Decrease In δ18O Research Articles

Triple oxygen and hydrogen isotopes of glacial diamictites record crustal maturation and changing surface conditions through geologic time

The triple oxygen and hydrogen stable isotopic compositions of 24 glacial diamictite composites with depositional ages from 2.9 to 0.3 Ga are used to reconstruct the evolution of the continental crust and surface environments. The δ18O of the diamictite composites increases from the Archean to the Phanerozoic by ∼4 ‰ (3–5 ‰ in Mesoarchean to 13.5 ‰ in Neoproterozoic), a trend that is comparable to those seen in shales and granites, although the diamictites plot on the lower δ18O end of the range seen in shales. The Δ’17O0.528 decreases from −0.04 to −0.15 ‰, overlapping with shales and granites. Nine zircon separates extracted from individual diamictites also show a ∼3 ‰ increase in δ18O from 3.75 to 4.5 ‰ to 7 ‰ between 2.9 and 0.6 Ga. The δ18O of diamictites and zircon separates negatively correlates with ɛNd(i) of the diamictites, suggesting that the longer the rocks reside in the upper continental crust (the lower the ɛNd), the more they are subjected to repeated weathering cycles, thus acquiring a higher δ18O and lower Δ’17O0.528 during each cycle. On a triple O isotope plot (δ17O vs δ18O), diamictites define an array with a slope of 0.523, analogous to granites and shales.The δD values of nearly all diamictites overlap with the typical mantle and crustal ranges of −58 ± 12 ‰ and − 69 ± 24 ‰, respectively, but are on the lower end of coeval shale values. Water, contained primarily in sheet silicates, ranges from 2.85 wt% (pre-2.4 Ga) and 2.35 wt% (post-2.4 Ga), showing no temporal change, likely reflecting post-depositional reset. The youngest diamictites (0.3 Ga Dwyka Group) exhibit low δD values of −105 to −111 ‰, lower than the typical crustal values, potentially reflecting the near-polar position of South Africa at the time of their deposition, which is corroborated by the reconstructed δ18O value of −26 ‰ based on triple oxygen isotopic systematics.The chemical index of alteration (CIA) allows inverting measured δ18O and Δ’17O values of diamictite composites into a pure weathering product (wp) end member by subtracting values of coeval unweathered crust based on tzircon δ18O. This reveals that the δ18Owp also increases with time, suggesting that the increase in δ18O (decrease in Δ’17O) of the maturing crust is insufficient to fully explain the trends. Therefore, at least part of the increasing δ18Owp likely reflects increasing δ18O of the hydrosphere. Reconstructed surface temperatures around the time of diamictite deposition using δ18Owp and Δ’17Owp display a general decrease since the Archean. Given the diagenetic and post-diagenetic history experienced by the diamictites, the temporal and global trends should be interpreted with caution. Nonetheless, reconstructed δ18Owater shows an increase from −21 ‰ at 2.9 Ga to −11 ‰ at 0.6 Ga. Thus, diamictites, like shales, broadly monitor the evolution of surface conditions on Earth from warmer to colder and from lower to higher δ18Owater values of the hydrosphere.

What caused the lag between oxygen-18 and deuterium excess in atmospheric vapor and precipitation during the earlier summer season in southwest China?

A combined interpretation of oxygen-18 (δ18O) and deuterium excess (d-excess) in various archives in the Indian monsoon-dominated region may add value to the isotopic climate significance. Yet, coupling water δ18O and d-excess together is rarely discussed. Here, we present measurements for both δ18O and δ2H in water vapor and precipitation samples collected in the Nujiang River valley, an important moisture corridor for the Indian monsoon in southwestern China, aiming for an improved understanding of the hydrological cycle on seasonal scales. The observation found a significant drop occurred in the earlier summer season for δ18O and d-excess in vapor and precipitation. However, an approximately one-month lag between vapor δ18O and vapor d-excess is observed in seasonal variations, revealing different drivers for the seasonal patterns of vapor d-excess and δ18O. Spatial correlation analysis found that d-excess is mainly modulated by the relative humidity over the core moisture source over the Bay of Bengal (BOB), while the sharp decrease of vapor δ18O corresponds to the onset of the Indian summer monsoon associated with large-scale convective activities, which significantly depleted the vapor heavy isotopes. The finding of the phase difference in the seasonal isotope signals and the underlying mechanism will benefit the study in particularly using both δ18O and d-excess for an improved understanding of the hydrological processes in the Indian monsoon-dominated region, and will provide new insight into studies of paleoclimate rebuilding by using isotope-based proxy as well.

