Carbonate δ18O Values Research Articles

Looking upstream with clumped and triple oxygen isotopes of estuarine oyster shells in the early Eocene of California, USA

Abstract The δ18O of carbonate minerals that formed at Earth's surface is widely used to investigate paleoclimates and paleo-elevations. However, a multitude of hydrologic processes can affect δ18O values, including mixing, evaporation, distillation of parent waters, and carbonate growth temperatures. We combined traditional carbon and oxygen isotope analyses with clumped (Δ47) and triple oxygen isotopes (Δ′17O) analyses in oyster shells (Acutostrea idriaensis) of the Goler Formation in southern California (USA) to obtain insights into surface temperatures and δ18O values of meteoric waters during the early Eocene hothouse climate. The Δ47-derived temperatures ranged from 9 °C to 20 °C. We found a correlation between the δ18O of growth water (δ18Ogw) (calculated using Δ47 temperatures and δ18O of carbonate) and the δ13C values of shells. The Δ′17O values of shell growth waters (0.006‰–0.013‰ relative to Vienna standard mean ocean water–standard light Antarctic precipitation [VSMOW-SLAP]) calculated from Δ′17O of carbonate (−0.087‰ to −0.078‰ VSMOW-SLAP) were lower than typical meteoric waters. These isotopic compositions are consistent with oyster habitation in an estuary. We present a new triple oxygen isotope mixing model to estimate the δ18O value of freshwater supplying the estuary (δ18Ofw). The reconstructed δ18Ofw of −11.3‰ to −14.7‰ (VSMOW) is significantly lower than the δ18Ogw of −4.4‰ to −9.9‰ that would have been calculated using “only” Δ47 and δ18O values of carbonate. This δ18Ofw estimate supports paleogeographic reconstructions of a Paleogene river fed by high-elevation catchments of the paleo–southern Sierra Nevada. Our study highlights the potential for paired Δ47 and Δ′17O analyses to improve reconstructions of meteoric water δ18O, with implications for understanding ancient climates and elevations.

Ultramafic-alkaline-carbonatite Tajno intrusion in NE Poland: A new hypothesis about the massif formation and related mineralization

This manuscript presents results of the newest petrographic, mineralogical and bulk chemical, as well as H, C and O stable isotope study of carbonatites and associated silicate rocks from the Tajno Massif (NE Poland). The Tajno Intrusion is a Tournaisian-Visean ultramafic-alkaline-carbonatite body emplaced within the Paleoproterozoic rocks of the East European Craton (EEC). Carbonatites of the Tajno Massif can be subdivided into the calciocarbonatite (calcite), ferrocarbonatite (ankerite), and breccias with an ankerite-fluorite matrix. Due to location at the cratonic margin and abundance in the REE, Tajno classifies (Hou et al., 2015) as the carbonatite-associated REE deposit (CARD), and more precisely as the Dalucao-Style orebody (the breccia-hosted orebody). High Fe2O3 (13.8 wt%), MnO (2.1 wt%), total REE (6582 ppm), Sr (43895 ppm), Ba (6426 ppm), F (greater than10000 ppm) and CO2 contents points for the involvement of the slab – including pelagic metalliferous sediments – in the carbonatites formation. Spatial relations and Sr isotope composition ((87Sr/86Sr)i = 0.7043–0.7048; Wiszniewska et al., 2020) of alkali clinopyroxenite and syenite suggest that these are products of differentiation of the magma, generated by the initial melting of the SCLM due to influx of F-rich fluids from subducted marine sediments. Carbonatites Sr isotope composition ((87Sr/86Sr)i = 0.7037–0.7038), and Ba/Th (16–20620) and Nb/Y (0.01–6.25) ratios, link their origin with a more advanced melting of the SCLM, triggered by CO2-rich fluids from the subducted AOC and melts from sediments. The Tajno Massif – and coeval mafic-alkaline intrusions – age, high potassic composition, and location along the craton margin nearly parallel the Variscan deformation front, are suggesting Variscan subduction beneath the EEC. The oxygen isotope compositions of clinopyroxene (δ18O value = 5.2‰) and alkali feldspar (δ18O value = 5.7‰), from alkali clinopyroxenite and foid syenite, respectively, are consistent with mantle-derived magmas. Isotopic compositions of carbonatites and breccias (carbonate δ18O = 8.7‰ to 10.7‰; δ13C = -4.8‰ to −0.4‰) span from values of primary carbonatites to carbonatites affected by a fractionation or sedimentary contamination. The highest values (δ18O = 10.7‰; δ13C = -0.4‰) were reported for breccia cut by numerous veins confirming post-magmatic hydrothermal alteration. The lowest carbonate δ18O (9.3‰ to 10.7‰) and δ13C (−5.0‰ to −3.8‰) values are reported for veins in alkali clinopyroxenites, whereas the highest δ18O (11.2‰) and δ13C (−1.2‰ to −1.1‰) values are for veins in syenites and trachytes. Isotopic composition of veins suggests hydrothermal origin, and interaction with host mantle-derived rocks, as well as country rocks. In silicate rocks of the Tajno Massif, fluid influx leads to the development of Pb, Zn, Cu, Ag, Au sulfide mineralization-bearing stockwork vein system, with carbonate, silicate and fluorite infilling the veins. Bulk-rock contents of molybdenum (925 ppm), rhenium (905 ppb) and palladium (29 ppb) are notable. The Re-rich molybdenite association with galena, pyrite and Th-rich bastnäsite in carbonate veins is similar as in Mo deposits associated with carbonatites, implying the mantle source of Mo and Re.

