Isotopic Variations Research Articles

Elemental-organic geochemical evidence for the lacustrine metalimnetic oxygen minimum dynamics in the Mid-Late Triassic Chang 7 shales

The Middle-Late Triassic Ordos Basin has gained increasing attention due to its significance in recording the earliest known recovery-radiation of terrestrial aquatic ecosystems following the Permian-Triassic mass extinction (PTME) and its abundant non-renewable resources. Despite numerous attempts to elucidate mechanisms for high organic carbon burial, the pattern of carbon cycle and its associated environmental variations remain largely unknown, probably ascribed to multi-periodic deposition of volcanic ashes and complex lacustrine depositional architectures. Herein, we reconstructed environmental variations within basinal and global contexts and identified the formation of metalimnetic oxygen minimum area in paleo-lacustrine environments. Our results demonstrate a shift of oxygen-deficient and organic matter hyper-enriched areas from the central basin in the lower part of Chang 7-3 (interval I) towards the periphery in the upper part (interval II). This transition is ascribed to enhanced weathering intensity but weakened volcanic activity, which is contemporaneous with terrestrial bio-evolution across the Mid-Late Triassic boundary. In addition, the contemporary variations in carbon isotopes and relatively sea and lacustrine levels around the Paleo-Tethys Ocean, along with the rare occurrence of high redox metal abundances in lacustrine environments, indicate a (intermittent) water connection between the Ordos Basin and Paleo-Tethys Ocean during the latest Middle Triassic to early Late Triassic. We hypothesize that mild to moderate volcanic activity induced genetic disturbance and stimulated bio-irradiation in the late Ladinian. In contrast, intensified weathering resulted in biological proliferation but also transiently elevated morality in the early Carnian, which appears to be the prelude of spectacular Triassic terrestrial radiation that dominates modern ecosystems.

Reduced magnitude of Early Pleistocene intensification of Northern Hemisphere Glaciation

Increased magnitude and expanding geographic distribution of ice-berg rafted debris in deep ocean sediment cores and increasing amplitude of variability in the benthic oxygen isotope (δ18O) proxy-global ice volume from ∼2.7 Ma marks the intensification of the Northern Hemisphere glaciation (iNHG). However, the location, extent and volume of Northern Hemisphere Ice Sheets (NHISs) is poorly constrained by proximal geologic evidence, and global sea-level records cannot determine individual polar ice sheet contributions alone.Quantitative relationships between sediment transport, water depth and grain size on a wave-graded continental shelf were previously applied to Pliocene shallow-marine sedimentary deposits in Whanganui Basin, New Zealand to provide an independent relative sea level record (PlioSeaNZ; 3.3–2.5 Ma). Here, we extend the duration of the sea-level record from 3.3 to 1.7 Ma (X-PlioSeaNZ) using a well-documented, shallow-marine sedimentary succession, that outcrops in the Rangitikei River Valley of the Whanganui Basin.The resulting glacial-interglacial, relative sea-level fluctuations are up to 45 ± 12.5 m paced by 41 kyr-obliquity frequency during the Early Pleistocene, but also contain modulation of 100 kyr-eccentricity cycle. These maximum sea-level amplitudes are more comparable to the mean of those previously reconstructed from calibrated benthic δ18O (e.g. Miller et al., 2020), and are significantly lower than estimates provided by albeit limited geological evidence proximal to the NHIS (Batchelor et al., 2019). Here we suggest that while NHISs acquired continental extent by 2.6 Ma, their collective volume may be overestimated in benthic δ18O calibrations and area-volume reconstructions, by as much as 50%.We propose that a reduced aspect ratio of ice sheets during iNHG was driven by 41-kyr changes in integrated summer insolation and enhanced by a subglacial regolith feedback reducing resistance to basal sliding. Regardless, the lower ice volume through the iNHG implies a lower ice sheet sensitivity (ice equivalent sea-level per degree Celsius change in temperature) under higher atmospheric CO2 of 4–6 m/°C compared to ∼20 m/°C for the period since the Last Glacial Maximum. This ice sheet sensitivity still has significant implications for society and sea-level will continue to rise under current emissions.

