Stable Oxygen Isotope Composition Research Articles

Intra-tooth isotopic analysis shows seasonal variability in the high-elevation context of Melka Kunture (Upper Awash Valley, Ethiopia) during the Early Pleistocene

In order to investigate seasonal changes in diet, environment and climate, we analyzed the stable carbon and oxygen isotopic composition of intra-tooth sequential profiles (14 teeth, 282 enamel samples) of Hippopotamidae, Equidae, Bovidae and Suidae from Melka Kunture, Upper Awash Valley, central Ethiopian Highlands (2000–2200 m a.s.l.).We found that during the Early Pleistocene, between 1.95 and 1 Ma, most of the analyzed hippos display a seasonally stable C4 diet, even if the δ13C values within hippos show a degree of variability that we interpret as the outcome of feeding on plants that use different C4 photosynthetic pathways. Several hippo specimens display a seasonal shift from C4 to mixed C3-C4 diets. The sampled equid, bovid and suid specimens recorded both stable C4 diets and mixed C3-C4 feeding with a seasonal progressive increase of δ13C values. When affected by seasonal changes, the serially analyzed taxa show different niche partitioning: hippos increase the consumption of C3 vegetation, whereas equids and suids include more C4 vegetation in their diets. The intra-individual δ18O variability in the analyzed taxa is interpreted as the outcome of different water sources, depending on animal habitat, behavior and mobility patterns.Our data are placed in controlled stratigraphic and chronological sequences and combined with the outcome of other proxies, allowing us to evaluate the site paleoecology comprehensively. We suggest that the central Ethiopian Highlands, where MK is located, possibly acted as a refugium-like area during the Early and Middle Pleistocene, characterized by a specific type of montane vegetation (DAF) and diverse faunal and hominin species that demonstrated their resilience and adaptability to changing environments and climates.

Microscale δ34S and δ18O variations of barite as an archive for fluid mixing and microbial sulphur metabolisms in igneous rock aquifers

ABSTRACT The stable isotope compositions of sulphur (δ34S) and oxygen (δ18O) in barite are frequently used as proxies for microbial sulphate reduction (MSR) in diverse environments, such as in relation to anaerobic oxidation of methane in marine cold seeps. There, isotopically heavy barite is used as a marker for MSR from a sulphate pool that has undergone semi-closed system conditions. Closed-system MSR is also a commonly observed feature in igneous rock hosted fracture aquifers, as shown by extremely 34S-enriched pyrite. What is less well-constrained is whether δ34S in barite can be used as a proxy for MSR in such systems. Here we explore the microscale heterogeneity of δ34S and δ18O via secondary ion mass spectrometry and the trace element Sr via LA-ICP-MS maps in barite precipitated in granite-hosted boreholes during a 17-year experiment, at Äspö, Sweden. We compare it with δ18Osulfate, δ34Ssulfate, and δ34Ssulfide of the fracture fluids and with paragenetic pyrite with δ34S values reflecting closed system MSR. The δ18O values in barite (+9.4 to +16.9 ‰) represent two generations of barite, one with low values and one with high values. The latter are likely impacted by sulphur disproportionating or -oxidizing bacteria. The barite reflects a much smaller span in δ34S (+14.5 to +28.6 ‰) than the pyrite (−47.2 to +53.3 ‰). This lack of extremely high δ34Sbarite values is proposed to be due to that barite saturation only occurred in the early parts of the Rayleigh cycle. Additionally, fluid migration has affected the δ34S values to lower values, accompanied by higher Sr concentrations. Taken together, barite δ34S values cannot be regarded as a reliable independent proxy for MSR in deep sulphate-poor igneous rock hosted aquifers. However, the relation between the δ34S values of coeval barite and pyrite is regarded as a useful proxy for MSR-related fractionation during early stages of MSR.

