Increase In δ18O Values Research Articles

Stable isotope of Mesolithic remains of Pila wernei of the Nile area, Sudan: A tool for palaeoenvironment reconstruction

The article deals with stable isotopes of oxygen and carbon on current and archaeological semi-aquatic gastropod shell of the Pila wernei from Sudan and reports some new 14C analyses on archaeological Pila specimens. This, with the aim of obtaining new information on climatic and environmental changes in the al-Khiday area during the first phase of Holocene. The Pila shells come from a well-preserved shell midden present at al-Khiday and dated 8700-7000 cal yr BP. The area of al-Khiday was inhabited since at least 10 ka. Pila wernei species has never been studied in detail; this study demonstrates that the O and C isotope composition of aragonite shell is able to record the environmental and diet changes during the gastropod life. The comparison between the isotope data obtained on present-day and archaeological shells show a prevalence of C4 plants and higher rainfall during the Mesolithic. Two climatic fluctuations have been recognised: the first one indicating wetter conditions from 8650-8400 to 8500-8050 cal yr BP (decrease of the δ18O values), the second one pointing less wetter conditions from 8500-8050 to 7850-7600 cal yr BP (increase of δ18O values). Despite the difficulties in studying semi-aquatic gastropods due to their lifestyle habit, the identification of climatic fluctuations in the Early Holocene confirms the reliability of this proxy for climatic reconstructions. Despite the high number of isotopic analyses (1556) used in this work, they were not sufficient to mirror the fast climate changes characterising the studied period and finally obtain a detailed reconstruction for nearly the two millennia considered. Nonetheless, future projects may benefit greatly from unveiling the shell-environment relationship of the species.

Isotopic evidence of an environmental shift at the fall of the Kushite kingdom of Meroë, Sudan

Between c. 300 BC and AD 350, the Meroitic kingdom dominated the Middle Nile Valley; following its breakdown, it was replaced by a series of smaller successor polities. Explanation for this change centres on socio-political and economic instability. Here, the authors investigate the role of climate and environment using stable carbon and oxygen isotope analyses of human and faunal dental enamel from 13 cemeteries. The results show increasing δ18O values towards the end of the Meroitic kingdom and in the post-Meroitic period, combined with less negative δ13C values. These trends suggest a shift towards more arid conditions associated with changes in agricultural practices and land use that may have contributed to the kingdom's dissolution.

Zircon U–Pb ages and O–Hf isotopes of Quaternary trachytes from the East Sea: Implications for the genesis of low-δ18O magmas

Quaternary intraplate magmatism formed several volcanic islands and seamounts, including Dokdo (DD), Ulleungdo (UD), Simheungtack (ST), Anyongbok, and Isabu in the southwest of the East Sea back-arc basin. In this study, we present whole-rock geochemical, zircon U–Pb age, and in situ O–Hf isotope data for the submerged volcanic rocks from DD, UD, and ST to provide new insights into the eruption timing of these volcanoes and constrain the magma evolution processes. All samples used in this study were trachytes and exhibited ferroan, alkalic, and metaluminous to weakly peraluminous characteristics. They showed light rare earth element (REE)-enriched patterns with (La/Yb)N ratios of 25.3–31.7 and mostly negative Eu anomalies in a chondrite-normalized REE plot. In addition, they were enriched in large-ion lithophile elements and high field strength elements; they exhibited positive Pb anomalies and strongly negative Ba, Sr, P, and Ti anomalies. The zircons yielded a weighted-mean 206Pb/238U age of 2.61, 0.348–0.704, and 2.76–2.94 Ma for the DD, UD, and ST trachytes, respectively. All zircons exhibited lower δ18O values than normal depleted mantle values, regardless of the crystallization age and spatial distribution of volcanoes. The δ18O values showed no correlation with U contents or Th/U ratios, indicating that the low δ18O signatures were of primary magmatic origin. The Hf isotopic compositions of the zircons were relatively heterogeneous but predominately characterized by positive εHf values. Binary O–Hf mixing modeling revealed that low-δ18O rocks with positive εHf values from the UD and ST volcanoes were derived from a hybrid source of recycled juvenile crustal materials with low-δ18O and positive εHf signatures and an enriched mantle source with normal δ18O and negative εHf values. The juvenile oceanic crust in the source was likely metasomatized by seawater at high temperatures prior to melting. In contrast, the felsic magma that formed the DD volcanoes may have assimilated with regional basement rocks (Triassic–Jurassic granitoids), resulting in increased δ18O values and decreased εHf values relative to those of the UD and ST volcanoes. Our study highlights the significant contribution of recycled oceanic crust materials to the generation of the Quaternary magmas.

