a record of the reduction of these two metals in seawater 4 dolomitisation of two devonian limestones does not shift their fe isotope compositions 5 rare earth element and yttrium rey patterns in all limestones are similar to seawater rey sources this suggests that the bulk of the fe in the samples occurs in hydrogenous carbonates not in primary or secondary fe and mn hydroxides because the latter have different rey patterns the modern microbialites are unfractionated in fe relative to crustal rocks thus these hydrogenous precipitates either incorporated unfractionated fe of the ambient seawater composition or they modified the fe composition upon incorporation from a fractionated ocean fe reservoir in either case provided diagenetic alteration has not modified the fe isotope composition in a major way a potential open ocean fe isotope proxy may exist that may allow reconstruction of secular variations in seawater fe isotope compositions our preliminary results suggest that neoarchean microbial carbonate fe may have been lighter than today s microbialite fe and that the carbonates fe isotopes potentially record the efficiency of transition metal reduction in ocean basins through geologic time

was phanerozoic reef history controlled by the distribution of non enzymatically secreted reef carbonates microbial carbonate and biologically induced cement throughout most of the phanerozoic reef rigidity resulted as much or more from early lithification by microbial carbonates and biologically induced cements non enzymatic carbonates than from biological encrustation of or by large enzymatically secreted metazoan skeletons reef framework is divided into four categories 1 skeletal metazoan 2 non skeletal microbialite stromatolite and thrombolite 3 calcimicrobe and 4 biocementstone in which small or delicate organisms serve as scaffolds for rigid cement crusts the last three categories are dominated by non enzymatic carbonates skeletal framework and non skeletal microbialite framework were the most abundant framework types through the phanerozoic the composition and abundance of skeletal framework was controlled largely by mass extinction events but most reefs consisted of both microbialite and skeletal organisms in a mutually beneficial relationship microbialite framework was abundant throughout the palaeozoic and early mesozoic but declined after the jurassic calcimicrobe framework was important during the cambrian early ordovician and devonian and biocementstone framework was important from the late mississippian to the late triassic the phanerozoic history of reefs does not correlate well with the stratigraphic distribution of large skeletal reef builders or with a variety of physicochemical parameters including sea level history wilson cycle or global climate cycles because non enzymatic carbonates result from induction by non obligate calcifiers and not enzymatic precipitation by obligate calcifiers the distribution of these carbonates was controlled to a larger extent by temporal changes in physicochemical parameters affecting the saturation state of sea water with respect to carbonate minerals changes in pco ca mg ratios cation concentrations and temperature may have affected the abundance of non enzymatic carbonates and hence reefs independently from the effects of these same parameters and mass extinction events on skeletal reef biota the decline in abundance of reefal microbialite and absence of calcimicrobe and biocementstone reef framework after the jurassic may be a result of relatively low saturation states of sea water owing to increased removal and sequestration of finite marine carbonate resources by calcareous plankton since the jurassic reef history is difficult to correlate with temporal changes in specific global parameters because these parameters affect skeletal biota and biologically induced carbonate precipitation independently hence reef history was regulated not just by skeletal reef biota but by parameters governing non enzymatic carbonates

lagoon microbialites on isla angel de la guarda and associated peninsular shores gulf of california mexico abstract examples of two closed lagoons with extensive growth of recent microbialites showing variable surface morphology and internal structure are found on isla angel de la guarda in the gulf of california comparable lagoonal microbialites also occur ashore from ensenada el quemado on the adjacent peninsular mainland of baja california the perimeters of all three lagoons feature crusted structures indicative of thrombolites with a knobby surface morphology 2 cm to 3 cm in relief and internal clotting without any sign of laminations outward from this zone thrombolitic construction thins to merge with a white calcified crust below which a soft substratum of dark organic material 4 cm to 6 cm in thickness is concealed the substratum is laminated and heavily mucilaginous as observed along the edges of extensive shrinkage cracks in the overlying crust the thrombolitic crust is anchored to the shore while the thinner crust and associated stromatolitic mats float on the surface of the lagoons laboratory cultures of the dark organic material yielded the solitary cyanobacterium chroococcidiopsis as the predominant taxon interspersed with filamentous forms in decreasing order of abundance other morphotypes present include phormidium oscillatoria geitlerinema chroococus and probably spirulina the larger of the two island lagoons follows an east west azimuth and covers 0 225 km2 while the smaller lagoon has a roughly north south axis and covers only 0 023 km2 the salinity of water in the smaller lagoon was measured as148 ppt pliocene strata along the edge of the smaller modern lagoon include siltstone bearing calcified platelets suggestive of a microbial origin dry lagoons abandoned during the later quaternary occur inland at higher elevations on the island but retain no fossils except for sporadic white crusts cemented on cobbles around distinct margins raised quaternary lagoons parallel to the big lagoon on isla angel de la guarda are partly obscured by flood damage but still easily mapped from aerial photos these features suggest that isla angel de la guarda experienced quaternary uplift similar in scale to many other gulf islands on which marine terraces are preserved closed lagoons around the gulf of california represent a stable oligotrophic ecosystem affected by extreme aridity and hypersalinity punctuated episodically by the injection of floodwater from tropical storms the taxonomic and geographic ranges of microbial communities throughout the larger region remain to be explored

sedimentation rates and growth potential of tropical cool water and mud mound carbonate systems carbonate fixation in the ocean proceeds in three basic modes abiotically biotically induced with organic trigger and biotically controlled where organisms determine timing location and composition of the product the three modes combine in a variety of ways to produce carbonate sediment when viewed on the scale of formations and global facies belts three benthic carbonate production systems or factories emerge 1 the tropical shallow water system dominated by biotically controlled mainly photoautotrophic and abiotic precipitates 2 the cool water system dominated by biotically controlled mainly heterotrophic precipitates and 3 the mud mound system dominated by abiotic and biotically induced mainly microbial precipitates the sedimentation rates of all three factories decrease as the time span of observation increases this scaling trend is not just an artifact of ratio correlation it is almost certainly related to the episodic pulsating nature of sedimentation the growth potential of the three systems can be estimated by drawing the envelope of the maximum observed rates of aggradation the tropical system shows the highest rates 10

