ice age simulations with a calving ice sheet model abstract variations of ice sheet volume during the quaternary ice ages are simulated using a simple ice sheet model for the northern hemisphere the basic model predicts ice thickness and bedrock deformation in a north south cross section with a prescribed snow budget distribution shifted uniformly in space to represent the orbital perturbations an ice calving parameterization crudely representing proglacial lakes or marine incursions can attack the ice whenever the tip drops below sea level the model produces a large 100 000 yr response in fair agreement correlation coefficient up to 0 8 with the 18o deep sea core records to increase confidence in the results several of the more uncertain model components are extended or replaced using an alternative treatment of bedrock deformation a more realistic ice shelf model of ice calving and a generalized parameterization for such features as the north atlantic deglacial meltwater layer much the same ice age simulations and agreement with the 18o records as with the original model are still obtained the model is run with different types of forcing to identify which aspect of the orbital forcing controls the phase of the 100 000 yr cycles first the model is shown to give a 100 000 yr response to nearly any kind of higher frequency forcing although over the last 2 million yrs the model phase is mainly controlled by the precessional modulation due to eccentricity over just the last 500 000 yr the observed phase can also be simulated with eccentricity held constant a definite conclusion on the phase control of the real 100 000 yr cycles is prevented by uncertainty in the deep sea core time scales before 600 000 yr b p the model is adapted to represent west antarctica and yields unforced internal oscillations with periods of about 50 000 yr

evaporitic and biosiliceous cyclic sedimentation in the miocene of the gulf of suez depositional and diagenetic aspects abstract the middle to late miocene evaporite bearing formations which outcrop along the gebel zeit and gems highs in the southwestern gulf of suez egypt consist of thick calcium sulphate beds rhythmically interbedded with marlstones siltstones and claystones containing various amounts of biogenic silica as well as pure diatomites the calcium sulphate beds are composed mainly of gypsum and anhydrite subaqueous crustallisation in subpermanent brine ponds of marine origin predominated during deposition of the selenite and laminated gypsum early diagenetic interstitial growth of nodular anhydrite occurred episodically when the water level dropped in response to increasing salinity associated with the precipitation of halite in the deeper troughs nevertheless most of the nodular facies resulted from the burial conversion of gypsum to anhydrite which has been sometimes rehydrated into gypsum a last phase of gypsum dehydration has occurred at the surface related to the present hot and dry climatic conditions the biosiliceous deposits contain either well preserved opal a or diagenetically altered opal ct clinoptilolite quartz and even clays diatom frustules such diagenetic changes are commonly described in the oceanic biosiliceous deposits the fossil content of these diatomitic layers implies shallow marine conditions on the shelves of highs induced by block faulting the cyclic succession of evaporited and biosiliceous deposits recorded the alternation of high sea level stands associated with high organic productivity probably enhanced by seasonal input of nutrients and low sea level stands associated with hypersaline conditions and evaporite deposition locally the sharp contact between the diatomites and gypsum argues for a rapid evolution of salinity towards high concentration up to gypsum saturation and more the formation of finely laminated diatomitic sediments is known to require either high organic productivity or anoxic conditions in bottom intermediate waters and eventually both processes in the gebel zeit and gemsa areas a local association of the biosiliceous sediments with diagenetic carbonates and native sulphur resulting from processes of bacterial sulphate reduction confirms that anoxic conditions occurred in the deep water body or in the sediments large amounts of organic compounds were involved in these diagenetic processes immature organic matter was consumed during an early diagenetic stage of transformation it is assumed that this organic matter was supplied either by the biosilica rich interbeds or the organic rich laminated carbonates the initial organic content of the biogenic deposits was impoverished as early as the first stages of sedimentation and diagenesis although a large part of the organic matter could have been destroyed by these early diagenetic processes and further oxidation in outcrops these sediments can be considered as potential source rocks in nearby less exposed sequences