Rising bottom-water temperatures induced methane release during the middle Holocene in the Okinawa Trough, East China Sea

It is not currently well understood how a changing climate controls methane releases in the Okinawa Trough. In this study, a piston core was collected from a seep-impacted area at a water depth of approximately 1000 m from the Okinawa Trough, East China Sea, and was studied using comprehensive analyses of carbon and oxygen stable isotopes, lipid biomarkers, and major and trace element geochemistry. Extremely low δ13C values of total inorganic carbon and organic carbon, increased Methane Index, and molybdenum (Mo) and uranium (U) enrichments at depths of 225–255 cm below the sea-floor (bsf) and 75–142.5 cm bsf were found. Combined with published pore water data, fossil and current sulfate-methane transitions (SMTs) were identified. Mass balance equations were applied to estimate the fraction and content of authigenic carbonates at the two SMTs and their corresponding δ18O values. The calculations revealed that the estimated δ18O values of authigenic carbonates for the fossil SMT (from 4.9‰ to 5.8‰) and the current SMT (from −2.1‰ to 4.4‰) were higher and lower than the theoretical equilibrium δ18O value (4.8‰), respectively. These results suggest that fossil seepage is possibly induced by the dissociation of gas hydrates. The fluids of the current seepage may be derived from the equilibrium between the ambient seawater and the fluids of gas hydrate dissociation. Based on the accumulation of authigenic carbonate in sediments, previously published pore-water calcium and magnesium fluxes, and foraminifera 14C dating, the fossil methane seepage was confirmed to have occurred during the period 8.2 to 4.5 ka B.P., with its SMT depth less than 105 cm bsf, while the present-day methane seepage with a low fluid intensity was estimated to have started before 1.0 ka and formed its SMT at approximately 250 cm bsf. The timing of this fossil methane seepage suggests that the rising temperature of the North Pacific Intermediate Water during the early and middle Holocene mainly controlled the methane release in the Okinawa Trough, whereas the ongoing methane emission was likely induced by decreased pressure driven by back-arc extension. The geochemical data demonstrate past methane release as a response to environmental changes and suggest that the Okinawa Trough gas hydrates were sensitive to temperature fluctuations during the middle Holocene.

Oxygen Isotope Equilibrium of the Shallow-Water Benthic Foraminifer Hanzawaia nipponica Asano in Tosa Bay, Southwest Japan

Oxygen isotopic compositions (δ18O) of benthic foraminifer tests are widely used for reconstructing paleoceanographic changes, such as global ice volumes during glacial–interglacial cycles. Although deep-sea benthic foraminifers have been well characterized and are considered reliable indicators, little attention has been paid to the geochemistry of shallow-water benthic foraminifers. In this study we evaluated δ18O in the shallow-water benthic foraminifer Hanzawaia nipponica Asano, which lives in surface sediments on continental shelves and upper slopes under the influence of two warm currents, the Kuroshio and Tsushima currents, in the East China Sea, northwest Pacific, and southwestern Japan Sea. To evaluate oxygen isotope equilibrium, we analyzed δ18O of H. nipponica and ambient seawater on the continental shelf in Tosa Bay, southwest Japan. Seawater δ18O and salinity in Tosa Bay are similar to those of surface and subsurface waters in the Kuroshio region in the Okinawa Trough and the northwest Pacific. Vertical profiles of seawater δ18O show no variation with water depth (0–200 m) in Tosa Bay. However, tests of living H. nipponica (as determined by staining with Rose Bengal) and fossil (non-stained) H. nipponica, picked from samples of the top centimeter of seafloor sediment, yielded carbonate δ18O values that clearly increase with water depth, suggesting a temperature-dependent relationship. A comparison of carbonate δ18O values in living H. nipponica and those predicted on the basis of seawater δ18O and annual mean bottom temperature shows that H. nipponica tests are in oxygen isotopic equilibrium with ambient seawater. We determined the linear equations of δ18O–temperature relationship, and the slope of −5.26 (0.19‰°C−1) for living and −4.50 (0.22‰°C−1) for the fossil H. nipponica, respectively. The carbon isotopic compositions (δ13C) of H. nipponica also closely match seawater δ13C. Thus, we propose that the carbonate δ18O and δ13C of H. nipponica are useful proxies to reconstruct shallow-water paleoenvironmental changes in the northwest Pacific and its marginal seas.

