Positive Oxygen Isotope Research Articles

Eocene Tectonic Uplifts Caused the Early Oligocene Intrusion of the Indian Monsoon Into the Tuotuohe Basin and the Increased Evaporation, Central‐Northern Tibet: Insights From the Oxygen Isotope Record

AbstractIt is not clear how the Asian monsoon evolved after leaving the spatial range of the Intertropical Convergence Zone in the Late Eocene and before the modern‐like Asian monsoon formed at ∼26 Ma. In this study, the results of a well‐dated, robust, and continuous paleoenvironmental carbonate oxygen isotope record from the Tuotuohe section in the Tuotuohe Basin on the central–northern Tibetan Plateau indicate that the carbonate oxygen isotope showed positive bias at 30.5 Ma. Evidence concerning the temperature, precipitation, paleolatitude, paleoelevation, salt mines and global temperature changes at this time suggest that the positive oxygen isotope shift was mainly due to a combination of the intrusion of the Indian monsoon into the basin and increased evaporation, both of which were ultimately induced by the tectonic uplift of the Tuotuohe Basin before 30.5 Ma and the retreat of the Paratethys Sea after the Eocene.

Comet 81P/Wild 2 dust impactors of Stardust turnip-like tracks analogous to cluster IDPs

We measured oxygen isotope ratios of 16 silicate fragments from seven aerogel tracks (turnip-like type B tracks 77, 149, 172, 191, and 220; carrot-like type A tracks 22 and 175) of the comet 81P/Wild 2 collector from NASA’s Stardust mission using secondary ion mass spectrometry. Thirteen were prepared by ultramicrotomy; three from track 220 were prepared by sputtering resin blocks using a SIMS Kohler beam, a new procedure aiming to mine as many cometary particles encased in aerogel/resin as possible. Combining new and literature results, we recognized that most silicate fragments of individual type B tracks have diverse mineralogy but consistent mass-independent fractionation of oxygen isotopes (Δ17O = δ17O − 0.52 × δ18O). These observations suggest that their impactors are loosely bound aggregates of unequilibrated materials originating mainly from similar protoplanetary disk regions, resembling the cluster IDP U2-20-GCA. Furthermore, silicate fragments from type A track 22 have almost identical mineralogy and Δ17O values, confirming that its impactor is a single chondrule-like fragment. The terminal particle of type A track 175 is pure forsterite with Δ17O of ∼ –23 ‰.Six iron-rich fragments of this study have positive oxygen isotope ratios (Δ17O∼ +2 ‰) and ordinary chondrite chondrule-like olivine compositions. Together with five similar fragments in the literature, a unique population (Mg# ≤ 86) of Wild 2 fragments that resemble chondrules from the inner solar system (O-E-R) chondrites or the outer solar system CH-CB chondrites was identified. The remaining 16O-poor Wild 2 fragments are Mg# ≥ 79 silicates with Δ17O ∼ –2 ‰ and a small amount of Mg# ≤ 79 silicates with Δ17O ∼ 0 ‰, which are most consistent with CR chondrite chondrules. Thus, we conclude that in addition to the possible major source of CR chondrite chondrule-like materials, the inner solar system or CH-CB chondrule-like materials are a minor component of comet Wild 2, like the cluster IDP U2-20-GCA.

Calcareous nannofossils across the Eocene-Oligocene transition: Preservation signals and biostratigraphic remarks from ODP Site 1209 (NW Pacific, Shatsky Rise) and IODP Hole U1411B (NW Atlantic Ocean, Newfoundland Ridge)

