Carbon Isotope Variations Research Articles

High benthic methane flux in low sulfate oceans: Evidence from carbon isotopes in Late Cretaceous Antarctic bivalves

The shell material of marine benthic bivalves provides a sensitive archive of water chemistry immediately above the sediment–water interface, which in turn is affected by sedimentary geochemistry and redox reactions. Sulfate has a major controlling effect on sedimentary carbon cycling, particularly the processes of methane production and oxidation, with lower concentrations of sulfate likely resulting in an increase in sedimentary methane production. Whilst it is accepted that ocean sulfate varied markedly across the Phanerozoic, evidence of changes in methane production in sediments has so far been lacking. There is potential for the oxidation products of sedimentary methane to be preserved and detected in marine fossils. Here we present the results of high resolution carbonate isotope records from two taxa of well-preserved shallow-infaunal bivalve (Lahillia and Cucullaea) collected from the marine shelf succession across the Cretaceous–Paleogene (K–Pg) boundary in Seymour Island, Antarctica. The succession has pre-existing subtle indications of more abundant methane, and the time period is characterized by much lower marine sulfate concentrations than modern.These shell carbonate–carbon isotope records vary widely: at one extreme, shells have typical average values and small ranges compatible with a contemporaneous marine dissolved inorganic carbon (DIC) source and modern-style sedimentary carbon cycling. At the other, the shells have large-amplitude annual cycles of carbon isotopic variability of up to 23.8‰ within a single year of growth and shell carbonate δ13C compositions as negative as −34‰. Shells with these increased ranges and unusually negative values are found at discrete intervals and across both bivalve taxa. The contribution of methane required to explain the most negative carbonate–carbon isotopic values in the bivalve shells is extremely high (between 30 to 85% of bottom-water DIC based on mass balance calculations). Records of organic-carbon isotopes from the same succession remained between −26.1 and −21.7‰ throughout, suggesting that methane influence was restricted to bottom-waters. A lack of authigenic carbonate in the section indicates that methane oxidation progressed aerobically and may have provided a significant driver for transient bottom water de-oxygenation.Where methane addition is indicated, the seasonal sensitivity precludes control by methane hydrates. We argue that these data represent the increased importance and sensitivity of methanogenesis in the sediments, enabled by lower ocean sulfate concentrations during the Late Cretaceous. The tendency towards a more dynamic role for marine methane production and oxidation is likely to apply to other times of low marine sulfate in Earth's history.

Authigenic carbonate formation rates in marine sediments and implications for the marine δ13C record

Carbon isotope (δ13C) variations measured in carbonates have been attributed to large-scale phenomena throughout Earth history, such as changes in atmospheric oxygen or global glaciations. These interpretations follow from a model wherein the δ13C of marine dissolved inorganic carbon (DIC) is controlled by the relative sedimentary burial rates of biogenic carbonate (BC) and organic carbon (OC). A new model proposes authigenic carbonate (AC) as a third major sedimentary C pool, implying that δ13C anomalies are not necessarily indicative of extreme changes in the global carbon cycle and/or atmospheric oxygen. Two conditions are required for AC formation to significantly alter bulk carbonate δ13C: the AC isotopic composition must be at least ∼3‰ different from that of BC and the AC/BC ratio must be >0.1. We use pore fluid Ca and Sr concentrations to estimate rates of AC formation in Late Cenozoic marine sediments, then calculate relative fractions of AC, OC, and BC. Today AC is not expected to constitute a significant fraction of total sedimentary carbon (AC+OC+BC) globally; however, there are modern sites where local conditions promote elevated AC/BC and anaerobic metabolisms can alter the δ13C of pore fluids. We investigate these sites to determine what conditions might enable AC to alter δ13C of marine DIC. We find there is very little net addition of AC relative to BC, but large quantities of AC form today across many settings via recrystallization. In settings where remineralization of organic matter causes recrystallized carbonate to form with modified δ13C, AC/BC is generally too low for this recrystallization to significantly shift the δ13C of the bulk carbonate. However, exceptions are found in sites with very low BC and extensive methane oxidation, suggesting that this environment type would need to be globally extensive in the past in order for AC formation to change the δ13C of marine DIC.