the influence of organic matter in organogenic dolomitization we hypothesize that the decomposition of organic debris derived from plants planktonic animals and algae by sulfate reducing bacteria and specifically by the enzymatic degradation of protein is vital to organogenic dolomitization the role of this degradation is to increase both alkalinity and ph of the pore waters providing the necessary solution and surface chemistries for dolomitization to occur ammonia derived by the enzymatic breakdown of proteins is the only simple base available to supply the ph conditions necessary for dolomitization protein is abundant in organic matter derived from a marine source because of the large enthalpies of hydration of mg super 2 and ca super 2 ions we propose that dolomitization proceeds with the transfer of neutral ion pairs caco 3 and mgco 3 to and from the dolomite surface within a replacement zone for this reason dolomitization will occur only when the concentration of the co 3 super 2 ion is high and in large excess to the hco 3 super ion in solution and at the carbonate crystal surface these conditions will be met when the ph and or ionic strength is high and the carbonate alkalinity is also high

mineral biosignatures in evaporites presence of rosickyite in an endoevaporitic microbial community from death valley california rosickyite is a rare form of sulfur sulfur monoclinic symmetry that is not thermodynamically predicted to be stable at earth s surface temperatures instead it reverts to the more common sulfur form orthorhombic symmetry here we show for the first time that rosickyite exists and is stably maintained within an endoevaporitic microbial community from the salt pan of death valley california we hypothesize that this mineral is formed by a cycle of microbial dissolution of gypsum caso4 2h2o to sulfide and reoxidation of the sulfide to elemental sulfur rosickyite within a stable oxygen sulfide gradient maintained by the organisms furthermore we report a microstratigraphic layering of mineral types that correlates with layering of the microbial community knowledge of how microbial communities can affect the mineral assemblages of evaporite deposits on earth can help us to identify potential markers of the past or present existence of life on extraterrestrial bodies bearing evidence of ancient seas or lakes

bacterially mediated formation of diagenetic aragonite and native sulfur in zechstein carbonates upper permian central germany abstract neoformed diagenetic aragonite associated with organic rich aggregations and native sulfur occur within cavities of the ca2 and ca3 zechstein carbonates in central germany the cavities were formerly filled with gypsum bacterial sulfate reduction favored the precipitation of the carbonate phase this can be attributed to an increase in alkalinity accompanied with sulfate reduction the high sr concentrations of the neoformed aragonites compared to the low concentrations in the carbonate host rocks point to sulfate dissolution as the cation supplying process for the precipitation of aragonite low 13c values 10 pdb of the aragonite indicate that some of its carbon is derived from organic matter that has been oxidized by bacterial sulfate reduction aragonite inclusions bear rhomb shaped crystals of calcite replacing former dolomite elevated mg ca ratios due to this dedolomitization may have promoted the precipitation of aragonite instead of calcite the aragonite precipitated in the near surface meteoric vadose zone in recent times aragonite crystals display a platy habit sem analyses show that two types of micro rods are associated with these plates the mineralized micro rods are interpreted to be fossilized bacteria aragonite inclusions most of which contain organic rich aggregations yield a distinctive biomarker pattern high concentrations of specific unsaturated fatty acids are clearly indicative of newly produced organic matter and reflect the presence of a discrete microbial community being associated with the formation of the aragonite at one of the studied localities the aragonite is accompanied by native sulfur the formation of sulfur was mediated by h2s oxidizing bacteria this is corroborated by the presence of densely packed curved rods representing permineralized bacterial cells on and within the sulfur