Organic matter removal for continuous flow isotope ratio mass spectrometry analysis of carbon and oxygen isotope compositions of calcite or dolomite in organic‐rich samples

Abstractδ13C and δ18O values of carbonate in organic‐rich samples such as soils, biofilms, and lake sediments have been scarcely used so far in paleo‐environmental, biomineralization, or diagenetic studies because organic matter in high proportions is suspected to alter carbonate isotope analysis. Yet, with the improvement of analytical capabilities and in particular the use of CF‐IRMS, this may not be an issue anymore. To evaluate this, δ13C and δ18O values of calcite or dolomite mixed in various proportions with yeast (used here as a model for immature organic matter) were measured. The results indicate that measurements of calcite or dolomite δ13C and δ18O by CF‐IRMS require organic matter removal only when its weight proportion exceeds that of carbonates. Yeast generated an unidentified molecule during phosphoric acid digestion at 25°C (none at 80°C), which shifted the carbonate δ18O values for yeast proportions higher than 50%. It also generated CO2, but with a noticeable shift in δ13C and δ18O values only at 80°C (none at 25°C) and for yeast proportions higher than 75%. Three methods of organic matter removal were tested: the well‐established NaOCl and H2O2 wet treatments as well as low temperature plasma ashing (LTA) so far seldom examined. LTA removed the isotope shifts, although imperfectly for yeast proportions of 95%, without carbonate dissolution. NaOCl efficiently removed the isotope shifts in all cases but dissolved part of the calcite and dolomite. H2O2 treatment caused severe dissolution of calcite and dolomite while worsening the δ13C and δ18O shifts and should thus be avoided.

Paleoclimate and paleoenvironment reconstruction of paleosols spanning the Lower to Upper Cretaceous from the Rukwa Rift Basin, Tanzania

The Cretaceous Period is considered the archetypical greenhouse interval, yet there is mounting evidence for intermittent cooler climatic phases throughout this geologic span. Few continental climate histories exist for the Cretaceous south of the paleoequator, and fewer still for Africa during this time. The Cretaceous Galula Formation is one of the few exposed fossiliferous continental sedimentary successions from what is now subequatorial Africa that also contains paleosols and spans both the Early and Late Cretaceous series. Bulk sediment and pedogenic carbonate nodules were sampled from paleosols located throughout ~500 m of the Galula Formation stratotype section, including four from the lower Mtuka Member (Aptian-Cenomanian) and four from the upper Namba Member (Cenomanian-Campanian), to reconstruct the paleoclimate and paleoenvironment of the Rukwa Rift Basin. The Mtuka paleosols developed on channel fill deposits in a semi-arid climate amidst well-defined wet and dry seasons that predominantly generated vertic Calcisols. Geochemical and morphological climofunctions determined that mean annual precipitation (MAP) increased up-section from 454 to 860 mm/yr, whereas the δ18O values of pedogenic carbonates and Bt horizon major-element paleothermometry estimated mean annual temperature (MAT) remained relatively steady, ranging between 12.9 and 13.9 °C throughout the Mtuka Member. The paleosols of the overlying Namba Member formed on channel fill and overbank deposits proximal to the main channels, in a semi-arid to sub-humid climate with little seasonality, forming mostly calcic Argillisols. The Namba paleosols formed under slightly wetter and cooler conditions, recording MAP values between 602 and 987 mm/yr, and MAT estimates between 10.3 and 12.5 °C. These data are consistent with the Mtuka paleosols corresponding to the cool greenhouse period that spanned the Aptian-Albian, with the Namba paleosols corresponding to Late Cretaceous cooling following the Cretaceous Thermal Maximum. This work demonstrates that temperate conditions prevailed in the Rukwa Rift Basin during the mid-Late Cretaceous.

Differentiating early from later diagenesis in a Cretaceous sandstone and petroleum reservoir of the Cedar Mountain Formation, Utah

ABSTRACT Previously published anomalous whole-rock stable isotopic values from the Poison Strip Sandstone Member of the Cretaceous Cedar Mountain Formation (CMF) of eastern Utah are of uncertain origins. This study investigated the diagenetic history and the processes responsible for these anomalous data. Accordingly, we integrated photomicroscopic techniques including polarized light microscopy, epifluorescence and cathodoluminescence (CL) imaging, micromilling of stable isotope samples, and fluid-inclusion heating and freezing measurements to this end. The key observations involved the microscopic mapping of calcite cement stratigraphy using CL imaging to permit the analysis of stable isotopes of calcite cements that crystallized during early and late diagenesis. The mapping of calcite cement zones of sufficient submillimeter size to mill out and isolate microgram-sized stable isotope samples enabled this discrimination. Early diagenetic calcite cements have the most positive δ18O values (-10 to -8.5‰ Vienna Pee Dee Belemnite [VPDB]) of all components. The pattern of δ13C and δ18O variation in this early diagenetic cement indicates affinities with early meteoric diagenesis previously documented in published literature on the CFM. The late diagenetic calcite cements yield the most negative δ18O values (-18 to -16‰ VPDB). We interpret the late diagenetic cements to be responsible for the anomalously low whole-rock δ18O values previously reported from the Poison Strip Sandstone Member. Our discoveries of bitumen in late-stage pore fillings and liquid petroleum in the fluid inclusions of late diagenetic calcite cements of the Poison Strip Sandstone Member explain the lower whole-rock organic matter δ13C values and anomalous Δ13C values reported from the unit. Comparatively lower carbonate δ18O and organic δ13C values originally derived from whole-rock analyses of samples from the Poison Strip Sandstone Member resulted from high-temperature basinal diagenesis (hydrothermal circulation and/or petroleum migration), rather than the alternative interpretation of early diagenesis related to a Cretaceous paleoclimatic perturbation. Our results are illustrative of methods to resolve the long-standing geologic problem of discriminating and characterizing products of early vs. late diagenesis in terrigenous clastic sedimentary strata.

