fractionation of sulfur isotopes during bacterial sulfate reduction in deep ocean sediments at elevated temperatures abstract a numerical model of sulfate reduction and isotopic fractionation has been applied to pore fluid so 4 2 and 34 s data from four sites drilled during ocean drilling program odp leg 168 in the cascadia basin at 48 n where basement temperatures reach up to 62 c there is a source of sulfate both at the top and the bottom of the sediment column due to the presence of basement fluid flow which promotes bacterial sulfate reduction below the sulfate minimum zone at elevated temperatures pore fluid 34 s data show the highest values 135 yet found in the marine environment the bacterial sulfur isotopic fractionation factor is severely underestimated if the pore fluids of anoxic marine sediments are assumed to be closed systems and rayleigh fractionation plots yield erroneous values for by as much as 15 in diffusive and advective pore fluid regimes model results are consistent with 1 077 0 007 with no temperature effect over the range 1 8 to 62 c and no effect of sulfate reduction rate over the range 2 to 10 pmol cm 3 d 1 the reason for this large isotopic fractionation is unknown but one difference with previous studies is the very low sulfate reduction rates recorded about two orders of magnitude lower than literature values that are in the range of mol cm 3 d 1 to tens of nmol cm 3 d 1 in general the greatest 34 s depletions are associated with the lowest sulfate reduction rates and vice versa and it is possible that such extreme fractionation is a characteristic of open systems with low sulfate reduction rates

the belize margin revisited 1 holocene marine facies belize is best known for its 260 km long barrier reef and associated lagoon reef shoals but also exhibits a complete transition from near shore siliciclastics to pure carbonate deposits across a narrow shelf lagoon and in addition has three of the very few occurrences of caribbean atolls published holocene facies relationships on the belize shelf have been semi quantitative in that they are based on quantitative point count thin section data in the northern shelf lagoon and on qualitative estimates of thin section composition in the southern shelf lagoon this contrast has been rectified by point counting southern shelf lagoon thin sections and incorporating the result with published point count data from the northern shelf supplemented by additional information to produce a modified factor analysis facies distribution for the entire shelf the resulting nine facies are extended seaward to include previously published analysis of sediment on the three belize off shelf atolls resulting in a total of 11 facies in addition the carbonate mud distribution has been mapped over both the shelf and atolls with the not unexpected result that in places the distribution pattern clearly indicates a barrier platform source of lagoon deposited carbonate mud dunham s limestone classification terminology has been used as a basis for description to make these relationships more useful to those reconstructing ancient limestone depositional environments

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

late quaternary reef growth and sea level in the maldives indian ocean abstract based on rotary drilling and radiometric and u series dating we present the first comprehensive data on holocene reef anatomy and sea level rise as well as nature and age of underlying pleistocene limestone in the maldives holocene reefs in rasdhoo atoll central maldives are composed of four facies including 1 robust branching coral facies 2 coralline algal facies 3 domal coral facies and 4 detrital sand and rubble facies branching coral and coralline algal facies predominate the marginal reefs and domal corals and detrital facies preferentially occur in a lagoon reef in addition microbialite crusts are found in lower core sections of marginal reefs microbialites formed during the early holocene in reef cavities holocene reef thickness ranges from 14 5 m to 22 m reef growth started as early as 8 5 kyr bp marginal reefs accreted in the keep up mode with rates of 15 m kyr rate of sea level rise significantly slowed down from 7 6 kyr bp and subsequently gradually rose with rates

microbial mats as bioreactors populations processes and products microbial mats are dynamic and complex ecosystems exhibiting spatial and temporal heterogeneity the physical chemical environment is typified by steep gradients and distinct microenvironments these microenvironments support a great diversity of species with a wide range of metabolic processes these processes often result in coupled reactions and biogeochemical cycles and produce important end products such as trace gases and mineral precipitates the latter can impact the composition and character of the sediment imparting a biosignature these biosignatures can be preserved in the rock record and are useful in the interpretation of fossil record on earth and possibly as an indication of life on other planetary bodies the modern marine stromatolites of the exuma cays bahamas provide an ideal system for studying the populations processes and products in a microbial ecosystem using a multidisciplinary approach in order to acquire redox energy microbial populations need to carry out metabolic reactions at rates faster than the equivalent chemical abiotic reactions as such microbes can be viewed as bioreactors that preferably oxidize carbon to co2 to maximize the energy yield the study of the microbial role in carbonate sedimentation and lithification in these stromatolites provides a picture of microbial mats as bioreactors producing a biosignature

breccia and carbonate hosted au cu ag replacement mineralization associated with the homestake porphyry intrusive complex new world district montana the miller creek homestake and daisy pass resources contain 7 406 260 metric tons t of mineralized rock averaging 7 44 to 11 97 g t au and 0 44 to 0 90 percent cu they are spatially associated with the 41 ma homestake intrusive complex within the new world district near cooke city park county in south central montana the homestake intrusive complex consists of numerous dacitic porphyry intrusive rocks and phreatomagmatic breccia bodies that have intruded cambrian sedimentary rocks and precambrian crystalline rocks diatreme breccia cuts earlier intrusion breccia and contains small zones rich in pulverized and altered rock flour matrix and volumetrically more abundant clast rich zones clasts within diatreme breccia include cambrian sedimentary rocks precambrian crystalline rocks and porphyritic intrusive rocks although moderate mixing and milling of clasts is present within diatreme breccia relict cambrian stratigraphy is commonly preserved alteration may be separated into three stages initial contact metamorphism of sedimentary rocks related to the emplacement of the intrusive rocks and intrusion breccias an early hydrothermal stage that occurred after emplacement of the intrusive rocks and a late hydrothermal stage that is postdiatreme early hydrothermal alteration of sedimentary rocks and breccia is laterally zoned relative to the homestake intrusive complex from narrow zones of proximal prograde skarn to more widespread distal calc silicate hornfels calc silicate hornfels consists of banded epidote actinolite chlorite and orthoclase lies stratigraphically above and below the mineralized replacement zones occurs up to 3 3 km away from the intrusive complex and is barren of au cu mineralization intrusive rocks intrusion breccia and diatreme breccia are dominated by late vertically zoned alteration assemblages that destroyed early alteration and consist of deep advanced argillic alteration including hydrothermal breccia intermediate level intermediate argillic alteration and distal upper level propylitic alteration trace fine grained epidote associated with early alteration is present locally in the mineralized homestake replacement zones but is absent at miller creek replacement zones within the homestake resource are hosted by diatreme and intrusion breccia containing abundant clasts of cambrian limestone and are spatially associated with intermediate argillic alteration tabular subhorizontal replacement zones at miller creek and daisy pass are hosted by dolomitic limestone of the cambrian meagher formation at the contact with the intrusive complex and are spatially associated with sills and dikes with argillic and propylitic alteration replacement zones associated with late alteration are dominated by magnetite rutilated i e rutile bearing quartz and were partially replaced by later pyrite clear quartz carbonate minerals chlorite clay and chalcopyrite almost all magnetite in the mineralized replacement zones partially to completely replaced earlier acicular hematite and retains the distinct acicular habit native gold and electrum typically occur as micron sized inclusions and fracture fillings in pyrite and to a lesser extent in chalcopyrite the abundance of chlorite carbonate magnetite and gold increase in the upper part of the hydrothermal system whereas kaolinite cu au 1 000 and ag au increase in the lower part homogenization temperatures of fluid inclusions in clear quartz associated with au cu ag mineralization suggest a thermal zoning from the proximal homestake resource 369 c to the peripheral miller creek resource 344 c a phreatomagmatic event and related structures created permeable zones and conduits for the hydrothermal fluids that selectively replaced nearby limestone rich protoliths the alteration mineralogy geochemistry fluid inclusion homogenization temperatures and paragenesis of the replacement zones and breccia hosted resources are similar suggesting that they were related to the same hydrothermal system centered on the homestake intrusive complex

