Evaporitic Environments Research Articles

Minerals formed by the weathering of sulfides in mines of the Czech part of the Upper Silesian Basin

AbstractThis study describes the occurrences of sulfate minerals in mines of the Czech part of the Upper Silesian Basin. This mineralization originates from the oxidation of Fe disulfides contained in the coal matrix and enclosing sediments. The oxidation occurs in the presence of formational brines and chemotrophic bacteria. The brines have a pH between 6.3 and 8.5 and total solute content of up to 300 g/l. They are rich in Na, Ca, K, Mg, Ba and Sr and Cl– is the major anion. The minerals of the pickeringite–halotrichite series with coexisting magnesiocopiapite are formed primarily in drier places and areas where the water is only slightly mineralized. In more humid places where the brines are more concentrated, a diverse assemblage of up to 20 different sulfates are found (e.g. natrojarosite, sideronatrite, metasideronatrite, tamarugite, magnesiocopiapite, bílinite, starkeyite, blödite, rozenite and siderotil). These sulfates are accompanied by halite, sulfur, goethite and a number of phases of uncertain identity, such as sulfates containing Sr and REE. This is an example of mineral paragenesis formed by weathering in a saline evaporite environment, which is extremely rare in Europe but is found in arid regions elsewhere (e.g. in the Atacama Desert in Chile).

A palynological study from Sweden reveals stable terrestrial environments during Late Silurian extreme marine conditions

ABSTRACTA palynological study of the upper Silurian Öved–Ramsåsa Group in Skåne, Sweden yields a well preserved spore assemblage with low relative abundances of marine microfossils. In total, 26 spore taxa represented by 15 genera were identified. The spore assemblage is dominated by long-ranging cryptospore taxa, and the trilete spore Ambitisporites avitus-dilutus. However, key-species identified include Artemopyra radiata, Hispanaediscus lamontii, H. major, H. verrucatus, Scylaspora scripta and Synorisporites cf. libycus. Importantly, Scylaspora klintaensis was identified, allowing correlation with the Klinta 1 drillcore (Skåne). A Ludlow age is inferred for the exposed succession, which agrees well with previous conodont stratigraphy. The organic residue is dominated by phytodebris and spores, but with high relative abundances of acritarchs at two levels, possibly related to flooding surfaces.Based on the palynofacies analysis, a near-shore marine environment is proposed. The close proximity to land is inferred by the high proportions of spores, and the dispersed assemblage most likely represents the local flora growing on delta plains. The palynological signal also infers a stable terrestrial environment and vegetation, in contrast to unstable conditions in the marine environment characterised by ooid formation in an evaporitic environment. Comparisons with coeval spore assemblages from Gotland, Avalonia and Laurentia show relatively close similarities in taxonomic composition at the generic level.

The gypsum–anhydrite paradox revisited

Despite much experimentation precipitation of anhydrite from solution in conditions similar to those occurring in sedimentary environments has not yet been reproduced in the laboratory. To resolve this long-standing contradiction we have monitored the precipitation and stability behaviors of calcium sulfate during experiments lasting up to two years. Calcium sulfate was precipitated from solution between 40 and 120°C at three different salinities and the formed solid phase was sampled at different time intervals (from 2min up to 2yr). We found that below 80°C gypsum is the sole primary phase and in the range of 80 to 120°C gypsum and bassanite are the primary phases. The stability of the latter increased with increasing salinity. As expected, we did not observe primary anhydrite precipitation, but over time phase transition occurred and anhydrite eventually appeared at temperatures >80°C. We show that intrinsic thermodynamic and kinetic properties severely constrain the precipitation of anhydrite (compared to gypsum and bassanite), and consequently, a considerable amount of time (e.g. >2yr at 60°C) is needed for anhydrite to form. Even so, at a geological time-scale, anhydrite can be considered as a pseudo-primary phase thus resolving the long-standing paradox of our inability to directly precipitate anhydrite in the laboratory at temperatures below 120°C and the abundant presence of anhydrite in evaporitic environments. Our results also show that at low water activity, bassanite becomes an important phase, which could be relevant to explain its presence on the surface of Mars.

