Variations In Isotopic Composition Research Articles

Age, duration, and geochemical signatures of paleo-exposure events in Cenomanian–Santonian sequences (Sarvak and Ilam formations) in SW Iran: Insights from carbon and strontium isotopes chemostratigraphy

This study addresses the timing implications of chemical stratigraphy and chronostratigraphic correlation of unconformity-related shallow water (neritic) carbonate rock units. Cenomanian–Santonian sequences (i.e., Sarvak and Ilam formations) host considerable volumes of hydrocarbon in the Zagros region of SW Iran, which represent good examples of unconformity-related reservoirs worldwide. Geochemical traits including carbon, oxygen, and strontium isotopes of these sequences are measured in three wells from the Abadan Plain and one well from the Dezful Embayment. The altered intervals below the exposed erosional surfaces are characterized by remarkable variations in both elemental and isotopic compositions. These variations are marked by increasing in Mg, Al, Fe, Mn, and Rb elemental contents as well as decreasing in Sr and Ca elemental contents. The altered intervals show a positive shift in 87Sr/86Sr isotopic values and a negative trend in δ13Ccarb and δ18Ocarb isotopic values. The paleontological age that is available from previous studies provide a valuable basis for numerical dating of the studied intervals using the rubidium-corrected 87Sr/86Sr isotopic values. The duration of the stratigraphic hiatuses at the Cenomanian–Turonian and mid-Turonian paleo-exposures are calculated as 0.53 Myr and 2.7 Myr, respectively. The calculated durations corroborate the results of previous studies of these paleo-exposures within the Zagros region of western Iran. The concluded (carbon and Sr-isotope) chemostratigraphic correlation of the Cenomanian–Santonian sequences in the Abadan Plain and other parts of the Zagros region can provide a more reliable chronostratigraphic framework of larger scale. The latter can be used in regional correlation and interpretation of the geological-structural evolution of the Zagros region during Cenomanian–Santonian, particularly in western Iran.

Characterization of spatiotemporal patterns of soil water stable isotopes at an agricultural field

Spatiotemporal variations in soil water content (SWC) and soil water stable isotopic compositions (SWSIC; 2H/1H (δD) and 18O/16O (δ18O)) provide critical information on elucidating land surface processes across scales. Meanwhile, little is known about the spatiotemporal characteristics of SWSIC and its driving factors. Therefore, it's necessary to improve tracer techniques of SWSIC by interpreting their spatiotemporal variability patterns as well as the correlations with other factors such as texture, soil depth and vegetation. To this end, the spatiotemporal variations in SWC and SWSIC along with their controlling factors were jointly investigated based on seven field campaigns over roughly a two-year period at an agricultural field in North China Plain. Two transects, vegetated and bared, were considered. The results of vegetated transect showed that both SWC and SWSIC exhibited considerable spatiotemporal variabilities at the field scale of ~100 m, with SWSIC displaying more complex patterns. Overall, the spatial variations in SWSIC were larger in wet seasons than in dry seasons, which decreased with increasing soil depth, largely due to less impacts of precipitation inputs and soil evaporation on SWSIC dynamics at deeper depths. The temporal stability analysis (TSA) showed that there existed temporal persistence of the spatial structure of SWSIC, particularly at deeper soil depths. Moreover, the SWSIC data in our study showed that the effect of vegetation on the SWSIC dynamics was noticeable with shading effects, root distribution and water uptake, which caused much lesser degrees of soil evaporation at the vegetated transect. What's more, the representative sites for monitoring spatial average δD values were identified, demonstrating the viability of using the TSA method to estimate the spatial average SWSIC values at field scales. These findings can improve the interpretation of SWSIC data for practical applications.

Diagnoses of Antarctic Inland Water Cycle Regime: Perspectives From Atmospheric Water Vapor Isotope Observations Along the Transect From Zhongshan Station to Dome A