the calcium carbonate saturation state in cyanobacterial mats throughout earth s history through early lithification cyanobacterial mats produced vast amounts of caco3 on precambrian carbonate platforms before 540 myr ago the superposition of lithified cyanobacterial mats forms internally laminated macroscopic structures known as stromatolites similar structures can be important constituents of phanerozoic carbonate platforms 540 myr to present early lithification in modern marine cyanobacterial mats is thought to be driven by a metabolically induced increase of the caco3 saturation state xcaco3 in the mat however it is uncertain which microbial processes produce the xcaco3 increase and to which extent similar xcaco3 shifts were possible in precambrian oceans whose chemistry differed from that of the modern ocean i developed a numerical model that calculates xcaco3 in cyanobacterial mats and used it to tackle these questions the model is first applied to simulate xcaco3 in modern calcifying cyanobacterial mats forming at highborne cay bahamas it shows that while cyanobacterial photosynthesis increases xcaco3 considerably sulphate reduction has a small and opposite effect on mat xcaco3 because it is coupled to h2s oxidation with o2 which produces acidity numerical experiments show that the magnitude of the xcaco3 increase is proportional to dic in dic limited waters dic 3 10 mm is proportional to ph when ambient water dic is not limiting and always proportional to the concentration of ca2 in ambient waters with oceanic ca2 concentrations greater than a few millimolar an appreciable increase in xcaco3 occurs in mats under a wide range of environmental conditions including those supposed to exist in the oceans of the past 2 8 gyr the likely lithological expression is the formation of the microsparitic stromatolite microtexture indicative of caco3 precipitation within the mats under the control of microbial activity which is found in carbonate rocks spanning from the precambrian to recent the model highlights the potential for an increase in the magnitude of the xcaco3 shift in cyanobacterial mats throughout earth s history produced by a decrease in salinity and temperature of the ocean a decrease in atmospheric pco2 and an increase in solar irradiance such a trend would explain how the formation of the microsparitic stromatolite microtexture was possible as the xcaco3 of the ocean decreased from the paleoproterozoic to the phanerozoic

sedimentary pyrite a window into the microbial past early diagenetic pyrite forms as a result of microbial processes in sediments but has received little attention as a potential source of microbial fossils sedimentary pyrite grains archean to jurassic in age were examined by scanning and transmission electron microscopy tem ion milling was essential for the preparation of electron transparent tem samples most pyrite grains revealed coccoid rod shaped and even filamentous features that are interpreted as microbial identification of microbial remains in sedimentary pyrite may greatly expand our knowledge of microbial life in the sedimentary record combined with geochemical characterization the study of pyritized microbes in the geologic record may in the future enable us to interpret changes in geochemical signatures more realistically in terms of different sulfate reducing communities sedimentary pyrite grains may also represent a good prospect to further our knowledge of past life on mars

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

microbe mineral interactions early carbonate precipitation in a hypersaline lake eleuthera island bahamas microbialites benthic microbial carbonate deposits were discovered in a hypersaline alkaline lake on eleuthera island bahamas from the edge towards the centre of the lake four main zones of precipitation could be distinguished 1 millimetre sized clumps of mg calcite on a thin microbial mat 2 thicker and continuous carbonate crusts with columnar morphologies 3 isolated patches of carbonate crust separated by a dark non calcified gelatinous mat and 4 a dark microbial mat without precipitation in thin section the precipitate displayed a micropeloidal structure characterized by micritic micropeloids strong autofluorescence surrounded by microspar and spar cement no fluorescence observations using scanning electron microscopy sem equipped with a cryotransfer system indicate that micrite nucleation is initiated within a polymer biofilm that embeds microbial communities these extracellular polymeric substances eps are progressively replaced with high mg calcite discontinuous eps calcification generates a micropeloidal structure of the micrite possibly resulting from the presence of clusters of coccoid or remnants of filamentous bacteria at high magnification the microstructure of the initial precipitate consists of 200 500 nm spheres no precipitation is observed in or on the sheaths of cyanobacteria and only a negligible amount of precipitation is directly associated with the well organized and active filamentous cyanobacteria in deeper layers of the mat indicating that carbonate precipitation is not associated with co2 uptake during photosynthesis instead the precipitation occurs at the uppermost layer of the mat which is composed of eps empty filamentous bacteria and coccoids gloeocapsa spp two dimensional mapping of sulphate reduction shows high activity in close association with the carbonate precipitate at the top of the microbial mat in combination these findings suggest that net precipitation of calcium carbonate results from a temporal and spatial decoupling of the various microbial metabolic processes responsible for caco3 precipitation and dissolution theoretically partial degradation of eps by aerobic heterotrophs or uv fuels sulphate reducing activity which increases alkalinity in microdomains inducing caco3 precipitation this degradation could also be responsible for eps decarboxylation which eliminates ca 2 binding capacity of the eps and releases ca 2 ions that were originally bound by carboxyl groups at the end of these processes the eps biofilm is calcified and exhibits a micritic micropeloidal structure the eps free precipitate subsequently serves as a substrate for physico chemical precipitation of spar cement from the alkaline water of the lake the micropeloidal structure has an intimate mixture of

early middle triassic stromatolites from the luoping area yunnan province southwest china geobiologic features and environmental implications abstract an early middle triassic stromatolite deposit is documented from the guanling formation of the luoping area yunnan sw china the luoping stromatolite shows five types of constructional microbial forms in various magnifications 1 typical stratified columnar structures up to 20 cm high with crinkled laminae dark coloured laminae 1 mm thick are composed of upright filamentous tubes averagely 29 4 m in diameter showing a vertical growth fabric 2 laminoid fenestrae 0 5 1 mm wide and 3 prostrate filaments which are reflected by strong fluorescence in sharp contrast to dolomite cement in fluorescent images 4 rod like aggregates 4 6 18 m in diameter composed of minute dolomite rhombs are very common in stromatolitic laminae they resemble present day cyanobacterial trichome and thus may represent putative fossilized cyanobacteria 5 moreover small pits coccoid like spheroids calcified biofilms and fibrous structures are also common in stromatolitic laminae the last two may represent calcified extracellular polymeric substances eps that contributed to the development and lithification of stromatolites authigenic quartz grains are common and may also have involved biological processes in stromatolite formation of these microbial functional groups driving accretion and lithification processes of stromatolite documented in literature both lithified cyanobacteria oxygenic phototrophs and possible sulfate reducing bacteria srb which induced microbial formation of dolomite and contributed to the accretion of the luoping stromatolite are suggestive of biogenic origin the luoping stromatolite differs from the permian triassic boundary microbialites ptbms in having abundant filamentous structures and growing in an oxic marine environment both sedimentary facies analysis and abundant fossilized cyanobacteria may indicate proliferation of oxygenic phototrophs in a normal oxic habitat during the middle anisian early middle triassic a period when hospitable environments coupled with biotic diversification prevailed in south china and set an agenda for the full recovery of marine ecosystems in middle late anisian however the post extinction stromatolites and other anachronistic facies are not necessarily indicative of anoxic or oxic conditions and their environmental settings are much more complex than previously thought