Climate and anthropogenic controls on the carbon cycle of Xingyun Lake, China

The organic and inorganic carbon cycles in lakes are responsive to both natural climate variability and human-induced environmental changes. Here we present an 8600-year sediment record from Xingyun Lake in Yunnan, China that provides insight into carbon cycling and lake primary productivity using stable isotope measurements of organic carbon and nitrogen, carbon to nitrogen ratio, inorganic carbon isotopes, and mass accumulation rates of carbon, nitrogen, and calcite. The early to middle Holocene is characterized by relative stability in most proxies although some variation in organic carbon isotopes reflects fluctuating terrestrial organic matter inputs. The middle to late Holocene, from 5300 to 3300 years BP, is marked by amplified variability in nearly all proxies and suggests either increased C4 vegetation or increased primary productivity. These changes are coincident with declining lake levels and increased aridity throughout the Indian Summer Monsoon region. After 3300 years BP, the greatest variability is in nitrogen isotopes and coincides closely with human settlement in the region. These changes are accompanied by further indications of increased primary productivity, suggesting eutrophic lake conditions. While we cannot definitively link human activity to these changes, the balance of evidence suggests that other mechanisms are unlikely. This study supports previous work that anthropogenic activities began measurably impacting the landscape in southwestern China within the last few millennia.

Isotopic evidence for ceremonial provisioning of Late Bronze age khirigsuurs with horses from diverse geographic locales

Khirigsuurs are communal ritual and mortuary monuments that featured prominently on Late Bronze Age pastoralist landscapes of the Mongolian steppe through the mid-late second millennium to early first millennium cal BC. Khirigsuurs sustained ceremonies that legitimized the relationship between the deceased and the participants, facilitated the formation of new alliances, and emphasized integration and cohesion between mobile pastoralist communities through monument building, ritual horse slaughter, and feasting. Horses played a prominent role in ceremonial activities conducted at khirigsuurs, their heads and hooves regularly deposited in small stone satellite mounds as part of publically visible ceremonies associated with mortuary celebrations that simultaneously integrated mobile pastoralist communities. Here, strontium and oxygen isotopic analyses of sequentially sampled teeth of horses from khirigsuurs located in the Khanuy Valley, a major center of monumental activity situated north of the Khangai mountains in Mongolia, indicate horses from distant locales were ceremonially placed in khirigsuur satellite mounds, while patterned seasonal variation in carbon isotopes suggests horses were fodder provisioned during the winter months. These isotopic data suggest horses were well cared for, reflecting their status as a prestige animal, and were key to facilitating regionally integrative ceremonial activities conducted at khirigsuurs that brought together people from geographically distant mobile communities.

Late Maastrichtian carbon isotope stratigraphy and cyclostratigraphy of the Newfoundland Margin (Site U1403, IODP Leg 342)

Earth’s climate during the Maastrichtian (latest Cretaceous) was punctuated by brief warming and cooling episodes, accompanied by perturbations of the global carbon cycle. Superimposed on a long-term cooling trend, the middle Maastrichtian is characterized by deep-sea warming and relatively high values of stable carbon-isotope ratios, followed by strong climatic variability towards the end of the Cretaceous. A lack of knowledge on the timing of climatic change inhibits our understanding of underlying causal mechanisms. We present an integrated stratigraphy from Integrated Ocean Drilling Program (IODP) Site U1403, providing an expanded deep ocean record from the North Atlantic (Expedition 342, Newfoundland Margin). Distinct sedimentary cyclicity suggests that orbital forcing played a major role in depositional processes, which is confirmed by statistical analyses of high resolution elemental data obtained by X-ray fluorescence (XRF) core scanning. Astronomical calibration reveals that the investigated interval encompasses seven 405-kyr cycles (Ma4051 to Ma4057) and spans the 2.8 Myr directly preceding the Cretaceous/Paleocene (K/Pg) boundary. A high-resolution carbon-isotope record from bulk carbonates allows us to identify global trends in the late Maastrichtian carbon cycle. Low-amplitude variations (up to 0.4‰) in carbon isotopes at Site U1403 match similar scale variability in records from Tethyan and Pacific open-ocean sites. Comparison between Site U1403 and the hemipelagic restricted basin of the Zumaia section (northern Spain), with its own well-established independent cyclostratigraphic framework, is more complex. Whereas the pre-K/Pg oscillations and the negative values of the Mid-Maastrichtian Event (MME) can be readily discerned in both the Zumaia and U1403 records, patterns diverge during a ~ 1 Myr period in the late Maastrichtian (67.8–66.8 Ma), with Site U1403 more reliably reflecting global carbon cycling. Our new carbon isotope record and cyclostratigraphy offer promise for Site U1403 to serve as a future reference section for high-resolution studies of late Maastrichtian paleoclimatic change.