Fisheries shocks provide an opportunity to reveal multiple recruitment sources of sardine in the Sea of Japan

The abrupt decline in sardine catches in the Sea of Japan and the East China Sea (SJ-ECS) in 2014 and 2019 and the recovery in the following years call into question the current assumption that sardines in the SJ-ECS form a self-recruiting subpopulation. To test this hypothesis, we analysed otolith stable oxygen and carbon isotope profiles (δ18O, δ13C) of age-0 and age-1 sardines from 2010 and 2013–2015 year-classes captured in the SJ-ECS, as geographic markers for nursery areas. Age-0 sardines generally showed a significant ontogenetic decrease in otolith δ18O from larval to juvenile stages. However, the majority of age-1 captured in spring 2011, 2015 and 2016 showed non-decreasing otolith δ18O profiles, suggesting that the age-0 off the Japanese coast were not the main source of recruitment. Different migration groups were thus indicated: the “locals” growing up off the Japanese coast and the migrating “nonlocals”. The isotope profiles of the “nonlocals” overlapped with those of age-0 captured in the subarctic North Pacific, suggesting that they may be migrants from the Pacific, or perhaps an unobserved northward migration group in the SJ-ECS. Our results highlight the considerable uncertainty in the population structure assumed in current stock assessment models for Japanese sardine.

Neoproterozoic tectonic shift along the western margin of the Yangtze block, South China craton: Implications for its paleogeographic position during the Rodinia−Gondwana transition

Neoproterozoic igneous rocks in the Panxi region from the western margin of the Yangtze block, South China craton, record major changes in tectonic evolution and crustal thickness, and provide important constraints on the paleogeographic position and climate changes of the block in Rodinia and Gondwana reconstructions. Our new dating results show that the Pengguan complex from the Panxi region comprises ca. 743 Ma adakitic granodiorites and ca. 741 Ma A-type granites. The former is plagioclase-rich and has relatively low SiO2 content (65.22−67.36 wt%), high Mg# values (44.40−52.33), Sr/Y (23.22−61.94) and (La/Yb)N ratios (normalized to chondritic values; 13.41−24.00), with positive whole-rock εNd(t) values (+1.17 to +5.15), and high zircon εHf(t) (+5.53 to +8.83) and low zircon δ18O (4.21‰ to 5.51‰) values, indicating derivation from the partial melting of thickened juvenile mafic lower crust. By contrast, the granites are rich in K-feldspar and have high SiO2 contents (78.71−84.62 wt%), 10,000 × Ga/Al ratios (>2.6), and Fe/Mg ratios, with negative εNd(t) values (−1.47 to −0.02), high zircon εHf(t) (+2.94 to +9.77), and high saturation temperatures (828−920 °C). These features indicate fractional crystallization and partial melting of continental crust in a low-pressure, high-temperature, extensional environment. Integrated with the zircon Eu anomalies (Eu/Eu*) data and Hf-O isotopic data on pre-740 Ma magmatism in the Panxi region, we find that a rapid increase of crustal thickening (820−740 Ma) was synchronous with a gradual decrease of zircon δ18O and an increase of zircon εHf(t) values. These variations indicate an increase in the degree of hybridization between mantle sources and subducting slab-derived materials, thus generating a great number of low-δ18O melts, and continuous crustal thickening through the underplating of juvenile magmas. In addition, thinning of the crust since ca. 740 Ma, along with the gradual increase in zircon δ18O and εHf(t), suggests a shift from regional compression to extension. This documented shift, together with regional data from along the western margin of the Yangtze block, indicates a change from the well-developed and long-established Panxi convergent plate margin arc system (at least ca. 870−750 Ma) to passive-margin development and Cryogenian (720−635 Ma) glaciation. The long-lived subduction zone requires a peripheral location of South China that was linked to northern India during the Rodinia breakup. The subsequent and contemporaneous passive-margin development of the Yangtze block and India, along with the similar tectonic affinities of the Cathaysia block with India, indicate that South China lay outboard of the northern Gondwana margin during the assembly of this supercontinent.