deglacial sea level record from tahiti corals and the timing of global meltwater discharge the timing of the last deglaciation is important to our understanding of the dynamics of large ice sheets1 and their effects on the earth s surface2 3 moreover the disappearance of the glacial ice sheets was responsible for dramatic increases in freshwater fluxes to the oceans which probably disturbed the ocean s thermohaline circulation and hence global climate4 7 sea level increases bear witness to the melting of continental ice sheets but only two such records from barbados8 9 and new guinea10 11 corals have been accurately dated but these corals overlie active subduction zones where tectonic movements are large and often discontinuous especially in new guinea so the apparent sea level records may be contaminated by a complex tectonic component here we date fossil corals from tahiti which is far from plate boundaries and thus is likely to be tectonically relatively stable and remote from the locations of large former ice sheets the resulting record indicates a large sea level jump shortly before 13 800 calendar years bp which corresponds to meltwater pulse 1a in the barbados coral records8 9 the timing of this event is more accurately constrained in the tahiti record revealing that the meltwater pulse coincides with a short and intense climate cooling event12 15 that followed the initiation of the bolling allerod warm period12 16 but preceded the younger dryas cold event by about 1 000 years

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

holocene development of the belize barrier reef previously knowledge of the holocene development of the belize barrier reef bbr the largest reef system in the atlantic ocean was limited to one location carrie bow cay we present new data from 11 rotary drill cores taken at 9 locations and 36 radiometric ages that indicate that the bbr was established from 8 26 to 6 68 ky bp on pleistocene reef limestones presumably deposited during oxygen isotope stage 5 the nonsynchronous start of holocene reef growth was a consequence of variation in elevation of antecedent topography largely controlled by underlying nne trending structures from north to south pleistocene elevation decreases along these structural trends probably reflecting differential subsidence and variations in karst topography reef anatomy is characterized by three facies in order of decreasing abundance these facies are represented by corals mainly acropora palmata and members of the montastraea annularis group by unconsolidated sand and rubble and by well cemented coral grainstones rudstones holocene reef accumulation rates average 3 25 m ky the degree of reef consolidation is negatively correlated with holocene thicknesses indicating that slowly growing reefs are better cemented than fast growing ones we present a holocene sea level curve for belize based on 36 dates from this study and 33 dates from our previous studies in the area

production and cycling of natural microbial exopolymers eps within a marine stromatolite extracellular polymeric secretions eps that are produced by cyanobacteria represent potential structuring agents in the formation of marine stromatolites the abundance production and degradation of eps in the upper layers of a microbial mat forming shallow subtidal stromatolites at highborne cay bahamas were determined using 14 c tracer experiments and were integrated with measurements of other microbial community parameters the upper regions of a type 2 reid r p visscher p t decho a w stolz j bebout b macintyre i g dupraz c pinckney j paerl h prufert bebout l steppe t des marais d 2000 the role of microbes in accretion lamination and early lithification of modern marine stromatolites nature london 406 989 992 stromatolite mat exhibited a distinct layering of alternating bgreenq cyanobacteria rich layers layers 1 and 3 and bwhiteq layers layers 2 and 4 and the natural abundance of eps varied significantly depending on the mat layer the highest eps abundance occurred in layer 2 the production of new eps as estimated by the incorporation of 14 c bicarbonate into eps occurred in all layers examined with the highest production in layer 1 and during periods of photosynthesis i e daylight hours a large pool i e up to 49 of the total 14 cbicarbonate uptake was released as low molecular weight mw dissolved organic carbon doc this doc was rapidly mineralized to co2 by heterotrophic bacteria eps degradation as determined by the conversion of 14 c eps to 14 co2 was slowest in layer 2 results of slurry experiments examining o2 uptake following additions of organic substrates including eps supported this degradation trend and further demonstrated selective utilization by heterotrophs of specific monomers such as acetate ethanol and uronic acids results indicated that natural eps may be rapidly transformed post secretion by heterotrophic degradation specifically by sulfate reducing bacteria to a more refractory remnant polymer that is relatively slow to accumulate a mass balance analysis suggested that a layer specific pattern in eps and low mw doc turnover may contribute to major carbonate precipitation events within stromatolites our findings represent the first estimate of eps

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

variations in neoarchean microbialite morphologies clues to controls on microbialite morphologies through time the neoarchean carawine formation hamersley group western australia is a carbonate ramp that preserves diverse microbial structures which are characteristic of specific depositional environments these distinctive structures are distributed in five shallow subtidal and two deeper water facies in the oakover area of the carawine formation the shallow subtidal facies are composed of biohermal and bedded stromatolites centimetre scale ridge shaped microbialites and wavy laminated microbialites the deeper water facies are composed of fenestrate microbialites planar laminated dolostone and dolostone with rolled up microbial laminae microbialites in the carawine formation lie within a continuum of archean to proterozoic microbial facies some shallow water microbial facies in the carawine formation are similar to proterozoic facies such as large bioherms internally composed of a variety of stromatolite morphologies in contrast fenestrate microbialites grew in quiet subtidal environments and are common in archean rocks but have not been documented in similar proterozoic environments the similarity of shallow water facies across the archean proterozoic transition before and after the oxidation of the atmosphere and surface oceans indicates that stromatolite growth in shallow subtidal environments was not strongly affected by the chemical changes associated with oxidation of the oceans or by biological responses to those chemical changes rather stromatolite morphology was controlled mostly by the physical environment and the corresponding biological responses to that environment in contrast the absence of fenestrate microbialites from proterozoic deep subtidal environments suggests that the morphology of deep subtidal microbial structures was influenced by chemical or biological changes that occurred in association with oxidation of the surface oceans

glacial interglacial changes in southern africa compound specific δ13c land plant biomarker and pollen records from southeast atlantic continental margin sediments this is part 2 of a study examining southwest african continental margin sediments from nine sites on a north south transect from the congo fan 4 s to the cape basin 30 s representing two glacial mis 2 and 6a and two interglacial stages mis 1 and 5e contents distribution patterns and molecular stable carbon isotope signatures of long chain n alkanes c27 c33 and n alkanols c22 c32 as indicators of land plant vegetation of different biosynthetic types were correlated with concentrations and distributions of pollen taxa in sediments of the same time horizons selected single pollen type data reveal details of vegetation changes but the overall picture is best illustrated by summing pollen known to predominantly derive from c4 plants or c4 plus cam plants the c4 plant signals in the biomarkers are recorded in the 13c data and in the abundances of c31 and c33n alkanes and the c32n alkanol calculated clusters of wind trajectories for austral summer and winter situations for the holocene and the last glacial maximum afford information on the source areas for the lipids and pollen and their transport pathways to the ocean this multidisciplinary approach provides clear evidence of latitudinal differences in leaf wax lipid and pollen composition with the holocene sedimentary data paralleling the current major phytogeographic zonations the northern sites congo fan area and northern angola basin get most of their terrestrial material from the congo basin and the angolan highlands dominated by c3 plants airborne particulates derived from the western and central south african hinterland dominated by deserts semideserts and savannah regions are rich in organic matter from c4 plants as can be expected from the present and glacial positions of the phytogeographic zones the carbon isotopic signatures of n alkanes and n alkanols both become isotopically more enriched in 13c from north to south in the northern part of the transect the relative importance of c4 plant indicators is higher during the glacials than in the interglacials indicating a northward extension of arid zones favoring grass vegetation in the south where grass rich vegetation merges into semidesert and desert the difference in c4 plant indicators is small