This work provides a detailed biostratigraphic correlation through the Eocene-Oligocene Transition (EOT), based on an integrated stratigraphic approach and the study of calcareous nannofossils, between two disparate sites, one in the NW Atlantic (IODP Hole U1411B) and one in the NW Pacific (ODP Site 1209).The precise site-to-site correlation provided by these data allows for a comparison of carbonate preservation across the EOT including identification of the main post-depositional processes that impact the calcareous nannofossil ooze at Site 1209. The main aim of this work is to understand the extent to which the bulk δ18O and δ13C records and their sources (mainly calcareous nannofossils) are altered by diagenesis. Our detailed SEM study highlights some differences before, during and after the EOT, suggesting local diagenetic dynamics.At Site 1209, a distinctive change, both in nannofossil assemblage composition and preservation state, is observed from the pre-EOT phase to the Late Eocene Event (LEE), with a shift in the dominant process from dissolution to recrystallisation. Surprisingly, despite the overall poor preservation, only the interval between 141 and 142.4 (adj. rmcd) was compromised in term of isotopic values and assemblage diversity and abundance. This interval, recorded in the upper Eocene, was characterized by severe dissolution, concomitant with deposition of secondary calcite on solution-resistant forms. Diagenetic processes have strongly biased the δ18O isotopic signal, resulting in a positive oxygen isotope anomaly through the upper Eocene that is difficult to reconcile with other published trends. For the remaining time intervals, diagenesis seems not to have altered the bulk δ18O profile, which closely resembles that of other sites across the world, and is particularly consistent with other data from the Pacific Ocean. In summary, the impact of diagenesis on nannofossil preservation even if clearly visible both in SEM and optical microscope observations does not always cause a pervasive alteration of the primary isotopic signal and can instead provide important clues on local depositional dynamics.

The timing, duration and magnitude of the 8.2 ka event in global speleothem records

Abrupt events are a feature of many palaeoclimate records during the Holocene. The best example is the 8.2 ka event, which was triggered by a release of meltwater into the Labrador Sea and resulted in a weakening of poleward heat transport in the North Atlantic. We use an objective method to identify rapid climate events in globally distributed speleothem oxygen isotope records during the Holocene. We show that the 8.2 ka event can be identified in >70% of the speleothem records and is the most coherent signal of abrupt climate change during the last 12,000 years. The isotopic changes during the event are regionally homogenous: positive oxygen isotope anomalies are observed across Asia and negative anomalies are seen across Europe, the Mediterranean, South America and southern Africa. The magnitude of the isotopic excursions in Europe and Asia are statistically indistinguishable. There is no significant difference in the duration and timing of the 8.2 ka event between regions, or between the speleothem records and Greenland ice core records. Our study supports a rapid and global climate response to the 8.2 ka freshwater pulse into the North Atlantic, likely transmitted globally via atmospheric teleconnections.

Oxygen isotope insights into the Archean ocean and atmosphere

Accurately reconstructing the temperature of the ocean through time carries implications for Earth's climate and early habitability. Attempts to build these reconstructions using oxygen isotope records have led to three end-member interpretations. Namely, that the observed enrichment over time in 18O relative to 16O in ancient chemical sediments reflects a change in sea-surface temperatures (SST), a change in the 18O composition of the contemporaneous water, or that the primary signal has been subsequently overprinted. These questions become most salient in the Archean, where estimates of the isotopic composition of the ocean span ∼20‰, with a correspondingly wide range in estimated SSTs. Here, we introduce barite (BaSO4) as a robust new proxy for the oxygen isotope composition of the Archean ocean. We compile new and existing triple oxygen isotope and sulfur isotope data from the Fig Tree Group barite deposits in the Barberton Greenstone Belt, South Africa, with the goal of identifying the primary sources of sulfate to the Archean ocean. Using a simple Monte Carlo approach, we then constrain the possible isotopic composition of contemporaneous seawater. Our results suggest that microbial sulfur cycling played a limited role in setting the isotopic composition of Archean seawater sulfate. Additionally, it is likely that a significant flux of sulfate to the marine reservoir was atmospherically derived and carried a significant positive triple oxygen isotope signal. Importantly, our results support an Archean ocean with a somewhat enriched oxygen isotope composition (∼0-5‰), with the exact composition dependent on the relative contribution from each sulfate production pathway. This result points either to the decreased significance of low-temperature weathering and/or to elevated SSTs in the early Archean.

Hypersalinity accompanies tectonic restriction in the eastern Mediterranean prior to the Messinian Salinity Crisis

