Carbon Isotope Variations Research Articles

Investigating early husbandry strategies in the southern Caucasus: intra-tooth sequential carbon and oxygen isotope analysis of Neolithic goats, sheep, and cattle from Göytepe and Hacı Elamxanlı Tepe

The beginning of agriculture marks major changes in food acquisition technology in human history. In the southern Caucasus, agricultural villages emerged around 6000 cal. BC, 2000 to 3000 years later than in the Fertile Crescent despite their geographical closeness to each other. This study aims to provide isotopic evidence indicative of animal herding strategies in the early farming communities in the southern Caucasus. To analyze intra-individual sequential carbon and oxygen isotope variations, we used tooth enamel of five goats, four sheep, and three cattle from the Neolithic settlements, Göytepe and Hacı Elamxanlı Tepe (Azerbaijan). The analyses also included reference samples of modern domestic goat and sheep that are known to have grazed in the vicinity of the sites. As a result, the sequential carbon and oxygen isotope variations of the modern goat and sheep samples showed large amplitude and a synchronous pattern during the whole enamel maturation period. The same pattern was observed in some of the Neolithic goat and sheep samples, suggesting they may have grazed in the vicinity of the lowland site throughout the year. Other Neolithic goat and cattle samples exhibited relatively small amplitudes and inverse cyclical variation patterns. While these patterns may have been caused by multiple factors, such as drinking water and food/fodder, vertical transhumance has also been proposed for the similar isotope patterns. Despite the difficulty in specifying exact causes for the observed isotopic patterns, their diversity within/among the species implies multiple pastoral strategies that may have had adaptive advantages in new agricultural frontiers in the southern Caucasus.

Isotopic Evidence for Long-Distance Connections of the AD Thirteenth-Century Promontory Caves Occupants

The Promontory caves (Utah) and Franktown Cave (Colorado) contain high-fidelity records of short-term occupations by groups with material culture connections to the Subarctic/Northern Plains. This research uses Promontory and Franktown bison dung, hair, hide, and bone collagen to establish local baseline carbon isotopic variability and identify leather from a distant source. The ankle wrap of one Promontory Cave 1 moccasin had a δ13C value that indicates a substantial C4component to the animal's diet, unlike the C3diets inferred from 171 other Promontory and northern Utah bison samples. We draw on a unique combination of multitissue isotopic analysis, carbon isoscapes, ancient DNA (species and sex identification), tissue turnover rates, archaeological contexts, and bison ecology to show that the high δ13C value was not likely a result of local plant consumption, bison mobility, or trade. Instead, the bison hide was likely acquired via long-distance travel to/from an area of abundant C4grasses far to the south or east. Expansive landscape knowledge gained through long-distance associations would have allowed Promontory caves inhabitants to make well-informed decisions about directions and routes of movement for a territorial shift, which seems to have occurred in the late thirteenth century.

Carbon and sulfur isotope variations through the Upper Ordovician and Lower Silurian of South China linked to volcanism

Pronounced excursions of carbon and sulfur isotopes (δ13C and δ34S) spanning the Ordovician–Silurian (O–S) transition have been recognized globally, yet the causes of these isotope events and their causal relationships to environmental changes, especially during Early Silurian time, remain the topics of debate. In this study, we present organic carbon and pyrite sulfur isotope (δ13Corg and δ34Spyr) records and elemental concentrations from a drill core spanning from the Upper Ordovician (Katian) to Lower Silurian (Aeronian) succession of the Yangtze Platform, China. The newly presented δ13Corg profile records previously recognized positive excursions, including the Hirnantian isotopic carbon excursion (HICE), the middle Rhuddanian excursion, and the early Aeronian positive carbon isotope excursion (EACIE). In addition, two negative δ13Corg excursions are recognized during the Katian to early Hirnantian and late Hirnantian to middle Rhuddanian stages. The δ34Spyr profile exhibits a great amount of variation (−17.8‰ to +19.6‰) but mimics the δ13Corg record. Coupled positive δ13Corg and δ34Spyr excursions during the middle Hirnantian and late Rhuddanian to Aeronian are associated with decreased total organic carbon (TOC) content and less reducing water column conditions, whereas the negative δ13Corg and δ34Spyr excursions during the Katian to early Hirnantian and late Hirnantian to middle Rhuddanian are associated with enhanced TOC values and the establishment of more strongly anoxic to euxinic conditions. The δ13C and δ34S excursions during the O–S transition, then, cannot be explained by changes in redox conditions or the burial of organic carbon and pyrite. The heavier and less variable middle Hirnantian and late Rhuddanian to Aeronian δ34Spyr values probably reflect low marine sulfate concentration. In contrast, negative Katian to early Hirnantian and late Hirnantian to middle Rhuddanian δ34Spyr excursions may reflect increased sulfate concentration. The two negative δ13Corg and δ34Spyr excursions associated with the Katian to early Hirnantian and late Hirnantian to middle Rhuddanian correspond with episodes of volcanism. We therefore propose that volcanism was the main driver of coupled negative δ13Corg and δ34Spyr excursions. As volcanism diminished during the middle Hirnantian and late Rhuddanian to Aeronian, both δ13Corg and δ34Spyr returned to more positive values. Our results indicate that volcanism played a crucial role in triggering carbon and sulfur isotope perturbations and environmental changes during the Late Ordovician and Early Silurian.

