variability of the north atlantic thermohaline circulation during the last interglacial period studies of natural climate variability are essential for evaluating its future evolution greenland ice cores suggest that the modern warm period the holocene has been relatively stable for the past 9 000 years much less is known about other warm interglacial periods which comprise less than 10 of the climate record during the past 2 5 million years here we present high resolution ocean sediment records of surface and deep water variables from the bermuda rise spanning the last interglacial period about 118 000 127 000 years ago in general deep water chemical changes are coincident with transitions in surface climate at this site the records do not show any substantial fluctuations relative to the much higher variability observed during the preceding and subsequent cool climates the relatively stable interglacial period begins and ends with abrupt changes in deep water flow we estimate using 230 th measurements to constrain the chronology that transitions occur in less than 400 years

climate instability during the last interglacial period recorded in the grip ice core isotope and chemical analyses of the grip ice core from summit central greenland reveal that climate in greenland during the last interglacial period was characterized by a series of severe cold periods which began extremely rapidly and lasted from decades to centuries as the last interglacial seems to have been slightly warmer than the present one its unstable climate raises questions about the effects of future global warming

diversity of iron and silica precipitation by microbial mats in hydrothermal waters iceland implications for precambrian iron formations direct examination of microbial mats from icelandic hot springs with transmission electron microscopy and energy dispersive x ray spectroscopy revealed a consortium of bacterial cells in varying stages of mineralization differences in observed mineralogy largelyreflectdifferencesinthechemistryofthehydrothermalwaters silica richspheroids formedepicellularlyoncellwallsandsurroundingsheathsandcapsulesofmicroorganisms and insomecases intracellularlywhenpresumablythecell s hadlysed commonly these precipitateswereobservedcoalescingtoformamatrixofamorphoussilicathatcompletely encapsulated the cells and or replaced their cytoplasmic material however in other cells the precipitates were composed of amorphous granules made exclusively of iron and silica inapproximatelyequalproportions atonelocality thebacteriaformedseveralepicellular iron minerals ranging from iron mineralized capsules tofine grained spheroids of amorphous ferric hydroxide and acicular aggregates of goethite the complete encrustation of bacterial cells by silica iron or a combination of both may greatly enhance their preservation potential such that these mineralized microorganisms may conceivably represent future microfossils thus we may be witnessing contemporaneous biomineralization processes that are similar to those of the geologic past particularly with regard to the origin of some precambrian banded iron formations

new age determinations for the eocene oligocene boundary sediments in the fayum depression northern egypt the fayum depression of northern egypt contains the richest collection of fossil vertebrates known from the afro arabian paleogene this fauna has been collected for over a century and yet the sediments containing the fossil localities have remained undated paleomagnetic sampling was carried out through the fossil bearing jebel qatrani formation and the underlying dir abu lifa member of the qasr el sagha foundation to determine a correlation between the paleomagnetic reversal stratigraphy and the geomagnetic reversal time scale grts a new date of 23 64 pm 0 035 ma was determined for the overlying widan el faras basalt by the laser incremental heating 40 ar 39 ar dating method the results of this study indicate that the majority of the vertebrate fossil localities occur in chrons c13 c15r late eocene to early oligocene in age these age determinations provide a minimum date for the origin of higher primates

nonuniform response of the major surface currents in the nordic seas to insolation forcing implications for the holocene climate variability 1 high resolution sediment cores from the voring plateau the north iceland shelf and the east greenland shelf have been studied to investigate the stability of major surface currents in the nordic seas during the holocene results from diatom assemblages and reconstructed sea surface temperatures ssts indicate a division of the holocene into three periods the holocene climate optimum 9500 6500 calendar cal years bp the holocene transition period 6500 3000 cal years bp and the cool late holocene period 3000 0 cal years bp the overall climate development is in step with the decreasing insolation on the northern hemisphere but regional differences occur regarding both timing and magnitude of sst changes sites under the direct influence of the norwegian atlantic current and the irminger current indicate sst cooling of 4 5 c from early holocene to present compared to 2 c recorded under the east greenland current superimposed on the general holocene cooling trend there is a high frequency sst variability which is in the order of 1 1 5 c for the voring plateau and the east greenland shelf and 2 5 3 c on the north iceland shelf