the first microbialite coral mounds in the cenozoic uppermost paleocene from the northern tethys slovenia environmentally triggered phase shifts preceding the petm abstract upper thanetian microbialite coral mounds from the adriatic carbonate platform sw slovenia are described herein for the first time representing an important case study of extensively microbially cemented boundstones in the early paleogene the mounds are constructed primarily by microbialites associated to small sized coral colonies forming metric bioconstructions in a mid ramp setting detailed macroscopic and microscopic studies show that microbes are the major framework builders playing a prominent role in the stabilization and growth of the mounds with corals being the second most important component microbial carbonates represent up to 70 of the mounds forming centimetric thick crusts alternating with coral colonies the microbial nature of the crusts is demonstrated by their growth form and internal microfabrics showing accretionary binding and encrusting growth fabrics often with gravity defying geometries thin sections and polished slabs reveal a broad range of mesofabrics with dense structureless micrite leiolite laminated crusts stromatolites and clotted micritic masses thrombolites a first layer of micro encrusters including leiolites and thrombolites occurs in cryptic habitats whereas discontinuous stromatolites encrust the upper surface of corals a second encrustation the major mound construction phase follows and is dominated by thrombolites encrusting corals and other micro encrusters this sequence represents the basic constructional unit horizontally and vertically interlocked in an irregular pattern to form the mounds the processes which favored the deposition of these microbial carbonates were mainly related to in situ precipitation with minor evidences for grain agglutination and trapping processes scleractinian corals comprise moderately diversified community of small centimetric colonial massive platy encrusting and branching forms coral colonies are distributed uniformly throughout the mounds without developing any ecological zonation these features indicate that coral development remained at the pioneer stage throughout the mound growth the spatial relationships between corals and microbialites as well as the characteristics of microbial crusts and coral colonies indicate a strong ecological competition between corals and microbes a model for the evolution of the trophic structures during the mound growth is proposed with changes in the paleoecology of the main bioconstructors triggered by frequent environmental perturbations turbidity and nutrient pressure interpreted here as related to frequent recurrences of wet phases during the warm humid climate of the uppermost thanetian might have promoted temporary dominance of microbes over corals causing rapid environmentally driven phase shifts in the dominant biota

environmental significance of microbialites in reef environments during the last deglaciation in situ microbialites occurring in reef rocks dredged between 80 and 130 m water depth on the modern fore reef slopes of tahiti and the marquesas islands yield ages ranging from 17 100 2900 to 4410 2250 years bp suggesting that they played a prominent role during the last deglacial sea level rise microbialites developed in both shallow and deep water depositional environments where they characterize various zones of the reef tracts reef crests upper reef slopes deep fore reef slopes reflecting contrasting scenarios of microbialite development involving reefal microbialites in shallow water settings and slope microbialites that formed in environments deeper than 10 20 m and extending down to more than 100 m reefal microbialites correspond to a late stage of encrustation of the dead parts of coral colonies or more commonly of related encrusting organisms red algae and foraminifers thus forming surface crusts slope microbialites generally form the ultimate stage of a biological succession indicating a deepening sequence whereby shallow water corals and associated encrusting organisms are replaced by deeper water assemblages of red algae and foraminifers before microbialite growth the precipitation of phosphatic iron manganese crusts and the deposition of planktonic micritic limestones on the microbialites characterize a deepening upward sequence the widespread development of microbialites in reef sequences from the last deglaciation characterizes a period of environmental degradation consequential on the rapid sea level rise and abrupt climatic changes of that time the reported biological succession reflects changes in water quality and especially an increase in nutrients in shallow water settings increased alkalinity and nutrient availability in interstitial waters were related to surface fluxes and terrestrial groundwater seepage while slope environments were exposed to continuous upwelling of nutrient rich deeper waters during the last deglacial sea level rise the age differences between corals and overlying slope microbialites range from 1600 to 8400 years based on high precision u series age measurements of both corals and microbialites and indicates that a significant time several thousand years elapsed between the development of the coralgal frameworks and the growth of slope microbialite crusts microbialites cannot be considered as part of the drowning event some 14 000 years ago that resulted in the demise of reef frameworks in the 90 110 m present depth range but are substantially younger

occurrence and significance of microbialites in the uplifted tasmaloum reef sw espiritu santo sw pacific abstract in the sw pacific ocean subduction of the d entrecasteaux ridge system has caused rapid uplift of the central new hebrides island arc the maximum uplift rate of 6 mm yr 1 occurs along the southwest coast of espiritu santo island near tasmaloum the tasmaloum uplifted reef sequence internal structure which is strongly linked to its tectonic context was investigated through a series of drill holes to depths up to 42 m although a stable tropical coast would undergo approximately 120 m of post glacial sea level rise the net relative sea level rise on such a rapidly uplifting coast is only about 20 m colonization of the tasmaloum fringing reef occurred by 24 ka upon a pre reef substrate composed of a thick bioclastic sand formation accumulated during the last glacial period during the post glacial sea level rise the vertical succession of reef assemblages reflects environmental and bathymetric variations controlled by the interplay of rapid but variable rates of sea level rise and more or less constant uplift of 5 6 mm yr 1 microbialite crusts composed of high magnesian calcite laminae occur in the tasmaloum reef from 20 to 6 ka and are particularly abundant from 16 to 10 ka the development of microbialite crusts is related to nutrient enrichment of interstitial waters through mixing with meteoric groundwater after 6 ka when sea level ceased rising in the region and continuing uplift caused rapid emergence of the reef microbialites disappear within the subtidal assemblages several explanations can be put forward for their disappearance in particular nutrient input changes are a likely cause a new hydrologic and oceanographic regime was established when sea level ceased rising this change was accompanied by warming of tropical waters

stromatolite reef crusts early cretaceous spain bacterial origin of in situ precipitated peloid microspar peloidal crusts are significant components of early cretaceous aptian reef carbonates in eastern spain the crusts form steep sided laminated deposits on coral and other skeletal surfaces their microfabric consists almost entirely of silt sized peloids in fenestral microspar matrix this microfabric contrasts with more poorly sorted and generally finer grained detrital wackestone packstone fabrics of the adjacent reef matrix scarcity of incorporated grains indicates that the crusts did not trap many particles it is proposed that the crusts are stromatolites and that peloids and inter peloid space were created concurrently by bacterial degradation of organic matter as they developed inter peloid voids were protected from infiltration of extraneous sediment by the organic rich exterior surface of the stromatolite even spacing of the peloids within microspar may reflect self organization of bacterial colonies in the decaying organic matrix compressed and partly amalgamated peloids marginal to burrows in the stromatolites suggest that the peloid fabrics were initially only partially lithified the grainstone like peloid fabric is therefore interpreted as having formed in situ by very early diagenetic processes driven by heterotrophic bacteria