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The European eel, a catadromous endangered species, displays plasticity in phenotypic traits and in trophic behaviour, features that can eventually affect growth. The aim of this study was to compare variation in carbon (δ13C) and nitrogen (δ15N) stable isotopes in muscle tissue of the adult eel life stages (i.e. yellow and silver eel), as function of growth. Eel samples were obtained from nine sites across the Mediterranean region, that included seven coastal lagoons of different size and ecological structure, and two freshwater habitats corresponding to one river and one lake. A large variability was found in both δ13C and δ15N values, suggesting that eels fed over a broad range of potential food sources. Eels from each location showed specific range values of δ13C, in most sites irrespective of stage, reflecting lagoon- or freshwater-specific carbon sources. A positive correlation was found between individual δ15N values and annual growth rates, suggesting that eels generally seem to feed on prey of higher δ15N values among the trophic resources available in each foraging habitat. This allows individuals to grow faster and to attain earlier the pre-spawning phase (i.e. the silver stage). A better understanding of eel trophic ecology across different life stages and different habitats is one of the prerequisites for the effective management of local stocks.

Gas composition and carbon isotopic variation during shale gas desorption: Implication from the Ordovician Wufeng Formation - Silurian Longmaxi Formation in west Hubei, China

This paper focuses on the gas composition and carbon isotopic variation during gas desorption from the shale in Ordovician Wufeng Formation-Silurian Longmaxi Formation. The results show that the desorbed shale gases are predominately comprised of methane (content: 98.36–98.76%) and thus are typical dry gas. As the desorption proceeded, the content of methane gradually decreased, and the drying coefficient (C1/C1-5) of the desorbed shale gases also gradually decreased. The drying coefficient decreased from 98.58 to 99.04% (98.81% on average) of the initial desorbed samples to 88.72–96.78% (94.15% on average) of the final desorbed samples. Based on the variation patterns of gas component ratios during the shale gas desorption, the order of the shale's absorption capacity for different gas components is determined as CO2 > H2 > C3H8 > C2H6 > CH4 > He. The drying coefficient of the shale gas is negatively correlated with the total shale gas content due to the different adsorption capability of shale for gaseous hydrocarbons. The δ13C values tended to become heavier during desorption in general. The δ13C1 values of the initial desorbed samples were the lowest, ranging from −30.71‰ to −25.63‰. In detail, the δ13C values of methane increased by 5.41–25.93‰, while those of ethane increased within a relatively smaller range. The carbon isotopic fractionation is mainly ascribed to diffusion and adsorption/desorption. Moreover, the carbon isotopic fractionation degree of methane was correlated with the shale gas content during the field desorption, suggesting that the carbon isotopic fractionation degree of methane is a potential tool for the identification and selection of favorable sweet spots of shale gas. In addition, the shale gas from Wufeng-Longmaxi Formation in west Hubei consists of highly mature oil-cracking gas and kerogen-cracking gas. All of the desorbed shale gas is characterized by isotopic reversal and the extent of the isotopic reversal increased as desorption continued. This is primarily caused by the mixing of the gas from thermal degradation of kerogen with the gas from residual oil cracking and wet gas as well as the diffusion of the shale gas. The carbon isotope reversal of methane and ethane is not directly related to shale gas content.

