Shift In δ13C Values Research Articles

Evolutionary palaeoecology of European rhinocerotids across the Oligocene-Miocene transition.

The Oligocene-Miocene transition witnessed great environmental and faunal changes, spanning from late Oligocene to early Miocene (MP28-MN3). Its drivers and consequences on mammals are, however, poorly understood. Rhinocerotoids are among the most affected taxa, reflected by great taxonomical and morphological changes. However, potential associated changes in ecology have not been explored. Here, we investigated the palaeoecology of 10 rhinocerotid species coming from 15 localities across Western Europe and ranging from MP28 to MN3. We explored evolutionary trends for diet, physiology and habitat via dental wear, hypoplasia, body mass and stable isotopy. All rhinocerotids studied were C3 feeders, whether browsing or mixed-feeding, but clear dietary differences were observed at some localities and between Oligocene and Miocene rhinocerotids. The prevalence of hypoplasia was low (less than 10%) to moderate (less than 20%), but there were great differences by loci, species and localities. Body mass covaried with hypoplasia prevalence, suggesting that larger species might be more susceptible to stresses and environmental changes. We reconstructed similar warm conditions at all localities except Gaimersheim, but found greater variations in precipitation. Indeed, a clear shift in δ13C values was noticed at the end of the Oligocene, consistent with climatic and vegetation changes reported at that time.

Geochemical analysis of bituminous samples from the Pre-Pottery Neolithic B site of Nahal Efe (Northern Negev, Israel): Earliest evidence in the region and an example of alteration of the Dead Sea bitumen

Thirty-six bituminous samples from the Pre-Pottery Neolithic site of Nahal Efe (8000–7700 cal. BCE, Negev, Israel) were analysed to determine their origins. Most samples are mixtures of bitumen with minerals, predominantly quartz, calcite, dolomite and aragonite. The aragonite is from marine shells, which were crushed to be incorporated into the mixtures.Dead Sea bitumen was identified in all samples. The Nahal Efe geochemical data were compared to samples from Tell Yarmuth (2800–1100 BCE), Tall Dhiban (700–800 cal. BCE), Tel Gezer (1250–700 cal. BCE) and to floating asphalt blocks from the Dead Sea. The stable carbon isotope values of resins and asphaltenes of the Nahal Efe bitumens differ from those of the Dead Sea floating blocks, Tell Yarmuth, Tel Gezer and Tall Dhiban, apparently a consequence of diagenesis. Thus, δ13C values of asphaltenes may not always be a reliable representative of the stable carbon isotope composition of the unaltered source bitumen. Surprisingly, the sterane and terpane patterns of the Nahal Efe samples are not severely biodegraded and clearly indicate a Dead Sea bitumen source. No difference was recorded between mixtures and pure bitumen. Some changes have, however, been observed when using quantitative data on biomarkers. The shifts in δ13C values of resins and asphaltenes is not related to biodegradation but rather to oxidation of the Nahal Efe samples.This is the earliest and most complete evidence of Dead Sea bitumen exploitation and use, which included the preparation of different ‘recipes’, in the Negev and Sinai arid regions.

Corals nitrogen and carbon isotopic signatures alters under Artificial Light at Night (ALAN)

This study examines the impact of Artificial Light at Night (ALAN) on two coral species, Acropora eurystoma and Pocillopora damicornis, in the Gulf of Aqaba/Eilat Red Sea, assessing their natural isotopic responses to highlight changes in energy and nutrient sourcing due to sensory light pollution. Our findings indicate significant disturbances in photosynthetic processes in Acropora eurystoma, as evidenced by shifts in δ13C values under ALAN, pointing to alterations in carbon distribution or utilization. In Pocillopora damicornis, similar trends were observed, with changes in δ13C and δ15N values suggesting a disruption in its nitrogen cycle and feeding strategies.The study also uncovers species-specific variations in heterotrophic feeding, a crucial factor in coral resilience under environmental stress, contributing to the corals' fixed carbon budget. Light measurements across the Gulf demonstrated a gradient of light pollution which possess the potential of affecting marine biology in the region. ALAN was found to disrupt natural diurnal tentacle behaviors in both coral species, crucial for prey capture and nutrient acquisition, thereby impacting their isotopic composition and health.Echoing previous research, our study underscores the need to consider each species' ecological and physiological contexts when assessing the impacts of anthropogenic changes. The findings offer important insights into the complexities of marine ecosystems under environmental stress and highlight the urgency of developing effective mitigation strategies.