accretion patterns in holocene tropical coral reefs do massive coral reefs in deeper water with slowly growing corals accrete faster than shallower branched coral reefs with rapidly growing corals it is a widely held concept that tropical coral reefs in shallower water with branched acroporid corals should accrete faster than those in deeper water dominated by massive corals results from a study of holocene development of the largest atlantic reef system including paleo waterdepth data challenge these concepts in belize barrier and atoll reefs reef accretion rates range from 0 46 to 7 50 m kyr and average 3 03 m kyr as measured along 33 dated reef sections interestingly accretion rates increase with increasing paleo waterdepth and sections dominated by massive corals accumulated even slightly faster than those with branched acroporids published data from some other reef locations reveal no significant trends when plotting reef accretion rate versus paleo waterdepth also indicating that the above mentioned concepts should be questioned massive corals apparently are more resistant and accrete in lower disturbance conditions in slightly deeper water 5 10 m and higher accomodation space available for sediment deposition as compared to shallow water 0 5 m branched acroporids which repeatedly get broken and leveled out during tropical cyclones

non hawaiian lithostratigraphy of louisville seamounts and the formation of high latitude oceanic islands and guyots guyots are large seamounts with a flat summit that is generally believed to form due to constructional biogenic and or erosional processes during the formation of volcanic islands however despite their large abundance in the oceans there are still very few direct constraints on the nature and formation of guyots in particular those formed at high latitude that lack a thick cap of shallow marine carbonate rocks it is largely accepted based on geophysical constraints and surficial observations sampling that the summit platform of these guyots is shaped by wave abrasion during post volcanic subsidence of volcanic islands here we provide novel constraints on this hypothesis and the summit geology of guyots with a lithostratigraphic analysis of cores from three louisville seamounts south pacific collected during expedition 330 of the integrated ocean drilling program iodp thirteen lithofacies of sedimentary and volcanic deposits are described which include facies not previously recognized on the top of guyots and offer a new insight into the formation of high latitude oceanic islands on a fast moving plate our results reveal that the lithostratigraphy of louisville seamounts preserves a very consistent record of the formation and drowning of volcanic islands with from bottom to top i volcaniclastic sequences with abundant lava fed delta deposits ii submarine to subaerial shield lava flows iii post volcanic shallow to deeper marine sedimentary rocks lacking thick reef deposits iv post erosional rejuvenated volcanic rocks and v pelagic sediments recognition of erosional boundaries between subaerial lava flows and shallow marine sedimentary rocks provides novel support for post volcanic wave planation of guyots however the summit geology of louisville seamounts is dissimilar to that of high latitude hawaiian emperor guyots that have emplaced in a similar tectonic and environmental setting and that include thicker lava stacks with apparently little lava fed delta deposits to explain observed lithostratigraphic discrepancy we propose that louisville seamounts represent a distinct type of intraplate ocean volcano characterized by formation of a smaller island with a central shield volcano surrounded by extended shallow marine shelves formed by lava fed deltas in this interpretation the summit platform of louisville type guyots results from early syn volcanic subaerial to shallow marine constructional volcanic processes and marine erosion enhanced by later post volcanic wave planation this contrasts with larger hawaiian edifices that are capped by thicker shield volcanoes and that develop an extended wave planation surface during post volcanic subsidence in the absence of efficient coral growth the difference between hawaiian and louisville type volcanic islands and guyots can be explained by contrasted dynamic disequilibrium between magmatic growth erosion and subsidence during the island building stage unlike hawaiian type volcanoes louisville seamounts are characterized by alkaline magmatism that extends from the late seamount to island stages this supports more limited magmatic growth during the formation of louisville islands and we hypothesize that this promotes the formation of ephemeral shallow marine platforms and extended lava fed deltas hawaiian type volcanoes and guyots are unusually large in the population of intraplate ocean volcanoes louisville type guyots as defined in this study could therefore represent a very common but yet poorly documented mode of oceanic island formation in the pacific ocean and other similar fast moving plate settings

the belize margin revisited 2 origin of holocene antecedent topography the importance of antecedent topography in dictating holocene facies patterns has been generally recognized there is however disagreement as to origin or lithology of the antecedent topography particularly with respect to the siliciclastic or carbonate nature of the underlying topography and structural patterns to help resolve these problems published and unpublished information have been compiled to produce a structural fabric map of onshore and offshore belize that includes a new geologic map of the country the map along with illustrated seismic lines demonstrates the occurrence of a number of nne trending transpressional faults in which landward directed thrusting is consistently displayed along with tectonic inversion offshore wells in conjunction with the seismic lines document the inversion as post eocene suggesting a similar age for transpressional fault movement presumably the landward directed thrusting reflects the opposing force of eastward directed subduction along the western margin of central america relative to the westward seafloor spreading of the caribbean cayman ridge the belize faults show little current seismicity but nevertheless the resulting structures have affected quaternary carbonate deposition as evidenced on an illustrated seismic line by both seaward and landward bi directional progradation of the reef margin from an underlying structural high the structural influence on the development of holocene antecedent topography is further suggested by the occurrence of a bouguer gravity plateau in the same shelf area that marks the occurrence of the belize lagoon rhomboid shelf atolls the youngest documented lithology of 12 illustrated offshore exploration wells is miocene carbonate in some wells the carbonate is interpreted as extending into the pliocene and holocene although no age diagnostic criteria are in evidence in other wells siliciclastics of unknown age and thickness are identified as overlying neogene carbonates the regional distribution and age of onshore limestones suggests that unroofing of the cretaceous carbonate cap of the maya mountains siliciclastic sediment source did not occur until late in neogene time perhaps no earlier than late pliocene consequently the maya mountains could not have been a major offshore source of siliciclastics until the quaternary information on the lithology of the immediately underlying pre holocene is provided by limited penetration core data and shallow resolution seismic lines these show that antecedent pleistocene limestones beneath the holocene reefs were deposited around 130 000 b p isotope stage 5e the thickness of the overlying holocene shelf margin reef capped carbonates increases along depositional strike from a few meters in the north to more than 25 m in the south in contrast piston cores and seismic data from the southern shelf lagoon collectively document the occurrence of antecedent siliciclastic topography the southward dip of both carbonate and siliciclastic antecedent surfaces is presumably a reflection of increasing southerly subsidence increasing antecedent erosional relief in the same direction reflects the pronounced southerly increase in paleo rainfall that presumably paralleled that of present day rainfall

flow behavior in a dual fracture network a model that incorporates a pseudo random process controlled by mechanical rules of fracturing is used to generate 3d orthogonal joint networks in tabular stratified aquifers the results presented here assume that two sets of fractures each with different conductivities coexist this is the case in many aquifers or petroleum reservoirs that contain sets of fractures with distinct hydraulic properties related to each direction of fracturing constant rate pump tests from partially penetrating wells are simulated in synthetic networks the transient head response is analyzed using the type curve approach and plots as a function of time of pressure propagation in the synthetic network are shown the hydrodynamic response can result in a pressure transient that is similar to a dual porosity behavior even though such an assumption was not made a priori we show in this paper that this dual porosity like flow behavior is in fact related to the major role of the network connectivity especially around the well and to the aperture contrast between the different families of fractures that especially affects the earlier hydrodynamic response flow characteristics that may be interpreted as a dual porosity flow behavior are thus related to a lateral heterogeneity large fracture or small fault accordingly when a dual porosity model matches well test data the resulting reservoir parameters can be erroneous because of the model assumptions basis that are not necessarily verified finally it is shown both on simulated data and well test data that such confusion in the interpretation of the flow behavior can easily occur well test data from a single well must therefore be used cautiously to assess the flow properties of fractured reservoirs with lateral heterogeneities such as large fractures or small faults