Carbon and oxygen dual-isotopes in tree rings indicate alternative physiological responses opted by European beech trees to survive drought stress

ABSTRACT Poor drought tolerance of European beech trees raised concerns in Europe. We hypothesized that beech could show an opposite physiological response to the same level of climatic drought with change in edaphic drought. We performed a combined analysis of δ13C and δ18O in tree rings to reveal retrospective temporal physiological responses of trees to drought. The edaphic drought was assessed by quantifying the capacity of soil to store water in plots (classified as “dry” and “less-dry”) near the drought limit of the species in three near-natural oak-beech ecotones in Germany and Switzerland. Neighbourhood competition was quantified. A climatic drought index was calculated from meteorological records and related to the δ13C and δ18O values of the trees. Trees from dry plots showed a higher response to drought and climatic dependency than less-dry plots. Neighbourhood competetion increased δ18O values significantly. Dual isotope analysis shows a tendency of greater stomatal resistance in dry plots and higher stomatal conductance in less-dry plots. We conclude that beech trees belonging to the same population under changing soil water availability can show different physiological responses under climatic drought stress. Our finding indicates the high plasticity of the beech trees to survive drought stress with changing site conditions.

Recharge variability in Australia's southeast alpine region derived from cave monitoring and modern stalagmite δ18O records

Oxygen isotopic (δ18O) variations in stalagmite records have the potential to provide new insights about past climates beyond the instrumental record. This paper presents the first high-resolution oxygen isotope time series of three coeval stalagmite records from the alpine region of south-eastern Australia covering the period 1922–2006 CE. We use extended surface and cave monitoring datasets, petrographic investigation, modelled recharge time series and farmed calcite precipitates to assess the controls on speleothem δ18O and investigate the coherence between three records from Harrie Wood Cave. The drip water response to recent interannual rainfall variability shows that cave drip water Cl−, δ18O and drip rate display a clear response to an increase in rainfall recharge. It is demonstrated that stalagmites from the same drip sites also record variability in interannual recharge, where an increase in δ18O values is observed with lower recharge, while a decrease in δ18O values correspond to higher recharge amounts. The three stalagmite δ18O records are in broad agreement, showing common responses to relatively higher recharge between 1945 and 1995 CE and the low recharge periods between 1937 and 1945 CE (World War II drought) and late 1996 to 2006 CE (beginning of the Millennium Drought). However, differences in the magnitude of the relative response of each stalagmite δ18O record varies. Based on evidence from our cave monitoring study and farmed calcites, we conclude that the differences between the three stalagmite records is attributed to variability in the contribution of preferential flows during recharge events and the store reservoir volume supplying the drip site. When the δ18O decreases in response to enhanced recharge, the speleothem δ13C also decreases, and this is interpreted to reflect a soil respiration response to changes in soil moisture availability due to recharge. Hence, stalagmite δ18O from the Australian alpine region can be applied to reconstruct periods of relatively higher and lower rainfall recharge and thus extend our knowledge of the timing and relative magnitude of droughts as well as past periods of higher recharge in this region.

Holocene Millennial‐Scale Precipitation Variations in the Asian Summer Monsoon Margin of Northwest China and Their Relation to Migrations of Monsoon Northern Boundary via Oxygen Isotope Analysis of Calcareous Root Tubes in Deserts

AbstractThe Asian summer monsoon (ASM) margin of northwest China is a critical region for studying Asian monsoon variability during the Holocene. However, the pattern of precipitation variations and migration of the ASM northern boundary in arid regions during the Holocene remains unclear. In this study, we analyzed oxygen isotopes of 70 calcareous root tube (CRT) samples from the desert hinterlands in the ASM margin of northwest China, including the Badain Jaran, Tengger, Ulan Buh, and Kubuqi deserts. The CRT samples were analyzed to reconstruct millennial‐scale precipitation variation patterns during the Holocene and further explored the location of the ASM northwestern boundary during the middle Holocene. Our results demonstrated that the δ18O values of CRTs during the late Holocene (4.0–1.0 cal kyr BP) were all less negative than those during the middle Holocene (8.0–4.0 cal kyr BP), which revealed that the precipitation decreased from the middle Holocene to the late Holocene. Moreover, an increase in δ18O values from the southeast to northwest revealed that the precipitation decreased. We argued that the Holocene millennial‐scale precipitation changes in this region were influenced by the migrations in the ASM northern boundary. During the middle Holocene, due to the strong summer solar radiation and reduced ice sheets, the ASM northern boundary in the arid region advanced by at least 250 km to the northwest, which resulted in increased precipitation. During the late Holocene, the ASM northern boundary retreated southeastward, resulting in decreased precipitation.

