δ13Ccarb Values Research Articles

Extremely 13C-enriched dolomite records interval of strong methanogenesis following a sulfate decline in the Miocene Ries impact crater lake

In impact crater lakes, the lacustrine sedimentary records may not solely reflect climatic changes but also potential erosional effects from lithologically distinct impactite formations. The hydrochemical and biogeochemical processes during the deposition of the Nördlinger Ries impact crater lake, which fall in the range of the mid-Miocene Climate Transition, were studied by analysing microcrystalline authigenic carbonates in a drill core succession, using stable oxygen and carbon isotopes in conjunction with biomarkers. These investigations revealed an early sulfidic interval characterized by thiophenes, iso- and anteiso-C15:0 acids derived from sulfate reducing bacteria, and dolomites with low to intermediate δ13Ccarb values. The subsequent distinctive interval is characterized by extremely 13C-enriched dolomite (δ13Ccarb up to +20.93 ± 0.05‰ V-PDB), decline of iso- and anteiso-C15:0 acids and is rich in an Archaea-derived archaeol that is 13C-enriched (−14.7‰), indicating extensive methanogenesis in sulfate-depleted lake porewater during early diagenesis. The sulfate decline results from successive sulfate reduction when replenishment by sulfate-bearing inflow water is limited. The carbonates exhibit enriched 18O due to pronounced evaporation in a long-resided water body and enriched 13C by methanogenesis. A change in provenance of water derived from the sulfur-rich suevite (impact melt-bearing breccia) and crystalline source rocks to the sulfur-poor Bunte Breccia (continuous ejecta blanket) is required. Intermittently high Si/Al and Zr/Al at the high δ13C interval suggests sporadic short-term runoff increase, leading to fluctuating physiochemical lake conditions. A subsequent decline in both δ13Ccarb and archaeol indicates a decreasing lake level with intermittent subaerial exposure events, supported by bioturbation and mud cracks. The concomitant lake oxygenation is well supported by increasing Pr/Ph ratios and lipids derived from aerobic methanotrophs (13C-depleted 3-methyl-hopanoids). In the youngest unit, allochthonous lignites and biomarkers from lacustrine/soil sources appear, high total sulphur contents and thiophenes recur, and stable C and O isotope values decrease again. These observations suggest another major provenance change of the inflowing solutes, with increasing influx from weathered pyrite-bearing Jurassic claystones. These findings demonstrate that the climatic record expected from the stable carbon and oxygen isotopes of the Ries carbonates is strongly overprinted by hydrochemical and biogeochemical processes due to changing ion influx from substrate rocks, along the course of the successive ejecta erosion and catchment changes. Such an intrinsic control of lacustrine biogeochemical processes is also expected for other hydrologically closed impact crater lake basins, where catchment rocks with distinctively different lithologies are present.

High positive carbonate carbon isotope excursion identified in the North China Craton: Implications for the Lomagundi–Jatuli Event

AbstractRecords of the Lomagundi–Jatuli Event (LJE) are well preserved globally, but high δ13Ccarb carbonates have not been identified in the North China Craton (NCC). Our results on ~3–4 km thick carbonates from the newly confirmed Palaeoproterozoic successions in Fanhe Basin in the northeastern NCC show that the ~2.20–2.06 Ga carbonates have positive carbon isotope excursion and those deposited after 2.06 Ga have normal carbon isotope. Specially, carbonates from the Daposhan Formation have δ13Ccarb values of 10.2‰–11.8‰, which is the largest positive carbon isotope excursion in the NCC. The ~2.20–2.06 Ga carbonates in Fanhe Basin have similar δ13Ccarb values as those contemporaneously deposited in other cratons and their δ13Ccarb values exhibit a decreasing trend from ~2.20 Ga to 2.06 Ga. Our identification of carbonates with high positive δ13Ccarb values in Fanhe Basin not only casts new lights on records of the LJE in the NCC, but also provides important constraints on global significance of the positive δ13Ccarb excursion of LJE.

The Roles of Various Types of Crustal Contamination in the Genesis of the Jinchuan Magmatic Ni-Cu-PGE Deposit: New Mineralogical and C-S-Sr-Nd Isotope Constraints