Water stable isotopes are crucial for paleoclimate reconstruction and water cycle tracing in Antarctica. Accurate measurement of atmospheric water vapor isotopic composition of hydrogen and oxygen is required urgently for understanding the processes controlling the atmosphere–snow interaction and associated isotope fractionation. This study presents in situ real-time measurements of water vapor isotopes along the transect from Zhongshan Station to Dome Argus (hereafter Dome A) in East Antarctica for the first time. The results reveal that the surface vapor stable isotopes of δ18O and δ D showed a gradual decreasing trend in the interior plateau region with the distance away from the coast, with significant δ18O-temperature correlation gradient of 1.61‰°/C and δ18O-altitude gradient of –2.13‰/100 m. Meanwhile, d-excess gradually arises with elevation rise. Moreover, the spatial variation of vapor isotopic composition displays three different characters implying different atmosphere circulation backgrounds controlling the inland water cycle; it can be divided as the coastal steep area below 2,000 m, a vast inland area with an elevation varied between 2,000 and 3,000 m, and high central plateau. Thirdly, observed high inland Antarctica water vapor d-excess quantitatively confirms stratosphere air intrusion and vapor derived from low latitudes by Brewer–Dobson circulation. Finally, the diurnal cycle signals of interior area water vapor isotopes δ18O, δ D, and air temperature highlighted the substantial domination of the supersaturation sublimation/condensation effect in inland, and this suggests that fractionation occurs during sublimation and vapor–snow exchanges should no longer be considered insignificant for the isotopic composition of near-surface snow in Antarctica.

Thermal springs and active fault network of the central Colca River basin, Western Cordillera, Peru

Thermal waters and vapor discharges (hot springs, geysers, solfataras, and fumaroles) are common phenomena in volcanic regions at active plate boundaries, and the Central Andes are no exception. The Colca River basin in S Peru is a highly diversified and complex thermal region with unresolved questions on the origin of thermal fluids, reservoir temperature, and connections with tectonic and/or volcanic activity. To answer these, we used hydrogeochemical analysis of 35 water samples from springs and geysers, together with isotopic (δ18O and δD) analysis, chemical and mineral studies of precipitates collected in the field around these outflows, and field observations. We aimed (1) to recognize the geochemistry of thermal waters and precipitates in the central part of the Colca River basin, (2) to identify fluid sources and their origin, (3) to estimate the temperature of a potential geothermal reservoir, and (4) to discuss the regional active tectonic and volcanic framework of this geothermal region and mutual relationships.Our results corroborate a heterogeneous and complex geothermal system in the central part of the Colca River basin, with contrasting hydrogeochemical and physical properties, variable isotope composition, different reservoir temperatures, and associated precipitates around thermal springs. Processes controlling water chemistry are closely related to the Ampato-Sabancaya magmatic chamber's activity and tectonic structures that allow complex interactions of meteoric waters with magmatic fluids and gases. With a considerable gradient of pressure owing to local relief and deep incision in the Colca Canyon, these processes led to the differentiation of the thermal waters into three main groups. (1) Chloride-rich, mainly sodium chloride, thermal waters are of meteoric origin but mature within the geothermal reservoir possibly fed by magma degassing. These waters' chemical and isotopic composition results from water-rock interaction and mixing with magmatic waters within the reservoir. These waters discharge at the bottom of the Colca Canyon and Valley, presenting a broad hydrogeochemical spectrum and highly variable mineral phases precipitating at the outflows. The reservoir temperature estimated for these waters ranges from 180 to 200 °C. The group of hottest springs and geysers at the bottom of the Colca Canyon waters are fully equilibrated, with the reservoir temperature ~ 240 °C. (2) Sulfate-rich waters are shallow meteoric waters heated by ascending gases that form an independent group referring to the local water circulation, often controlled by tectonic barriers. (3) Bicarbonate-rich waters are the intermediate meteoric waters, divided into two hydrochemical groups: waters partially equilibrated with reservoir rocks and more similar to chloride-rich waters or additionally enriched with SO4 and more similar to sulfate-rich waters.Studied thermal springs show a clear spatial correlation with active and seismogenic crustal W- to NW-tracing normal and strike-slip faults. These act as barriers to infiltrating meteoric waters, provide pathways to hydrothermal solutions and gases assisting in meteoric water heating, and yield passages for pressured by lithostatic load and heated waters to ascend to the surface.