Late Cretaceous Paleoceanographic Evolution and the Onset of Cooling in the Santonian at Southern High Latitudes (IODP Site U1513, SE Indian Ocean).

The latest Cenomanian to Santonian sedimentary record recovered at IODP Expedition 369 Site U1513 in the Mentelle Basin (SE Indian Ocean, paleolatitude 60°S at 85 Ma) is studied to interpret the paleoceanographic evolution in the Southern Hemisphere. The planktonic foraminiferal assemblage changes, the depth ecology preferences of different species, and the surface and seafloor temperature inferred from the stable isotopic values measured on foraminiferal tests provide meaningful information to the understanding of the Late Cretaceous climate. The hothouse climate during the Turonian‐Santonian, characterized by weak latitudinal temperature gradients and high atmospheric CO2 concentrations, is followed by a progressive cooling during the Campanian. At Site U1513 the beginning of this climatic transition is nicely recorded within the Santonian, as indicated by an ∼1‰ increase in δ18O values of planktonic foraminifera suggesting a decline in surface water paleotemperatures of 4°C. The onset of cooling is mirrored by changes in the planktonic foraminiferal assemblages including extinctions among surface and deep dwellers, appearances and diversification of newly evolving taxa, and changes from predominantly epifaunal oxic to infaunal dysoxic/suboxic taxa among co‐occurring benthic foraminifera. Overall, the data presented here document an interval in the Santonian during which the rate of southern high latitude cooling increased. Both surface and bottom waters were affected, although the cooling signal is more evident in the data for surface waters. This pattern of cooling ascribes the deterioration of the Late Cretaceous climate to decreased CO2 in the atmosphere and changes in the oceanic circulation correlated with enhanced meridional circulation.

Oxygen Kinetic Isotope Effects in the Thermal Decomposition and Reduction of Ammonium Diuranate.

Oxygen stable isotopes in uranium oxides processed through the nuclear fuel cycle may have the potential to provide information about a material’s origin and processing history. However, a more thorough understanding of the fractionating processes governing the formation of signatures in real-world samples is still needed. In this study, laboratory synthesis of uranium oxides modeled after industrial nuclear fuel fabrication was performed to follow the isotope fractionation during thermal decomposition and reduction of ammonium diuranate (ADU). Synthesis of ADU occurred using a gaseous NH3 route, followed by thermal decomposition in a dry nitrogen atmosphere at 400, 600, and 800 °C. The kinetic impact of heating ramp rates on isotope effects was explored by ramping to each decomposition temperature at 2, 20, and 200 °C min–1. In addition, ADU was reduced using direct (ramped to 600 °C in a hydrogen atmosphere) and indirect (thermally decomposed to U3O8 at 600 °C, then exposed to a hydrogen atmosphere) routes. The bulk oxygen isotope composition of ADU (δ18O = −16 ± 1‰) was very closely related to precipitation water (δ18O = −15.6‰). The solid products of thermal decomposition using ramp rates of 2 and 20 °C min–1 had statistically indistinguishable oxygen isotope compositions at each decomposition temperature, with increasing δ18O values in the transition from ADU to UO3 at 400 °C (δ18OUO3 – δ18OADU = 12.3‰) and the transition from UO3 to U3O8 at 600 °C (δ18OU3O8 – δ18OUO3 = 2.8‰). An enrichment of 18O attributable to water volatilization was observed in the low temperature (400 °C) product of thermal decomposition using a 200 °C min–1 ramp rate (δ18OUO3 – δ18OADU = 9.2‰). Above 400 °C, no additional fractionation was observed as UO3 decomposed to U3O8 with the rapid heating rate. Indirect reduction of ADU produced UO2 with a δ18O value 19.1‰ greater than the precipitate and 4.0‰ greater than the intermediate U3O8. Direct reduction of ADU at 600 °C in a hydrogen atmosphere resulted in the production of U4O9 with a δ18O value 17.1‰ greater than the precipitate. Except when a 200 °C min–1 ramp rate is employed, the results of both thermal decomposition and reduction show a consistent preferential enrichment of 18O as oxygen is removed from the original precipitate. Hence, the calcination and reduction reactions leading to the production of UO2 will yield unique oxygen isotope fractionations based on process parameters including heating rate and decomposition temperature.