Abstract Addition of crustal sulfur to the Jinchuan magma or oxidation of the magma associated with carbonate assimilation has been considered to be the main cause of sulfide saturation in the magma by two opposing groups of researchers. To address this controversy, we have carried out an integrated mineralogical and C-S-Sr-Nd isotope study of the Jinchuan magmatic Ni-Cu-platinum group element (PGE) sulfide ore deposit. Pure marble, olivine marble, serpentine marble, and hybrid rocks occur in the contact zone. The δ13Ccarb values of the Jinchuan sulfide-bearing ultramafic rocks containing calcite xenocrysts are from –4.7 to –2.9‰, which are similar to or slightly lower than those of the marbles and associated hybrid rocks (–3.9 to 0.8‰) but significantly higher than those of the calcite-barren intrusive rocks (–9.3 to –8.0‰). This, together with the calcite xenocrysts and calcium-silicate minerals in hybrid rocks and some intrusive rocks, indicates that carbonate assimilation took place during magma-carbonate interaction. Only less than several hundred ppm of the CO2 gas produced in the reaction zone could be added to the Jinchuan magma. Such a trace amount of additional CO2 was rapidly swamped by the much greater mass of FeO in the magma, resulting in little change in the FeO/Fe2O3 of the magma, and thereby negligible change of magma redox state. This is evident from similar calculated fO2 values for the calcite-bearing (~QFM+0.7) and calcite-barren (~QFM+0.6) intrusive rocks. The new results rule out the possibility that sulfide saturation in the Jinchuan magma resulted from in situ carbonate assimilation. The Sr-Nd isotope data from this study and previous studies are consistent with up to 20% bulk contamination with siliceous crustal materials at depth, followed by minor amounts of in situ carbonate assimilation by the Jinchuan magma. Our new sulfur isotope data expand the range of δ34S for the Jinchuan deposit significantly. The new range is from −7.6 to 3.0‰, with an average of −1.8‰, which is generally lower than the mantle value (0 ± 2‰). The new result supports the premise that crustal sulfur was involved in the genesis of the Jinchuan deposit. Very high, crustal-like S/Se ratios (as high as 8080) for some of the samples from the deposit provide additional support for the interpretation. The country rocks in the vicinity of the Jinchuan deposit analyzed to date have δ34S values varying from −4.0 to 11.3‰, with an average of 2.9‰, which is higher than both the mantle value and the average value of the Jinchuan deposit, suggesting that the Jinchuan magma acquired some crustal sulfur at depth, likely concurrent with the siliceous assimilation. Numerical modeling of δ34S-S/Se of sulfide ores and country rocks further illustrates that the observed variations of δ34S and S/Se ratios are related to the assimilation of S-rich rocks located at depth, followed by progressive dilution of the contaminated δ34S-S/Se signature. Based on the new results, we conclude that contamination with siliceous crustal materials as well as addition of crustal sulfur at depth played a critical role in triggering sulfide saturation in the Jinchuan magma.

Constraining the onset and environmental setting of metazoan biomineralization: The Ediacaran Nama Group of the Tsaus Mountains, Namibia

The advent of animal (metazoan) biomineralization marks a fundamental transition in Earth's history, facilitating the diversification of novel body plans as well as the biological control of carbonate sediment production, resulting in a permanent restructuring of the global carbon cycle. The oldest known skeletal metazoans, the tubular ‘cloudinids’ e.g., Cloudina, appeared during the terminal Ediacaran of the Nama Group, Namibia, but precisely when, where, and why metazoans first acquired the ability to biomineralize and their timing relative to the regional δ13Ccarb profile is undocumented. Assessing possible environmental triggers for this key event therefore requires accurate constraint of the age, paleoenvironmental setting and the geochemical context of the earliest cloudinids.Here we present new stratigraphic, sedimentological, and geochemical (δ13Ccarb and δ18Ocarb) data from the oldest strata (Dabis Formation) of the Nama Group from the Witputs Sub-basin, exposed in the Tsaus Mountains of the Tsau Khaeb National Park (formerly Sperrgebiet). This constrains the oldest confirmed occurrence of Cloudina to limestones of the lower Kliphoek Member, ca. 551–550 Ma, that were deposited laterally-equivalent to the more proximal fossiliferous Kliphoek quartzite. These limestones have dominantly negative δ13Ccarb values that immediately precede recovery from the basal Nama negative δ13Ccarb excursion (BANE), and were deposited in shallow waters after a transition from semi-restricted, evaporitive-dolomitic to open marine carbonate settings. Regional correlation shows that Cloudina first appeared during an interval of dominantly low oxygen and unstable, regional marine redox conditions. We conclude that data do not support a long-term shift towards more stable, oxygenated conditions as a driver for the first appearance of skeletonization in metazoans, but do suggest that open marine carbonate settings with high carbonate supersaturation were required to support the calcifying Cloudina. In the Nama Group, the appearance of Cloudina also notably coincided with a major sea level lowstand, which we hypothesise may have enabled oxygenation of these more distal settings. Such an expansion of shallow marine oxygen may therefore have provided an evolutionary driver for innovations that were not metabolically costly, such as the onset of Cloudina biomineralization.

Did sea level change drive carbon isotopic trends in the Madison Shelf? Sequence stratigraphy and carbon isotopes in the Mississippian Lodgepole Formation of southwest Montana

The Lower Mississippian Lodgepole Formation of Montana and Wyoming records one of the largest positive carbon isotopic excursions of the Phanerozoic. This globally recognized up to 7‰ increase in δ13Ccarb values occurs across the North American Kinderhookian-Osagean boundary (referred to as the KO excursion). It has been argued to reflect significant organic carbon burial, possibly linked to the onset of the Late Paleozoic Ice Age. Previously proposed correlations between carbon isotopic patterns and the sequence stratigraphic framework within these strata suggests that changes in sea level could have played a significant role in the expression and/or magnitude of the KO excursion in the Madison Shelf. This study explores the relationship between carbon isotopic values and sea level change at multiple scales. To accomplish this, we provide a comprehensive overview of the sedimentological and stratigraphic framework and address uncertainty about the number of sequences in the Lodgepole Formation. Our results support a three-sequence model for the Lodgepole Formation. Based on the number of sequences and the placement of sequence stratigraphic surfaces, we see little evidence of statistically significant correlation between carbon isotopic trends and the sequence stratigraphic framework. We argue that sea level change was not the primary driving mechanism for carbon isotopic trends in the Madison Shelf, nor the KO excursion. Instead, we support models that invoke global ocean anoxia and/or destabilization of the global carbon cycle due to land plants.