the role of sulphate reducing bacteria and cyanobacteria in dolomite formation in distal ephemeral lakes of the coorong region south australia abstract the dolomitic distal ephemeral lakes of the coorong region of south australia are dynamic systems in which the concentrations of solutes generally increase during late spring and summer when the rate of evaporation greatly exceeds aqueous input during late evaporation active bacterial sulphate reduction occurs in uppermost anoxic lake sediments as rising salinity and then h2s toxicity kills off the macrobiota these hypersaline lakewaters are characterised by high ph and elevated concentrations of carbonate and sulphate ions magnesium concentrated in cyanobacterial cells and sheaths is released into the strongly electrolytic alkaline lake brines from lake margin stromatolites and mats during desiccation and may explain the remarkable increase in mg levels recorded at this time continued evaporation of the lakewaters produces a halite crust overlying the dolomite sulphate free residual puddles and the absence of gypsum indicates that all sulphate is removed by sulphate reducing bacteria dolomite forms in situ as a primary precipitate from these microbially mediated lake porewaters and both the morphology and size distribution of dolomite grains is identical to bacterial cells it is not proven exactly when dolomite precipitation occurs but the submicron sized grains and multiple defects in a disordered lattice indicate that precipitation is rapid and follows bacterial removal of sulphate which is known to inhibit dolomite formation bacterial sulphate reduction and cyanobacterial degradation may thus lead to the removal of all kinetic inhibitors to dolomite formation and provide a mechanism for dolomite precipitation around bacterial cells in shallow subsurface sediments during the late stages of evaporation in adjacent non dolomitic lakes sulphate is present throughout evaporation so that other minerals principally aragonite and hydromagnesite are precipitated

textural and mineralogical biosignatures in an unusual microbialite from death valley california abstract for in situ astrobiological studies of mars or other planets we must employ strategies that will enable us to verify whether our approach and prototype instruments are actually capable of distinguishing life from non life this must be done against a background of rigorously conducted scientific characterization of the environment or sample types being considered for measurement by the instruments under development in this study we show how a combination of mineralogical and textural features can be considered a biosignature in an early mars analogue environment death valley california we propose that it is a combination of features in context of the geologic matrix which allows determination of biogenicity to be made polymineralic microbialites organosedimentary formations constructed by microorganisms from a spring pool at badwater within death valley national park are composed of alternating biogenic and abiogenic minerals in a distinct triplet sequence related to wet and dry seasons a microbial community occurring as a black biofilm produced paired layers of two different mineral types manganese oxyhydroxides and calcite these biogenic layers are separated from the next pair by a gypsum layer and appear to be laid down in the wet season with the gypsum a mineral positively identified on mars precipitating in the dry part of the year abiogenically i e not dependent on microbial metabolic activity for its deposition in addition textural features smaller grain size and less geometric morphology unique to the biogenic vs the abiogenic layers were consistently observed so that texture served as a biosignature in this environment

cyanobacterial precipitation of gypsum calcite and magnesite from natural alkaline lake water results from transmission electron microscopy provide direct evidence for cyanobacterial biomineralization of gypsum and calcite in aquatic environments laboratory simulations using filter sterilized natural lake water inoculated with synechococcus sp isolated from fayette ville green lake new york revealed epicellular biomineralization of gypsum calcite and magnesite experimental electron microscopical and sedimentological evidence indicates that synechococcus is responsible for a major proportion of the marl sediment and carbonate bioherms in green lake the elucidated role of synechococcus in biomineralization and its ubiquitous distribution in nature have widespread implications for cyanobacterial mineralization in marine and freshwater environments since late archean time

biological influences on modern sulfates textures and composition of gypsum deposits from guerrero negro baja california sur mexico abstract gypsum caso 4 2h 2 o deposits from a range of sedimentary environments at guerrero negro baja california sur mexico were investigated for microscale texture and composition in order to differentiate features formed under substantial microbial influence from those for which microbial effects were relatively minor or absent gypsum deposits were classified according to their sedimentary environment textures crystal habit brine composition and other geochemical factors the environments studied included subaqueous sediments in anchialine pools and in solar salterns as well as subsurface sediments of mudflats and saltpans gypsum that developed in the apparent absence of biofilms included crystals precipitated in the water column and subsedimentary discs that precipitated from phreatic brines subsedimentary gypsum developed in sabkha environments exhibited a sinuous microtexture and poikilitically enclosed detrital particles water column precipitates had euhedral prismatic habits and extensive penetrative twinning gypsum deposits influenced by biofilms included bottom nucleated crusts and gypsolites developing in anchialine pools and saltern ponds gypsum precipitating within benthic biofilms and in biofilms within subaerial sediment surfaces provided compelling evidence of biological influences on crystal textures and habits this evidence included irregular high relief surface textures accessory minerals s ca carbonate sr ca sulfate and mg hydroxide and distinctive crystal habits such as equant forms and crystals having distorted prism faces