This study describes the hydroclimate evolution of the eastern Mediterranean Basin during the early Messinian (7.2 to 6.5 Ma) time-interval based on analysis of a succession at Agios Myron (Crete, Greece), prior to the onset of the Messinian Salinity Crisis (5.96–5.33 Ma). Specifically, we report sea surface temperature and salinity reconstructions based on a combined analysis of biomarkers and oxygen isotopes of planktonic foraminifera. Data reveal that a negative water budget and strong hydrologic and climate variability characterized the eastern Mediterranean Basin at this time, and we identify three distinct phases. In Phase 1 (7.2–6.9 Ma), a shift to more positive oxygen isotope values in planktonic foraminifera at ~7.2 Ma is attributed to progressive gateway restriction of Mediterranean–Atlantic corridors and subsequent cooling until 6.9 Ma. In Phase 2 (6.9–6.7 Ma), distinct warm and hypersaline events (at 6.9–6.82 and 6.72 Ma) resulted in stressed marine microfauna during periods of strong evaporation. An important step-change in the Mediterranean restriction at 6.72 Ma may have resulted from shallowing of the Mediterranean gateways and reduced Mediterranean marine outflow. During Phase 3 (6.7–6.5 Ma) this gateway shallowing reduced the oceanic input into the Mediterranean Basin causing significant hydrological changes, reflected in a wide range of temperature and salinity fluctuations accompanied by enhanced water-column stratification. The data presented here counterbalance the general lack of quantitative temperature and especially salinity estimates available for the Mediterranean Messinian, time interval where we still highly rely on modelling for such evaluations. This study highlights the severity of preconditioning stages leading to the Messinian Salinity Crisis in the Mediterranean and sets values for extreme salinity conditions that could still host marine life. • Three restriction phases affected the eastern Mediterranean between 7.2 and 6.5 Ma. • Colder water and normal salinity characterized the Mediterranean during 7.2–6.9 Ma. • Periods of strong evaporation observed for the 6.9–6.7 Ma interval. • Warm-hypersaline events (6.9–6.82 and 6.72 Ma) caused extreme environmental stress. • Restriction at 6.7–6.5 Ma was induced by tectonics and modulated by climate.

Oxygen isotope evidence for input of magmatic fluids and precipitation of Au-Ag-tellurides in an otherwise ordinary adularia-sericite epithermal system in NE China

Abstract Tellurium-rich (Te) adularia-sericite epithermal Au-Ag deposits are an important current and future source of precious and critical metals. However, the source and evolution of ore-forming fluids in these deposits are masked by traditional bulk analysis of quartz oxygen isotope ratios that homogenize fine-scale textures and growth zones. To advance understanding of the source of Te and precious metals, herein, we use petrographic and cathodoluminescence (CL) images of such textures and growth zones to guide high spatial resolution secondary ion mass spectroscopy (SIMS) oxygen isotope analyses (10 µm spot) and spatially correlated fluid inclusion microthermometric measurements on successive quartz bands in contemporary Te-rich and Te-poor adularia-sericite (-quartz) epithermal Au-Ag vein deposits in northeastern China. The results show that large positive oxygen isotope shifts from –7.1 to +7.7‰ in quartz rims are followed by precipitation of Au-Ag telluride minerals in the Te-rich deposit, whereas small oxygen isotope shifts of only 4‰ (–2.2 to +1.6‰) were detected in quartz associated with Au-Ag minerals in the Te-poor deposits. Moreover, fluid-inclusion homogenization temperatures are higher in comb quartz rims (avg. 266.4 to 277.5 °C) followed by Au-Ag telluride minerals than in previous stages (~250 °C) in the Te-rich deposit. The Te-poor deposit has a consistent temperature (~245 °C) in quartz that pre- and postdates Au-Ag minerals. Together, the coupled increase in oxygen isotope ratios and homogenization temperatures followed by precipitation of Au-Ag tellurides strongly supports that inputs of magmatic fluid containing Au, Ag, and Te into barren meteoric water-dominated flow systems are critical to the formation of Te-rich adularia-sericite epithermal Au-Ag deposits. In contrast, Te-poor adularia-sericite epithermal Au-Ag deposits show little or no oxygen isotope or fluid-inclusion evidence for inputs of magmatic fluid.

Climatic fluctuations and seasonality during the Kimmeridgian (Late Jurassic): Stable isotope and clay mineralogical data from the Lower Saxony Basin, Northern Germany