climatic bisection of the last interglacial warm period in the polar north atlantic new multiproxy marine data of the eemian interglacial mis5e from the norwegian sea manifest a cold event with near glacial surface ocean summer temperatures 3 4 c this mid eemian cooling divided the otherwise relatively warm interglacial climate and was associated with widespread expansions of winter sea ice and polar water masses due to changes in atmospheric circulation and ocean stability while the data also verify a late rather than early last interglacial warm peak which is in general disharmony with northern hemisphere insolation maximum and the regional climatic progression of the early holocene the cold event itself was likely instrumental for delaying the last interglacial climate development in the polar north when compared with regions farther south such a climatic decoupling of the polar region may bear profound implications for the employment of eemian conditions to help evaluate the present and future state of the arctic cryosphere during a warming interglacial

the role of endolithic cyanobacteria in the formation of lithified laminae in bahamian stromatolites the microboring activity of endolithic cyanobacteria plays a major role in the formation of the dominant lithified laminae in modern marine stromatolites in the exuma cays bahamas these stromatolites are composed primarily of fine grained carbonate sand that is trapped and bound by the filamentous cyanobacteria schizothrix sp periodic introduction of coccoid endolithic cyanobacteria solentia sp during hiatuses in stromatolite growth associated with very low rates of sedimentation results in the formation of lithified horizons 200 1000 lm thick these layers consist of micritized grains that are welded together at point contacts the micritization is caused by extensive microboring and carbonate precipitation within boreholes concurrent with endolithic activity grain welding occurs when boreholes cross from one grain to another at point contacts thus microboring destroys original grain textures but at the same time plays a constructional role in stromatolite growth by forming lithified layers of welded grains these lateral bands of fused carbonate grains help to stabilize and preserve the stromatolite deposits

the role of microorganisms and biofilms in the breakdown and dissolution of quartz and glass three different types of silica 1 quartz sand 2 crystalline scepter quartz and 3 commercial glass subjected to biological growth were investigated for evidence of biologically induced breakdown the results of laboratory experiments with biofilms on quartz sand and glass were compared with material from the field microscopic optical and sem analysis of quartz sand weathered in vitro by a microbial community of cyanobacteria diatoms and heterotrophic bacteria demonstrate grain diminution of a sand fraction of north sea sediment the microbial community changes the minerals in the vicinity of the living cells which leave their mark on the quartz surface in form of imprints depressions and pits subsequently microbial growth on the surface brought about a general decrease of the grain size in the layer beneath a biofilm imprints of diatoms on glass surfaces colonized by diatoms and heterotrophic bacteria confirm the chemical etching activity of these organisms mixed cultures of diatoms and bacteria show depressions corresponding to the shape of individual cells our experiments confirm that diatoms heterotrophic bacteria and cyanobacteria from natural biofilms can actively attack quartz and glass microscopic analysis of an idiomorphic scepter quartz crystal from a tepui weathering environment reveals that the associated biofilms can create a local shift in the ph from 3 4 ph of water on the tepui to evidently higher than 9 necessary for quartz dissolution the quartz covered with a biofilm is partially perforated to a depth of more than 4 mm we conclude that biofilm growth in marine sub aquatic and terrestrial sub aerial conditions can significantly increase the breakdown of silica in the amorphous glass sub crystalline chert crystalline and granular forms of quartz microbial growth may therefore substantially modify the processes of transformation of a major rock forming mineral of great chemical and physical resistance

evidence for early warming and cooling in north atlantic surface waters during the last interglacial in depth analysis of planktic foraminiferal census data paired with 18o records of specific indicator species provides new insight into the surface ocean evolution of the northeast atlantic during the previous interglacial warm period oxygen isotope stage ois 5e full interglacial conditions existed at the study site for a maximum of only 8 kyr between 125 and 117 ka highest sea surface temperatures ssts occurred during early ois 5e concomitant with high summer insolation but after the main phase of ice sheet melting of the preceding glaciation saalian this early peak sst interval is marked by the appearance of tropical subtropical species and lasted for 4 kyr until 121 ka as corroborated by a major change in planktic 18o relative stability in global ice volume continued for another 3 4 kyr before ssts dropped further toward the next stadial during early ois 5e the situation of the surface water vertical structure appears to have been different from the early holocene for ois 5e it is therefore suggested that the particular melting history of late saalian ice had a long lasting and profound effect on both postdeglacial surface water mass configuration in the north atlantic and heat moisture transfer into europe