Isotopic and mineralogic bias introduced by pulverization of aragonite

RationaleStable carbon and oxygen isotope data of biogenic and abiogenic aragonite are of fundamental relevance in paleoclimate research. Wet‐chemical analysis of such materials requires well‐homogenized, fine‐grained powder. In the present study, the effect of different grinding/milling methods on sample homogeneity and the potential risk of unintentional calcite formation and isotope shift were evaluated.MethodsShells of Arctica islandica and aragonite sputnik crystals were pulverized using a set of commonly used methods, including a hand‐held drill, a vibromill operated at various settings (with and without liquid nitrogen cooling, changes in ball diameters, frequencies, and processing durations), and an agate mortar and pestle. Stable isotope values were measured using an isotope ratio mass spectrometer operated in continuous flow mode. Identification of mineral phases was obtained by powder X‐ray diffraction (PXRD), Raman spectroscopy, and attenuated total reflectance‐Fourier transform infrared (ATR‐FTIR) spectroscopy. Calcite content was quantified by PXRD Rietveld refinement.ResultsSamples showed substantial homogeneity, in particular after vibromilling (duration 3–10 min). More vigorous grinding resulted in larger fractions of calcite (0.5–4.2 wt%) and a concomitant δ18O and δ13C decrease, specifically in bivalve shells. The only method for producing pure aragonite powder was by pounding the aragonite sputniks manually with an agate mortar and pestle.ConclusionsNone of the studied, commonly used machine‐based pulverization methods produced pure aragonite powder from samples consisting originally of aragonite. These findings have significant implications for light‐stable isotope‐based paleoclimate reconstructions. Except for abiogenic aragonite powder produced by pounding in an agate mortar, paleotemperatures would be overestimated.

A seed-borne endophyte mediates plant drought responses and intergenerational effects on seed characteristics

Global warming is expected to increase drought severity in diverse environments, impacting plant performance. Plants acclimate to drought by mechanisms like stomatal closure and osmotic adjustment. Maternally inherited symbiotic microorganisms with capacity to regulate these mechanisms have the potential to influence intergenerational plant drought responses. We studied how a seedborne endophyte affects maternal drought effects on seed germination water requirements and specialized metabolites. Isotopic analysis of seed cellulose indicated that drought led to improved water use efficiency (WUE; higher δ13C) in endophyte-free mother plants, seemingly without affecting stomatal conductance (non-significant δ18O change). Alternatively, endophyte-symbiotic plants did not exhibit a change in WUE but apparently an increase in stomatal conductance (significant δ18O decrease). Regardless of the symbiosis, drought reduced seed production but not seed size. Endophyte symbiosis improved the seed concentration of mannitol and sorbitol, but this increment was higher under drought. Maternal plant responses to drought did not increase seed germination under reduced water potential but induced dormancy in seeds from endophyte-free but not endophyte-symbiotic plants. Our findings suggest that although differential accumulation of metabolites in seeds results from how endophyte-symbiotic plants perceive and respond to drought, this response may not form part of a mechanism that would enhance seed performance of progeny under drought.

Mantle-like to low oxygen isotopes in zircon from the mid-Cretaceous high-silica granites reveal unweathered basement recycling along the present coastal area of SE China

Zircon oxygen isotope compositions can provide potential constraints on the origin of granitic magmas. In this work, we report new data of zircon UPb dating, trace elements and HfO isotopes for two types of mid-Cretaceous high-silica granites from Zhoushan archipelago in Zhejiang Province, SE China. The in-situ zircon data are carefully screened for radiation damage and post-magmatic alteration. The analyzed magmatic zircon grains indicate that the Zhoushan calc-alkaline granites (ZCAG; 101–92 Ma) have mantle-like δ18O values of 5.29–5.88 ‰, in contrast to the Zhoushan peralkaline granites (ZPAG; 91–89 Ma) with relatively low δ18O of 4.42–4.6 ‰. The slight decreases in δ18O and εHf(t) from zircon center to periphery of both groups suggest that they may have consistent oxygen isotopic compositions in their parental magma sources and have gone through assimilations to hydrothermally altered wall rocks in the shallow crust. The mantle-like δ18O values in zircon are widespread in the Cretaceous felsic rocks in the present coastal area of SE China, which, in combination Hf isotopes, indicates the possible existence of a buried unweathered Proterozoic basement in the eastern part of the Cathaysia Block. This work highlights the role of deep crustal magma sources, assimilation and post-magmatic radioactive damage on the formation of mantle-like to low δ18O signals in zircons, and has fundamental implications for understanding the formation of widespread mid-Cretaceous felsic rocks along the present coastal area of SE China.