In previous palaeoclimatic models, the Kimmeridgian stage has been defined as a typical greenhouse-time interval with weak latitudinal gradients. However, palaeoclimatic information based on biogenic low-Mg calcite δ18O for the Kimmeridgian is still limited. Here, shell materials (n = 81) precipitated by brachiopods, oysters and Trichites bivalves from the Lower Saxony Basin, Northern Germany are evaluated for their potential to act as archive for marine sea-surface temperatures. Furthermore, the associated clay mineral assemblages based on bulk materials are used to infer hinterland weathering patterns and overall climatic conditions simultaneously. The established sea-surface temperature curve reveals an overall slightly warming trend through the Kimmeridgian. Weak seasonality (~4 °C) in sea-surface temperatures is documented by oxygen isotope variations measured along the growth lines of a large Trichites shell. Distinctly higher δ18O values observed in the Lower Kimmeridgian (late Baylei Chron) are interpreted to be related to the short-term influx of cooler boreal water masses. Judging from the corresponding smectite-dominated interval, however, the positive oxygen isotope anomaly may also be partly explained by a relatively drier climate causing enhanced δ18Oseawater values due to intensified evaporation. The kaolinite/(illite + chlorite) ratio points to a slightly long-term decrease in humidity through the Kimmeridgian on the landmasses surrounding the LSB. The short-term fluctuations in humid/arid conditions correlate significantly with sea-level changes, with humid climates accompanying high sea-levels and arid climates accompanying low sea-levels. Results from this study provide new insights into Late Jurassic climatic dynamics and help to establish a reliable Subboreal Late Jurassic sea-surface temperature curve.

Geochemical features of the Menteşe Formation dolostones (Rhaetian) in the Karacahisar-Kasımlar area (Isparta-Turkey)

The purpose of this study is to determine the geochemical characteristics of the Mentese dolostones (Rhaetian) in the west Taurus Mountains, Turkey. The isotope characteristics of the Mentese dolostones show a variation in the δ18O values of the dolomites between + 1.35 and − 2.72‰ (VPDB) and a variation in the δ13C values of the dolomites between + 1.6 and + 3.33‰ (VPDB). Most of the Mentese dolostone samples have positive oxygen isotope values; others have weak negative isotope values, which indicate early diagenetic development. In addition, oxygen, carbon, and strontium isotopes and trace element analyses suggest that these dolostones were probably resulted from insignificant recrystallizaion process than chemically similar fluids. Furthermore, Mn/Sr values of the Mentese dolostones indicate that there is no significant diagenetic alteration. The total REE contents of the Mentese Formation dolostones display a strong positive correlation with Si, Al, Fe, K, Ti, Ba, Rb and negative correlation with Mg, hence REE content is associated with in the first degree titanium-bearing minerals (probably sphene, rutile, anatase), second degree phosphate (apatite), and barium and rubidium-bearing minerals (probably barite). Ce anomalies versus Nd concentrations of the Mentese dolostone samples indicate that these dolostones are formed in an anoxic environment. In addition, the K/Rb ratios of the Mentese dolostones are not exposed to any metamorphism and indicate some diagenetic alteration. Calculated temperature values using δ18O values of the Mentese dolostones yield precipitation temperatures of 17 to 42 °C, and suggest burial depths of 67 to 900 m. Sources of magnesium for dolomitization of the Mentese Formation carbonates were most likely fluids from seawater and/or diagenetic brines (slightly modified seawater). The Mentese dolostones formed as early diagenetic (Rhaetian to Cenomanian) in shallow burial areas at lower temperatures than seawater and/or diagenetic brines (slightly modified seawater).

Turonian Sea Level and Paleoclimatic Events in Astronomically Tuned Records From the Tropical North Atlantic and Western Interior Seaway

AbstractMultiple hiatuses punctuate the Middle to Upper Turonian Substages of the Western Interior Basin (WIB, USA), interrupting an otherwise highly resolved and relatively conformable Upper Cretaceous stratigraphic record. To determine the duration and possible causes of these hiatuses, while simultaneously reconstructing Turonian carbon cycling, we develop a new astronomically tuned carbon isotope (δ13C) chemostratigraphy from time‐equivalent organic carbon‐rich shales of the tropical North Atlantic (Demerara Rise—Ocean Drilling Program Leg 207). A Gaussian kernel smoothing and cross‐correlation technique is also introduced to quantitatively correlate the astronomically tuned Demerara Rise δ13C chemostratigraphy to coeval records globally. This analysis reveals a general consistency among ages for Turonian δ13C excursions from astronomical and radioisotopic timescales. It also indicates that a positive oxygen isotope excursion at Demerara Rise, previously interpreted to signify cooling, is synchronous (~±200 ka) with the Ogbourne Hardground in the English Chalk, the Bass River—Magothy sequence boundary, and an expansive mid‐Turonian hiatus in the WIB. However, the event is not concurrent with maximum regression in the WIB. Trends in δ13C, and additional geochemical data from Demerara Rise (weight percent total organic carbon, weight percent carbonate, and C/N), are consistent with oceanic upwelling conditions modulated by monsoonal winds, which drove significant organic carbon burial instep with several prominent positive δ13C excursions (e.g., Hitchwood #2‐3). Moreover, δ13C and weight percent total organic carbon preserve strong obliquity power (~1 Ma). This suggests that higher cross‐equatorial insolation gradients resulting from changes in Earth's axial tilt intensified monsoonal winds, strongly influencing tropical Atlantic organic carbon burial and Turonian carbon cycling dynamics following Ocean Anoxic Event 2.