Differentiation of Carbon Isotope Composition and Stratabound Mechanism of Gas Desorption in Shallow-Buried Low-Rank Multiple Coal Seams: Case Study of Well DE-A, Northeast Inner Mongolia

The desorbed gas composition and δ13C(CH4) and δD(CH4) of five coal groups in Well DE-A of the Labudalin Basin in northeastern Inner Mongolia show that the δ13C(CH4) and δD(CH4) are heavy, and significantly heavier in some layers, whereas the overall δ13C(CO2) is abnormally light. Based on coalfield exploration data, this study analyzes the characteristics and control factors of methane isotope alienation of shallow-buried low-rank multiple coal seams in a small coal-bearing depression. The maximum buried depth of the coal seam in the Derbu coalfield is less than 600 m, and the coal rank is sub-bituminous coal. The carbon isotope composition (δ13C(CH4)) of CBM varies from − 56.3 to − 39.8‰, wherein the arithmetic average is − 48.72‰. Compared with coal seams of the same metamorphic degree, the δ13C(CH4) in Derbu is relatively heavy, wherein different coal groups and different coal seams of the same coal group have different characteristics, and the δ13C(CO2) is relatively light. Studies have shown that the existence of thermogenic gas, anaerobic oxidation of methane microorganisms, and desorption-fractionation of methane isotopes in Well DE-A are causes for the heavier δ13C(CH4) and δD(CH4). Note that the thermal cause is the basis for the heavier δ13C(CH4) of the gas sample, while the carbon isotope fractionation of methane and oxidation of microorganisms further aggravate the δ13C(CH4). The preservation conditions, such as lithofacies combination sequence and hydrogeological conditions, of different coal formations are also important factors influencing δ13C(CH4) differentiation, and the superposition and coupling of various reasons make this differentiation more obvious. By comparing the geological characteristics of Well DE-A with the vertical variation characteristics of carbon isotopes in the desorbed methane of the Surat Basin, it was concluded that the vertical variation of the desorbed gas sample in shallow-buried low-rank multiple coal seams has complex characteristics. In addition to maturity, local horizons are more obviously controlled by lithologic association and hydrological characteristics. Especially for small coal-bearing depressions or coal-bearing strata near outcrops with shallow-buried depth, the horizontal distance between the outcrop line of different coal formations and target well locations is obviously short. Thus, surface water and surface microorganisms can migrate along the short high permeability layers (coal seam or sandstone layer), and influence the coal-bearing seam. Meanwhile, different groundwater dynamic conditions also result in the transformation of desorbed gas in different coal groups, revealing variation in the methane carbon isotopes.

Rapid flood intensification and environmental response of the Lower Meuse during the Allerød-Younger Dryas climate transition

Lateglacial climatic oscillations exerted profound impacts on the Meuse fluvial system. In the Lower Meuse (southern Netherlands), geomorphological studies in the last decades mainly centred on Lateglacial vegetation evolution, channel pattern changes and river terrace formation. Little information has been reported about the paleohydrology and its relation with the regional climate conditions. This study investigates a sediment core that contains flood sediments deposited from the early Allerød up to the middle Holocene. We conducted grain size analysis, thermogravimetric analysis (organic matter and calcium carbonate content), pollen and macrofossil analysis, and determined the oxygen and carbon stable isotope ratios of biogenic carbonate (opercula of the freshwater gastropod Bithynia tentaculata). The chronology of the core is based on AMS 14C dating and pollen biostratigraphical correlation. The pollen and macrofossil studies reveal that the core site was a lake environment during the Allerød and Younger Dryas periods. The oxygen isotope record in conjunction with organic matter and carbonate content are believed to have captured the intra-Allerød Cold Period (IACP). The synchronous variation of oxygen and carbon isotopes with calcium carbonate content indicates a dominant evaporation effect on carbonate chemistry of the lake environments during the warm Allerød interstadial. End-member modelling decomposes the grain-size distributions into two sandy end members (bed load) and two silty-clayey end members (suspended load). In order to highlight the flood signal, we constructed two flood energy indexes (FEI-1 and FEI-2) that reflect the coarseness of the suspended load and bed load, respectively. Both indexes show a relatively high flood condition during the IACP, followed by a low flood phase in the Late Allerød and quickly intensified flood conditions at the onset of the Younger Dryas. In the second phase of the Younger Dryas, deposition of sandy aeolian sediments to the core site complicates the paleoflood identification using FEI-2 (bed load). This study shows a high sensitivity of the hydrological process and sedimentary environment at the Lower Meuse to the regional climate system.

