High δ18O Research Articles

Tibetan Plateau growth linked to crustal thermal transitions since the Miocene

Abstract The topographic transition of central–northern Tibet since the early Miocene has created a consistently high and flat plateau similar to that of today. However, to date, the associated deep crust and/or /mantle events are poorly understood, mainly due to an early Miocene metamorphic–magmatic lull within the Qiangtang Block. To address this issue, we undertook a study of crustal xenoliths and zircon xenocrysts in 6.0–2.3 Ma lavas in the Qiangtang Block. The occurrence of 22.6–12.9 Ma high-temperature–low-pressure granulite xenoliths implies that the middle crust of the block has been very hot since that time. Zircon xenocrysts and granitic xenoliths from 6.0–2.3 Ma lavas were studied and shown to have high δ18O values, which supports Miocene crustal melting and the formation of unexposed, coeval felsic plutons. Combined with paleoelevation data from the Tibetan Plateau, our results suggest that the early Miocene cold–hot thermal transition of the middle–lower crust was near-synchronous with topographic evolution from high-relief mountains to a flat plateau, which supports crustal flow as the main topographic smoothing mechanism for central–northern Tibet.

Neoproterozoic Metabasalts of the Tyya Complex of the Olokit Rift Trough (Baikal–Muya Belt): Composition, U–Pb Age, Isotope-Geochemical Characteristics, and Geodynamic Effects

Abstract—We present results of new mineralogical, geochemical, geochronological, and isotope (Sm–Nd, Rb–Sr, and O) studies of the volcanic rocks of the Tyya complex in the Olokit trough. These are normal tholeiitic basalts and scarcer basaltic andesites forming a fractionated series with mg# = 45–65, medium TiO2 contents (0.73–1.62 wt.%), low P2O5 contents (0.04–0.25 wt.%), and a significant domination of Na over K (Na2O/K2O = 2.1–50.0). The rocks are metamorphosed to greenschists, which are composed of chlorite, actinolite, epidote, and albite with quartz, titanite, ilmenite, and magnetite impurity. The metabasalts have is an age of 915 ± 5 Ma (zircon U–Pb dating) and are characterized by wide variations in εNd(T) (–3.5 to –11.9) and 87Sr/86Sr (0.70602–0.70732) and high δ18O values (9.0–15.2‰) as compared to the mantle ones. According to the isotope-geochemical characteristics, the studied metabasalts have features of both IAB and E-MORB. The Tyya metabasalts might have resulted from the melting of the lithospheric mantle with a subductional component. Comparison of the studied rocks with volcanic rocks of recent geodynamic settings shows their similarity to basalts of back-arc basins. The Tyya metabasalts might belong to a back-arc basin of the late Mesoproterozoic Nyurundukan island arc system.

Ultramafic-alkaline-carbonatite Tajno intrusion in NE Poland: A new hypothesis about the massif formation and related mineralization