Effects on global warming by microbial methanogenesis in alkaline lakes during the Late Paleozoic Ice Age (LPIA)

Abstract Methane (CH4) is an important greenhouse gas, but its behavior and influencing factors over geological time scales are not sufficiently clear. This study investigated the Late Paleozoic Ice Age (LPIA), which is thought to have experienced an interval of rapid warming at ca. 304 Ma, that may have been analogous to modern warming. To explore possible causes of this warming event, we investigated ancient alkaline lakes in the Junggar Basin, northwestern China. Results show that microbial CH4 cycling here was strong, as evidenced by carbonate δ13C (δ13Ccarb) values of >5‰, ~+0.6‰ offsets between pristane δ13C (δ13CPr) and phytane δ13C (δ13CPh) values, a 3β-methylhopane index of 9.5% ± 3.0%, and highly negative δ13C values of hopanes (–44‰ to –61‰). Low sulfate concentrations in the alkaline lakes made methanogenic archaea more competitive than sulfate-reducing bacteria, and the elevated levels of dissolved inorganic carbon promoted methanogenesis. Biogenic CH4 emissions from alkaline lakes, in addition to CO2, may have contributed to rapid climate warming.

Molecular mechanism of metamorphic alteration on traces of early life in banded iron formations

It is well-known that multi-stage metamorphism can result in the alteration of indigenous biological molecules, limiting our understanding of early life on Earth. However, the physiochemical mechanisms involved in these processes are still poorly understood. In this study, we present petrographic observations and micro- to nano-geochemical investigations on the carbonaceous matter (CM) in representative Neoarchean banded iron formations (BIFs) from North China, which have undergone significant alteration during lower amphibolite-facies prograde metamorphism, and subsequent retrograde alteration. The CM is in paragenetic equilibrium with prograde mineral phases, and is often associated with apatite that occurs in Fe-rich bands parallel to layering. This implies that the CM is most likely inherited from syn-depositional biomass, as confirmed by the nanoscale infrared spectroscopy, which shows the presence of CC, CH, and CN/NH bonds. Raman spectroscopic analyses reveal that the maximum metamorphic temperature of CM is 479 ± 50°C, which is consistent with the metamorphic peak conditions of the host BIFs from petrographic constraints (i.e., garnet-bearing amphibolite-face metamorphism). The BIFs possess average bulk δ13Corg values of −20.0 ± 0.9‰ (1σ) and δ13Ccarb values of −12.9 ± 1.8‰ (1σ), further indicating syngenetic biomass graphitization during prograde metamorphism. This thermal cracking process may have released gaseous hydrocarbons, as shown by secondary CH4 fluid inclusions in quartz. We further use quantum mechanical simulations to constrain dissociative tendencies for functional groups (CC, CH, CO, and CN) of original organic molecules to assess the stability of organic chemical bonds during prograde metamorphism (0–600°C, 0–15 kbar). The relatively high thermal durability of CH and the armoring effects of primary organic-phyllosilicate complexes may account for CH preservation in BIFs. Furthermore, the electron microscopy combined with elemental analysis reveals widespread nano-chlorite infiltration into CM during retrograde metamorphism (i.e., partial replacement of garnet by chlorite). The pervasive CM alteration is likely responsible for the absence of CO bonds, where nanopore-scale reactions might have played a key role. Altogether, we suggest that multi-stage metamorphic processes, involving mineral-organic reactions and nanoscale interface interactions, may have governed the preservation of ancient biosignatures in BIFs. Our findings highlight the importance of evaluating metamorphic effects when using molecular signals to reconstruct early life behaviors, and shed new light on the study of primordial microorganisms, particularly those found in iron-rich sediments on early Earth.

Paired carbon isotope chemostratigraphy across the Ordovician–Silurian boundary in central East Baltic: Regional and global signatures

