Negative Carbon Isotope Values Research Articles

Synglacial carbonate records of snowball Earth ocean composition—Evidence from the Nantuo Formation, South China

The Cryogenian (ca. 717−635 Ma) snowball Earth glaciations ended with the precipitation of “cap” carbonate successions with negative carbon isotope (δ13Ccarb) values, which have been explained by the addition of various 13C-depleted carbon sources in the deglacial process. These arguments assumed that marine dissolved inorganic carbon (DIC) was enriched in 13C in the synglacial ocean. However, this assumption has not yet been tested, because the synglacial ocean chemistry is unknown. In this study, we carried out detailed analyses of the petrology, carbonate carbon (δ13Ccarb) and oxygen (δ18Ocarb) isotopes, organic carbon (δ13Corg) isotopes, major and minor elemental compositions (Ca, Mg, Mn, Fe, Sr), and iron speciation (total Fe, highly reactive Fe, pyrite Fe) of the carbonate layers (also called synglacial carbonate layers) from the Nantuo Formation (ca. 650−635 Ma) on the Yangtze block, South China. Petrographic observations indicated that the synglacial carbonate comprises dolomicrite, mud-crystal powder dolomite, lime dolomite, and dolomitic limestone, supporting an authigenic carbonate origin, and thus, it potentially recorded the ocean chemistry during the Marinoan ice age. The synglacial carbonate is characterized by extreme Mn enrichment, low Fe/Mn ratios, and low δ13Ccarb (−7‰) values. High Mn contents and low Fe/Mn ratios imply marine redox conditions favoring Mn2+ accumulation and Fe2+ oxidation, while low δ13Ccarb values might be attributed to CO2 degassing of submarine volcanoes as well as low primary burial during the glaciation. Since the δ13CDIC value of the synglacial ocean was lower than the δ13Ccarb values of most cap carbonates, we infer the addition of 13C-enriched DIC or removal of 12C during cap carbonate precipitation, such as through carbonate weathering or organic carbon burial. These findings provide new insights into the nature of Cryogenian glaciation, the origin of cap carbonates, and the aftermath of global glaciation.

Newly-found ferns in an artificially illuminated deep karst cave have the most negative known carbon isotope values

Newly-found ferns in an artificially illuminated deep karst cave have the most negative known carbon isotope values

Influence of methane seep activities and sea-level changes on elemental and isotopic compositions and abundance of carbonates in sediments of the Okinawa Trough since the last glacial period

The elemental and isotopic compositions and abundance of carbonates in marine sediments are used to reconstruct ancient climate and ocean conditions. These carbonates usually include biogenic and authigenic carbonates in origin, which are controlled by sedimentary and diagenetic processes, respectively. However, the specific mechanisms involved in controlling the origin of biogenic and authigenic carbonates are still unclear. To address this knowledge gap, we analyzed core CSHC-4 sediment from the Okinawa Trough to examine the elemental and isotopic compositions of carbonates and their relationship to methane seepage and sea-level changes. Our results indicate that high-Mg calcite is the dominant authigenic carbonate, indicating moderate methane seep intensity. Negative carbon isotope values as low as −4.06‰ V-PDB of the bulk sediments deposited in the glacial period suggest a connection between the formation of authigenic carbonates and methane seepage. Negative oxygen isotope values as low as −2.6‰ V-PDB may be attributed to the input of 18O-depleted fluids influenced by gas hydrate formation. Biogenic carbonates contribute the majority (96.3%) of the total carbonate contents in sediments. Biogenic carbonate contents are lower during glacial periods and higher during interglacial periods. The reduced carbonate content during glacial periods may be due to lower export productivity influenced by the deepening of North Pacific Intermediate Water, as well as terrestrial inputs and anoxic conditions during low sea level periods. Our findings highlight the importance of considering both biogenic and authigenic carbonates when reconstructing paleoclimate and paleoceanography based on their geochemical characteristics.