Diel variations in chemical and isotopic compositions of a stream on King George Island, Antarctica: Implications for hydrologic pathways of meltwater

Antarctica is highly susceptible to climate and environmental change. In particular, climate change can lead to the warming of permafrost and the development of active layers in permafrost areas, resulting in variations in hydrological characteristics. This study investigated the hydrological process associated with a stream in a snow-dominated headwater catchment on King George Island, maritime Antarctica, during austral summer using the chemical and isotopic compositions. During the cold period, as the snowmelt rate decreased, the amount of new water also decreased. Hence, the electrical conductivity (EC) increased because the contribution of supra-permafrost groundwater (“old” water), which occurs in the active layer, increased more during the cold period than during the warm period. Moreover, diel variations in the stable isotopic compositions (δ18O and δD) of snowmelt (“new” water) were clearly observed in the stream water, indicating that runoff was the dominant flow path of snowmelt during the cold period. In contrast, during the warm period, the amount of snowmelt increased and the EC value decreased as a result of the dilution effect. In addition, compared with the cold period, diel variations in the isotopic compositions of the stream water were attenuated during the warm period. This attenuation effect was not due to the increased contribution of old water; instead, it was due to the contribution of new water with a low-amplitude signal in the diel variations of the isotopic compositions. Thus, the observed diel variations in the isotopic compositions of the stream water during cold and warm periods suggest that this catchment is dominated by new water. These findings are helpful for improving our understanding of climate-related changes in the hydrological pathways and water-related ecosystems of polar catchments.

Deciphering multiple ore-forming processes of the Shuangqishan orogenic gold deposit, Southeast China by in situ analysis of pyrite

Gold deposits are characteristically formed via multiple mineralization and remobilization events. Inferences on the complex formation history of such deposits from ore textures can be ambiguous in the lack of chemical and isotopic evidence as exemplified by the Shuangqishan orogenic gold deposit in Southeast China. We, therefore, conducted LA-ICP-MS spot analysis coupled with LA-MC-ICP-MS sulfur and lead isotope analysis on texturally complex gold-bearing pyrite from the Shuangqishan deposit. Gold distribution in different syn- to late-ore pyrite generations indicate initial evenly deposition of nano- to micro-inclusions of Au-Ag-Pb-Bi tellurides within pyrite grains and subsequent Au remobilization following brittle deformation and fluid-mediated coupled dissolution-reprecipitation (CDR) reactions. Gold dissolved from the pyrite during the reworking event became re-concentrated and re-precipitated as visible gold either as inclusions in pyrite or larger free gold grains within microfractures of pyrite. The distribution of Bi-Te-Pb-Ag assemblage is comparable to that of Au, illustrating that these trace metals became redistributed in a similar fashion. Early, undeformed pyrite has the least radiogenic Pb isotopic compositions. Late, porous and inclusion-rich pyrite, brecciated pyrite, and recrystallized Te-Bi minerals have variable but in general more radiogenic Pb isotope ratios (higher 206Pb/204Pb and 207Pb/204Pb ratios), which are attributed to modification and precipitation from a more evolved crustal fluid during the re-precipitation of native gold. The Pb isotopic signature of the younger pyrite generations was likely inherited from Pb released during modification of earlier proto-pyrite and a second, crustal lead source that was involved in the hydrothermal system during later remobilization. The δ34S values for the early proto-pyrite and the reworked pyrite generations within the individual vein stage are broadly similar. The variations in sulfur isotope compositions reflect the change in physicochemical conditions (temperature and oxygen fugacity) during proto-ore formation. Our study highlights the importance of metal recycling from early proto-auriferous pyrite during later reworking and gold remobilization, which led to visible gold and high-grade ore shoot formation. The extensive variations of trace element concentrations and S-Pb isotope compositions of proto and reworked pyrites record the complicated Au mobilization and deposition processes and help to understand how gold ores evolved.

Seasonal Variations of the Mercury Multiple Isotopic Compositions of Subrural and Urban Aerosols Highlight an Additional Atmospheric Hg0 Oxidation Pathway

The mechanisms triggering the large variations in the mercury (Hg) multiple isotopic compositions of atmospheric particle-bound Hg worldwide still remain unclear. The comparison of Hg isotopic compositions in aerosols collected in urban and rural areas may help distinguish the effects of natural processes from those of anthropogenic inputs. We thus investigated the Hg isotopic compositions of PM10 aerosols collected seasonally during 2015 at two monitoring stations on Montreal Island, one located downtown and the other in its westernmost subrural part, barely impacted by the city anthropogenic emissions. Our results show that, while Hg isotopic compositions present no seasonality at the urban station, possibly due to constant anthropogenic emissions, the subrural samples display clear seasonal variations, with higher Δ199Hg and Δ200Hg values (up to 0.77 and 0.12‰, respectively) during summer and close to 0‰ during the rest of the year, that cannot solely be explained by anthropogenic primary emissions. Besides, Δ200Hg measured in the subrural aerosols display a positive correlation with O3 suggesting the implication of secondary processes involving ozone. We propose that the significant summer shift in the multiple Hg isotopic compositions may reflect a transition in the corresponding Hg0 oxidation pathway, from halogens-triggered to ozone-dominated reactions. Still, this hypothesis needs to be further tested. Nevertheless, it demonstrates that Hg isotopes are effective at characterizing secondary processes that control its atmospheric budget, even at a local scale (i.e., urban vs subrural) and could thus be used to better constrain its atmospheric chemistry in various environments.