The latest Ordovician and earliest Silurian carbonate succession of the East Baltic region has served as a global reference for studying the Hirnantian mass extinction and global environmental changes recorded in geochemical archives. However, the data from the area have uneven spatial distribution, with the deeper shelf Livonian Basin poorly covered by integrated geochemical-paleontological data sets. Moreover, virtually no paired organic carbon (δ13Corg) and carbonate carbon (δ13Ccarb) isotope records are available from the Ordovician of Baltica, limiting the possibilities for chemostratigraphy and geochemical modelling. Here we studied the Likėnai 396 drill core from northern Lithuania to establish a late Katian to Rhuddanian bio- and chemostratigraphic reference section for the central East Baltic region. Our primary focus was on the Hirnantian carbon isotopic event (HICE) interval, together with the late Katian Parovėja excursion. The latter occurs within the Parovėja Formation, for which the Likėnai core serves as a neostratotype. The drill core was sampled for paired carbonate and organic carbon isotope analysis, other geochemical proxies and four groups of microfossils: chitinozoans, conodonts, ostracods and scolecodonts. The integrated data set allowed us to identify the main Baltic δ13Ccarb excursions in the Likėnai core: Moe, Parovėja and HICE. Additionally, a small-scale but distinct Jonstorp peak was distinguished between the Moe and Parovėja excursions. The Likėnai core is a suitable reference section for the Parovėja excursion as well as the falling limb of the HICE in Baltoscandia. Except for the HICE, the δ13Ccarb excursions were not clearly defined in the δ13Corg curve, and the correlation between δ13Corg and δ13Ccarb values appeares to be weak. However, the paired records co-vary in the HICE interval but have a negative correlation in the Parovėja excursion interval. This suggests different forcing mechanisms and poses challenges for chemostratigraphic correlations using mixed δ13Corg and δ13Ccarb records. On the other hand, the remarkable similarity between the δ13Corg curves from different deeper shelf sites indicates a time-constrained and at least basin-scale nature of these events. Our study highlights the existence of global, basin-scale as well as likely local features, but additional paired δ13Corg and δ13Ccarb data are needed to better discriminate between different scales.

Geochemical records and environmental analysis of the Ediacaran-Cambrian boundary in Eastern Yunnan, South China

The Ediacaran–Cambrian transition is characterized by an unprecedented change in biosphere, lithosphere, and atmosphere. However, the identification of this transition and its global correlation remains debated. East Yunnan is a region of utmost importance in the study of the Ediacaran-Cambrian transition. Although strata from this region have been well studied, recognition of the base of the Cambrian continues to be controversial. This paper presents, a carbon chemostratigraphic isotope and trace element profile through the Dengying and Zhujiaqing formations in the Sujiawa section in East Yunnan. Through carbon isotope and trace element analyses of the Dengying and Zhujiaqing formations we attempt to regionally correlate the section and discuss the paleo-marine redox environment changes during this period. The Fe content of samples is low and the Mn/Sr ratios of the majority of the Sujiawa section samples is lower than 10 and shows no correlations with δ13Ccarb and δ18Ocarb values, indicating that the samples retain near-primary δ13Ccarb characteristics. The carbon isotope profile from the Sujiawa section, shows a prominent negative δ13Ccarb in the basal Zhongyicun Member (N1’, −7.3‰) that most likely represents the prominent negative excursion recorded in Ediacaran-Cambrian strata across the world. REE patterns and Y/Ho ratios were used to screen samples, to ensure that the primary seawater REE features and Ce anomalies were preserved. The samples satisfying this screening process show seawater-like REE distribution patterns in leached carbonates and may have recorded Ce anomalies of the seawater from which the carbonates precipitated. These data show that the Baiyanshao Member has Ce/Ce* values between 0.57 and 0.88, the Daibu Member between 0.56 and 0.83, and the Zhongyicun Member between 0.60 and 0.96. The Ce anomalies through the section indicates that the region experienced suboxic-oxic-anoxic conditions throughout the Ediacaran-Cambrian period. Carbon isotope and trace element analyses indicate that the strata record a negative carbon isotope excursion event and anoxic conditions during the period of deposition. This study complements the carbon chemostratigraphic information and trace element data in the region and provides valuable clues for understanding the lithofacies changes through the Ediacaran and Cambrian in different regions of South China.

Paleoenvironmental significance of the carbon isotope record across the Cenomanian–Turonian transition and the Oceanic Anoxic Event 2 (OAE2) in the southeastern Neotethys, Zagros, Iran

A high–resolution carbon isotope record of pelagic carbonates (δ13Ccarb) from the Zagros Mountains, Iran, documents a 1.8‰ positive carbon isotope excursion (CIE) in the southeastern Neotethys during the Cenomanian–Turonian transition, corresponding to Ocean Anoxic Event (OAE2). The succession is controlled by biostratigraphy that includes the Rotalipora cushmani, Whiteinella archaeocretacea, and Helvetoglobotruncana helvetica biozones. The CIE is characterized by a positive shift culminating in a first positive peak (at 3.3‰), followed by a 1‰ negative excursion, accompanied by the highest δ18Ocarb values that can be interpreted as the result of a short–lived global cooling, the Plenus Cold Event. A subsequent second positive peak and a plateau of higher δ18Ocarb values are followed by relatively low values in the earliest Turonian akin to pre–excursion values. The biostratigraphic positions of the three diagnostic maximum points, referred to as peaks A, B, and C, within the generally positive CIE indicate a synchronous δ13Ccarb variability worldwide. The average δ13Ccarb values in southeastern Neotethys are generally lower compared to the open–ocean records of the same period. We attribute this offset to a substantial regional regression combined with recycling of isotopically light organic carbon. The diachronous deposition of black shales/mudstones after the onset of positive CIE suggests that its formation strongly depends on local paleogeography and paleoenvironmental factors.

