δ18O Values Of Calcite Research Articles

Depth variation of carbon and oxygen isotopes of calcites in Archean altered upperoceanic crust: Implications for the CO2 flux from ocean to oceanic crust in the Archean

Middle Archean greenstones with mid-ocean ridge basalt affinity and overlying bedded chert/banded iron formation (BIF) are exposed in the Cleaverville area, Pilbara Craton, Western Australia. On the basis of the hydrothermal carbonation of these Cleaverville greenstones, we estimated the potential CO2 flux from ocean to oceanic crust and the physical–chemical conditions of the subseafloor hydrothermal system for the middle Archean. The greenstones exhibit various extents of carbonation, and the igneous minerals contained in the greenstones are partly or completely replaced by calcite and other secondary minerals. The degreeof carbonation correlates with stratigraphy; the volume concentration of calcite in greenstones decreaseswith increasing depth below the chert/BIF horizon. Our results clearly indicate that the hydrothermal carbonation occurred along the axial zones of a middle Archean mid-ocean ridge. Both δ13C and δ18O values of calcite also change with the depth below the chert/BIF horizon. The δ18O value of calcite decreases with increasing depth, reflecting increasing temperature, while the δ13C value of calcite changes from positive to negative stratigraphically downward. A model reconstructing the δ13C of calcite suggests that the observed isotopic variation reflects the thermal structure and that the carbon source for calcite was derived from seawater. These mineralogical, geochemical, and geological features of calcite provide the amount of CO2 in the whole oceanic crust per seafloor unit area (1.2×107mol/m2). Based on this value, CO2 flux from the ocean to the oceanic crust in the middle Archean can be estimated to be 1.5×1014mol/yr when the spreading rate (m2/yr) of Archean oceanic crust is assumed to have been three times higher than it is today. The estimated CO2 flux into the oceanic crust is two orders of magnitude higher than the modern value, which points to the significance of sea-floor hydrothermal carbonation in the Archean carbon cycle.

Carbon- and oxygen-isotope compositions of a modern deep-water brachiopod Campagea japonica collected off Aguni-jima, Central Ryukyu Islands, southwestern Japan

We present carbon- and oxygen-isotope compositions in a shell (δ13CS and δ18OS, respectively) of a modern brachiopod Campagea japonica collected at a water depth of 606 m off Aguni-jima, Central Ryukyu Islands, southwestern Japan. The δ13CS and δ18OS profiles of the outermost portions of the secondary shell layer along the axis of shell growth do not display significant seasonal variations. The δ13CS values display a gradual decrease with growth, whereas the δ18OS values are nearly constant. The δ13CS and δ18OS values along a particular growth line and from the inner shell surface are rather constant. The δ13CS values near the posterior shell edge of the ontogenetic series and the δ18OS values near the anterior shell edge of the inner series fall within the ranges of δ13C and δ18O values of calcite in equilibrium with ambient seawater (equilibrium calcite) (δ13CEC and δ18OEC, respectively). Therefore, they are the best portions to use as proxies of δ13C values of dissolved inorganic carbon (δ13ODIC) and temperature and/or δ18O values of seawater (δ18OSW), respectively. The δ13CS values of the ontogenetic series near the anterior shell edge and the entire isochronous and inner series display the smallest decrease (less than ~0.2‰) compared with the δ13CEC values. The δ18OS values of the entire ontogenetic series and an interior part of the inner series are slightly greater (less than ~0.3‰) than the δ18OEC values. Thus, these portions can be used as paleoenvironmental proxies if appropriate corrections are made. A significant positive correlation is recognized between the δ13CS and δ18OS values for the ontogenetic-series samples, which can be ascribed to a kinetic isotope effect. A metabolic isotope effect is not evident for the δ18OS values in these samples. Our study provides useful information for the appropriate selection of brachiopod taxa and shell portions that reflect isotopic composition of ambient seawater for paleoenvironmental studies.

Fluid flow through the sedimentary cover in northern Switzerland recorded by calcite–celestite veins (Oftringen borehole, Olten)

Abundant veins filled by calcite, celestite and pyrite were found in the core of a 719 m deep borehole drilled in Oftringen near Olten, located in the north-western Molasse basin, close to the thrust of the Folded Jura. Host rocks are calcareous marl, argillaceous limestone and limestone of the Dogger and Malm. The δ18O values of vein calcite are lower than in host rock carbonate and, together with microthermometric data from fluid inclusions in vein calcite, indicate precipitation from a seawater-dominated fluid at average temperatures of 56–68°C. Such temperatures were reached at the time of maximum burial of the sedimentary pile in the late Miocene. The depth profile of δ13C and 87Sr/86Sr values and Sr content of both whole-rock carbonate and vein calcite show marked trends towards negative δ13C, high 87Sr/86Sr, and low Sr content in the uppermost 50–150 m of the Jurassic profile (upper Oxfordian). The 87Sr/86Sr of vein minerals is generally higher than that of host rock carbonate, up to very high values corresponding to Burdigalian seawater (Upper Marine Molasse, Miocene), which represents the last marine incursion in the region. No evidence for internally derived radiogenic Sr (clay minerals) has been found and so an external source is required. S and O isotope composition of vein celestite and pyrite can be explained by bacterial reduction of Miocene seawater sulphate. The available data set suggests the vein mineralization precipitated from descending Burdigalian seawater and not from a fluid originating in the underlying Triassic evaporites.