Triple oxygen and clumped isotopes in modern soil carbonate along an aridity gradient in the Serengeti, Tanzania

The isotopic composition of paleosol carbonates is used extensively to reconstruct past vegetation, climate, and altimetry, but poor constraints on soil evaporation and temperature have limited the utility of oxygen isotopes in the studies. Recent advances in carbonate clumped isotope thermometry (TΔ47) allow for independent controls on temperature, but the influence of evaporation remains unresolved. However, the sensitivity of 18O-17O-16O distributions to kinetic fractionation makes it possible to use triple oxygen isotopes (Δ′17O) to track evaporation in water. Recent work shows the sensitivity of Δ′17O to evaporation in lakes and lacustrine carbonates, but little is known about variation of Δ′17O in soil carbonates and their potential to track evaporation. For this study, we sampled soils across an aridity gradient in the Serengeti, Tanzania to evaluate how soil carbonate Δ′17O tracks soil water evaporation. Modern soil carbonates were collected from 11 sites across a transect of the Serengeti Ecosystem where mean annual precipitation and aridity index range from 499 to 846 mm yr−1 and 0.33 to 0.55, respectively. δ13C values range from −2.7 to 1.8‰ and reflect C4 dominated grasslands, whereas δ18O values of soil carbonates vary by ∼8‰ along a gradient in aridity. TΔ47 from these soil carbonates average 23°C (1σ± 4°C), which does not vary significantly across sites or with depth, likely due to minimal annual variation in temperature at the equator. Using these temperatures for each carbonate, reconstructed δ18O values of soil water are up to 6‰ higher than δ18O values of local precipitation and springs, indicating considerable soil water evaporation. The Δ′17O values of these soil carbonates range from −162 to −106 per meg and decrease as both aridity and δ18O values increase. Our results support the hypothesis that soil water evaporation drives the variance in δ18O and Δ′17O of soil carbonate in arid climates, demonstrating the potential for soil carbonate Δ′17O to track paleoaridity and constrain interpretations of paleosol carbonate δ18O records.

Internal Water Facilitates Thermal Resetting of Clumped Isotopes in Biogenic Aragonite

AbstractBiogenic and inorganic calcium carbonates contain considerable amounts of internal water, both as free and organically associated water. The oxygen isotopic compositions (δ18O) of internal water and hosting carbonate are analyzed for various carbonates before and after heating at 175°C for 90 minutes. During heating, the δ18O values of internal water significantly increased in biogenic aragonites and speleothem calcite, whereas the δ18O carbonate values were lowered. Correspondingly, an aragonitic bivalve’s clumped‐isotope distribution (Δ47) changed during heating, increasing reconstructed paleotemperatures. In contrast, an inorganic aragonite crystal, containing a comparable amount of internal water, showed no oxygen isotope exchange, and its Δ47 values remained unaltered during heating, implying that there is a link between internal oxygen isotope exchange and Δ47 resetting. This alteration process occurred without any detectable transformation from aragonite to calcite. Our results therefore reveal a mechanism that facilitates oxygen isotope exchange between biogenic aragonite and its internal water, while simultaneously resetting the Δ47 values, without affecting mineralogy. Future studies may therefore apply coupled water‐carbonate analyses to scrutinize these kinds of diagenetic alteration processes. It appears that in biogenic aragonites, more carbonate is available for exchange reactions with the internal water reservoir than in inorganic aragonites, a feature that can be attributed to the distribution of organic‐associated water and/or high surface area fluid inclusions. This water‐aragonite exchange occurs at lower temperatures than those required for solid‐state bond reordering at the same timescale, and thus likely has occurred earlier during the burial of biogenic aragonites.

Climatic and limnological changes 12,750 to 3600 years ago in the Issyk-Kul catchment, Tien Shan, based on palynology and stable isotopes

Arid Central Asia is at the border between the prevailing Westerlies and monsoonal climates. Lake Issyk-Kul, a large mountain lake, and its sediment record are key to reconstructing complex interactions between these two major climatic systems over time. We present for the first time sedimentological (magnetic susceptibility, CaCO3, δ13C and δ18O values of benthic ostracods and fine-grain carbonates) and palynological investigations of a sequence covering the last 12,750 years, with additionally some proxies at a lower resolution above 3590 years ago. From 12,750 until 10,000 years ago, vegetation reflects desert to semi-desert conditions around the lake. Soil erosion is intensive as reconstructed from high Glomus percentages and high magnetic susceptibility. The values of fine-grain carbonate δ18O are very negative. This is explained by intensive glacial meltwater runoff from surrounding mountain tops and slopes. Negative δ13C values of carbonates reflect the presence of C3 plants near the glacial edge, such as Kobresia sedges. From 10,000 to c. 6500 years ago, the increase of the Artemisia steppe occurring at the same time as an increasingly positive trend in fine-grained carbonate δ18O values are apparently contradictory. This is interpreted by a Westerly trend (increasing humidity) at the altitude of the lake (1607 m) and a monsoonal trend (increasingly dry) at the elevation of the glaciated mountain tops (a reconciliation hypothesis developed for multiple sites, illustrated here within one site only). Whereas the steppe is widespread around the lake, the fine-grain carbonates are precipitated from surface waters strongly influenced by meltwater having spilled down the surrounding mountains that are >2 km higher. Additionally after low lake levels until 10,000 years ago, our data suggest a lake level rise between 10,000 and 8000 years ago.