oceanic mechanisms for amplification of the 23 000 year ice volume cycle situated adjacent to the largest northern hemispher ice sheets of the ice ages the mid latitude north atlantic ocean has an important role in the earth s climate history it provides a significant local source of moisture for the atmosphere and adjacent continents forms a corridor that guides moisture bearing storms northward from low latitudes and at times makes direct contact along its shorelines with continental ice masses evidence of major ice ocean air interactions involving the north atlantic during the last 250 000 years is summarized outflow of icebergs and meltwater initially driven by summer insolation over the ice sheets affects midlatitude ocean temperatures summer heat storage winter sea ice extent and global sea level these oceanic responses in turn influence the winter moisture flux back to the ice sheets as well as ablation of land ice by calving spectral data indicate that the oceanic moisture and sea level feedbacks in part controlled by glacial melt products amplify milankovitch insolation forcing of the volumetrically dominant mid latitude ice sheets at the 23 000 year precessional cycle

christmas island lagoonal lakes models for the deposition of carbonate evaporite organic laminated sediments the atoll of christmas island now known as kiritimati in the kiribati republic central pacific lies at about 28n in the intertropical convergence zone much of the surface area of the atoll ca 360 km 2 is occupied by numerous lakes in which carbonate evaporite calcium sulfate halite and organic layers are deposited observations suggest that deposition of these different laminae is controlled by climatic and biologic factors it is thought that periodic climatic variations such as el nino southern oscillations enso events which bring heavy rainfall to the atoll result in the succession of the precipitation of carbonate minerals during periods after dilution of hypersaline waters by heavy rains followed by evaporitic minerals carbonate calcium sulfate halite when salinity increases through evaporation thick up to 5 cm microbial essentially cyanobacterial mats develop continuously on the lake bottom surfaces providing the sediment with an important total organic carbon 2 5 organic contribution in the form of an internal geometrically structured network in which the authigenic minerals precipitate the high bioproductivity of these microbial populations is reflected in low d 13 c values of sedimentary organic carbon 214 to 217 interpreted as being the result of high atmospheric co2 demand geochim cosmochim acta 56 1992 335 the well laminated organic layers present in the sediment profile result from the death and burial of microbial populations at the time of severe climatic events storms heavy rainfall these lagoonal lakes provide a model for the deposition of carbonate and organic matter in an evaporitic environment the high ratio of deposited carbonate vs sulfate 1 chloride when compared to low ratio in evaporitic salinas results from both a lack of limitation of calcium magnesium and carbonate ions in a carbonate reef environment and active processes of high mg calcite precipitation organomineralization q 2001 elsevier science b v all rights reserved

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

a model study of the role of high latitude topography in the climatic response to orbital insolation anomalies abstract the response of a combined global zonal averaged energy balance model and a continental ice sheet model to insulation anomalies produced by orbital perturbations is examined although the role of ice sheet dynamics is included in the model this paper focuses mostly on the response of the atmosphere hydrosphere to the perturbations the model developed from that of suarez and held 1979 explicitly incorporates surface topography surface energy balances snow and sea ice budgets and variable static stability the ice sheet model is based on a power law rheology it is found that the presence of a high latitude plateau significantly increases the sensitivity of the climate to the insolation perturbation the sensitivity is maximized when the elevation of the summer snowline is near the elevation of the plateau as appears to be the case with baffin island today there appears to be a rapid response of the seasonal cycle to an insolation anomaly through albedo temperature feedback and with the