Sedimentary mercury as a proxy for redox oscillations during the Cambrian SPICE event in western Newfoundland

The Steptoean Positive Isotopic Carbon Excursion, or SPICE event, was a globally recorded +2‰ to +6‰ shift in δ13C values during the later Cambrian (ca. 500 Ma). The excursion is associated with regional sea level change and trilobite extinctions documented from multiple paleocontinents, yet the mechanism(s) for these events and the excursion itself are not fully understood. Recent studies have suggested that the SPICE is accompanied by global changes in redox conditions, which may have served as an extinction driver. Here, we present new geochemical and mineralogical data from an outcrop of the SPICE located along the Port au Port Peninsula in western Newfoundland. We focus on paired sedimentary mercury and glauconite enrichments as a potential redox indicator in an effort to evaluate local redox conditions during the SPICE in Newfoundland and assess the validity of Hg as a redox proxy. These strata record small mercury enrichments (<20 ppb) relative to SPICE background, sometimes coupled with elevated glauconite abundance. This coupling occurs after the carbon isotope peak, suggesting that the environment during the falling limb of SPICE experienced redox oscillations. Findings from this study supplement existing global proxies by adding information on redox conditions during the SPICE in a shallow shelf environment. Further, the coupling of Hg enrichments with elevated glauconite provides some support for the use of sedimentary Hg as a redox indicator when enrichments are elevated compared to background.

Earliest known Cambrian calcimicrobial reefs occur in the Gobi-Altai, western Mongolia: Intriguing geobiological products immediately after the Ediacaran–Cambrian boundary

A dramatic shift in microbial reefs occurred around the Ediacaran–Cambrian boundary. Here we describe changes in the composition, construction, and texture of microbial reefs in the Zavkhan Terrane of Gobi-Altai Province, western Mongolia, during the late Ediacaran and early Cambrian. Stromatolites consisting of peloids, micritic clots, and homogeneous lime mud without calcified microbes (calcimicrobes) are characteristic of the upper Ediacaran (units 9 and 16A of the Zuun-Arts Formation). In contrast, abundant thrombolites with stromatolites occur in the lowest Terreneuvian (units 17A and 17 of the Bayan Gol Formation). These Cambrian microbial reefs are made up of micritic clots and homogeneous lime mud in close association with calcimicrobes including Korilophyton, Renalcis, and Tarthinia. The thrombolites and calcimicrobial reefs studied herein occur directly stratigraphically above strata that record a strong negative shift in δ13C values and are dominated by small shelly fossils; these are the earliest known calcimicrobial reef representatives of the Phanerozoic. These microbial reefs changed almost simultaneously with drastic fluctuations in environmental conditions (e.g., seawater chemistry, Ca concentration, carbonate saturation, and oxygen level). These changes would have been influenced by the evolution of calcimicrobes and skeletal metazoans across the Ediacaran–Cambrian boundary. The present work provides crucial geobiological information on substantial shifts at the Ediacaran–Cambrian boundary and how calcimicrobes and related textures appeared in tandem with the innovation of biomineralisation.

Sedimentary evolution and chemostratigraphy of the post-Sturtian cap carbonate-like Dolomie Tigrée Formation (Katanga Supergroup) in the Democratic Republic of the Congo

This study provides new constraints on the paleoenvironmental, eustatic and carbon isotopic models of the cap carbonate-like rocks of the Dolomie Tigrée Formation in the Katanga region, Democratic Republic of the Congo. The study integrates detailed lithofacies analysis combined with sequence stratigraphy to reveal the high-resolution architecture framework of deposition. Eight lithofacies types are defined: shale, siltstone and sandstone (LF1), dolosiltite (LF2), dolarenite (LF3), massive dolomudstone (LF4), laminated dolomudstone (LF5), intraclastic dolopackstone (LF6), laminar dolomudstone (LF7), and fenestral dolopackstone (LF8). These lithofacies types show that the Dolomie Tigrée carbonate succession accumulated on a distally steepened slope to proximal carbonate ramp setting. The sequence stratigraphy analysis revealed 17 elementary parasequence sets (5th order) that are grouped into 3 distinct successive marine transgressive cycles (PSQ1 to PSQ3) of 4th order magnitude. Each cycle records, from the base to the top, a dominant transgressive system tract, vertically-upward evolving to short time highstand and lowstand system tracts bounded by a sequence boundary. PSQ1 records an early marine transgression marked by the negative shift in δ13C values near −5‰ at the base, then returned to positive values in the upper part of PSQ1 and PSQ2. This carbon isotopic excursion is here interpreted as the hydrological and tectonic reorganizations of outer-ramp-derived allopadic materials from deeper to shallow carbonate-oversaturated reservoirs, coupled with a glacial-isostatic rebound, which enhanced the development of biogenic carbonates in a proximal carbonate ramp setting. Our results show that the changes of sea level triggered the carbonate deposition occurring during the marine transgression. This trangression was related to extensional tectonics of the Nguba rift basin, which were possibly enhanced by the isostatic deglacial rebound in the aftermath of the Sturtian glaciation event.