Individual isotopic niche variation of herbivores and palaeoenvironmental reconstruction of the Mid-Pleistocene Site of Kathu Pan 1 (South Africa)

Individual niche variation occurs when individuals within a population adopt unique behavioural or ecological traits, giving the population a distinct niche structure. Although widespread amongst natural populations, most of niche theory and its applications in ecological and palaeoenvironmental contexts remains focused on species level patterns, treating variation within the population as noise. Using serial stable isotope profiles sampled along teeth of herbivore fossils recovered from the Mid-Pleistocene site of Kathu Pan 1, Northern Cape (South Africa), enabling partitioning of isotopic niche variation into between- and within-individual components. Stable carbon isotope data revealed significant dietary differences between-individuals within most of the taxa sampled, but no dietary differences between taxa. By contrast, stable oxygen isotope compositions were more consistent amongst individuals, but varied across taxa. In both isotope systems, as well as in bivariate space, most taxa comprised individuals with both narrow and broad isotope niches, indicating flexibility in dietary strategies. These results indicate sufficient opportunity for individual level specialization within the Kathu palaeocommunity, afforded by a palaeoenvironment that provided a consistent supply of resources year-round. Given the herbivore sample is strongly associated with fossil hominin activities, our results imply that hominins in these landscapes would have profited from year-round accessibility to resources, accounting for the record of occupancy of Kathu Pan I during the Mid-Pleistocene.

Radiocarbon dating of the natural groundwater in the Ob-Zaisan folded region (Russia)

Groundwater in the Ob-Zaisan folded region (Russia) has significant differences in the stable isotope composition of oxygen and hydrogen, which cannot be explained by the geographical and relief features of the region. A probable reason for these differences could be climatic changes in the study area over the past tens of thousands of years. The method of the radiocarbon dating can be perfectly suited in order to determine such small geological ages. The dating of waters using 14C data gives an understanding of their residence time. It will make it possible to differentiate periods of recharge and accumulation of water in aquifers and track the changes of the water stable isotope composition over time. The estimated water age ranges from 650 to 19,000 years. The enrichment of δD and δ18O values with the decreasing of the water age indicates a gradual warming of the Novosibirsk region climate. These results logically complement the meteorological observations over the last century and may be useful for paleoclimate reconstructions of the region.

Unveiling spatial variations in atmospheric CO2 sources: a case study of metropolitan area of Naples, Italy

In the lower atmosphere, CO2 emissions impact human health and ecosystems, making data at this level essential for addressing carbon-cycle and public-health questions. The atmospheric concentration of CO2 is crucial in urban areas due to its connection with air quality, pollution, and climate change, becoming a pivotal parameter for environmental management and public safety. In volcanic zones, geogenic CO2 profoundly affects the environment, although hydrocarbon combustion is the primary driver of increased atmospheric CO2 and global warming. Distinguishing geogenic from anthropogenic emissions is challenging, especially through air CO2 concentration measurements alone. This study presents survey results on the stable isotope composition of carbon and oxygen in CO2 and airborne CO2 concentration in Naples’ urban area, including the Campi Flegrei caldera, a widespread hydrothermal/volcanic zone in the metropolitan area. Over the past 50 years, two major volcanic unrests (1969–72 and 1982–84) were monitored using seismic, deformation, and geochemical data. Since 2005, this area has experienced ongoing unrest, involving the pressurization of the underlying hydrothermal system as a causal factor of the current uplift in the Pozzuoli area and the increased CO2 emissions in the atmosphere. To better understand CO2 emission dynamics and to quantify its volcanic origin a mobile laboratory was used. Results show that CO2 levels in Naples’ urban area exceed background atmospheric levels, indicating an anthropogenic origin from fossil fuel combustion. Conversely, in Pozzuoli's urban area, the stable isotope composition reveals a volcanic origin of the airborne CO2. This study emphasizes the importance of monitoring stable isotopes of atmospheric CO2, especially in volcanic areas, contributing valuable insights for environmental and public health management.

Tracing sulfate sources in a tropical agricultural catchment with a stable isotope Bayesian mixing model