Assessing geographic controls of hair isotopic variability in human populations: A case-study in Canada.

Studying the isotope variability in fast-growing human tissues (e.g., hair, nails) is a powerful tool to investigate human nutrition. However, interpreting the controls of this isotopic variability at the population scale is often challenging as multiple factors can superimpose on the isotopic signals of a current population. Here, we analyse carbon, nitrogen, and sulphur isotopes in hair from 590 Canadian resident volunteers along with demographics, dietary and geographic information about each participant. We use a series of machine-learning regressions to demonstrate that the isotopic values in Canadian residents' hair are not only influenced by dietary choices but by geographic controls. First, we show that isotopic values in Canadian residents' hair have a limited range of variability consistent with the homogenization of Canadian dietary habits (as in other industrialized countries). As expected, some of the isotopic variability within the population correlates with recorded individual dietary choices. More interestingly, some regional spatial patterns emerge from carbon and sulphur isotope variations. The high carbon isotope composition of the hair of eastern Canadians relative to that of western Canadians correlates with the dominance of corn in the eastern Canadian food-industry. The gradient of sulphur isotope composition in Canadian hair from coast to inland regions correlates with the increasing soil pH and decreasing deposition of marine-derived sulphate aerosols in local food systems. We conclude that part of the isotopic variability found in the hair of Canadian residents reflects the isotopic signature associated with specific environmental conditions and agricultural practices of regional food systems transmitted to humans through the high consumption rate of intra-provincial food in Canada. Our study also underscores the strong potential of sulphur isotopes as tracers of human and food provenance.

Variations in stable carbon isotopes in different components of aquatic macrophytes from Taihu Lake, China

Understanding the fractionation mechanism and factors controlling carbon isotopes (δ13C) in macrophytes is significant for studying the biogeochemical carbon cycle and changes in nutrients within lakes. However, whether the environmental implications of δ13C are different in the different components (α-cellulose, holocellulose and whole sample) of macrophytes is unclear. In this paper, the δ13C composition of different macrophytes tissue components in Taihu Lake was analysed on spatial and temporal scales. The effects of aquatic environmental variables on the δ13C values of macrophytes were also explored. The results showed that emergent plants have the most negative stable carbon isotope signals, with a mean value (±SD) of −27.85‰ ± 0.95‰. Submerged macrophytes have large variations with significantly more positive δ13C values (−15.19‰ ± 3.08‰). The δ13C values of floating-leaved plants (−25.32‰ ± 1.02‰) fall between those of the two groups above. In the one-way analysis of variance (ANOVA), the different components (α-cellulose, holocellulose and whole sample) of δ13C exhibited significant differences (p-value <0.01) among the three groups of macrophytes. However, there was no significant seasonal difference in the δ13C of macrophytes. In addition, the δ13C of macrophytes in rivers were lighter than those in lakes. Correlation analyses indicated that the water pH was the main factor affecting δ13C variability in macrophytes. The δ13C values of α-cellulose showed stronger sensitivity to environmental variation than did those of the whole samples and holocellulose. Hence, it is necessary to extract α-cellulose when using the δ13C of aquatic macrophytes to study the changes in lake ecological environments. These results provided a theoretical reference and experimental data support for a modern process of using δ13C in macrophytes for palaeoenvironmental changes.

Carbon and hydrogen isotopic variations in gold tube gas pyrolysates from an Athabasca oil sand