Origin of the hydrocarbon gases carbon dioxide and hydrogen sulfide in Dodan Field (SE-Turkey)

Gas occurrences consisting of carbon dioxide (CO2), hydrogen sulfide (H2S), and hydrocarbon (HC) gases and oil within the Dodan Field in southeastern Turkey are located in Cretaceous carbonate reservoir rocks in the Garzan and Mardin Formations. The aim of this study was to determine gas composition and to define the origin of gases in Dodan Field. For this purpose, gas samples were analyzed for their molecular and isotopic composition. The isotopic composition of CO2, with values of −1.5‰ and −2.8‰, suggested abiogenic origin from limestone. δ34S values of H2S ranged from +11.9 to +13.4‰. H2S is most likely formed from thermochemical sulfate reduction (TSR) and bacterial sulfate reduction (BSR) within the Bakuk Formation. The Bakuk Formation is composed of a dolomite dominated carbonate succession also containing anhydrite. TSR may occur within an evaporitic environment at temperatures of approximately 120–145 °C. Basin modeling revealed that these temperatures were reached within the Bakuk Formation at 10 Ma. Furthermore, sulfate reducing bacteria were found in oil–water phase samples from Dodan Field. As a result, the H2S in Dodan Field can be considered to have formed by BSR and TSR.As indicated by their isotopic composition, HC gases are of thermogenic origin and were generated within the Upper Permian Kas and Gomaniibrik Formations. As indicated by the heavier isotopic composition of methane and ethane, HC gases were later altered by TSR. Based on our results, the Dodan gas field may have formed as a result of the interaction of the following processes during the last 7–8 Ma: 1) thermogenic gas generation in Permian source rocks, 2) the formation of thrust faults, 3) the lateral-up dip migration of HC-gases due to thrust faults from the Kas Formation into the Bakuk Formation, 4) the formation of H2S and CO2 by TSR within the Bakuk Formation, 5) the vertical migration of gases into reservoirs through the thrust fault, and 6) lateral-up dip migration within reservoir rocks toward the Dodan structure.

The Fethiye–Burdur Fault Zone: A component of upper plate extension of the subduction transform edge propagator fault linking Hellenic and Cyprus Arcs, Eastern Mediterranean

The Hellenic and Cyprus Arcs, that mark the convergent boundary of the African and Aegean–Anatolian plates, are offset along a subduction transform edge propagator (‘STEP’) fault running NE–SW along the Pliny and Strabo Trenches. The continuation of the fault to the northeast through the Rhodes Basin and into SW Anatolia is assessed. Seismic reflection profiles show that the structural architecture of the northern sector of the Rhodes Basin includes a large crustal-scale fold–thrust belt which is overprinted by numerous faults with small extensional stratigraphic separations. A protracted episode of convergence in the Miocene resulted in the development of a prominent NE–SW-striking and NW-verging fold–thrust belt in the Rhodes Basin. The absence of evaporites in the Rhodes Basin and several seaward prograded vertically stacked Quaternary delta successions resting at 2500–3500m water depth collectively suggest that the Rhodes Basin must have remained above the depositional base of marine evaporite environment during the Messinian and that the region must have subsided very rapidly during the Pliocene-Quaternary. During the Pliocene-Quaternary, a NE–SW-trending belt developed across the Rhodes Basin: while the structural framework of this belt was characterised by reactivated thrusts in the central portion of the basin, a prominent zone of NE–SW-striking and NW- and SE-dipping faults with extensional separations developed in the northern portion of the basin. Two seismic profiles running parallel to the present-day coastline provide the much needed linkage between the Fethiye–Burdur Fault Zone onland and the reactivated thrusts in central Rhodes Basin, and show that the Pliocene-Quaternary zone of high-angle faults with extensional separations clearly link with the similarly trending and dipping strike–slip faults onland in the Eşen Valley, thus providing the continuity between the Pliny–Strabo Trenches in the southwest and the Fethiye–Burdur Fault Zone in the northeast. Mapping of many faults in parts of the Fethiye–Burdur Fault Zone shows evidence for sinistral strike–slip but total displacement across the fault zone is at maximum a few tens of kilometres. The STEP fault thus appears to have diminishing displacement associated with it as it propagates upwards into the upper plate from its originating tear in the subducting plate.

