Oxygen Isotope Composition Of Phosphate Research Articles

Water utilization of the Cretaceous Mussentuchit Member local vertebrate fauna, Cedar Mountain Formation, Utah, USA: Using oxygen isotopic composition of phosphate

While the oxygen isotopic composition of pedogenic carbonate has successfully been used to address the effects of global climate change on the hydrologic cycle, detailed regional paleohydrologic studies are lacking. Since the hydrologic cycle can vary extensively on local or regional scales due to events such as such as mountain building, and since pedogenic carbonates (calcite) form in a narrow moisture regime, other proxies, such as vertebrate remains, must be used to decipher local versus regional variations in paleohydrology. In this study, the oxygen isotopic composition (δ 18O p) of phosphatic remains from a diverse set of vertebrate fossils (fish, turtles, crocodiles, dinosaurs, and micro-mammals) from the Mussentuchit Member (MM) of the Cedar Mountain Formation, Utah, USA (Aptian to Cenomanian) are analyzed in order to determine differences among the available water reservoirs and water utilization of each taxon. Calculated changes in water reservoir δ 18O w over time are then used to determine the effects of the incursion of the Western Interior Seaway (WIS) and the Sevier Mountains on paleohydrology during the MM time. Calculation of δ 18O w from the results of isotopic analysis of phosphate oxygen suggests that turtles and crocodiles serve as another proxy for meteoric water δ 18O that can be used as a measure of average local precipitation δ 18O w similar to pedogenic calcite. Pedogenic calcites can be slightly biased toward higher values, however, due to their formation during evaporative conditions. Turtles and crocodiles can be used in place of pedogenic calcite in environments that are not conducive to pedogenic carbonate formation. Remains of fish with rounded tooth morphology have δ 18O p values that predict temperatures consistent with other estimates of mean annual temperature for this latitude and time. The δ 18O p of ganoid scales and teeth with pointed morphology, however, indicates that these skeletal materials were precipitated from water that is 18O-enriched due to migration to either evaporatively enriched water, or 18O-enriched estuarine waters of the Western Interior Seaway (WIS). Another possibility that cannot be discounted and assuming all morphological remains are from the same taxon, is that the pointed teeth and ganoid scales precipitated at different temperatures than rounded teeth. Mammal and herbivorous dinosaur δ 18O p suggests they primarily drank isotopically depleted river water. Co-existence of crocodiles, turtles, and mammals allows for calculation of relative humidity from site to site and these calculations suggest humidity averaged ~ 58% and ranged between ~ 42% and ~ 76%. The δ 18O w values estimated from semi-aquatic taxa and pedogenic calcite suggest dominance of WIS-derived moisture during their growth. Herbivorous dinosaurs particularly indicate that altitude and catchment effects from the Sevier Mountains are seemingly important for river water δ 18O w in the fall through early spring. These data suggest that temporal changes in the isotopic composition of the MM fauna are produced by the small-scale regressive–transgressive cycles of the WIS.

Triple oxygen isotope analysis of bioapatite as tracer for diagenetic alteration of bones and teeth

The detection of diagenetic alteration is critical for palaeoclimate reconstruction that is based on the oxygen isotope composition of fossil bones and teeth. So far, no direct chemical proxy has been found to track diagenetic modification of the oxygen isotope ratios. Here, a new approach to identify diagenetic changes of δ 18O PO4 values in skeletal apatite of small mammals by means of triple oxygen isotope analysis ( 16O, 17O and 18O) is presented. Our method is based on the fact that inhaled air oxygen (O 2) has an isotope anomaly on its rare isotope 17O. Inhaled air O 2 is a major source of oxygen in small land-living mammals. A fraction of the anomaly is transferred via body water to skeletal apatite, where it can be detected by means of δ 17O and δ 18O analyses. The approach, considering the current analytical uncertainty, is restricted to small mammals with body masses ≤ 1 kg. This is due to the low specific metabolic rates of large mammals, resulting in a lower fraction of oxygen inhaled via breathing relative to oxygen from other sources in their body water. Remnant negative 17O anomalies derived from in vivo inhaled O 2 have been detected in enamel bioapatite of Eocene to Miocene rodent teeth while dentine of the same teeth lacks significant 17O anomalies. This suggests preservation of the original phosphate oxygen isotope composition in enamel of these small mammal teeth. In contrast, 17O anomalies in dentine have been erased due to diagenetic alteration with isotopically normal diagenetic fluids. Triple oxygen isotope analysis of bioapatite thus seems to be a useful new proxy to directly detect diagenetic alterations of the δ 18O PO4 values of small mammal teeth.