The Jurassic–Cretaceous boundary in the Kurovice section (Southern Moravia, Czech Republic): trace fossils, stable isotopes, and magnetic susceptibility

In the Kurovice Quarry (southern Moravia), the paleoenvironmental conditions around the Jurassic–Cretaceous boundary were deciphered using geochemical, magnetic, macrofaunal, microfaunal, and ichnologic evidence. In addition to weakly bioturbated horizons, four discernible ichnologic assemblages (IA1–IA4) were distinguished. Geochemical changes were recorded both between individual ichnologic assemblages and within them. IA1 is dominated by Chondrites, Thalassinoides, and Nereites, accompanied by Bergaueria and Phycodes, and occurring in pelagic sediments. IA1 also occurs in an impoverished form, in deposits characterized by a low ichnofabric index (i.e. impoverished IA1). IA2 consists of Trypanites in layers with microbially induced sedimentary structures; it is present in beds 20–21. IA3 is represented by subvertical shafts filled with pyrite and Trypanites isp., found in beds 30 and 63–72. IA4 consists of common Balanoglossites and rare Trypanites, Thalassinoides, and Planolites; it occurs in beds 102–104, 126–127, and 137–140. Four intervals with prominent Balanoglossites (IA4) and occasional Trypanites (IA2) mark probable hardgrounds and/or condensed deposits. These deposits are also characterized by slightly increasing magnetic susceptibility and enhanced biogenic reworking. Enhanced bioturbation/bioerosion commonly concords with higher susceptibility intervals. Geochemically, the beds hosting impoverished IA1 show a slight decrease in the δ13Ccarb followed by an increase in the δ13Ccarb values; this may reflect a general rise in productivity accompanied by organic matter burial. Similar trends in the δ13Ccarb values are also associated with the IA2 and IA3 characterized by the presence of vertical pyrite-filled tubes. Increases in δ18Ocarb values within IA2 and IA3 are interpreted to express cooling episodes suggesting a rise in oceanic productivity. Therefore, this primary isotopic signal reflects nutrient changes within an open ocean, which is not (in this case) influenced by bioturbation activity. IA4 (Balanoglossites) is characterized by an apparent increase in δ18Ocarb values (the trend is similar to that in IA2 and IA3) but δ13Ccarb values are stable.

Famennian negative carbon isotope excursion in the Izyayu River section (Tchernyshev Uplift, Cis-Uralian Foredeep)

Research subject. The article is focused on manifestations of the Early-Middle Famennian negative excursion in carbon isotope composition in the Izyayu River section (south of the Tchernyshev Uplift). Aim. The study is aimed to reconstructing of the probable causes of this isotope excursion. Materials and methods. The article is based on the results of study of the deep-shelf lower-middle Famennian sequence of the Izyayu River section. The carbon isotopic composition was studied for the carbonate samples and conodont elements (32 and 9 samples respectively). Results. The lightening of the carbon isotopic composition of carbonates by 2.5 %, and of the carbon isotopic composition of conodont elements by 4 % were found in the Pal. gracilis gracilis–Pal. marginifera marginifera zonal interval. The difference between the carbon isotopic compositions of carbonates and conodont elements increases in this interval by more than 2.5 %. Possible reasons for the observed variations could be the intensification of terrigenous runoff due to regression, a relatively high content of carbon dioxide in the atmosphere, and, possibly, a local decline in primary bioproductivity. Conclusions. The negative carbon isotope shift spans Pal. gracilis gracilis–Pal. marginifera marginifera zonal interval in the Izyayu River section. The shift corresponds to the global decreasing in the δ13Ccarb values in the early/middle Famennian boundary interval. The increase in supply of the isotopically light carbon with terrigenous runoff and decrease in the primary bioproductivity are considered as probable causes of the carbon isotope excursion.

Uncovering the temporal carbon isotope (δ13C) heterogeneity in seep carbonates: a case study from Green Canyon, northern Gulf of Mexico

Authigenic carbonates that form at hydrocarbon seeps, known as seep carbonates, are direct records of past fluid flow close to the seafloor. Stable carbon isotopes of seep carbonates (δ13Ccarb) have been widely used as a proxy for determining fluid sources and seepage mode. Although the spatial heterogeneity of δ13C in seep carbonates is increasingly understood, the temporal heterogeneity of δ13C in seep carbonates is not well studied. In this study, we report δ13C values of different components (clasts, matrix, and pore-filling cements) for 124 subsamples drilled across an authigenic carbonate block from Green Canyon block 140 (GC140) of the northern Gulf of Mexico continental slope. High-Mg calcite is the dominant mineral regardless the types of components. The δ13Ccarb values range from −39.6‰ to 3.6‰, indicating multiple dissolved inorganic carbon (DIC) sources that include methane carbon (13C-depleted), seawater DIC, and residual CO2 from methanogenesis (13C-enriched). Specifically, the clasts show large variability in δ13C values (−39.6‰ to 2.3‰; mean: −27.6‰, n = 71), demonstrating the dominance of methane-derived fluids during formation at the initial seepage stage. The δ13C values of the matrix vary between −29.4‰ and 3.4‰ (mean: −11.6‰, n = 21). The carbon isotopes of pore-filling cements that formed most recently vary narrowly, with δ13C values of −3.2‰ to 3.6‰ (mean: 1.7‰, n = 28). Isotopic variations within individual samples were observed in seep carbonate. However, common trends occur across components of carbonates that formed during different seepage stages. This suggests that the temporal evolution of local fluid sources may play an important role in determining carbonate isotope geochemistry. Studies regarding seeps must take into account the highly variable fluids that leave their geochemical imprints on the seep carbonate.