Variation in isotopic composition of precipitation with identification of vapor source using deuterium excess as tool

Variation in isotopic composition of precipitation with identification of vapor source using deuterium excess as tool

Variation in carbon isotope composition of plants across an aridity gradient on the Loess Plateau, China

Carbon isotope composition (indicated by leaf δ13C) reflects plants' strategies to balance carbon acquisition and water loss, which is directly linked to water use efficiency. At a large scale, leaf δ13C has been demonstrated to be closely related to climate and plant functional types; however, at a regional scale, its variation across an aridity gradient and species categories and its covariation with traits in leaf economic spectrum (LES) traits are still unclear. Here, a multivariate statistical analysis was carried out with 271 observations of leaf δ13C from 22 sampling sites on the Loess Plateau. Our results showed that (1) mean leaf δ13C did not differ significantly among life forms in C3 plants, with the maximum mean values occurring in geophytes (−26.31‰) and the minimum values occurring in lianas (−28.29‰); (2) the leaf δ13C responses of different life forms, families and species categories to climates were consistent for C3 plants, namely, they presented similar response rates to changes along the aridity gradient (represented by the moisture index); (3) climate, soil, species category, and leaf economic spectrum traits jointly explained 78.4% of the variation in leaf δ13C for C3 plants. Species category was the largest determinant of leaf δ13C variations for both C3 and C4 plants. Revealing the variation in leaf δ13C of the Loess Plateau plants not only helps us to understand plant adaptation strategies under arid conditions but also provides scientific support for ecological management in vegetation restoration regions.

The Zn, S, and Cl isotope compositions of mare basalts: Implications for the effects of eruption style and pressure on volatile element stable isotope fractionation on the Moon

AbstractWe compare the stable isotope compositions of Zn, S, and Cl for Apollo mare basalts to better constrain the sources and timescales of lunar volatile loss. Mare basalts have broadly elevated yet limited ranges in δ66Zn, δ34S, and δ37ClSBC+WSC values of 1.27 ± 0.71, 0.55 ± 0.18, and 4.1 ± 4.0‰, respectively, compared to the silicate Earth at 0.15, –1.28, and 0‰, respectively. We find that the Zn, S, and Cl isotope compositions are similar between the low- and high-Ti mare basalts, providing evidence of a geochemical signature in the mare basalt source region that is inherited from lunar formation and magma ocean crystallization. The uniformity of these compositions implies mixing following mantle overturn, as well as minimal changes associated with subsequent mare magmatism. Degassing of mare magmas and lavas did not contribute to the large variations in Zn, S, and Cl isotope compositions found in some lunar materials (i.e., 15‰ in δ66Zn, 60‰ in δ34S, and 30‰ in δ37Cl). This reflects magma sources that experienced minimal volatile loss due to high confining pressures that generally exceeded their equilibrium saturation pressures. Alternatively, these data indicate effective isotopic fractionation factors were near unity.Our observations of S isotope compositions in mare basalts contrast to those for picritic glasses (Saal and Hauri 2021), which vary widely in S isotope compositions from –14.0 to 1.3‰, explained by extensive degassing of picritic magmas under high-P/PSat values (>0.9) during pyroclastic eruptions. The difference in the isotope compositions of picritic glass beads and mare basalts may result from differences in effusive (mare) and explosive (picritic) eruption styles, wherein the high-gas contents necessary for magma fragmentation would result in large effective isotopic fractionation factors during degassing of picritic magmas. Additionally, in highly vesiculated basalts, the δ34S and δ37Cl values of apatite grains are higher and more variable than the corresponding bulk-rock values. The large isotopic range in the vesiculated samples is explained by late-stage low-pressure “vacuum” degassing (P/PSat ~ 0) of mare lavas wherein vesicle formation and apatite crystallization took place post-eruption. Bulk-rock mare basalts were seemingly unaffected by vacuum degassing. Degassing of mare lavas only became important in the final stages of crystallization recorded in apatite—potentially facilitated by cracks/fractures in the crystallizing flow. We conclude that samples with wide-ranging volatile element isotope compositions are likely explained by localized processes, which do not represent the bulk Moon.