Making sense of variation in sclerochronological stable isotope profiles of mollusks and fish otoliths from the early Eocene southern North Sea Basin

Stable isotope sclerochemistry of biogenic carbonate is frequently used for the reconstruction of paleotemperature and seasonality. Yet, few studies have compared intra-and inter-taxon isotope variability and variation within a single depositional environment. We measured seasonal changes in δ18O and δ13C compositions in multiple specimens of two carditid bivalve species, a turritelline gastropod species, and two species of otoliths from demersal fish, from two early Eocene (latest Ypresian, 49.2 Ma) coquinas in the inner neritic Aalter Formation, located in the Belgian part of the southern North Sea Basin (paleolatitude ~41°N). Results demonstrate considerable variation among taxa in the mean, amplitude, and skewness of δ18O and δ13C values from sequentially sampled growth series. We attribute this variation to factors including differences in seasonal growth over ontogeny, mixing of depositional settings by sediment transport, differences between sedentary and mobile organisms, and differences in longevity of the taxa in question. Growth cessation during winters in turritellines and fishes in particular lead to an incomplete representation of the seasonal cycle in their growth increments, in comparison to carditid bivalves. Ophidiid fish otolith isotope records appear to reflect environmental conditions over a wider range of habitats and environments, and we infer this is due to a combination of sedimentary transport, as these are small structures, and postmortem transport by free-swimming predatory fish. Our study highlights the potential variability encompassed by taxa in the shallow marine realm even when they are found in the same deposits. While this has significant implications for seasonality reconstructions based on conventional isotope profiles, we show that careful study of the ecology and ontogeny of multiple taxa and specimens within a death assemblage can reveal sources of variation and yield a close approximation of conditions in the setting of interest.

Sclerochronology and oxygen isotope variations in modern Conomurex luhuanus shells: An archive for reconstructing palaeotemperature and shellfish gathering on the Great Barrier Reef, Australia

The marine gastropod Conomurex luhuanus is a common component of Holocene archaeological shell deposits in the Western Indo-Pacific region. Oxygen isotope ratios recorded in shells of C. luhuanus from the Great Barrier Reef potentially provide high-resolution data on palaeoclimatic conditions and the exploitation of coastal marine resources. However, the application of this species as a palaeoclimate and seasonal foraging archive has yet to be investigated. We employ sclerochronology techniques to analyse oxygen isotope variations in modern, live-collected shells to evaluate the efficacy of this species in recording modern environmental factors, such as sea-surface temperature (SST). We collected live C. luhuanus during the cold season (August and September), in two separate years (2019 and 2023), from reef-flat intertidal zones. Shell growth increments were studied in section and high-resolution carbonate samples were collected from the lip portion and along the body whorl to test whether shell carbonate is deposited in oxygen isotopic equilibrium with seawater and to decipher the shell growth pattern of this species. Measured δ18Oshell ranges between −0.2 and − 1.8 ‰ (VPDB) in modern samples. Despite showing variability in growth rates, including a remarkable growth stop/slowdown during the cool season that is punctuated by a red zone with closely spaced growth increments, our findings reveal that C. luhuanus deposits its aragonite shells at (or close to) equilibrium with its surrounding environment. When sampled at high resolution (sub-seasonally), oxygen isotopes can be used to reconstruct the full annual SST range. The estimated SSTs closely match the range of instrumental SSTs measured in the study area, whilst the last deposited (edge) shell-calculated SST can be used to determine the season of animal death. This study shows that well-preserved shells of C. luhuanus can serve as excellent high-resolution palaeotemperature archives for the tropical reef-flat zone. This research underpins the use of this species in future research to determine human-environment interactions and reconstruction of palaeoclimate from archaeological and fossil samples in the Western Pacific Ocean.