Late Pleistocene climatic change in the French Jura (Gigny) recorded in the δ 18O of phosphate from ungulate tooth enamel

Oxygen isotope compositions of phosphate in tooth enamel from large mammals (i.e. horse and red deer) were measured to quantify past mean annual air temperatures and seasonal variations between 145 ka and 33 ka in eastern France. The method is based on interdependent relationships between the δ 18O of apatite phosphate, environmental waters and air temperatures. Horse ( Equus caballus germanicus) and red deer ( Cervus elaphus) remains have δ 18O values that range from 14.2‰ to 17.2‰, indicating mean air temperatures between 7°C and 13°C. Oxygen isotope time series obtained from two of the six horse teeth show a sinusoidal-like signal that could have been forced by temperature variations of seasonal origin. Intra-tooth oxygen isotope variations reveal that at 145 ka, winters were colder (− 7 ± 2°C) than at present (3 ± 1°C) while summer temperatures were similar. Winter temperatures mark a well-developed West–East thermal gradient in France of about − 9°C, much stronger than the −4°C difference recorded presently. Negative winter temperatures were likely responsible for the extent and duration of the snow cover, thus limiting the food resources available for large ungulates with repercussions for Neanderthal predators.

How cold was it for Neanderthals moving to Central Europe during warm phases of the last glaciation?

Precise estimates of mean annual temperature (MAT) for when Neanderthals occupied Central Europe are critical for understanding the role that climatic and associated environmental factors played in Neanderthal migrations and in their ultimate extinction. Neanderthals were continuously present in the relatively warm regions of southern and Western Europe in the Pleistocene but only temporarily settled Central Europe (CE), presumably because of its colder and less hospitable climate. Here, we present a new approach for more spatially and temporally accurate estimation of palaeotemperatures based on the stable oxygen isotope composition of phosphates extracted from animal teeth found at sites linked directly to concurrent Neanderthal occupation. We provide evidence that Neanderthals migrated along the Odra Valley of CE during warmer periods throughout the Upper Pleistocene. The MATs during these migrations were about 6.8 °C for the warm phase of Oxygen Isotope Stage OIS 5a–d (prior to the OIS4 cold event) at ∼115–74,000 yr BP and about 6.3 °C during the early OIS 3 warm phase ∼59–41,000 yr BP. Our results show that temperatures during these phases peaked 2–4 °C above longer term estimates from ice cores and pollen records. We argue that our approach can provide valuable insights into evaluating the role of climate in human migration patterns in the Pleistocene.

Advances in the Measurement Methods and Fractionation Mechanism of the Oxygen Isotope Composition of Phosphate

Oxygen isotope composition of phosphate is an important tracer for paleoclimatology and phosphorus cycle studies.The measurement methods and the fractionation mechanism are the basis of this kind of study.There have been a lot of these kinds of research.The measurement methods are enveloped from the indirectly methods,to high temperature reduction or pyrolyze,then to fluorination,to improved high temperature reduction,at last,to TC/EA-IRMS,and even to the laser in situ technology.The samples are extended from pure chemical regents to the geology samples.The measurement precision,sample amount,data output,safety and technical requirement are all improved greatly.For the isotope fractionation mechanism,① Although Longinelli′s formula was confirmed by nature samples,and was regarded as equilibrium fractionation,but the results of culture experiments did not agree with the formula(not equilibrium).②For the nature temperature and pH of the earth surface,non-biological processes can not make the oxygen exchange between the phosphate and water.But at high temperature(70℃),phosphate and water can exchange oxygen intensively at different pH,even without biological activity.And the results of different laboratory disagree with each other.③ The phosphate can exchange oxygen water intensively under biological process.But all are away from equilibrium fractionation except the PPase.④ The oxygen isotope of apatite was higher than the dissolved phosphate by 1‰～1.4‰.Therefore,when the Longinelli′s formula was used to the dissolved phosphate attention should be paid to this.The equilibrium fractionation is condition depended,so the disagreement of non-biological experiment results at high temperature(70℃),and the non-equilibrium fractionation of the biological culture experiments,are all the results of condition change,including pH,concentration of phosphate,kinds of organism,biomass and so on.