Closed system gold tube pyrolysis experiments were conducted on an oil sand bitumen from the Athabasca region in western Canada to investigate gas isotope variation with heating temperatures. Experiments were carried out at 350–650 °C and a pressure of 50 MPa, with heating rates of 2 °C/h and 20 °C/h. The progressive cracking of bitumen and liquid oil lead to continuous release of methane with increasing pyrolysis temperatures, while the yields of wet gas components increase initially at the lower temperature range and then decrease drastically at high temperatures. A general trend of 13C- and 2H-enrichment in the gas products is seen with increasing pyrolysis temperatures due to thermal cracking of various hydrocarbon and non-hydrocarbon components. Isotopic variations can be quite extensive, commonly occurring for both hydrocarbon gases and CO2 as pyrolysis progresses. A carbon isotopic rollover (isotope of gas component shifting from 13C enrichment to 12C-enrichment with increasing temperature) occurs for bulk hydrocarbon gas components at low temperature (< 450 °C), while rollover only occurs for ethane and propane at high temperature (> 550 °C), where partial reversal (lower carbon numbered gas component becomes more 13C-enriched relative to higher carbon numbered ones) also exists. The isotopic values of CO2 show a wide range of variation with the highest δ13C value seen at the lowest temperature while the lowest δ13C value occurs at 500 °C and 550 °C for 2 °C/h and 20 °C/h, respectively. Relative 13C-enrichment occurs only in the high temperature range. The unusual carbon isotope ratios of CO2 in pyrolysates is likely related to biodegradation influence on the precursors, but further investigation is still called for. Hydrogen isotopic values of hydrocarbon gases show even more dramatic variation than that seen for carbon. While methane is progressively 2H-enriched to 650 °C, ethane and propane are relatively 2H-enriched when the pyrolysis temperature is < 500 °C, while the trend is inverted at higher temperatures. Data presented here clearly deviate from the trajectories expected by simple equilibrium or Rayleigh-type kinetic cracking models. Dynamic generation, destruction and mixing processes govern the final isotopic signature in an associated gas product. Isotopic rollover and reversal seem to be a common phenomenon during thermal evolution where heterogeneous precursors are available. While the reaction system of pyrolysis at high temperature differs completely from source rock evolution at low temperature and some artifacts are inevitably involved, the isotope behavior observed here may provide some insights into the complexity in natural cases and help our understanding of the mechanisms of isotopic evolution during thermal maturation.

High-resolution correlation between contrasting early Tonian carbonate successions in NW Canada highlights pronounced global carbon isotope variations

Carbonate successions older than the 800 Ma Bitter Springs isotope anomaly are sparse and this time is consequently poorly understood. The Shaler and Mackenzie Mountains supergroups of northwestern Canada, however, contain thick ca. 1050–750 Ma carbonate successions recording both deep- and shallow-water depositional environments that are interpreted to have been deposited in two separate, extensive epicratonic basins on the Rodinia supercontinent. When the contrasting sedimentology (benthic- versus pelagic-dominated carbonate production) and basin dynamics of two correlative carbonate successions from these basins are accounted for, a precise regional correlation can be made using sequence stratigraphic and carbon isotope data. This paper presents a regionally correlatable carbon isotope profile, containing substantial excursions between −2 and 7‰vpdb, that can be used to extend the global curve into early Neoproterozoic time prior to the Bitter Springs anomaly.

Mobility and origin of camels in the Roman Empire through serial stable carbon and oxygen isotope variations in tooth enamel

Although camels are not indigenous to Europe, they have been found at several sites from several Roman provinces dating from the beginning of the 1st century AD onwards. It must have been beneficial to bring them there. Based on finds of remains from juvenile individuals (e.g. from Tanais), it has been suggested that the Romans might have systematically bred camels within Europe. For this study, we took serial samples of the enamel of four camels from European sites (Innsbruck-Wilten, Mamer-Bertrange, Tongeren, and Trier) dating to the 2nd - 4th century AD. We measured the relative abundances of carbon and oxygen isotopes of the carbonate fraction from the tooth enamel. The continuous record of oxygen and carbon isotopic composition of the intra-tooth enamel serial samples reflects the climate and habitat in which an individual lived during the time of tooth mineralization. We used these data to make a rough evaluation of the areas of origin consistent with the relative abundances of the isotopes from the enamel of the camels and attempt to reconstruct their life history and mobility behavior based on the different ecological characteristics of the habitats represented in the isotopic data. Furthermore, the data can function as an additional proxy for species determination, due to the different habitats of Camelus bactrianus and Camelus dromedarius. This work also yields interesting insights on the similarities in the mobility pattern of the camels from Mamer-Bertrange and Trier. In combination with archaeological evidence, it was possible to tentatively connect them with specific military units, i.e. the detachments of the Legio VIII Augusta.