Manifestation of the Late Famennian Dasberg Event in the shelf-batial transition (Pai-Khoi sequences)

Research subject. Regional manifestations of the Dasberg eustatic event in the shelf and bathyal Pai-Khoi successions. The event appears in the Lower-Middle expansa zones interval (Upper Devonian, Famennian).Aim. To evaluate the manifestations of the event in the realm of transition from the shallow-water shoal succession of the Pai-Khoi carbonate parautokhtone towards the deep-water (bathyal) successions of the Kara shale allokhtone.Materials and methods. A number of successions comprising different facies and located in different parts of Pai-Khoi were studied: the Silova-Yakha River section and Tal’beyshor Creek section (south-western Pai-Khoi), the Lymbad’yakha section and the Peschanaya River section (northern Pai-Khoi). The interpretation of facies and the reconstruction of transgression-regression couplets were conducted based on the previously developed models of shoal and bathyal sedimentation. The stratigraphic framework comprised data on conodonts, transgression acmes, and carbonate carbon isotopic record.Results. The Pai-Khoi successions comprising Lower–Middle expansa zones demonstrate four transgression-regression cycles. The transgression acme of the third cycle marks the Dasberg eustatic event. The absence of anoxia is characteristic of this event in the region under consideration. The carbonate carbon isotope record of the Silova-Yakha River section shows a structure similar to that of North American successions. Variations in δ13Cкарб were likely to be caused by climate changes and perturbations of the global oceanic circulation.Conclusions. The stratigraphic interval comprising Lower–Middle expansa or Lower–Upper expansa (expansa s. l.) zones is detected clearly in different facies. A more detailed subdivision and correlation in the region under consideration is possible on the basis of evaluating manifestations of the Dasberg eustatic event: a characteristic eustatic succession and variations of the carbonate carbon isotopic composition.

Quantifying the growth of continental crust through crustal thickness and zircon Hf-O isotopic signatures: A case study from the southern Central Asian Orogenic Belt

Abstract Accretionary orogens function as major sites for the generation of continental crust, but the growth model of continental crust remains poorly constrained. The Central Asian Orogenic Belt, as one of the most important Phanerozoic accretionary orogens on Earth, has been the focus of debates regarding the proportion of juvenile crust present. Using published geochemical and zircon Hf-O isotopic data sets for three belts in the Eastern Tianshan terrane of the southern Central Asian Orogenic Belt, we first explore the variations in crustal thickness and isotopic composition in response to tectono-magmatic activity over time. Steady progression to radiogenic zircon Hf isotopic signatures associated with syn-collisional crustal thickening indicates enhanced input of mantle-derived material, which greatly contributes to the growth of the continental crust. Using the surface areas and relative increases in crustal thickness as the proxies for magma volumes, in conjunction with the calculated mantle fraction of the mixing flux, we then are able to determine that a volume of ~14–22% of juvenile crust formed in the southern Central Asian Orogenic Belt during the Phanerozoic. This study highlights the validity of using crustal thickness and zircon isotopic signatures of magmatic rocks to quantify the volume of juvenile crust in complex accretionary orogens. With reference to the crustal growth pattern in other accretionary orogens and the Nd-Hf isotopic record at the global scale, our work reconciles the rapid crustal growth in the accretionary orogens with its episodic generation pattern in the formation of global continental crust.

Highly siderophile element and Os isotope systematics of the Cenozoic volcanic rocks from the Eastern Pontides, NE Turkey: Constraints on the origin and evolution of subcontinental mantle-derived magmas