Upper Paleocene–lower Eocene calcareous nannofossil biostratigraphy at Nukhul, West Central Sinai, Egypt

This research offers a comprehensive examination of calcareous nannofossils, isotopic ratios (δ18O and δ13C), and carbonate content spanning the critical Paleocene-Eocene transition at Gebel Nukhul. The study covers three lithostratigraphic formations: Tarawan, Esna, and Thebes. By meticulous analysis, we’ve identified five distinct calcareous nannofossil zones, including NP7/8 and NP9-12. Zone NP9 is further divided based on the lowest occurrences of specific nannofossils (Discoaster anartios, D. araneus, and/or Rhomboaster spp.), while Zone NP10 is subdivided using Tribrachiatus contortus as a marker. We discussed the significance of various calcareous nannofossil taxa in biostratigraphy, highlighting their role as valuable stratigraphic markers. Our research precisely locates the base of the Eocene at the onset of Subzone NP9b within the Esna Formation, revealing no discernible lithological changes coinciding with this boundary. Additionally, our findings align with the global record, indicating significant decreases in δ13C, δ18O, and calcium carbonate (CaCO3) content during the Paleocene–Eocene transition. These changes underscore the climatic and environmental perturbations during this pivotal geological phase. This study provides valuable insights into the stratigraphy and paleoclimatology of the Paleocene–Eocene transition, contributing to our understanding of past climatic variations on Earth.

Tree growth, wood anatomy and carbon and oxygen isotopes responses to drought in Mediterranean riparian forests

Mediterranean riparian forests have been altered by past use and are also negatively impacted by climate and hydrological droughts. However, we lack data on their historical changes in extent combined with multi-proxy, long-term assessments of tree growth and leaf gas exchange responses to climate, drought severity and river flow. These evaluations must also consider their current stand structure and the amount of lying deadwood which are proxies of river dynamics and forest maturity. To fill these research gaps, we studied four riparian tree species (Populus alba, Populus nigra, Fraxinus angustifolia and Ulmus minor) inhabiting a Mediterranean riparian forest located in north-eastern Spain. We quantified and analyzed: stand structure; lying deadwood; radial growth; relationships between growth, climate variables, the SPEI drought index and river flow; stable C (δ13C) and O (δ18O) isotopes in wood of P. alba, P. nigra and F. angustifolia; and earlywood anatomy in F. angustifolia. Mature sites were dominated by P. nigra and F. angustifolia and showed the highest amount of decayed lying deadwood. Radial growth was reduced by drought and low spring-summer river flow. We found the highest growth responses to 3- (P. nigra, r = 0.62; P. alba, r = 0.46) or 12-month SPEI (F. angustifolia, r = 0.54; U. minor, r = 0.53). The coordinated decrease in δ18O and intrinsic water-use efficiency (WUE) in P. alba and P. nigra could involve an increase in stomatal conductance rate. P. alba and P. nigra were more enriched in δ18O than F. angustifolia, suggesting the former used more enriched shallow groundwater in dry periods. The F. angustifolia WUE and P. nigra δ18O series were positively and negatively correlated with the SPEI, respectively. The F. angustifolia hydraulic diameter decreased in response to drought, whereas its vessel density and WUE were positively associated. Overall, P. nigra and F. angustifolia were the species most responsive to drought.