Linking the Bitter Springs carbon isotope anomaly and early Neoproterozoic oxygenation through I/[Ca + Mg] ratios

The ca. 810 Ma Bitter Springs carbon isotope anomaly is an abrupt and long-lived (5–10 Myr.) departure from the positive carbon isotope values (δ13Ccarb) that otherwise characterize early-middle Neoproterozoic carbonate rocks. The onset of this isotope anomaly is manifested by a globally expressed ~8‰ negative shift in δ13C values in shallow marine carbonate strata. Given increasing evidence that metazoan diversification began well before the Ediacaran Period, the Bitter Springs anomaly is a logical interval to explore the potential relationship between biological innovation, perturbations to the carbon cycle, and oxygenation. The iodine-to-calcium+magnesium ratio (I/[Ca + Mg]) in marine carbonates is a sensitive proxy for seawater redox conditions that is increasingly being applied to reconstruct the oxygenation of the global surface ocean through Earth's history. We report I/[Ca + Mg] ratios, along with carbon and oxygen isotope ratios and major and minor element concentrations from carbonate sections spanning the Bitter Springs anomaly in Svalbard, East Greenland, and the Mackenzie Mountains (northwestern Canada). The results from Svalbard and Greenland collectively show two prominent features. The early stage of the Bitter Springs anomaly is characterized by a negative δ13C shift coupled to low I/[Ca + Mg] ratios compared to carbonates pre- and post-dating the Bitter Springs anomaly. The last stage of the Bitter Springs anomaly displays a positive excursion in I/[Ca + Mg], with the highest values at ~7 μmol/mol yet documented in rocks older than 580 Ma. In contrast, carbonates from the Mackenzie Mountains are uniformly low and display no variation in I/[Ca + Mg] across the Bitter Springs anomaly, which we interpret to be the consequence of alteration of primary signatures during diagenesis. The observed geochemical variations in our Svalbard and Greenland datasets are interpreted to be the result of a shift from pre-Bitter Springs ocean conditions, defined by well oxygenated surface waters and anoxic-ferruginous bottom waters, to syn-Bitter Springs ocean conditions, characterized by expanded euxinia.

The Permian-Triassic transition in ocean island setting: Environmental disturbances and new high-resolution carbon-isotope record from the Qiangtang Basin, NW China

The Permian-Triassic boundary (PTB) events such as carbon-isotope excursion, volcanism, and environmental disturbances are not well understood in an ocean island setting. Here, we develop a new case study for the Permian-Triassic transition in the Qiangtang Basin, Tibet, by combining existing biostratigraphy, new carbon-isotope data, sedimentological data, UPb zircon age, mineralogical and geochemical data. These new data defined the Permian-Triassic transition age as about 252.3 ± 0.9 Ma, corresponding to the lowermost sea level. A long-term negative carbonate carbon isotope trend is present at the same stratigraphic level in different depositional environments, strongly suggesting its global nature. However, our new ocean island setting is characterized by an abrupt shift in carbon isotope values across the level, which is different from many chemostrtigraphic studies that show a gradual shift in carbon isotope values across the event horizon. The most likely explanation for the abrupt shift in δ13C values is a sedimentary hiatus at this level in the study section. Three-stage climatic models through the PTB in the ocean island setting are identified: Stage 1 is characterized by a warm and humid climate with moderate chemical weathering; while a hot and humid climate with intense chemical weathering dominates stage 2; the climate in stage 3 is a relatively hot and arid climatic condition with weak chemical weathering.