Average Time-Delays for the Scattering of O Atoms from O2 Molecules.

We report full quantum-computed average microcanonical, initial state-specific, and canonical cumulative time-delays associated with the O + O2 scattering, presented as a function of total energy (in relation to an idealized molecular beam experiment) or temperature (for the properties of the gas phase in bulk conditions). We show that these quantities are well-defined and computable, with a temperature-dependent (canonical) time-delay presenting a smooth, monotonic decreasing behavior with temperature, despite an energy-dependent (microcanonical) time-delay of apparent chaotic character. We discuss differences in behavior when considering isotopic variations, 18O + 16O16O and 16O + 16O18O, with respect to the reference process 16O + 16O16O and reveal a greater magnitude of the cumulative time-delay when genuinely reactive events can take place, in the presence of 18O. These results constitute an addition to more conventional fashions (like cross sections and rate constants) of displaying information related to collisions in various experimental contexts.

Isotopic variability of the invasive blue crab Callinectes sapidus in the Gulf of Cadiz: Impacts and implications for coastal ecosystem management.

The variability in trophic position and carbon isotopic signatures can provide information about their dietary flexibility and its ability to adapt to changing environmental conditions. The impact of the invasive blue crab Callinectes sapidus was assessed by estimating its trophic position and isotopic niche using stable isotopes (δ1³C, δ1⁵N, δ³⁴S) across different invaded Atlantic coastal areas. This study, the first of its kind in the eastern Atlantic range, reveals the crab's omnivorous behavior with a wide trophic position (TP=2-4), consistent with findings from its native range. Significant spatial variability in trophic roles was observed between habitats, with salt marshes showing enriched δ1³C and lower TP due to differences in primary producers like Spartina detritus and phytoplankton. Estuaries exhibited higher and broader δ1⁵N and TP ranges, influenced by prey availability and anthropogenic nitrogen inputs, that implies a significant impact on marine benthic and pelagic animal communities. Seasonal changes in the Guadalquivir estuary, linked to overwintering and brooding migrations, showed sexual differences in isotopic signatures, with males more enriched in δ1⁵N than females, indicating varied prey availability or metabolic differences. Overall, the Atlantic invaded range displayed broader and lower TP compared to the Mediterranean, underlining the blue crab's flexible and diverse trophic role across different habitats and trophic levels. The variability in trophic position and carbon isotopic signatures reflects the dietary flexibility of this invasive species and its potential effects on native fauna. These findings emphasize the need to consider trophic interactions in ecosystem management and conservation efforts.

Intra-Individual Stable Isotope Variation Tracks Brazilian Contemporary Dietary and Nutritional Transition.

Contemporary dietary and nutritional transitions are commonplace, but difficult to study directly. In Brazil, and Latin America, this generalized process, leading to current obesity and malnutrition problems, started more than four decades ago. Although body weight and food availability are used to measure changes, not much information on food consumption and nutrition exist. Stable isotope analysis allows for the study of modern individual diets because it reflects the proportional contribution of different foods, general dietary patterns, and the effect of metabolism. Furthermore, when samples from tissues reflecting different time points are used, it allows for the assessment of individual transitions. To explore intra-person isotopic variation for the first time in the Southern Hemisphere for modern humans, and examine the nutritional transition reported for Brazil in the past four decades. Stable carbon and nitrogen isotope values from 68 14C-dated bone samples (vertebra, occipital, parietal, femur) from 17 individuals born in 1963, from three cemeteries. Data reflect chronologically ordered high intra-individual δ15N variation tracking the dietary and nutrition transition over the last few decades, while no relationship between δ13C values and time was found. Vertebrae, reflecting diets from the mid 2000s, showed lower δ15N values than other bones reflecting the mid 1980s and early 1990s. We show how different bones capture nutritional transitions over the lifespan of modern individuals. Nitrogen isotope values were lower in recent tissues as a consequence of the changes in the agri-food industry and worldwide consumption patterns that have intensified in Latin America in the last decades.