The Cenozoic subalkaline to alkaline volcanic rocks from the Eastern Pontides Orogenic Belt (EPOB) in NE Turkey exhibit wide range of highly siderophile element (HSE) contents and Os isotope compositions that can be related to their mantle sources and parental melt evolution. Although some Eocene rocks show signs of late-stage modification, most of the studied volcanic rocks possess primary HSE and Os systematics. The Eocene volcanic rocks have largely variable PdN/IrN (~0.25–751), PdN/PtN (~0.25–87) and (187Os/188Os)i ratios (~0.113 to 0.51). The Miocene volcanic rocks have high PdN/IrN (~2.6–441), low PdN/PtN (~0.11–1.7) and radiogenic (187Os/188Os)i ratios ranging from ~0.166 to 0.52 while the Mio-Pliocene volcanic rocks show high PdN/IrN of ~7.2–29, low PdN/PtN (~1.2–2.7), and only mildly radiogenic (187Os/188Os)i values between ~0.132 and 0.177. Finally, the Quaternary volcanic rocks have low PdN/IrN (~2.3–3.6), PdN/PtN (~0.9–1.6) and radiogenic (187Os/188Os)i values ranging from ~0.133 to 0.194. Overall, the primitive mantle-normalized HSE patterns of the studied volcanic rocks are characterized by PPGE (Pt, Pd) and Re enrichment with respect to IPGE (Os, Ir, Ru). The variations in HSE and radiogenic Os isotope compositions of the relatively differentiated volcanic rocks indicate that crustal assimilation played active role in their parental melt evolution, especially in the Eocene and Miocene period. Besides, the obtained HSE data also suggest that the parental magmas of all the studied volcanic rocks were generated mainly in the S-saturated conditions. The temporal and spatial HSE and Os isotope variations observed in the studied Cenozoic volcanic rocks reflect the heterogeneity of the mantle source beneath the EPOB, which could be attributed to variations in response of different fluid/melt metasomatism of the underlying SCLM.

Role of sediment in generating contemporaneous, diverse “type” granitoid magmas

Abstract Granite typology categorizes granitoid rocks based upon distinguishing characteristics that are interpreted to indicate sources, conditions of generation, and, by implication, tectonic setting. Complexities of elemental and isotopic geochemistry, however, commonly preclude simple typological interpretation and suggest more complex petrogenetic histories. Granitoids from the Songpan-Ganzi terrane in the eastern Tibetan Plateau were emplaced within a short interval (~15 m.y.). They display mineralogical and geochemical characteristics that are consistent with a wide range of proposed typologies (I-, S-, and A-type; high Ba-Sr and adakitic variants). Despite their close spatial and temporal association, these granitoids exhibit diversity in geochemical characteristics that indicates a broad spectrum of contributing sources. Radiogenic isotope data reveal a continuum from primitive to evolved crustal compositions; i.e., 87Sr/86Sr(t) = 0.704–0.715 and εNd(t) = +2 to −11. All granitoid “types” have variable but commonly high zircon δ18O (+4.1‰ to +11.6‰) and low whole-rock Li-B-Mg isotopic ratios compared to mantle and/or seawater (δ7Li = +5.1‰ to −3.2‰; δ11B = −10.7‰ to −16.5‰; δ26Mg = −0.23‰ to −0.59‰). These stable isotopic compositions suggest that the Songpan-Ganzi granitic magmas of all “types” had contributions from sediment, ranging from minor to dominant. The highly variable isotopic compositions of the granitoids rule out a single homogeneous source for these diverse yet contemporaneous granitoids. Their compositional variability may have been significantly influenced by sedimentary contributions, and these results demonstrate the difficulty of straightforward assignment and interpretation of granitoids using conventional typology.

Magma evolution of the South China Sea basin from continental-margin rifting to oceanic crustal spreading: Constraints from In-situ trace elements and Sr isotope of minerals

The South China Sea (SCS) has undergone a complete Wilson cycle. We studied the volcanic rocks from IODP Sites U1502, U1500, U1431, U1433, and U1434 that record the exact mantle dynamics of the SCS basin from continental rifting to oceanic crustal spreading. We present in-situ major and trace element data for minerals (olivine, clinopyroxene, and plagioclase) and melt inclusions; and in-situ Sr isotopic ratios for plagioclases from the volcanic samples. All SCS volcanic rocks belong to the tholeiite series. Most Site U1431 basalts are slightly depleted in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) and similar to that of normal mid-oceanic ridge basalts (N-MORB). All clinopyroxenes and olivines, and the calculated equilibrium melts of clinopyroxene cores display depletion of LREEs and enrichment of heavy rare earth elements (HREEs) similar to N-MORB-type primary magma. Thus, these volcanic rocks are likely formed by the extension decompression partial melting of shallow depleted magma. The Sites U1502, U1500, U1433, and U1434 bulk-rocks and melt inclusions show flat LREE and HREE patterns similar to that of enriched MORB (E-MORB), likely attributed to the partial melting of a depleted mantle source with different degrees of lower continental crustal contamination. The increase in 87Sr/86Sr ratios from core to rim in Site U1500 plagioclases further prove the crustal contamination during magmatism. Site U1431 clinopyroxene and Site U1500 plagioclase show normal zoning textures and slight enrichment of incompatible elements in their rims but depletion in their cores, likely indicating significant crystallization of late-stage differentiated melts. Although some clinopyroxene and plagioclase were likely crystallized in a relatively closed environment, the existence of melt inclusions and the large Sr isotopic variations between the plagioclase rim and groundmass suggest a dynamic and open magmatic system that likely experienced melt–rock interactions and magma mixing processes. Melt supply rate changes from low to high during the SCS evolution process from initial rifting (Site U1500) to later spreading (Sites U1431 and U1433) indicating that: (1) the number of high-An plagioclase-hosted melt inclusions has gradually decreased; and (2) the zoning textures of clinopyroxene and plagioclase have gradually changed from simple to complex. The influence of mantle plumes and the upwelling of fertile subduction-related asthenospheric magma were negligible during the continental-margin breakup and initial SCS spreading, but played an important role during the late SCS spreading period, as revealed by the variation in Sr isotopic compositions of high-An plagioclase cores, the different mantle potential temperatures and source lithologies.