Early hydrosphere-rock interactions and intra-crustal recycling recorded by remarkably high-δ18O Mesoarchean granitoids in the Sulu orogenic belt, eastern China

When, why and how global plate tectonics were initiated, as well as secular changes of plate tectonics over time are fundamental issues in the earth sciences. In this study, we present a combined U-Pb, Hf and O isotope dataset for complex zircons from the high-grade granitic (TTG) gneisses of the Haiyangsuo complex in the Sulu orogenic belt, eastern China. This combined dataset reveals that the granitoids formed by at least ca. 3.22, 2.81, 2.70 and 2.58 Ga and recorded multiple metamorphic thermal events that occurred at ca. 3.05, 2.60–2.50 and 1.93–1.85 Ga. The 3.22 and 2.58 Ga granitoids have negative εHf(t) values (averages of −2.08 and −10.81, respectively) and were derived from remelting of evolved Eoarchean crust, in combination of 3.47 Ga inherited zircon, reflecting occurrence of matured felsic continent crust in the Paleoarchean, whereas the 2.81 and 2.70 Ga granitoids show positive εHf(t) values (averages of + 4.32 and + 2.23, respectively) and formed by remelting of juvenile crust derived from depleted mantle at ca. 3.0 Ga. The 3.22, 2.70 and 2.58 Ga granitoids have mantle-like or slightly enriched zircon δ18O values (averages of 5.53 ± 0.15, 5.41 ± 0.18 and 6.10 ± 0.22‰, respectively). However, the 2.81 Ga granitoid displays remarkably high zircon δ18O values ranging from 9.65 to 7.02‰ with an average of 8.48 ± 0.29‰ (1 SD), this demonstrates occurrences of early hydrosphere-rock interactions and intra-crustal recycling in the Mesoarchean. Synthetically, here we propose the following tectonic model for the formation of the high-δ18O Mesoarchean granitoids: (1) Generation of voluminous high-density lower crust along with the formation of early felsic continent dominated by TTGs; (2) gravity-driven subduction of denser oceanic crust relative to continental crust induced by delamination of high-density lower crust together with lithospheric mantle; and (3) subsequently partial melting of altered high-δ18O hydrated oceanic crust caused by delamination-induced hot asthenospheric mantle upwelling to form the high-δ18O Mesoarchean granitoid. Furthermore, we argue that gravity-driven subduction occurred in the Archean, and subduction may be responsible for the trend towards increasing δ18O values within zircons since the Mesoarchean.

Warmer–wetter climate drives shift in δD–δ18O composition of precipitation across the Queen Elizabeth Islands, Arctic Canada

We examine how recent increases in air temperature and precipitation, together with reductions in sea ice extent, may have affected the regional δD–δ18O composition of precipitation. In spring 2014, 80 snow samples were collected from six glaciers and ice caps across the Queen Elizabeth Islands, and in 2009 and 2014, two shallow ice cores were collected from Agassiz Ice Cap and White Glacier, respectively. The snow samples showed average δ18O values from 2013 to 2014 to be approximately 2‰–3‰ higher than those recorded in 1973–1974 in nearby locations, with the ice cores showing similar trends in δ18O values. A zonal average water isotope model was used to help understand the causes of the increased δ18O values, using inputs calibrated for observed changes in temperature, vapour flux, and sea ice extent. Model results indicate that atmospheric temperature changes account for <1‰ of the observed change in δ18O values, and that changes in local water input and precipitation driven by changes in sea ice extent only have an effect in coastal regions. Enhanced meridional vapour flux to the Queen Elizabeth Islands is, therefore, also required to explain the observed increases in δ18O values, with fluxes ∼7% higher today than in the 1970s, consistent with the change in precipitation.