Terrestrial carbonate oxygen isotopes constraints on the interplay between westerlies and monsoonal rains modulating the Cenozoic climate on the northeastern Tibetan Plateau

The relative contribution of the westerlies and the Asian monsoon has played a critical role in regulating the evolution of the Cenozoic Asian interior environment in response to regional tectonic uplift and global climate change. Here, we reconstruct a long-term record of atmospheric moisture oxygen isotopes in the Asian interior using the oxygen isotopic composition of authigenic carbonates in terrestrial sediments from the Xining Basin on the northeastern Tibetan Plateau spanning the last 52 Myrs. On the VPDB scale, the average carbonate δ18O is characterised by a long-term decrease from −1.5‰ at ca. 43 Ma to −7.5‰ at 33 Ma and smaller variations from −7‰ to −8.5‰ after 33 Ma, with the exception of one period that contains samples with higher δ18O values at −6‰ at 17–14 Ma. At the same time, there is a corresponding long-term rise in average carbon isotope δ13C values from −9.5‰ at ca. 43 Ma to −3.5‰ at 33 Ma, followed by a less variable but similar variation in δ18O values after 33 Ma. The oxygen and carbonate isotopic compositions are negatively correlated before 33 Ma and positively after, suggesting a transition at ∼33 Ma in the context of atmospheric circulation coupled with regional aridity. The δ18O record can be explained by a simple binary mixing model of the atmospheric moisture between the recycling moisture with a positive δ18O value in the westerlies-dominated region and the monsoon rainfall or related moisture with a negative δ18O value from the Indo-Pacific Ocean. In that case, the first-order carbonate δ18O decrease from ca. 43 Ma to 33 Ma was primarily controlled by the retreat of the Para-Tethys Sea, which induced a remarkable decline in westerly rainfall relative to the monsoon. After ∼33 Ma, the contribution from the westerlies could be constant and the lower limits of the carbonate δ18O values are modelled by episodic increases in monsoon rainfall corresponding to enhanced Asian monsoon periods at 26–24 Ma, 17–14 Ma, and 9–8 Ma induced by the tectonic uplift of the Tibetan Plateau and the global climate. Our study, also, suggests that the Oligocene was a transitional stage of moisture sources from the dominance of Eocene westerlies to the Miocene-Quaternary monsoon in continental Asian environments.

Variable Response in Alpine Tree-Ring Stable Isotopes Following Volcanic Eruptions in the Tropics and Iceland

The importance of the stable isotopes in tree rings for the study of the climate variations caused by volcanic eruptions is still unclear. We studied δ18O, δD, δ13C stable isotopes of larch and cembran pine cellulose around four major eruptions with annual resolution, along with a superposed epoch analysis of 34 eruptions with 5-year resolution. Initial analysis of the tropical Tambora (1815 CE) and Samalas (1257 CE) eruptions showed a post-eruption decrease in δ18O values attributed to post-volcanic cooling and increased summer precipitation in Southern Europe, as documented by observations and climate simulations. The post-volcanic cooling was captured by the δD of speleothem fluid inclusion. The δ18O decrease was also observed in the analysis of 34 major tropical eruptions over the last 2000 years. In contrast, the eruptions of c. 750, 756, and 764 CE attributed to Icelandic volcanoes left no significant responses in the cellulose isotopes. Further analysis of all major Icelandic eruptions in the last 2000 years showed no consistent isotopic fingerprints, with the exception of lower post-volcanic δ13C values in larch. In summary, the δ18O values of cellulose can provide relevant information on climatic and hydroclimatic variations following major tropical volcanic eruptions, even when using the 5-year resolution wood samples of the Alpine Tree-Ring Isotope Record database.

Relief and climate influence on isotopic composition of Atlantic‐Mediterranean karst spring waters (Andalusia, southern Spain)

AbstractThe geographic location of Andalusia, bordered by the Atlantic Ocean and the Mediterranean Sea, and its great topographic and climate diversity enable the analysis of different factors that condition the isotopic composition of precipitation and groundwater. A total of 42 karst springs representative of the main carbonate aquifers were selected in order to sample the base flow waters during the summer of 2020. Isotopic analyses revealed that groundwater δ18O was between −5.04 and −9.85‰ while that of δ2H was between −26.39 and −62.69‰. Weighted mean isotopic data for precipitation, from six IAEA stations in the study area, was between −4.67 and −9.06‰ for δ18O, and between −24.93 and −58.99 ‰ for δ2H. The results showed that diffuse flow groundwaters are lighter and have higher d‐excess than meteoric waters, which may be due to preferential recharge during winter months and the sub‐cloud evaporation effect. The spatial variation of d‐excess, with average values around 14 ‰ in the Atlantic watershed and 17 ‰ in the Mediterranean watershed, has showed for the first time the effect of the Atlantic‐Mediterranean divide on the isotopic composition of spring waters. Furthermore, a progressive decrease in δ18O towards the east of the study area seems to be related to the effects of continentality and altitude. This also allowed us to conclude that, in the areas with highest rainfall, springs drain waters with a relatively higher content of heavy isotopes than expected from the regional altitudinal gradient, while springs in drier areas discharge lighter waters. These differences can be attributed to a longer recharge period in the former, which extends in the warmer months or autumn and spring.