late jurassic coral microbial reefs from the northern paris basin facies palaeoecology and palaeobiogeography abstract during the late middle oxfordian patch reefs grew on the northern margin of the paris basin according to the facies analysis of the reef and inter reef sediments the environment was a warm clear and agitated sea with highly episodic sedimentation the bioherms were a short lived phenomenon during the third phase of regional reefal development sequence stratigraphically they are associated with a highstand system tract volumetrically and trophically dominant organisms were microbes now represented by massive clotted leiolite stalactitic hemispheroids with purely thrombolitic texture are restricted to open caves corals were of structural reef building importance due to their rapid upward growth the patch reefs are characterised by thickets of ramose corals which developed a very open framework in the vicinity of these patch reefs though in hydrodynamically higher energy environments grew thickets of more stoutly branched corals however they are rarely preserved in situ and are generally represented as abundant coral rubble the reef taxa are characterised by the notable absence of several groups e g oysters serpulids bryozoans pectinids occurring at other localities where reefs of similar age developed in similar environmental conditions the reefs also have strikingly modern aspects to them in particular the presence of cryptic elements within caves and a sponge dominated borer association dwellers belong to various life form types although encrusting taxa are exceedingly rare this may be explained by the presence of soft microbial films on most surfaces the palaeoecological analysis suggests that the major controls on faunal composition and high diversity were elevated nutrient levels highly episodic sedimentation and probably seasonal environmental disturbances structural and functional aspects of the reef community grazers trigger framebuilders borers trigger binders binders hamper borers allow ecological comparisons to be made with contemporaneous as well as recent reefs the unique combination of ecological factors resulted in a specialised previously undescribed community which differs from both tethyan and northern localities in various aspects these include cavities with cryptofauna prominence of grazing gastropods and high faunal diversity in a microbially dominated build up

hauterivian shallow marine calcareous biogenic mounds s e spain abstract hauterivian biogenic deposits from the prebetic northern margin near caudete albacete province southeastern spain are represented by low domed bodies less than 10 m thick surrounded by bioclastic sediments and capped by siliciclastics they consist of a coral stromatoporoid and microbial framework with cavities filled by mud rapidly lithified intermound bioclastics derived from the mound organic community reflect high energy conditions and shallow water settings mound growth and architecture were controlled by nutrient fluxes and internal light gradients comparisons with closely related structures from the palaeozoic and the recent show some similarities but also significant differences the example documented here also departs from other cretaceous occurrences

high energy coralgal stromatolite frameworks from holocene reefs tahiti french polynesia drill cores from holocene reefs on tahiti french polynesia reveal a framework composed of massive branching acroporids encrusted by coralline algae associated with sessile vermetid gastropods and arborescent foraminifers laminated micritic crusts form coatings over coral branches or more commonly over related encrusting organisms throughout the cored reef sections these crusts appear as a major structural and volumetric component of the reef framework the microbial nature of these micritic crusts is inferred from their typical organic growth forms and geometry the occurrence of microbial remains and stable isotope measurements the reef communities accumulated at depths less than 5 m below mean sea level in a high energy environment throughout vertical growth from 7140 170 yr bp to the present the nature of the involved benthic communities stable isotope data and high calcification rates of microbially encrusted corals strongly suggest that local environmental conditions have been optimal for reef development for the last 7000 years the causes of the predominance of microbial communities over actual encrusters red algae foraminifers remain problematic and could be related to short term fluctuations in ecological parameters microbial micritic crusts seemingly played a prominent role in protecting the coralgal colonies from bioeroders and grazers and possibly in strengthening the framework due to rapid lithification the record of similar microbial crusts in other quaternary reef tracts suggests that microbial communities may have played a more prominent role in quaternary reefs than presently recognized

origins and development of holocene coral reefs a revisited model based on reef boreholes in the seychelles indian ocean until recently concepts of coral reef growth and accumulation have been predominantly based on a darwinian model in this the upwards and outwards growth of a reef core a coral framework takes place over a foreslope consisting of reef talus with the simultaneous filling of the back reef lagoon by reef derived debris the principal adaptations of this pattern relate to the influence of relative changes in sea level and commonly ignore oceanographic factors such as storm frequency and severity boreholes through the outer edge of a fringing reef in the seychelles western indian ocean reveal a record of holocene sediment accumulation first established approximately 8 ka ago faunal and floral associations show that growth of this body began in relatively deep water but that this shallowed to 5 m within 1 ka subsequent accumulation was of keep up style but as the rate of sea level rise slowed shoaling became more frequent and aggradation was limited by reducing accommodation space constructional facies are characterised either by massive corals including leptastrea porites and faviids or by branching corals typically acropora of the danai robusta group coral surfaces may be encrusted by red algae foraminifera and vermetids and are commonly bored by filamentous algae clionids and molluscs however detrital facies are volumetrically dominant and the paucity of a constructional framework requires a re evaluation of models of reef accretion new models relate the geometry of accretion to the interplay between extreme storm events and fairweather hydrodynamic conditions these suggest that a contiguous framework forms in areas of moderate fairweather energy without extreme storm events severe storms destroy the continuity of reef structures and generate increasing volumes of coarse detritus low storm severity coupled with low fairweather hydrodynamic energy may promote the accumulation of fine grained reef derived sediments that inhibit framework growth while ecology reflects year by year sea conditions lithology and structure are controlled by exceptional storms with the effects of changing sea level superimposed

coral reef response to quaternary sea level and environmental changes state of the science there is considerable uncertainty about future sea level projections due to the current poor understanding of the vulnerability and sensitivity of ice sheets to global warming fossil coral reefs represent a unique archive of sea level and associated environmental changes that extend far beyond instrumental records here recent advances in this field are summarized focusing on periods of earth history typified by rapid sea level rise and short term climate excursions sea level and reef response records from the last deglaciation and periods characterized by higher sea level stands and short term and low magnitude sea level oscillations such as the mid to late holocene and the last interglacial are synthesised special emphasis is given to understanding how coral reefs have responded to these changes in terms of reef development architecture and composition finally seven major challenges and future research directions are identified

controls on the spatio temporal distribution of microbialite crusts on the great barrier reef over the past 30 000 years abstract calcification of microbial mats adds significant amounts of calcium carbonate to primary coral reef structures that stabilizes and binds reef frameworks previous studies have shown that the distribution and thicknesses of late quaternary microbial crusts have responded to changes in environmental parameters such as seawater ph carbonate saturation state and sediment and nutrient fluxes however these studies are few and limited in their spatio temporal coverage in this study we used 3d and 2d examination techniques to investigate the spatio temporal distribution of microbial crusts and their responses to environmental changes in integrated ocean drilling program iodp expedition 325 great barrier reef environmental changes fossil reef cores that span 30 to 10 ka at two locations on the gbr reef margin our gbr microbialite record was then combined with a meta analysis of 17 other reef records to assess global scale changes in microbialite development i e presence absence thickness over the same period the 3d results were compared with 2d surface area measurements to assess the accuracy of 2d methodology the 2d technique represents an efficient and accurate proxy for the 3d volume of reef framework components within the bounds of uncertainty average 9 45 4 5 we found that deep water reef frameworks were most suitable for abundant microbial crust development consistent with a previous exp 325 study braga et al 2019 we also found that crust ages were broadly coeval with coralgal communities in both shallow water and fore reef settings however in some shallow water settings they also occur as the last reef framework binding stage hundreds of years after the demise of coralgal communities lastly comparisons of crust thickness with changes in environmental conditions between 30 and 10 ka show a temporal correlation with variations in partial pressure of co2 pco2 calcite saturation state calcite and ph of seawater particularly during the past 15 kyr indicating that these environmental factors likely played a major role in microbialite crust development in the gbr this supports the view that microbialite crust development can be used as an indicator of ocean acidification