Integrated Litho-, Chemo- and Sequence Stratigraphy of the Ediacaran Gametrail Formation Across a Shelf-Slope Transect in the Wernecke Mountains, Yukon, Canada

The Ediacaran Gametrail Formation of northwestern Canada chronicles the evolution of a complex carbonate ramp system in response to fluctuations in relative sea level and regional tectonic subsidence alongside exceptional global change associated with the Shuram carbon isotope excursion (CIE). Here, we use extensive outcrop exposures of the Gametrail Formation in the Wernecke Mountains of Yukon, Canada, to construct a shelf-slope transect across the Shuram CIE. Twelve stratigraphic sections of the Gametrail Formation are combined with geological mapping and a suite of geochemical analyses to develop an integrated litho-, chemo-, and sequence stratigraphic model for these strata. In the more proximal Corn/Goz Creek region, the Gametrail Formation represents a storm-dominated inner to outer ramp depositional setting, while slope depositional environments in the Nadaleen River region are dominated by hemipelagic sedimentation, turbidites, and debris flows. The magnitude of the Shuram CIE is largest in slope limestones which underwent sediment-buffered diagenesis, while the CIE is notably smaller in the inner-outer ramp dolostones which experienced fluid-buffered diagenesis. Our regional mapping identified a distinct structural panel within the shelf-slope transect that was transported ~30 km via strike-slip motion during the Mesozoic–Cenozoic Cordilleran orogeny. One location in this transported structural block contains a stromatolite reef complex with extremely negative carbon isotope values down to ~ -30‰, while the other location contains an overthickened ooid shoal complex that does not preserve the characteristic negative CIE associated with the Shuram event. These deviations from the usual expression of the Shuram CIE along the shelf-slope transect in the Wernecke Mountains, and elsewhere globally, provide useful examples for how local tectonic, stratigraphic, and/or geochemical complexities can result in unusually large or completely absent expressions of a globally recognized CIE.

Selective removal of non-woody species released water limitation on vegetation community stagnated at early successional stages in a humid karst region

The use of plantations and natural restoration enhance vegetation coverage and contribute significantly to the hydrologic functions of degraded terrestrial ecosystems. Researchers have investigated the development and use of plantations, however, few studies looked at naturally-restored vegetation, especially for early successional areas. We applied a series of treatments and their impacts on water status and the dominant woody species were monitored. Our goal is to enhance the hydrologic functions of naturally-restored karst vegetation that stagnated as early successional shrubby-grassland. Our results show that vegetation on both the upper and lower slopes intercepted approximately 50 % of the rainfall because of the dense community structure. With more than 60 % of the community-level root biomass concentrated in the upper 20 cm soil depth, shallow soil water content (SSWC) decreased continuously even when there was scattered light rain events. On the lower slopes, competition-reducing oriented removal of non-woody species significantly reduced canopy interception of water, which contributed to the increase of SSWC and water availability for the remaining dominant woody species. Moreover, dominant woody species in the treated plot exhibited more negative leaf carbon isotope values than the same species in the control plot, indicating released water limitation on plant growth because of the treatment. On the contrary, this treatment exhibited negligible effects on SSWC on the upper slope, environmental water status could only be improved by improving soil water storage capacity. Our results highlight strong competition on SSWC for naturally-restored vegetation on karst hillslopes that stagnated during the early successional stages. In lower slope position where soil is relatively deep, competition-reducing oriented treatment alone can release water limitations on plant growth and provide opportunities for the settlement of species with different hydrologic niches, thus providing a path for enhancing hydrologic functions of naturally-restored karst vegetation.

Dynamic biogeochemical cycling and mineralization of manganese of hydrothermal origin after the Marinoan glaciation

The aftermath of the Marinoan glaciation is an important period in the evolutionary history of Earth. Here we report on the dynamically biogeochemical cycling and organic mineralization processes of Mn in the Doushantuo Formation, Yangtze Block, South China, which was sourced hydrothermally during this time. This process occurred in an extensional tectonic setting as the water depth became shallower. Mn(+2) ascended with deep reducing hydrothermal fluids and accumulated in the form of Mn(+4) near the redox interface of locally oxidized water, which may be associated with metabolism by autotrophic microorganisms. Mn(+4) was reduced to Mn(+2), with organic matter acting as an electron donor during its degradation, which may be mediated by heterotrophic microorganisms. Mn(+2) in the water combined with CO32− generated by the oxidation and degradation of organic matter to form rhodochrosite. Algae and bacteria provided nucleation sites for the crystallization of rhodochrosite. Finally, spherical and ellipsoidal rhodochrosite with a biological structure and negative carbon isotope values (average δ13CPDB = −7.04‰) under the influence of Mn bacteria was formed. The input of hydrothermal material, mineralization involving the participation of organic matter, and mixed enrichment during syngenesis and syngenetic–early diagenetic stage were unique factors in the formation of Mn deposits in the Doushantuo Formation along the northern margin of the Yangtze Block. This represents the first report of Mn mineralization in carbonate form in the Ediacaran globally (Doushantuo Formation), and provides new insights into the biogeochemical cycling of Mn in the Ediacaran.