This manuscript presents results of the newest petrographic, mineralogical and bulk chemical, as well as H, C and O stable isotope study of carbonatites and associated silicate rocks from the Tajno Massif (NE Poland). The Tajno Intrusion is a Tournaisian-Visean ultramafic-alkaline-carbonatite body emplaced within the Paleoproterozoic rocks of the East European Craton (EEC). Carbonatites of the Tajno Massif can be subdivided into the calciocarbonatite (calcite), ferrocarbonatite (ankerite), and breccias with an ankerite-fluorite matrix. Due to location at the cratonic margin and abundance in the REE, Tajno classifies (Hou et al., 2015) as the carbonatite-associated REE deposit (CARD), and more precisely as the Dalucao-Style orebody (the breccia-hosted orebody). High Fe2O3 (13.8 wt%), MnO (2.1 wt%), total REE (6582 ppm), Sr (43895 ppm), Ba (6426 ppm), F (greater than10000 ppm) and CO2 contents points for the involvement of the slab – including pelagic metalliferous sediments – in the carbonatites formation. Spatial relations and Sr isotope composition ((87Sr/86Sr)i = 0.7043–0.7048; Wiszniewska et al., 2020) of alkali clinopyroxenite and syenite suggest that these are products of differentiation of the magma, generated by the initial melting of the SCLM due to influx of F-rich fluids from subducted marine sediments. Carbonatites Sr isotope composition ((87Sr/86Sr)i = 0.7037–0.7038), and Ba/Th (16–20620) and Nb/Y (0.01–6.25) ratios, link their origin with a more advanced melting of the SCLM, triggered by CO2-rich fluids from the subducted AOC and melts from sediments. The Tajno Massif – and coeval mafic-alkaline intrusions – age, high potassic composition, and location along the craton margin nearly parallel the Variscan deformation front, are suggesting Variscan subduction beneath the EEC. The oxygen isotope compositions of clinopyroxene (δ18O value = 5.2‰) and alkali feldspar (δ18O value = 5.7‰), from alkali clinopyroxenite and foid syenite, respectively, are consistent with mantle-derived magmas. Isotopic compositions of carbonatites and breccias (carbonate δ18O = 8.7‰ to 10.7‰; δ13C = -4.8‰ to −0.4‰) span from values of primary carbonatites to carbonatites affected by a fractionation or sedimentary contamination. The highest values (δ18O = 10.7‰; δ13C = -0.4‰) were reported for breccia cut by numerous veins confirming post-magmatic hydrothermal alteration. The lowest carbonate δ18O (9.3‰ to 10.7‰) and δ13C (−5.0‰ to −3.8‰) values are reported for veins in alkali clinopyroxenites, whereas the highest δ18O (11.2‰) and δ13C (−1.2‰ to −1.1‰) values are for veins in syenites and trachytes. Isotopic composition of veins suggests hydrothermal origin, and interaction with host mantle-derived rocks, as well as country rocks. In silicate rocks of the Tajno Massif, fluid influx leads to the development of Pb, Zn, Cu, Ag, Au sulfide mineralization-bearing stockwork vein system, with carbonate, silicate and fluorite infilling the veins. Bulk-rock contents of molybdenum (925 ppm), rhenium (905 ppb) and palladium (29 ppb) are notable. The Re-rich molybdenite association with galena, pyrite and Th-rich bastnäsite in carbonate veins is similar as in Mo deposits associated with carbonatites, implying the mantle source of Mo and Re.

The importance of alkyl nitrates and sea ice emissions to atmospheric NO<sub><i>x</i></sub> sources and cycling in the summertime Southern Ocean marine boundary layer

Abstract. Atmospheric nitrate originates from the oxidation of nitrogen oxides (NOx=NO+NO2) and impacts both tropospheric chemistry and climate. NOx sources, cycling and NOx to nitrate formation pathways are poorly constrained in remote marine regions, especially the Southern Ocean, where pristine conditions serve as a useful proxy for the pre-industrial atmosphere. Here, we measured the isotopic composition (δ15N and δ18O) of atmospheric nitrate in coarse-mode (>1 µm) aerosols collected in the summertime marine boundary layer of the Atlantic Southern Ocean from 34.5 to 70∘ S and across the northern edge of the Weddell Sea. The δ15N–NO3- decreased with latitude from −2.7 ‰ to −42.9 ‰. The decline in δ15N with latitude is attributed to changes in the dominant NOx sources: lightning at the low latitudes, oceanic alkyl nitrates at the mid-latitudes and photolysis of nitrate in snow at the high latitudes. There is no evidence of any influence from anthropogenic NOx sources or equilibrium isotope fractionation. Using air mass back trajectories and an isotope mixing model, we calculate that oceanic alkyl nitrate emissions have a δ15N signature of -21.8±7.6 ‰. Given that measurements of alkyl nitrate contributions to remote nitrogen budgets are scarce, this may be a useful tracer for detecting their contribution in other oceanic regions. The δ18O–NO3- was always less than 70 ‰, indicating that daytime processes involving OH are the dominant NOx oxidation pathway during summer. Unusually low δ18O–NO3- values (less than 31 ‰) were observed at the western edge of the Weddell Sea. The air mass history of these samples indicates extensive interaction with sea-ice-covered ocean, which is known to enhance peroxy radical production. The observed low δ18O–NO3- is therefore attributed to increased exchange of NO with peroxy radicals, which have a low δ18O, relative to ozone, which has a high δ18O. This study reveals that the mid- and high-latitude surface ocean may serve as a more important NOx source than previously thought and that the ice-covered surface ocean impacts the reactive nitrogen budget as well as the oxidative capacity of the marine boundary layer.