Tellurium stable isotope composition in the surface layer of ferromanganese crusts from two seamounts in the Northwest Pacific Ocean

We present the first report of the concentrations and stable isotope compositions of Te in the surface layer of ferromanganese crusts collected from the slope of two seamounts in the Northwest Pacific Ocean, Takuyo-Daigo and Takuyo-Daisan, which cover water depths from 1000 to 5500 m. The Te concentration in the surface layer of the ferromanganese crusts sharply decreases with increasing depth at shallow regions, but this decrease becomes more gradual at deeper regions. The Te isotope composition exhibits two trends that become (1) lighter with increasing water depth at shallow depths and (2) heavier with increasing water depth at greater depths. These profiles are the same for the two seamounts and indicate two types of correlations between the concentration and isotope composition with water depth. The turning points are located at approximately 2000 m for Takuyo-Daigo and 3200 m for Takuyo-Daisan, which likely correspond to different oxygen minimum zones. Co-precipitation processes with Fe and oxidation involving Mn may be responsible for the variation of Te concentration and isotope composition with water depth. These processes are also related to a change of the dissolved oxygen concentration in ambient seawater. The Te isotope compositions in ferromanganese crusts may therefore serve as a potential proxy for changes of the oxygen minimum zone of paleoceans.

Relationship between precipitation isotopic compositions and synoptic atmospheric circulation patterns in the lower reach of the Yangtze River

In this study, the stable isotopic compositions (δ18O, δ2H and d-excess) in daily precipitation from October 2017 to May 2020 in the lower reach of the Yangtze River (LYR) were examined to understand the relationship between stable isotopic compositions in precipitation and synoptic atmospheric circulation patterns (SACPs). Atmospheric circulations over the LYR were classified into seven patterns according to the Kirchhofer method with three, one and three SACPs in the East Asian winter monsoon period (EAWM), the monsoon transition period and the East Asian summer monsoon period (EASM), respectively. Precipitation events between most SACPs are distinguishable for either δ18O or d-excess, except for two SACPs in the EAWM, and two SACPs in the EASM. Local meteorological variables are primary factors determining the variation of isotopic compositions in precipitation for three SACPs in the EAWM. Though the precipitation-producing air masses come from different seas under two different SACPs in the EASM, i.e., the South China Sea for one and the western Pacific for the other, neither δ18O nor d-excess is distinguishable. Nevertheless, the relationships between isotopic composition and meteorological variables appear to be distinguishable between these two SACPs. The SACP dominant in the monsoon transition tends to have higher δ18O (mean value) and δ2H (mean value) in precipitation than all other SACPs. The d-excess in precipitation of this SACP is more likely altered by subcloud evaporation, and the retrieval of moisture source d-excess information should be treated with cautions. Similarly, d-excess in Meiyu rain, classified in one of the SACPs in the EASM period, is very likely altered by moisture recycling along the atmospheric moisture transport pathway, evident from the significant negative relationship between d-excess in precipitation and mean specific humidity along the backward trajectories. Thus d-exess of Meiyu rain is very likely positively deviated from its oceanic moisture source. Overall, the findings can provide valuable insights for understanding the relationship between stable isotopes in precipitation and SACPs in the East Asian monsoon region.