mineralogical footprints of microbial life earth s geosphere is intimately tied to its biosphere a major link between the two lies in the microbial realm microorganisms grow in and upon rocks and minerals often relying on their substratum for critical compounds needed in order to produce cellular energy the presence of a metabolizing cell on a mineral substrate has a significant effect on the mineral texture and on the geochemistry of the surrounding microenvironment in nature microorganisms exist in microbial commu nities as mats or biofilms growing upon a solid substrate as such they cover a vast surface area both within and below the surface of earth s land and sea the following review will provide a glimpse into the latest findings in the field of geomicrobiology and is intended to convey a sense of the profound influence microorganisms can have upon the geological environment they inhabit

a paleoclimate model of northern hemisphere ice sheets abstract a model for predicting the growth and decay of ice sheets based on the astronomical theory of climate change is presented the purpose of the study in part is to isolate the role of the ice sheet physics and earth response under varying ice load by simplifying to the extreme the role of the hydrosphere atmosphere ice sheet physics and the response of the lithosphere asthenosphere under the ice load are modeled explicitly insolation anomalies taken at a fixed latitude directly force latitudinal displacement of the snow line accumulation rate a and ablation rate a evaluated at mean sea level are specificed as external constants a a decrease linearly with ice sheet elevation rough tuning of the model to the general shape of the ice volume record of the last two major glacials determines the external constants model predictions of the ages of several events in the last major glaciation compare well with the radiological ages the six glacial terminatios i vi over the last 600 000 yr are identified and the predicted ages compare reasonably well with the 18 o record for two deep sea cores a direct comparison of model power spectra of ice volume as a function of period with spectra of the 18 o record shows apparent underprediction of power near 100 000 yr when a quantitative but heuristic method for taking into account the red noise spectrum evident in the geological records is used a much more favorable comparison is possible the model prediction lends support to the hypothesis that the nonlinearity of the ice sheet physics is responsible for the 100 000 yr periodicity in the geological record of the late pleistocene

brunhes matuyama magnetic reversal dated at 790 000 yr b p by marine astronomical correlations abstract the presence of the brunhes matuyama magnetic reversal in deep sea core sediments makes possible an alternative to the usual k ar radioistope method of dating the reversal as found in rocks the alternative method uses correlations of northern hemisphere summer insolation with oxygen isotope ratios from tropical cores the latitude dependent insolation variations are calculated from planetary mechanics and thus provide a highly accurate astronomical time scale the insolation variations strongly influence glacial ice volume fluctuations that dominate the oxygenisotope ratio changes recorded in core sediments the summer half year insolation variations are identified with corresponding isotope ratio changes in cores from the present through glacial stage 20 misleading effects of discontinuities or major nonuniformities of sediment deposition are avoided by an analysis of the uniformity of v28 238 and v28 239 the principal cores studied and by comparisons with other cores the top section stages 1 to 10 of v28 238 is uniformly deposited and for this section an isotope ratio time scale is chosen that agrees with the thorium uranium date for the high sea stand of the last interglacial extreme over this interval major glacial extremes strong isotope ratio minima coincide consistently with major insolation minima at times of low orbital eccentricity in addition interstadials are directly associated with precessional insolation peaks and the envelope of isotope ratio peaks resembles the envelope of precessionally dominated insolation peaks the assumption that the glacial extremes depended similarly on insolation minima during stages 10 to 20 permits minor age shifts of strong isotope ratio minima in the two cores relative to ages based on uniform overal deposition to match the ages of low eccentricity insolation minima the age shifts reflect residual nonuniformities of deposition the validity of this matching procedure is supported by a resulting consistent identification of principal isotope ratio peaks with high and low latitude coincident insolation maxima the brunhes matuyama reversal is found intinterglacial stage 19 and is dated on the astronomical time scale at 790 000 5000 yr b p