Sedimentary environments and geochemistry of Upper Eocene and Lower Oligocene rocks in the northeastern Caucasus

Upper Eocene and Lower Oligocene rocks in the northeastern Caucasus were examined in the most representative Chirkei section (Sulak River basin). Sharp lithogeochemical distinctions between them were revealed. The results of the study of nannoplankton demonstrated that the Eocene/Oligocene interface occurs slightly below the boundary between the Belaya Glina and Khadum formations. The studied section revealed a series of nannoplankton bioevents facilitating its stratigraphic subdivision. It has been established that organic matter (OM) in rocks of the Khadum Formation is characterized by a relatively high degree of maturity. Probably, the material of mainly marine genesis contains a terrigenous OM admixture. Positive oxygen isotope anomaly in the upper part of the Belaya Glina Formation reflects global climate changes (cooling) near the Eocene/Oligocene interface. Limitation of the anomaly by the upper boundary of the Belaya Glina Formation is likely related to changes in water salinity variations in the Early Oligocene basin and intense early diagenetic processes in rocks therein. Lithological, geochemical, and paleoecological data suggest that the Khadum paleobasin was depleted in oxygen. Such environment was unstable with periodic intensification or attenuation. Paleoecology in the Belaya Glina basin was typical of normally aerated basins.

Examining the interplay of climate and low amplitude sea‐level change on the distribution and volume of massive dolomitization: Zebbag Formation, Cretaceous, Southern Tunisia

AbstractDuring the Cretaceous, a humid global climate, calcitic seas, high relative sea‐level and low amplitude changes in relative sea‐level largely prevented large‐scale dolomitization in many carbonate successions. However, the well‐exposed shallow‐water carbonate sediments of the Upper Albian–Lower Turonian Zebbag Formation on the Jeffara Escarpment, southern Tunisia, are pervasively dolomitized. This study considers why dolomitization was so widespread in this region during a period of Earth history when platform‐scale dolomitization is rare. Marine conditions were established in the Upper Albian, evidenced by stacked upward‐shallowing packages of shallow subtidal to peritidal carbonate sediments in the basal Rhadouane Member. A gradual increase in the volume of subtidal sediments in the Cenomanian Kerker Member, culminated in deposition of laterally extensive marls, during maximum flooding of the platform in the Lower Turonian. The overlying Gattar Member was then deposited in shallower water as relative sea‐level fell. The entire Zebbag Formation is pervasively replaced by stratabound, fabric‐retentive, dolomite, except within the marl at the top of the Kerker Member, which is only partially dolomitized. Petrographic textures indicate dolomitization largely post‐dated marine cementation and platform emergence but pre‐dated chemical compaction. Slightly more positive oxygen isotope signatures, slightly elevated concentrations of Sr and a near‐absence of evaporites are consistent with dolomitization by reflux of mesohaline sea water. An upward‐decrease in major element concentrations and higher 87Sr/86Sr compared to Upper Cretaceous sea water suggest that basal, Albian siliciclastic beds acted as aquifers facilitating dolomitization by fluxing fluids offshore. Dolomitization is interpreted to have resulted from multiple fluxes of sea water over periods of 0·5 to 2·5 Ma. The unusually high volume of dolostone for a platform of this age most probably reflects deposition within an arid climate belt, where an efficient reflux system was facilitated by basal, permeable siliciclastic strata.