Volcanic dust veils from sixth century tree-ring isotopes linked to reduced irradiance, primary production and human health

The large volcanic eruptions of AD 536 and 540 led to climate cooling and contributed to hardships of Late Antiquity societies throughout Eurasia, and triggered a major environmental event in the historical Roman Empire. Our set of stable carbon isotope records from subfossil tree rings demonstrates a strong negative excursion in AD 536 and 541–544. Modern data from these sites show that carbon isotope variations are driven by solar radiation. A model based on sixth century isotopes reconstruct an irradiance anomaly for AD 536 and 541–544 of nearly three standard deviations below the mean value based on modern data. This anomaly can be explained by a volcanic dust veil reducing solar radiation and thus primary production threatening food security over a multitude of years. We offer a hypothesis that persistently low irradiance contributed to remarkably simultaneous outbreaks of famine and Justinianic plague in the eastern Roman Empire with adverse effects on crop production and photosynthesis of the vitamin D in human skin and thus, collectively, human health. Our results provide a hitherto unstudied proxy for exploring the mechanisms of ‘volcanic summers’ to demonstrate the post-eruption deficiencies in sunlight and to explain the human consequences during such calamity years.

Variations in carbon and nitrogen stable isotopes and in heavy metal contents of mariculture kelp Undaria pinnatifida in Gijang, southeastern Korea

Variations in carbon and nitrogen stable isotopes and in heavy metal contents of mariculture kelp Undaria pinnatifida in Gijang, southeastern Korea

Organic carbon content and carbon isotope variations across the Permo-Triassic boundary in the Gartnerkofel-1 borehole, Carnic Alps, Austria

The Gartnerkofel borehole is one of the most thoroughly studied and described Permo-Triassic sections in the world. Detailed bulk organic carbon isotope studies show a negative base shift from − 24‰ to − 28‰ in the Latest Permian which latter value persists into the Earliest Triassic after which it decreases slightly to − 26‰. Two strongly negative peaks of > − 38‰ in the Latest Permian and a lesser peak of − 31‰ in the Early Triassic are too negative to be due to a greater proportion of more negative organic matter and must be due to very negative methane effects. The overall change to more negative values across the Bulla/Tesero boundary fits the relative rise in sea level for this transition based on the facies changes. A positive shift in organic carbon isotope values at the Late Permian Event Horizon may be due to an increase in land-derived organic detritus at this level—a feature shown by all Tethyan Permo-Triassic boundary sections though these other sections do not have the same values. Carbonate carbon isotope trends are similar in all sections dropping by 2–3 units across the Permo-Triassic boundary. Gartnerkofel carbonate oxygen values are surprisingly, considering the ubiquitous dolomitization, compatible with values elsewhere and indicate reasonable tropical temperatures of 60 °C in the Latest Permian sabkhas to 20–40 °C in the overlying marine transition beds. Increased land-derived input at the Late Permian Event Horizon may be due to offshore transport by tsunamis whose deposits have been recognized in India at this level.

Latest Permian carbonate carbon isotope variability traces heterogeneous organic carbon accumulation and authigenic carbonate formation

Abstract. Bulk-carbonate carbon isotope ratios are a widely applied proxy for investigating the ancient biogeochemical carbon cycle. Temporal carbon isotope trends serve as a prime stratigraphic tool, with the inherent assumption that bulk micritic carbonate rock is a faithful geochemical recorder of the isotopic composition of seawater dissolved inorganic carbon. However, bulk-carbonate rock is also prone to incorporate diagenetic signals. The aim of the present study is to disentangle primary trends from diagenetic signals in carbon isotope records which traverse the Permian–Triassic boundary in the marine carbonate-bearing sequences of Iran and South China. By pooling newly produced and published carbon isotope data, we confirm that a global first-order trend towards depleted values exists. However, a large amount of scatter is superimposed on this geochemical record. In addition, we observe a temporal trend in the amplitude of this residual δ13C variability, which is reproducible for the two studied regions. We suggest that (sub-)sea-floor microbial communities and their control on calcite nucleation and ambient porewater dissolved inorganic carbon δ13C pose a viable mechanism to induce bulk-rock δ13C variability. Numerical model calculations highlight that early diagenetic carbonate rock stabilization and linked carbon isotope alteration can be controlled by organic matter supply and subsequent microbial remineralization. A major biotic decline among Late Permian bottom-dwelling organisms facilitated a spatial increase in heterogeneous organic carbon accumulation. Combined with low marine sulfate, this resulted in varying degrees of carbon isotope overprinting. A simulated time series suggests that a 50 % increase in the spatial scatter of organic carbon relative to the average, in addition to an imposed increase in the likelihood of sampling cements formed by microbial calcite nucleation to 1 out of 10 samples, is sufficient to induce the observed signal of carbon isotope variability. These findings put constraints on the application of Permian–Triassic carbon isotope chemostratigraphy based on whole-rock samples, which appears less refined than classical biozonation dating schemes. On the other hand, this signal of increased carbon isotope variability concurrent with the largest mass extinction of the Phanerozoic may provide information about local carbon cycling mediated by spatially heterogeneous (sub-)sea-floor microbial communities under suppressed bioturbation.