Isotope geochemistry of volcanic rocks from the Barton Peninsula, King George Island, Antarctica

C, O, H, S and Sr isotopes were investigated to characterize the isotopic composition of the Paleocene-Eocene volcanic rocks in Barton Peninsula, King George Island, Antarctica. The analyzed samples of volcanic rocks cover a range from basalt to andesite. The δ 13C and δ 18O values of calcite in volcanic rocks range from −7.5‰ to −3.2‰ and 0.4‰ to 5.1‰, respectively. The homogenous δ 13Ccal and depleted δ 18Ocal values relative to primary magmatic values reflect the effect of high temperature alteration of volcanic rocks. The δ 18O of volcanic rocks ranges from −1.1‰ to 7.2‰, and the majority of values are significantly lower than those of primary magmatic values. The low δ 18OWR values are compatible with high temperature alteration. The initial values of 87Sr/86Sr of volcanic rocks span a narrow range from 0.703 312 to 0.703 504 which belong to the present mid-oceanic ridge basalt. The values of 87Sr/86Sr of volcanics in the Barton Peninsula are similar to those determined in the Fildes Peninsula, King George Island. The δDWR and H2O contents of volcanic rocks range from −74‰ to −66‰ and 0.67 wt.% to 2.74 wt.%, respectively. The higher δDWR and H2O wt.% of volcanic rocks relative to fresh basalts also result from high temperature alteration. Sulfur isotope compositions of the volcanic rocks range from −12.5‰ to −7.0‰; these values may represent fractionation accompanying partial oxidation of magmatic S or incorporation of some bacteriogenic sulfide. The low S contents and negative correlation with C and H2O reflect the formation of calcite and breakdown of sulfide as a result of high temperature water-rock interaction.

Indications for the past redox environments in deep groundwaters from the isotopic composition of carbon and oxygen in fracture calcite, Olkiluoto, SW Finland

In paleohydrogeological studies, the geochemical and isotope geochemical composition of fracture calcites can be utilised to gain information about the evolution of the composition of deep groundwaters in crystalline bedrock. The aim of our study was to investigate the latest hydrogeochemical evolution of groundwaters in the crystalline bedrock at Olkiluoto, which is the planned site for deep geological disposal of spent nuclear fuel. Samples were collected from drill cores intercepting water-conducting fractures at the upper ∼ 500 m of the bedrock. The latest fracture calcite generations were identified using optical microscopy and electron microprobe. They occur as thin ∼ 10–200 μ m crusts or small euhedral crystals on open fracture surfaces. These latest calcite fillings were carefully sampled and analysed for the isotopic composition on carbon and oxygen. In addition, fluid inclusion homogenisation temperatures were determined on selected calcite samples. Fluid inclusion data indicated a low temperature of formation for the latest fracture calcite fillings. The δ18O values of calcite in these fracture fillings vary only slightly, from−7.3 to−11.5 ‰ (Vienna Pee Dee Belemnite, VPDB), whereas the δ13C values fluctuate widely, from−30 to+31 ‰ (VPDB). The δ13C values of latest calcite fillings show a systematic pattern with depth, with high and variable δ13C values below 50 m. The high δ13C values indicate active methanogenesis during the formation of the latest calcite fillings. In contrast, the present-day methanic redox environment is restricted to depths below 200–300 m. It is possible that the shift in the redox environment at Olkiluoto has occurred during infiltration of -rich marine waters, the latest of such events being the infiltration of brackish waters of the Littorina Sea stage of the Baltic Sea at ∼ 8000–3000 BP.