Analysis of changes in hydrological cycle of a pristine mountain catchment. 2. Isotopic data, trend and attribution analyses

Abstract δ18O in precipitation at station Liptovský Mikuláš (about 8.5 km south from the outlet of the Jalovecký Creek catchment) remains constantly higher since 2014 that might be related to greater evaporation in the region of origin of the air masses bringing precipitation to the studied part of central Europe. Increased δ18O values are reflected also in the Jalovecký Creek catchment runoff. Seasonality of δ18O in the Jalovecký Creek became less pronounced since 2014. The most significant trends found in annual hydrological data series from the catchment in the study period 1989–2018 have the correlation coefficients 0.4 to 0.7. These trends are found in the number of flow reversals (change from increasing to decreasing discharge and vice versa), June low flow, number of simple runoff events in summer months (June to September) and the flashiness index. The attribution analysis suggests that drivers responsible for the changes in these data series include the number of periods with precipitation six and more days long, total precipitation amount in February to June, number of days with precipitation in June to September and total precipitation in May on days with daily totals 10 mm and more, respectively. The coefficients of determination show that linear regressions between the drivers and supposedly changed data series explain only about 31% to 36% of the variability. Most of the change points detected in the time series by the Wild Binary Segmentation method occur in the second and third decades of the study period. Both hydrometric and isotopic data indicate that hydrological cycle in the catchment after 2014 became different than before.

SIMS oxygen isotopes indicate Phanerozoic fluids permeated a Precambrian gold deposit

The Reef Deposit is an anomalous AuCu occurrence in the Paleoproterozoic terranes of northern Wisconsin, better known as host to significant CuZn volcanogenic massive sulfide (VMS) deposits. Previous work using lead isotopes and fluid inclusions has identified a protracted development of the mineralization from initial formation as the root zone veins of a VMS deposit during the Penokean orogeny (~1.9–1.8 Ga), with the most recent mineralization/remobilization activity associated with late Paleozoic Mississippi Valley-type (MVT) fluid flow (Haroldson et al., 2018a, 2018b). Here we use the oxygen isotope history of the Reef Deposit to verify and further examine the deposit's protracted development.Laser fluorination oxygen isotope measurements of primary mineralized quartz veins range in δ18O from 6.8 to 10.0‰ (VSMOW), and a trend is observed of increasing δ18O values of parallel vein zones along a 400-m traverse from northwest to southeast, likely from a temperature gradient during initial formation. Temperature estimates for a VMS deposit setting (230 to 400 °C) are consistent with a hydrothermally shifted formation fluid, using seawater δ18O estimates during the Penokean Orogeny and evolving to higher δ18O by incorporation of magmatic fluids or interaction with local crust.In situ oxygen isotope measurement by Secondary Ion Mass Spectrometer (SIMS) of cross-cutting quartz and carbonate range in δ18O from 19.4 to 28.4‰ for quartz, 25.3 to 28.1‰ for dolomite, and 9.6 to 29.3‰ (VSMOW) for calcite. High δ18O values (>19‰) are measured in a crustiform-textured quartz stockwork microveinlet, in dolomite observed in late carbonate microveinlets, and calcite in settings associated with the late quartz and dolomite and a separate calcite-only setting directly linked with gold mineralization/remobilization.

Oxygen and carbon isotopes and trace-element/Ca ratios in Late Quaternary ostracods Loxoconcha lepida and Palmoconcha agilis from the Black Sea: Paleoclimatic and paleoceanographic implications

Oxygen and carbon isotopes and trace-element/Ca ratios are presented for the valves of benthic ostracods Palmoconcha agilis and Loxoconcha lepida extracted from two composite sediment cores from the Black Sea shelf. The two cores together provide a complete latest Pleistocene–Holocene sedimentary record, with 16 radiocarbon-dated shells in core M02-45 and 9 in core M05-50. Results reveal environmental changes and evolution of water chemistry associated with the last reconnection of the Black Sea to the global ocean. The δ18O values in composite cores M02-45 and M05-50 exhibit ranges of ~8.0‰ and ~3.0‰, respectively; δ13C values show ranges of ~2.5‰ and ~2.0‰. There are three isotopic substages: (a) substage 1c (12.1–9.4 cal ka) represents conditions in the Neoeuxine Lake with relatively high δ13C and low δ18O, (b) substage 1b (9.4–6.3 cal ka) represents the transition from lacustrine to marine conditions with an increase in δ18O values as small amounts of Mediterranean water flowed into the Black Sea, and (c) substage 1a (6.3 cal ka to Present) represents the establishment of marine conditions like today with near-constant δ18O values and a trend of slowly increasing δ13C. Trace-element/Ca ratios show downcore trends consistent with the three isotopic substages. At ~6.3 cal ka, isotopic data and trace-element/Ca ratios exhibit a major shift, with changes in the Mg/Ca ratio suggesting an increase in water temperature from 12–14 °C to 17–19 °C. These mid-Holocene changes in ostracod-valve composition and environmental conditions coincide with the weakening of Black Sea outflow and the proliferation of marine organisms.