East-west asymmetry in the distribution of rainfall in the Chinese Loess Plateau during the Holocene

As the cradle of Chinese civilization the Chinese Loess Plateau (CLP) and its climatic and environmental history have attracted much research attention; however, the spatial characteristics of Holocene climatic conditions across the CLP remains unclear. We investigated five loess sections from the Ganjia Basin in the western CLP, and used paleoclimatic indexes from one well-dated loess-paleosol sequence to reconstruct precipitation variation during the past ~15,000 years. The chronology of the loess section is well established, based on 12 AMS 14C dates from bulk organic matter. Loess deposited before 8 ka (1 ka = 1000 years before 1950 CE) indicates that precipitation was low from ~15 to 8 ka. Subsequently, a paleosol, characterized by higher magnetic susceptibility and more negative organic carbon isotopic values, developed under the moist conditions of the middle to late Holocene. Comparison with paleoclimatic records from the CLP revealed that precipitation across the CLP was low during the early Holocene and reached its highest level during 8–3 ka, against the background of an enhanced East Asian summer monsoon (EASM). In the eastern CLP (the region east of the Liupan Mountains), the climate became dry after 3 ka because of a weakening of the EASM; however, precipitation in the western CLP was relatively high at this time. This observation of an east–west asymmetry in precipitation in the CLP during the Holocene based on geological data is supported by numerical modeling results from PMIP3, which indicate that it could have been caused by the westward extension of the western Pacific subtropical high. Our findings provide an analog for the study of interglacials on a longer time-scale and they demonstrate that the pattern of moisture conditions between the eastern and western CLP may differ on sub-orbital timescales.

The Ca and Mg isotope record of the Cryogenian Trezona carbon isotope excursion

The Trezona carbon isotope excursion is recorded on five different continents in platform carbonates deposited prior to the end-Cryogenian Marinoan glaciation (>635 Ma) and represents a change in carbon isotope values of 16–18‰. Based on the spatial and temporal reproducibility, the excursion previously has been interpreted as tracking the carbon isotopic composition of dissolved inorganic carbon in the global ocean before the descent into a snowball Earth. However, in modern restricted shallow marine and freshwater settings, carbon isotope values have a similarly large range, which is mostly independent from open ocean chemistry and instead reflects local processes. In this study, we combine calcium, magnesium, and strontium isotope geochemistry with a numerical model of carbonate diagenesis to disentangle the degree to which the Trezona excursion reflects changes in global seawater chemistry versus local shallow-water platform environments. Our analysis demonstrates that the most extreme carbon isotope values (∼-10‰ versus +10‰) are preserved in former platform aragonite that was neomorphosed to calcite during sediment-buffered conditions and record the primary carbon isotope composition of platform-top surface waters. In contrast, the downturn and recovery of the Trezona excursion are recorded in carbonates that were altered during early fluid-buffered diagenesis and commonly are dolomitized. We also find that the nadir of the Trezona excursion is associated with a fractional increase in siliciclastic sediments, whereas the recovery from the excursion correlates with a relative increase in carbonate. This relationship suggests that the extreme negative isotopic shift in platform aragonite occurred in concert with periods of increased input of siliciclastic sediments, changes in water depth, and possibly nutrients to platform environments. Although the process for generating extremely negative carbon isotope values in Neoproterozoic platform carbonates remains enigmatic, we speculate that these excursions reflect kinetic isotope effects associated with CO2 invasion in platform waters during periods of intense primary productivity.

Siliceous sponge expansion and phosphogenesis in a shallow water environment in the Malyi Karatau Range (Kazakhstan) during the Precambrian-Cambrian transition