Reconstructing Past Elevations From Triple Oxygen Isotopes of Lacustrine Chert: Application to the Eocene Nevadaplano, Elko Basin, Nevada, United States

Triple oxygen isotope measurements are an emerging tool in paleoclimate reconstructions. In this contribution we develop the application of triple oxygen isotope measurements to lacustrine sediments to reconstruct past elevations. We focus on a well-constrained sample set from the Eocene North American Cordillera (Cherty Limestone Formation, Elko Basin, NV, United States, 42–43.5 Ma) on the east side of the elevated Nevadaplano. We present triple oxygen isotope measurements on freshwater lacustrine chert samples from the Cherty Limestone Formation. Across an evaporation trend spanning 6.5‰ in δ18O values we observe a negative correlation with Δ′17O ranging from −0.066 to −0.111‰ (λRL= 0.528), with an empirical slope (λchert, δ′17O vs. δ′18O) of 0.5236. Additionally, we present new carbonate clumped isotope (Δ47) temperature results on the overlying fluvial-lacustrine Elko Formation, which indicate an error-weighted mean temperature of 32.5 ± 3.8°C (1σ), and evaporatively enriched lake water spanning δ18O values of −3.7 to +3.5‰ (VSMOW). Paired chert and carbonate δ18O values demonstrate that co-equilbrium among the carbonate and chert phases is unlikely. Thus, as also previously suggested, it is most likely that Elko Basin chert formed during early diagenesis in equilbirium with pore waters that reflect evaporatively18O-enriched lake water. Using this scenario we apply a model for back-calculating unevaporated water composition to derive a source water of δ′18O = −16.1‰ (VSMOW), similar to modern local meteoric waters but lower than previous work on paired δ18O- δD measurements from the same chert samples. Further, this back-calculated unevaporated source water is higher than those derived using δD measurements of Late Eocene hydrated volcanic glass from the Elko Basin (average δ′18O equivalent of approximately −18.4‰, VSMOW). This suggests, assuming Eocene meteoric water Δ′17O values similar to today (∼0.032‰), either that: (1) the hypsometric mean elevation recorded by the lacustrine Cherty Limestone was lower than that derived from the average of the volcanic glass δD measurements alone; or (2) there was hydrogen exchange in volcanic glass with later low δD meteoric fluids. Nonetheless, our new findings support a relatively high (∼2.5–3 km) plateau recorded in the Elko Basin during the mid-Eocene.

Diagenesis and fluid evolution in the third member of the Eocene Shahejie Formation, Bonan Sag, Bohai Bay Basin, China

Diagenesis significantly influences sandstone reservoir quality, so that a detailed understanding of diagenesis is essential for the identification and prediction of favorable reservoirs. Several studies have been conducted on individual sandstone reservoirs in the Bonan Sag, a typical half-graben, oil-producing sag in eastern China, but few have explicitly considered the potential impact of the asymmetric shape of the sag, which is steep in the north and gentle in the south. This research takes full consideration of the tectonic setting and distribution of source rocks in the sag to compare the style, magnitude and timing of diagenesis in the northern Steep Slope and Sag Zone (SSSZ) and the southern Gentle Slope Zone (GSZ) by means of drill core observations, thin section petrography, high resolution scanning electron microscopy, fluid inclusion microthermometry, as well as cathodoluminescence and carbonate stable isotope analysis. Petrographic observations identify the wide-spread occurrence of authigenic quartz and clay minerals in association with dissolved feldspar, as well as abundant carbonate cements occurring as several distinct generations due to evolving pore fluid composition. The SSSZ has variable carbonate mineralogy while ankerite dominates in the GSZ. The homogenization temperature of fluid inclusions in authigenic quartz in the SSSZ has a wider range (93°C–157.9 °C) and lower starting temperature (93 °C) compared with the GSZ (106.2 °C) and is characterized by more lighter carbonate δ13C and δ18O values (δ18O: −14.8‰~-11.3‰, δ13C: 1.1‰–2.7‰ for the SSSZ; δ18O: −11.9‰~-5.8‰, δ13C: 2.9‰–5.5‰ for the GSZ). The results indicate that (1) Diagenetic conditions in the Bonan Sag evolved from alkaline to acidic to alkaline in general, but with distinct late stage carbonate cementation in SSSZ vs GSZ; (2) the diagenetic processes as well as the nature of pore waters varied across different zones and also temporally. Acidic conditions in the SSSZ started earlier and persisted for longer than in the GSZ. (3) The regional differentiation of the diagenetic processes and pore fluid evolution result from the source rock distribution and burial histories which differ across the Bonan Sag because of the asymmetric shape of the half-graben. We propose a model of fluid evolution over space and time which may provide a new framework for understanding the diagenetic processes and the fluid evolution of oil-producing sags and also contribute to the prediction of favorable reservoirs.