microbialites in last glacial maximum and deglacial reefs of the great barrier reef iodp expedition 325 ne australia abstract microbialites are volumetrically abundant components in last glacial maximum and deglacial reefs in the australian great barrier reef sampled by iodp expedition 325 in 34 holes from 17 sites m0030 m0058 along four transects on the shelf edge detailed radiometric datings show that four distinct reef phases developed between 28and 10 ka displaying offlapping and then backstepping patterns the reef boundstone facies include coralgal coralgal microbialite and microbialite boundstone the microbialite consists of combinations of micrite microspar bioclasts siliciclastic grains up to 14 5 fenestrae and encrusting epibionts the micrite microspar is high magnesian calcite commonly irregularly clotted fenestral and peloidal mesoscale microbialite fabrics include laminated structureless digitate intraskeletal and boring filling and coatings on debris intraskeletal and boring filling is the first fabric to develop in skeletal voids and borings it is usually followed by structureless and laminated microbialite locally overlain by digitate fabric microbialite coated debris can occur at any stage in this succession including in bioclastic accumulations where the scarce in situ framework builders are mainly encrusting corals lipid biomarkers of intermediate to high specificity for sulfate reducing bacteria together with 13c values of these lipids indicate that microbialite formation was favored by sulfate reducing bacteria in anoxic microenvironments probably under high nutrient levels the microbialite in fore reef deposits accumulated in the photic zone in water depths of a few to several tens of meters within small spaces generated by large bioclasts and encrusting corals in the topmost centimeters of the sediment these crusts that formed on the illuminated surface constitute a previously unrecognized style of microbialite formation in quaternary reefs as with the cryptic crusts described from other reef locations its greatest development occurred during the last glacial maximum and early deglaciation microbial carbonate formation during this interval may reflect elevated seawater carbonate saturation corresponding with relatively low levels of atmospheric co2

cryptic intertidal microbialites in beachrock heron island great barrier reef implications for the origin of microcrystalline beachrock cement microbialites with microcolumnar growth morphology occur on the surfaces of solution enlarged crypts in modern beachrock an heron island great barrier reef australia the microbialites consist predominantly of felted aragonite but blocky mg calcite and fringes of acicular aragonite also occur a variety of microbial bodies and filaments occur within and on the surface of the microbialites although most heron beachrock is cemented by isopachous fringes of acicular aragonite irregular micritic cement is common adjacent to the microbialites the micritic cement consists predominantly of felted aragonite and contains microbial filaments and organic matter similar to those in the microbialites both the microbialite and the micritic cement are interpreted to be biologically induced although physico chemical models including co2 degassing and evaporation are commonly held to be sufficient to promote the formation of beachrock the similarity of heron cryptic beachrock microbialite with micritic beachrock cements supports the contention that some beachrock cements are biologically induced additionally the primary difference between fringes of acicular aragonite and the felts of aragonite in microbialites appears to be that the former nucleated on a well defined surface of organic matter whereas the latter nucleated within a framework of organic matter that contained irregularly disposed nucleation sites hence aragonite fringe cements may also be localized and or induced by films of organic matter c 1999 elsevier science b v all rights reserved

lithified and unlithified mg calcite precipitates in tropical reef environments abstract push cores from the branching coral framework of lagoonal reefs in belize and panama indicate that the micritic mg calcite in the unlithified mud matrix is similar in composition to the well documented submarine lithified micritic crusts and infillings found in open reef framework the sediment matrix of the lagoonal framework contains significant concentrations of 12 to 13 mole mgco3 calcite that shows almost no signs of skeletal origins as indicated by a fairly consistent mineralogy and independent distribution patterns in sand silt and clay size fractions its sedimentological and mineralogical similarity to cemented micritic crusts and infillings suggests that the precipitation of micritic mg calcite is widespread in tropical reef waters and becomes lithified when precipitated in open frameworks but remains unlithified when trapped in impermeable lagoonal muddy sediments

microbial domes and megaoncoids in miocene reefs in the mahakam delta in east kalimantan indonesia abstract coral patch reefs in the miocene mahakam delta in east kalimantan borneo indonesia developed in shallow marine turbid waters these reefs grew in delta front to prodelta environments under fluctuating siliciclastic input langhian patch reefs are well exposed in limestone quarries of the air putih area near the city of samarinda and include two different types of microbial carbonates in two sections 2 km apart in the northern section microbialites occur as low relief domes up to 2 m wide and 0 5 m high the domes show internal lamination and coralline algae and encrusting foraminifers interlayered with microbial micrite they developed around large coral fragments at the transition from reef deposits to fine grained siliciclastics microbialites in the southern section formed decimetre to metre scale nodules here termed megaoncoids around nuclei of large coral fragments microbial micrite with laminated to digitate fabrics intergrew with coralline algae and encrusting foraminifers to form the thick covers of megaoncoids small nodules were bound together in larger ones which are usually strongly asymmetric internally megaoncoids accumulated in a carbonate breccia laterally changing into coral boundstones in both sections microbialites grew on the flanks of shallow water low relief patch reefs in poorly illuminated low energy conditions flat domed microbialites formed on a nearly horizontal stable seafloor seawards of the patch reef the megaoncoids in the southern section formed as a result of downslope displacement of coral fragments coated by microbialite coralline algal encrusting foraminifer crusts the slope of the patch reef s flank favoured the falling and rolling of encrusted corals with the continued growth of microbial crusts on all sides of nodules in contrast to quaternary and recent occurrences of microbialites in indo pacific and caribbean reefs the samarinda microbialites are not components of the reef framework nor formed in deep water instead they developed in shallow turbid waters of the miocene mahakam delta in periods of arrested siliciclastic input