Oxygen isotope (δ18O, Δ′17O) insights into continental mantle evolution since the Archean

Oxygen isotopic ratios are largely homogenous in the bulk of Earth’s mantle but are strongly fractionated near the Earth’s surface, thus these are robust indicators of recycling of surface materials to the mantle. Here we document a subtle but significant ~0.2‰ temporal decrease in δ18O in the shallowest continental lithospheric mantle since the Archean, no change in Δ′17O is observed. Younger samples document a decrease and greater heterogeneity of δ18O due to the development and progression of plate tectonics and subduction. We posit that δ18O in the oldest Archean samples provides the best δ18O estimate for the Earth of 5.37‰ for olivine and 5.57‰ for bulk peridotite, values that are comparable to lunar rocks as the moon did not have plate tectonics. Given the large volume of the continental lithospheric mantle, even small decreases in its δ18O may explain the increasing δ18O of the continental crust since oxygen is progressively redistributed by fluids between these reservoirs via high-δ18O sediment accretion and low-δ18O mantle in subduction zones.

Tree‐ring stable isotopes and radiocarbon reveal pre‐ and post‐eruption effects of volcanic processes on trees on Mt. Etna (Sicily, Italy)

AbstractEarly detection of volcanic eruptions is of major importance for protecting human life. Ground deformation and changes in seismicity, geochemistry, petrology, and gravimetry are used to assess volcanic activity before eruptions. Studies on Mt. Etna (Italy) have demonstrated that vegetation can be affected by pre‐eruptive activity before the onset of eruptions. During two consecutive years before Mt. Etna's 2002/2003 flank eruption, enhanced vegetation index (NDVI) values were detected along a distinct line which later developed into an eruptive fissure. However, the mechanisms by which volcanic activity can lead to changes in pre‐eruption tree growth processes are still not well understood. We analysed δ13C, δ18O and14C in the rings of the survived trees growing near to the line where the pre‐eruptive increase in NDVI was observed in order to evaluate whether the uptake of water vapour or fossil volcanic CO2could have contributed to the enhanced NDVI. We found a dramatic decrease in δ18O in tree rings formed before 2002/2003 in trees close to the eruption fissure, suggesting uptake of volcanic water by trees during pre‐eruptive magma degassing. Moist conditions caused by outgassing of ascending magma may also have led to an observed reduction in tree‐ring δ13C following the eruption. Furthermore, only ambiguous evidence for tree uptake of degassed CO2was found. Our results suggest that additional soil water condensed from degassed water vapour may have promoted photosynthesis, explaining local increases in NDVI before the 2002/2003 Mt. Etna flank eruption. Tree‐ring oxygen stable isotopes might be used as indicators of past volcanic eruptions.

Can We Better Constrain the Timing of GNAIW/UNADW Variability in the Western Equatorial Atlantic and Its Relationship to Climate Change During the Last Deglaciation?

AbstractWe have revisited the well‐trod VM28‐122 core retrieved from the deep Colombian Basin, which includes sediments that reflect modern Upper North Atlantic Deep Water and extends through the last deglaciation into the last glacial period when the site was bathed in Glacial North Atlantic Intermediate Water. Here, we leverage the nearby Cariaco Basin's surface water radiocarbon reconstruction (reservoir age, and ΔR) on the IntCal20 timescale to recast the period of the last deglaciation with a newly constrained age model. Based on the revised age model, we observe that the multimillennial decrease in benthic δ13C and B/Ca (which record δ13C of dissolved inorganic carbon and Δ[CO3−2], respectively) began at 18,100 ± 240 (95% CI) calibrated years BP, synchronous with Termination 1, as identified by changes in the Antarctic Ice Sheet composite and by the onset of rapid glacier recession in the Southern Hemisphere (Denton et al., 2010, https://doi.org/10.1126/science.1184119; Denton et al., 2021, https://10.1016/j.quascirev.2020.106771). The beginning of the decrease in benthic δ18O is concurrent with the changes in carbon chemistry or at most, a few hundred years later. It is no later than 17,700 ± 300 (95% CI) yrs BP in our record, at the putative start of Heinrich Stadial 1. With sufficient data density (more than 2–3 control points per kyr) and an independent record of past surface water radiocarbon variations, it is possible to achieve late glacial and deglacial chronologies with fidelities similar to those of ice cores. Doing so in more oceanographic locations should shed light more broadly on the mechanisms instrumental to abrupt climate change.