Coastal primary productivity changes over the last millennium: a case study from the Skagerrak (North Sea)

Abstract. A comprehensive multi-proxy study on two sediment cores from the western and central Skagerrak was performed in order to detect the variability and causes of marine primary productivity changes in the investigated region over the last 1100 years. The cores were dated by Hg pollution records and AMS 14C dating and analysed for palaeoproductivity proxies such as total organic carbon, δ13C, total planktonic foraminifera, benthic foraminifera (total assemblages as well as abundance of Brizalina skagerrakensis and other palaeoproductivity taxa) and palaeothermometers such as Mg∕Ca and δ18O. Our results reveal two periods with changes in productivity in the Skagerrak region: (i) a moderate productivity at ∼ CE 900–1700 and (ii) a high productivity at ∼ CE 1700–present. During ∼ CE 900–1700, moderate productivity was likely driven by the nutrients transported with the warm Atlantic water inflow associated with a tendency for a persistent positive NAO phase during the warm climate of the Medieval Climate Anomaly, which continues into the LIA until ∼ CE 1450. The following lower and more variable temperature period at ∼ CE 1450–1700 was likely caused by a reduced contribution of warm Atlantic water, but stronger deep-water renewal, due to a generally more negative NAO phase and a shift to the more variable and generally cooler climate conditions of the Little Ice Age. The productivity and fluxes of organic matter to the seafloor did not correspond to the temperature and salinity changes recorded in the benthic Melonis barleeanus shells. For the period from ∼ CE 1700 to the present day, our data point to an increased nutrient content in the Skagerrak waters. This increased nutrient content was likely caused by enhanced inflow of warm Atlantic water, increased Baltic outflow, intensified river runoff, and enhanced human impact through agricultural expansion and industrial development. Intensified human impact likely increased nutrient transport to the Skagerrak and caused changes in the oceanic carbon isotope budget, known as the Suess effect, which is clearly visible in our records as a negative shift in δ13C values from ∼ CE 1800. In addition, a high appearance of S. fusiformis during the last 70 years at both studied locations suggests increased decaying organic matter at the sea floor after episodes of enhanced primary production.

Integrated graptolite-conodont biostratigraphy and organic carbon chemostratigraphy of the Llandovery of Kallholn quarry, Dalarna, Sweden

A revised graptolite and conodont biostratigraphy together with new organic carbon isotope data for the middle Aeronian to lower Telychian (Llandovery, lower Silurian) of Kallholn quarry, central Sweden, are presented. The base of the section is marked by an unconformity between the Ordovician Boda Limestone and Aeronian shales with limestone nodules belonging to the Pribylograptus leptotheca Biozone. These are overlain by strata assigned to the lower Lituigraptus convolutus Biozone. The Stimulograptus sedgwickii Biozone, previously recorded from the section, is absent and marked by an unconformity which is overlain by radiolarian-rich graptolitic shales belonging to the Stimulograptus halli Biozone. The overlying Telychian strata are assigned to the Spirograptus guerichi, Spirograptus turriculatus and Streptograptus crispus biozones. Conodonts indicate that the boundary between the Distomodus staurognathoides Biozone and Pterospathodus eopennatus Superzone lies within the Sp. turriculatus Biozone, as it does also in the Ohesaare core, Estonia. The δ13Corg record shows mostly minor fluctuations but with a distinct twin-peaked positive excursion in the upper Sp. turriculatus Biozone, close to the boundary with the overlying Streptograptus crispus Biozone. The excursion in the Kallholn section may correlate with the earliest positive shift in δ13C values at the onset of the excursion associated with the Valgu Event or it may represent a new excursion (provisionally named the Kallholn excursion) preceding this. Many more bentonites (36) occur in the section than previously recorded – geochemical studies are needed to confirm whether the Osmundsberg bentonite (which may or may not be equivalent to the “O” bed in Estonia and Latvia) is present.