Sulfate (SO42−) is an essential anion in drinking water and a vital macronutrient for plant growth. However, elevated sulfate levels can impact ecosystem or human health and could be an important indicator of acid rock drainage or pollution. Therefore, monitoring SO42− sources and transport is important for water quality assessments. This study focused on exploring the sources and transformations of SO42− as well as estimating the proportional contribution of the potential SO42− pollutant sources to groundwater and surface water in a tropical river basin, the Densu River Basin. The study used major ions combined with stable sulfur and oxygen isotope compositions and a Bayesian isotope mixing model, MixSIAR. The major ion characteristics indicate that SO42− concentrations remain stable throughout the rainy and dry seasons but originate from diverse sources. The multi-isotope model (δ34SSO4, δ18OSO4) identified four potential SO42− sources: detergent, precipitation, sewage, and sulfate fertilizer. However, the δ34SSO4 and δ18OSO4 values of the fertilizer source signatures overlapped with those of precipitation and sewage. Nevertheless, the contributions from each source were disentangled using the MixSIAR model, which revealed sewage as the most dominant SO42− pollutant in the Densu Basin, accounting for ~47 % of sulfate in groundwater and ~ 56 % of sulfate in surface water. Sulfate fertilizer (~33 %) was the second most important source after sewage for groundwater, while detergent (~23 %) was the second most important source for surface water. The redox processes of bacterial sulfate reduction and sulfide oxidation were determined to have a minimal impact on the sulfur isotope fractionation within the basin. This study highlights the benefits of combining major ions, sulfur isotopes and the MixSIAR model for identifying sources of sulfate. This approach accounts for uncertainties in source contributions which allows for more robust and reliable apportionment of sulfate sources. The study emphasizes the need for effective waste management and pollution control measures to protect water quality and provides vital guidelines on how to partition sulfate sources on a large catchment scale and evidence for making pollution management decisions on water resources.

Tracing the orogenic sulfur cycle in the Andes using stable isotope composition of dissolved sulfate in thermal springs

The cycling of sulfur (S) to the upper crust and surface via thermal springs at convergent margins has not been explored outside areas with active arc volcanism, even though subduction plays a key role in the Earth's long-term S cycle. To address this knowledge gap, we analyzed stable sulfur and oxygen isotope compositions (δ34S and δ18O values) of dissolved sulfate (SO42−) in 55 thermal springs from five distinct settings in the Andean orogen. These regions are the Peruvian flat slab and backarc, transition between these two, Argentinian backarc, and Chilean forearc. Although the flat-slab settings had lower SO42− concentrations (<2000 mg/L) compared to the steep-slab settings (<12,700 mg/L), there was no significant relationship between isotope composition of SO42− and slab geometry. The δ34S and δ18O values of SO42− varied widely across the studied areas (+0.2 to +23.5 ‰ and − 3.3 to +16.0 ‰, respectively) and reflected the isotope compositions of local bedrock endmembers from dissolution of marine evaporites (+5 to +25 ‰ and + 10 to +20 ‰, respectively) and oxidation of magmatic and/or hydrothermal S and ore sulfide minerals with variable δ34S (0 to +16 ‰). The δ18O and δ2H values of thermal spring water (−18.5 to −3.3 ‰ and − 141.1 to −23.7 ‰, respectively) were consistent with meteoric precipitation, and in most cases decreased with increasing altitude following precipitation in the Andes. Generally, our isotope results do not support the direct transfer of slab-derived S/SO42− to thermal springs in the investigated settings. Rather, the δ34S and δ18O of SO42− in the thermal springs are a sensitive indicator of local water-rock interactions that remobilize bedrock S originating from a complex orogenic cycle reflecting tectonic uplift, erosion, weathering, and exhumation history across the duration of Andean Mountain building.

Landscape and paleoenvironmental change in stream valleys of the Central Great Plains, North America, during Marine Isotope Stage 3 (ca. 59–27 ka)