Evolution of the Neoarchean lower crust beneath Western Liaoning of the North China Craton: Evidence from U-Pb-Hf-O isotopes in zircon

A comprehensive analysis of U-Pb geochronology and Hf-O isotopes of zircons from granulite xenoliths hosted in the Cretaceous basalt and zircon xenocrysts in the Jurassic andesitic volcanic rocks of Western Liaoning within the North China Craton (NCC) provides an opportunity to understand the evolution of the lower crust in the region. Zircons from these granulites yield 207Pb/206Pb ages of 1678–2646 Ma and consistent upper intercept ages of 2514–2544 Ma. In contrast, zircon xenocrysts yield two periods of 207Pb/206Pb ages of Neoarchean (2500–2591 Ma) and Paleoproterozoic (1733–2499 Ma), which produce consistent upper intercept ages of 2543–2549 Ma and lower intercept ages of 1761–1789 Ma, respectively. The Neoarchean zircons from the granulites and xenocrysts generally have similar positive εHf(t) values resulting in Hf depleted mantle model ages (TDM) of 2.5–2.9 Ga with a mode of 2.7 Ga and variable δ18O (5.05‰ to 8.81‰) values. However, the δ18O values of zircons from the xenocrysts are a little bit higher than those of zircons from granulites (peak of 8.00‰ for the xenocrysts and 6.00‰ for the xenoliths). Some Paleoproterozoic zircon xenocrysts have negative εHf(t) values (−12.8 to −9.39) and similar 176Hf/177Hf ratios and δ18O values to those of the Neoarchean zircons, whereas the other Paleoproterozoic zircons show chondritic εHf(t) (−4.34 to 1.16) and higher δ18O (7.70‰ to 10.19‰) values. All these data demonstrate that the primordial lower crust beneath Western Liaoning originally formed in the Neoarchean, perhaps around 2.6–2.7 Ga. The ubiquitous presence of ca. 2.5 Ga magmatic zircons illustrates that an important melting event indeed happened in the lower crust beneath Western Liaoning, similar to the thermal event throughout the entire NCC. In addition, the Neoarchean lower crust underwent another important thermal event during the Paleoproterozoic, which was considered to be associated with the final amalgamation of the Western and Eastern blocks of the NCC. Thus, we can conclude that there indeed existed an Archean lower crust beneath the Western Liaoning until to the Late Mesozoic, and this Archean lower crust underwent a similar multiple-stage (Neoarchean and Paleoproterozoic) evolutionary history as those throughout the NCC.

Palaeoceanography of the Japan Sea Across the Mid‐Pleistocene Transition: Insights From IODP Exp. 346, Site U1427

AbstractLarge‐scale atmospheric circulation patterns, such as the East Asian monsoon, have been proposed as possible feedbacks of the mid‐Pleistocene transition (MPT). Marine sediments of the Japan Sea (JS) record variations in the East Asian monsoon over long timescales and may be crucial for understanding of the MPT. To interpret these sediments correctly an understanding of the JS palaeoceanography is necessary. So far, the JS palaeoceanography has been extrapolated across the MPT from studies of the most recent glacial‐interglacial cycles. These suggest a good connection and unrestricted water‐mass exchange with the open ocean during interglacial sea‐level highstands, while during glacial sea‐level lowstands the JS is nearly isolated. Glacial isolation often results in poor carbonate preservation and unusually low oxygen isotope (δ18O) ratios from low‐saline/low‐δ18O waters accumulating in the basin. Using the sediments of Integrated Ocean Drilling Program (IODP) Site U1427, a shallow‐water site in the southern JS, we present a continuous foraminiferal δ18O record encompassing the MPT. This record shows the JS‐typical low glacial δ18O values in the late phase of the MPT, across Marine Isotope Stages (MIS) 24‐17, while earlier MPT glacials, across MIS 39‐25, are characterized by high δ18O values. We propose that high glacial δ18O values are the result of an improved connection between the shallow, southern JS and adjacent ocean during early MPT glacials. The impact of this palaeoceanographic mode, if continued to deep‐water sites, would make the interpretation of dark/light sediment layers as glacial/interglacial deposits uncertain.