Ocean oxygenation during the Precambrian-Cambrian transition is thought to be one of the key factors for the rapid diversification of animals, known as the ‘Cambrian Explosion’. Siliceous sponges inhabiting continental slope settings were capable of shifting the respiratory oxygen demand in the water column to depth, resulting in an increase in oxygen levels of shallow water. To assess the global importance of the impact of ecosystem engineering by sponges, we evaluated the palaeo-environmental conditions that confine sponge expansion using multiple geochemical proxies including silicon and carbon stable isotopes as well as major- and trace elements in cherts and siliceous sedimentary rocks that were deposited in shallow water around the Precambrian-Cambrian transition. We investigated three lithosections in the Malyi Karatau Range in southern Kazakhstan. The lowermost chert samples of these sections are typically characterized by comparably low δ30Si values and exhibit molds of siliceous sponge spicules. Upsection the occurrence of cross-bedding structures and granular phosphorites document the shallowing of the depositional environment, where overall increasing δ30Si values and increasing Al/Si indicate less settlement of sponges and higher suspended particle loads. Corresponding carbon isotope records of the carbonate fraction decrease from near-zero to negative carbon isotope values averaging −5.9‰. The δ13Ccarb values overall coincide with Y/Ho, the redox indicator Ce/Ce*, and mass fractions of carbonate fluorapatite, suggesting that these ratios are predominantly controlled by the mineralization of organic matter in phosphatic sediment. The studied sections document the ecological requirements for sponge settlement and proliferation, such as the presence of a hard substrate and sufficiently low suspended particle concentrations and/or low wave energy. This work suggests that the widespread appearance of karsted surfaces around the Precambrian-Cambrian transition that were subsequently submerged during the Cambrian transgression would have created permissive habitats for sponges and large phosphate deposits. The capability of sponges to oxygenate shallow seawater and extensive phosphorous burial in epicontinental basins are thus feasible mechanisms for the seawater oxygenation around the Precambrian-Cambrian transition.

Biomineralization of Carbonate Minerals Induced by The Moderate Halophile Staphylococcus Warneri YXY2

Although biomineralization of minerals induced by microorganisms has been widely reported, the mechanisms of biomineralization and the characteristics of the biominerals precipitated needs to be studied further. In this study, Staphylococcus warneri YXY2, a moderate halophile, was used to induce the precipitation of carbonate minerals at various Mg/Ca molar ratios. To investigate the biomineralization mechanism, the growth curve, pH changes, ammonia test, the concentration of bicarbonate and carbonate ions, and the activity of carbonic anhydrase (CA) and alkaline phosphatase (ALP) were determined. X-ray powder diffraction (XRD), scanning electron microscopy - energy disperse spectroscopy (SEM-EDS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and stable carbon isotope analyses were used to characterize the minerals. The obtained biotic minerals were calcite, vaterite, Mg-rich calcite, and aragonite crystals. The crystallinity of aragonite decreased with increasing Mg/Ca ratios. The preferred orientation, diverse morphologies, organic substances, and more negative stable carbon isotope values proved the biogenesis of these carbonate minerals. The presence of Mg in the biotic aragonite crystals was likely related to the acidic amino acids which also facilitated the nucleation of minerals on/in the extracellular polymeric substances (EPS). Mg2+ and Ca2+ ions were able to enter into the YXY2 bacteria to induce intracellular biomineralization. Dynamics simulation using Material Studio software proved that different adsorption energies of Glutamic acid (Glu) adsorbed onto different crystal planes of aragonite led to the preferred orientation of aragonite. This study helps to deepen our understanding of biomineralization mechanisms and may be helpful to distinguish biotic minerals from abiotic minerals.

Small microbialites from the basal Triassic mudstone (Tieshikou, Jiangxi, South China): Geobiologic features, biogenicity, and paleoenvironmental implications

We report small microbialites from the calcareous mudstone slightly above the Permian–Triassic boundary (PTB) in the Tieshikou section, southern Jiangxi Province, South China. The newly found microbialite is a bowl-like structure, which contrasts with the surrounding calcareous mudstone. The small microbialite is composed of columnar forms and fan-shaped structures. Mini-columnar structures resemble mini stromatolites. In plane view, most branches are patchy or strip-shaped, with clotted structures, resembling that of a thrombolite. Fan-shaped cement precipitates are comprised of multiple crystal fans that have a radiating texture and that show distinct growth laminae. The radiating fabrics are represented by rod-shaped filaments under SEM, which are interpreted as calcified trichomes/filaments. Prominent micropores occur within the rods, and they may be the plane views of multiple sheaths of the false branching zone of trichomes. Thus, both the columnar forms and radiating fans are probably microbial in origin. Moreover, three types of nano-sized structures: intraparticle micropores, fibrous biofilms, and filamentous sheaths are also pronounced in both the columnar structures and cement fans of the Tieshikou microbialite, all suggestive of biogenicity. These well-preserved fibrous biofilms probably represent calcified extracellular polymeric substances (EPS), and linear sheaths are interpreted as the traces of bacterial activities during the precipitation of dolomite crystals. The Tieshikou microbialite therefore shares similar biogenetic mechanisms with other PTB microbialites. The combination of the absence of pyrite framboids, slightly negative carbon isotope values, and an association with abundant ammonoids, bivalves, and gastropods indicates oxic conditions during the growth of the Tieshikou microbialite. Moreover, the nodular preservational state resembles widely distributed nodular mudstones or muddy limestones of the Lower Triassic successions worldwide. This implies that nodular mudstone or argillaceous limestones may also have been deposited in microbe-rich environments, in which terrigenous supply was abundant.