A biogeochemical study of sediments from the eutrophic Lake Lugano and the oligotrophic Lake Brienz, Switzerland

The biomarker composition and stable isotope (C, O) ratio values of organic matter (OM) and carbonate from sediment cores from the oligotrophic Lake Brienz and the eutrophic Lake Lugano (both in Switzerland) are compared, in order to obtain information about OM sources and transformation processes. Eutrophic conditions at Lake Lugano are reflected in elevated total organic carbon (TOC) content and hydrogen index (HI) values, as well as higher lipid concentrations. Parallel down core trends in δ13C values of TOC and calcite in the Lake Lugano sediments reflect bioproductivity cycles. Variations in δ18O values of calcite are consistent with changes in mean summer temperature over the time interval covered by the core. In contrast, such a correlation does not exist for Lake Brienz and there the stable isotope composition of calcite reflects its allochthonous origin. In the sediments of both lakes, fatty acid (FA) distributions and the composition of n-alkanols and n-alkanes indicate highly variable proportions of autochthonous OM sources (algae, zooplankton, bacteria) and OM from land plants. Enhanced in situ microbial synthesis during sediment deposition in Lake Lugano is suggested by the higher TOC-normalised concentrations of branched chain FAs (C15–C17), hopanoic acids and triterpenoid alcohols (i.e. tetrahymanol, diplopterol). Variations in the concentrations of cholesterol are related to contributions from zooplankton and/or green algae, while sitosterol concentrations reflect the input of vascular plants. Periods of increased input of OM from diatoms are evidenced by high 24-methylcholesta-5,22-dien-3β-ol (either epibrassicasterol or brassicasterol) and/or highly branched isoprenoid (HBI) alkenes concentrations. High relative concentrations of diplopterol in Lake Lugano sediments are consistent with the predominance of cyanobacteria commonly observed in eutrophic lakes. The presence of archeol and hydroxyarcheol in very low concentrations in the Lugano sediments argues for the activity of methanogens and/or anaerobic methanotrophs.Differences in OM degradation processes are reflected in higher chlorin index values in the Brienz sediments but higher saturated vs. unsaturated n-FAs in the core from Lugano. Higher concentrations of branched chain FAs and 16:1ω7 n-FA, as well as enhanced 18:1ω7/18:1ω9 n-FA, are consistent with enhanced bacterial biomass in the Lugano water column or sediments. The preservation of phytol seems to be enhanced in sediments with a high relative contribution of land plant OM. Major factors affecting OM accumulation in the lakes are differences in OM sources (i.e. terrestrial OM vs. autochthonous production), extent of bacterial activity and most likely oxygen availability in the water column.

Environmental change in the subtropics during the late middle Eocene greenhouse and global implications

Oxygen isotope records from ODP Site 1052 (Blake Nose, subtropical North Atlantic Ocean) indicate significant short‐term, high‐amplitude variability (up to 1.4‰ in about 2500 to 4000 years) during the late middle Eocene (37.9–37.5 Ma). These variations reflect a combination of changes in sea surface temperature and the oxygen isotope composition of the regional seawater. In order to independently evaluate the magnitude of SST changes at Blake Nose and better understand the nature of environmental change during the late middle Eocene, we present planktonic foraminiferal Mg/Ca data combined with δ18O data from the same samples. The calculated Mg/Ca paleotemperatures indicate a decrease in SSTs over the study interval from ∼33 to 28°C at the same time that previously published foraminiferal δ18Ocalcite values from the same samples also decrease. Thus, the δ18Ocalcite values must reflect a significant component of seawater δ18O change in order to reconcile the opposite paleotemperature trends reflected by both proxies. Calculated δ18Oseawater values decreased from ∼3‰ at 37.83 Ma to ∼2‰ at 37.6 Ma. The combined trends of the SST and δ18Oseawater cannot be explained by an increase of continental ice over this time interval. Instead, the data favor an overall weakening of the hydrological cycle as global climate transitioned from the greenhouse to icehouse.

Mechanisms of oxygen isotopic exchange and isotopic evolution of 18O/16O-depleted periclase zone marbles in the Alta aureole, Utah: insights from ion microprobe analysis of calcite

Oxygen isotope ratios have been measured by ion microprobe and millimeter-scale dental drill along detailed sampling traverses across the boundary between periclase-bearing (δ18O = 11.8‰) and periclase-free (δ18O = 17.2‰) marble layers in the periclase (Per) zone of the Alta Stock aureole, Utah. These data define a steep, coherent gradient in δ18O that is displaced a short distance (~4 cm) into the periclase-free (Cal + Fo) layer. SEM and ion microprobe analyses show two isotopically and texturally distinct types of calcite at the grain scale. Clear (well polished) calcite grains are isotopically homogeneous (within analytical uncertainty; ±0.27‰, 2SD). More poorly polished (pitted), texturally retrograde ‘turbid’-looking calcite has lower and more variable δ18O values, and replaces clear calcite along fractures, cleavage traces or grain boundaries. Despite significant lowering of the δ18O values in calcite throughout both layers during prograde metamorphism, ion microprobe analyses indicate that individual clear calcite grains are now isotopically homogeneous across the entire gradient in δ18O. Diffusion calculations indicate that conservative time scales required for isotopic homogenization of calcite grains by volume diffusion, 30,000–62,000 years at 575–600°C, exceed significantly the timescale (~1,250 years) estimated for the prograde development of the δ18O gradient at the boundary between these two marble layers. The ion microprobe data and these diffusion calculations suggest instead that surface reaction mechanisms accompanying recrystallization are responsible for the observed oxygen isotope homogeneity of these calcite grains. Thus, the ion microprobe data are consistent with the formation of calcite in oxygen isotope exchange equilibrium with infiltrating fluid during prograde reaction and recrystallization.