dimension of greater india in the early mesozoic paleomagnetic constraints from triassic sediments in the tethyan himalaya abstract the northern dimension of greater india defined as the continental part of india from which the himalayan fold thrust belt was derived is difficult to determine due to major tectonic deformations of the himalaya during the tertiary caused by the india eurasian collision paleomagnetic data from the tethyan himalaya can potentially help determine this dimension however the tectonic deformations also led to widespread remagnetization of rocks there and thus previously published paleomagnetic poles from the lower mesozoic himalayan rocks may not accurately reflect their original paleomagnetic directions to better understand the triassic history of india in a gondwana configuration we report new paleomagnetic data after tilt correction d 345 5 i 43 0 95 12 0 k 17 3 n 10 sites and the corresponding paleopole at 34 5 n 282 2 e with a95 11 7 of triassic limestone and sandstone from the tulong group in the tethyan himalaya the primary features of the magnetization are supported by the positive fold test our new triassic paleomagnetic pole overlaps with 230 250 ma poles from india suggesting the close affinity between the tethyan himalaya and cratonic india since the triassic assuming that the tethyan himalaya was a part of the great india since triassic the differences between the tethyan himalaya paleolatitude obtained by this study and those previously reported for the cratonic india imply 1300 km dimension of the tethyan himalaya has been absorbed after the eurasia india collision

pendant bioconstructions cemented by microbial carbonate in submerged marine caves holocene se sicily abstract unusual pendant bioconstructions occur within two submerged caves in the plemmirio marine protected area south of syracuse se sicily these cm to dm sized bioconstructions project downward from the cave roofs the main framework builders are serpulids belonging to the genera protula semivermilia and josephella encrusting bryozoans corals hydrozoans and sponges are subordinately present whereas bivalves and brachiopods are overall rare these faunal associations are characteristic of the studied caves but only in some places they form small bioconstructions that are cemented by microbial carbonates made up by autochthonous peloidal to clotted peloidal and aphanitic micrite the autochthonous micrite stabilizes the pendant bioconstructions herein referred to as biostalactites the microbial carbonate reveals high contents of bacterial lipid biomarkers among the most abundant compounds are mono o alkyl glycerol ethers mages and branched fatty acids 10 me c16 0 iso and anteiso c15 0 and c17 0 interpreted as biomarkers of sulfate reducing bacteria other compounds preserved in the autochthonous carbonates include lipids derived from marine zoo and phytoplankton brassicasterol dinosterol and monounsaturated short chain fatty acids unspecified marine bacteria saturated and monounsaturated short chain fatty acids and land plants long chain fatty acids long chain alcohols stigmasterol sitosterol and campesterol the observed lipid biomarker signatures are remarkably similar to those of post glacial reefal microbialites where microbialite formation in cavities of the coral framework was also mediated by sulfate reducing bacteria in the submerged caves of plemmirio marine protected area serpulid colonies apparently occur preferentially where freshwater is seeping through crevices in the limestone the consequent nutrification is believed to favor serpulid aggregation and the growth of other skeletal i e bryozoans and corals and soft bodied organisms i e sponges and ascidiae the resultant accumulation of biomass and its taphonomy provide niches for sulfate reducing bacteria which induce carbonate precipitation cementing and stabilizing the biostalactites the finding of pendant cemented bioconstructions in the mazzere and granchi caves of the plemmirio marine protected area reveals that sulfate reducing bacteria are involved in microbialite formation in various cryptic environments the formation of such peloidal to clotted peloidal microcrystalline carbonates in cryptic ecosystems is a significant factor for the stabilization of different kinds of bioconstructions spanning from small biostalactites to large reefs

biological control of paleomagnetic remanence acquisition in carbonate framework rocks of the tahiti coral reef abstract integrated ocean drilling program iodp expedition 310 recovered 632 meters of post last glacial max post lgm and late pleistocene coral reef framework rocks 37 holes at 22 sites from the current fore reef slopes surrounding the island of tahiti the cores were collected primarily to constrain the timing and amplitude of rising sea levels associated with the end of the last global glaciation 10 18 000 ybp to identify the climatic and environmental changes during that period and to evaluate the reef response to those combined changes the cores which were collected in three regions faaa tiarei and maraa are remarkable in that more than 60 of the total core volume is associated with microbialites rather than macroscopic coral or coralline algae assemblages we present here paleomagnetic data from 316 oriented paleomagnetic samples collected from the post lgm framework rocks of the maraa region 12 holes at 6 sites 96 of our samples were recovered from microbialites that have filled the primary cavities of the coral coralgal framework all samples appear to accurately record geomagnetic field variability at tahiti average inclination 30 6 alpha 95 2 9 site axial dipole inclination 32 6 and yet the samples are composed of 99 precipitated calcium carbonate our rock magnetic data indicate that the nrm resides almost entirely in detrital titanomagnetite grains derived from the tahiti volcanic edifice we present magnetic and microscopic evidence that the natural remanent magnetization in the carbonate rocks was locked in through mediation of organic biofilms mostly bacterial that held the grains in place until they were trapped by carbonate precipitation

wasp waisted hysteresis loops from a pyrrhotite and magnetite bearing remagnetized triassic limestone remagnetized samples of the triassic prezzo limestone from northern italy contain a mixture of pyrrhotite and magnetite as deduced by thermal unblocking characteristics of triaxial isothermal remanent magnetizations irm s and low temperature cycling of saturation irm s hysteresis loops are commonly wasp waisted and remanent coercivity curves contain a break in slope as a result of the contrast in coercivity between remanence carrying pyrrhotite and magnetite the relative proportion of the high to low remanent coercivity fractions as deduced by the study of the remanent coercivity curves seems to control the degree of wasp waistedness of the hysteresis loops samples that are dominated by one of the two coercivity end members have lower bcr bc values as well as hysteresis loops that have a less pronounced constricted waist compared to samples with higher bcr bc values maximum bcr bc values and thus maximum degrees of wasp waistedness in the hysteresis loops are attained when the low remanent coercivity fraction contributes 15 35 to the bulk remanent coercivity curves

magnetic reorientation induced by pressure solution a potential mechanism for orogenic scale remagnetizations abstract new paleomagnetic analyses of triassic cretaceous and eocene strata in the south central pyrenees show evidence for a widespread remagnetization located along the southern border of the axial zone the internal sierras and the northern part of the jaca pamplona basin this remagnetization always reversed in polarity was acquired after an extensive period of late eocene early oligocene folding and tilting in the area and affects limestones sandstones marls and red beds in addition a characteristic prefolding component was identified in 30 of upper cretaceous and triassic red beds these results together with a revaluation of previously published paleomagnetic data from the central pyrenees indicate that the spatial distribution of the postfolding remagnetization coincides with that of a domain of pressure solution cleavage a relationship between the intensity of the remagnetization and the characteristic prefolding component with respect to the density of cleavage surfaces leads us to propose a mechanism for the remagnetization related to the development of pressure solution cleavage that is framed within the tectonic evolution of the central pyrenees partial dissolution of rock under tectonic compression leads to the liberation and subsequent accumulation of insoluble minerals in cleavage planes magnetic grains are part of the relatively insoluble residue and they reorient in the presence of the ambient field after they are freed during dissolution of the rock matrix chemical reequilibrium dissolution and or neoformation of magnetic carriers during this process cannot be excluded the remagnetization mechanism we propose can help to explain widespread remagnetizations in low to moderately deformed rocks without the need of large scale migration of orogenic fluids