Impact of the Southern Ecuadorian Andes on Oxygen and Hydrogen Isotopes in Precipitation

Determining how the elevation of the Northern Andes has evolved over time is of paramount importance for understanding the response of the Northern Andes to deformational and geodynamic processes and its role as an orographic barrier for atmospheric vapor transport over geologic time. However, a fundamental requirement when using stable isotope data for paleoaltimetry reconstructions is knowledge about the present-day changes of δ18O and δD with elevation (isotopic lapse rate). This study defines the present-day river isotopic lapse rate near the Equator (∼3°S) based on analysis of δ18O and δD of surface waters collected from streams across the Western Cordillera and the Inter-Andean depression in Southern Ecuador. The results for the two domains show a decrease of δ18O with elevation which fits a linear regression with a slope of −0.18‰/100 m (R2= 0.73,n= 83). However, we establish a present-day lapse rate of −0.15‰/100 m for δ18O (R2= 0.88,n= 19) and -1.4‰/100 m for δD (R2= 0.93,n= 19) from water samples collected along the west facing slopes of the Western Ecuadorian Cordillera which is mainly subject to moisture transport from the Pacific. We argue that this empirical relationship, consistent with those obtained in different tropical areas of the world, can inform stable isotope paleoaltimetry reconstructions in tropical latitudes.

Large-scale climate indices teleconnections with hydrochemical and isotopic characteristics of a karst spring using wavelet analysis

This study investigated the large-scale El Nino–Southern Oscillation and Pacific Decadal Oscillation indices teleconnections with the spring chemographs and δ18O time series in the Sarabkalan Spring, Zagros Mountains, Iran. It was intended to document a karst system by identifying synchrony and coupling between climate indices and its precipitation, spring discharge, chemographs, and isotopes. Here, we used the continuous wavelet transform and wavelet coherence. The findings demonstrated that El Nino Southern Oscillation is more correlated with EC, Ca + Mg, K, and Cl while Pacific Decadal Oscillation is largely teleconnected with EC, Na, and K. Also, the El Nino phenomena coupled with either positive or neutral Pacific Decadal Oscillation phased appeared to cause a decrease in δ18O. It is further found that in addition to its quick pulses in the spring, intense precipitation can also substantially affect the spring discharge even up to ~ 1.5-years later. The discharge and physicochemical parameters of EC, HCO3 + SO4, HCO3, Mg, and Na are out of phase, indicating that the constituent concentrations generally decrease over wet years of high discharge. This work highlighted the necessity of a combination of hydrogeological and climatic-oceanic-atmospheric factors from the local to global scales to improve water resources planning and operations in karst water resources.

Cherts, spiculites, and collapse breccias – Porosity generation in upper Permian reservoir rocks, Gohta discovery, Loppa High, south-western Barents Sea