Petrography, geochemistry and genesis of dolomites in the upper Cambrian Sanshanzi Formation of the western Ordos Basin, northern China

The Cambrian Sanshanzi Formation is an important dolomite horizon in the Ordos Basin of northern China, and is well exposed in the western part of that basin. This paper focuses on the petrography and geochemistry of dolomites in this formation with an aim to provide a better understanding of their genesis. Reflux seepage, burial, and hydrothermal dolomites are identified in the Sa nshanzi Formation. Reflux seepage dolomites include dolomitic grainstone and powder crystal dolomite, and have high Sr and low Mn contents, a high CaO/MgO ratio and low δ13 C values, and the reflux seepage dolomitization was formed by high-salinity and Mg2+-enriched fluid penetrates downward into the limestone. Burial dolomites are the main type of dolomite distributed through the Sanshanzi Formation. Burial dolomites are composed of fine- to coarse-crystal dolomite and have low Sr and high Mn contents, a low CaO/MgO ratio, and high δ13C values. The features of high salinity, strong reducibility and negative δ18O values indicate the burial dolomites formed under high-temperature and high-pressure conditions, the burial dolomitization was due to the limestone deeply buried during the later diagenetic stage. Hydrothermal dolomites are mainly characterized by saddle dolomite features, a positive δEu anomaly, and low δ18O values, and the hydrothermal dolomitization occurred during Mg2+-rich hydrothermal fluid invaded. The reflux seepage and hydrothermal dolomite developed in local areas, with the hydrothermal dolomite affected by the distribution of faults.

Lithium, Oxygen and Magnesium Isotope Systematics of Volcanic Rocks in the Okinawa Trough: Implications for Plate Subduction Studies

Determining the influence of subduction input on back-arc basin magmatism is important for understanding material transfer and circulation in subduction zones. Although the mantle source of Okinawa Trough (OT) magmas is widely accepted to be modified by subducted components, the role of slab-derived fluids is poorly defined. Here, major element, trace element, and Li, O and Mg isotopic compositions of volcanic lavas from the middle OT (MOT) and southern OT (SOT) were analyzed. Compared with the MOT volcanic lavas, the T9-1 basaltic andesite from the SOT exhibited positive Pb anomalies, significantly lower Nd/Pb and Ce/Pb ratios, and higher Ba/La ratios, indicating that subducted sedimentary components affected SOT magma compositions. The δ7Li, δ18O, and δ26Mg values of the SOT basaltic andesite (−5.05‰ to 4.98‰, 4.83‰ to 5.80‰ and −0.16‰ to −0.09‰, respectively) differed from those of MOT volcanic lavas. Hence, the effect of the Philippine Sea Plate subduction component, (low δ7Li and δ18O and high δ26Mg) on magmas in the SOT was clearer than that in the MOT. This contrast likely appears because the amounts of fluids and/or melts derived from altered oceanic crust (AOC, lower δ18O) and/or subducted sediment (lower δ7Li, higher δ18O and δ26Mg) injected into magmas in the SOT are larger than those in the MOT and because the injection ratio between subducted AOC and sediment is always >1 in the OT. The distance between the subducting slab and overlying magma may play a significant role in controlling the differences in subduction components injected into magmas between the MOT and SOT.

Investigating hydrometeorology of the Western Himalayas: Insights from stable isotopes of water and meteorological parameters

The Himalayas govern the hydrometeorology of the entire Indian subcontinent and feed 19 major rivers on which a large population is dependent. Despite its vast socio-economic relevance, there exist knowledge gaps in the detailed understanding of Himalayan hydrometeorology. The present study attempts to understand subtle hydrometeorological processes concerning precipitation in Western Himalaya (WH), which is further complex due to several mountain ranges in addition to the Great Himalaya and varied land cover. In this backdrop, oxygen and hydrogen isotopic analyses of daily precipitation samples collected from Jammu was done. Major processes and features identified are: (1) Raindrop re-evaporation is dominant, indicated by lower δ18O – δD regression slope (6.6 ± 0.2). (2) Maximum raindrop re-evaporation occurs during Western Disturbances (WD), evident from the low d-excess (<10‰) and high δ18O (>0‰) with low rainfall. (3) Contrary to expectations, negligible moisture is derived from the Mediterranean region (MR) through WD. In contrast, maximum moisture (~97%) is derived from local terrestrial sources, estimated through backward wind trajectories and confirmed by vertical wind velocity (ω) and OLR. (4) The Arabian Sea (AS) moisture contributes more (~11%) compared to the Bay of Bengal (BOB) moisture (~8%) during the monsoon. (5) Isotopically, the most depleted precipitation during July-Aug is associated with continental recycling from the Indo-Gangetic plains and not with the BOB moisture. (6) Isotopic enrichment in precipitation towards end of monsoon is attributed to raindrop re-evaporation, lesser rainout fraction and moisture derived from enriched local sources. (7) Annually, 87% of the moisture for precipitation is derived through continental recycling, and only 13% of the moisture is derived through marine sources (11% from AS and 2% from BOB). Moisture from MR (<0.1%) is negligible.