Recent studies of late Quaternary landscape evolution in stream valleys throughout the central Great Plains of North America have identified thick packages of MIS 3-age alluvium and colluvium beneath terraces, alluvial fans, and footslopes. Here, we review and build on these studies to highlight the geomorphic history of river systems in the Central Plains during MIS 3 and reconstruct the paleoclimatic conditions and vegetation communities that existed in the region at that time.A suite of 50 radiocarbon and optically stimulated luminescence ages indicate that late MIS 3 was a period of widespread landscape stability and soil development in the Central Plains. In general, river systems were actively aggrading during early MIS 3, but aggradation slowed after ca. 40 ka, allowing soils to develop until ca. 30–27 ka. In some river valleys, a second pulse of aggradation occurred shortly after that time followed by a period of landscape stability and pedogenesis during early MIS 2 (ca. 24–19 ka). Evidence for widespread landscape stability in stream valleys implies that high magnitude flooding was minimal. Atmospheric circulation patterns that favor these conditions include zonal airflow that inhibits incursions of moist, maritime air from the Gulf of Mexico.Paleoenvironmental conditions during late MIS 3 were reconstructed using the stable carbon, oxygen, and hydrogen isotopic composition of pedogenic phyllosilicates, carbonates, and organic matter. Estimated paleotemperatures ranged from 6 to 9 °C ± 3 °C and are approximately 4–7 °C lower than modern mean annual temperature. The estimated δ18O values of soil water from phyllosilicates range from −10.6‰ to −9.6‰. These estimates are lower (18O-depleted) by about 1.5–3‰ relative to modern meteoric δ18O values and we estimate that the δ18O of paleoprecipitation was likely 18O-depleted by up to 8.5‰ relative to modern values. Our estimated δ18O values for soil waters in the Central Plains during late MIS 3 can be explained by climatic conditions characterized by 1) cooler temperatures, 2) precipitation being sourced primarily from Pacific/Arctic air masses, 3) seasonal changes in precipitation patterns (i.e., predominantly winter precipitation), or 4) a combination of the above. δ13C values indicate the presence of a notable proportion of C4 vegetation in the Central Plains during late MIS 3. We suggest that C4 productivity was likely favored at that time by a low atmospheric CO2 environment that had sufficiently warm temperatures and effective precipitation during the growing season.

Heterogeneous shell growth of the neustonic goose barnacle Lepas anserifera

Floating materials of both natural and anthropogenic origin affect marine ecosystems and human economic activities. Although the tracking of floating materials is important to manage the economic risks, it is difficult to trace them back to the events of origin, such as tsunamis and underwater volcanic eruptions. The gooseneck barnacle Lepas anserifera, a rapid colonizer in pelagic environments, is a potential “natural logger” of floating materials. In this study, we performed temperature-controlled culture experiments and growth line staining in the laboratory to quantify the growth increments of shells (scutum and tergum) consisting the capitulum of L. anserifera separately, and to examine the effects of the temperature on their growth. Following calcein staining, the growth lines of L. anserifera were visualized under a fluorescent microscope, and gross (capitular length and width) and individual (scutum and tergum) shell growth were compared. Shells grew in twice as much in the capitular length direction than in the capitular width direction owing to the larger growth increases in the scutum than in the tergum. Growth increments were unaffected by temperatures in the range from 20°C to 30°C, although the growth appeared to slow down in September and October compared with August. The stable oxygen isotope composition (δ18O) of the shells represented the water temperature as previously known, and the present results showed that 18O enriched in scutum than tergum in most cases. Further understanding for the biomineralization process of barnacles is required for the precise application of environmental proxies in barnacle shells.

Oxygen and sulfur stable isotope ratios of Late Devonian vertebrates trace the relative salinity of their aquatic environments

Late Devonian aquatic environments hosted the fin-to-limb transition in vertebrates. Upper Devonian (ca. 365−360 Ma) strata in Pennsylvania, USA, preserve a diversity of fishes and tetrapods in coastal marine to fluvial depositional environments, making this region ideal for investigating the ecology and evolution of Late Devonian vertebrates. A key unresolved issue has been reconstructing the specific aquatic habitats that hosted various vertebrates during this period. Specifically, the salinity of environments spanning fresh to shallow marine water is difficult to discern from sedimentological and paleontological analyses alone. Here, we analyze rare earth elements and yttrium (REY) as well as stable oxygen and sulfur isotope compositions (δ18O, δ34S) in fossil vertebrate bioapatite from late Famennian (ca. 362−360 Ma) strata of the Catskill and Lock Haven formations in the Appalachian Basin, USA, to determine the relative salinity of their aquatic environments. These results confirm the ecological euryhalinity of several taxa (Bothriolepis sp., tristichopterids, and Holoptychius sp.). Our results are the first demonstrating that some early tetrapod species occupied unequivocally freshwater habitats by late Famennian time (ca. 362−360 Ma). Our study shows that integrating sedimentological and paleontological data with combined oxygen and sulfur isotope analysis allows precise tracing of the relative salinity of vertebrate habitats deep in the past.