Middle–Upper Ordovician (Darriwilian–Early Katian) positive carbon isotope excursions in the northern Tarim Basin, northwest China: Implications for stratigraphic correlation and paleoclimate

Three positive carbon isotope excursions are reported from Middle–Upper Ordovician in Tahe oil-gas field, northern Tarim Basin. Based on conodont biostratigraphy, the Middle Darriwilian Isotope Carbon Excursion (MDICE) and the Guttenberg Carbon Isotope Excursion (GICE) are identified from Darriwilian to Early Katian by the aid of whole-rock carbon isotope data from two well cores. Positive excursion within conodont Pygodus anserinus zone is developed in Early Sandbian, and the fluctuation range is no less than MDICE. Because the range of this excursion in the generalized global carbon isotope curve is short, previous studies paid little attention to it, and named Early Sandbian Isotope Carbon Excursion (ESICE) in this paper. Furthermore, these positive excursions are not directly related to sea level fluctuations and the MDICE and GICE identified in northern Tarim can be globally correlated to that in southern China, North America, South America, and Europe. The Saergan Fm. source rocks of Middle-Upper Ordovician in Kalpin Dawangou outcrop are in accord with the geologic time of MDICE and ESICE, and GICE have strong ties to the source rock of Lianglitag Fm. in basin. Abundant organic carbon burial is an important factor in genesis of positive isotope carbon excursions. Positive oxygen isotope excursion, conodont fauna turnover, decreased conodont total diversity, and the change of sedimentary facies indicated that dramatic changes of paleoceanographic environment of Early-Middle Ordovician in Tarim Basin started from the end of Darriwillian, and an obvious icehouse climate of Late Ordovician occurred in ESICE.

Expansion and diversification of high-latitude radiolarian assemblages in the late Eocene linked to a cooling event in the southwest Pacific

Abstract. The long-term cooling trend from middle to late Eocene was punctuated by several large-scale climate perturbations that culminated in a shift to "icehouse" climates at the Eocene–Oligocene transition. We present radiolarian micro-fossil assemblage and foraminiferal oxygen and carbon stable isotope data from Deep Sea Drilling Project (DSDP) sites 277, 280, 281, and 283 and Ocean Drilling Project (ODP) Site 1172 to identify significant oceanographic changes in the southwest Pacific through this climate transition (~ 40–30 Ma). We find that the Middle Eocene Climatic Optimum at ~ 40 Ma, which is truncated but identified by a negative shift in foraminiferal δ18O values at Site 277, is associated with a small increase in radiolarian taxa with low-latitude affinities (5 % of total fauna). In the early late Eocene at ~ 37 Ma, a positive oxygen isotope shift at Site 277 is correlated with the Priabonian Oxygen Isotope Maximum (PrOM). Radiolarian abundance, diversity, and preservation increase within this cooling event at Site 277 at the same time as diatom abundance. A negative δ18O excursion above the PrOM is correlated with a late Eocene warming event (~ 36.4 Ma). Radiolarian abundance and diversity decline within this event and taxa with low-latitude affinities reappear. Apart from this short-lived warming event, the PrOM and latest Eocene radiolarian assemblages are characterised by abundant high-latitude taxa. High-latitude taxa are also abundant during the late Eocene and early Oligocene (~ 38–30 Ma) at DSDP sites 280, 281, 283 and 1172 and are associated with very high diatom abundance. We therefore infer a northward expansion of high-latitude radiolarian taxa onto the Campbell Plateau in the latest Eocene. In the early Oligocene there is an overall decrease in radiolarian abundance and diversity at Site 277, and diatoms are scarce. These data indicate that, once the Antarctic Circumpolar Current was established in the early Oligocene (~ 30 Ma), a frontal system similar to present day developed, with nutrient-depleted Subantarctic waters bathing the area around DSDP Site 277, resulting in a more restricted siliceous plankton assemblage.

Influence of West Antarctic Ice Sheet collapse on Antarctic surface climate

AbstractClimate model simulations are used to examine the impact of a collapse of the West Antarctic Ice Sheet (WAIS) on the surface climate of Antarctica. The lowered topography following WAIS collapse produces anomalous cyclonic circulation with increased flow of warm, maritime air toward the South Pole and cold‐air advection from the East Antarctic plateau toward the Ross Sea and Marie Byrd Land, West Antarctica. Relative to the background climate, areas in East Antarctica that are adjacent to the WAIS warm, while substantial cooling (several ∘C) occurs over parts of West Antarctica. Anomalously low isotope‐paleotemperature values at Mount Moulton, West Antarctica, compared with ice core records in East Antarctica, are consistent with collapse of the WAIS during the last interglacial period, Marine Isotope Stage 5e. More definitive evidence might be recoverable from an ice core record at Hercules Dome, East Antarctica, which would experience significant warming and positive oxygen isotope anomalies if the WAIS collapsed.