The Middle to Late Cretaceous marine incursion of the Proto-Paratethys Sea and Asian aridification: A case study from the Simao-Khorat salt giant, Southeast Asia

Asian aridification during the Cenozoic was strongly related to moisture flux modulated by sea incursion and the retreat fluctuations of the proto-Paratethys Sea via the Westerlies. However, the extents of the sea incursions are poorly constrained, and there is a lack of sedimentary records beyond the Central Asian basins. The Middle to Late Cretaceous Simao Basin and the Khorat Plateau in Southeast Asia contain thick continental sequences of red beds, evaporites, and aeolian deposits, but whether the depositional environment of the evaporites was marine or continental has been hotly debated. In this study, we compiled multiple sedimentology proxies, biomarkers, element geochemistry, and isotopic geochemistry of the fine-grained sediments of the Mohan section in the Simao Basin. Sedimentological analyses show that this section is composed of subtidal flat fine-grained siliciclastic and carbonate rocks and supratidal flat conglomerates and evaporites. The paleosalinity proxies show that the B concentration and B/Ga and Srcorrected/Ba ratios range from 30.3 to 81.7 ppm, 2.4 to 5.0, and 0.05 to 0.60, respectively, which indicate brackish conditions. The two typical increases in these proxies are consistent with the stratigraphic horizons of the diagnostic marine biomarkers 24-n-propyl- and 24-iso-propylcholestanes, which suggests seawater recharge. The carbonate δ13C and δ18O values (−4.84‰ to −1.75‰ and −8.25‰ to −3.69‰, respectively) and the bulk δ13Corg values (−26.5‰ to −23.22‰) also indicate the influence of seawater. We propose that the seawater likely originated from the proto-Paratethys Sea during a marine incursion. We propose for the first time that the proto-Paratethys seawater incursions extended into the Simao Basin and the Khorat Plateau. These results provide a novel paleogeographic configuration for East Asia during the Middle to Late Cretaceous. The retreat of the proto-Paratethys Sea, the northeast Trade Winds, and the tectonic uplift dramatically decreased the moisture and intensified the extreme aridification in the Simao Basin and the Khorat Plateau.

The mineralogic and isotopic fingerprint of equatorial carbonates: Kepulauan Seribu, Indonesia

Kepulauan Seribu is an isolated patch reef complex situated in the Java Sea (Indonesia) and is a typical example for a humid, equatorial carbonate system. We investigate the mineralogical and isotopic fingerprint of Panggang, one of the reef platforms of Kepulauan Seribu, to evaluate differences to other carbonate systems, using isotope in combination with XRD and SEM analysis. A characteristic property of shallow water (< 20 m) sediments from Kepulauan Seribu is their increased LMC content (~ 10%) derived from some genera of rotaliid foraminifers and bivalves. The relative abundance of these faunal elements in shallow waters might be related to at least temporary turbid conditions caused by sediment-laden river runoff. This influence is also evidenced by the presence of low amounts of siliciclastic minerals below the regional wave base. Kepulauan Seribu carbonates are characterized by very low δ13C and δ18O values. This is related to the isotopically depleted riverine input. The δ13CDIC in riverine water is reduced by the contribution of 12C from riverside mangroves. Deep atmospheric convection and intensive rains contribute 18O-depleted freshwater in the river catchments, finally reducing salinity in the Java Sea. The depleted δ13C signature in carbonates is further enhanced by the lack of green algae and inorganic carbonates and abundance of coral debris. Low δ18O values in carbonates are favored by the high water temperatures in the equatorial setting. Since equatorial carbonates in SE Asia, including the Java Sea, are typically influenced by high turbidity and/or river runoff, the observed distinctively low isotope values likely are characteristic for equatorial carbonate systems in the region.

Isotopic fractionation accompanying CO2 hydroxylation and carbonate precipitation from high pH waters at The Cedars, California, USA