Geochemistry of forty‐one eclogitic and pyroxenitic mantle xenoliths from the Central Slave Craton, Canada (Ekati Diamond Mine)

AbstractThis article describes a novel dataset on non‐diamondiferous eclogite and garnet pyroxenite xenoliths from four kimberlite pipes of the Ekati Diamond Mine (Central Slave Craton, Canada). Xenoliths brought to the surface by kimberlite eruptions are direct sources of information on the composition and evolution of the Earth's mantle. Eclogite and garnet pyroxenite xenoliths, specifically, are testimony of subduction into, and metasomatism of, the mantle beneath cratons. Furthermore, these rocks are major hosts for diamond and thus an important part of the deep carbon cycle. The sample suite consists of 41 small xenoliths (2–5 cm) recovered from drill cores. The dataset includes major and trace element concentrations for garnet, clinopyroxene and ilmenite, as well as stable oxygen isotope compositions of garnets. Strontium and neodymium isotopic compositions are reported for garnet and clinopyroxene for four samples which were large enough to allow for analysis. Overall, this dataset significantly expands and complements existing datasets on diamondiferous and non‐diamondiferous xenoliths from the Slave Craton in Canada, furthering our understanding of the composition of the Slave subcratonic lithosphere. The dataset includes several samples with rare mineral assemblages, including an olivine‐bearing eclogite as well as ilmenite and apatite‐bearing garnet‐pyroxenites, and thus provides data shedding light on rarely reported compositional nuances in xenolith suites found in kimberlites.

Stable isotope evidence for resource partitioning in extinct marine carnivores

Living pinnipeds play essential roles in marine ecosystems and display divergent lineage-specific foraging and habitat preferences. The origin of these ecological strategies remains unclear. We analyzed original (n = 22) and published (n = 93) stable carbon (δ13C) and oxygen (δ18O) isotope compositions of tooth enamel from fossil pinnipeds and coeval marine and terrestrial mammals from the late Early Miocene to early Middle Miocene Temblor Formation, the early Middle Miocene Round Mountain Silt Formation at Sharktooth Hill Bonebed, the Middle Miocene of lower levels of the Monterey Formation, and the late Middle Miocene Santa Margarita Formation from the eastern North Pacific Ocean; and original (n = 43) δ13C and δ18O values from the Early Pliocene Yorktown Formation in the western North Atlantic. As expected for aquatic mammals, pinnipeds and control marine mammals had low δ18O variability relative to coeval terrestrial mammals, indicating minimal diagenetic alteration. Among late Early Miocene and Middle Miocene pinnipeds, Allodesmus had lower δ13C values than coeval taxa indicating offshore foraging, in agreement with predictions based on skeletal morphology. However, Allodesmus from the late Middle Miocene Santa Margarita Formation had overlapping δ13C values with co-occurring cetaceans and pinnipeds suggesting preferential nearshore habitat. The stem otariid Pithanotaria had higher enamel δ13C values than co-occurring pinnipeds, indicating nearshore foraging. The stem odobenid Neotherium had intermediate δ13C values supporting foraging between nearshore and offshore predators. Conversely, the stem odobenid cf. Imagotaria had lower enamel δ18O but comparable δ13C values to coexisting pinnipeds, suggesting the exploitation of estuarine resources. At least two ecologically distinct pinniped groups also occurred in the Yorktown Formation. In this unit, the odobenid Ontocetus and some monachinae phocids were predominantly nearshore foragers, with higher enamel δ13C and δ18O values than simultaneously occurring marine mammals. The second group was exclusively composed of phocids, which had comparatively low δ13C and δ18O values, compatible with northward foraging movements along the western North Atlantic coast. The paleoecological profiles of fossils bear strong similarities to the life modes of extant pinniped communities, implying that extant foraging modes appeared early in pinniped evolution and that resource partitioning contributed to their community structure over time.

West African Monsoon Dynamics and Its Control on the Stable Oxygen Isotopic Composition of Precipitation in the Late Cenozoic