Seawater carbon and strontium isotope variations through the late Ediacaran to late Cambrian in the Tarim Basin

The radiation of early animals during the Cambrian explosion was accompanied by widespread perturbations in oceanic and atmospheric conditions. However, the cause-effect relationships between evolutionary innovations, carbon (C) cycling, and continental weathering are still a matter of debate. Further paleoenvironmental reconstructions of poorly studied paleo-continents covering the Precambrian/Cambrian (PC/C) transition may improve the correlation of the events and their impact on the Earth system on a global scale. The Tarim Basin was presumably located proximal to the well-studied South China craton during the Ediacaran-Cambrian boundary and is expected to contribute equally to our paleoenvironmental reconstructions. Here we apply carbon and strontium (Sr) isotope records (δ13C and 87Sr/86Sr) obtained from well-preserved marine carbonates spanning the late Ediacaran to the late Cambrian from the Keping region in the northwestern Tarim Basin, NW China. By comparison to other carbonate successions from various paleo-continents, our obtained seawater 87Sr/86Sr curve in this study shows a stepwise first-order increase to more radiogenic values superimposed by second-order presumably regional variations. Our obtained δ13C variations exhibit four prominent negative and three positive excursions identical to the global δ13C record. Based on existing paleontological evidence and our new Tarim δ13C and 87Sr/86Sr curves, we propose a stratigraphic correlation from the late Ediacaran to the late Cambrian between the Tarim Basin and other paleo-continents. Besides, our paleoenvironmental reconstruction reveals both regional and global sea-level changes that may have controlled the influx of radiogenic Sr and presumably the influx of nutrients controlling primary production and the rate of organic carbon burial in the Tarim Basin at the time of deposition. These feedbacks may have ultimately influenced the rapid diversification of metazoans and the oxygenation of the Cambrian ocean and atmosphere approaching-present pO2 levels.

Strontium, oxygen, and carbon isotope variation in modern human dental enamel.

ObjectivesIsotopic analyses using human dental enamel provide information on the mobility and diet of individuals in forensic and archeological studies. Thus far, no study has systematically examined intraindividual coupled strontium (Sr), oxygen (O), and carbon (C) isotope variation in human enamel or the effect that caries have on the isotopic integrity of the enamel. The inadequate quantification of isotopic variation affects interpretations and may constrain sample selection of elements affected by caries. This study aims to quantify the intraindividual isotopic variation and provides recommendations for enamel sampling methods.Material and MethodsThis study presents the first systematic results on intraindividual variation in Sr–O–C isotope composition and Sr concentration in modern human dental enamel of third molars (affected and unaffected by caries). A multiloci sampling approach (n = 6–20) was used to analyze surface and inner enamel, employing thermal ionization mass spectrometry (TIMS) and isotope ratio mass spectrometry (IRMS). Third molars were analyzed from 47 individuals from the Netherlands, Iceland, the United States, the Caribbean, Colombia, Somalia, and South Africa.ResultsIntradental isotopic variation in modern Dutch dental elements was recorded for Sr, O, and C and exceeded the variation introduced by the analytical error. Single loci and bulk sampling approaches of third molars established that a single analysis is only representative of the bulk Sr isotope composition in 60% of the elements analyzed. Dental elements affected by caries showed twice the variation seen in unaffected dental elements. Caries did not consistently incorporate the isotopic composition of the geographical environment in which they developed.DiscussionThe isotopic variability recorded in unaffected inner enamel indicates that variations greater than 0.000200 for 87Sr/86Sr and larger than 2‰ for δ18O and δ13C are required to demonstrate changes in modern Dutch human diet or geographic location.

Suspension feeders as natural sentinels of the spatial variability in food sources in an Antarctic fjord: A stable isotope approach

• δ 13 C and δ 15 N signatures of benthic suspension feeders differed across Potter Cove. • Food sources presented obvious small-scale spatial heterogeneity across Potter Cove. • Mixing models showed different contributions of food sources across Potter Cove. • Glacial melt-related environmental conditions govern the spatial heterogeneity of food sources. We examined the effect of glacial melt-related retreat environmental conditions on the spatial heterogeneity of food sources for Antarctic nearshore marine benthic communities of Potter Cove, a representative fjord on King George Island on the West Antarctic Peninsula. Focusing on Antarctic suspension feeders, we surveyed the variation of carbon (δ 13 C) and nitrogen (δ 15 N) stable isotopes in three different stations with contrasting characteristics in terms of glacier-sedimentary regimes and distance from the glacier. Bayesian mixing models were used to estimate the relative contribution of eight potential food sources at the study sites and to the consumers' diet. The isotopically signatures of suspension feeders displayed a broad range of assimilate available food sources with distinct contributions at each site. Overall, for most of the suspension feeders analysed, contributions of suspended particulate matter and macroalgae detritus tended to prevail over the rest of the potential food sources. Additionally, we confirm the assimilation of krill faeces by benthic organisms reinforcing its relevance as food source in nearshore Antarctic ecosystems. Mixing models corroborated the influence of glacier discharge on the spatial distribution of available food sources, since their contribution in the suspension feeders diet was different and more heterogeneous in the internal stations (ice-proximal zone) than in the external one (ice-distal zone). The analysis of isotopic signatures of primary benthic consumers (suspension feeders) indicated the spatial heterogeneity of food sources and its composition in coastal Antarctic nearshore, as consequence of the glacial influence on the local environment and hydrology.