Decoupling of isotopes between magmatic zircons and their mafic host rocks: A case study from the ca. 830 Ma Jiabang dolerite, South China

Zircon is widely used to constrain the crystallization age and magma source of the host rocks. However, in recent years an increasing number of studies have demonstrated that Hf-O isotopes of zircon grains are not always in equilibrium with the whole-rock isotopes of the host rocks. Here we present a case from the Jiabang dolerite from southeastern Guizhou Province, South China, to demonstrate the decoupling of isotopes between magmatic zircons and their mafic host rocks as a result of limited crustal contamination. In-situ SIMS U-Pb zircon dating shows that the Jiabang dolerite formed at 838 ± 7 Ma. The Jiabang dolerite has low SiO2 of 47.6 – 54.2 wt%, high MgO of 4.93 – 15.6 wt%, and Fe2O3 of 8.51 – 11.9 wt%, typical of mantle-derived magma. All whole-rock samples in this study have depleted Nd-Hf isotopes, with εNd(t) values ranging from + 2.6 to + 3.8 and εHf(t) values from + 6 to + 10, with whole-rock εNd(t) values similar to those of contemporary OIB-like mantle-derived melts in South China. Such whole-rock chemical features imply that the source of the Jiabang dolerites is mantle-dominated and that any crustal contribution should be insignificant. However, zircons from the dolerite display negative εHf(t) values of −6 to −1 and high δ18O values of 9.7–11.5 ‰ that are similar to those of the Guibei granitoids in the study area, suggesting a crustal origin. Our AFC simulation based on Magma Chamber Simulator (MCS) demonstrates that AFC played a limited role in the generation of the Jiabang dolerite and indicates that the decoupling between zircon Hf-O and the whole-rock Nd-Hf isotopes likely reflects a contribution from wallrock-derived melts during emplacement of the dolerites. We propose that the zircons crystallized in interstitial liquids that were contaminated by crustal-derived fluids/melts, resulting in decoupling between the whole-rock isotopes and the magmatic zircon isotopes. Due to mass balance requirements, the crustal melts/fluids would have insignificant effect on the whole-rock isotopes, but will strongly modify the Hf-O isotopes of zircons that crystallized in the interstitial liquids. This case study shows that caution needs to be applied when comparing zircon Hf-O isotopes from mafic intrusive rocks to constrain their source.

An exploration of whether Earth can be built from chondritic components, not bulk chondrites

Here, two of a range of possible models are explored that assume that: (i) two of the main chondritic components (chondrules and refractory inclusions) dominated the Earth’s building blocks, (ii) that their relative abundances differed from those of known chondrites, and (iii) that the elemental compositions of all components, as well as the isotopic compositions of refractory inclusions, resembled those of the components in carbonaceous chondrites.In terms of the elemental abundances, the chondrules can explain the moderately volatile element fractionations in the Bulk Silicate Earth (BSE), except for a few elements like F and In, while the refractory inclusions explain the BSE’s refractory lithophile element enrichment. Accretion of a CM- or EL-like late veneer reproduces the S, Se, Te and highly siderophile element abundances in the BSE. The accretion of CI- or CM-like material prior to the late veneer, along with small amounts of cometary and implanted solar wind material, can explain the BSE elemental and isotopic abundances of the noble gases as well as the elemental abundances of most other highly volatile elements (e.g., H, C and N) provided that variable fractions of them can be sequestered into the core or into hidden mantle reservoirs. This CI- or CM-like material provides an upper limit for the contribution to the BSE from matrix that is much less than in any chondrites. In the models, Si, O and S are the dominant light elements in the core, in that order, and their abundances are constrained to be consistent with the first principles calculations of Umemoto and Hirose (2020). The core compositions of the models are also consistent with most geochemical constraints.Satisfying all isotopic constraints is a challenge. The BSE Os isotopes are consistent with an EL dominated late veneer, but Ru isotopic evidence is best explained by the addition of CM-like material in the late veneer. Either CI- or CM-like material, in combination with small amounts of cometary and implanted solar wind material, can reproduce the BSE Ne, Ar and Kr isotopic compositions. CM-like, but not CI-like, material can roughly reproduce the BSE’s H and C isotopic compositions, but neither material can explain the BSE N isotopic composition. The BSE composition requires that the O isotopic compositions of the refractory inclusions (initially Δ17O ≤ −20‰) in Earth’s building blocks were reset in the nebula by interaction with high Δ17O H2O, perhaps during chondrule formation. To plot on the inner Solar System ε54Cr vs. ε50Ti (or ε48Ca) trend, either the average Cr content of the chondrules was roughly half of that assumed here or most of the CI/CM-like and refractory-inclusion-rich materials were accreted late.