Geochemical characteristics of natural gases from different petroleum systems in the Longgang gas field, Sichuan Basin, China

Located in the Sichuan Basin, China, the Longgang gas field consists of three vertically developed petroleum systems with the Triassic Leikoupo Formation as a dividing interface. There is one marine petroleum system below the interface and one continental petroleum system above it. The marine petroleum system is composed of coal measures, the main source rock in the Longtan Formation, and marine reef reservoirs in the Changxing and Feixianguan formations. The continental petroleum system can also be subdivided into two sets. One is the Xujiahe petroleum system sourced from the Xujiahe coal measures in the Upper Triassic formation. The other is a Jurassic petroleum system that is sourced from Jurassic lacustrine black shales. The gas pools in the marine system contain H2S gas. The gases are very dry and the δ13C1 and δ13C2 values display less negative values with an average of −29.2 and −25.0‰, respectively. The gases are humic origin generated at highly to over mature stages from coal measures of the Longtan Formation. The natural gas in the continental petroleum system does not contain H2S. The natural gases from the Xujiahe petroleum system are mainly wet gases with a few dry gases, and belong to typical humic type sourced from coal measures of the Xujiahe Formation. All the gases from this Jurassic petroleum system are wet gases and the alkane gases show more negative carbon isotopic values typical of sapropels. These are derived from the lower Jurassic lacustrine black mudstone. The three sets of petroleum systems in the Longgang gas field are vertically well separated. Each system has its own source rock, and there are no gases from other sources despite multiple tectonic events in the past. The reservoirs had been in a relatively stable tectonic condition with excellent seals by cap rocks during the gas accumulation period.

Sediment biomarkers elucidate the Holocene ontogeny of a shallow lake.

We carried out geochemical analyses on a sediment core from Lake Harris, Florida (USA) to identify sources of organic matter to the sediment throughout the Holocene, and relate changes in those sources to shifts in past climate and environmental conditions. We hypothesized that the sources of organic matter changed in response to regional hydrologic shifts following de-glaciation, and to human population expansion in the state during the 20th century. Hydroclimate shifts in Florida were related to: 1) a steady rise in relative sea level and the fresh water table that began in the early Holocene, 2) wetland formation and expansion ca. 5,000 cal yrs BP, and 3) the onset of the modern El Niño (ENSO) cycle ~3,000 cal yrs BP. Stratigraphic changes in sediment variables from Lake Harris reflect each of these hydroclimate periods. Early in the Holocene, Lake Harris was a marsh-like system in a relatively dry, open-prairie environment. Organic sediments deposited at that time were derived largely from terrestrial sources, as inferred from high TOC/TN ratios, a dominance of longer-chain of n-alkanes (n-C29-31), relatively negative organic carbon isotope values (δ13CTOC), and low biogenic silica concentrations. In the middle Holocene, a positive shift in δ13CTOC coincided with the onset of wetter conditions in Florida. Submerged macrophyte biomarkers (n-C21-23) dominated, and during that period bulk organic carbon isotope values were most similar to δ13C values of mid-chain-length n-alkanes. In the late Holocene, δ13CTOC values declined, CaCO3 levels decreased to trace amounts, organic carbon concentrations increased and diatom biogenic silica concentrations increased from 10 to 120 mg g-1. Around 2,900 cal yrs BP, the effects of ENSO intensified and many Florida lakes deepened to their current limnetic state. Concentrations of algal and cyanobacterial biomarkers in the Lake Harris core increased by orders of magnitude after about AD 1940, in response to human-induced eutrophication, an inference supported by values of δ15N that fluctuate around zero.

Stable isotope analysis of the dietary habits of a Greek community in Archaic Syracuse (Sicily): a pilot study

ABSTRACTThe study and reconstruction of ancient dietary habits has become a very significant topic in archaeological research. Most chemical analysis studies for diet at Greek sites are limited to prehistory, with very few examples of studies for the Classical period. This paper represents a pilot study of stable isotope analysis carried out on a group of 15 individuals selected among the population interred in the Archaic necropolis of Scala Greca at Syracuse (Sicily), a cemetery discovered in 2010-2011. Based on the funerary assemblages and certain features such as shape, dimensions, coverage system and architectural features, a preliminary distinction has been made based on apparent wealth on a diagnostic sample of 15 tombs to test possible variations in diet. Carbon and nitrogen isotope analyses were conducted on bone collagen, which represents dietary protein, and bone apatite, which represents the whole diet. The highly negative collagen carbon isotope values indicate little if any seafood in the diet, and this is supported by the nitrogen isotope modest values, which are consistent with terrestrial foods and do not suggest higher trophic level fish.