Middle Permian environmental changes and shale oil potential evidenced by high-resolution organic petrology, geochemistry and mineral composition of the sediments in the Santanghu Basin, Northwest China

Shales in the Middle Permian Lucaogou Formation have been considered as one of the most important unconventional reservoirs in NW China, due to their excellent shale oil and tight oil potential. Based on the data of previous organic geochemical studies, the overall evolution of the ecosystem in the lake, water conditions and organic matter sources have been outlined. Here we present the results of the mineralogical and organic-petrological examinations and the Rock-Eval pyrolysis, on samples of a fine-grained, mixed carbonate-clastic sedimentary succession from the Santanghu Basin, providing new detailed insights into paleoenvironmental evolution and lake level changes. In addition, shale oil potential was further evaluated on this oil-prone shale.Analytical results from a total of 196 shale samples from the second member of the Lucaogou Formation, collected from one newly drilled core, indicate algal/microbial sources of organic matter, classified by Rock-Eval pyrolysis as type I kerogen. Samples characterized by lower hydrogen index (HI) values (type II kerogen) contain increased contributions of terrigenous organic matter, evidenced by enhanced inertinite and vitrinite contents. Vertical variations in mineralogical and bulk geochemical composition reflect an evolution from a saline, anoxic to a deep, freshwater lake. Two environmental changes subdivided the sequence into three units. The boundary between the lower Unit I and the middle Unit II is characterized by the drastic decrease of smectite and K-feldspar contents and a sharp increase in total organic carbon (TOC) content. The upper Unit III is distinguished by high TOC/S ratios. Based on variations in petrography and carbon isotopes of hopanes, the middle Unit II can be further subdivided into two parts reflecting changes in water level of the lake. All the periods of high-water level are characterized by higher lamalginite contents (>40vol%) and higher HI values than those obtained from sediments deposited during periods of shallower water levels. Deposition in a shallow lake is evidenced by increased dolomite and decreased lamaginite contents. Lake level fluctuations, indicated by mineralogical and geochemical parameters, are associated with changes in organic matter sources, leading to the great variability in mineral content, organic petrography and hydrocarbon generation potential. High TOC and brittle minerals contents, micro-nanopores system and large thickness of the shale show that the Lucaogou Formation holds a significant shale oil potential in areas where oil window maturity is reached.

Paired carbonate-organic carbon and nitrogen isotope variations in Lower Mississippian strata of the southern Great Basin, western United States

The Early Mississippian K-O (Kinderhookian-Osagean) carbon isotope (δ13C) excursion or TICE (mid-Tournaisian carbon isotope excursion) is one of the most prominent positive δ13C excursions of the Phanerozoic. Recent studies raise uncertainties about the representative shape (single vs. double spikes) and magnitude of this δ13C excursion (3‰ to ≥6‰ in South China; ≥5.5‰ in Europe; and ≥7‰ in North America) and the 3‰ unidirectional increase in nitrogen isotopes across the δ13C excursion, which is unanticipated considering the amount of organic carbon burial required to form the δ13C excursion and the resultant oxygen increase and global cooling. To test if stratigraphic completeness and spatial isotope variations caused such uncertainties, we have conducted paired carbonate carbon (δ13Ccarb), organic carbon (δ13Corg) and nitrogen (δ15N) isotope analyses across the K-O interval in two well-exposed sections of the southern Great Basin, western United States. The two sections represent proximal shallow-water and distal deep-water depositional settings of a west-dipping carbonate ramp. In the distal ramp section where no exposure surface is present, both δ13Ccarb and δ13Corg show double spikes with peak δ13Ccarb values up to 7‰ and a negative shift down to 4‰ between the peaks. In the proximal shallower-water section where two karstic disconformities are observed, δ13Corg shows similar double spikes but δ13Ccarb displays only a single peak with the highest value of 5.5‰. The missing δ13Ccarb spike is likely caused by diagenetic alteration below a karstic disconformity that lowered δ13Ccarb but not δ13Corg values, resulting in smaller magnitude of the δ13Ccarb excursion. These features suggest that the 7‰ magnitude and double spikes are more representative of the K-O δ13C excursion in the southern Great Basin. The smaller magnitude of the K-O δ13Ccarb excursion in some sections of the Great Basin and the TICE in other sections globally may have overprinted with local environmental/diagenetic signal or resulted from stratigraphic hiatus/truncation, which needs to be clarified in future research.The δ15N across the K-O δ13C excursion in the distal ramp section is decoupled from δ13C, with the majority of δ15N values around 4±1‰ that do not show any obvious temporal trend. In contrast, δ15N values in the shallow-water section is coupled with the K-O δ13C excursion, with a 3‰ positive shift from 4‰ to 7‰ at the rising limb of the δ13C excursion and a negative shift from 7‰ to 1–2‰ at the falling limb of the δ13C excursion. The δ15N trend from the distal ramp section is, in some extent, comparable with that documented from a section in South China, while the coupled δ13C–δ15N pattern in the proximal section seems better explain the potential redox change across a prominent δ13C excursion. Considering the sensitivity of δ15N to redox conditions of depositional environments, a more comprehensive δ15N study in a broader paleogeographic context is required to better understand the interactions between carbon and nitrogen cycles across the K-O interval—a critical transition from the mid-Paleozoic greenhouse clime to Late Paleozoic Ice Age (LPIA).