Stable isotope geochemistry of magnesite from Holocene salt lake deposits, Taoudenni, Mali

Stable isotope data (δ18O, δ13C) were obtained for magnesite (MgCO3) from Holocene salt lake deposits of the Taoudenni–Agorgott basin, Mali, in which it is the only carbonate mineral present. The deposits have a high glauberite content (Na2Ca(SO4)2), representing a type of continental evaporite formation that is commonly magnesite‐bearing. Samples from seven different levels in a 5 m long sequence were analysed. δ18OV‐PDB values are between +3.17 and +5.91‰, which is lower than might be expected for evaporitic environments, recording reduced 18O enrichment at high salinity. δ13CV‐PDB values are between −1.32‰ and −4.79‰, showing an influence of carbon derived from the decomposition of organic matter rather than from exchange with atmospheric CO2. Covariance between δ18O and δ13C is strong for the lower part of the sequence, recording coupled 18O and 13C enrichment as commonly observed for closed basins. Covariance is lacking for the upper part of the sequence, with only periodic massive magnesite deposition in a more shallow lake. Copyright © 2012 John Wiley & Sons, Ltd.

Paleoenvironmental and paleoecological implications of Permian (Guadalupian) radiolarian and geochemical variations in the Lamar Limestone, Delaware Basin, West Texas (USA)

Geochemical proxies integrated with well-preserved radiolarian data from the Lamar Limestone and lower part of the Reef Trail Member of the Bell Canyon Formation provide insights into paleoceanographically and climatically driven controls on radiolarian distribution in the northern part of the Delaware Basin of west Texas, during late Guadalupian time. Data from a section of the Lamar Limestone (~8.9m; ~500kyr interval), indicate that major variations between sphaerellarian-dominated and Follicucullus-dominated faunas appear to be controlled by fluctuations in nutrient supply and salinity. Geochemical parameters of bulk carbonate carbon and oxygen isotopes (δ13Ccarb and δ18Ocarb), organic carbon isotope (δ13Corg), and total organic carbon (TOC) were analyzed and show variations on two scales. On a fine‐scale, sphaerellarian-dominated beds are associated with a relatively siltier lithology, higher TOC, and higher radiolarian richness but lower diversity, and are interpreted as the result of a rise in productivity stimulated from increased terrestrial input. The radiolarian variations and geochemical evidence imply that the Permian spongiose spumellarians are more opportunistic than Follicucullus (albaillellids) in response to eutrophication. On a broad-scale, δ18Ocarb variations in limestone samples are interpreted mainly as a proxy for paleosalinity in response to changes in basin circulation. Mg/Ca ratios from the carbonate fraction co-vary with δ18Ocarb and are used as an additional paleosalinity proxy in this basin. The limestone beds in the middle part of the section have lighter δ18Ocarb values compared to limestone beds above and below, and likely represent the interval when the Delaware Basin was less restricted and had relatively normal marine salinity. Impacts of diagenesis are mild but are likely the cause for systematically light δ18Ocarb values in silty limestone beds in the middle interval. The Mg/Ca ratios and δ18Ocarb values in the limestone samples both increase from the middle to the top of the section, implying a gradually enhanced evaporitic environment coupled with increased salinity towards the top section. Collectively, these data paint a picture of intermittent restriction in the Delaware Basin, prior to the change to a dominantly evaporative regime in the late Permian, and on a finer scale, show pronounced and frequent ecological fluctuations that appear to be driven by fluxes in terrestrial input and, to some extent, paleoproductivity.