An improved understanding of the origin and mechanism of Permian-Triassic natural gas-condensate accumulations in the Gavbendi High, Southwest Iran: An integrated approach

Numerous gas-condensate discoveries have been made in the Permian-Triassic reservoirs in the southwest of Iran. However, the origin and post-generation alterations of these gas-condensate accumulations remain unresolved. Thus, a multifaceted approach was utilized to investigate their origin and formation mechanism. For this study, molecular and isotopic compositions of 11 gases and their associated condensate samples together with source rock extracts were integrated to better address the source of the gases. The molecular data on associated condensates suggested that the gas-condensates were derived from two different source rocks. The reservoirs in central and northern parts of the study area with their low concentrations of tetracyclic terpanes and high extended tricyclic terpane ratios were generated from the Early Triassic Kangan source rocks, whereas the condensates at eastern and western parts with higher contents of tetracyclic terpanes and lower values of extended tricyclic terpane ratios showed good affinity with the Silurian Sarchahan Formation. The gas-condensates at central parts of the Gavbendi High with high amounts of saturate to aromatic ratios together with high fractionation index were accumulated through migration of gas from the Kangan source kitchen in northern part of the Gavbendi High. However, the gas accumulations in western side of the Gavbendi High were formed by migration of gas from the western Silurian source kitchen. Moreover, the Permian-Triassic gas reserves with their relatively low differences between carbon isotope values of ethane and propane and less negative carbon isotope values of methane together with less negative carbon isotope values of CO2 and negligible H2S contents were subjected to low extent of oil cracking and generated mainly by kerogen cracking through progressive maturation of source rock. In addition, multiple charging of the reservoirs in northern part of the study area plays an important role in the Permian-Triassic sequence which has not been previously identified.

Carbon cycle history through the Jurassic–Cretaceous boundary: A new global δ13C stack

We present new carbon and oxygen isotope curves from sections in the Bakony Mts. (Hungary), constrained by biostratigraphy and magnetostratigraphy in order to evaluate whether carbon isotopes can provide a tool to help establish and correlate the last system boundary remaining undefined in the Phanerozoic as well provide data to better understand the carbon cycle history and environmental drivers during the Jurassic–Cretaceous interval. We observe a gentle decrease in carbon isotope values through the Late Jurassic. A pronounced shift to more positive carbon isotope values does not occur until the Valanginian, corresponding to the Weissert event. In order to place the newly obtained stable isotope data into a global context, we compiled 31 published and stratigraphically constrained carbon isotope records from the Pacific, Tethyan, Atlantic, and Boreal realms, to produce a new global δ13C stack for the Late Oxfordian through Early Hauterivian interval. Our new data from Hungary is consistent with the global δ13C stack. The stack reveals a steady but slow decrease in carbon isotope values until the Early Valanginian. In comparison, the Late Jurassic–Early Cretaceous δ13C curve in GTS 2012 shows no slope and little variation. Aside from the well-defined Valanginian positive excursion, chemostratigraphic correlation durSchning the Jurassic–Cretaceous boundary interval is difficult, due to relatively stable δ13C values, compounded by a slope which is too slight. There is no clear isotopic marker event for the system boundary. The long-term gradual change towards more negative carbon isotope values through the Jurassic–Cretaceous transition has previously been explained by increasingly oligotrophic condition and lessened primary production. However, this contradicts the reported increase in 87Sr/86Sr ratios suggesting intensification of weathering (and a decreasing contribution of non-radiogenic hydrothermal Sr) and presumably a concomitant rise in nutrient input into the oceans. The concomitant rise of modern phytoplankton groups (dinoflagellates and coccolithophores) would have also led to increased primary productivity, making the negative carbon isotope trend even more notable. We suggest that gradual oceanographic changes, more effective connections and mixing between the Tethys, Atlantic and Pacific Oceans, would have promoted a shift towards enhanced burial of isotopically heavy carbonate carbon and effective recycling of isotopically light organic matter. These processes account for the observed long-term trend, interrupted only by the Weissert event in the Valanginian.