Exploring Summer Precipitation Stable Isotopes in the Monsoon Border Zone: Environmental Drivers and Moisture Source Tracing

ABSTRACTPrecipitation processes in the monsoon border zone (MBZ) of China remain unclear because of the complex hydro‐climatic interactions. This study analyzes the temporal variation of δ2H and δ18O in precipitation and its influencing factors based on samples of precipitation events in Lake Dali during summer from 2018 to 2020. The results showed significant monthly isotopic variability, characterised by lower δ2H and δ18O values in July with averages of −103.62‰ and −14.87‰, respectively. This clear isotopic variability is largely related to the East Asian summer monsoon (EASM) activity during the prevailing summer monsoon season and hydrometeorological processes. The isotopic composition of precipitation is mainly determined by the amount of precipitation and relative humidity, but not related to temperature. The results of backward trajectory modelling show that precipitation primarily originates from northern China, inland Xinjiang, and the western Pacific with different monthly contributions. The increase in δ2H and δ18O isotopes from June to August indicates sub‐cloud evaporation. The mean sub‐cloud evaporation rate was 3.5% during the summer. Sub‐cloud evaporation is associated with relative humidity and precipitation. Recycled moisture accounts for 12% of local precipitation and 2.5%–31.6% of total monthly precipitation. After considering the contribution of evaporation, the estimated average recycling ratio increased from 12% to 24%, suggesting the important role of evaporation processes in lakes in the formation of precipitation.

Dietary preferences and collagen to collagen prey-predator trophic discrimination factors (Δ13C, Δ15N) in late Pleistocene cave hyena

AbstractThe spotted hyena (Crocuta crocuta) was an important large carnivore of Pleistocene ecosystems in Africa and Eurasia. Like its modern relatives, this obligate carnivore was adapted to crush and digest bones of its prey and absorb organic matter from bones more efficiently than any other carnivore. This difference in the nutrient resource use between hyenas and most other carnivores led to differences in the isotope flux and variation in the carbon and nitrogen isotopic composition. In our paper, we assess the prey-to-hyena collagen-to-collagen Δ13C and Δ15N trophic discrimination factor (TDF), a key parameter needed in mixing models used for quantitative reconstruction of diet. We analyzed a Pleistocene hyena den bone accumulation in Perspektywiczna Cave (Poland), with a preserved assemblage of remains containing both hyenas and a wide spectrum of their prey represented by digested bones. With the use of proteomics-based taxonomic identification (ZooMS), we estimated the proportion of prey species in the hyena diet. The modeled collagen-to-collagen TDFs are around +1.6‰ to +1.7‰ for δ13C and around +3.4‰ to +3.5‰ for δ15N. This study provides new data on the dietary habits of this large carnivore and allows for a more accurate use of isotopic signals in modeling past hyena diets.

In situ elemental and sulfur isotopic variations of Au-bearing iron–sulfides from the Alin Au deposit in the Shuixie Cu–Co Orefield, western Yunnan Province, SE Tibet: Insights into enrichment processes of invisible Au

In situ elemental and sulfur isotopic variations of Au-bearing iron–sulfides from the Alin Au deposit in the Shuixie Cu–Co Orefield, western Yunnan Province, SE Tibet: Insights into enrichment processes of invisible Au

Physical and biogeochemical drivers of multi-year isoscape in the California upwelling system