eocene krabi basin southern thailand paleontology and magnetostratigraphy we present the magnetostratigraphy of the late eocene krabi section from the krabi basin thailand the investigated section is about 105 m thick and consists of red and yellow siltstones sandstones gray claystones lignites and limestones we collected 86 samples at 43 different stratigraphic levels from the 105 m thick section the samples yielded a paleomagnetic direction and paleomagnetic analyses give favorable demagnetization behaviors two magnetic components can be isolated rock magnetic experiments show that the natural remanent magnetization generally results from the presence of titanomagnetite the first component has roughly the direction of the present day field the second component displays only reversed polarity with a mean direction declination 192 34 inclination 15 5 n 71 95 4 7 k 14 after bedding correction mammalian biostratigraphy indicates a late eocene age correlation to the geomagnetic polarity time scale relies on biostratigraphic data previously proposed for the fauna found in the same section these correlations have been achieved by comparing variations in the sedimentation rate derived from the alternative correlation the proposed correlation puts the krabi section in chron c12r or chron c13r this suggests sedimentation rates of 4 9 and 9 4 cm k y respectively this correlation suggests that the krabi section is older than 31 ma and younger than 34 ma

surface and deep ocean interactions during the cold climate event 8200 years ago evidence from a north atlantic deep sea sediment core reveals that the largest climatic perturbation in our present interglacial the 8200 year event is marked by two distinct cooling events in the subpolar north atlantic at 8490 and 8290 years ago an associated reduction in deep flow speed provides evidence of a significant change to a major downwelling limb of the atlantic meridional overturning circulation the existence of a distinct surface freshening signal during these events strongly suggests that the sequenced surface and deep ocean changes were forced by pulsed meltwater outbursts from a multistep final drainage of the proglacial lakes associated with the decaying laurentide ice sheet margin

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

a palaeo tibet myanmar connection reconstructing the late eocene drainage system of central myanmar using a multi proxy approach strain resulting from the collision of india with asia has caused fundamental changes to asian drainage patterns but the timing and nature of these changes are poorly understood one frequently proposed hypothesis involves the connection of the palaeo tsangpo drainage to a precursor to the irrawaddy river of central myanmar in the palaeogene to test this hypothesis we studied the provenance of palaeogene fluvio clastic sedimentary rocks that crop out in central myanmar namely the late middle eocene early oligocene pondaung and yaw formations isotopic analysis on bulk rock and petrographic data indicate a primary magmatic arc source and a secondary source composed of recycled metamorphosed basement material although the exact location of both sources is hardly distinguishable because burmese and tibetan provinces share common lithological features the presence of low grade metamorphic fragments the heterogeneity in sr nd isotopic values of bulk sediments and westward directed palaeoflow orientations indicate a proximal source area located on the eastern asian margin central myanmar was the locus of westward prograding deltas opening into the indian ocean supplied by the unroofing of an andean type cordillera that extended along the burmese margin we found no evidence to support a palaeo tsangpo irrawaddy river at least during the late eocene supplementary material data locations and isotopic and petrographic results are available at www geolsoc org uk sup18655