heating and freezing experiments on aqueous fluid inclusions in anhydrite recognition and effects of stretching and the low temperature formation of gypsum aqueous fluid inclusions in anhydrite that are frozen during microthermometry commonly develop new daughter crystals heating these inclusions induces the new daughter crystals to decompose repeated cycles of crystal growth and decomposition may result in morphological changes of the fluid inclusions this phenomenon of new mineral growth presumed to represent the low temperature hydrate gypsum has been recognized for over 20 years nevertheless its occurrence and its significance to the determination of inclusion salinities are not widely appreciated similarly the potential of inclusions in anhydrite to stretch when overheated is commonly assumed yet tests demonstrating the phenomenon have not been reported changes in the ice melting temperature of aqueous inclusions in anhydrite before and after gypsum formation were documented showing that hydration of the host anhydrite may significantly lower the ice melting temperature and increase the salinity of the residual inclusion fluid heating inclusions to induce gypsum to decompose results in the release of bound water restoring the inclusions to their original salinity the homogenization temperatures of inclusions that have undergone these phenomena are generally unchanged indicating that the inclusions specific volumes are unchanged however heating aqueous inclusions in anhydrite more than ten degrees or so above their homogenization temperature commonly results in stretching therefore those who study aqueous inclusions in anhydrite must avoid the determination of ice melting temperatures if new gypsum daughter crystals are present and must determine homogenization temperatures sequentially with rising temperature to avoid overheating and stretching

a climate model with cryodynamics and geodynamics we present a highly simplified zero dimensional model of the climatic system the model attempts to incorporate mechanisms important on the time scale of glaciation cycles namely 104 105 years in particular the radiation balance of the ocean atmosphere the plastic flow of continental ice sheets and the viscous flow of the upper mantle are taken into account the stress is on the interaction between the cryosphere ice sheets and the asthenosphere upper mantle which had not been taken into account in previous energy balance or ice sheet models of climate weertman 1976 kallen et al 1979 the model exhibits free self sustained oscillations of an amplitude and period comparable to those found in the paleoclimatic record of glaciations viz o 10 deg k and o 104 years respectively such oscillations had already been found in the simpler model of kallen et al 1978 1979 the fact that nonlinear self sustained oscillations also obtain for the present slightly more complex model is interesting it offers mild support to the idea that unforced oscillations can actually exist in the real climatic system itself these oscillations are characterized by a quarter phase lag between ice extent and global temperature given that the climatic system is a nonlinear oscillator with a period of o 104 years it is expected that astronomical forcing at 19 000 23 000 and 41 000 years will lead to subharmonic oscillations with a period close to 100 000 years the dominant period of glaciation cycles such effects have already been noticed in the work of birchfield and weertman 1978 with an even simpler model including only glacial dynamics the careful study of the interplay between internal mechanisms and external forcing with comparable time scales represents an interesting challenge to the theory of ice ages the results of such theoretical studies can also provide guidance for observational work in particular periodicities of o 104 years in the climate record and phase lags between ice volume and temperature appear to be supported by the most recent paleogeochemical investigations ruddiman and mclntyre 1981

endolithic cyanobacteria in soil gypsum occurrences in atacama chile mojave united states and al jafr basin jordan deserts 1 soil sulfates are present in arid and hyperarid environments on earth and have been found to be abundant in soils on mars examination of soil gypsum from the atacama desert chile the mojave desert united states and al jafr basin jordan revealed endolithic cyanobacteria communities just below the surface of soil gypsum samples optical and scanning electron microscope observations of the colonized layers indicated that the unicellular chroococcidiopsis is the dominant cyanobacterium in all studied communities 16s rrna gene analysis revealed that in addition to chroococcidiopsis a few other cyanobacteria are present heterotrophic bacteria are also abundant in the colonized zones of the fine grained gypsum from the atacama and mojave desert but insignificant in the fibrous gypsum from the jordan desert endolithic life forms similar to these described here may exist or have existed on mars and should be targeted by the mars science laboratory and future in situ missions