Stable isotopes of carbon (δ13C) and nitrogen (δ15N) are commonly employed to reconstruct past change in marine ecosystems and nutrient cycling. However, multiple biogeochemical and physical drivers govern spatiotemporal variability of these isotopic signals, particularly in dynamic coastal systems, complicating interpretation. Here, we coupled a modern multi-year (2010–2019) δ13C and δ15N isoscape record from intertidal mussels (Mytilus californianus) with high-resolution ocean model output and satellite chlorophyll-a observations in the California Current System (32°–43° N) to identify major drivers of isotopic variability. Our results show that spatial variations in δ13C are largely related to primary production, whereas spatial δ15N variability is driven by water mass mixing. Major isotopic change was also related to ocean climate variability; however, these effects vary regionally. In northern and central California, δ15N values are predominantly a function of nitrate utilization, whereas in southern California, δ15N varies due to shifts in water mass composition. In all regions, δ13C values are driven by productivity, with the largest changes occurring in southern California. Our findings provide novel insight into regional differences in predominant drivers of isotopic variability, and links to modern ocean climate variability. These findings offer crucial information needed for robust interpretations of California Current palaeoceanographic δ13C and δ15N records.

Using stable-isotope analysis to assess recent diet and habitat use of stranded green turtles (Chelonia mydas)

Context Gaining a better understanding of the foraging ecology of green turtles (Chelonia mydas) in different regions can improve the effectiveness of conservation and management strategies for this species. New South Wales (NSW), in eastern Australia, has been identified as a region requiring improved information on foraging ecology and habitat use by green turtles. Aims The aim of this study was to use stable-isotope analyses to investigate diet composition and foraging ecology of green turtles found stranded along the NSW coastline from 2018 to 2021. Methods This study measured δ13C, δ15N, and δ34S in macrophyte (mangrove, seagrass, and algae) samples collected along the NSW coastline, between March and June 2021, and epidermis tissue from green turtles that stranded in the same region between 2018 and 2021. In addition, stable-isotope values for animals known, or expected, to be consumed by green turtles were sourced from the literature. Macrophyte stable-isotope values were mapped to display spatial variations, then combined with literature-sourced animal matter and measured epidermis isotope values into stable-isotope mixing models. Mixing models were used to determine diet structure for each stranded green turtle, and their similarity to regional macrophyte isotope values to infer potential foraging locations. Key results High variation was present in δ13C, δ15N, and δ34S values of macrophyte sources along the NSW coastline, limiting the potential to infer the recent foraging locations of stranded green turtles. High δ15N values in green turtle epidermis samples indicated that most sampled turtles were feeding two to three trophic levels above primary producers within algal-based food webs. Mixing models suggested that these food webs were predominantly supported by algae (mean ± s. d. = 51.2 ± 6.1%) and animal material (31.4 ± 4.7%), with mangrove (13.4 ± 2.0%) and seagrass (4.0 ± 2.5%) representing much smaller proportions of overall diet composition. Conclusions The results showed that green turtles stranded along the NSW coastline feed primarily in algae-dominated food webs, with evidence of omnivory. These findings suggest that sampled individuals display diet compositions alternative to those of their Queensland counterparts, who feed primarily in seagrass-dominated habitats. Management and conservation programs in NSW should focus on limiting anthropogenic disturbances in algae-dominated habitats. Implications This novel finding demonstrated the importance of algae-dominated habitats for green turtle conservation and management in south-eastern Australia. The generated isoscapes are useful in examining isotopic variations in macrophytes along the NSW coastline.

Patagonian toothfish (Dissostichus eleginoides) stocks in South American waters and its implications for fishery management

Patagonian toothfish (Dissostichus eleginoides) is a highly prized resource in markets due to the quality of its meat. The Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) implemented controls and regulations in response to the sharp rise in the illegal fishing of D. eleginoides in the 1990s. Today, four fisheries in waters close to the southern tip of South America are managed in accordance with stringent sustainability standards. Even though they are separate management units, both abundance assessments and annual catch allocations are conducted using different criteria regarding stock considerations across the region, leaving one of the fundamental premises of fisheries management unclear. This study examines historical data and recent research to explore the potential differentiation between Patagonian toothfish populations in South American waters, which is crucial for the management of diverse fisheries. Genetic studies, otolith microchemistry, morphometry, parasitic fauna, tagging programs, reproductive characteristics, and the impact of ocean circulation on dispersal and recruitment were analyzed. Tagging studies in the southern hemisphere oceans confirm the species’ affinity to specific habitats, suggesting minimal fish exchange between South American fishing grounds. From a fisheries perspective, this review suggests the existence of distinct stocks of the species structured along the shelf and slope of the southern cone of America based on reproduction areas along the continental shelves and slope, the diverse parasitic fauna, the variability in stable isotopes and trace elements of otoliths, and the little significant exchange of fish between current fishing grounds. All of this could lead to considering the D. eleginoides fishery as independent exploitation units.