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.

Seasonal and Inter-Annual Variations of Stable Isotopic Characteristics of Rainfall and Cave Water in Shennong Cave, Southeast China, and Its Paleoclimatic Implication

Speleothem calcite stable oxygen isotope (δ18OC) is one of the most widely used proxies in paleoclimate research, and understanding its seasonal-annual variability is very significant for palaeoclimate reconstruction. Five-year precipitation and karst cave water from 2016 to 2021 were monitored in Shennong cave, Jiangxi Province, Southeast China. The local meteoric water line (LMWL) is δD = 8.20 × δ18O + 13.34, which is similar to the global meteoric water line. The stable hydrogen and oxygen isotope (δD and δ18O) characteristics of precipitation and cave water were studied. δ18O and δD of precipitation and cave water show obvious seasonal variations. Lower precipitation δ18O and δD generally occur during summer and autumn compared with higher δ18O and δD values during winter and spring. Meanwhile, low precipitation δ18O values do not only appear in June–July when precipitation is the highest of the year but also appear in August–September when precipitation is limited. The back-trajectory analysis of monsoon precipitation moisture sources shows that the moisture uptake regions vary little on inter-annual scales; the water vapor of rainfall in June–July comes from the South China Sea and the Bay of Bengal, while the moisture source in August–September is mainly from the West Pacific and local area. The El Niño-Southern Oscillation is an important factor affecting the value of δ18O by modulating the percentage of summer monsoon precipitation in the annual precipitation and moisture source. The relationship between amount-weighted monthly mean precipitation δ18O and Niño-3.4 index shows that the East Asian summer monsoon (EASM) intensifies during La Niña phases, resulting in more precipitation in monsoon season (May to September, MJJAS) and lower δ18O values, and vice versa during El Niño phases.

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.

Recharge sources and hydrogeochemical evolution of groundwater in a heterogeneous karst water system in Hubei Province, Central China

Although numerous studies on hydrogeochemistry in karst catchments have been performed, some challenges in understanding their hydrogeochemical behavior remain due to the unique architecture of karst groundwater systems and the high heterogeneity of the karst medium. Here we used a multicriteria approach combining hydrodynamic, hydrochemical, and stable isotopic tools to investigate the impact of heterogeneity on karst hydrogeochemistry in a mountainous area of Hubei Province, Central China. Our results showed that groundwater was dominated by Ca2+, Mg2+, HCO3−, and SO42−, and typically characterized by HCO3–Ca·Mg type in the shallow aquifer and by SO4–Ca·Mg type in the deep aquifer. With increasing circulation depth, water-rock interactions and the heterogeneity of the karst medium produced water with increased Ca2+, Mg2+, Sr2+, and SO42− concentrations, and also increased the dissolved total solids. Calcite, dolomite, and gypsum dissolution in local and intermediate flows were important geochemical processes in groundwater evolution, and calcite precipitation occurred in the regional flow due to dedolomitization, supported by PHREEQC modeling of minerals. The decreased major ions and increased nitrate with increasing δ18O due to the dilution of shallow groundwater by high δ18O recharge and nitrogen fertilizer required during the summer rainy season were found relying on Plots of δ18O vs major ions, which offer an alternative approach to learn seasonal effects of hydrogeochemistry. All advances presented were integrated into a hydrogeochemical conceptual model, including local, intermediate, and regional flows, hydrogeochemical evolution, horizontal and vertical karst zones, hydrochemistry, and stable isotopes of groundwater, providing a typical model for multiple karst groundwater systems in Central China.