The Cedars ultramafic block hosts alkaline springs (pH > 11) in which calcium carbonate forms upon uptake of atmospheric CO2 and at times via mixing with surface water. These processes lead to distinct carbonate morphologies with “floes” forming at the atmosphere-water interface, “snow” of fine particles accumulating at the bottom of pools and terraced constructions of travertine. Floe material is mainly composed of aragonite needles despite CaCO3 precipitation occurring in waters with low Mg/Ca (<0.01). Precipitation of aragonite is likely promoted by the high pH (11.5–12.0) of pool waters, in agreement with published experiments illustrating the effect of pH on calcium carbonate polymorph selection.The calcium carbonates exhibit an extreme range and approximately 1:1 covariation in δ13C (−9 to −28‰ VPDB) and δ18O (0 to −20‰ VPDB) that is characteristic of travertine formed in high pH waters. The large isotopic fractionations have previously been attributed to kinetic isotope effects accompanying CO2 hydroxylation but the controls on the δ13C-δ18O endmembers and slope have not been fully resolved, limiting the use of travertine as a paleoenvironmental archive. The limited areal extent of the springs (∼0.5 km2) and the limited range of water sources and temperatures, combined with our sampling strategy, allow us to place tight constraints on the processes involved in generating the systematic C and O isotope variations.We develop an isotopic reaction–diffusion model and an isotopic box model for a CO2-fed solution that tracks the isotopic composition of each dissolved inorganic carbon (DIC) species and CaCO3. The box model includes four sources or sinks of DIC (atmospheric CO2, high pH spring water, fresh creek water, and CaCO3 precipitation). Model parameters are informed by new floe Δ44Ca data (−0.75 ± 0.07‰), direct mineral growth rate measurements (4.8 to 8 × 10−7 mol/m2/s) and by previously published elemental and isotopic data of local water and DIC sources. Model results suggest two processes control the extremes of the array: (1) the isotopically light end member is controlled by the isotopic composition of atmospheric CO2 and the kinetic isotope fractionation factor (KFF (‰) = (α − 1) × 1000) accompanying CO2 hydroxylation, estimated here to be −17.1 ± 0.8‰ (vs. CO2(aq)) for carbon and −7.1 ± 1.1‰ (vs. ‘CO2(aq) + H2O’) for oxygen at 17.4 ± 1.0 °C. Combining our results with revised CO2 hydroxylation KFF values based on previous work suggests consistent KFF values of −17.0 ± 0.3‰ (vs. CO2(aq)) for carbon and −6.8 ± 0.8‰ for oxygen (vs. ‘CO2(aq) + H2O’) over the 17–28 °C temperature range. (2) The isotopically heavy endmember of calcium carbonates at The Cedars reflects the composition of isotopically equilibrated DIC from creek or surface water (mostly HCO3-, pH = 7.8–8.7) that occasionally mixes with the high-pH spring water. The bulk carbonate δ13C and δ18O values of modern and ancient travertines therefore reflect the proportion of calcium carbonate formed by processes (1) and (2), with process (2) dominating the carbonate precipitation budget at The Cedars. These results show that recent advances in understanding kinetic isotope effects allow us to model complicated but common natural processes, and suggest ancient travertine may be used to retrieve past meteoric water δ18O and atmospheric δ13C values. There is evidence that older travertine at The Cedars recorded atmospheric δ13C that predates large-scale combustion of fossil fuels.

Global significance of oxygen and carbon isotope compositions of pedogenic carbonates since the Cretaceous

Few global syntheses of oxygen and carbon isotope composition of pedogenic carbonates have been attempted, unlike marine carbonates. Pedogenic carbonates represent in-situ indicators of the climate conditions prevailing on land. The δ18O and δ13C values of pedogenic carbonates are controlled by local and global factors, many of them not affecting the marine carbonates largely used to probe global climate changes. We compile pedogenic oxygen and carbon isotopic data (N= 12,167) from Cretaceous to Quaternary-aged paleosols to identify potential trends through time and tie them to possible controlling factors. While discrete events such as the Paleocene-Eocene Thermal Maximum are clearly evidenced, our analysis reveals an increasing complexity in the distribution of the δ18O vs δ13C values through the Cenozoic. As could be expected, the rise of C4 plants induces a shift towards higher δ13C values during the Neogene and Quaternary. We also show that the increase in global hypsometry during the Neogene plays a major role in controlling the δ18O and δ13C values of pedogenic carbonates by increasing aridity downwind of orographic barriers. Finally, during the Quaternary, an increase of 3‰ in δ18O values is recorded both by the pedogenic carbonates and the marine foraminifera suggesting that both indicators may be used to track global climate signal.

The Formation of Authigenic Carbonates at a Methane Seep Site in the Northern Part of the Laptev Sea

Authigenic carbonates from cold seeps are unique archives for studying environmental conditions, including biogeochemical processes associated with methane-rich fluid migration through the sediment column. The aim of this research was to study major oxide, mineralogical, and stable isotopic compositions of cold-seep authigenic carbonates collected in the northern part of the Laptev Sea. These carbonates are represented by Mg-calcite with an Mg content of 2% to 8%. The δ13C values range from −27.5‰ to −28.2‰ Vienna Peedee belemnite (VPDB) and indicate that carbonates formed due to anaerobic oxidation of methane, most likely thermogenic in origin. The authigenic pyrite in Mg-calcite is evidence of sulfate reduction during carbonate precipitation. The δ18O values of carbonates vary from 3.5‰ to 3.8‰ VPDB. The calculated δ18Ofluid values show that pore water temperature for precipitated Mg-calcite was comparable to bottom seawater temperature. The presence of authigenic carbonate in the upper horizons of sediments suggests that the sulfate–methane transition zone is shallowly below the sediment–water interface.

Stable oxygen and carbon isotope record from a drill core from the Hetao Basin in the upper reaches of the Yellow River in northern China and its implications for paleolake evolution