Grain-scale stable carbon and oxygen isotopic variations of the international reference calcite, IAEA-603.

The new international reference material IAEA-603 (calcite) for stable carbon and oxygen isotopes (δ13 C and δ18 O values) was released in 2016 to replace the previous reference material, NBS19 (exhausted). We examined the grain-scale isotopic variations in IAEA-603 for application to microscale isotopic analysis of carbonate samples. Individual grains of IAEA-603 were analyzed with an IsoPrime100 isotope ratio mass spectrometer with a customized continuous-flow gas preparation system (MICAL3c). The individual grains of IAEA-603 were observed by optical and scanning electron microscopy, and their observational characteristics (grain color and size) were compared with their stable isotope compositions. Translucent grains (main component of IAEA-603; grain weight, 4-132μg) had homogeneous isotopic ratios, comparable with the grain-scale isotopic homogeneity of NBS 19. Their average δ13 C and δ18 O values were the same as the recommended values determined by the IAEA. Opaque (whitish) grains (1-2 per 100 grains; grain weight, 8-63μg) were significantly more depleted in 13 C and 18 O than the translucent grains. Low-abundance opaque grains (1-2 grains out of 100 grains) have lower δ13 C and δ18 O values, suggesting that these grains should be eliminated when using IAEA-603 for single-grain (microscale) isotope analysis.

Position-specific 13C distributions within propane from experiments and natural gas samples

Site-specific carbon isotope measurements of organic compounds potentially recover information that is lost in a conventional, ‘bulk’ isotopic analysis. Such measurements are useful because isotopically fractionating processes may have distinct effects at different molecular sites, and thermodynamically equilibrated populations of molecules tend to concentrate heavy isotopes in one molecular site versus another. Most recent studies of site-specific 13C in organics use specialized Nuclear Magnetic Resonance (NMR) techniques or complex chemical degradations prior to mass spectrometric measurements. Herein we present the first application of a new mass spectrometric technique that reconstructs the site-specific carbon isotope composition of propane based on measurements of the 13C/12C ratios of two or more fragment ions that sample different proportions of the terminal and central carbon sites. We apply this method to propane from laboratory experiments and natural gas samples to explore the relationships between site-specific carbon isotope composition, full-molecular δ13C, thermal maturity, and variation in organic matter precursors. Our goal is to advance the understanding of the sources and histories of short-chain alkanes within geologic systems. Our findings suggest that propane varies in its site-specific carbon isotope structure, which is correlated with increasing thermal maturity, first increasing in terminal position δ13C and then increasing in both center and terminal position δ13C. This pattern is observed in both experimental and natural samples, and is plausibly explained by a combination of site-specific, temperature-dependent isotope effects associated with conversion of different precursor molecules (kerogen, bitumen, and/or oil) to propane, differences in site-specific isotopic contents of those precursors, and possibly distillation of reactive components of those precursors with increasing maturity. We hypothesize that the largest changes in site-specific isotopic content of propane occur when bitumen and/or oil replace kerogen as the dominant precursors. If correct, this phenomenon could have significant utility for understanding gas generation in thermogenic petroleum systems.