Late Pleistocene and Holocene vegetation changes in the arid Andean piedmont of central Argentina inferred from sediment stable carbon isotopes and C/N ratios

Stable carbon isotopes (δ13C) and C/N ratios in bulk sediment organic matter were used as indicators of C3 and C4 vegetation functional types in a ca. 40-kyr, predominantly alluvial sequence from Arroyo La Estacada (33.5°S, 69°W), in the arid Andean piedmont of central Argentina. Although this sediment sequence has the potential to contribute to the knowledge of past vegetation dynamics through its fossil pollen content, previous palynological studies provided scant information, constrained to a few time windows, because of poor pollen preservation. Stable isotope results for the total organic carbon fraction from three lithostratigraphic sections that span the past 40 kyr show shifts in δ13C values associated with changing contributions from C3 and C4 plants, suggesting this proxy can detect changes in these photosynthetic pathways, representative of Patagonian (and/or stream-margin macrophytes) and Monte phytogeographic units, respectively. Climate and local factors, such as water availability in the basin, are inferred to be the main drivers of vegetation dynamics. The C/N ratios in a few samples show that organic matter sources are algae and/or phytoplankton, not solely C3 or C4 vascular plants. The δ13C record from Arroyo La Estacada is an example of how isotope geochemistry has overcome difficulties with pollen analysis and provided a reliable tool to investigate past vegetation changes in regions where C3 and C4 are differentially distributed. Isotope values during part of the Marine Isotope Stage 3 (MIS 3) and the middle Holocene are attributed to C3 local floodplain plants, with a little or no contribution from regional terrestrial (Patagonian) vegetation. Late Pleistocene–Holocene palaeosols have a clear C4 signal of terrestrial dominance, representative of Monte plants, thus, carbon isotope values constrain the early times of MIS 1, particularly where the Pleistocene–Holocene transition occurs. A mixture of C3 and C4 values during the last 3–4 kyr is indicative of environmental conditions that prevail in the region today.

Chemostratigraphy of the Cenomanian-Turonian shallow-water carbonate: new correlation for the rudist levels from north Sinai, Egypt

The present study aims to provide carbon-isotope curves for the Cenomanian to Turonian rudist-dominated successions in north Sinai. The high-resolution carbon-isotope curves obtained from north Sinai sections provide new insight for calibrating the age of rudists as well as for evaluating the effects of the oceanic anoxic event 2 (OAE2) on rudist communities. The primary goals are (1) to provide a high-resolution sequence stratigraphic framework for the Cenomanian-Turonian succession, (2) to use rudist and ammonite biostratigraphic data to distinguish the stratigraphic levels of the rudist species, and (3) to integrate the chemostratigraphic (δ13C) profile and the rudist levels to improve the biostratigraphy based on the rudist distributions and the carbon-isotope data. The recognition of three ammonite zones through the Cenomanian-Turonian succession was utilized to identify four temporally significant rudist levels indicative of the Lower Cenomanian, Middle Cenomanian, Upper Cenomanian, and Middle Turonian, respectively. Most of the rudists occur in the highstand deposits of medium-scale sequences. Carbon- and oxygen-isotopic analyses were carried out on both rudists and surrounding carbonate units. Based on the variations in the carbon-isotope signals, 12 chronostratigraphic segments were identified in the studied sections. The Cenomanian carbon-isotope segments (C23–C30) were obtained from the Halal Formation at Gabal Yelleg and Gabal Maaza sections, while the Turonian segments (C30–C34) were measured from the Wata Formation at Gabal Yelleg section. The carbon-isotope record from the studied sections is consistent with the trends documented in previous studies of the Tethyan realm. The Cenomanian-Turonian boundary is placed at the onset of falling carbon-isotope values (δ13C) from 2.61 to −0.25‰ in the upper part of OAE2 with the carbon-isotope segment C30 at Gabal Yelleg. The negative shift in δ13C values (C33) occurred in the Middle Turonian lowstand deposits characterizing the global sea level fall during this interval.

Distortion of carbon isotope excursion in bulk soil organic matter during the Paleocene-Eocene thermal maximum

The Paleocene-Eocene thermal maximum was a period of abrupt, transient global warming, fueled by a large release of 13C-depleted carbon and marked globally by a negative carbon isotope excursion. While the carbon isotope excursion is often identified in the carbon isotope ratios of bulk soil organic matter (δ13Corg), these records can be biased by factors associated with production, degradation, and sources of sedimentary carbon input. To better understand these factors, we compared δ13Corg values from Paleocene-Eocene thermal maximum rocks in the southeastern Bighorn Basin, Wyoming, with those derived from leaf wax n -alkanes (δ13C n -alk). While both δ13C n -alk and δ13Corg records indicate an abrupt, negative shift in δ13C values, the carbon isotope excursions observed in bulk organic matter are smaller in magnitude and shorter in duration than those in n -alkanes. To explore these discrepancies, we modeled predicted total plant tissue carbon isotope (δ13CTT) curves from the δ13C n -alk record using enrichment factors determined in modern C3 plants. Measured δ13Corg values are enriched in 13C relative to predicted δ13CTT, with greater enrichment during the Paleocene-Eocene thermal maximum than before or after. The greater 13C enrichment could reflect increased degradation of autochthonous organic matter, increased input of allochthonous fossil carbon enriched in 13C, or both. By comparing samples from organic-rich and organic-poor depositional environments, we infer that microbial degradation rates doubled during the Paleocene-Eocene thermal maximum, and we calculate that fossil carbon input increased ∼28%−63%. This approach to untangling the controls on the isotopic composition of bulk soil carbon is an important development that will inform not only future studies of global carbon cycle dynamics during the Paleocene-Eocene thermal maximum hyperthermal event, but also any study that seeks to correlate or estimate duration and magnitude of past events using soil organic carbon.