AbstractThis study presents an overview of the Late Cenozoic evolution of the West African Monsoon (WAM), and the associated changes in atmospheric dynamics and oxygen isotopic composition of precipitation (δ18Op). This evolution is established by using the high‐resolution isotope‐enabled GCM ECHAM5‐wiso to simulate the climatic responses to paleoenvironmental changes during the Mid‐Holocene (MH), Last Glacial Maximum (LGM), and Mid‐Pliocene (MP). The simulated responses are compared to a set of GCM outputs from Paleoclimate Model Intercomparison Project Phase 4 (PMIP4) to assess the added value of a high resolution and model consistency across different time periods. Results show WAM magnitudes and pattern changes that are consistent with PMIP4 models and proxy reconstructions. ECHAM5‐wiso estimates the highest WAM intensification in the MH, with a precipitation increase of up to 150 mm/month reaching 25°N during the monsoon season. The WAM intensification in the MP estimated by ECHAM5‐wiso (up to 80 mm/month) aligns with the mid‐range of the PMIP4 estimates, while the LGM dryness magnitude matches most of the models. Despite an enhanced hydrological cycle in MP, MH simulations indicate a ∼50% precipitation increase and a greater northward extent of WAM than the MP simulations. Strengthened conditions of the WAM in the MH and MP result from a pronounced meridional temperature gradient driving low‐level westerly, Sahel‐Sahara vegetation expansion, and a northward shift of the Africa Easterly Jet. The simulated δ18Op values patterns and their relationship with temperature and precipitation are non‐stationarity over time, emphasizing the implications of assuming stationarity in proxy reconstruction transfer functions.

Authigenic carbonate and native sulfur formation in Messinian (upper Miocene) marine sediments: Sedimentological, petrographical and geochemical constraints

Carbonate concretions accompanied by elemental sulfur were found in an early Messinian (Late Miocene) marine succession of NW Italy. The rocks were studied with an integrated approach including sedimentological, petrographical, stable isotope (carbon, oxygen, and multiple sulfur isotopes), and lipid biomarker analyses. Unlike other examples from Messinian strata of the Mediterranean area, the studied carbonate and sulfur concretions did not derive from the diagenetic replacement of sulfate minerals. Three lithofacies were distinguished: a) laminated lithofacies representing aphotic carbonate stromatolites enclosing fossils of filamentous sulfide-oxidizing bacteria; b) brecciated lithofacies deriving from the brecciation of carbonate stromatolites by mud injections; c) sulfur-bearing lithofacies deriving from the precipitation of thin laminae of elemental sulfur at or close to the sediment-water interface. The carbon and oxygen stable isotope composition of authigenic carbonate minerals and lipid biomarkers indicate that the initial formation of the laminated lithofacies was favored by organoclastic sulfate reduction in the shallow subsurface close to the sediment-water interface, producing sulfide that sustained dense microbial mats of sulfide-oxidizing bacteria at the seafloor. Calcification of the mats and consequent formation of stromatolites were possibly favored by nitrate-driven sulfide oxidation at the seafloor. The subsequent brecciation of the stromatolites was apparently the consequence of sulfate-driven anaerobic oxidation of methane (SD-AOM) in an underlying sulfate-methane transition zone (SMTZ). Focused fluid flow from below, possibly resulting from destabilization of gas hydrates, was not only responsible for the brecciation of the stromatolites, but also for the delivery of bicarbonate ions and the consequent precipitation of additional, 13C-depleted calcite (δ13C values as low as −52‰). Along with bicarbonate, also hydrogen sulfide was produced by SD-AOM at the SMTZ and was transported upwards. The oxidation of hydrogen sulfide at or close to the seafloor promoted the formation of elemental sulfur characterized by δ34S values and Δ33S values close to coeval seawater sulfate.

Characterizing the stable oxygen isotopic composition of the southeast Indian Ocean

New seawater stable oxygen isotope (δ18O) samples were collected from the southeast Indian Ocean as part of the Coring to Reconstruct Ocean Circulation and Carbon dioxide Across 2 Seas (CROCCA-2S) expedition in November – December of 2018. These data fill a gap in the δ18O sampling coverage of the southern Indian basin, providing new insights into the hydrologic and oceanographic processes influencing the δ18O distribution of the region and its relationship to salinity in the upper ocean. Our surface ocean data (<100 m)—in combination with decades of observations from the broader south Indian Ocean—show distinct δ18O – salinity characteristics on either side of ∼85°E. The balance between evaporation and precipitation yields a strong, robust δ18O – salinity relationship west of 85°E (δ18O = 0.50(±0.01) * S – 17.2(±0.22)). However, within the mesoscale eddy field initiated by the Leeuwin Current further east (∼85–120°E), our observations fall along a mixing line between the southwest Indian Ocean and data collected from the Australian coastal margin, illustrating for the first time how the unique eastern boundary system of the south Indian Ocean drives regional-scale variability in the δ18O – salinity relationship of the surface ocean. A comparison between our observations in the shallow subsurface (100–1000 m) and those from neighboring surveys reinforces this upper ocean connection across the Indo-Australian basin. Antarctic Intermediate Water from the Indian Ocean can be isotopically distinguished from the more regional Tasman Intermediate Water occupying the South Australian Bight, suggesting exchange between the two regions is most prevalent at surface and mode water depths. In deeper waters (> ∼1500 m), we observe a notable 0.87‰ spread in δ18O. This variability may represent interactions between distinct deep water masses in the region, although additional data are needed to confirm. Overall, our data provide a new look at the hydrography and isotopic chemistry of the southeast Indian Ocean, emphasizing the impact of the region's mesoscale eddy field and its interconnectivity with neighboring basins.