Carbon and oxygen isotope variation and its implication for marine sequence: A case study of Ordovician in Tarim Basin

Carbon and oxygen isotopes of marine carbonate rocks or fossil shells could indicate global sea-level relative change. Generally, heavier carbon isotopes or lighter oxygen isotopes reflected rise of global sea-level relative changes, and vice versa. For carbonate rocks deposited before Tertiary, carbon isotope was more stable than oxygen isotope, and thus it was more suitable to indicate global sea-level relative fluctuation. Within time limit of the 3rd-order sequence (1-10Ma), large scale migration of carbonate platforms would not occur and carbonate productivity would not change greatly, therefore, a research method of marine tectonic sequence (i.e, a contrastive analysis method of carbon and oxygen isotopes-sedimentary cycles) was proposed. Thus, the marine 3rd-order sequences were divided the global sea-level sequence which was controlled by global sea-level change and the tectonic sequence which was controlled by regional tectonic subsidence and uplift. The global sea-level fluctuation indicated by carbon and oxygen isotopes were consistent with variation of sedimentary cycles and water depth of the global sea-level sequence, but were not consistent with variation of sedimentary cycles and water depth of the tectonic sequence. As a case study of Ordovician in Tarim Basin, two 3rd-order global sea-level sequences were identified in the Lower Ordovician, while six 3rd-order tectonic sequences controlled by collision and compression between Arkin island arc, Kudi active continental margin uplift and Tarim Plate were identified in the Middle and Upper Ordovician. Particularly, OSQ6 in the Upper Ordovician (Lianglitage Formation) was a typical tectonic sequence.

Tracking sources of selected diamonds from Southern Africa based on carbon isotopic and chemical impurities

AbstractThe morphological, chemical impurities and carbon isotope properties of diamonds may reveal subtle details of their mantle source and growth characteristics, supporting efforts towards identifying their original place of harvesting. Here we investigate the mantle carbon and nitrogen sources and growth patterns from selected diamonds mined from four kimberlites: macro-sized diamonds from River Ranch kimberlite in Zimbabwe and the Swartruggens and Klipspringer kimberlitic deposits from South Africa, and micro-sized diamonds from the Klipspringer and Premier kimberlite intrusions in South Africa. Type IaAB diamonds are found in all the samples; Type IaB diamonds only occur in samples from the Swartruggens, River Ranch and Premier kimberlites. A single Type II diamond (nitrogen below the detection limit) was also observed in the River Ranch and Premier kimberlites. Both the micro- and macro-sized diamonds from Klipspringer have similar nitrogen contents. Based on the % B-defect, the diamonds from Klipspringer are grouped into low- and high-nitrogen aggregates (i.e. % of B-defect <40% and >56%, respectively) that likely represent two different diamond forming episodes. Time averaged mantle storage temperatures for Type IaAB diamonds are calculated to have been: 1060°C for Swartruggens; 1190°C for River Ranch; 1100°C (low aggregated); and 1170°C (highly aggregated) for Klipspringer, and 1210°C for Premier diamonds. The CL-images of the River Ranch, Klipspringer and Premier diamonds reveal multi-oscillatory growth zones. The carbon isotopic analyses on the diamonds reveal an average δ13CVPDB value of: -4.5‰ for Swartruggens; -4.7‰ for River Ranch; -4.5‰ for Klipspringer; and -3‰ for Premier. With the exception of the diamond from Premier, the average δ13C value of the diamonds are similar to the average δ13C value of the mantle (-5‰), which is similar to the occurrence of diamonds in the other kimberlites. The internal carbon isotopic variation of individual diamonds from Swartruggens, Klipspringer and Premier are less than 4‰, which is similar to the variability of most other diamond occurrences reported from elsewhere in the world. Up to 6.7‰ internal carbon isotopic variation was observed in a single diamond from River Ranch. The internal carbon isotopic studies of the diamonds reveal that the primary carbon in the Swartruggens and Klipspringer was derived from an oxidation of CH4-bearing fluid, whereas in the River Ranch the primary carbon was derived from the reduction of carbonate-or CO2-bearing fluids. The Swartruggens diamonds also reveal a secondary carbon sourced from a reduction of CO2- or carbonate-rich fluid or melt. Diamonds from Klipspringer exhibit a cyclic change in δ13C values that reflects fluctuation in a complex mantle perturbation system or periodic change in fugacity of the mantle. Based on this study, we conclude that, in principle, a selected range of diamond signatures might be used to fingerprint their origins; especially when linked to their other physical properties such as a low temperature magnetic signature.