The influence of hydrology and climate on the isotope geochemistry of playa carbonates: a study from Pilot Valley, NV, USA

AbstractCarbonates often accompany lake and lake‐margin deposits in both modern and ancient geological settings. If these carbonates are formed in standing water, their stable isotope values reflect the aquatic chemistry at the time of precipitation and may provide a proxy for determining regional hydrologic conditions. Carbonate rhizoliths and water samples were collected from a playa lake in eastern Nevada. Pilot Valley (∼43°N) is a closed‐basin, remnant playa from the Quaternary desiccation of palaeo‐Lake Bonneville. Water is added to the playa margin by free convection of dense brines to the east and forced convection of freshwater off the alluvial fan to the west. Both freshwater and saline springs dot the playa margin at the base of an alluvial fan. Water samples collected from seven springs show a range from −16 to −0·2‰ (Vienna Standard Mean Ocean Water), and are consistent with published values. The δ18Ocalcite values from rhizolith samples range from −18·3 to −6·7‰ (Vienna Pee Dee Belemnite), and the average is −12‰ V‐PDB (1 − σ SD 2‰). With the exception of samples from Little Salt Spring, the range in the δ18Ocalcite values collected from the rhizoliths confirms that they form in equilibrium with ambient water conditions on the playa. The initial geochemical conditions for the spring waters are dictated by local hydrology: freshwater springs emerge in the northern part of the basin to the east of a broad alluvial fan, and more saline springs emerge to the south where the influence of the alluvial fan diminishes. Rhizoliths are only found near the southern saline springs and their δ13Ccalcite values, along with their morphology, indicate that they only form around saltgrass (Distichlis sp.). As the residence time of water on the playa increases, evaporation, temperature change and biological processes alter the aquatic chemistry and initiate calcite precipitation around the plant stems. The range in δ18Ocalcite values from each location reflects environmental controls (e.g. evaporation and temperature change). These rhizoliths faithfully record ambient aquatic conditions during formation (e.g. geochemistry and water depth), but only record a partial annual signal that is constrained by saltgrass growth and the presence of standing water on the playa margin.

Thermal history of the unsaturated zone at Yucca Mountain, Nevada, USA

Abstract Secondary calcite, silica and minor amounts of fluorite deposited in fractures and cavities record the chemistry, temperatures, and timing of past fluid movement in the unsaturated zone at Yucca Mountain, Nevada, the proposed site of a high-level radioactive waste repository. The distribution and geochemistry of these deposits are consistent with low-temperature precipitation from meteoric waters that infiltrated at the surface and percolated down through the unsaturated zone. However, the discovery of fluid inclusions in calcite with homogenization temperatures (Th) up to ∼80 °C was construed by some scientists as strong evidence for hydrothermal deposition. This paper reports the results of investigations to test the hypothesis of hydrothermal deposition and to determine the temperature and timing of secondary mineral deposition. Mineral precipitation temperatures in the unsaturated zone are estimated from calcite- and fluorite-hosted fluid inclusions and calcite δ18O values, and depositional timing is constrained by the 207Pb/235U ages of chalcedony or opal in the deposits. Fluid inclusion Th from 50 samples of calcite and four samples of fluorite range from ∼35 to ∼90 °C. Calcite δ18O values range from ∼0 to ∼22‰ (SMOW) but most fall between 12 and 20‰. The highest Th and the lowest δ18O values are found in the older calcite. Calcite Th and δ18O values indicate that most calcite precipitated from water with δ18O values between −13 and −7‰, similar to modern meteoric waters. Twenty-two 207Pb/235U ages of chalcedony or opal that generally postdate elevated depositional temperatures range from ∼9.5 to 1.9 Ma. New and published 207Pb/235U and 230Th/Uages coupled with the Th values and estimates of temperature from calcite δ18O values indicate that maximum unsaturated zone temperatures probably predate ∼10 Ma and that the unsaturated zone had cooled to near-present-day temperatures (24–26 °C at a depth of 250 m) by 2–4 Ma. The evidence of elevated temperatures persisting in ash flow tuffs adjacent to parent calderas for as much as ∼8 Ma is a new finding, but consistent with thermal modeling. Simulations using the HEAT code demonstrate that prolonged cooling of the unsaturated zone is consistent with magmatic heat inputs and deep-seated (sub-water table) hydrothermal activity generated by the large magma body ∼8 km to the north that produced the 15–11 Ma ash flows and ash falls that make up Yucca Mountain. The evidence discussed in this and preceding papers strongly supports unsaturated zone deposition of the secondary minerals from descending meteoric waters. Although depositional temperatures reflect conductive (and possibly vapor-phase convective) heating of the unsaturated zone related to regional magmatic sources until perhaps 6 Ma, depositional conditions similar to the present-day unsaturated zone have prevailed for at least the past 2–4 Ma.