the identification and biogeochemical interpretation of fossil magnetotactic bacteria magnetotactic bacteria which most commonly live within the oxic anoxic transition zone oatz of aquatic environments produce intracellular crystals of magnetic minerals specifically magnetite or greigite the crystals cause the bacteria to orient themselves passively with respect to the geomagnetic field and thereby facilitate the bacteria s search for optimal conditions within the sharp chemical gradients of the oatz the bacteria may also gain energy from the redox cycling of their crystals because magnetotactic bacteria benefit from their magnetic moments natural selection has promoted the development of traits that increase the efficiency with which the intracellular crystals impart magnetic moments to cells these traits also allow crystals produced by magnetotactic bacteria called magnetofossils when preserved in sediments to be distinguished from abiogenic particles and particles produced as extracellular byproducts of bacterial metabolism magnetofossils are recognizable based on their narrow size and shape distributions distinctive morphologies with blunt crystal edges chain arrangement chemical purity and crystallographic perfection this article presents a scheme for rating magnetofossil robustness based on these traits the magnetofossil record extends robustly to the cretaceous and with lesser certainty to the late archean because magnetotactic bacteria predominantly live in the oatz the abundance and character of their fossils can reflect environmental changes that alter the chemical stratification of sediments and the water column the magnetofossil record therefore provides an underutilized archive of paleoenvironmental information several studies have demonstrated a relationship between magnetofossil abundance and glacial interglacial cycles likely mediated by changes in pore water oxygen levels more speculatively a better developed magnetofossil record might provide constraints on the long term evolution of marine redox stratification more work in modern and ancient settings is necessary to explicate the mechanisms linking the abundance and character of magnetofossils to ancient biogeochemistry

coral microbialite environment in a middle miocene reef of hungary abstract the presence of microbial crusts microbialites is reported here for the first time from a middle miocene badenian coral reef in hungary the succession of initiation and development of the microbial crusts is described in relation to the reef architecture the main features of the palaeoenvironment are discussed with focus on the associated invertebrate faunas mostly corals molluscs bryozoans and crabs we conclude that the microbial crusts developed under normal marine conditions and were a significant contributor to reef framework development including its strengthening through calcification of the microbial organisms microbialites are anticipated in other early and middle miocene coral reef occurrences of the mediterranean paratethys realm and to date may have simply escaped positive recognition

mammalian responses to pleistocene climate change in southeastern australia resolving faunal responses to pleistocene climate change is vital for differentiating human impacts from other drivers of ecological change while 90 of australia s large mammals were extinct by ca 45 ka their responses to glacial interglacial cycling have remained unknown due to a lack of rigorous biostratigraphic studies and the rarity of terrestrial climatic records that can be related directly to faunal records we present an analysis of faunal data from the naracoorte caves in southeastern australia which are unique not only because of the species richness and time depth of the assemblages that they contain but also because this faunal record is directly comparable with a 500 k y speleothem based record of local effective moisture our data reveal that despite significant population fluctuations driven by glacial interglacial cycling the species composition of the mammal fauna was essentially stable for 500 k y before the late pleistocene extinctions larger species declined during a drier interval between 270 and 220 ka likely reflecting range contractions away from naracoorte but they then recovered locally persisting well into the late pleistocene because the speleothem record and prior faunal response imply that local conditions should have been favorable for megafauna until at least 30 ka climate change is unlikely to have been the principal cause of the extinctions

late quaternary barrier and fringing reef development of bora bora society islands south pacific first subsurface data from the darwin type barrier reef system the universally known subsidence theory of darwin based on bora bora as a model was developed without information from the subsurface to evaluate the influence of environmental factors on reef development two traverses with three cores each on the barrier and the fringing reefs of bora bora were drilled and 34 uranium series dates obtained and subsequently analysed sea level rise and to a lesser degree subsidence were crucial for holocene reef development in that they have created accommodation space and controlled reef architecture antecedent topography played a role as well because the holocene barrier reef is located on a pleistocene barrier reef forming a topographic high the pedestal of the fringing reef was pleistocene soil and basalt barrier and fringing reefs developed contemporaneously during the holocene the occurrence of five coralgal assemblages indicates an upcore increase in wave energy age depth plots suggest that barrier and fringing reefs have prograded during the holocene the holocene fringing reef is up to 20 m thick and comprises coralgal and microbial reef sections and abundant unconsolidated sediment fringing reef growth started 8780 50 yr bp accretion rates average 5 65 m kyr 1 the barrier reef consists of 30 m thick holocene coralgal and microbial successions holocene barrier reef growth began 10 030 50 yr bp and accretion rates average 6 15 m kyr 1 the underlying pleistocene reef formed 116 900 1100 yr bp i e during marine isotope stage 5e based on pleistocene age depth and coralgal palaeobathymetry the subsidence rate of bora bora was estimated to be 0 05 to 0 14 m kyr 1 in addition to subsidence reef development on shorter timescales like in the late pleistocene and holocene has been driven by glacioeustatic sea level changes causing alternations of periods of flooding and subaerial exposure comparisons with other oceanic barrier reef systems in tahiti and mayotte exhibit more differences than similarities

biosedimentology of a silurian thrombolite reef with meter scale growth framework cavities a completely dolomitized silurian reef at maumee ohio u s a is uncommon among early paleozoic reefs in that its framework consists of about 95 thrombolite boundstone and 5 corals the reef lithified syndepositonally as evidenced by centimeter to meter scale growth framework cavities within it that are completely filled with internal sediment the largest cavities 2 3 5 m in diameter are prob ably the largest ever recorded in a thrombolite reef four main types of components are distinguished in the reef frame work 1 dense bushy mesoclots 2 dense rounded to subrounded mesoclots 3 saccate microstructures and 4 crusts mostly struc tureless but also containing dense bushy mesoclots the dense bushy components are abundant rounded to sub rounded mesoclots less so and the remaining components are rare the dense bushy and rounded to subrounded mesoclots are interpreted to represent the calcified re mains of what were initially either aggregates of the probable filamen tous cyanobacterum angulocellularia or aggregates of renalcid calci microbes or both and the saccate mesoclots as the calcimicrobe ren alcis sensu stricto it is speculated that syndepositonal lithification of the reef framework was accomplished by impregnation of unlithified protomesoclots by micrite to create mesoclots along with the formation of micritic cement around and between mesoclots that served to fuse together successive generations of mesoclots the direction of branching of bushy mesoclots in the reef indicates that unlithified protomesoclots were able to grow upward laterally and downward this ability along with syndepositional lithification to create mesoclots and to cement them together enabled the protome soclots and mesoclots to create reef knobs and eventually to roof over spaces that were centimeters to three or more meters in width and height to create growth framework cavities features indicative of rap id deposition including graded bedding poor sorting of bioclasts and textural inversion are common in internal sediment in meter size cav ities these features suggest that storms possibly hurricanes were at least as important and probably more so than everyday wave en ergy in transporting sediment into all cavities in the reef regardless of size