U-Pb and oxygen isotope characteristics of Timanian- and Caledonian-age detrital zircons from the Brooks Range, Arctic Alaska, USA

AbstractThe Devonian connection between the Brooks Range of Alaska, USA, with the continental margin of Arctic Canada and its subsequent Jurassic–Cretaceous counterclockwise rotation to form the Amerasian Basin, is a highly debated topic in Arctic tectonics. This resource-rich region was assembled from terranes that formed part of Laurentia or Baltica, or were juvenile oceanic arcs in the early Paleozoic that were brought together during Caledonian Orogenesis and the subsequent collision that formed Pangea (Uralide Orogeny). Elements of these orogens, as well as older ones, are predicted to occur in the Brooks Range of Arctic Alaska. This study presents the first combined zircon U-Pb and oxygen data from six Brooks Range metasedimentary units with assumed Neoproterozoic to Devonian ages. Three distinct detrital zircon patterns are identified in these units: (1) those with Neoproterozoic maximum depositional ages characteristic of the Timanide Orogen of northern Baltica and adjacent parts of Siberia, (2) an almost unimodal Siluro–Ordovician (443.5 ± 2.3 Ma) detrital zircon population consistent with the oceanic Apoon arc believed to have existed off shore of northern Laurentia and to have accreted to the North Slope subterrane during the Caledonian event, and (3) those with Middle Devonian maximum depositional ages consistent with post-accretion extension during the final (Scandian) phase of Caledonian Orogenesis. Oxygen isotopes from the same zircons reveal minor to significant crustal contamination with approximately two thirds (n = 255/405) having δ18O values >5.9‰ (above the mantle field of 5.3 ± 0.6‰). Pattern 1 units exhibit a progressive increase in δ18O values throughout the Proterozoic (5.99 to 9.29‰) indicative of increasing crustal growth and Timanide age zircons yield average δ18O values of 7.18 ± 0.64‰ (n = 26) suggestive of more crustal influence than Caledonian age zircons, possibly reflecting northern Baltica signatures. The unimodal population in Pattern 2 yields average δ18O values of 5.49 ± 0.66‰ (n = 17) and 6.02 ± 0.27‰ (n = 23) prior to and during, respectively, the main Caledonian event and suggest derivation from Devonian juvenile arc sources possibly representing the initiation of the collision between Laurentia and Baltica. Similar to Pattern 1, the δ18O values associated with Pattern 3 show a progressive increase in δ18O values throughout the Proterozoic (5.00 to 9.39‰). However, Pattern 3 also exhibits a distinct juvenile fingerprint (6.13 ± 0.24‰, n = 51) during the main Caledonian event and a slight increase to 7.12 ± 1‰ (n = 7) in post-Caledonian zircons possibly suggest correlating with a post-accretion phase in which proximally sourced zircon-bearing detritus was deposited in extension-related basins marking the joining of Laurentia and Baltica.

The δ18O of primary and secondary waters in hydrous volcanic glass

We present analyses of phyllosilicates, volcanic glass, and a series of experiments and observations on the δ18O of water extracted by rapid thermal pyrolysis at 1450 °C using the TCEA (Thermal Conversion Elemental Analyzer). The study includes the same hydrous glasses that were previously analyzed for their δD values of extracted water (Seligman et al., 2016). We utilize natural phyllosilicates (where water is present as OH– only), natural magmatic and experimentally quenched glasses, (in which water occurs as H2Om and OH−), and glasses that underwent low-temperature secondary hydration by meteoric water (almost completely as H2Om). Our study documents that: 1) thermal extraction and simultaneous pyrolysis of H2Ot into CO and H2 produces little or no exchange (<1‰) in δ18O between hydrogen-bound and silica-bound oxygen; 2) water extracted from different natural phyllosilicates have 103lnαsilicate-OH (~δ18Osilicate – δ18OOH) values ranging from −3 to 6‰, which is in agreement with previous results using partial fluorination, Density-Functional Theory, and the increment method; 3) water extracted from eruptively or experimentally quenched hydrous magmatic glasses have 103lnαsilicate-H2Ot values that decrease with increasing quench temperature from 10‰ (900 °C) to 3‰ (1100 °C); 4) during progressive volcanic degassing, the δ18O of remaining water in glass decrease as the departing H2Om is more positive in δ18O, especially at lower temperature; this oxygen isotope trend correlates with δD as the departing H2Om is also more positive in δD than the remaining magmatic water. 5) the δ18O of water extracted from glass hydrated at low-temperature by secondary water yields isotopically negative δ18O values that are closer to the δ18O values of local meteoric water than values appropriate for low temperature silicate-water 18O/16O equilibrium; this suggests that upon hydration water simply dissolves into glass as H2O; 6) subsequently, secondarily hydrated glasses appear to show an increase in δ18O values that trend towards equilibrium with continued secondary hydration, with time, and at higher temperatures. This research demonstrates that the δ18O of extracted water, despite its 1–2 per mil- analytical precision, provides an isotopic tool for investigations of first order trends of both magmatic degassing, and of secondary hydration by meteoric waters. The trends described here aids hydrogen isotopic variations in the same processeses and combined use of both O and H isotopic variation of water in glass to help fingerprint sources of meteoric water, extent and stages of alteration of glass, and magmatic degassing.