Isotopic evidence for channeled fluid flow in low‐grade metamorphosed Jurassic accretionary complex in the Northern Chichibu belt, western Shikoku, Japan

AbstractThe low‐grade metamorphosed Jurassic accretionary complex of the Northern Chichibu Belt, Hijikawa area, western Shikoku, is divided into two units, the Hijikawa and Kanogawa units, that are separated by an out‐of‐sequence thrust (OOST), the Ozu‐Kawabegawa Fault. The Kanogawa unit south of the Ozu‐Kawabegawa fault consists mainly of sandstone, shale, broken formation of alternating sandstone and shale, greenstone, chert, and pelitic melange, while the Hijikawa unit is characterized by a stack of subunits separated by small‐scale thrusts. The subunits are mainly made up of basic, pelitic and psammitic semischists, schistose pelitic melange, and chert. Petrological and mineralogical constraints suggest peak metamorphic conditions of 204–247 °C at 1–3 kbar in the Kanogawa unit, and 228–289 °C at 3–5.6 kbar in the Hijikawa unit. Quartz and quartz‐calcite veins are widely developed in the study area, especially in the Hijikawa unit. Regional variations in stable isotopic data show that the δ18Oquartz and δ18Ocalcite values in veins tend to increase towards the Ozu‐Kawabegawa Fault. The δ18Owhole rock values are remarkably high in some subunits close to OOSTs within the Hijikawa unit. Oxygen isotopic compositions from vein quartz indicate that a higher δ18O fluid migrated upward from depth along the Ozu‐Kawabegawa Fault within the accretionary prism during prehnite‐pumpellyite facies metamorphism. The fluid source is inferred to be pelitic rocks at higher temperatures and greater depths within the accretionary prism.

Reconstruction of the Middle Pleistocene climate of south Mediterranean using the Wadi Sannur speleothem, eastern Desert, Egypt

The Wadi Sannur speleothems record climate changes spanning 188 and 136 kyr before present. Petrographically, the studied speleothem laminae are made up entirely of fibrous calcite except for one lamina that displays microspar fabric. Time-series analysis of the data set reveals regular changes in Sr/Ca and Mg/Ca at a scale that matches the thickness of the different laminae. δ18O values of the stalactite laminae range from −7.2 to −10.1 VPDB‰ The lower δ18Ocalcite values, which are correspond to the second oldest lamina (WSS5), indicate that the drip-waters were likely affected by evaporative fractionation and that the speleothem activity has probably stopped due to the very arid conditions that followed the pluvial period. The variations of the δ13C values between −5.0 and −2.3‰ VPDB argue that the drip water composition is influenced by the interaction with the overlying grass-covered ecosystem and the degree of aridity rather than the bedrock (δ13C −0.9‰ VBDP).87Sr/86Sr ratios of the interglacial lamina are low (0.70781–0.70808), whereas the glacial lamina display higher ratios (0.70826–0.70859) in comparison with the nummulitic limestone bedrock (0.70753).

Stalagmite stable isotope record of recent tropical cyclone events

Research Article| February 01, 2007 Stalagmite stable isotope record of recent tropical cyclone events Amy Benoit Frappier; Amy Benoit Frappier 1Department of Geology & Geophysics, Boston College, Chestnut Hill, Massachusetts 02467, USA Search for other works by this author on: GSW Google Scholar Dork Sahagian; Dork Sahagian 2Environmental Initiative and Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA Search for other works by this author on: GSW Google Scholar Scott J. Carpenter; Scott J. Carpenter 3Center for Global and Regional Environmental Research and Department of Geoscience, University of Iowa, Iowa City, Iowa 52242, USA Search for other works by this author on: GSW Google Scholar Luis A. González; Luis A. González 4Department of Geology, University of Kansas, Lawrence, Kansas 66045, USA Search for other works by this author on: GSW Google Scholar Brian R. Frappier Brian R. Frappier 5Department of Natural Resources, University of New Hampshire, Durham, New Hampshire 03857, USA Search for other works by this author on: GSW Google Scholar Geology (2007) 35 (2): 111–114. https://doi.org/10.1130/G23145A.1 Article history received: 28 Jun 2006 rev-recd: 12 Sep 2006 accepted: 16 Sep 2006 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Amy Benoit Frappier, Dork Sahagian, Scott J. Carpenter, Luis A. González, Brian R. Frappier; Stalagmite stable isotope record of recent tropical cyclone events. Geology 2007;; 35 (2): 111–114. doi: https://doi.org/10.1130/G23145A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We present a 23 yr stalagmite record (1977–2000) of oxygen isotope variation, associated with 11 tropical cyclones (TCs), from Actun Tunichil Muknal cave in central Belize. High-resolution microsampling yielded a record of monthly to weekly temporal resolution that contains abrupt decreases (negative excursions) in calcite δ18O values that correspond with recent TC rain events nearby. A logistic discriminant model reliably identified TC proxy signals using the measurable parameters δ18O and δ13C values, and single point changes in δ18O value. The logistic model correctly identified 80% of excursions as TC events and incorrectly classified only 1 of nearly 1200 nonstorm sampling points. In addition to enabling high-resolution TC frequency reconstruction, this geologic proxy also provides information about the intensity of individual TCs. A multiple regression predicted TC intensity (R2 = 0.465, p = 0.034) using sampling frequency and excursion amplitude. Consistent with previous low-resolution studies, we found that the decadal average δ18O value was lower during the 1990s when several TCs produced rainfall in the area, but higher during the 1980s when only one TC struck. Longer, accurately dated, high-resolution speleothem stable isotope records may be a useful new tool for paleotempestology, to clarify associations between highly variable TC activity and the dynamic range of Quaternary climate. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geology and geochemistry of jasperoids from the Gold Bar district, Nevada