Spatial and Temporal Variations of Hydrogen and Oxygen Isotopes in Multiple Water Sources of Handan Eastern Plain

Spatial and Temporal Variations of Hydrogen and Oxygen Isotopes in Multiple Water Sources of Handan Eastern Plain

Reappraising the eruptive history of the Alchichica Maar Volcano (Mexico) based on Sr–Nd isotopes: Understanding the role of the magma source region on the growth of small-volume volcanoes

Alchichica Maar is in the eastern sector of the Trans-Mexican Volcanic Belt, where volcanics range from basalts to rhyolites, with 87Sr/86Sr and 143Nd/144Nd values between 0.70293 to 0.70447 (εSr = −21.1 to −4.5) and 0.512733 to 0.512949 (εNd = +3.4 to +6.2), respectively. In this study, Nd and Sr isotopic data and new major and trace elements compositions are provided for the lava flow, the scoria cone, and the phreatomagmatic deposits composing Alchichica Maar Volcano. These results are used in combination with published data to reexamine its eruptive history, though to be formed by more than one eruptive event. The cogeneticy of the magmas that fed the eruptions of these deposits is examined to infer the role of the deep magma source region and post-magmatic alterations on the construction of the Alchichica Maar magmatic system. Alchichica samples present 87Sr/86Sr values between 0.70387 and 0.70447 (εSr = −9.0 to −0.4) and 143Nd/144Nd values from 0.512749 to 0.512773 (εNd = +2.0 to +2.8). While the 87Sr/86Sr values suggest a slight increment in radiogenic 87Sr from the pre-maar lower lava flow to the upper maar eruptions indicative of magma contamination within the upper crust and by hydrodynamic mingling during the phreatomagmatic phases, the slight variation in Nd isotopes suggest an overall homogeneous source where small batches of magmas formed episodically to feed monogenetic eruptions, which subsequently yielded the pre-maar lava flow, the pre-maar surge, the scoria cone, and the maar units. This study reinforces the idea that an already “tapped” source region of a monogenetic volcano may become fertile again, and new eruptions can occur from near or old vent sites. It also highlights the hazardous potential of small-volume volcanoes.

Deciphering the mineral code of urinary stones: A first look at zinc isotopes.

Zinc (Zn) is an essential element for all living organisms, and Zn isotopes play a key role in studying the formation of disease. Despite extensive studies on Zn isotopes in healthy and diseased human tissues, the role of Zn isotopes in urinary stones remains unexplored. This study investigates Zn isotopes in 37 urinary stones using multi-collector inductively coupled plasma mass spectrometry. The δ66Zn values of urinary stones range from -0.15‰ to 0.47‰, with a mean value of 0.11‰. Carbonate apatite (CA) stones exhibit lighter Zn isotopic compositions (δ66Zn=-0.15‰∼-0.03‰) compared to calcium oxalate (CO) stones (δ66Zn=-0.11‰ ∼ 0.47‰). The variation in Zn isotopic compositions between CO and CA stones may result from urinary pH differences during stone formation. At higher urinary pH, CA stones are enriched in lighter Zn isotopes compared to CO stones. Urinary stones are enriched in lighter Zn isotopes compared to blood and urine. This study identifies two steps influencing Zn isotope variations during kidney transport. The first step involves kidney filtration and reabsorption, enriching heavy Zn isotope in the urine. The second step is the deposition process of urinary stones, where light isotopes, due to their lower bond energy, are more prone to breaking. This kinetic fractionation effect leads to an enrichment of light Zn isotope in urinary stones. Overall, this study offers preliminary insights into the geochemical mechanisms that influence the Zn isotopic composition in urinary stones.