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

deposition and diagenesis of marine oncoids implications for development of carbonate porosity abstract microbially mediated carbonates form in numerous environments and produce a range of lithologic fabrics and textures while the environmental and biological factors controlling microbial carbonate precipitation are becoming more established the influence of precursor microbial textures on diagenetic processes is less understood here we describe microbial oncolites from mare new caledonia to assess their formation textures and subsequent diagenetic alteration calcified filaments and sheets are interpreted as microbial mat constituents bacterial sheaths and eps sheets within a constructive micritic cortex of uniform crystal size 5 10 m and shape formed around a nucleus grain thin section analyses show trapped and bound grain inclusions and irregular laminations within the cortex also consistent with a microbial origin the combination of a dense micrite cortex subrounded shape and irregular laminations are consistent with oncoid formation under periodic wave agitation in a back reef setting the mare oncolites also display an unexpected pseudomorphic stabilization of the nucleus grains and cortex likely due to the original mineralogy and low permeability of the cortex micrite to our knowledge this is the first example of microbial coatings leading to pseudomorphic stabilization of aragonite and high mg calcite skeletal grains this textural preservation lies in stark contrast to the extensive dissolution and moldic porosity found outside the oncolite beds and common to many tropical shallow water carbonates these findings have implications for development of meteoric porosity the interpretation of 13c values in marine carbonates and potential controls on fluid flow

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

postglacial growth history of a french polynesian barrier reef tract tahiti central pacific bthe internal structure and growth pattern of tahiti reefs over the last 14 ka is reconstructed using sedimentological morphological and palaeobiological data coupled with radiometric dates in drill cores through the modern barrier reef flooding of the volcaniclastic deposits or the karst surface of a pleistocene reef started at 14 ka bp and coral growth began shortly after inundation the sequence in the tahiti barrier reef edge has formed predominantly through long term keep up growth controlled by stable environmental conditions while the adjacent backreef deposits did not start to accumulate before sea level stabilization around 6 ka the dominance of porites communities and the coeval occurrence of branching gracile lithophyllum in the lowermost part of the postglacial reef sequence 14 11 ka suggest the prevalence of uniformly moderate to low energy conditions and or growth in slightly deeper waters all over the drflled area during the early reef stages during the last 11 ka the reef frameworks developed in a high energy environment at maximum water depths of 5 6 m and were dominated by an acropora robusta danaihydrolithon onkodes association the local interlayering of other coralgal assemblages dominated by tabular acropora or domal porites reflects distinct diversification stages resulting either from the palaeotopographic control of the substrate or from slight and episodic environmental changes

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 cretaceous maastrichtian shallow water hydrocarbon seeps from snow hill and seymour islands james ross basin antarctica fossil hydrocarbon seeps are present in latest cretaceous maastrichtian volcaniclastic shallow shelf sediments exposed on snow hill and seymour islands james ross basin antarctica the seeps occur in the snow hill island formation on snow hill island and are manifest as large sized cement rich carbonate bodies containing abundant thyasirid bivalves and rarer ammonites and solemyid bivalves these bodies have typical seep cement phases with 13c values between 20 4 and 10 7 and contain molecular fossils indicative of terrigenous organic material and the micro organisms involved in the anaerobic oxidation of methane including methanotrophic archaea and sulphate reducing bacteria on seymour island the seeps occur as micrite cemented burrow systems in the lopez de bertodano formation and are associated with thyasirid solemyid and lucinid bivalves and background molluscan taxa the cemented burrows also have typical seep cement phases with 13c values between 58 0 and 24 6 there is evidence from other data that hydrocarbon seepage was a common feature in the james ross basin throughout the maastrichtian and into the eocene the snow hill and seymour island examples comprise the third known area of maastrichtian hydrocarbon seepage but compared to most other ancient and modern seep communities the james ross basin seep fauna is of very low diversity being dominated by infaunal bivalves all of which probably had thiotrophic chemosymbionts but which were unlikely to have been seep obligates absent from the james ross basin seep fauna are typical obligate seep taxa from the cretaceous and the cenozoic reasons for this may have been temporal palaeolatitudinal palaeobathymetric or palaeoecological

calcification of cyanobacterial mats in solar lake sinai pore water samples were obtained from the shallow water part of solar lake sinai where luxurious cyanobacterial mats grow these samples were analyzed for ca sup 2 mg sup 2 sr sup 2 cl sup so sub 4 sup 2 and titration alkalinity ta to determine the role of cyanobacterial growth and degradation on the calcification of the mats the data are modeled thermodynamically to predict mineral pore water equilibria the data support earlier bacterial and sedimentological studies suggesting that the degradation of the cyanobacterial mat via sulfate reduction is of major importance in the calcification process 34 references 1 figure 2 tables

new observations on mid plate volcanism and the tectonic history of the pacific plate tahiti to easter microplate abstract we describe the geology and tectonics of a continuous swathe of seafloor between tahiti and the western edge of the easter microplate imaged by gloria and sea beam on two separate cruise transits in 1987 and 1988 the data reveal that mid plate volcanism is common in this region even on deep seafloor hundreds of kilometres from major lines of seamounts and islands this supports the idea of a thin weak lithosphere over the pacific superswell and the idea that the tops of major mantle plumes may spread out over diameters of the order of 1000 km the mid plate volcanism occurs in two distinct forms over most of our traverse it appears as fields of relatively young and acoustically strongly backscattering lava flows often accompanied by groups of numerous small circular volcanoes east of 122 w about chron 5a however we observed a distinct form major sharp crested constructional volcanic ridges many tens of kilometres long individually trending ene but lying en echelon along an e w regional trend these ridges appear morphologically identical to the cross grain ridges seen elsewhere in the pacific we attribute their formation to magma supplied from the regionally hot mantle leaking along tectonic lines of weakness however although these ridges are parallel to fracture zone trends seen farther west they are morphologically very different from any known fracture zone moreover individual ridges are somewhat oblique to the tectonic spreading fabric around them and so do not seem to follow actual fracture zone traces the whole line of en echelon ridges lies along part of the predicted trace of fracture zone 2 of okal and cazenave 15 and is probably its morphological expression however nowhere did we see a convincing conventional fracture zone trace in or following the predicted position or orientation we suggest instead that magma from an independent source has used lines of weakness along minor fracture zones to produce these en echelon features the austral fracture zone is the only major fracture zone crossed in our transit and here is characterised by four fossil transform strands its marked position on the aapg and gebco maps is found to be in error finally we found that the expected change from nnw to nne trending spreading fabric at chron 6c did not occur in a clear cut way as predicted by earlier tectonic histories of the pacific instead the post chron 6c fabric oscillates in a confused way between nne and nnw suggesting to us that this area has been characterised by an unstable plate boundary probably associated with a succession of propagating rifts or microplates from chron 6c to the present