Sedimentological characteristics and geochemical evolution of Nabq sabkha, Gulf of Aqaba, Sinai, Egypt

Nabq sabkha exists 16 km north of Sharm El Sheikh City occupying the low land topography in the alluvial fan zone along the coastal area, Gulf of Aqaba, Sinai, Egypt. The long axis of the sabkha trends NW–SE receiving water from two different sources: meteoric water drained from the surrounding mountainous area and seawater seepage. Field observations help to divide the area into raised beach, hill slopes, sabkha basin, and coastal area. The sabkha basin can be subdivided from its center outward into (1) basin center hypersaline lake flourished with microbial mat and precipitation of halite as rafts, cumulates, and chevrons, (2) saturated saline sand and/or mud flat zone with the extensive growth of gypsum and halite crystals growing displacively as well as different forms of petee structures, and (3) an elevated marginal dry zone with tepee structures. Mineralogical analysis reveals that quartz, halite, and gypsum are the dominant minerals with subordinate amount of aragonite, anhydrite, thenardite, and/or polyhalite. In addition, clay minerals in the mudflat zone are presented by illite and smectite, indicating derivation of soil from the surrounding basement rocks. Chemical analysis of the collected brine samples reveals alkali character in the saline lake (pH = 7.6) and high concentrations of Na+ (680 meq/l), Cl− (940 meq/l), Mg2+ (208 meq/l), Ca2+ (70 meq/l), SO42+ (30 meq/l), and HCO3− (6 meq/l). The high salinity values are due to the aridity of the area, which favors precipitation of halite. Using comparative sedimentological, chemical, and mineralogial methods between such modern and ancient evaporitic environments and by detailed field, petrographic and mineralogical studies of modern evaporite environments help to interpret paleo-depositional environments of ancient evaporites sequences still in debate.

Continental evaporites and the search for evidence of life on Mars

Continental evaporites are deposits that originate from the evaporation of saline waters in the low areas of saline lakes from all continents, except Europe, and mainly consist of chloride, sulphate and potash minerals. In recent years, the discovery on the Martian surface of hydrated salt minerals, including sulphates and chlorides, interpreted as deriving from the desiccation of preexisting large bodies of water, such as lakes, has provided further convincing evidence of liquid water activity on the surface of Mars and, consequently, it has reinforced the plausibility of finding life. Because evaporites require short‐term aqueous processes for their formation, they can trap and preserve over geologic times a biological record made up of halophilic extremophiles—such as microalgae, bacteria, and their remains—that recent research on Earth has shown to be characterized by unexpectedly high biodiversity. This record may consist of varying types of fossils, including morphological fossils, chemofossils and biominerals. As a consequence, continental evaporite environments and their saline deposits are now a primary target for the near future astrobiology missions devoted to the search for fossil Martian life. Lacustrine evaporite deposits and minerals have, therefore, been identified as primary targets for the NASA–ESA joint programme of the Mars sample return, planned for the end of the current decade. Copyright © 2011 John Wiley & Sons, Ltd.

Study of Hypersaline Deposits and Analysis of Their Signature in Airborne and Spaceborne SAR Data: Example of Death Valley, California

Field measurements of dielectric properties of hypersaline deposits were realized over an arid site located in Death Valley, CA. The dielectric constant of salt and water mixtures is usually high but can show large variations, depending on the considered salt. We confirmed values observed on the field with laboratory measurements and used these results to model both the amplitude and phase behaviors of the synthetic aperture radar (SAR) signal at C- and L-bands. Our analytical simulations allow reproducing specific copolar signatures observed in both Airborne SAR (AIRSAR) and Spaceborne Imaging Radar (SIR-C) data, corresponding to the saltpan of the Cottonball Basin. More precisely, the main objective of the present paper is to understand the influence of soil salinity as a function of soil moisture on the dielectric constant of soils and then on the backscattering coefficients recorded by airborne and spaceborne SAR systems. We also propose the copolarized backscattering ratio and phase difference as indicators of moistened and salt-affected soils. More precisely, we show that these copolar indicators should allow monitoring of the seasonal variations of the dielectric properties of saline deposits at both C- and L-bands. Because of the frequency dependence of the ionic conductivity, we also show that L-band SAR systems should be efficient tools for detecting both soil moisture and salinity, while C-band SAR systems are more suitable for the monitoring of soil moisture only. Through the study of terrestrial evaporitic environments by means of spaceborne SAR systems, our results could also be of great interest for defining future planetary missions, particularly for the exploration of Mars.