Olivine Ni isotopes of Ni-Cu deposits in NW China: A potential sulfide segregation indicator in basaltic magmatic systems

Sulfide segregation from silicate magmas is essential in forming magmatic sulfide deposits, but a robust Ni-isotopic indicator has been lacking. This study investigates the Ni isotopic compositions of olivine and coexisting sulfide from three magmatic Ni-Cu deposits in East Tianshan, NW China, to reveal the behavior of Ni isotopes during magmatic differentiation and sulfide segregation processes. The δ60/58Ni values in olivine separates (+0.15 to +0.68 ‰, median 0.40 ‰) are notably higher than typical mantle and basalt values (+0.11 to +0.23 ‰). Decreased Ni at the rim of Fe-Mg unzoned olivine indicates certain degrees of post-crystallization Fe-Ni diffusion between sulfide and olivine. The intragrain diffusion modeling results show that the δ60/58Ni values in olivine can change up to 0.5 ‰ to 1 ‰ when the diffusional modification of Ni extends to the core. However, the observed thin Ni gradient at the rim and relatively flat Ni profiles in the cores of the olivine samples suggest limited kinetic modifications. Moreover, the δ60/58Ni values negatively correlate with the Ni content and Ni/Co ratio in olivine. The sulfide separates are isotopically lighter than the coexisting olivine grain, with δ60/58Niolivine – δ60/58Nisulfide values ranging from +0.10 to +0.40 ‰ (median +0.22 ‰). Given minimal Ni isotope fractionation between olivine and silicate melt, the observed Ni isotopic differences between olivine and sulfides are attributed to Ni isotope fractionation between silicate melt and sulfide melt at magmatic temperatures, efficiently incorporating isotopically light Ni into the sulfide melt. The sulfide segregation-driven Ni isotope fractionation aligns with the distinct Ni isotopic compositions observed in olivine grains from two highly differentiated mafic units of the Huangshandong deposit with different sulfide evolution histories. Olivine grains from mineralized gabbronorite have higher δ60/58Ni values (+0.68 ‰, Phase III) than those from sulfide-barren olivine gabbro (δ60/58Ni = +0.23 ‰, Phase I). Systematic stratigraphic Ni isotope variations in the Huangshanxi olivine from intervals with varying degrees of sulfide segregation and magma replenishment further support this interpretation. These findings demonstrate that Ni isotopes in olivine are reliable for tracing sulfide segregation, provided that diffusional modification is carefully assessed. With limited diffusional modification, the widespread isotopically heavy Ni signature in olivine (δ60/58Ni > 0.3 ‰) serves as a valuable indicator for sulfide segregation and exploring magmaticNi-Cu deposits.

Marine redox evolutions and carbon isotope variations during the Guadalupian-Lopingian transition in the Yangtze Platform, South China

Marine redox evolutions and carbon isotope variations during the Guadalupian-Lopingian transition in the Yangtze Platform, South China

Early Cretaceous to Eocene magmatic evolution of Livingston Island, South Shetland Archipelago: Geochronological and isotope geochemical constraints from intrusive suites