mediation of endoevaporitic microbial communities in early replacement of gypsum by dolomite a case study from miocene lake deposits of the madrid basin spain abstract based on petrographic mineralogical isotope and facies assemblage analysis a microbial origin is established for the formation of dolomite associated with gypsum in miocene evaporite lake deposits of the madrid basin central spain in these deposits dolomite is present as both intercalated carbonate beds locally showing domal stromatolite structures between packages of selenite christmas tree like gypsum and patches replacing macrocrystalline gypsum texture of the dolomite is characterized by crystal aggregates showing a variety of crystal sizes and morphologies e g platelets rhombs micro rods and rings whilst larger crystals are commonly spherical and or wheat grain shaped organic remains in the form of filaments shrubs micro fibrils and strands are also common and contain significant amounts of carbon these textural features are also recognized in dolomite replacing gypsum where fe oxide and sulfide as well as celestite are ubiquitous mixed with the dolomite groundmass the dolomite whether primary or replacing gypsum is poorly ordered and slightly ca rich thus non stoichiometric stable isotope compositions are characterized by negative values for both oxygen and carbon dolomite beds featuring domal stromatolites have 18o values ranging from 2 99 and 3 79 and 13c values ranging from 4 67 and 7 35 whilst 13c values determined in the dolomite replacive of gypsum shows a small range of variation between 5 70 and 6 96 by contrast 18o values of replacive dolomite oscillate in a wider range from 3 04 to 7 99 formation of the dolomite was associated mainly with microbial mats having taken place in relatively diluted lake water further evaporative concentration resulted in precipitation of gypsum crystals sealing the mats and creating endoevaporitic microenvironments in which endolithic cyanobaterial activity produced extensive boring and corrosion of the gypsum crystals hiatuses in gypsum growth caused an intensification of the corrosion process and favored the precipitation of dolomite mediated by microbes resulting in pervasive replacement of the sulfate

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

fluid inclusion analysis of twinned selenite gypsum beds from the miocene of the madrid basin spain implication on dolomite bioformation this research work is centred on continental lacustrine gypsum deposits of miocene age cropping out in the easternmost part of the madrid basin these gypsum deposits accumulated in a continental saline lake are characterized by a spectacular distinctive christmas tree morphology and a peculiar dolomite replacement a combination of microscopic petrography and scanning electron microscopy and analytical techniques fluid inclusion microthermometry x ray energy dispersive spectroscopy and x ray diffractometry was used in order to study the crystallographic distribution and the composition of the fluid inclusions within the gypsum the objectives were to characterize the continental brine from which the mineral precipitated and to detect mineral and element traces that could indicate early diagenetic processes altering the gypsum deposits data from primary fluid inclusions indicated that gypsum precipitated from an aqueous fluid lake water of low to moderate total salinity between 20 and 90 g l nacl secondary fluid inclusions represent interstitial lake brine in contact with gypsum slightly enriched in total salt content as crystal formation proceeded textural ultrastructural and microanalytical analysis indicate that the presence of dolomite precipitates inside the gypsum layers is related to the microbial colonization of the gypsum deposits and the biomineralization of the cell walls and extracellular polymeric substances around the cells our investigation emphasizes necessity of a multidisciplinary approach to assess geobiological processes

endomycorrhizae in miocene paleosols implications in biotite weathering and accumulation of dolomite in plant roots sw madrid basin spain abstract this work provides evidence of arbuscular mycorrhizae or endomycorrizhae and their role in mineral weathering and soil carbonate production from miocene paleosols formation of the paleosols bearing mycorrhizae took place in marginal lacustrine and distal fan deposits in the madrid basin endomycorrizhae fossils are preserved as carbonate in biotite filled rhizoliths fossilized mycorrhizae are morphologically identical to those of living counterparts and consist of two linked parts the extraradicular segment surrounding the root is represented by a calcitic mycelium comprising spores and two types of hyphae that are seen to attach and to corrode the mineral grains the intraradicular part is made of a network of branched filaments hyphal coils arbuscules and likely vesicles that are mixed with the cortical root cells of the plants fossilized as dolomicrosparite aggregates the preferential formation of the dolomite aggregates on biotite grains in rhizoliths is interpreted to reflect a focused interest of the plants on this mineral and indicates the mineral may have been used as a source of plant nutrients the close association of the carbonate with the silicate further suggests that the precipitation of dolomite in the root cells and the weathering of the biotite resulted from interrelated processes main mechanisms to produce mineral dissolution include organic acids and chelate secretions by the plant roots and fungi which would remove nutrients these results confirm that carbonate paleosols provide an excellent reservoir of information on ecological interactions and biogeochemical cycling and expand the range of biogenic processes and their resultant products involved in the formation of carbonate soils this approach has broad applications given the abundance of carbonate paleosols and the endomycorrhiza record dating back 400 million years