Geologic reconnaissance of the island of Velika Palagruža (central Adriatic, Croatia)

Velika Palagruža (Pelagosa) is the largest island of the Palagruža archipelago (central Adriatic Sea, Croatia). Despite its minute size the island bears a certain geological interest being the only exposed piece of land in the central part (Mid-Adriatic ridge) of the common Adriatic foreland of the Apenninic and the Dinaridic orogenic domains. The litho-, bio-, and chemostratigraphic (strontium and sulphur isotopes) characteristics of the sedimentary units, along with tectono-structural and geomorphic characteristics of the island, are described in this paper. The oldest Žalo unit is composed of highly deformed siliciclastics containing gypsum, and carbonates of Middle Triassic (Ladinian) age. This unit represents a transitional fl uvial-to-shallow marine, occasionally evaporitic environment, typical of the Middle Triassic rifting phase of the Adriatic microplate. Soft and strongly deformed Žalo unit deposits are found along a probably still active, WNW–ESE striking, subvertical, oblique-slip fault that crosses the entire length of the island. The Žalo unit is probably in diapiric contact with the Lanterna unit, poorly defi ned as Late Triassic, and characterized by dolomite with chert and dolomite breccia, presumably deposited in a transitional platform-to-basin environment of an evolving Adriatic basin. The Lanterna unit deposits are capped by Miocene biocalcarenites of the Salamandrija unit over an almost perpendicular discordance, possibly representing an unconformity, suggesting that an early deformational phase preceded a Miocene marine transgression. Talus, landslide deposits, and humic soil make up the cover of the bedrock sedimentary succession, and they represent the ultimate phase of emersion of the island, which probably occurred during Pliocene(?) to Quaternary times. An active neotectonic regime of the central Adriatic is evidenced by present-day seismicity, while recent uplifting of the island is shown by the presence of remnants of pebbly palaeobeach deposits, marine (erosional) straths, and cyanobacterial supratidal encrustations (pelagosite) currently observed at various elevations above mean sea level.

Geochemistry of coesite-bearing "pyrope quartzite" and related rocks from the Dora-Maira Massif, Western Alps

A complete set of geochemical data for “pyrope quartzite”, various phengite-schist and jadeite-bearing rock intercalations, as well as for enclosing orthogneiss from different localities within the southern Dora-Maira Massif is presented, in order to establish the nature of the different protoliths and their primary relationships. For comparison, Mg-rich rocks from the Eastern Alps occurring along shear zones within granitic gneiss, as well as metaevaporitic rocks from Sar e Sang, Afghanistan, were also studied. Typically, the “pyrope quartzite” and its retrograde derivative are enriched in Mg and strongly depleted in Na, Ca, Fe, Cu, P, Rb, Ba, and Sr with respect to the enclosing orthogneiss of granitic bulk composition. This orthogneiss is usually peraluminous, has normative corundum, and may be best related to S-type granite protoliths. Trace-element contents of the different phengite schists included in the “pyrope quartzite” confirm their close overall kinship to the enclosing orthogneiss. Jadeite-bearing rocks, which occur as conformable layers and boudins within “pyrope quartzite”, in general differ by having lower contents in K, Mg, Rb, and higher contents in Na, Fe, Ca, Mn, P, and Zn. The present study indicates that the “pyrope quartzite” has a metasomatic origin, whereas an evaporitic nature of the protolith can be ruled out. Discrimination plots demonstrate that the “pyrope quartzite” can clearly be distinguished chemically from metasedimentary whiteschist derived from former evaporite environments. The field relations of trails of “pyrope quartzite”, now forming lensoid inclusions within an orthogneiss (former granite), would also make a metasedimentary origin rather unlikely. On the other hand, consistent genetic relationships between “pyrope quartzite” and its enclosing orthogneiss were obtained that virtually match those of Mg-metasomatic leucophyllites from the Eastern Alps occurring along shear zones within granitic gneisses. Similar Mg-rich rocks within the Tauern Window such as within the Monte Rosa and Gran Paradiso Massifs of the Western Alps indicate that local processes of Mg metasomatism occurred throughout the entire range of the Alps.