Advances in Limnogeology: The lake‐basin‐type model revisited 25 years after…anomalies, conundrums and upgrades

AbstractThe lake‐basin‐type model classified the stratigraphic record of ancient lake systems according to rates of potential accommodation relative to sediment + water supply. The model convolved all modes and paths of water supply (direct fall, surficial, subsurface) with amounts and types of sediment supply (clastic, biogenic, chemical) into a single basin‐filling volume term (sediment + water); its major strength was its widespread applicability. This was supported by subsequent investigations confirming the utility of this approach, but it also revealed some important limitations due to simplifications in the original model. The model has been expanded here to address all inland waters (lakes, ponds, wetlands, playas) as well as adding two major subdivisions of the sediment + water term: (1) water supply paths and (2) the volume of water supply relative to sediment supply. Water supply flow paths in the subsurface are subdivided into ‘throughflow’, ‘recharge’ and ‘discharge’. Each of these groundwater hydrology states can be defined quite precisely by the ratio of net outflow to inflow, from persistently open to consistently closed. These paths can be deciphered using stable carbonate and oxygen isotope composition of primary lacustrine limestones, detailed sedimentology, stratigraphy, palaeontology and mineralogy. Distinguishing water supply paths provides additional insights into playa systems and the occurrence and character of evaporites and carbonates. The volume ratio of water to sediment supply most directly influences the water depths of lakes, ponds and wetlands, which affect water body hydrodynamics and ecosystem behaviour as well as the details of stratal stacking and depositional sequences. It helps fine‐tune estimates of the distribution of sediment texture, bedding, composition and organic matter content. The aim of this contribution is to address questions about the fundamental types of inland water bodies and to explain the new lake‐basin subtypes and provide examples that illustrate their potential to enable higher‐resolution, robust analysis of inland water systems and their stratigraphic records.

Middle Eocene Climatic Optimum (MECO) and its imprint in the continental Escanilla Formation, Spain

Abstract. The Middle Eocene Climatic Optimum (MECO) is a global warming event mainly recognized in the marine domain and described less in the terrestrial environment. Here we present a comprehensive geochemical record of the MECO from the Escanilla Formation, a fluvial sedimentary succession in the southern Pyrenees, Spain, based on a suite of sampled paleosols, fluvial stromatolites, and pedogenic nodules. Our multiproxy approach involves using carbon and oxygen stable isotope compositions to identify the regional preservation of the MECO, calculate chemical weathering intensity and mean annual precipitation, perform clumped isotopes on carbonates, and identify clay mineralogy assemblages of paleosols. Results indicate that the Middle Eocene interval in the southern Pyrenees was characterized by low weathering rates under warm and arid climatic conditions. This is further supported by the presence of smectite, palygorskite, illite, and chlorite, which suggest seasonal rainfall but under generally dry conditions resulting in weak chemical weathering. Importantly, a negative organic carbon isotopic excursion indicates the regional, terrestrial impact of the MECO, highlighting that fluvial sedimentary successions even in active foreland basins can represent particularly interesting terrestrial archives of past changes in global climate.

Impact of Noodle Formulation, Boiling Methodology and Their Interactions on Stable Hydrogen and Oxygen Isotope Ratios.