Late Pleistocene–Holocene climatic implications of high‐resolution stable isotope profiles of a speleothem from south‐central Anatolia, Turkey

ABSTRACTHigh‐resolution 230Th ages and stable isotope (δ18O and δ13C) records from a stalagmite that grew between 39 and 2 ka in Incesu Cave located in south‐central Anatolia allow us to evaluate paleoclimate conditions for growth periods during Marine Isotope Stage 3 (MIS3) and the Holocene. High δ18O values and Heinrich events H3 and H4 are observed during the MIS3 interval. After a dry period in the Younger Dryas, low values between ca. 10 and 5.3 ka suggest a transition to wet mid‐ to early Holocene conditions. In the early Holocene, there are drier periods at 9.4 and 10.3 ka, coincident with cooling events recorded in the North Atlantic sediments and, after 5.3 ka a relatively dry late Holocene is seen. The IN‐01 isotope record is in phase with general trends of speleothem records in Turkey and the Eastern Mediterranean but differs in detail at the millennial scale. The more depleted δ18O values of IN‐01 compared to those of Eastern Mediterranean speleothems during the Holocene indicate that central Anatolian winter rainfall was isotopically influenced by the same air mass trajectories derived from the North Atlantic and Mediterranean with an isotopic rain shadow effect.

Paleoproterozoic tectonic evolution from subduction to collision of the Khondalite Belt in North China: Evidence from multiple magmatism in the Qianlishan Complex

This study presents detailed petrology, major and trace element chemistry, Sr-Nd isotope, and in-situ zircon U-Pb dating and Hf-O isotopes on granites from the Qianlishan Complex of the Khondalite Belt in North China. Zircon SIMS U-Pb data indicate three episodes of Paleoproterozoic magmatism at ca. 2.06 Ga, 1.95 Ga and 1.92 Ga. Of those, the ca. 2.06 Ga hornblende monzogranites and fine-grained potassic granites display typical features of A-type granite, represented by metaluminous affinity, high FeOT/MgO ratios (3.29–5.63), and high Ga/Al and Zr + Nb + Ce + Y values with high zircon saturation temperatures (916–1000 °C). Moreover, they have positive zircon εHf values of +0.29 to +5.01 and homogenous δ18O values of 5.11–5.87 ‰, probably derived from the partial melting of calc-alkaline granitoids without addition of supracrustal rocks. The ca. 1.95 Ga garnet-bearing granites contain peraluminous minerals of garnet and muscovite and have high ASI index (>1.1) and δ18O values (7.14–9.18 ‰), indicating S-type granite. Their negative whole-rock εNd(t) values (−2.87 to −2.81) and zircon εHf(t) values (−4.06 to +1.29) suggest a source of ancient supracrustal rocks. The ca. 1.92 Ga coarse-grained potassic granites present metaluminous and magnesian signatures of I-type granite, including low A/CNK and FeOT/MgO ratios. Their negative εHf(t) values (−5.00 to −1.64) and homogenous δ18O values (5.20–6.00%) suggest that the original magma was derived primarily from enriched lower crust. The results, combined with regional geology and previous studies, establish a prolonged magmatic-metamorphic evolution of the Khondalite Belt from subduction to collision, involving ca. 2.3–2.0 Ga subduction-related arc magmatism, ca. 1.95 Ga syn-collisional high-pressure high-temperature crustal anatexis, and ca. 1.92 Ga post-collisional magmatism and synchronous ultrahigh temperature metamorphism.

A low-temperature, meteoric water-dominated origin for smectitic clay minerals in the Chicxulub impact crater upper peak ring, as inferred from their oxygen and hydrogen isotope compositions

The peak-ring of the 66 Ma, ~180 km Chicxulub impact structure in the northern Yucatán peninsula and southern Gulf of Mexico was sampled during the International Ocean Discovery Program and International Continental Scientific Drilling Program (IODP–ICDP) Expedition 364 at Site M0077 (21.45° N, 89.95° W). Secondary clay minerals are pervasive throughout the upper peak-ring lithologies as a product of ubiquitous altered glass present throughout the impact melt and melt-bearing breccia sequence. Here we present the first detailed study of the clay mineralogy (microprobe, pXRD, spectral reflectance from 350 to 2500 nm) and isotope geochemistry (δ2H and δ18O) of the <0.2 μm size-fraction from upper peak-ring lithologies. The clay mineralogy is dominated by smectitic clay minerals, whose composition varies with stratigraphic position. Trioctahedral MgFe smectite (var. saponite) is most common in Units or Subunits 2A, 2C, 3 and 4, while a section of Subunit 2B contains a more dioctahedral, Al-rich smectite. Higher porosity regions of the lower to mid, dioctahedral smectite-dominated intervals have higher δ18O (+14.2 to +18.6‰) whereas intervals dominated by trioctahedral smectite have lower δ18O (+10.4 to +14.1‰). The range of smectite δ2H (−105 to −87‰), in comparison to that of oxygen isotopes, is proportionally much less variable and unrelated to smectite mineralogy. When combined, the oxygen and hydrogen isotope compositions of the smectitic clay minerals suggest low temperature (~20 to 50 °C) formation from meteoric water-dominated fluids. The lower end of this temperature range is below current ambient conditions, which conceivably could suggest smectite formation before much of the overlying sedimentary rocks were deposited (~56 Ma?). Calculated temperatures are generally lower than those associated with impact-generated hydrothermal alteration.Calculated δ18O and δ2H of meteoric water-dominated fluids associated with low-temperature formation of these clay minerals are lower than known for modern meteoric water in the Yucatán region. The simplest explanation for the source of these ancient fluids is meteoric water-dominated Gulf Coast brines. A more remote possibility is orogenically-driven, long-distance transport of groundwater from highlands to the east via an artesian aquifer formed in part by fractured Mesozoic rocks extending laterally beneath the impact structure.