Beta-carotene—A possible biomarker in the Martian evaporitic environment: Raman micro-spectroscopic study

Due to the discovery of the evaporitic environment on the Martian surface, there is a reasonable possibility that evaporites served (or still serve) as habitats for microbial life if ever present on Mars. At the very least, if no signatures of extant life exist within these rocks, it may sustain molecular remnants as evidence for living organisms in the past. β-Carotene, among other carotenoids, could be such a suitable biomarker. In this study, Raman micro-spectroscopy was tested as a nondestructive method of determining the presence of β-carotene in experimentally prepared evaporitic matrices. Samples prepared by mixing β-carotene with powdered gypsum (CaSO 4·2H 2O), halite (NaCl) and epsomite (MgSO 4·7H 2O) were analyzed using a 785 nm excitation source. Various concentrations of β-carotene in the matrices were investigated to determine the lowest β-carotene content detectable by Raman micro-spectroscopy. Mixtures were also measured with a laser beam permeating the crystals of gypsum and epsomite in order to evaluate the possibility of identifying β-carotene inside the mineral matrix. We were able to obtain a clear β-carotene signal at the 10 mg kg −1 concentration level—the number of registered β-carotene Raman bands differed depending on the particular mineral matrix. Spectral signatures of β-carotene were detected even when analyzing samples containing 1 mg kg −1 of this molecule. The 10–100 mg kg −1 of β-carotene in mineral matrices (halite, epsomite) was detected when analyzed through the monocrystal of gypsum and epsomite, respectively. These results will aid both in-situ analyses on Mars and sample analyses on Earth.

Structural architecture of the Rhodes Basin: A deep depocentre that evolved since the Pliocene at the junction of Hellenic and Cyprus Arcs, eastern Mediterranean

Interpretation of ~ 1500 km of multi-channel seismic reflection profiles shows that the Rhodes Basin evolved in two distinct tectonic stages. A protracted Miocene convergence (episode P1) resulted in the development of a mainly SSE-verging fold–thrust belt. This period corresponds with the emplacement of the Lycian Nappes to the northeast and fold/thrust complexes on the Island of Rhodes to the west. This phase of deformation ceased in the late Miocene. The absence of evaporites in the Rhodes Basin suggests that this region must have remained above the depositional base of marine evaporite environment during the Messinian. The morphology of the M-reflector, and the architecture of the lowermost Pliocene growth strata in the central portion of the Rhodes Basin demonstrate the existence of rugged paleo-relief over the pre-existing Miocene fold–thrust belt in early Pliocene. The middle Pliocene–Quaternary phase of deformation is characterised by NE–SW sinistral transpression (P2), and rapid regional subsidence. The P2 deformation occurred in response to an actively curving Hellenic Arc and the increasing obliquity of its eastern limb, the Pliny–Strabo Trenches (or fault zones) to the convergence vector of the African Plate. During this time, the P1 structures became re-activated as transpressional faults, further facilitating the rapid subsidence. We envisage the evolution of the Rhodes Basin as the result of subsidence associated with loading of the large imbricate thrust panels that carry the western Tauride Mountains in the north. At present, the Pliny–Strabo fault zones are delineated as a NE–SW trending 30–50 km-wide zone that continue into the western segment of the Rhodes Basin. This fault zone is further correlated with the similar trending ~ 50 km-wide Burdur–Fethiye Fault zone in southwestern Turkey.

A rare natural gypsum ooide (Gypsolites) in an evaporitic playa lake of late miocene (?) to pliocene age in central Anatlia, Turkey

Gyposolites are found in the western part of Polatli town, Polatli-Sivrihisar basin, about 90 km from Ankara, Turkey. Gypsolites are observed in a transition zone between a siliciclastic marginal lake and an evaporitic inner lake. The ooide-bearing strata have a small to moderate thickness (10 to 50 cm), are laterally restricted, and partially exhibit a sandy appearance, including various amounts of volcanoclastic grain and mud pellets. They sometimes show micro-gradations, parallel or cross-laminations and rippled surfaces, in which macroscopically there are evidences of biogenic activity such as fossil tracks and burrows, but microbial activity is intensively present within the gypsum cortex, yielding micro-moldic porosity. Gypsolites fabrics suggest either (I) they occur under bacterial activity in local salines or short-lived ponds that occupied the low-energy part of the lake or (ii) weak currents or wind activity exert a significant influence on the gypsolites fabric. Some traces of microbial mats within the cortex and micro-gradations, parallel or cross-laminations indicate that the concomitance of bacterial activity and inorganic processes contributed to the construction of gypsum ooide structures in the Neogene evaporitic lake environment.