GEOCHEMICAL CHARACTERIZATION OF THE PERMIAN–TRIASSIC TRANSITION AT OUTCROP, CENTRAL SAUDI ARABIA

This study presents the results of chemostratigraphic analyses and spectral gamma‐ray logging integrated with sedimentological data across the Permian‐Triassic boundary at a measured outcrop section in central Saudi Arabia. The studied section encompasses the uppermost part of the Midhnab Member and the Lower and Upper Khartam Members of the Khuff Formation. Lithofacies were interpreted to have been deposited in subtidal, tidal to supratidal, lacustrine and meandering fluvial / flood plain, marginal marine and lagoonal depositional environments. Integration of bulk geochemical and carbon isotope (δ13C) data allowed the identification of a stratigraphic interval with a negative shift in δ13C ratio values, which was interpreted to correspond to the end‐Permian mass extinction event. The end of this “first negative shift in δ13C values” is taken to mark the Permian‐Triassic boundary. Above this boundary and just below an interval containing scattered thrombolites, a second negative shift in δ13C ratios was observed, and corresponds to an interval with long‐term uranium depletion as indicated by the bulk sediment geochemical and spectral gamma‐ray uranium data.The Permian‐Triassic boundary (PTrB) was placed at the transition between marginal‐marine and subtidal deposits. This stratigraphic position corresponds to the end of the “first negative δ13C shift” and the point of greatest uranium depletion. Although previous studies on outcrops of the Khuff Formation in Saudi Arabia identified a major sequence boundary between the Lower and Upper Khartam Members and interpreted it as the P TrB, no evidence is presented in this study for exposure and dissolution at this surface. Accordingly, the Permian‐Triassic transition is placed in the transgressive portion of the Upper Khartam Member, while the sequence boundary below is interpreted to correspond to the end‐Permian extinction.Correlation of Khuff time‐equivalent units in the Arabian Plate is challenging, and this study will contribute to an improved understanding of this important stratigraphic unit, which contains prolific non‐associated gas reservoirs. The identification of the Permian‐Triassic boundary in central Saudi Arabia will help in the construction of a sequence‐stratigraphic scheme for the Khuff, and with the correlation of lithofacies within this heterogeneous reservoir unit.

Paleoenvironmental changes across the Carnian/Norian boundary in the Black Bear Ridge section, British Columbia, Canada

The Black Bear Ridge section in northeastern British Columbia, Canada, consists of a continuously exposed sequence of Upper Carnian through Lower Norian (Upper Triassic) continental margin strata. The section has been proposed as a candidate Global Stratotype Section and Point (GSSP) for the Carnian/Norian boundary (CNB). In order to assess Late Carnian to Early Norian environmental changes recorded in the section, we examined stratigraphic variations in 87Sr/86Sr, δ13C, and δ18O values, and also values of redox sensitive elements (V, Ni and Cr), in the CNB interval. The study section is located along the north shore of Williston Lake, northeastern British Columbia. The Black Bear Ridge section was deposited in a distal ramp environment on the passive western margin of the North American craton.The strata across the CNB display a positive shift in δ13C values and a corresponding increase in the redox indices V/(V+Ni) and V/Cr. The synchronous increase in δ13C values and redox indices suggests that burial rates of marine organic carbon increased in response to the development of anoxic conditions in the water column. An increase in δ13C values in carbonate rocks across the CNB has also been reported from Upper Triassic sections in the western Tethys (e.g., in the Pizzo Mondello section, Sicily), which suggests that the development of anoxic conditions within the CNB interval was widespread, affecting both the Panthalassan Ocean and Tethyan Sea. The geochemical data from this study, as well as from research into conodont biostratigraphy in the Black Bear Ridge section, show that the onset of oceanic anoxic conditions may have been responsible for the faunal turnover event at the CNB. The cause of this anoxic event is unknown, but the 87Sr/86Sr and δ13C isotope data largely exclude the possibility that the event was triggered by dissociation of methane hydrates and degassing related to large-scale volcanic activity.