The evolution of the Arabian-Nubian Shield and survival of its zircon U-Pb-Hf-O isotopic signature: A tale from the Um Had Conglomerate, central Eastern Desert, Egypt

In this study, we present in situ U-Pb, Hf and O isotopic analyses of detrital zircon grains from the Um Had Conglomerate from the Wadi Hammamat area in the central Eastern Desert of Egypt. The youngest detrital age population is 625 Ma, which gives an estimate to the maximum depositional age of the Um Had Conglomerate. Based on their Hf isotopic signature and U-Pb ages, detrital zircons fall into two distinct groups: a 625–680 Ma group and a 720–840 Ma group, with minor early Tonian-Stenian and Paleoproterozoic-Archean populations. These groups are consistent with derivation from local sources in the central Eastern Desert of Egypt. The 720–840 Ma group is characterized by exclusively positive εHf(t) values and it reflects the main juvenile Tonian intra-oceanic island arc stage characterizing the formation of the Arabian-Nubian Shield. On the contrary, εHf(t) values of the 625–680 Ma zircon group range between −17.5 and +12.8, which indicates involvement of Archean-Paleoproterozoic crust during the Cryogenian-Ediacaran magmatism associated with the accretion and assembly of the arc materials. The transition from juvenile intra-oceanic island arc stage to mixed juvenile magma and older crustal materials during the arc assembly stage in the central Eastern Desert is supported by the increase in δ18O values of detrital zircons from dominant mantle-like values in the Tonian to higher values in the Cryogenian-Ediacaran group. A wide variation in the εHf(t) values (−25.8 to +10) is recorded also in the early Tonian-Stenian population, which results in the double plunge U-Pb-Hf patterns of the detrital zircons. This pattern is similar to the Nile sediments and to the Paleozoic and Mesozoic Nubian sandstone, which may indicate substantial recycling of ANS crustal material by these sediments and survival of its original zircon U-Pb-Hf-O signature despite repeated recycling episodes.

Precipitation changes over the eastern Bolivian Andes inferred from speleothem (δ18O) records for the last 1400 years

Here we present high-resolution δ18O records obtained from speleothems collected in the eastern Bolivian Andes. The stable isotope records are related to the regional- to large-scale atmospheric circulation over South America and allow interpreting changes in δ18O during the last 1400 yr as a function of changes in precipitation regimes over the southern tropical Andes. Two distinct phases with more negative δ18O values, interpreted as periods of increased convective activity over the eastern Andean Cordillera in Bolivia are observed concomitantly with periods of global climate anomalies during the last millennium, such as the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA) respectively. Changes in the Bolivian δ18O record during the LIA are apparently related to a southward displacement of the Intertropical Convergence Zone (ITCZ), which acts as a main moisture driver to intensify convection over the tropical continent. During the MCA, however, the increased convective activity observed in the Bolivian record is likely the result of a different mechanism, which implies moisture sourced mainly from the southern tropical Atlantic. This interpretation is consistent with paleoclimate records further to the north in the tropical Andes that show progressively drier conditions during this time period, indicating a more northerly position of the ITCZ. The transition period between the MCA and the LIA shows a slight tendency toward increased δ18O values, indicating weakened convective activity. Our results also reveal a non-stationary anti-phased behavior between the δ18O reconstructions from Bolivia and northeastern Brazil that confirms a continental-scale east–west teleconnection across South America during the LIA.