Non-marine evaporites with both inherited marine and continental signatures: The Gulf of Carpentaria, Australia, at ∼ 70 ka

Changes in sea-level and associated climatic fluctuations resulted in extreme and cyclic changes in depositional environments in the Gulf of Carpentaria region (N. Australia). Disconnection from the sea led to the establishment of a “Lake Carpentaria”, perched above sea-level. In this environment, evaporitic conditions at about 70 ka produced a repetitive alternation of μm to mm-thick evaporitic and micritic laminae with a varve-like appearance. These precipitates are interpreted as primary features, deposited in a shallow lake that retained limited water in its centre (core MD-32). Elemental and isotope geochemistry of gypsum and micrite laminae show a complex evaporitic environment where initially marine waters evaporated with the input of continental waters compensating for evaporative losses. Reduced continental input could not support a lake of the initial dimensions and the lake contracted to the deepest part of the basin along the north-eastern side of the basin. In a lake with smaller water volume, continental solutes became apparent. While Sr contents and sulfur isotopes indicate marine contributions, strontium isotopes and oxygen isotopes in sulfates reveal continental inputs and other processes such as recycling of previously precipitated evaporites, sulfate reduction and potential reservoir effects. Carbonate- δ 13C and δ 18O values in micritic levels also reveal a continental influence and perhaps variations in organic matter signatures associated with climatic variations and vegetation changes. The REE-normalized patterns in gypsum samples are like those found in northern Cape York rivers, restricting the potential continental inputs into the evaporatic basin to a limited geographical area. The small depletion in LREE-normalized patterns between gypsum and river samples is interpreted as a marine influence while depletions in HREE are considered to be the result of fractionation of HREE during gypsum crystallization. The thickness of the calcite-gypsum couplets is consistent with those precipitated annually in modern evaporitic environments. This and the marked fluctuation between dry (gypsum laminae) and wet (micritic layer) environments, suggests a reduced monsoon-like rainfall pattern operating in northern Australia during evaporite precipitation.

Implications of a 3.472-3.333 Gyr-old subaerial microbial mat from the Barberton greenstone belt, South Africa for the UV environmental conditions on the early Earth.

Modelling suggests that the UV radiation environment of the early Earth, with DNA weighted irradiances of about three orders of magnitude greater than those at present, was hostile to life forms at the surface, unless they lived in specific protected habitats. However, we present empirical evidence that challenges this commonly held view. We describe a well-developed microbial mat that formed on the surface of volcanic littoral sediments in an evaporitic environment in a 3.5-3.3Ga-old formation from the Barberton greenstone belt. Using a multiscale, multidisciplinary approach designed to strongly test the biogenicity of potential microbial structures, we show that the mat was constructed under flowing water by 0.25 microm filaments that produced copious quantities of extracellular polymeric substances, representing probably anoxygenic photosynthesizers. Associated with the mat is a small colony of rods-vibroids that probably represent sulphur-reducing bacteria. An embedded suite of evaporite minerals and desiccation cracks in the surface of the mat demonstrates that it was periodically exposed to the air in an evaporitic environment. We conclude that DNA-damaging UV radiation fluxes at the surface of the Earth at this period must either have been low (absorbed by CO2, H2O, a thin organic haze from photo-dissociated CH4, or SO2 from volcanic outgassing; scattered by volcanic, and periodically, meteorite dust, as well as by the upper layers of the microbial mat) and/or that the micro-organisms exhibited efficient gene repair/survival strategies.