magnetic properties of pelagic marine carbonates abstract pelagic carbonates are deposited far from continents usually at water depths of 3000 6000 m at rates below 10 cm kyr and are a globally important sediment type recent advances with recognition of widespread preservation of biogenic magnetite the inorganic remains of magnetotactic bacteria have fundamentally changed our understanding of the magnetic properties of pelagic carbonates we review evidence for the magnetic minerals typically preserved in pelagic carbonates the effects of magnetic mineral diagenesis on paleomagnetic and environmental magnetic records of pelagic carbonates and what magnetic properties can tell us about the open ocean environments in which pelagic carbonates are deposited we also discuss briefly late diagenetic remagnetisations recorded by some carbonates despite recent advances in our knowledge of these phenomena much remains undiscovered we are only at early stages of understanding how biogenic magnetite gives rise to paleomagnetic signals in sediments and whether it carries a poorly understood biogeochemical remanent magnetisation recently developed techniques have potential for testing how different magnetotactic bacterial species which produce different magnetite morphologies respond to changing nutrient and oxygenation conditions future work needs to test whether it is possible to develop proxies for ancient nutrient conditions from well calibrated modern magnetotactic bacterial occurrences a tantalizing link between giant magnetofossils and paleogene hyperthermal events needs to be tested much remains to be learned about the relationship between climate and the organisms that biomineralised these large and novel magnetite morphologies rather than being a well worn subject that has been studied for over 60 years the magnetic properties of pelagic carbonates hold many secrets that await discovery

stromatolites associated with coralgal communities in holocene high energy reefs thick laminar to columnar stromatolitic growths encrust typical high energy coralgal reef communities in a french polynesian outer reef edge the reef communities accumulated during vertical growth from 5870 100 yr b p to the present at depths never exceeding 5 m below mean sea level oxygen and carbon isotope records and high calcification rates of stromatolite encrusted corals strongly suggest that for about the past 6000 yr the local environmental conditions were optimal for reef development accordingly stromatolites can grow in healthy holocene reefs because similar stromatolites have been recorded in other cenozoic reefs we suggest that microbial structures may have played a more prominent role in cenozoic reef growth than is currently recognized

reef coralgal assemblages as recorders of paleobathymetry and sea level changes in the indo pacific province the coralgal framework within the outer reef margin of many indo pacific reefs exhibits three main shallow water communities the environmental significance of which can be inferred by comparison with their modern counterparts a community dominated by tabular acropora gr hyacinthus cytherea with branching pocillopora damicornis p eydouxi montipora digitata occasional domal faviids and mm thick crusts of the coralline algae lithophyllum and mesophyllum mainly typical of the 6 15 m paleodepth range a community including robust branching acropora gr danai robusta a humilis a digitifera and subordinate favia stelligera echinopora gemmacea associated to vermetid gastropods and thick coralline crusts of hydrolithon cf onkodes and neogoniolithon cf fosliei flourishing in depths less than 6 m in medium to high water energy settings a community composed of domal porites cf lutea and p cf lobata with occasional acropora gr danai robusta and cm thick crusts of coralline algae in sheltered habitats in depths less than 10 m these biological assemblages allow us to determine relationships between reef growth and paleobathymetry and consequently to reconstruct regional relative sea level curves high water energy reefal assemblages provide stronger evidence for reconstructing sea level curves than low energy buildups because they have generally been controlled by a keep up growth mode subsiding reef sites seem to be more reliable indicators of sea level variations because they usually present expanded reef sequences

nature and environmental significance of microbialites in quaternary reefs the tahiti paradox drill cores from the bamer reef edge of tahiti exhibit 85 93 m thick coralgal sequences recording at least 13 500 years of continuous reef growth in optimal environmental conditions the cored reef sequences form an overall shallowingupward succession with assemblages of branching or massive colonies of porires at the base overlain by a robust branching community acropom gr dmmi robirstn heavily encrusted by coralline algae sessile vermetid gastropods and arborescent foraminifers which grew at depths less than 6 m microbialites generally form the last stage of encrustation of coral colonies or more commonly of related encrusting organisms thus appearing as a major structural component of the reef sequence where they may locally form 809 of the rock by volume they developed in an open cavity system of the reef framework with freely circulating normal marine water microbialites include laminated crusts and clotted micritic masses commonly associated in compound crusts probably feflecting differences in the composition of the involved biological communities and in biomineralization processes that controlled the accretion of the crusts the isotopic composition of the microbialites 2 05 to 3 92 i they underwent also indivitlual processes of biominrlali2ation besicles the overall decrease in lizht and enqy conditions rttlecting progressive burial by conil growth the widespread development of microbialites within the reef framework may be related to increased alkalinity and niitrient availability in interstitial waters due to temstrial groundwwr seepage and puiodic runoffs the tlevelopment of micrubialites in the cryptic niches of the reef framework ceased about 6000 years

holocene reef accretion southwest molokai hawaii u s a abstract two reef systems off south molokai hale o lono and hikauhi separated by only 10 km show strong and fundamental differences in modern ecosystem structure and holocene accretion history that reflect the influence of wave induced near bed shear stresses on reef development in hawaii both sites are exposed to similar impacts from south kona and trade wind swell however the hale o lono site is exposed to north swell and the hikuahi site is not as a result the reef at hale o lono records no late holocene net accretion while the reef at hikauhi records consistent and robust accretion over late holocene time analysis and dating of 24 cores from hale o lono and hikauhi reveal the presence of five major lithofacies that reflect paleo environmental conditions in order of decreasing depositional energy they are 1 coral algal bindstone 2 mixed skeletal rudstone 3 massive coral framestone 4 unconsolidated floatstone and 5 branching coral framestone bafflestone at hale o lono 10 cores document a backstepping reef ranging from 8 100 cal yr bp offshore to 4 800 cal yr bp nearshore a depauperate community of modern coral diminishes shoreward and seaward of 15 m depth due to wave energy disrupted recruitment activities and physical abrasion evidence suggests a change from conditions conducive to accretion during the early holocene to conditions detrimental to accretion in the late holocene reef structure at hikauhi reconstructed from 14 cores reveals a thick rapidly accreting and young reef maximum age 900 cal yr bp living coral cover on this reef increases seaward with distance from the reef crest but terminates at a depth of 20 m where the reef ends in a large sand field the primary limitation on vertical reef growth is accommodation space under wave base not recruitment activities or energy conditions interpretations of cored lithofacies suggest that modern reef growth on the southwest corner of molokai and by extension across hawaii in general is controlled by wave induced near bed shear stress related to refracted north pacific swell holocene accretion patterns here also reflect the long term influence of wave induced near bed shear stress from north swell during late holocene time this finding is consistent with other studies e g grigg 1998 cabioch et al 1999 that reflect the dominance of swell energy and sea level in controlling modern and late holocene accretion elsewhere in hawaii and across the pacific and indian oceans notably however this result is refined and clarified for hawaii in the hypothesis of rooney et al 2003 stating that enhancement of the el nino southern oscillation beginning approximately 5000 years ago led to increased north swell energy and signaled the end to net accretion along exposed coastlines in hawaii the exposure of hale o lono to north swell and the age of sea floor there ca 4 800 cal yr bp coupled with the lack of north swell incidence at hikauhi and the continuous accretion that has occurred there over the last millennium strongly supports the enso reef hypothesis as outlined by rooney et al 2003 other factors controlling holocene reef accretion at the study site are relative sea level position and rate of rise and wave sheltering by laau point habitat suitable for reef accretion on the southwest shore of molokai has shrunk throughout the holocene