Spatio-temporal variation in stable isotope and elemental composition of key-species reflect environmental changes in the Baltic Sea

Carbon and nitrogen stable isotope ratios are increasingly used to study long-term change in food web structure and nutrient cycling. We retrospectively analyse elemental composition (C, N and P) and stable isotopes (δ13C, δ15N) in archived monitoring samples of two important taxa from the bottom of the food web; the filamentous ephemeral macroalgae Cladophora spp. and the blue mussel Mytilus edulis trossulus from three contrasting regions in the Baltic Sea (coastal Bothnian Sea and Baltic Proper, open sea central Baltic). The aim is to statistically link the observed spatial and interannual (8–24 years’ time-series) variability in elemental and isotope baselines with their biomass trends and to the oceanographic monitoring data reflecting the ongoing environmental changes (i.e., eutrophication and climate) in this system. We find clear differences in isotope baselines between the two major Baltic Sea basins. However, the temporal variation in Mytilus δ13C was similar among regions and, at the open sea station, mussels and algae δ13C also correlated over time, likely reflecting a global (Suess) effect, whereas δ15N of both taxa varied with local and regional dissolved nitrogen concentrations in water. δ15N in source amino acids allowed detection of diazotrophic N in Mytilus, which was masked in bulk δ15N. Finally, Cladophora N:P reflected regional nutrient levels in the water while P%, which differed for both taxa, was linked to food quality for Mytilus. This study highlights the potential of a multi-taxa and multi-stable isotope approach to understand nutrient dynamics and monitor long-term environmental changes.

Spatial distribution of stable isotopes in surface water on the upper Indus River basin (UIRB): Implications for moisture source and paleoelevation reconstruction

Appropriate characterisation of isotope signatures (δ18O and δD) of groundwater, glaciers, and surface water in the Himalayan catchment are important for refining the sources of moisture and developing a modern isotope–elevation relationship for paleoelevation reconstructions. The Indus River located in Western Himalaya is previously unexplored in respect of the variation in water isotopic composition with elevation. In this study, water samples (n = 96) were collected from mainstream, tributaries, streams, glaciers, groundwater, lakes, hot springs, and pond water during the summer season. Results showed that δ18O signatures of the mainstream, tributaries and groundwater range from −15.1‰ to −12.3‰, −16.5‰ to −12‰, −13.8‰ to −12‰, with a mean value of −13.6‰, −14.7‰, −12.8‰, respectively. There was a northward rise in δ18O by 1.30‰ per degree of latitude in the tributary waters. The d-excess and δ18O values suggested that the key source of moisture in the studied river basin was mainly from westerlies and southwest monsoons. Thus, regional moisture recycling and subcloud evaporation contributed little in supplying moisture to the precipitation. The isotopic signature (δ18O) of the tributary stream and its surrounding regions conformed to a best-fit second-order polynomial relationship between δ18O and elevation over 3500 masl. Moreover, an δ18O elevation slope of −2‰/km is also established in this study area. This isotope–elevation relationship will be highly useful and can be applied to the Indus River and other Himalayan rivers for paleoelevation reconstruction.

Replenishment and mean residence time of root-zone water for woody plants growing on rocky outcrops in a subtropical karst critical zone

Quantifying root-zone water replenishment (β) is essential to understanding how root-zone water is partitioned and released, and the associated residence time of water in Earth’s critical zone. However, dynamics of such water replenishments and mean residence time in karst environments remain poorly understood. In this study, stable isotope compositions of precipitation and plant xylem water were investigated for eight woody plant species growing on two rocky outcrops in a subtropical karst critical zone. A new method was proposed for estimating mean residence time of root-zone water with a consideration of precipitation replenishment. The results show that all these species had seasonal water sources during the growing seasons, reflecting by seasonal variations in isotopic compositions of stem xylem water and β. Rapid increases in β were generally found for most species during the early wet season. The high values of β (>80%) appeared frequently during the main growing seasons accompanied with frequent precipitation events, indicating that precipitation frequency is particularly important for these plants growing on the outcrops. Meanwhile, precipitation was likely to bypass the root zone (β = 0) after several high replenishment events, which suggests that not all precipitation events evenly contribute to root-zone water. In addition, mean residence time of root-zone water varied seasonally and differed among species, with a range from 2 to 80 days for most species. The differences of mean residence time of root-zone water among these species during the main growing seasons may suggest that less competition exists among these species in such rocky outcrops. This study provides insights into understanding vegetation response to future changes in precipitation patterns, and ecohydrological dynamics of karst ecosystems.