Organic Carbon and Stable C-O Isotopic Study of the Lower Silurian Longmaxi Formation Black Shales in Sichuan Basin, SW China: Paleoenvironmental and Shale Gas Implications

In order to better decipher the paleoenvironment of the Early Silurian black shale in the southern Sichuan Basin and constrain their shale gas potential, organic carbon and stable C-O isotopic studies have been performed on the Longmaxi Formation from Well N203. The organic carbon isotopic studies show a relative low δ13Corgvalue at the lower part of the Longmaxi Formation, corresponding to the negative stable carbon isotope (δ13C) excursion and positive oxygen isotope (δ18O) excursion. These isotopic features indicate an anoxic environment at the lower part of the Longmaxi Formation, which might be related to the high atmosphere temperature and sea-level rise. The anoxic environment is also proved by framboidal pyrites studies, which present diameter of most framboids in the range of 3–5 μm. The anoxic environment promoted the deposition of organic carbon, leading to the high TOC content at the lower part. This anoxic environment only lasted for a short period, with a gradually evolution to oxic environment at the middle and upper part of the Longmaxi Formation, evidenced by very low TOC content, increasing δ13Corgvalues, positive δ13C values and negative δ18O values. Our studies have further constrained the best source of rocks for shale gas at the lower part of the Longmaxi Formation.

Unusual isotope effects on the pseudogap in high-[formula omitted] cuprate superconductors as support for the BCS-like pairing theory of large polarons above [formula omitted

The BCS-like pairing theory is extended to the intermediate coupling regime and to the cases of exotic cuprate superconductors with large and small Fermi surfaces, so as to describe the pairing correlations above Tc, the opening of a pseudogap (PG) at a mean-field temperature T∗>Tc and the unusual isotope effects on the PG in these materials within the large polaron model and two different BCS-like approaches. We argue that unconventional electron–phonon interactions are responsible for the polaron formation and the separation between temperatures T∗ (the onset of precursor Cooper pairing) and Tc (the onset of the superconducting transition) in exotic cuprate superconductors. Using the extended BCS-like approaches, we calculate the PG formation temperature T∗, isotope shifts ΔT∗, oxygen and copper isotope exponents and show that isotope effects on the PG basically depend on strengths of Coulomb and electron–phonon interactions, doping levels and dielectric constants of the cuprates. The new BCS-like pairing theory of polaronic carriers predicts the existence of small and sizable positive oxygen isotope effect and very large negative oxygen and copper isotope effects on the PG in the cuprates with large Fermi surfaces. The calculated results for T∗, isotope shifts and exponents are compared with experimental data on various cuprate superconductors. For all the considered cases, a good quantitative agreement was found between theory and experimental data. We also predict the existence of small and sizable negative isotope effects on T∗ in deeply underdoped cuprates with small Fermi surfaces. Further, we find that the isotope effects on T∗(=Tc) in heavily overdoped cuprates just like in some metals are relatively small positive or become even negative.

Oxygen stable isotope ratios from British oak tree-rings provide a strong and consistent record of past changes in summer rainfall

United Kingdom (UK) summers dominated by anti-cyclonic circulation patterns are characterised by clear skies, warm temperatures, low precipitation totals, low air humidity and more enriched oxygen isotope ratios (δ18O) in precipitation. Such conditions usually result in relatively more positive (enriched) oxygen isotope ratios in tree leaf sugars and ultimately in the tree-ring cellulose formed in that year, the converse being true in cooler, wet summers dominated by westerly air flow and cyclonic conditions. There should therefore be a strong link between tree-ring δ18O and the amount of summer precipitation. Stable oxygen isotope ratios from the latewood cellulose of 40 oak trees sampled at eight locations across Great Britain produce a mean δ18O chronology that correlates strongly and significantly with summer indices of total shear vorticity, surface air pressure, and the amount of summer precipitation across the England and Wales region of the United Kingdom. The isotope-based rainfall signal is stronger and much more stable over time than reconstructions based upon oak ring widths. Using recently developed methods that are precise, efficient and highly cost-effective it is possible to measure both carbon (δ13C) and oxygen (δ18O) isotope ratios simultaneously from the same tree-ring cellulose. In our study region, these two measurements from multiple trees can be used to reconstruct summer temperature (δ13C) and summer precipitation (δ18O) with sufficient independence to allow the evolution of these climate parameters to be reconstructed with high levels of confidence. The existence of long, well-replicated oak tree-ring chronologies across the British Isles mean that it should now be possible to reconstruct both summer temperature and precipitation over many centuries and potentially millennia.