Gold Bar is one of several Carlin-type gold mining districts located in the Battle Mountain–Eureka trend, Nevada. It is composed of one main deposit, Gold Bar; five satellite deposits; and four resources that contain 1.6 Moz (50 t) of gold. All of the deposits and resources occur at the intersection of north-northwest- and northeast-trending high-angle faults in slope facies limestones of the Devonian Nevada Group exposed in windows through Ordovician basin facies siliciclastic rocks of the Roberts Mountains allochthon. Igneous intrusions and magnetic anomalies are notably absent. The Gold Bar district contains a variety of discordant and stratabound jasperoid bodies, especially along the Wall Fault zone, that were mapped and studied in some detail to identify the attributes of those most closely associated with gold ore and to constrain genetic models. Four types of jasperoids, J0, J1, J2, and J3, were distinguished on the basis of their geologic and structural settings and appearance. Field relations suggest that J0 formed during an early event. Petrographic observations, geochemistry, and δ18O values of quartz suggest it was overprinted by the hydrothermal event that produced ore-related J1, J2, and J3 jasperoids and associated gold deposits. The greater amount of siliciclastic detritus present in J0 jasperoids caused them to have higher δ18O values than J1,2,3 jasperoids hosted in underlying limestones. Ore-related jasperoids are composed of main-ore-stage replacements and late-ore-stage open-space filling quartz with variable geochemistry and an enormous range of δ18O values (24.5 and −3.7‰). Jasperoids hosted in limestones with the most anomalous Au, Ag, Hg, ±(As, Sb, Tl) concentrations and the highest δ18O values are associated with the largest deposits. The 28‰ range of jasperoid δ18O values is best explained by mixing between an 18O-enriched fluid and an 18O-depleted fluid. The positive correlation between the sizes of gold deposits and the δ18O composition of jasperoids indicates that gold was introduced by the 18O-enriched fluid. The lowest calculated δ18O value for water in equilibrium with late-ore-stage quartz at 200°C (−15‰) and the measured δD value of fluid inclusion water extracted from late-ore-stage orpiment and realgar (−116‰) indicate that the 18O-depleted fluid was composed of relatively unexchanged meteoric water. The source of the 18O-enriched ore fluid is not constrained. The δ34S values of late-ore-stage realgar, orpiment, and stibnite (5.7–15.5‰) and barite (31.5–40.9‰) suggest that H2S and sulfate were derived from sedimentary sources. Likewise, the δ13C and δ18O values of late-stage calcite (−4.8 to 1.5‰ and 11.5 to 17.4‰, respectively) suggest that CO2 was derived from marine limestones. Based on these data and the apparent absence of any Eocene intrusions in the district, Gold Bar may be the product of a nonmagmatic hydrothermal system.