Crustal thickening and continental formation in the Neoarchean: Geochemical records by granitoids from the Taihua Complex in the North China Craton

The Earth’s lithosphere changed dramatically in many aspects during the Neoarchean era. In this period, the crust became more felsic and shows significant diversification in the composition of granitoids. Although this change has been noticed for many years, the mechanism that drove the change remains unclear. Here we report a geochemical study of Neoarchean granitoids from the Taihua Complex in the North China Craton. Zircon U-Pb dating reveals two episodes of granitoid magmatisms at ca. 2.65 Ga and ca. 2.55–2.50 Ga. Sodic TTGs at ca. 2.65 Ga are characterized by low K2O/Na2O ratios of < 0.6, high Sr/Y and (La/Yb)N ratios, positive Eu anomalies, positive zircon ɛHf(t) values of 0.9 to 5.2, and zircon δ18O values of 6.3–6.5‰, suggesting that they were derived from partial melting of the thickened oceanic crust. In contrast, K-rich granitoids at ca. 2.65 Ga have higher K2O/Na2O ratios, low Sr/Y and (La/Yb)N ratios, negative Eu anomalies, mostly positive zircon ɛHf(t) values of −0.4 to 8.0, and high zircon δ18O values of 7.0–8.4‰. The K-rich granitoids at 2.55–2.50 Ga show similarly high Sr/Y and (La/Yb)N ratios to the sodic TTGs at ca. 2.65 Ga. All these geochemical evidences suggest that these Neoarchean K-rich granitoids were derived from partial melting of the low-T seawater-hydrothermally altered oceanic basalt at different depths. Therefore, the sodic TTG and K-rich granitoids are linked to each other through both similarity and difference in their source compositions rather than magmatic processes such as partial melting and crystal fractionation. On the other hand, high-K granites at ca. 2.55–2.50 Ga are highly siliceous, have very high K2O/Na2O ratios of 4.0–7.9, and contain many relict zircons with U-Pb ages of ca. 3.49–2.68 Ga, indicating their formation by reworking of the pre-existing continental crust. Using a filtered global geochemical database, we find that the Neoarchean is the most important period for the growth of both sodic TTGs and K-rich granitoids. The Sr/Y and (La/Yb)N ratios of these granitoids also reach the maximum during this period, suggesting global-scale thickening of the oceanic crust caused by warm plate convergence. Once the orogenic lithospheric mantle was foundered for thinning, the thickened oceanic crust experienced partial melting at different depths. This gave rise to the different types of granitoid for growth of the continental crust along convergent plate boundaries. Therefore, partial melting of the thickened crust at different depths is a viable mechanism for the compositional transition of continental crust in the Neoarchean.

Significant Increase of Continental Freeboard During the Early Paleoproterozoic: Insights From Metasediment‐Derived Granites

AbstractSubaerial continental crust plays a fundamental role in modulating the composition of the ocean, atmosphere, and biosphere, but the timing and rate of continental emergence above sea level remain unclear. Here, we use the zircon oxygen isotopic compositions of early Paleoproterozoic metasediment‐derived granitoids from the southwestern Yangtze Block to constrain the rapidity of continental emergence. Statistical analyses of compiled igneous and detrital zircon oxygen isotopic database show a rapid increase in zircon δ18O at ∼2.36 Ga. We suggest that this isotopic shift is best explained by a significant increase of continental freeboard between ∼2.43 and ∼2.36 Ga due to the increasing strength of the continental lithosphere since the late Archean, concomitantly yielding a high‐δ18O sedimentary reservoir. Subsequent melting of these high‐δ18O sediments in a variety of tectonic regimes results in high zircon δ18O.