silicate bioweathering and biomineralization in lacustrine microbialites ancient analogues from the miocene duero basin spain the miocene dolomite chert microbialites studied here offer a complete record of the geochemical cycles of silicate weathering and the subsequent formation of secondary products the microbialites were formed in lacustrine systems during the miocene of the duero basin central spain mineralogical chemical and petrographic results provide evidence of the mediation of microbes in early weathering and by product formation processes irrespective of the composition the surfaces of the grains were subject to microbial attachment and concomitant weathering palaeo weathering textures range from surface etching and pitting to extensive physical disaggregation of the minerals extreme silicate weathering led to the complete destruction of the silicate grains whose prior existence is inferred from pseudomorphs exhibiting colonial textures like those recognized in the embedding matrix detailed petrographic and microanalytical examinations of the weathering effects in k feldspars show that various secondary products with diverse crystallinity and chemical composition can coexist in the interior of a mineral the coexistence of by products is indicative of different microenvironmental conditions likely created by microbial reactions thus the presence of varied secondary products can be used as a criterion of biogenicity intensive alteration of p bearing feldspars suggests that mineral weathering may have been driven by the nutrient requirements of the microbial consortium involved in the precipitation of dolomite the rock record provides useful information on mineral weathering mediated by microbes

formation of fine grained metal and silicate precipitates on a bacterial surface bacillus subtilis abstract the ability of the gram positive bacterium b subtilis to bind and nucleate precipitates from silicate anions has been studied over 24 weeks in the presence of fe and a1 at concentrations close to those levels in soils and at slightly acid 5 5 and basic 8 0 ph in all cases formation of silicate crystallites quasi crystalline precipitates on the bacterial surfaces was observed bacterially mediated minerals were more diverse in composition and morphology less crystalline smaller and sometimes more abundant than those that were abiotically formed fe pretreatment of the bacterial cells enhanced the binding of silicate at ph 8 0 walls which were not pretreated with fe bound silicate more favourably at acid values when heavy metals pb cd zn cr ni cu were added to the mixture at ph 4 5 silicate retention was greatly favoured giving greater retention of either si or metals than was seen in abiotic controls experiments with only heavy metals showed a high affinity of the bacterial walls for the metals even at low temperatures 4 c it is postulated that a cationic bridging mechanism is involved in the binding of silicate anions by bacterial cell walls

trace fossils and rhizoliths as a tool for sedimentological and palaeoenvironmental analysis of ancient continental evaporite successions abstract recognition of evaporite formations from continental tertiary basins of spain provides evidence that trace fossils including rhizoliths can be abundant in some saline lake systems and their study helps in palaeoenvironmental interpretation of ancient continental evaporite sequences six main types of trace fossils have been distinguished and include 1 networks of small rhizoliths 2 large rhizoliths 3 tangle patterned small burrows 4 isolated large burrows 5 l shaped traces and 6 vertebrate tracks rhizoliths were related to both marginal areas of hypersaline lakes and lakes of moderately high saline waters in these settings pedoturbation resulted from colonization by grasses and bushes of distinct lake subenvironments the activity of burrowing invertebrate faunas was especially intense in lakes of moderately concentrated brines from which gypsum was the main evaporite mineral deposited a specific gypsum lithofacies bioturbated gypsum deposits forming thick massive beds has a widespread occurrence in many of the basins tangle patterned small burrows and minor isolated large burrows constitute the typical trace fossil types within the gypsum the traces are interpreted as having been caused by burrowing insect larvae probably chironomids coleopterans and annelids the behaviour of these organisms in recent lake environments yields information about the salinity range of lake waters from which gypsum precipitated concentration values averaging 100 150 g l may be thus deduced though some organisms involved in the formation of the traces can tolerate higher salinities the combined analysis of lithofacies and trace fossils from the lacustrine evaporite sequences contributes to the study of distinct saline lake subenvironments as well as changes in the sedimentary evolution of the lake systems consequently trace fossils can provide valuable insight for palaeoenvironmental analysis of at least some evaporite formations that accumulated in continental settings