Giant calcite‐cemented concretions, Dakota Formation, central Kansas, USA

AbstractGiant spheroidal concretions (cannonball concretions; some nearly 6 m in diameter) in fluvial channel‐fill sandstones at two localities of the Dakota Sandstone formed by import of cement constituents at a burial depth of <1 km. During cannonball concretion growth a self‐organizational process restricted concretions to a relatively few but widely spaced, and locally, evenly spaced, sites. Other forms of calcite cements at these localities are cement patches in the form of intergrown grape‐size concretions (grapestone), and, locally, pervasive cement. An early episode of invasion by thermogenically generated H2S, which reacted with iron oxides on detrital grains, generated scattered pyrite crystals and decimetre‐scale spheroidal pyrite concretions. Intergranular volumes (IGV) in the concretions range from 36% to 27%. The absence of a trend in IGV and of carbon and oxygen‐isotope ratios from cannonball centres to margins indicates that these concretions did not cement progressively outwards from the centre. Rather, the modern spheres represent the spatial extent of nucleation sites that were not otherwise organized within that volume. Carbon and oxygen‐isotope values for concretion calcites plot along a swath between depleted values of δ18C of −36‰ and δ18O of −13‰ and enriched values of −4‰ and −6‰, respectively. Four groups of calcites are evident on the basis of trace‐element content and suggest that the calcite precipitated across a range of oxidation conditions that do not correlate strongly with the isotopic compositions. Although fluvial overbank sandstones have some pedogenic calcite, the channel sandstones have at most a trace of pedogenic calcite and carbonate rock fragments, so that the bulk of cement components were imported to the sandstones. Carbon and calcium sources for calcite cement include marine limestone, carbonate shells, and anhydrite in addition to HCO derived from oxidized methane, most likely derived from beds underlying or laterally in communication with Dakota sandstones. HCO in ascending formation waters, released during compaction, mixed with meteoric water whose temperature and composition varied with time, to generate the 7‰ range in δ18Ocalcite values measured.

Jurassic septarian concretions from NW Scotland record interdependent bacterial, physical and chemical processes of marine mudrock diagenesis

AbstractSeptarian concretions in the Staffin Shales Formation (Kimmeridgian, Isle of Skye) allow controls on concretion rheology and septarian cracking to be investigated. Stratabound concretions consist of anhedral ferroan calcite microspar enclosing clay and minor pyrite. Intergranular volumes range from 77% to 88%, and calcite δ13C and δ18O values in most concretion bodies range from −10·0‰ to −17·3‰ and +0·3‰ to −0·6‰ respectively, consistent with rapid and pervasive cementation in marine pore fluids. Septarian rupture occurred during incipient cementation, with a sediment volume reduction of up to 43%. Crack‐lining brown fibrous calcite records pore fluid re‐oxygenation during a depositional hiatus, followed by increasing Fe content and δ13C related to bacterial methanogenesis. Brown colouration results from an included gel‐like polar organic fraction that probably represents bacterially degraded biomass. A new hypothesis for concretion growth and septarian cracking argues that quasi‐rigid ‘proto‐concretions’ formed via binding of flocculated clays by bacterial extracellular polysaccharide substances (EPS). This provided rheological and chemical conditions for tensional failure, subcritical crack growth, volume contraction, calcite nucleation, and incorporation of degraded products into crack‐lining cements. Bacterial decay of EPS and syneresis of host muds provided internal stresses to initiate rupture at shallow burial. Development of septarian (shrinkage) cracks in muds is envisaged to require pervasive in situ bacterial colonization, and to depend on rates of carbonate precipitation versus EPS degradation and syneresis. Subsequent modification of septarian concretions included envelopment by siderite and calcite microspar, hydraulic fracturing associated with Cretaceous shallow burial or Palaeogene uplift; and cementation by strongly ferroan, yellow sparry calcite that records meteoric water invasion of the host mudrocks. An abundance of fatty acids in these spars indicates aqueous transport of organic breakdown products, and δ13C data suggest a predominantly methanogenic bicarbonate source. However, the wide δ18O range for petrographically identical cement (−1·3‰ to −15·6‰) is difficult to explain.

Evidence for high-frequency late Glacial to mid-Holocene (16,800 to 5500 cal yr B.P.) climate variability from oxygen isotope values of Lough Inchiquin, Ireland

A 7.6-m core recovered from Lough Inchiquin, western Ireland provides evidence for rapid and long-term climate change from the Late Glacial period to the Mid-Holocene. We determined percentage of carbonate, total organic matter, mineralogy, and δ18Ocalcite values to provide the first high-resolution record of climate variability for this period in Ireland. Following deglaciation, rapid climate amelioration preceded large increases in GISP2 δ18Oice values by ∼2300 yr. The Oldest Dryas (15,100 to 14,500 cal yr B.P.) Late Glacial event is documented in this record as a decrease in δ18Ocalcite values. Brief warming at ∼12,700 cal yr B.P. was followed by characteristic Younger Dryas cold and dry climate conditions. A rapid increase in δ18Ocalcite values at ∼10,500 cal yr B.P. marked the onset of Boreal warming in western Ireland. The 8200 cal yr B.P. event is represented by a brief cooling in our record. Prior to general warming, a larger and previously undescribed climate anomaly between 7300 and 6700 cal yr B.P. is characterized by low δ18Ocalcite values with high-frequency variability.