A method for the analysis of theδ18O of inorganic phosphate extracted from soils with HCl

The oxygen isotope composition of phosphate (δ18O‐PO4) has successfully been used to study the biological cycling of phosphorus (P) in seawater and marine sediments. However, only a few studies have used this approach in soils. In order to analyseδ18O‐PO4, phosphate must be extracted from the soil, purified and converted to silver phosphate (Ag3PO4). The published extraction methods, successfully applied to marine waters and sediments, lead to the precipitation of impure Ag3PO4when used with soils or organic‐rich samples. Here we present an improved purification protocol, designed for soils and other organic‐rich samples. After extraction with HCl, phosphate is purified with multiple mineral precipitations that do not require extreme pH adjustments of the solutions. We show that contaminant‐free Ag3PO4can be produced from fertilizers and various soils with different chemical and physical characteristics. Our first isotopic results confirm that differences in P status and availability in soils are expressed in theδ18O‐PO4signal, indicating the potential of this isotopic tracer to understand P dynamics in soil systems.

Phosphorus dynamics in soils irrigated with reclaimed waste water or fresh water — A study using oxygen isotopic composition of phosphate

Transformations of phosphate (Pi) in different soil fractions were tracked using the stable isotopic composition of oxygen in phosphate ( δ 18O p) and Pi concentrations. Clay soil from Israel was treated with either reclaimed waste water (secondary, low grade) or with fresh water amended with a chemical fertilizer of a known isotopic signature. Changes of δ 18O p and Pi within different soil fractions, during a month of incubation, elucidate biogeochemical processes in the soil, revealing the biological and the chemical transformation impacting the various P pools. P in the soil solution is affected primarily by enzymatic activity that yields isotopic equilibrium with the water molecules in the soil solution. The dissolved P interacts rapidly with the loosely bound P (extracted by bicarbonate). The oxides and mineral P fractions (extracted by NaOH and HCl, respectively), which are considered as relatively stable pools of P, also exhibited isotopic alterations in the first two weeks after P application, likely related to the activity of microbial populations associated with soil surfaces. Specifically, isotopic depletion which could result from organic P mineralization was followed by isotopic enrichment which could result from preferential biological uptake of depleted P from the mineralized pool. Similar transformations were observed in both soils although transformations related to biological activity were more pronounced in the soil treated with reclaimed waste water compared to the fertilizer treated soil.

Phosphate oxygen isotopic evidence for a temperate and biologically active Archaean ocean

Oxygen and silicon isotope compositions of cherts and studies of protein evolution have been interpreted to reflect ocean temperatures of 55-85 degrees C during the early Palaeoarchaean era ( approximately 3.5 billion years ago). A recent study combining oxygen and hydrogen isotope compositions of cherts, however, makes a case for Archaean ocean temperatures being no greater than 40 degrees C (ref. 5). Ocean temperature can also be assessed using the oxygen isotope composition of phosphate. Recent studies show that (18)O:(16)O ratios of dissolved inorganic phosphate (delta(18)O(P)) reflect ambient seawater temperature as well as biological processing that dominates marine phosphorus cycling at low temperature. All forms of life require and concentrate phosphorus, and as a result of biological processing, modern marine phosphates have delta(18)O(P) values typically between 19-26 per thousand (VSMOW), highly evolved from presumed source values of approximately 6-8 per thousand that are characteristic of apatite in igneous rocks and meteorites. Here we report oxygen isotope compositions of phosphates in sediments from the 3.2-3.5-billion-year-old Barberton Greenstone Belt in South Africa. We find that delta(18)O(P) values range from 9.3 per thousand to 19.9 per thousand and include the highest values reported for Archaean rocks. The temperatures calculated from our highest delta(18)O(P) values and assuming equilibrium with sea water with delta(18)O = 0 per thousand (ref. 12) range from 26 degrees C to 35 degrees C. The higher delta(18)O(P) values are similar to those of modern marine phosphate and suggest a well-developed phosphorus cycle and evolved biologic activity on the Archaean Earth.

A preliminary in situ Raman spectroscopic study of the oxygen isotope exchange kinetics between H 2O and (PO 4) aq