Early pastoral mobility and seasonality in Kenya assessed through stable isotope analysis

Eastern African pastoralists today depend on mobility to access seasonally available pastures for their livestock. Here, we evaluate the importance of mobility strategies for maintaining herds during the Pastoral Neolithic era in southern Kenya through stable isotope analysis. We use carbon and oxygen isotope ratios of sequentially sampled modern and archaeological cattle, sheep, and goat tooth enamel to reconstruct seasonal vertical mobility strategies of early pastoralists in the region from 3000 to 1200 years ago. An altitudinal transition from C4 grasses at low elevations to C3 grasses at higher elevations permits tracking vertical mobility in the Kenya Rift Valley. Results from eight Pastoral Neolithic sites from southern Kenya show that cattle, sheep, and goats were almost exclusively herded at low elevations throughout the year. Higher variability in carbon isotopes of caprines compared to cattle suggests that caprines had greater seasonal dietary flexibility. Oxygen isotopes were not consistently useful for identifying seasonal patterns: intra- and interannual variability in tropical precipitation and plant water isotopic composition are confounding factors. Archaeological contexts representing local foragers that adopted pastoralism show herding strategies similar to those from established pastoral sites. These results suggest that lower elevation savannas were productive enough to support pastoralism year-round during the Pastoral Neolithic era.

Modeling the evolution of pulse-like perturbations in atmospheric carbon and carbon isotopes: the role of weathering–sedimentation imbalances

Abstract. Measurements of carbon isotope variations in climate archives and isotope-enabled climate modeling advance the understanding of the carbon cycle. Perturbations in atmospheric CO2 and in its isotopic ratios (δ13C, Δ14C) are removed by different processes acting on different timescales. We investigate these differences on timescales of up to 100 000 years in pulse-release experiments with the Bern3D-LPX Earth system model of intermediate complexity and by analytical solutions from a box model. On timescales from years to many centuries, the atmospheric perturbations in CO2 and δ13CO2 are reduced by air–sea gas exchange, physical transport from the surface to the deep ocean, and by the land biosphere. Isotopic perturbations are initially removed much faster from the atmosphere than perturbations in CO2 as explained by aquatic carbonate chemistry. On multimillennial timescales, the CO2 perturbation is removed by carbonate compensation and silicate rock weathering. In contrast, the δ13C perturbation is removed by the relentless flux of organic and calcium carbonate particles buried in sediments. The associated removal rate is significantly modified by spatial δ13C gradients within the ocean, influencing the isotopic perturbation of the burial flux. Space-time variations in ocean δ13C perturbations are captured by principal components and empirical orthogonal functions. Analytical impulse response functions for atmospheric CO2 and δ13CO2 are provided. Results suggest that changes in terrestrial carbon storage were not the sole cause for the abrupt, centennial-scale CO2 and δ13CO2 variations recorded in ice during Heinrich stadials HS1 and HS4, though model and data uncertainties prevent a firm conclusion. The δ13C offset between the Penultimate Glacial Maximum and Last Glacial Maximum reconstructed for the ocean and atmosphere is most likely caused by imbalances between weathering, volcanism, and burial fluxes. Our study highlights the importance of isotopic fluxes connected to weathering–sedimentation imbalances, which so far have been often neglected on glacial–interglacial timescales.