Sedimentology and paleoecology of an Eocene–Oligocene alluvial–lacustrine arid system, Southern Mexico

A depositional model of the Eocene–Oligocene Coatzingo Formation in Tepexi de Rodríguez (Puebla, Mexico) is proposed, based on facies analysis of one of the best-preserved sections, the Axamilpa Section. The sedimentary evolution is interpreted as the retrogradation of an alluvial system, followed by the progressive expansion of an alkaline lake system, with deltaic, palustrine, and evaporitic environments. The analysis suggests a change towards more arid conditions with time. Fossils from this region, such as fossil tracks of artiodactyls, aquatic birds and cat-like mammals, suggest that these animals traversed the area, ostracods populated the lake waters, and plants grew on incipient soils and riparian environments many times throughout the history of the basin. The inferred habitat for some fossil plants coincides with the sedimentological interpretation of an arid to semiarid climate for that epoch. This combined sedimentological–paleontological study of the Axamilpa Section provides an environmental context in which fossils can be placed and brings into attention important biotic episodes, like bird and camelid migrations or the origin of endemic but extinct plants in this area.

Geochemical characterisation, provenance, source and depositional environment of ‘Roches Argilo-Talqueuses’ (RAT) and Mines Subgroups sedimentary rocks in the Neoproterozoic Katangan Belt (Congo): Lithostratigraphic implications

The chemical characteristics of sedimentary rocks provide important clues to their provenance and depositional environments. Chemical analyses of 192 samples of Katangan sedimentary rocks from Kolwezi, Kambove–Kabolela and Luiswishi in the central African Copperbelt (Katanga, Congo) are used to constrain (1) the source and depositional environment of RAT and Mines Subgroup sedimentary rocks and (2) the geochemical relations between the rocks from these units and the debate on the lithostratigraphic position of the RAT Subgroup within the Katangan sedimentary succession. The geochemical data indicate that RAT, D. Strat., RSF and RSC are extremely poor in alkalis and very rich in MgO. SD are richer in alkalis, especially K 2O. Geochemical characteristics of RAT and Mines Subgroups sedimentary rocks indicate deposition under an evaporitic environment that evolved from oxidizing (Red RAT) to reducing (Grey RAT and Mines Subgroup) conditions. There is no chemical difference between RAT and fine-grained clastic rocks from the lower part of the Mines Subgroup. The geochemical data preclude the genetic model that RAT are syn-orogenic sedimentary rocks originating from Mines Group rocks by erosion and gravity-induced fragmentation in front of advancing nappes.

Building the Biomarker Tree of Life

It is the great challenge of geomicrobiology to study microorganisms in the context of their environments, both in Earth’s distant past and in the present. Planet Earth and its biosphere have evolved together, and a chronicle of Earth’s ecosystems and their geochemical cycles is recorded in sedimentary rocks spanning billions of years. A relatively new and very powerful approach to read these subtle microbial and environmental signatures in ancient rocks is the study of molecular fossils, or biomarkers, within the context of the biochemistry and phylogeny of their origins. Biomarkers are organic compounds (primarily lipids) that have particular biosynthetic origins and can be preserved in sediments and sedimentary rocks. The most valuable biomarkers are taxonomically specific, i.e., they can be assigned to a defined group of organisms, and are resistant to degradation. Reading the biomarker signatures in rocks can give information about the ancient record of anoxic conditions in the water column (e.g., Summons and Powell 1986), the intensity of UV radiation penetrating lakes (Leavitt et al. 1997), hypersalinity in evaporitic environments (Grice et al. 1998), and the function of microbial communities at methane seeps (Hinrichs et al. 1999). Biomarkers have helped to reconstruct the first appearance of major groups of organisms (e.g., McCaffrey et al. 1994; Moldowan et al. 1994; Moldowan and Talyzina 1998; Brocks et al. 2005), elucidate events of global climate change (e.g., Brassell et al. 1986), record major perturbations and reorganization of geochemical cycles (e.g., Logan et al. 1995; Kuypers et al. 1999) and document catastrophic losses in biodiversity (e.g., Grice et al. 2005). They are even used as tools to help in the discovery of major new petroleum reservoirs (for a review see Peters et al. 2004). The field of biomarker research is young and many new applications wait to be …