microbialite morphology structure and growth a model of the upper jurassic reefs of the chay peninsula western france abstract during the early kimmeridgian the northern margin of the aquitaine basin western france is characterised by a significant development of coral reefs the reef formation of the chay peninsula comprises two main reefal units in which the microbial structures can contribute up to 70 of framework the microbial crusts which played an important role in the stabilisation and growth of the reef body show the characteristic clotted aspect of thrombolitic microbialites corals are the main skeletal components of the build ups the bioconstructions of the chay area are thus classified as coral thrombolite reefs four main morpho structural types of microbial crusts are distinguished 1 pseudostalactitic microbialites on the roof of intra reef palaeocaves 2 mamillated microbialites found either on the undersides or on the flanks of the bioherms 3 reticular microbialites in marginal parts of the reefs and between adjacent bioconstructed units and 4 interstitial microbialites in voids of bioclastic deposits thrombolitic crusts developed on various substrates such as corals bivalves or bioclasts the thrombolites formed a dense clotted and or micropeloidal microbial framework in which macro and micro encrusters also occur variations in accumulation rate strongly influenced the reef morphology in particular its relief above the sediment surface the coalescence of the coral microbialite patches created numerous intra reef cavities of metre scale dimensions the direction of microbial growth which defined the macroscopic microbialite forms strongly depended on the position within the reef framework but was also controlled by water energy accumulation rate and light availability

postglacial reef development in the south west pacific case studies from new caledonia and vanuatu abstract analysis of reef development in the south west pacific provides valuable information on climatic variations and environmental changes during the past 20 ka here i compare fringing reef development from two sites in different geotectonic settings in new caledonia located on a stable passive margin a lag in reef development is observed and postglacial reefs initiated late between 7 2 and 4 ka in vanuatu located in an uplifting island arc reef development was continuous from 24 to 6 ka and there was no lag with the reef initiating as soon as substrate was available the initiation lag in new caledonia can be attributed to a lack of suitable substrate or to unfavorable oceanographic conditions such as cold sea surface temperature sst which did not allow reef initiation in vanuatu although sst during much of the postglacial sea level rise was probably cooler than present the reef was still able to develop as soon as the substrate was flooded

contribution of microbialites to the development of coral reefs during the last deglacial period case study from vanuatu south west pacific coral reefs have grown continuously during the last 24 ka on the island of espiritu santo vanuatu south west pacific in a setting characterized by frequent tectonic uplifts related to subduction of the australia plate beneath the vanuatu island arc sedimentological and geochemical studies of two neighbouring areas having much different mean uplift rates and environmental settings provides an opportunity to analyze the environmental factors that promoted the widespread development of microbialite crusts from about 24 to 6 ka b p nutrient supplies and probably the rapidity of the sea level rise are critical to an explanation of the distinctive distribution of microbialites in reef cores nutrient flux has been related to terrigenous fluxes in the tasmaloum reef system that fringed the south west uplifting coast of espiritu santo and possibly to upwelling in the urelapa sequence that was isolated from the main island during its growth a peak in microbialite abundance occurred between 16 and 10 ka b p a time window characterized by increasing erosional processes and or more intense upwelling during a very rapid rise in sea level

rock magnetism of remagnetized carbonate rocks another look authigenic formation of fine grained magnetite is responsible for widespread chemical remagnetization of many carbonate rocks authigenic magnetite grains dominantly in the super paramagnetic and stable single domain size range also give rise to distinctiverock magnetic prop erties now commonly used as a fingerprint of remagnetization we re examine the basis of this association in terms of magnetic mineralogy and particle size distribution in remagnetized carbon ates having these characteristic rock magnetic properties including wasp waisted hysteresis loops high ratios of anhysteretic remanence to saturation remanence and frequency dependent susceptibility new measurements on samples from the helderberg group allow us to quantify the proportions of superparamagnetic stable single domain and larger grains and to evaluate the mineralogical composition of the remanence carriers the dominant magnetic phase is magnetite like with sufficient impurity to completely suppress the verwey transition particle sizes are extremely fine approximately 75 of the total magnetite content is superparamagnetic at room temperature and almost all of the rest is stable single domain although it has been pro posed that the single domain magnetite in these remagnetized carbonates lacks shape anisotropy and is therefore controlled by cubic magnetocrystalline anisotropy we have found strong exper imental evidence that cubic anisotropy is not an important underlying factor in the rock magnetic signature of chemical remagnetization the usefulness of a palaeomagnetic remanence is directly related to the accuracy with which its age is known e g van der voo 1990 primary remanence acquiredduringorverysoonafterrockformation pro videsdirectinformationonpalaeofieldorientationand strengthatthattime thepossibilityofpartialorcom plete remagnetization at a later time complicates palaeomagneticinterpretation ashaslongbeenrecog nized e g graham 1949 every robust palaeomag netic study must therefore include some effort to constrain the ages of identified components of the natural remanent magnetization nrm relative dating with respect to sedimentary processes struc tural tilting or cross cutting relationships is the basis of the classical geometric palaeomagnetic tests fold test and conglomerate test graham 1949 baked contact test everitt clegg 1962 unconformity test kirschvink 1978 the special circumstances required for application of these tests are often not available however or positive test results may providerelativelylooseconstraintsallowingmagneti zation ages that significantly post date the age of the rock formation as a result other evidence although generally more indirect becomes essential in evaluating the age origin and significance of nrm components such evidence commonly includes petrographic observations isotopic andgeo chemical data and rock magnetic characterization by their very existence magnetic overprints provide evidence of some process or event of