Short-term changes in amino sugar-specific δ13C values after application of C4 and C3 sucrose

Amino sugars are useful indicators for the accumulation of microbial residues. A 14-day incubation experiment with C4 and C3 sucrose additions was carried out to investigate the relationships between amino sugar-specific shifts in δ13C values and those of CO2 production, microbial biomass C, K2SO4 extractable C and soil organic C (SOC). High performance anion exchange chromatography (HPAEC-IRMS) was able to measure amino-sugar specific δ13C values for muramic acid (MurN), galactosamine (GalN), and glucosamine (GlcN) in the range of natural abundance. At day 7, the initial application of C4 sucrose significantly increased the δ13C value of MurN by 1.0‰ in comparison with the non-amended control treatment, whereas that of GalN and GlcN remained unchanged. This significant increase had disappeared by day 14. This means that the HPAEC-IRMS method is not useful for short-term incubation experiments in the natural abundance range, as the pool size, especially of GalN and GlcN, was too large for a significant response in δ13C values. The δ13C values significantly decreased in the order MurN (−23.2‰) > GalN (−25.7‰) > GlcN (−26.5‰) in the control treatment. Similar δ13C values were measured in GlcN, microbial biomass C, and SOC. MurN exhibited δ13C values similar to the K2SO4 extractable fraction. These results may be caused by differences in the access of bacteria and fungi to different SOC fractions or differences in metabolic fractionation in bacteria and fungi. C3 sucrose application without further nutrient supply seven days after C4 sucrose application together with N and P led to strong mineralization of freshly formed microbial residues.

A Late Holocene environmental history of a bat guano deposit from Romania: an isotopic, pollen and microcharcoal study

A 1.5-m-long core from a bat guano deposit in Zidită Cave (western Romania) has provided a 900-year record of environmental change. Shifts in δ13C values of bulk guano (between −22.6 and −27.5‰) combined with guano-sourced pollen and microcharcoal information show significant changes in the structure of vegetation and plant biomass. Cave guano δ13C values reflect the dietary preferences of bats which are controlled by local vegetation dynamics, which in turn depend on local climatic conditions. Neither δ13C values nor pollen association in guano changed strikingly over the Medieval Warm Period (MWP) and Little Ice Age (LIA) transition. Instead, an overall decreasing trend of δ13C values between ca. AD 1200 and 1870–1900 defines the duration of LIA. A shift toward cooler and wetter conditions at ca. AD 1500 noticed in the pollen record by an increase in Fagus sylvatica and Alnus and the decrease of Carpinus betulus, may indicate the first major change at the beginning of the LIA. Evidence for two major cold spells occurring around AD 1500 and ca. AD 1870 comes from both δ13C and pollen record. In between these events, the cave region experienced a warmer and drier climate but colder and wetter than the MWP, favouring the expansion of Quercus, Fraxinus and Tilia simultaneously with the decrease of F. sylvatica and Poaceae. Human impact in the studied area is mainly related to agriculture, grazing and deforestation. The effects are most pronounced after AD 1845 when the pollen of cereals increases and Zea is recorded (AD 1845). Higher percentages of microcharcoal particles in the guano sequence are generally correlated with agricultural activities like land cleaning via controlled fires.

Origins and fates of H2S in the Cambrian and Ordovician in Tazhong area: Evidence from sulfur isotopes, fluid inclusions and production data