High resolution geochemical investigation of the bivalve shells (Glycymeris sp.) from shell mounds in Jeju Island, Korea: Late Holocene paleoclimatic implications related to East Asian Monsoon climate

Recent and shell mound bivalves (Glycymeris sp.) were collected from two sites (Sangmori and Udo, Jeju Island) to investigate paleoceanographic conditions using high resolution stable isotope and trace element data. The radiocarbon ages are about 3210 ± 50 and 4210 ± 60 BP for the Sangmori and Udo shell mounds, respectively. Both oxygen and carbon isotopic compositions of the shell mound bivalve shells show clear seasonal cycles with depleted values during summer and the coeval trend, which indicates that they were controlled by freshwater input. Similar ranges of both isotopes indicate that shallow seawater conditions near the Sangmori and Udo shell mounds are more or less similar. However, more negative oxygen and carbon isotope values of the shell mound bivalves than the recent counterpart suggest that surface seawater salinity near Jeju Island was lower due to the increase in Changjiang River Discharge. Mg contents of the recent bivalve reflect surface seawater temperature changes whereas Sr and Ba contents were controlled by nutrient concentrations directly influenced by Changjiang River Discharge. Geochemical data of the recent and shell mound bivalves may imply higher precipitation (more intense East Asian Summer Monsoon activity) at ∼3.2 and 4.2 ka under the influence of the terminal stage of Holocene climatic optimum in south China or may reflect smaller fluctuating peaks during its decline.

Carbon isotope geochemistry and geochronological constraints of the Neoproterozoic Sirohi Group from northwest India

The early Neoproterozoic carbonates of the Sirohi Group, northwestern India, were studied for geochronology as well as for oxygen and carbon isotopes across three different sections to understand their relationship with contemporaneous global events. New single zircon Pb–Pb evaporation data constrain the age of the Sirohi Group between ca. 920 and ca. 820Ma, though actual span for deposition of the carbonates may be little shorter taking into allowance the time for peneplanation. Narrow range in variation of δ18O values is generally observed in the carbonate which is consistent with generally well-preserved original compositions. Two sections (SA and SS) out of the three that were studied had similarly small variations in δ18O (about 2‰), whereas the third section (SN) had a slightly wider range of δ18O (around 5‰). The δ18O values of the SN section varied between −11.9 and −17.6‰, which can be attributed to marginal modifications in primary composition caused by contact metamorphism which is evident by the occurrence of wollastonite in the carbonates.The δ13Ccarb values of these carbonates varied between −4.1 and +4.7‰ in all sections. Positive and near-zero δ13Ccarb values are from SA and SS sections which are least affected by the metamorphism. The δ13Ccarb values of these two sections are interpreted as primary and can be correlated with contemporary sea-water compositions. The SN section showed negative δ13Ccarb values which might be marginally affected by contact metamorphism, though we avoided sampling from the contact aureole. However, the metamorphic equilibration with 13C-depleted fluids leading to insignificant alteration in isotope compositions cannot be totally ruled out for the analyzed samples.Positive and near-zero carbon isotope values of Sirohi carbonates (SA and SS sections) closely match the global trends of carbonate dated between 900 and 850Ma but the negative carbon isotope anomaly (SN section) shown by the Sirohi Group carbonates is unmatched. Since the SN section is stratigraphically lower to SA and SS sections we suggest that the negative carbon isotope values of the Sirohi Group as one of the oldest Neoproterozoic negative carbon anomaly possibly close to 900Ma and not associated with glaciation. The carbon isotopic compositions of carbonates from the Sirohi Group suggest a variable degree of preservation of original compositions that represent yet another locality with moderately heavy values between 850 and 900Ma and an unmatchable example of the associated negative values. This reflects a unique biogeochemical signature of changing carbon cycles prior to the onset of Snowball Earth Events in the Neoproterozoic.

Cenozoic stratigraphy of Taiwan: Window into rifting, stratigraphy and paleoceanography of South China Sea