Stable isotopes are commonly utilized for the geographical origin verification of foods, including wheat. However, assessing processed products poses a greater challenge due to the alterations that take place during processing and which have not been fully elucidated yet. In the current study, the effects of the formulation (the mass ratios of gluten to starch), boiling process and their interaction on the stable hydrogen (δ2H) and oxygen (δ18O) isotopic ratios of wheat noodles were evaluated. The δ2H and δ18O of noodles with different formulations (the mass ratios of gluten to starch) as raw materials, in uncooked and cooked (boiled in water) noodles, were examined. The results indicated that the δ2H of the boiled noodles ranged from -80.1‱ to -46.8‱ and were significantly lower than those of the raw materials, which ranged from -73.0‱ to -39.2‱, and the uncooked noodles, which ranged from -73.3‱ to -39.6‱. Oppositely, 18O was enriched in the boiled noodles, ranging from 27.7‱ to 31.3‱, compared with the uncooked noodles, ranging from 28.4‱ to 29.6‱. In addition, a significant interaction effect between the formulation and the boiling process was recorded for δ18O. This study shows that the hydrogen and oxygen stable isotopic compositions of noodles were significantly changed during the boiling process, and the isotopic fractionation varies with the different formulations.

The Pleistocene high-elevation environments between 2.02 and 0.6 Ma at Melka Kunture (Upper Awash Valley, Ethiopia) based upon stable isotope analysis

Pleistocene environments are among the most studied issues in paleoecology and human evolution research in eastern Africa. Many data have been recorded from archaeological sites located at low and medium elevations (≤ 1500 m), whereas few contexts are known at 2000 m and above. Here, we present a substantial isotopic study from Melka Kunture, a complex of prehistoric sites located at 2000—2200 m above sea level in the central Ethiopian highlands. We analyzed the stable carbon and oxygen isotopic composition of 308 faunal tooth enamel samples from sites dated between 2.02 and 0.6 Ma to investigate the animal diets and habitats. The carbon isotopic results indicate that the analyzed taxa had C4-dominated and mixed C3-C4 diets with no significant diachronic changes in feeding behavior with time. This is consistent with faunal and phytolith analyses, which suggested environments characterized by open grasslands (with both C3 and C4 grasses), patches of bushes and thickets, and aquatic vegetation. However, palynological data previously documented mountain forests, woodlands, and high-elevation grasslands. Additionally, the carbon isotopic comparison with other eastern African localities shows that differences in elevation did not influence animal feeding strategies and habitat partitioning, even though plant species vary according to altitudinal gradients. In contrast, the oxygen isotopic comparison suggests significant differences consistent with the altitude effect. Our approach allows us to detect diverse aspects of animal behavior, habitat, and vegetation that should be considered when reconstructing past environments.

Stable hydrogen and oxygen isotope geochemistry of Fe3+-rich, interstratified clay minerals from seafloor hydrothermal sites

Chemical and structural complexities in interstratified clay minerals arising from mineral transformations and evolving formation conditions should be recorded in their stable isotope signatures. This paper examines the controls on the stable hydrogen and oxygen isotope compositions of Fe3+-rich, glauconite-nontronite, talc-nontronite and talc-saponite from submarine hydrothermal sites, with a focus on the Red Sea Atlantis II Deep. Values of δ18O became less positive in the order nontronite > glauconite-nontronite > talc-nontronite > talc-saponite, yielding oxygen isotope temperatures ranging from ∼40 °C for nontronite to ∼135–300 °C for talc-saponite. These clay minerals have low δ2H, ranging from −145 to −127‰ for talc-nontronite, −135‰ for nontronite, −129 to −85‰ for glauconite-nontronite, and −95 to −59‰ for talc-saponite. The clay mineral-water hydrogen isotope fractionation is strongly affected by Fe3+ content, with increasing structural Fe3+ causing progressive weakening of O-H bonds. The large ionic radius and high charge of Fe3+ cause it to be more poorly shielded by surrounding oxygen from hydroxyl hydrogen relative to other octahedral or tetrahedral cations. The resulting lengthening of O-H bonds favours 1H over 2H. Our results confirm that Fe-rich phases, such as nontronite-glauconite, are produced at the lowest temperatures and Mg-rich phases, such as talc-saponite, are formed at the highest temperatures in seafloor hydrothermal environments. Evolving fluid chemistry and fluid pathways during clay mineral formation are also important. Glauconite layer formation from precursor nontronite likely occurred during diagenesis; redox changes causing Fe-reduction and an increase in layer charge facilitated interlayer K-fixation. Talc-saponite compositions vary with temperature and fluid chemistry, forming at variable sediment depths depending on access to the hottest hydrothermal fluids.