near infrared spectroscopy of lacustrine sediments in the great salt lake desert an analog study for martian paleolake basins the identification and characterization of aqueous minerals within ancient lacustrine environments on mars are a high priority for determining the past habitability of the red planet terrestrial analog studies are useful both for understanding the mineralogy of lacustrine sediments how the mineralogy varies with location in a lacustrine environment and for validating the use of certain techniques such as visible near infrared vnir spectroscopy in this study sediments from the pilot valley paleolake basin of the great salt lake desert were characterized using vnir as an analog for martian paleolake basins the spectra and subsequent interpretations were then compared to mineralogical characterization by ground truth methods including x ray diffraction automated scanning electron microscopy and several geochemical analysis techniques in general there is good agreement between vnir and ground truth methods on the major classes of minerals present in the lake sediments and vnir spectra can also easily discriminate between clay dominated and salt dominated lacustrine terrains within the paleolake basin however detection of more detailed mineralogy is difficult with vnir spectra alone as some minerals can dominate the spectra even at very low abundances at this site the vnir spectra are dominated by absorption bands that are most consistent with gypsum and smectites though the ground truth methods reveal more diverse mineral assemblages that include a variety of sulfates primary and secondary phyllosilicates carbonates and chlorides this study provides insight into the limitations regarding the use of vnir in characterizing complex mineral assemblages inherent in lacustrine settings

the depositional and diagenetic evolution of the coastal ridges of northwestern egypt since the lower pliocene successive generations of coastal ridges made up of mixed oolitic and biogenic grainstones formed along the mediterranean coastline of egypt west of alexandria and have undergone progressive stages of diagenesis the ridges are complex structures resulting from several phases of deposition in which shallow marine components are partially and progressively reworked as subaerial and aeolian deposits diagenetic alteration resulted from increasing freshening of the pore waters in the ridges diagenesis began with marine phreatic cementation followed by freshwater phreatic cements aggrading recrystallization of microcrystalline calcite to microspar and minor dolomitization non marine hypersaline diagenesis soil profiles developed on the upper surfaces of the oldest dunes

age and significance of aeolian sediment reworking on high plateaux in the scottish highlands holocene aeolian sand sheets at the margins of high level deflation surfaces in the scottish highlands commonly comprise two units a lower weathered unit representing slow accumulation through out much or all of the holocene and an upper unit of structureless sand that is inferred to represent recent erosion of aeolian deposits and aeolisols from adjacent plateaux osl dating of samples from above and below the upper lower unit contact at three sites in nw scotland places the onset of upper unit sediment accumulation within the interval ad 1550 1700 accumulation rates calculated from osl ages confirm rapid accumulation of upper unit deposits the timing of the onset of upper unit sand accumulation excludes expansion of sheep grazing intrinsic instability and atmospheric pollution as triggers of plateau surface vegetation degradation and consequent erosion but favours climatic deterioration increased wind stress and possibly prolonged snow lie during the little ice age as the likely cause this explanation does not however apply in all cases at an osl dated site in the grampian highlands upper unit aeolian sand accumulation commenced around ad 1900

mineralogy of a mudstone at yellowknife bay gale crater mars sedimentary rocks at yellowknife bay gale crater on mars include mudstone sampled by the curiosity rover the samples john klein and cumberland contain detrital basaltic minerals calcium sulfates iron oxide or hydroxides iron sulfides amorphous material and trioctahedral smectites the john klein smectite has basal spacing of similar to 10 angstroms indicating little interlayer hydration the cumberland smectite has basal spacing at both similar to 13 2 and similar to 10 angstroms the larger spacing suggests a partially chloritized interlayer or interlayer magnesium or calcium facilitating h2o retention basaltic minerals in the mudstone are similar to those in nearby eolian deposits however the mudstone has far less fe forsterite possibly lost with formation of smectite plus magnetite late noachian early hesperian or younger age indicates that clay mineral formation on mars extended beyond noachian time