Changing recharge pathways within an intensively pumped aquifer with high fluoride concentrations in Central Mexico

Fluoride (F), naturally found in aquifers around the world at toxic concentrations, causes disease in millions of people. The long-term stability, however, of those concentrations within intensively pumped aquifers is poorly characterized. We assessed long-term stability in the spatial distribution of F concentrations in an intensively pumped aquifer within the semi-arid, inter-montane Independence Basin in central Mexico between 1999 and 2016. Although stable in 16 re-sampled wells, F concentrations increased in some localities across the basin by as much as 4mg/L. Changes in recharge pathways to the deep aquifer were identified by analyzing changes in δ2H, δ18O and Cl/Br mass ratios. In 1999, δ2H and δ18O values suggested the aquifer was recharged in the mountains. In 2016, however, substantial increases in δ18O values in the center of the basin suggest recharge water is derived from rainfall that had experienced increased evaporation. In 1999, the mass ratio Cl/Br in groundwater was slightly enriched over local rainfall, and followed a single mixing line on a plot of Cl. vs. Cl/Br. In 2016, however, three distinct groupings of wells were evident, all following different mixing lines. These changes suggest input from new sources including urban sewage, evaporate dissolution, connate sea water and geothermal waters. Step-wise multiple regression was used to quantify the impact of physical and chemical parameters on F concentrations. In 1999, Li (6.8±1.7) and Na (0.01±0.004) drove F concentrations (R2=0.54). In 2016, Na (0.013±0.0018), HCO3 (0.004±0.001), Ca (−0.0018±0.00045), and Mg (−0.055±0.023) drove F concentrations (0.78). Irrigation pumping and urban expansion within semi-arid, groundwater-dependent, inter-montane basins drive mixing of disparate groundwater chemistries and introduces new sources of recharge to aquifers inducing changes in aquifer chemistry including increasing concentrations of geogenic toxic elements.

A paired apatite and calcite clumped isotope thermometry approach to estimating Cambro-Ordovician seawater temperatures and isotopic composition

The secular increase in δ18O values of both calcitic and phosphatic marine fossils through early Phanerozoic time suggests either that (1) early Paleozoic surface temperatures were high, in excess of 40 °C (tropical MAT), (2) the δ18O value of seawater has increased by 7–8‰ VSMOW through Paleozoic time, or (3) diagenesis has altered secular trends in early Paleozoic samples. Carbonate clumped isotope analysis, in combination with petrographic and elemental analysis, can deconvolve fluid composition from temperature effects and therefore determine which of these hypotheses best explain the secular δ18O increase. Clumped isotope measurements of a suite of calcitic and phosphatic marine fossils from late Cambrian- to Middle-late Ordovician-aged strata–the first paired fossil study of its kind–document tropical sea surface temperatures near modern temperatures (26–38 °C) and seawater oxygen isotope ratios similar to today’s ratios.

Testing the early Late Ordovician cool-water hypothesis with oxygen isotopes from conodont apatite

AbstractLatest Sandbian to early Katian sequences across Laurentia's epicontinental sea exhibit a transition from lithologies characterized as ‘warm-water’ carbonates to those characterized as ‘cool-water'carbonates. This shift occurs across the regionally recognized M4/M5 sequence stratigraphic boundary and has been attributed to climatic cooling and glaciation, basin reorganization and upwelling of open ocean water, and/or increased water turbidity and terrigenous input associated with the Taconic tectophase. Documentation of oxygen isotopic trends across the M4/M5 and through bracketing strata provides a potential means of distinguishing among these alternative scenarios; however, oxygen isotopic records generated to date have failed to settle the debate. This lack of resolution is because δ18O records are open to multiple interpretations and potentially confounding factors related to local environmental conditions have not been tested by examining the critical interval in multiple areas and different depositional settings. To begin to address this shortcoming, we present new species-specific and mixed assemblage conodont δ18O values in samples spanning the M4/M5 boundary from the Upper Mississippi Valley, Alabama, and Virginia. The new results are combined with previous studies, providing a record of δ18O variability across SE Laurentia. The combined dataset allows us to test for regional trends at a resolution not previously available. Our results document a ~1.5‰ decrease in values across Laurentia instead of increasing δ18O values across the M4/M5 as predicted in various ‘cool-water’ scenarios. In short, these results do not support a shift to ‘cool-water’ conditions as an explanation for changes in early Katian carbonates across the M4/M5.