Evidence for abundant organic matter in a Neoarchean banded iron formation

Abstract Microbial Fe(II) oxidation has been proposed as a major source of Fe minerals during deposition of banded iron formations (BIFs) in the Archean and Proterozoic Eons. The conspicuous absence of organic matter or graphitic carbon from BIFs, however, has given rise to divergent views on the importance of such a biologically mediated iron cycle. Here, we present mineral associations, major element concentrations, total carbon contents and carbon isotope compositions for a set of lower amphibolite-facies BIF samples from the Neoarchean Zhalanzhangzi BIF in the Qinglonghe supracrustal sequence, Eastern Hebei, China. Graphite grains with crystallization temperatures (~470 °C) that are comparable to that predicted for the regional metamorphic grade are widely distributed, despite highly variable iron (12.9 to 54.0 wt%) and total organic carbon (0.19 to 1.10 wt%) contents. The crystalline graphite is interpreted to represent the metamorphosed product of syngenetic bio-mass, based on its co-occurrence with apatite rosettes and negative bulk rock δ13Corganic values (–23.8 to –15.4‰). Moreover, the crystalline graphite is unevenly distributed between iron- and silica-rich bands. In the iron-rich bands, abundant graphite relicts are closely associated with magnetite and/or are preserved within carbonate minerals (i.e., siderite, ankerite, and calcite) with highly negative bulk rock δ13Ccarb values (–16.73 to –6.33‰), indicating incomplete reduction of primary ferric (oxyhydr) oxides by organic matter. By comparison, only minor graphite grains are observed in the silica-rich bands. Normally, these grains are preserved within quartz or silicate minerals and thus did not undergo oxidation by Fe(III). In addition, the close association of graphite with iron-bearing phases indicates that ferric (oxyhydr)oxides may have exerted a first order control on the abundance of organic matter. Combined, the biological oxidation of Fe(II) in the oceanic photic zone and subsequent burial of ferric (oxyhydr)oxides and biomass in sediments to form BIFs, suggests that a BIF-dependent carbon cycle was important in the Archean Eon. Although significant re-adsorption of phosphorus to ferric (oxyhydr)oxides and the formation of authigenic phosphate minerals at the sediment-water interface would be expected, oxidation of biomass in BIFs may have recycled at least a portion of the P (and other nutrients) released from reactions between organic matter and ferric (oxyhydr)oxides to the overlying water column, potentially promoting further primary productivity.

Frenchman Mountain Dolostone: A new formation of the Cambrian Tonto Group, Grand Canyon and Basin and Range, USA

Abstract We describe, interpret, and establish a stratotype for the Frenchman Mountain Dolostone (FMD), a new Cambrian stratigraphic unit that records key global geochemical and climate signals and is well exposed throughout the Grand Canyon and central Basin and Range, USA. This flat-topped carbonate platform deposit is the uppermost unit of the Tonto Group, replacing the informally named “undifferentiated dolomites.” The unit records two global chemostratigraphic events—the Drumian Carbon Isotope Excursion (DICE), when δ13Ccarb (refers to “marine carbonate rocks”) values in the FMD dropped to −2.7‰, and the Steptoean Positive Carbon Isotope Excursion (SPICE), when the values rose to +3.5‰. The forma tion consists of eight lithofacies deposited in shallow subtidal to peritidal paleoenvironments. At its stratotype at Frenchman Mountain, Nevada, the FMD is 371 m thick. Integration of regional trilobite biostratigraphy and geochronology with new stratigraphy and sedimentology of the FMD, together with new δ13Ccarb chemostratigraphy for the entire Cambrian succession at Frenchman Mountain, illustrates that the FMD spans ~7.2 m.y., from Miaolingian (lower Drumian, Bolaspidella Zone) to Furongian (Paibian, Dicanthopyge Zone) time. To the west, the unit correlates with most of the Banded Mountain Member of the ~1100-m-thick Bonanza King Formation. To the east, at Grand Canyon’s Palisades of the Desert, the FMD thins to 8 m due to pre–Middle Devonian erosion that cut progressively deeper cratonward. Portions of the FMD display visually striking, meterscale couplets of alternating dark- and light-colored peritidal facies, while other portions consist of thick intervals of a single peritidal or shallow subtidal facies. Statistical analysis of the succession of strata in the stratotype section, involving Markov order and runs order analyses, yields no evidence of cyclicity or other forms of order. Autocyclic processes provide the simplest mechanism to have generated the succession of facies observed in the FMD.