the rise of the diptera microbial mat interactions during the cenozoic consequences for the sedimentary record of saline lakes shoreline gypsiferous sediments of an inland lake in central spain furnishes valuable insight into reconstructions of early sedimentary changes related to shore fly microbial mat interactions in fossil gypsum precipitating saline lake systems the association of adult and larval forms of ephydra diptera with microbial matgrounds overlying the lake margin results in the formation of gypsiferous meniscate back filled burrows that provide an analogue for recurring extensively developed trace fossils that occur in cenozoic but not older lacustrine gypsum rocks in this setting sediment burrowing by ephydridae hinders significant preservation of microbialites the overwhelming rise of diptera at the onset of the cenozoic resulted in extensive feeding and dwelling activity and contributed to reshape the saline aquatic habitat where microbial mats thrived thus leading to the formation of specific trace fossils that are illustrative of the existence of microbes in the paleoenvironment

role of permeability barriers in alluvial hydromorphic palaeosols the eocene pondaung formation myanmar this study examines the lateral distribution of hydromorphy in the fine grained alluvial deposits of the eocene pondaung formation central myanmar through detailed outcrop analysis and using a combined sedimentological and pedological approach this study proposes a reconstruction of pondaung overbank floodplain palaeoenvironments the variations of hydromorphic features in the different overbank sub environments are then discussed and used to build a model of hydromorphic variability in alluvial deposits two main architectural associations with distinctive lithofacies and pedogenic features were identified corresponding to different sub environments heterolithic deposits and extensive mudstone successions the heterolithic deposits display variegated fine grained lithofacies and contain poorly developed palaeosols with gley and vertic features which are interpreted to reflect a seasonal wetlands landscape developed in actively aggrading avulsion belts extensive mudstone successions with vertisols that locally exhibit mukkara style pseudogley features are interpreted to represent a distal open forested environment the palaeosols of both sub environments display dense local hydromorphic variations they are also characterized by a gradual shift from gley dominated to pseudogley dominated features with increasing distance from the avulsion belt the clay dominated lithology of the floodplain parent material which forms numerous subsurface permeability barriers is shown to have acted as a fundamental control in limiting water table dynamics in coarse grained parts of the succession thereby favouring hydromorphic variability palaeosol sequences of the pondaung formation contrast with the soil landscape associations described in other studies and provide an alternative model with which to account for the hydromorphic variability in poorly drained alluvial soils the model proposed as an outcome of this study demonstrates that hydromorphic variations can be dramatic in floodplains where permeability barriers are numerous further the model stresses the importance of undertaking detailed lateral palaeosol analyses prior to making interpretations regarding hydromorphic variability

feldspars as a source of nutrients for microorganisms phosphorus and nitrogen are essential macronutrients necessary for the survival of virtually all living organisms in groundwater systems these nutrients can be quite scarce and can represent limiting elements for growth of subsurface microorganisms in this study we examined silicate sources of these elements by characterizing the colonization and weathering of feldspars in situ using field microcosms we found that in carbon rich anoxic groundwaters where p and n are scarce feldspars that contain inclusions of p minerals such as apatite are preferentially colonized over similar feldspars without p a microcline from s dakota which contains 0 24 p 2 o 5 but 1 mu mol g nh 4 super was heavily colonized and deeply weathered a similar microcline from ontario which has no detectable p or nh 4 super was barren of attached organisms and completely unweathered after one year anorthoclase 0 28 p 2 o 5 approximately 1 mu mol g nh 4 super was very heavily colonized and weathered whereas plagioclase specimens 0 01 p 1 mu mmol g nh 4 super were uncolonized and unweathered in addition the observed weathering rates are faster than expected based on laboratory rates we propose that this system is particularly sensitive to the availability of p and the native subsurface microorganisms have developed biochemical strategies to aggressively scavenge p or some other essential nutrient such as fe super 3 from resistant feldspars the result of this interaction is that only minerals containing p will be significantly colonized and these feldspars will be preferentially destroyed as the subsurface microbial community scavenges a limiting nutrient