Spatial variation in carbonate carbon isotopes during the Cambrian SPICE event across the eastern North China Platform

The Cambrian SPICE (Steptoean Positive Carbon Isotope Excursion) event is widely reported from the late Miaolingian to the early Furongian, but recent studies imply that the synchroneity and magnitude of the SPICE may be controversial. In order to test the spatial variation of the SPICE, here we present 367 carbonate δ13C data from six outcrop sections across a ~700 km transect along the eastern North China Platform. The SPICE is present in all the studied sections, but showing different features regarding the duration and magnitude. An abrupt increase in δ13C from ~1.5‰ to 3.5‰ occurs in three Shandong sections, with maximum value present only in a thin (0.5–1.2 m thick) transgressive lag deposit (crudely wave stratified, bioclastic grainstone). In contrast, the δ13C record of the Baijiashan and Shashan sections in the Liaoning region displays a gradual increase (~20 m thick) from ~1.5‰ to the maximum value of 4.7‰. Both facies analysis and trilobite collections suggest that the spatial variation of the SPICE in the North China Platform most likely resulted from missing of sedimentary record (with high δ13C values) in the Shandong sections as a result of erosion and non-deposition during sea-level lowstand, a possible coeval hiatus prior to the Sauk III transgression in the Laurentian basins. Further high-resolution, integrated studies of sedimentology, biostratigraphy, and sedimentary geochemistry on the SPICE event are required to better understand its nature.

Sequence stratigraphy of the ca. 1730 Ma Wollogorang Formation, McArthur Basin, Australia

Abstract The ca. 1730 Ma Wollogorang Formation is a mixed carbonate-siliciclastic unit in the southern McArthur Basin of northern Australia. It is an exploration target for base metals and may be a petroleum source unit and unconventional reservoir. Facies analysis indicates that the Wollogorang Formation records deposition in supratidal to offshore environments. Lateral consistency in facies, thickness, and stratigraphic architecture suggests deposition on an east-west trending (present coordinates) epeiric platform. According to recent tectonic models for northern Australia, this epeiric platform likely deepened to the present-day east. The Wollogorang Formation comprises one second-order and five nested third-order sequences. Despite the unavailability of biostratigraphy and biofacies, and the general poor time control in Precambrian basins, we are able correlate third-order (i.e., our lowest rank) sequences over tens to hundreds of kilometers using high-resolution facies and petrophysical logs. As the distinction between stratigraphic sequences (related to shoreline shifts) and sedimentological bedsets (not related to shoreline shifts) becomes more difficult with decreasing hierarchical level, sedimentological, stratigraphic, paleontological, and mineralogical (including diagenetic) criteria were recently proposed to distinguish these cycles. Our study suggests that sedimentological and stratigraphic criteria are the most useful in Precambrian sequence stratigraphy. In contrast, paleontological criteria are irrelevant in Precambrian successions. Mineralogical (including diagenetic) criteria have not been tested in this study. In addition to further study these criteria, we suggest testing geochemical criteria, such as the enrichment of redox-sensitive trace elements and minor carbonate carbon isotope variation, to distinguish stratigraphic sequences from sedimentological bedsets.

Carbon and nitrogen isotopic variability in foxtail millet (Setaria italica) with watering regime.

RationaleCarbonised plant remains are analysed for reconstruction of past climates and agricultural regimes. Several recent studies have used C4 plants to address related questions, and correlations between modern C4 plant δ13C values and rainfall have been found. The millets were important food crops in prehistoric Eurasia, yet little is known about causes of isotopic variation within millet species. Previous research has shown there to be significant isotopic variation between millet accessions. Here we compare isotope ratios from plants grown under different watering regimes. This allows for a consideration of whether or not Setaria italica is a good proxy for environmental reconstruction.MethodsWe compare stable isotope ratios of Setaria italica plants grown in a controlled environment chamber with different watering regimes. We compare the carbon isotope ratios of leaves and grains, and the nitrogen isotope ratios of grains, from 12 accessions of Setaria italica.ResultsWe find significant isotopic variability between watering regimes. Carbon isotope ratios are positively correlated with water availability, and on average vary by 1.9‰ and 1.7‰ for leaves and grains, respectively. Grain nitrogen isotope ratios also vary with watering regime; however, the highest isotope ratios are found with the 130‐mL watering regime.ConclusionsThe carbon isotope ratios of Setaria italica are strongly correlated with water availability. However, the correlation is the opposite to that seen in studies of C3 plants. The difference in isotopic ratio due to watering regime is comparable with that seen between different accessions; thus distinguishing between changing varieties of Setaria italica and changing climate is problematic. In terms of grain nitrogen isotope ratios, the highest δ15N values were not associated with the lowest watering regime. Again, δ15N variation is comparable with that which would be expected from an aridity effect or a manuring effect, and thus distinguishing between these factors is probably problematic.