late early triassic microbial reefs of the western united states a description and model for their deposition in the aftermath of the end permian mass extinction abstract research in the western united states to reinvestigate and characterize the previously reported stromatolites of the latest early triassic spathian virgin limestone member of the moenkopi formation has shown that these microbial build ups formed patch reef mounds detailed field analysis of these reef mounds and the surrounding beds has established that they attained 1 2 m of topographic relief above the seafloor a petrographic study of these microbial mounds illustrates that they consisted of microbial fabrics as well as voids with early marine cements and formed in the absence of in situ metazoans these reef mounds formed during the aftermath of the end permian mass extinction that spanned the early triassic due to the devastation of marine metazoans at this extinction the early triassic has often been deemed a reef gap because no metazoan reefs are found at this time whereas colonial metazoan reefs are globally absent from lower triassic strata microbial build ups similar to these described from the western united states have been found in south china armenia southern turkey and greenland a regional early permian to late early triassic unconformity exists in the study region however analysis of these microbial reefs within a global context suggests that conditions favouring microbial reef development must have existed as long as 4 8 m y after the end permian mass extinction a depositional model of these microbial mounds indicates that flooding of the shelf with anoxic and or co2 rich deep waters may have fostered their occurrence

microbialite formation in seawater of increased alkalinity satonda crater lake indonesia abstract the crater lake of the small volcanic island satonda indonesia is unique for its red algal microbial reefs thriving in marine derived water of increased alkalinity the lake is a potential analogue for ancient oceans sustaining microbialites under open marine conditions current reef surfaces are dominated by living red algae covered by non calcified biofilms with scattered cyanobacteria and diatoms minor caco3 precipitates are restricted to the seasonally flooded reef tops which develop biofilms up to 500 m thick dominated by the cyanobacteria pleurocapsa calothrix phormidium and hyella microcrystalline aragonite patches form within the biofilm mucilage and fibrous aragonite cements grow in exopolymer poor spaces such as the inside of dead lysed green algal cells and reef framework voids cementation of lysed hadromerid sponge resting bodies results in the formation of wetheredella like structures hydrochemistry data and model calculations indicate that co2 degassing after seasonal mixis can shift the carbonate equilibrium to cause caco3 precipitation increased concentrations of dissolved inorganic carbon limit the ability of autotrophic biofilm microorganisms to shift the carbonate equilibrium therefore photosynthesis induced cyanobacterial calcification does not occur instead passive diffusion controlled eps mediated permineralization of biofilm mucus at contact with the considerably supersaturated open lake water takes place in contrast to extreme soda lakes the release of ca2 from aerobic degradation of extracellular polymeric substances does not support caco3 precipitation in satonda because the simultaneously released co2 is insufficiently buffered subfossil reef parts comprise green algal tufts encrusted by microstromatolites with layers of fibrous aragonite and an amorphous unidentified mg si phase the microstromatolites probably formed when lake satonda evolved from seawater to ca2 depleted raised alkalinity conditions because of sulfate reduction in bottom sediments and pronounced seasonality with deep mixing events and strong co2 degassing the latter effect caused rapid growth of fibrous aragonite while mg si layers replaced the initially mg calcite impregnated biofilms this could be explained by dissolution of siliceous diatoms and sponge spicules at high ph followed by mg calcite dissolution and mg silica precipitation at low ph due to heterotrophic activity within the entombed biofilms

paleogeographic trends in late triassic reef ecology from northeastern panthalassa the late triassic was a pivotal period in reef evolution but the majority of information about reef ecology during this time comes from buildups in the alps e g the tethys ocean recent studies of reefs in western north america have recognized unique ecologies along the eastern margin of the panthalassa ocean although there are numerous twenty five localities with putative reef builders only four buildups had syndepositional relief and a rigid framework i e true reefs the most paleo northern true reefs were microbial patch reefs with only a few large skeletal bioconstructors hypercalcified sponges and spongiomorphs built the mid latitude reefs with secondary microbial encrustation and branching phaceloid retiophyllia corals corals are the primary bioconstructors in panthalassa s most paleo equatorial reefs and calcareous microbes are sparse when all reefal deposits are analyzed the n s gradient is also present with microbial and bivalve deposits in the north sponge coral deposits in the mid latitudes and coral deposits near the equator this ecological gradient is not apparent in the tethys ocean tethyan reefs thrived in oligotrophic tropical waters without strong latitudinal gradients by contrast paleoceanographic considerations suggest that cool nutrient rich waters swept south along the western north american borderlands in the late triassic the eastern boundary current is interpreted to have created a strong north south differentiation of environments in northeastern ne panthalassa that was manifested in both the biotic and abiotic characteristics of eastern panthalassic reefs reefs from equatorial panthalassa are similar to tethyan reefs warm water photozoan coral reef structures whereas higher paleo latitude reefs from panthalassa are interpreted as cool water heterozoan buildups with abundant calcareous microbes diminutive biocalcifiers and few large framework building corals

microbialites in a modern lagoonal environment nature and distribution tikehau atoll french polynesia abstract microbialites including stromatolitic structures gelatinous masses and mats develop in tikehau lagoon tuamotu french polynesia on the flanks and slopes of numerous pinnacles and motu islets between 0 and 25 m deep on coral colonies algal tufts or on lagoonal sediments individual stromatolitic structures consist of hemispherical domes and are produced by monospecific populations of filamentous cyanobacteria belonging to three distinctive phormidium species symploca and possibly to one new species of schizothrix gelatinous masses hanging from the ceiling of caverns are produced by phormidium laysanense microbial mats are well developed at all depths on pinnacle slopes and are produced by either monospecific or mixed cyanobacterial populations the study of micro and ultrastructure of stromatolites reveals that they result both from trapping and binding of sediments and from carbonate precipitation in addition to phototrophic biomineralization pervasive carbonate precipitation occurred in association with decaying organic matter the basic constructional elements of the precipitated carbonate consist of 0 1 0 2 m rounded bodies that form very fine anhedral to subhedral micrite their surfaces comprise extremely small carbonate granules 20 50 nm gradual changes ageing in the ultrastructure of carbonate precipitates include the precipitation of successive euhedral calcite layers around these grains widespread development of microbialites in this pristine lagoonal environment results from rapid blooms of microbial benthic communities possibly induced by temporary and local nutrient pulses nutrients seem to undergo a restricted circulation between the sediment and pinnacle reef frameworks pinnacles and reef islets are repeatedly affected by catastrophic storm events which cause redistribution of sediments and nutrients