Preservation of molecular fossils in carbonate concretions in cretaceous shales in the songliao basin, northeast China

Paleoenvironmental information is better preserved in carbonate concretions. In this study, carbonate concretions in the Cretaceous Nenjiang shale, Songliao Basin, were examined to determine whether molecular fossils reflective of the paleoenvironment were better preserved at these sites. Organic and inorganic geochemical characteristics of the concretions and surrounding rocks were analyzed using a series of techniques, including SEM, LA-ICP-MS, GC-MS-MS, and GC-IRMS. The concretions are composed of high content microcrystalline dolomite. The δ13Ccarb and δ18Ocarb values of the concretionary dolomite were significantly higher than those of the surrounding rocks. The dolomite show enrichment in the LREEs and have a negative Eu anomaly. The concretion biomarkers showed distribution characteristics similar to those of surrounding rocks. This suggested that the molecular fossils preserved in concretions were mainly inherited from surrounding rocks. However, the concretions contained more C27 sterane and hopanes, with the hopane/sterane ratio being significantly higher than that of surrounding rocks (1.49 v. 0.86). Moreover, the relative content of 2-methylhopane was 2.4–6.6 times that of the surrounding rocks. This indicated changes in the biological equilibrium of source organisms within and outside the concretions. It was possible that the unstable organic matter at the core increased the bacterial concentration and activity inside the concretions. Both the hydrogen index and biomarker-derived indicators implied that the transformation of organic matter in concretions was minimized when compared with their host rock. The isotope δ13C16-30 was 1‰–3‰ more prevalent in individual N-alkane hydrocarbons in the concretions than in surrounding rocks, likely owing to differences in lithology, bacterial action, and degree of weathering. The study concluded that carbonate concretions could preserve molecular fossils better than the surrounding rocks, and the in-depth organic geochemical analysis of concretions could provide a valuable reference for research into early life forms.

New insights into the carbon cycle and depositional models of the Eocene saline lake, Jianghan basin, China

Geochemical characteristics and sedimentary processes of sediments, especially mixed sediments, have long been hot topics of research in basin systems, which can be used to infer runoff, evaporation, productivity, precipitation patterns of the paleoenvironment, as well as records of a diverse range of climate-related signs. However, compared with shallow marine mixed siliciclastic-carbonate sediments, the study of lacustrine mixed sedimentary systems in combination with geochemical information is relatively limited. Also, model-based study on the reconstruction of the lacustrine carbon biogeochemical cycle is defective, especially in saline lacustrine environments. In this work, a total of 26 mixed siliciclastic–carbonate sediment core samples from Well BX-7 in the Qianjiang Depression were sequentially selected for scanning electron microscope observation, major and minor element test, and isotope measurements (e.g., δ13Ccarb, δ18Ocarb, and δ34SCAS). Two types of mixed sediments were generated in the overall inflow/outflow balance of water within the saline basin during the Eocene: siliciclastic-dominated mixed sediment (SDMS) and carbonate-dominated mixed sediment (CDMS). The δ13Ccarb values of SDMS and CDMS samples range from −7.13‰ to −4.52‰ (average −5.82‰) and −9.00‰ to −6.11‰ (average −7.48‰), respectively. The δ18Ocarb values of SDMS and CDMS samples range from −10.26‰ to −1.40‰ (average −4.73‰) and −3.80‰ to −0.96‰ (average −1.55‰), respectively. In addition, combining the geochemical parameters of sedimentation rate, Mn2+ content in carbonate lattice ([Mncarb]), carbonate mass fraction, and [Mg2+]/[Ca2+] ratio, we established a lacustrine δ13C variation model at the sediment-water interface (SWI) with the aim of accurately reflecting δ13Ccarb changes during carbonate formation. Based on the sedimentary processes and geochemical characteristics, we established two depositional models for these two types of mixed sediments in a saline lacustrine basin in the overall inflow/outflow balance of water. Model I illustrates SDMS sedimentary processes with a relative low salinity, low FluxDIC-O, high water inflow, and the organic carbon oxidation is dominated by Fe (III) reduction. Model II demonstrates CDMS sedimentary processes with a high salinity, high FluxDIC-O, high water outflow, and the organic carbon oxidation is dominated by sulfate reduction.

Interpretation of abnormally negative carbon isotope excursions in Eifelian–Givetian (Middle Devonian) sediments of South China and implications for paleo-environmental variation

Many negative carbon isotope excursions (NCIEs) have been employed to interpret global biogeochemical events in Earth history, but they are also influenced by local depositional settings and diagenetic alteration, thus making it difficult to link them to global events. Like the NCIEs in Middle Devonian sediments of North America, similar NCIEs were also observed in the age-equivalent sediments of South China, and therefore it is necessary to further address the logical significance of these intercontinentally comparable NCIEs. Here, we present major and trace elements as well as δ15N, δ13Ccarb, and δ13Corg records across the Middle Devonian Eifelian–Givetian transition in the intra-platform basin of South China. The changes in stable isotopes (δ13Ccarb, δ13Corg, δ18Ocarb, and δ15N) and their relationships may jointly indicate limited influence of diagenetic processes. Our abnormally negative δ13Ccarb values (−7 to −5‰), even lighter than the mantle input end-member value (∼ −5‰), cannot be easily explained by dramatic changes in the δ13C values of global dissolved inorganic carbon (DIC) reservoir. In a restricted platform basin, enhanced nutrient input may stimulate high paleo-productivity, resulting in the depletion of DIC concentrations in surface waters. CO2 is then replenished via invasion from the atmosphere, and intense evaporative conditions may promote the large isotopic fractionation of CO2 invasion owing to hydration reactions of aqueous CO2. Our NCIEs are most likely related to the local 13C-depleted values of platform DIC owing to CO2 invasion associated with large isotopic fractionation due to enhanced evaporation and primary productivity. The “productivity-anoxia feedback” mechanism can explain the coupled changes in our terrestrial clastics input, productivity, and redox proxies. By combining redox proxies and C and N isotopic records, we further propose a model for local carbon and nitrogen cycling in a restricted and evaporative intra-platform basin. Our study highlights local depositional controls on NCIEs, and these local factors should be considered before NCIEs are applied to understand the global carbon cycle perturbations during the Middle Devonian.