Intra-crystal co-variations of carbon isotopes and nitrogen contents in diamond from three North American cratons

Eighteen diamond samples from the A154 South kimberlite pipe (Diavik Mine), Slave Craton, Northwest Territories (Canada); sixteen diamond samples from the Lynx kimberlite dyke, Superior Craton, Quebec (Canada) and twelve diamond samples from the Kelsey Lake kimberlite pipe, Wyoming Craton, Colorado (USA), were cut through the core-zones, polished, imaged by cathodoluminescence (CL), and analyzed by secondary ion mass spectrometry (SIMS) for carbon isotope composition and nitrogen abundance. Twenty Kelsey Lake diamond plates, including the twelve crystals analyzed by SIMS, were analyzed by Fourier transform infrared spectrometry (FTIR) for nitrogen concentration and aggregation state.Diamond samples from Diavik and Kelsey Lake have average δ13CPDB and nitrogen contents (atomic ppm) similar to those found by earlier studies: averaging between −3.9‰ and 486ppm, and −7‰ and 308ppm, respectively. Samples from the Lynx dyke, investigated for the first time, are substantially different, having δ13C=−1.2‰ and nitrogen content=32ppm (averages). All three localities have examples of significant variations in nitrogen content (>100ppm) within single stones. Carbon isotope variation within individual stones is relatively minor (<2‰). In terms of nitrogen aggregation, samples from the Kelsey Lake kimberlite are dominated by zones of Type IaA, but mixed-type and Type IaB (less common) stones also occur. For the majority of samples, overall intra-diamond zonations of nitrogen abundances and carbon isotope ratios are not in agreement with modeled trends for single-event Rayleigh fractionation of diamond from fluid under nitrogen-compatible conditions at 1100°C. The involvement of fluids from subducted crustal reservoirs with exceptionally light, and in the case of Lynx samples, exceptionally heavy δ13CPDB values is necessary to explain the observed growth histories of all the samples studied here.

Climate response to the 8.2 ka event in coastal California

A fast-growing stalagmite from the central California coast provides a high-resolution record of climatic changes synchronous with global perturbations resulting from the catastrophic drainage of proglacial Lake Agassiz at ca. 8.2 ka. High frequency, large amplitude variations in carbon isotopes during the 8.2 ka event, coupled with pulsed increases in phosphorus concentrations, indicate more frequent or intense winter storms on the California coast. Decreased magnesium-calcium ratios point toward a sustained increase in effective moisture during the event, however the magnitude of change in Mg/Ca suggests this event was not as pronounced on the western North American coast as anomalies seen in the high northern latitudes and monsoon-influenced areas. Nevertheless, shifts in the White Moon Cave record that are synchronous within age uncertainties with cooling of Greenland, and changes in global monsoon systems, suggest rapid changes in atmospheric circulation occurred in response to freshwater input and associated cooling in the North Atlantic region. Our record is consistent with intensification of the Pacific winter storm track in response to North Atlantic freshwater forcing, a mechanism suggested by simulations of the last deglaciation, and indicates this intensification led to increases in precipitation and infiltration along the California coast during the Holocene.

Unradiogenic strontium and moderate-amplitude carbon isotope variations in early Tonian seawater after the assembly of Rodinia and before the Bitter Springs Excursion