Oxygen isotope anomaly observed in water vapor from Alert, Canada and the implication for the stratosphere

To identify the possible anomalous oxygen isotope signature in stratospheric water predicted by model studies, 25 water vapor samples were collected in 2003-2005 at Alert station, Canada (82°30'N), where there is downward transport of stratospheric air to the polar troposphere, and were analyzed for δ(17)O and δ(18)O relative to Chicago local precipitation (CLP). The latter was chosen as a reference because the relatively large evaporative moisture source should erase any possible oxygen isotope anomaly from the stratosphere. A mass-dependent fractionation coefficient for meteoric waters, λMDF(H2O) = 0.529 ± 0.003 [2σ standard error (SE)], was determined from 27 CLP samples collected in 2003-2005. An oxygen isotopic anomaly of Δ(17)O = 76 ± 16 ppm (2σ SE) was found in water vapor samples from Alert relative to CLP. We propose that the positive oxygen isotope anomalies observed at Alert originated from stratospheric ozone, were transferred to water in the stratosphere, and subsequently mixed with tropospheric water at high latitudes as the stratospheric air descended into the troposphere. On the basis of this ground signal, the average Δ(17)O in stratospheric water vapor predicted by a steady-state box model is ∼40‰. Seven ice core samples (1930-1991) from Dasuopu glacier (Himalayas, China) and Standard Light Antarctic Precipitation did not show an obvious oxygen isotope anomaly, and Vienna Standard Mean Ocean Water exhibited a negative Δ(17)O relative to CLP. Six Alert snow samples collected in March 2011 and measured at Laboratoire des Sciences du Climat et de l'Environnement, Gif sur Yvette, France, had (17)Oexcess of 45 ± 5 ppm (2σ SE) relative to Vienna Standard Mean Ocean Water.

Variations in deep-sea benthic foraminifera at ODP Hole 756B, southeastern Indian Ocean: Evidence for changes in deep ocean circulation

Seventy three core samples of 10cc volume were analysed from Ocean Drilling Program (ODP) Site 756, Hole B with an average interval of 236kyr per sample. Factor and cluster analyses of 37 high ranking species of benthic foraminifera enabled us to identify four clusters representing four biofacies. Biofacies Sl-Om is characterised by Stilostomella lepidula, Orthomorphina modesta, Robulus gibbus, Bulimina miolaevis and Cibicides cf. lucidus. This biofacies dominates the latest Oligocene to early Miocene interval (24–21Ma and 18.75–16.5Ma), indicating low to intermediate dissolved oxygen, organic carbon rich environment, warm and sluggish deep waters. During 21–18.75Ma, the biofacies Gp-Nu (Globocassidulina pacifica, Nuttallides umbonifera, Trifarina angulosa, Quinqueloculina weaveri, Robulus gibbus and Pullenia bulloides) dominates the benthic assemblage at Hole 756B. This biofacies also has a short lived presence at ~16.5Ma. This was an interval of intense circulation of cold and corrosive deep water similar to Antarctic Intermediate Water (AAIW), and intermediate to high flux of organic matter to Hole 756B. The Antarctic Circumpolar Current (ACC) initiated during this time. From 16.5 to 10.6Ma, the benthic assemblage at Hole 756B is dominated by biofacies Ub-Gs (characteristic species Uvigerina buzasi, Globocassidulina subglobosa, Laticarina pauperata), indicating intermediate to high food supply and cold deep water. This interval coincides with the middle Miocene positive oxygen isotope shift and permanent build up of Antarctic ice sheets. The late Miocene interval (10.6–6.5) is characterised by biofacies Ec-Bs (dominant species Ehrenbergina carinata, Bolivina spathulata, Osangularia culter and Pullenia osloensis) indicating low oxygen condition with high food supply in the study area. This corresponds to an interval of global biogenic bloom.