deposition and early alteration of evaporites summary the depositional settings for primary and early diagenetic evaporite deposits generally fall into three categories marginal mixed shallow subaqueous and subaerial shallow and deep subaqueous these three environmental groupings hold for both marine and nonmarine settings although the details of continental evaporites may be far more complex than in most marine fed water bodies the primary evaporite morphologies from many continental playa hypersaline marine and marine marginal depositional settings are reasonably well understood because of the numerous detailed studies of recent holocene and cenozoic deposits that serve as models for sedimentary interpretation the sedimentological features that develop in deeper water settings are inferred based on examination of unaltered cenozoic deposits each environmental setting develops characteristic depositional features and patterns and one facies grades into the next because there may be significant physiochemical changes in water composition during deposition as well as sudden change s in both water depth and basinal circulation description and interpretation of evaporative rocks should not be based on mineralogy alone but on the distinct sedimentary characteristics of each part of a deposit in cases where sediments still reflect their primary mineralogy and morphology most of the environments can now be recognized however geochemical studies are commonly required to determine the source s of the original water the determinative geochemical techniques that presently serve as a support to sedimentologic study include studies of fluid inclusions bromine content of chloride salts and the stable isotopes of strontium sulphur carbon and oxygen only after these and perhaps other chemical analyses are considered can the full depositional history of a region be reasonably unravelled

a coupled climate ice sheet model applied to the quaternary ice ages a coupled climate ice sheet model is used in an attempt to simulate the northern hemispheric ice sheet volume variations implied by deep sea core records over the last several 105 years the climate model consists of a global one level diffusive energy balance equation with seasonal variation and land ocean contrast using a severely truncated series of associated legendre functions the ice sheet model uses a standard vertically averaged flow law and predicts ice thickness in a north south cross section bedrock deformation under the ice is included as linear flow confined to a thin asthenospheric channel the climate model is tuned to the modern climate and tested against results from atmospheric general circulation models for 18 000 and 10 000 yr b p it is suggested that the lack of sea ice variations causes the biggest departures from the gcm results the sensitivities to the orbital perturbations of the coupled model and of the ice sheet model alone with a prescribed snow budget distribution are examined some qualitative differences are found in the latitudinal forms of these sensitivities despite these differences however the two types of models produce similar ice age simulations over the last 700 thousand years in response to the orbital perturbations in particular fairly realistic simulations with strong 100 kyr cycles similar to those obtained by adding an ice calving mechanism to the ice sheet model in pollard 1982a are again produced in the coupled model

biogenic carbonates are they a criterion for underlying hydrocarbon accumulations an example from the gulf of suez region egypt the gulf of suez petroliferous basin is characterized by the occurrence of a middle upper miocene evaporite cap field and subsurface investigations in the southwest gulf of suez region indicate that the evaporites usually contain two types of biogenic carbonates in addition to sulfur the first type of carbonate is biogenic syngenetic in origin and was formed in low temperature euxinic aquatic environments by the reduction of dissolved sulfates and the oxidation of organic matter by sulfate reducing bacteria the resulting carbonate and sulfur are stratigraphically restricted within the evaporite sequence and occur as interlaminated gypsum calcite sulfur or as sulfur ooids sulfur globules and stromatolitic sulfur that are enclosed in gypsum or calcite the biogenic syngenetic carbonate and sulfur did not originate from petroleum and have a significance of euxinic aquatic settings the second type of carbonate is a biogenic epigenetic carbonate that formed under low temperatures by biochemical reactions between seeping hydrocarbons and the overlying miocene evaporites the carbonate is vuggy is included within or caps the evaporites and usually shows hydrocarbon stains or emits a hydrocarbon smell the sulfur is usually yellow but may be brown to black due to bitumen staining carbonates replace gypsum or anhydrite and occur as aragonite calcite or dolomite the d13c mean values for carbonate minerals range from 7 8 to 27 2 pdb being relatively heavier than the d13c values of the nearby crude oil 28 1 to 29 3 pdb carbon isotope values indicate that these carbonates were formed from the heavier organic carbon dioxide under reducing conditions where the lighter methane is stable the occurrence of biogenic epigenetic carbonates and sulfur in the evaporite deposits of the gulf of suez petroliferous basin is similar to that recorded in other basins such as the gulf coast basins of texas louisiana and southern mexico the fergana and amudarya depression of central asia and the mesopotamian depression in iraq the biogenic epigenetic carbonates or sulfur in some regions is considered as a criterion for petroleum microseepage that may indicate a potential hydrocarbon bearing reservoir at depth

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