Knowledge of the evolution of the paleolakes that developed intermittently within the Hetao Basin in northern China is important for understanding both the evolution of the upper and middle reaches of the Yellow River drainage system and the spatio-temporal evolution of regional water vapor transport associated with the East Asian summer monsoon. However, the long-term history of these lakes and the driving mechanisms are poorly understood. In this study, we reconstructed the history of the paleolakes within the Hetao Basin over the past ~1.68 Myr, based on δ18O and δ13C records from lacustrine authigenic carbonates in a 274.60-m-long drill core from the centre of the Hetao Basin. The absence of covariation of carbonate δ18O and δ13C during the intervals of ~1.47–1.30 Ma and ~1.17–1.07 Ma suggests the existence of a hydrologically-open lake system in the Hetao Basin. The relatively low and invariant carbonate δ18O values are similar to those of the modern Yellow River water, suggesting that the paleolakes formed when the lakes and the Yellow River were connected. The moderately high correlation (r = 0.61) of carbonate δ18O and δ13C during ~0.68–0.60 Ma suggests an intermittently open hydrology of the lake probably with a tendency towards closure within certain intervals. The low correlation during 0.47–0.28 Ma suggests a hydrologically open lake, while the relatively high δ18O values suggest a longer residence time of the lake water. The high correlation (r = 0.92) of carbonate δ18O and δ13C from ~0.28 Ma to the last interglacial suggests a hydrologically-closed lake, although the Yellow River may have flowed through the Hetao Basin at that time. The interaction of the tectonically-controlled relative height of the basin spill point and the climatically-driven sediment and water supply of the Yellow River was responsible for the long-term evolution of the paleolake within the Hetao Basin. Strong regional tectonic activity which induced the uplift of the eastern part of the Hetao Basin and Jinshan Gorge and raised the relative height of the spill point was the primary cause of the formation of a hydrologically-closed lake within the basin.

No evidence for carbon enrichment in the mantle source of carbonatites in eastern Africa

Abstract Carbonatites are unusual, carbon-rich magmas thought to form either by the melting of a carbon-rich mantle source or by low-degree partial melting of a carbon-poor (&amp;lt;80 ppm C) mantle followed by protracted differentiation and/or immiscibility. Carbonate-bearing mantle xenoliths from Oldoinyo Lengai (East African Rift), the only active volcano erupting carbonatites, have provided key support for a C-rich mantle source. Here, we report unique microscale O and C isotopic analyses of those carbonates, which are present as interstitial grains in the silicate host lava, veins in the xenoliths, and pseudo-inclusions in olivine xenoliths. The δ18O values vary little, from 19‰ to 29‰, whereas δ13C values are more variable, ranging from –23‰ to +0.5‰. We show that such carbonate δ18O values result from the low-temperature precipitation of carbonate in equilibrium with meteoric water, rather than under mantle conditions. In this framework, the observed δ13C values can be reproduced by Rayleigh distillation driven by carbonate precipitation and associated degassing. Together with petrological evidence of a physical connection between the three types of carbonates, our isotopic data support the pedogenic formation of carbonates in the studied xenoliths by soil-water percolation and protracted crystallization along xenolith cracks. Our results refute a mechanism of C enrichment in the form of mantle carbonates in the mantle beneath the Natron Lake magmatic province and instead support carbonatite formation by low-degree partial melting of a C-poor mantle and subsequent protracted differentiation of alkaline magmas.

A trace metal, stable isotope (H, O, S), and geochronological (U-Pb titanite) characterization of hybridized gold orebodies in the Missanabie-Renabie district, Wawa subprovince (Canada)

Mineralized shear zones in the Archean Missanabie-Renabie gold district (~ 1.1 Moz Au; Wawa, Ontario, Canada) locally define composite orebodies that record three hydrothermal events: (1) a pre-orogenic Au1 event (pre-D1 and pre-prograde-metamorphic); (2) a syn-orogenic, post-peak-metamorphic Retrograde event (syn-D3); and (3) a late syn-to post-orogenic Au2 event (late syn- to post-D4). Genetic considerations indicate the orebodies are hybrids with early intrusion-related (Au1) and later orogenic (Retrograde + Au2) events. Pearson product-moment correlation coefficients (log10) of whole-rock and LA-ICP-MS pyrite trace metal datasets distinguish Au1 from Au2 mineralization by Au-Ag, Au-Bi, and Au-Te correlations > 0.7 (p < 0.05) in the former, irrespective of sample medium and analytical method. An Au-Mo correlation in whole rock data (0.58–0.76; p < 0.05) further distinguishes Au1 from Au2 and supports an independently inferred intrusion-related origin for Au1. Sulfur isotope data is similar for both Au1 and Au2 pyrite with average δ34S values of − 5.5‰ ± 0.2‰ (1σ) and − 3.5‰ ± 0.3‰ (1σ) and average Δ33S values of 0.4‰ ± 0.1‰ (1σ) and − 0.3‰ ± 0.2‰ (1σ), respectively. SIMS δ18Oquartz values for the Au1, Retrograde, and Au2 events largely overlap and, like δ18Ocarbonate values of previous studies, tend to be lower than values typical of Archean gold deposits. The results of this study suggest that correlation coefficients in trace metal datasets are useful in discriminating and characterizing different gold events. Caution is emphasized with the use of S- and O-isotope datasets for these purposes. The presence of low δ18O values in vein quartz and carbonate is best explained by an 18O-depleted fluid formed during the Retrograde hydrothermal event. The latter is inferred at 2580 ± 21 Ma based on U-Pb geochronlogy of hydrothermal titanite, and relates to deformation and metamorphism in the nearby, amphibolite- to granulite-grade Kapuskasing metamorphic belt. Geochronological and geochemical evidence suggest that the 18O-depleted fluid may have formed via the devolatilization of biotite-bearing granitoids during deep-crustal metamorphism.