Sedimentary geochemical evidence for recent eutrophication of Lake Chenghai, Yunnan, China

Geochemical anomalies and stable isotope ratios (δ18O, δ13C) in authigenic carbonates and organic matter (δ13C) from a 660-year sediment core from Lake Chenghai, southern China, provide a continuous history of recent lake eutrophication. The multi-proxy geochemical and isotopic record can be divided into a three-part history of contrasting limnological development, including: (1) a clear-water, oligotrophic open lake system (1340 and 1690 AD); (2) an environmentally unstable, hydrologically closed, oligotrophic lake system (1690–1940 AD); and (3) an increasingly eutrophic, closed lake system marked by higher organic matter, nitrogen, CaCO3, and pigment concentrations, and lower δ18O and δ13C values in authigenic calcite (1940–1999 AD). The unanticipated lowering of δ18O and δ13C of authigenic calcite during eutrophication is thought to be the result of disequilibrium water–carbonate fractionation of oxygen and carbon isotopes during periods of elevated primary production, pH, and [CO3 2−] activities in the water column. The recent eutrophication of Lake Chenghai indicated by these geochemical proxies is essentially simultaneous with large-scale human migration and the application of agricultural fertilizers in the catchment area during the 20th century.

Fracture‐fill calcite as a record of microbial methanogenesis and fluid migration: a case study from the Devonian Antrim Shale, Michigan Basin

AbstractThe Devonian Antrim Shale is an organic‐rich, naturally fractured black shale in the Michigan Basin that serves as both a source and reservoir for natural gas. A well‐developed network of major, through‐going vertical fractures controls reservoir‐scale permeability in the Antrim Shale. Many fractures are open, but some are partially sealed by calcite cements that retain isotopic evidence of widespread microbial methanogenesis. Fracture filling calcite displays an unusually broad spectrum of δ13C values (+34 to −41‰ PDB), suggesting that both aerobic and anaerobic bacterial processes were active in the reservoir. Calcites with high δ13C values (>+15‰) record cementation of fractures from dissolved inorganic carbon (DIC) generated during bacterial methanogenesis. Calcites with low δ13C values (<−32‰) are solely associated with outcrop samples and record methane oxidation during cement precipitation. Fracture‐fill calcite with δ13C values between −10 and −30‰ can be attributed to variable organic matter oxidation pathways, methane oxidation, and carbonate rock buffering. Identification of 13C‐rich calcite provides unambiguous evidence of biogenic methane generation and may be used to identify gas deposits in other sedimentary basins.It is likely that repeated glacial advances and retreats exposed the Antrim Shale at the basin margin, enhanced meteoric recharge into the shallow part of the fractured reservoir, and initiated multiple episodes of bacterial methanogenesis and methanotrophic activity that were recorded in fracture‐fill cements. The δ18O values in both formation waters and calcite cements increase with depth in the basin (−12 to −4‰ SMOW, and +21 to +27‰ PDB, respectively). Most fracture‐fill cements from outcrop samples have δ13C values between −41 and −15‰ PDB. In contrast, most cement in cores have δ13C values between +15 and +34‰ PDB. Radiocarbon and 230Th dating of fracture‐fill calcite indicates that the calcite formed between 33 and 390 ka, well within the Pleistocene Epoch.

Constraints on the application of carbon isotope thermometry in high‐ to ultrahigh‐temperature metamorphic terranes

AbstractNine marble horizons from the granulite facies terrane of southern India were examined in detail for stable carbon and oxygen isotopes in calcite and carbon isotopes in graphite. The marbles in Trivandrum Block show coupled lowering of δ13C and δ18O values in calcite and heterogeneous single crystal δ13C values (− 1 to − 10‰) for graphite indicating varying carbon isotope fractionation between calcite and graphite, despite the granulite facies regional metamorphic conditions. The stable isotope patterns suggest alteration of δ13C and δ18O values in marbles by infiltration of low δ13C–δ18O‐bearing fluids, the extent of alteration being a direct function of the fluid‐rock ratio. The carbon isotope zonation preserved in graphite suggests that the graphite crystals precipitated/recrystallized in the presence of an externally derived CO2‐rich fluid, and that the infiltration had occurred under high temperature and low fO2 conditions during metamorphism. The onset of graphite precipitation resulted in a depletion of the carbon isotope values of the remaining fluid+calcite carbon reservoir, following a Rayleigh‐type distillation process within fluid‐rich pockets/pathways in marbles resulting in the observed zonation. The results suggest that calcite–graphite thermometry cannot be applied in marbles that are affected by external carbonic fluid infiltration. However, marble horizons in the Madurai Block, where the effect of fluid infiltration is not detected, record clear imprints of ultrahigh temperature metamorphism (800–1000 °C), with fractionations reaching <2‰. Zonation studies on graphite show a nominal rimward lowering δ13C on the order of 1 to 2‰. The zonation carries the imprint of fluid deficient/absent UHT metamorphism. Commonly, calculated core temperatures are > 1000 °C and would be consistent with UHT metamorphism.