dolomite silica stromatolites in miocene lacustrine deposits from the duero basin spain the role of organotemplates in the precipitation of dolomite this research provides an ancient analogue for biologically mediated dolomite precipitation in microbial mats and biofilms and describes the involvement of highly structured extracellular polymeric secretion eps templates in dolomite nucleation the structure of eps is shown to match the hexagonal trigonal lattice geometry of dolomite which favoured the epitaxial crystallization of dolomite on the organic substrate this structure of eps also matches the arrangement of silica nanospheres in opal which further accounts for the organically templated formation of opal enabling the non replacive co existence of dolomite and silica the study is focused on a 50 m thick dolomite succession that is exposed in central areas of the tertiary duero basin and was deposited in a mudflat saline lake sedimentary complex during the middle to late miocene 9 to 15 ma in the intermediate intervals of the succession poorly indurated dolomite beds pass gradually into silica beds on the basis of sedimentological compositional geochemical and petrographic data silica and dolomite beds have been interpreted as mineralized microbial mats the silica beds formed in marginal areas of the lake in response to intense evaporative concentrations this resulted in the rapid and early precipitation of opal silicification accounted for the exceptional preservation of the microbial mat structure including biofilms filamentous and coccoid microbes and eps extracellular polymeric secretions have a layered structure each layer being composed of fibres which are arranged in accordance with a reticular pattern with frequent intersection angles at 120 and 60 therefore the structure of eps matches the lattice geometry of dolomite and the arrangement of silica nanospheres in opal additionally eps binds different elements with preference to si and mg the concurrence of suitable composition and surface lattice morphologies in the eps favoured the crystallization of dolomite on the substrate in some cases dolomite nucleation took place epicellularly on coccoid micro organisms which gave way to spheroid crystals organic surfaces enable the inorganic mineral precipitation by lowering the free energy barrier to nucleation most of the microbial mats probably developed on the lake floor under sub aqueous conditions where the decomposition of organic matter took place the subsequent formation of openly packed dolomite crystals with inter related si enriched fibrils throughout is evidence for the pre existence of fibrillar structures in the mats miocene dolomite crystals are poorly ordered and non stoichiometric with a slight ca excess up to 5 which is indicative of the low diagenetic potential the microbial dolomite has towards a more ordered and stoichiometric structure this confirms that microbial imprints can be preserved in the geological record and validates their use as biosignatures

first magnetostratigraphic study of the pondaung formation implications for the age of the middle eocene anthropoids of myanmar abstract we report the results of a magnetostratigraphic investigation to improve the stratigraphical and chronological resolution of the pondaung formation of central myanmar a total of 98 samples were collected from 45 sites through a 319 m thick section at the fossiliferous locality of yashe kyitchaung or the primate resort yielding primate species bahinia pondaungensis amphipithecus mogaungensis and myanmarpithecus yarshensis near the bahin village thermal and alternating field demagnetization allowed separation of two remanence components the high temperature component is interpreted as the characteristic magnetization rock magnetic experiments show that the remanence magnetization is mainly carried by magnetite this investigation documents normal polarity remanent magnetization with a mean direction documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usep

magnesite formation by microbial activity evidence from a miocene hypersaline lake abstract this paper provides an ancient analogue for biologically mediated magnesite in lacustrine hypersaline environments thin beds of massive to crudely laminated magnesitic marls occur interbedded with mudstone and evaporite facies deposited in a saline lake mudflat sedimentary system during the lower miocene in the madrid basin central spain exposure of this succession in a recently excavated tunnel and in collected cores offered a good opportunity to study magnesite precipitates that have been preserved in unaltered conditions by primary evaporitic minerals specifically magnesite arranged as host matrix between and enclosed by displacive halite and glauberite crystals occurs closely associated with abundant microfossils embedded in sheets of organic matter interpreted as the remains of extracellular polymeric substances eps even many magnesite clots are seen to have resulted from the agglutination of fossilized bacterial bodies forming a biofilm the close and pervasive association of microorganisms and magnesite is used as evidence that microbes played a fundamental role in the precipitation of this mineral additional traces of microorganisms and microbial activity observed in magnesite beds include the isotopic ratios in the carbonates with the 13c pdb value averaging 6 2 the presence of carbonaceous film like structures and the association with patchly distributed pyrite and celestite minerals on the basis of the combined sedimentological mineralogical chemical and morphological signatures of the magnesitic deposits it is postulated that carbonate crystals precipitated in microbial mats magnesite crystals together with celestite local barite and or pyrite have been also observed to replace silicate and sulphate minerals it is suggested that magnesite precipitation was biochemically coupled with the early dissolution of the associated sedimentary minerals microbes may have used the associated minerals as source of energy and or of essential elements