Neoproterozoic sedimentary records reveal a poorly documented, ∼150m.y. time interval, between ∼1000 and 850Ma, that limit our understanding of C and Sr isotope variations in seawater and thus operation of the biogeochemical carbon cycle, changes in surface redox state, and fluctuations in continental vs. hydrothermal fluxes to the oceans following assembly of the supercontinent Rodinia. Carbon and Sr isotope data for carbonates from the Karatau Group of the Southern Ural Mountains in Russia provides a record partially covering the younger portion of this time interval. The lower part of the Karatau Group (LKG) contains well-preserved carbonate strata of the Katav, Inzer, and Min’yar formations that are focus of this study. Pb-Pb isochron ages for carbonates from the Inzer and Min’yar formations are 844±24 and 820±77Ma, respectively, establishing an early Tonian age. Carbon isotope data for unaltered carbonates in the LKG show a range of moderately positive to negative values from −2.8 to +5.9‰, with the majority below +3.0‰. 87Sr/86Sr values range from 0.70522 to 0.70534 in the Lower Inzer Member, increasing in the overlying Upper Inzer Member and Min’yar Formation, from 0.70555 to 0.70600. The Sr isotope range of the Lower Inzer Member is similar to that typical for the 1.03–0.95Ga seawater (0.70519–0.70554), suggesting that the Sr isotope composition of seawater in the aftermath of Rodinian assembly was unradiogenic for 150m.y. This pattern indicates that supercontinent-scale orogenic events do not result in an enhanced, long-term flux of radiogenic Sr from continents once the supercontinent assembled. To account for the prolonged period characterized by unradiogenic Sr seawater composition and moderate-amplitude C isotope variations, we suggest a supercontinent configuration ringed by continental arcs and with predominantly internal runoff. In this model, accompanying supercontinent blanketing resulted in thermal perturbation in the mantle, emplacement of multiple Large Igneous Provinces, and high continental freeboard. Weathering of juvenile arcs would have provided unradiogenic Sr and a high flux of sediments to the oceans, enhancing organic carbon burial and progressive oxygenation of surface environments and contributing to instability in the biogeochemical carbon cycle. Mass anomalies induced by protracted mantle plume activity led to True Polar Wander and brought the supercontinent, covered with juvenile mafic volcanics and segmented by failed rift systems, to low latitudes thus enhancing chemical weathering, atmospheric CO2 consumption, and eventually ushering in Snowball Earth conditions. Comparison with the records for early Paleoproterozoic strata indicates that a similar sequence of events bracketed both ends of the Proterozoic, highlighting fundamental relationships among plate tectonics, mantle dynamics, biogeochemical carbon cycling, surface oxygenation, and climate change.

Facies, rock attributes, stratigraphy, and depositional environments: Yanchang Formation, Central Ordos Basin, China

The Late Triassic Yanchang Formation (Fm) is a major target of drilling for hydrocarbons in the Ordos Basin. Although most of the early focus on this thick succession of lacustrine rocks has been the dominant deltaic sandstones and siltstones, which act as local reservoirs of oil and gas, more recent consideration has been given to the organic-rich mudstone source rocks. We used modern chemostratigraphic analysis to define vertical facies successions in two closely spaced cores through the Chang 7 Member, the primary source rock for the Yanchang hydrocarbon system. We used integrated high-resolution X-ray fluorescence and X-ray diffraction measurements to define four dominant facies. Variations in stable carbon isotopes mimic facies stacking patterns, suggesting that terrigenous organic matter (although minor in volume) is associated with the arkoses and sandstones, whereas aquatic organic matter is dominant in the mudstones. Facies stacking patterns define three major depositional cycles and parts of two others, each defined by basal mudstone facies that document basin flooding and deepening (i.e., flooding surfaces). Unconfined compressive strength measurements correlate with clay mineral abundance and organic matter. Comparisons of core attributes with wireline logs indicate that although general variations in clay mineral volumes (i.e., mudstone abundance) can be discerned from gamma-ray logs, organic-matter distribution is best defined with density or resistivity logs. These findings, especially those established between the core and log data, provide a powerful linkage between larger scale facies patterns and smaller scale studies of key reservoir attributes, such as pore systems, mineralogy, diagenesis, rock mechanics, hydrocarbon saturation, porosity and permeability, and flow parameters. This first application of modern chemostratigraphic techniques to the Yanchang Fm reveals the great promise of applying these methods to better understand the complex facies patterns that define this lacustrine basin and the variations in key reservoir properties that each facies displays.

Using ocean models to predict spatial and temporal variation in marine carbon isotopes

AbstractNatural‐abundance stable isotope ratios provide a wealth of ecological information relating to food web structure, trophic level, and location. The correct interpretation of stable isotope data requires an understanding of spatial and temporal variation in the isotopic compositions at the base of the food web. In marine pelagic environments, accurate interpretation of stable isotope data is hampered by a lack of reliable, spatio‐temporally distributed measurements of baseline isotopic compositions. In this study, we present a relatively simple, process‐based carbon isotope model that predicts the spatio‐temporal distributions of the carbon isotope composition of phytoplankton (here expressed as δ13CPLK) across the global ocean at one degree and monthly resolution. The model is driven by output from a coupled physics‐biogeochemistry model, NEMO‐MEDUSA, and operates offline; it could also be coupled to alternative underlying ocean model systems. Model validation is challenged by the same lack of spatio‐temporally explicit data that motivates model development, but predictions from our model successfully reproduce major spatial patterns in carbon isotope values observed in zooplankton, and are consistent with simulations from alternative models. Model predictions represent an initial hypothesis of spatial and temporal variation in carbon isotopic baselines in ocean areas where a few data are currently available, and provide the best currently available tool to estimate spatial and temporal variation in baseline isotopic compositions at ocean basin to global scales.