molecular and isotopic abundances of long chain n fatty acids in open marine sediments of the western north pacific abstract molecular abundance and compound specific carbon isotope ratios of long chain n fatty acids lcfas ranging from c20 to c30 are reported for open marine sediments of the western north pacific a positive correlation r2 0 94 is observed between total organic carbon toc and lcfa concentrations although the toc concentrations varied from 0 34 to 2 62 wt in dry sediment the relatively uniform ratio of lcfas to toc 0 74 0 12 mg gc is similar to that of the central pacific reported by ohkouchi et al ohkouchi n kawamura k kawahata h taira a 1997 latitudinal distributions of terrestrial biomarkers in the sediments from the central pacific geochim cosmochim acta 61 1911 1918 and in a core sample from the tropical central pacific by prahl et al prahl f g muehlhausen l a lyle m 1989 an organic geochemical assessment of oceanographic conditions at manop site c over the past 26 000 years paleoceanography 4 495 510 on the other hand lcfa concentrations in riverine and estuarine sediments are apparently higher than in open marine sediments carbon isotopic compositions of individual lcfas in open marine sediments are similar 26 1 for n c24 and n c26 being more enriched in 13 c than bay and riverine sediments by up to 6 terrestrial c3 higher plants commonly have a 13 c value of 26 in bulk organic carbon and because lipid components including fatty acids are known to be depleted in 13 c by several per mil compared to bulk organic matter this study indicates that the open marine sedimentary lcfas are not derived from normal terrestrial c3 higher plants rather the isotopic composition is consistent with a mixed source of c4 and c3 terrestrial higher plants and or marine organisms low carbon preference index cpi values of the lcfas and the isotopic difference between the northern and southern samples suggest that the lcfas of this study are likely to be related to marine primary productivity even though lcfas have often been used to infer a terrestrial higher plant input to marine sediments

evidence for tropical south pacific climate change during the younger dryas and the bolling allerod from geochemical records of fossil tahiti corals abstract we present monthly resolved records of strontium calcium sr ca and oxygen isotope 18o ratios from well preserved fossil corals drilled during the integrated ocean drilling program iodp expedition 310 tahiti sea level and reconstruct sea surface conditions in the central tropical south pacific ocean during two time windows of the last deglaciation the two tahiti corals examined here are uranium thorium u th dated at 12 4 and 14 2 ka which correspond to the younger dryas yd cold reversal and the bolling allerod b a warming of the northern hemisphere respectively the coral sr ca records indicate that annual average sea surface temperature sst was 2 6 3 1 c lower at 12 4 ka and 1 0 1 6 c lower at 14 2 ka relative to the present with no significant changes in the amplitude of the seasonal sst cycle these cooler conditions were accompanied by seawater 18o 18osw values higher by 0 8 and 0 6 relative to the present at 12 4 and 14 2 ka respectively implying more saline conditions in the surface waters along with previously published coral sr ca records from the island cohen and hart 2004 deglacial sea surface temperatures of the western tropical pacific a new look at old coral paleoceanography 19 pa4031 doi 10 1029 2004pa001084 our new tahiti coral records suggest that a shift toward lower sst by 1 5 c occurred from 13 1 to 12 4 ka which was probably associated with a shift toward higher 18osw by 0 2 along with a previously published coral sr ca record from vanuatu correge et al 2004 interdecadal variation in the extent of south pacific tropical waters during the younger dyras event nature 428 927 929 the tahiti coral records provide new evidence for a pronounced cooling of the western to central tropical south pacific during the northern hemisphere yd event

bioerosion traces in the great barrier reef over the past 10 to 30 kyr abstract bioerosion traces preserved in coral reef framework provide insight into past environmental conditions and reef health however few studies have explored the relationship between bioerosion reef growth history and environmental changes the international ocean drilling program iodp expedition 325 sampled late glacial to deglacial reef sequences that developed from 30 to 10 ka on the shelf edge adjacent to the modern great barrier reef gbr this study quantifies spatiotemporal patterns of macrobioerosion intensity macrobioerosion intensity referred to as bi of trace ichnogenera entobia sponges gastrochaenolites bivalves and maeandropolydora trypanites worms based on detailed observations of 13 drill holes from two cross shelf transects in two regions hydrographers passage in the southern central gbr and noggin pass in the north our high resolution stratigraphic and temporal records of bioerosion activity are defined and compared to published multi proxy paleoenvironmental interpretations from the exp 325 cores coralgal assemblages geochronology and lithology are used to understand the controls on the multi scale spatiotemporal patterns of bioerosion across and along the gbr shelf and within individual holes noggin pass is characterized by significantly higher bioerosion activity than hydrographers at each transect changes in bioerosion intensity are observed to be coeval with some major reef sequence boundaries however due to the unconsolidated nature and limited recovery many boundaries have inconclusive trends statistical analysis is used to interpret the relationship of bioerosion intensity to various factors including paleo water depth lithology coral and algal assemblages transect location geomorphic feature vertical accretion and reef sequences terrigenous sediment flushing depositional regime and a pronounced latitudinal gradient in sea surface temperature are likely to have contributed to the significant regional differences in bioerosion intensity between northern and southern transects and also during reef demise bioerosion at both transects is highest in the proto barrier reef 4 sites when compared to all fringing terrace reefs reef 3a 3b and 4 higher bioerosion tends to occur when vertical accretion is at its extremes its lowest and highest limits the enhanced bioerosion alongside lowest reef accretion in a sea level rise scenario reef 3a 3b and 4 might have contributed to reef degradation prior to its demise

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

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

microbial carbonate abundance compared with fluctuations in metazoan diversity over geological time abstract secular variation in microbial carbonate abundance may be reflected by stromatolite morphotype diversity and reefal microbial carbonate abundance these datasets reveal long term changes over the past 3000 myr that include a peak of abundance 1250 myr ago late proterozoic decline cambrian resurgence and fluctuating decline during the remainder of the phanerozoic it is conceivable that proterozoic metazoan diversification coincided with inception of stromatolite decline 1250 myr ago but microbial carbonate increase during cambrian metazoan radiation together with failure of microbial carbonates to increase in the aftermaths of the end ordovician end triassic and end cretaceous mass extinctions suggest that factors in addition to metazoan competition significantly influenced long term changes in microbial carbonate abundance

microscale observations of sulfate reduction correlation of microbial activity with lithified micritic laminae in modern marine stromatolites we report for the first time micrometer scale correlation of geologic and microbial processes in modern marine stromatolites precipitation of micritic laminae in these stromatolites was studied by comparing microstructure as observed in petrographic thin sections with microbial sulfatereduction activity two dimensional mapping of sulfate reduction rates was implemented by incubating a vertical section of a stromatolite face on silver foil coated with 35 so 4 2 our results show that sulfate reduction activity is high in zones of caco 3 precipitation and indicate that microbial activity produces lithified micritic laminae near the surface of the stromatolites similarities with micritic laminae in ancient stromatolites suggest that sulfate reduction may also have been an important mechanism of carbonate precipitation in these fossilized structures

stromatolites in precambrian carbonates evolutionary mileposts or environmental dipsticks stromatolites are attached lithified sedimentary growth structures accretionary away from a point or limited surface of initiation though the accretion process is commonly regarded to result from the sediment trapping or precipitation inducing activities of microbial mats little evidence of this process is preserved in most precambrian stromatolites the successful study and interpretation of stromatolites requires a process based approach oriented toward deconvolving the replacement textures of ancient stromatolites the effects of diagenetic recrystallization first must be accounted for followed by analysis of lamination textures and deduction of possible accretion mechanisms accretion hypotheses can be tested using numerical simulations based on modern stromatolite growth processes application of this approach has shown that stromatolites were originally formed largely through in situ precipitation of laminae during archean and older proterozoic times but that younger proterozoic stromatolites grew largely through the accretion of carbonate sediments most likely through the physical process of microbial trapping and binding this trend most likely reflects long term evolution of the earth s environment rather than microbial communities