Spiculitic cherts are uncommon reservoir rocks and their porosity evolution is poorly understood compared to sandstones and carbonates. In the Gohta oil and gas discovery on the Loppa High in the south-western Barents Sea, the reservoir is in brecciated, silicified, and dolomitized Permian spiculites below the Permian/Triassic unconformity. It represents the infill of several collapsed cave systems with spiculite clasts in a micritic matrix, separated by intact cave roofs of shale and spiculite facies. The cave collapse was related to dissolution of less resistant (?limestone and ?spiculitic) beds, due to percolation of freshwater during latest Permian – earliest Triassic emersion. During later transformation of opal-A to opal-CT, associated growth of silica concretions left the margins of spiculite clasts depleted in SiO2 and thus highly porous. Subsequent transformation of opal-CT to quartz resulted in precipitation of texture-preserving quartz and chalcedony cements. The latter show a systematic decrease of δ18O from the first to last precipitated phase, implying crystallization under increasing temperatures during renewed Middle–Late Triassic burial. Later diagenesis includes in situ brecciation and fracturing, dolomitization affecting mainly the micritic matrix of cave-collapse facies, chemical compaction, and calcite cementation. The best reservoir properties are in cave-collapse facies (commonly 10–25% and 0.03–19 mD) where the pore system is dominated by (1) uncemented interspicule pores and central parts of spicule molds within clast margins, and (2) intercrystalline pores between dolomite crystals in the breccia matrix. The primary depositional facies have much lower porosity and permeability (rarely exceeding 10% and 1 mD, respectively). This study shows that porosity in the cave fill most likely formed by local redistribution of silica to form concretions and dissolution of the carbonate matrix to source the growth of dolomite crystals, while prolonged subaerial exposure only played an indirect role by isolating spiculite clasts and preventing their complete silicification during burial.

Controls on spatiotemporal variations of stable isotopes in precipitation across Bangladesh

Indian summer monsoon (ISM) driven moisture from the Indian Ocean is important for moisture transport to the Asian Water Towers and causing floods and droughts in Bangladesh. Stable isotopic compositions of precipitation (δ18O and δD) are crucial tracers of ISM moisture transport processes. Here we presented spatiotemporal variations of stable isotopes in precipitation at three stations over Bangladesh in 2017–2018 to evaluate the influence of ISM on intra-seasonal variations of stable isotopes in precipitation in the south of Himalaya, combined with local meteorological data, ERA5 reanalysis and HYSPLIT model. We found that precipitation δ18O displayed the highest values in May following gradual decrease till the lowest in October. The spatial decrease of δ18O and δD from southwest to northeast appeared in pre-monsoon and monsoon seasons together with increasing of d-excess eastwards. The weak temperature effect and amount effect were detected at Satkhira only at the daily scale in specific seasons, and the significantly negative correlations between relative humidity and δ18O were almost detected at three stations in both monsoon and non-monsoon seasons. We suggested that convections over Bay of Bengal (BOB) and tropical Indian Ocean and variable contributions of moisture sources are important factors controlling distinct variations of precipitation δ18O from west to east Bangladesh. Our results provided a better understanding of the ISM influence on precipitation stable isotopes and variations of the ISM moisture sources, which is crucial to address hydroclimatic interpretations relevant with ISM.

Petrographical and geochemical constraints on carbonate diagenesis in an epeiric platform deposit: Late Cretaceous Bagh Group in central India

A combined facies, petrography and isotope geochemistry reveal an extensive pedogenic alteration of platform carbonate deposits of the Upper Cretaceous Bagh Group in the western Narmada basin in central India. The highly fossiliferous carbonate sequence of the Bagh Group consists of two formations, the Nodular Limestone at the base, and the Bryozoan Limestone Formation at top. The Nodular Limestone Formation comprises primarily of mudstone and wackestone, with relics of bioclasts such as gastropods, echinoderms, molluscs, forams and calcispheres, reflecting a low-energy supratidal to upper intertidal environment of deposition. Prolonged pedogenesis leads to pervasive micritization, brecciation, desiccation and recrystallization. The overlying Bryozoan Limestone, dominated by packstone and planar laminated rudstone with bryozoans, gastropods, echinoderms/echinoid spines, and molluscs, indicates deposition within the lower intertidal environment with moderately high-energy conditions. Abundant meniscus cement and biomolds suggest meteoric vadose diagenetic modification, whereas, intense secondary micritization of sediments indicates prolonged calichification. The δ13C and δ18O ratios of both Nodular Limestone and Bryozoan Limestone formations are depleted than the normal marine values, indicating an extensive diagenetic resetting of the carbonates. While the meteoric cement is depleted in both δ13C and δ18O ratio, the burial cement shows a decrease of δ18O at constant δ13C. The pedogenetic horizons within the Bagh carbonates bear subtle evidence of subaerial exposures within the overall transgressive Bagh Group. A glauconite bed at the upper part of the Bryozoan Limestone Formation marks the top part of the transgressive deposit.