Gases, oils, solid bitumen samples and fluid inclusions were analyzed for chemical composition and δ13C and δ34S values, and production data were collated, to elucidate the origin and fates of the H2S in the Tazhong area. The H2S in the Ordovician was measured to have δ34S values from 12.1‰ to 23.4‰, which are significantly lighter than the counterpart in the Cambrian (33.0‰), indicating that the H2S did not migrate from the Cambrian, but may have been derived from thermochemical sulfate reduction in situ, as partially supported by 12C-rich CO2 in fluid inclusions and TSR calcites with δ13C values from −3.6‰ to −17.7‰ in the Ordovician. The H2S rich gas subsequently mixed with H2S-poor, 13C rich methane-dominated gas as indicated by the roughly negative correlations of H2S content to dryness and gas/oil ratio, and CH4 from fluid inclusions having δ13C values 1.2–3.6‰ lighter than the associated free gas. The H2S has been dissolved in formation water, precipitated as pyrite and incorporated into oils and solid bitumens, resulting in the generation of alkylthiolanes, alkylthiols and alkyl 2-thiaadamantanes, and increase in δ34S values and S/C ratios of the solid bitumen. The content of H2S produced from the water intervals or petroleum–water transition zones is significantly contributed from exsolved gas and thus higher than that of the gas-cap gas in the reservoirs. Thus, H2S contents cannot be used to indicate TSR extents in the Ordovician. However, in the Cambrian, whole oil and saturates show positive shifts in δ13C values with increasing H2S contents, indicating that the H2S in the Cambrian was derived from TSR by liquid hydrocarbons.

Organic geochemical studies of modern microbial mats from Shark Bay: Part I: Influence of depth and salinity on lipid biomarkers and their isotopic signatures.

The present study investigated the influence of abiotic conditions on microbial mat communities from Shark Bay, a World Heritage area well known for a diverse range of extant mats presenting structural similarities with ancient stromatolites. The distributions and stable carbon isotopic values of lipid biomarkers [aliphatic hydrocarbons and polar lipid fatty acids (PLFAs)] and bulk carbon and nitrogen isotope values of biomass were analysed in four different types of mats along a tidal flat gradient to characterize the microbial communities and systematically investigate the relationship of the above parameters with water depth. Cyanobacteria were dominant in all mats, as demonstrated by the presence of diagnostic hydrocarbons (e.g. n-C17 and n-C17:1). Several subtle but important differences in lipid composition across the littoral gradient were, however, evident. For instance, the shallower mats contained a higher diatom contribution, concordant with previous mat studies from other locations (e.g. Antarctica). Conversely, the organic matter (OM) of the deeper mats showed evidence for a higher seagrass contribution [high C/N, 13C-depleted long-chain n-alkanes]. The morphological structure of the mats may have influenced CO2 diffusion leading to more 13C-enriched lipids in the shallow mats. Alternatively, changes in CO2 fixation pathways, such as increase in the acetyl COA-pathway by sulphate-reducing bacteria, could have also caused the observed shifts in δ13C values of the mats. In addition, three smooth mats from different Shark Bay sites were analysed to investigate potential functional relationship of the microbial communities with differing salinity levels. The C25:1 HBI was identified in the high salinity mat only and a lower abundance of PLFAs associated with diatoms was observed in the less saline mats, suggesting a higher abundance of diatoms at the most saline site. Furthermore, it appeared that the most and least saline mats were dominated by autotrophic biomass using different CO2 fixation pathways.

Investigation of Late Pleistocene and Early Holocene palaeoenvironmental change at El Mirón cave (Cantabria, Spain): Insights from carbon and nitrogen isotope analyses of red deer

El Mirón Cave was occupied by humans for over 40,000years. Evidence of Late Mousterian, Gravettian, Solutrean, Magdalenian, Azilian, Mesolithic, Neolithic, Chalcolithic, Bronze Age and Mediaeval occupations has been found in the cave. Understanding the local environmental conditions during the occupations is crucial for gaining an insight into the lifeways of El Mirón's inhabitants as they relied on the surrounding region and its natural resources for their subsistence. 170 bones of hunted red deer recovered from the cave were sampled for carbon and nitrogen stable isotope analyses with the aim of reconstructing the palaeoenvironment and palaeoclimate during the human occupation. The results show that the surrounding landscape underwent considerable environmental change during the Late Pleistocene and Early to Mid-Holocene. Shifts in δ13C values between the Last Glacial Maximum, Heinrich stadial 1, Heinrich event 1, the Late-glacial interstadial and the onset of the Holocene are likely to reflect changes in water availability and temperature. Deer δ15N generally increased over time indicating the regeneration of soil biological activity and nitrogen cycling, which was temporarily halted during the Younger Dryas. Comparison of the El Mirón results with those of 300 deer from other regions of Europe shows geographical variations in the timing and magnitude of the variation in δ13C and δ15N values. This variation tracks local climate (temperature and water availability) and environmental (vegetation and forest development) changes.