The magmatic products exposed across Livingston Island comprise a part of the South Shetland magmatic arc built on an active continental margin. The main magmatic phases of Livingston Island include the Early to Late Cretaceous plutonic and volcanic rock associations, the Eocene plutons, and some sporadically distributed intrusions of Paleocene to Eocene dykes. The intrusions were emplaced into the strongly deformed turbiditic sedimentary rocks of the Miers Bluff Formation. 40Ar39Ar and zircon UPb geochronologic data presented here define four distinct episodes of intrusive activity represented by the emplacement of the Hesperides (125 Ma), the Cerro Mirador (104–97 Ma), the Siddons Point (80 Ma) and the Barnard Point suites (42 Ma), in addition to some dyke intrusions with emplacement ages of 105 to 40 Ma. The intrusions are represented by mafic to intermediate compositions characterized by middle potassic, calc-alkaline affinities, with only a few samples from the Eocene activity exhibiting tholeiitic character. Trace element, Sr-Nd-Pb isotope and zircon Hf isotope variations indicate magma generation in a subduction setting with extensive involvement of continental crust-derived melts for the Early to Late Cretaceous activity. The effects of crustal input on melt chemistry appear to have diminished in time, especially from the Paleocene onwards, leading to magma generation with typical oceanic island arc geochemical signatures. This change in melt chemistry is likely partly related to the roll-back induced overriding plate extension which resulted in a time-integrated increase in the rate of melt movement and a decrease in the residence time in crustal levels. Slab steepening caused by the systematic decrease in plate convergence rate led to an enhanced involvement of isotopically depleted mantle component, producing a heterogeneous source with variable subduction signature.

Fe-isotopic evidence for hydrothermal reworking as a mechanism to form high-grade Fe-Ti-V oxide ores in layered intrusions

The genesis of high-grade Fe-Ti-V oxide ores (up to >90 vol%) in layered intrusions remains highly debated. Here, on example of Hongge layered intrusion in China, we show that hydrothermal dissolution and precipitation of Fe-Ti-V oxides played a critical role in forming high-grade massive ore deposits as demonstrated by textural-compositional evidence and in-situ iron isotope data (δ56Fe), analyzed with femtosecond laser ablation multicollector (LA-MC-) ICP-MS. Hongge is a mafic layered intrusion composed of a Lower olivine clinopyroxenite Zone (LZ), a Middle clinopyroxenite Zone (MZ), where thick massive ore layers (with up to 90 % Fe-Ti-V oxides) formed, and an Upper gabbro Zone (UZ). Magnetite in Hongge exhibits two contrasting generations: 1) Mag1, observed in all lithological zones and formed at the magmatic stage, has extensive ilmenite exsolution lamellae and high Ti and Cr content. The δ56Fe of Mag1 shows considerable variations from −0.23 to 0.63 ‰ and strikingly an offset of ∼0.3 ‰ towards lower values in the massive ore zone compared to the zones below and above; 2) Mag2, concentrated mainly in thick massive ore layers in MZ without exsolution lamellae, is almost pure magnetite (with low Ti, Al content) and has extremely low δ56Fe values (−1.24 to −0.09 ‰), indicating precipitation from Fe-enriched hydrothermal fluids. Similarly, the δ56Fe of ilmenite shows significant variations from −1.08 to −0.27 ‰ and is significantly lower than typical values for igneous ilmenite (−0.4–0 ‰). Ilmenite displays a similar Fe isotope variation pattern to Mag1 along the stratigraphic position, i.e., with significantly lower δ56Fe in the massive ore zone. As magnetite and ilmenite together contain essentially all Fe, the isotopic shift of these minerals in the ore zone translates to a bulk isotopic offset of ∼−0.3 ‰ compared to the zones below and above. This requires a bulk flux of isotopically light Fe resulting in Fe enrichment in this zone to form massive or even monomineralic ores. The very light isotopic values, particularly hydrothermal magnetite (Mag2) and petrologic evidence, strongly indicate that the Fe flux into the massive ore layers occurred during hydrothermal reworking. This scenario is furthermore supported by magnetite-ilmenite elemental and isotopic thermometry, according to which Fe-Ti oxides experienced hydrothermal re-equilibration in a temperature range of 400–300 °C. Iron isotopic mass balance calculations imply that ∼20–30 % of the Fe in the thick massive ore layers may result from secondary enrichment through hydrothermal precipitation, significantly increasing the ore tonnages and grades. Potentially, other layered intrusions experienced similar mechanisms of hydrothermal Fe enrichment, which will have to be proven in future investigations.