role of autochthonous versus detrital micrite in depositional geometries of middle triassic carbonate platform systems middle triassic platforms formazione di contrin upper anisian in the italian dolomites southern alps record large changes in carbonate production and depositional geometry the changes include variation in the abundance of detrital micrite i e allochthonous calcareous mud relative to the autochthonous micrite which formed in situ and was syndepositionally lithified carbonate factory dynamics controlled the geometry of the clinostratified slope units and were probably associated with changes in oxygenation early anisian low relief platforms were followed by late anisian high relief buildups associated with basinal dysoxic anoxic environments the geometric evolution of the platform slopes records a progressive increase in the dip angle of clinostratifications matched by a deepening evolution of the basinal environments at the same time the slope sediments record a gradual change from loose detrital micrite characterizing the lower portion to autochthonous micrite dominating the upper part and recording massive syndepositional lithification the development of the autochthonous micrite was associated with the preservation of significant amounts of organic matter this sharp increase in automicrite formation was probably induced by a rapid change from oxic to suboxic conditions in the carbonate slope and margin environments while anoxic conditions developed in the adjacent organic rich basin moena formation the low oxygen level promoted the preservation of organic matter and the activity of sulfate reducing bacteria which in turn induced in situ deposition of autochthonous micrite through biomediated processes this pervasive early cementation and lithification induced the development of steep platform slopes the conceptual model distilled from this middle triassic case can support the interpretation of analogous buildups where the skeletal framework is subordinate to the micrite component

isotopic fingerprints of microbial respiration in aragonite from bahamian stromatolites authigenic aragonite preserves a carbon isotopic record of heterotrophic microbial influences on dissolved inorganic carbon dic in microenvironments within shallow subtidal stromatolites from highborne cay bahamas a greater amount of aragonite precipitates when and where respiration rather than photosynthesis influences local dic which is consistent with sulfate reduction promoting carbonate precipitation and calcium release during decay of exopolymeric substances thus heterotrophs play a more direct role than phototrophs in stromatolite lithification cyanobacteria are spatially associated with aragonite containing heterotrophic isotopic signatures hence the absence of an autotrophic isotopic signature in the rock record does not imply the absence of photosynthetic organisms

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

hypersulfidic deep biosphere indicates extreme sulfur isotope fractionation during single step microbial sulfate reduction coexisting dissolved sulfide and sulfate from hypersulfidic interstitial waters of a 380 m long sediment core show a large isotopic difference of up to 72 caused by in situ microbial sulfate reduction this is considerably larger than the assumed biological maximum of 46 derived from laboratory studies with pure cultures of sulfate reducing bacteria similar high fractionations inferred from sedimentary metal sulfides have been previously explained by a multistage process involving sulfide reoxidation and disproportionation of sulfur intermediates our data show that extreme isotopic differences between sulfate and the reduced sulfur species can also be generated during microbial single step fractionation this result indicates that the sulfate reducing communities and or their cellular metabolic activities in the deep biosphere may differ from those observed in near surface sediments or the water column

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

molecular and isotopic analysis of anaerobic methane oxidizing communities in marine sediments convergent lines of molecular carbon isotopic and phylogenetic evidence have previously indicated hinrichs k u hayes j m sylva s p brewer p g delong e f 1999 methane consuming archaebacteria in marine sediments nature 398 802 805 that archaea are involved in the anaerobic oxidation of methane in sediments from the eel river basin offshore northern california now further studies of those same sediments and of sediments from a methane seep in the santa barbara basin have confirmed and extended those results mass spectrometric and chromatographic analyses of an authentic standard of sn 2 hydroxyarchaeol hydroxylated at c 3 in the sn 2 phytanyl moiety have confirmed our previous tentative identification of this compound but shown that the previously examined product was the mono tms rather than di tms derivative further analyses of 13 c depleted lipids appreciably more abundant in samples from the santa barbara basin have shown that the archaeal lipids are accompanied by two sets of products that are only slightly less depleted in 13 c these are additional glycerol ethers and fatty acids the alkyl substituents in the ethers mostly monoethers with some diethers are non isoprenoidal the carbon number distributions and isotopic compositions of the alkyl substituents and of the fatty acids are similar suggesting strongly that they are produced by the same organisms their structures n alkyl and methyl branched n alkyl require a bacterial rather than archaeal source the non isoprenoidal glycerol ethers are novel constituents in marine sediments but have been previously reported in thermophilic sulfate and nitrate reducing organisms which lie near the base of the rrna based phylogenetic tree based on previous observations that the anaerobic oxidation of methane involves a net transfer of electrons from methane to sulfate it appears likely that the non archaeal 13 c depleted lipids are products of one or more previously unknown sulfate reducing bacteria which grow syntrophically with the methane utilizing archaea their products account for 50 of the fatty acids in the sample from the santa barbara basin at all methane seep sites examined the preservation of aquatic products is apparently enhanced because the methane oxidizing consortium utilizes much of the sulfate that would otherwise be available for remineralization of materials from the water column

geobiology of microbial carbonates metazoan and seawater saturation state influences on secular trends during the phanerozoic abstract microbial carbonates are long ranging essentially bacterial aquatic sediments their calcification is dependent on ambient water chemistry and their growth is influenced by competition with other organisms such as metazoans in this paper these relationships are examined by comparing the geological record of microbial carbonates with metazoan history and secular variations in caco3 saturation state of seawater marine abundance data show that microbial carbonates episodically declined during the phanerozoic eon past 545 myr from a peak 500 myr ago this abundance trend is generally inverse to that of marine metazoan taxonomic diversity supporting the view that metazoan competition has progressively limited the formation of microbial carbonates lack of empirical values concerning variables such as seawater ionic composition atmospheric partial pressure of co2 and ph currently restricts calculation of caco3 saturation state for the phanerozoic as a whole to the use of modeled values these data together with palaeotemperature data from oxygen isotope analyses allow calculation of seawater caco3 saturation trends microbial carbonate abundance shows broad positive correspondence with calculated seawater saturation state for caco3 minerals during the interval 150 545 myr ago consistent with the likelihood that seawater chemistry has influenced the calcification and therefore accretion and preservation of microbial carbonates these comparisons suggest that both metazoan influence and seawater saturation state have combined to determine the broad pattern of marine microbial carbonate abundance throughout much of the phanerozoic in contrast for the major part of the precambrian it would seem reasonable to expect that seawater saturation state together with microbial evolution was the principal factor determining microbial carbonate development interrelationships such as these with feedbacks influencing organisms sediments and the environment are central to geobiology

high isotope fractionations during sulfate reduction in a low sulfate euxinic ocean analog a detailed record of the early earth sulfur s cycle is chronicled by the s isotope values of sulfide and sulfate preserved in the rock record interpretation of this record rests on our understanding of sulfur cycling in modern systems experiments and the resulting isotopic signatures very large fractionations in 34s of 70 are commonly measured between sulfide and sulfate in modern systems and in ancient sediments theoretical calculations suggest that sulfate reducing prokaryotes are capable of producing such large fractionations during the reduction of sulfate to sulfide although they have only been demonstrated to generate fractionations up to 48 here we report the first direct determination of 60 70 fractionations by natural populations of sulfate reducers these high fractionations occur under the relatively low sulfate conditions 1 1 2 mm of meromictic lago di cadagno in switzerland the major and minor isotopic composition of sulfide and sulfate in the lake water is consistent with sulfide produced by sulfate reduction with little evidence for modification by further oxidative sulfur cycling these observations help us to constrain the evolution of seawater sulfate concentrations

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