Negative δ13Ccarb values at the Jurassic-Cretaceous boundary – Vaca Muerta Formation, Neuquén Basin, Argentina

The stable carbon isotopic compositions of carbonate sediments (δ13Ccarb) have been widely used to reconstruct the global carbon cycle and as a stratigraphic correlation tool. This work investigates a δ13Ccarb record of marine organic-rich, mixed siliciclastic‐carbonate mudstones accumulated from the Tithonian (late Jurassic) to the early Valanginian (early Cretaceous) in a retro-arc basin connected to the Proto-Pacific Ocean (Neuquén Basin, Argentina). The high-resolution record of δ13Ccarb values of these strata from outcrops at various locations in the basin differ from those in the coeval pelagic strata of the Tethys. The δ13Ccarb profile in the Puerta Curaco area shows an overall increasing trend from average values of −7‰ (early Tithonian) to approximately 0‰ (early Valanginian). The negative δ13Ccarb values occur in a ~ 400 m thick TOC-rich succession (TOC > 2%) and are arranged in 3 large-scale increasing-decreasing cycles. Although absolute values are offset, main trends are similar in other outcrops and in subsurface wells. The co-occurrence of the highest U/Th (>1.4) and the most negative δ13Ccarb values suggests that the major trends in the δ13Ccarb profile might be modulated by oscillations in oxygen content within the basin probably driven by sea-level changes. Clastic-dominated transgressive hemicycles have higher TOC and more negative δ13Ccarb values while the opposite occurs in carbonate-rich regressive hemicycles. A trend from “normal” Tithonian surface-water values (avg. +2‰) in the southern shelf to increasingly negative δ13Ccarb values towards the basin center indicates a mixing between platform-derived carbonate material unaffected by diagenesis and carbonate precipitated or altered in a dysoxic/anoxic basin. The δ13Ccarb profile in the Tethyan realm does not record this episode of high burial of organic carbon occurred in the Neuquén basin during the Tithonian and Berriasian but display a slightly decreasing trend between +2.5 and +1.5‰.

Cold seep activity in the early Cambrian: Evidence from the world-class shale-hosted Tianzhu barite deposit, South China

Barite (BaSO4) is widely formed in modern and ancient sedimentary environments by the interaction of barium- and sulfate-bearing fluids from a variety of sources. A word-class barite deposit, hosted by lower Cambrian black shale in the Tianzhu area of the Yangtze block, is a well-known stratiform barite ore deposits. Its origin remains highly debated. A variety of genetic models have been proposed, including biological enrichment, cold seep and submarine exhalative hydrothermal activity, which have differing implications for paleo-oceanic environmental interpretation and resource exploration. We conducted a detailed sedimentological, mineralogical and geochemical study of the Tianzhu barite deposit. It dominated by anhedral barite with variable carbonate-pyrite-hyalophane associations and siliciclastic admixtures. These typically display porous and pore-filling structures, as well as dendritic and rosette-like growth forms, indicating in situ precipitation near the sediment-water interface. Based on vertical variation in sedimentary and mineralogical features, four vertically successive Units (I–IV) can be recognized. Barite samples in Units I and III display 87Sr/86Sr ratios within a very narrow range (0.7084 to 0.7085), nearly identical with coeval seawater values; while others display elevated ratios (0.7086 to 0.7279) that likely reflect contributions from modified porewater and/or siliciclastic admixture. Barites also display seawater-like and higher δ34Sbrt values (32.3 ‰ to 48.6 ‰) with respect to coeval seawater sulfate, and δ34Sbrt-δ18Obrt co-variation trends extending from seawater sulfate to higher δ-values. Co-occurring authigenic dolomites display moderately negative δ13Ccarb values (−18.3 ‰ to −7.2 ‰). These features suggest precipitation at the sulfate-methane transition zone (SMTZ) near the sediment-water interface. Overall, these characteristics of the Tianzhu barite deposit most closely resemble features associated with barite enrichment in modern and ancient cold seeps. This genetic model differs from previous interpretations, and does not require a restricted basin with seawater sulfate-depletion. This study has implications for understanding of oceanic barium‑sulfur cycling and environmental change in the early Cambrian, and formation of shale-hosted stratiform barite deposit in the geological record.