Shallow marine sequences of the northern South China Sea (SCS) are uplifted and exposed by plate convergence in the Taiwan mountain belt. These deposits provide detailed geological information about the rifting event, stratigraphy, sedimentology, paleoclimate and paleoceanography of the shallow SCS to compare with what are recorded in the ODP 1148 deep-sea core. Seismic surveys and marine micropalentological studies show that Eocene sequences in the offshore Taiwan Strait and onland Taiwan mountain belt are all deposited in rifting basins and are covered unconformably by the Late Oligocene-Neogene post-rifting strata. Between syn-rifting and post-rifting sequences, there is a regional break-up unconformity throughout the island. Early Oligocene and Late Eocene strata are missing along the break-up unconformity equivalent to the T7 unconformity in the Pearl River Mouth Basin off south China. This may suggest that the SCS oceanic crust could have initiated between 33 and 39 Ma. Neither obvious stratigraphic gap nor slumping features are found in the Oligocene-Miocene transition interval of Taiwan. This observation highly contrasts with what has been documented from the ODP 1148 deep-sea core. This suggests that the stratigraphic gap and slumping features could only be recorded in the SCS deep sea region, but not in the shallow shelf near Taiwan. Compared to the Middle Miocene paleoceanographic re-organization events in the SCS deep sea, the geological history of the Taiwan shallow sequence shows changes of in sedimentation and faunal composition. Due to the Antarctic glacial expansion at ∼14 Ma, Middle to late Miocene strata of the Western Foothills show progressive regression sedimentations associated with a decrease of benthic foraminiferal abundance and a sharp faunal turnover event. Many Early-Middle Miocene endemic benthic foraminifers were extinct in 14-13 Ma and new benthic foraminifers of the Kuroshio Current fauna appeared from 10.2 Ma, comparable with new occurrence of Modern benthic foraminifers at 9 Ma in the Java Sea area. This reveals that the Western Boundary Kuroshio Current in the North Pacific could initiate from 10-9 Ma due to closures of the Indo-Pacific seaways by convergent tectonics between the Australian Continent and the Indonesian Arc in 12-8 Ma. Subduction of the SCS oceanic lithosphere since the Middle Miocene resulted in formation of the Hengchun Ridge accretionary prism and the North Luzon Arc. Occurrence of these two bathymetric highs (−2400 m) since the Middle Miocene and closures of the inter-arc passages in the North Luzon arc in the last 3.5 Ma would control the water exchanges between the West Pacific and the deep SCS. Accordingly, the tectonic evolution in the Central Range-Hengchun Peninsula accretionary prism and the arc-forearc Coastal Range not only control directly the route for water exchanges between the West Pacific and the SCS, but also indirectly shows a great influence on the geochemistry of deep SCS waters. The latter is best shown by much negative carbon isotope values of benthic foraminifers in the ODP 1148 deep-sea core than the West Pacific records in the last 14 Ma.

Influence of climate and dolomite composition on dedolomitization: insights from a multi-proxy study in the central Oman Mountains

Abstract Dedolomitization is an important diagenetic process that can yield information on the history of paleo-fluids in a given aquifer or outcrop, and by extension it offers a window into past hydrologic conditions. Dedolomitization is also relevant economically as it can alter the porosity and permeability of carbonates, thus affecting reservoir quality. Despite considerable research, the process of dedolomitization is still not entirely understood. Here, new findings from the central Oman Mountains highlight the importance of dolomite chemistry and fluctuating climate on dedolomitization. Petrographic, mineralogical, and geochemical data from both altered and pristine dolomite hosted in Jurassic carbonate rocks from the Sahtan Group and outcropping at Wadi Mistal in the Jebel Akhdar tectonic window reveal two dolomite bodies with different characteristics, stratabound and fault-related dolostone. The (ferroan) dolomite crystals are larger in the fault-related dolostone, whereas the (non-ferroan) dolomite crystals are smaller in the stratabound dolostone. Petrography reveals a complex suite of alteration textures, including pristine dolomite, recrystallized dolomite, and calcitized dolomite (dedolomite). Iron oxides are present pervasively in the altered rocks, and different alteration textures co-occur in the same sample. Relative to unaltered dolomite in this outcrop, the recrystallized dolomite is characterized by similar positive carbon isotope values but less negative oxygen isotope values, and the calcitized dolomite is characterized by even less negative oxygen isotope values and more negative carbon isotope values. Based on field data and petrography, two phases of alteration are evident, an earlier dedolomitization event affecting the stratabound dolomite only, and a more pervasive Pleistocene-Holocene alteration phase associated with surface weathering that affected both the fault-related dolomite and to a lesser extent the stratabound dolomite. Stable isotope results further suggest that the more recent subaerial weathering phase happened under two alternating climate regimes: dedolomitization was triggered by meteoric fluids that interacted with soil-related carbon during humid climatic period(s), whereas recrystallization of ferroan fault-related dolomite happened during more arid climatic period(s) with less developed soils and lower fluid–rock interaction. These results suggest that weathering of ferroan fault-related dolomite can lead to formation of goethite and recrystallized, non-stoichiometric dolomite (with a reset stable oxygen isotope signature), and sometimes to dedolomitization.