Studies on the mechanism and kinetics of the oxygen isotope exchange reaction between H2O and (PO4)aq are essential for the interpretation of natural variations in the oxygen isotope composition of biogenic and inorganic phosphate and utilization of the oxygen isotope composition of phosphates as a geochemical indicator. To gain a deeper understanding of the exchange kinetics we have started an in situ Raman spectroscopy study using an in-house-made Teflon-based fluid cell. Raman spectroscopy allows monitoring the exchange reaction because the substitution of O in aqueous phosphate species by O should cause a shift of those vibrational modes to lower frequencies which involve the motion of oxygen due to the different masses of both isotopes. First in situ exchange experiments have been carried out with a 2 M (NH4)2HPO4 solution (room temperature pH of 8) at 150(±1) °C and 175(±1) °C for 112 and 45 h, respectively. Speciation calculations show that under these conditions HPO4 is the main aqueous phosphate species. The phosphate solution was prepared with H2O enriched with 97 at.% O. At the beginning of the reaction at the experimental temperatures the Raman spectrum of the HPO4 phosphate solution is characterised by distinct bands near 310, 321, 366, 384, 398, 533, 867, 983, and 1076 cm. The latter three bands are the most intense bands and can be assigned to the ν(P–OH), νs(PO3), and the νas(PO3) vibration of the HPO4 molecule (based on a C3v symmetry), respectively. All bands involving oxygen motions are

Phosphate oxygen isotope analysis on microsamples of bioapatite: removal of organic contamination and minimization of sample size

Modern and fossil teeth record seasonal information on climate, diet, and migration through stable isotope compositions in enamel and dentine. Climatic signals such as seasonal variation in meteoric water isotopic composition can be recovered through a microscale histology-based sampling and isotopic analysis of enamel phosphate oxygen. The phosphate moiety in bioapatite is particularly resistant to post mortem diagenesis. In order to determine the phosphate oxygen isotope composition of enamel, phosphate must be chemically purified from other oxygen sources in the enamel lattice and matrix, mainly hydroxyl and carbonate ions, and trace quantities of organics. We present a wet chemical technique for purifying phosphate from microsampled enamel and dentine. This technique uses a sodium hypochlorite oxidation step to remove interferences from residual organic constituents of the enamel and/or dentine scaffold, isolates phosphate as relatively large and easily manipulated Ag(3)PO(4) crystals by using a strongly buffered, moderate-temperature microprecipitation, and preserves the oxygen isotope composition of the initial tooth phosphate. The reproducibility of phosphate oxygen isotope compositions thus determined (measured as delta(18)O, V-SMOW scale) is typically 0.2-0.3 per thousand (1 s.d.) on samples as small as 300 microg of enamel or dentine, a considerable improvement over available techniques for analyses of bioapatite phosphate oxygen.

Is small beautiful? A review of the advantages and limitations of using small mammal teeth and the direct laser fluorination analysis technique in the isotope reconstruction of past continental climate change

Oxygen isotopes in mammalian tooth enamel have become a widely used proxy for reconstructing past continental climate change. However, owing to sample size constraints in the chemical separation and precipitation of the phosphate prior to oxygen isotope analysis, this technique has been, until recently, limited to relatively large mammal teeth. As the result of recent developments in chemical and analytical techniques, including the development of a novel pre-treatment technique and a direct laser fluorination methodology of analysis, small mammal teeth (rodents), which are relatively more abundant in the fossil record than large mammal teeth, can now be used. Published examples from the recent literature are provided with the aim of examining the advantages and limitations of using small mammal teeth in palaeoclimate reconstruction studies. Issues that are addressed include 1) the rationale behind the calculation of the mean oxygen isotope composition of local water and the problems associated with the intake of 18O enriched water from plant food and from small water bodies fractionated as a result of evaporation; 2) the methodology, justification and limitations in determining mean annual temperatures (MATs) from calculated δ 18O local water values and also combining the δ 18O local water values with δ 18O values from freshwater biota to calculate, using established thermometry equations, multiple palaeotemperatures of differing climatic significance; 3) specific published examples of how the phosphate oxygen isotope composition of small mammal teeth are being used in studies of continental climate change.

Oxygen isotope compositions of phosphate from Middle Miocene–Early Pliocene marine vertebrates of Peru

Phosphatic remains of marine vertebrates recovered from five fossil sites of the Pisco Formation ranging from the latest Middle/earliest Late Miocene (Ca 11–13 Ma) to the Early Pliocene (Ca 3.5 Ma) have been analysed for their oxygen isotope compositions ( δ 18O p). Coexisting seals, dolphins, whales, penguins and sharks from each locality have distinct δ 18O p values reflecting ecology and physiology differences, ranging from 18.2‰ to 21.4‰ for marine mammals, from 19.5‰ to 21.5‰ for marine birds and from 20.9‰ to 23.1‰ for sharks. Systematic offsets observed between dolphin teeth and bones as well as between dolphin and whale bones indicate that the fractionation equation established by using data from extant cetaceans may not be directly applicable to Miocene cetaceans in order to estimate water δ 18O w values. Assuming that polar ice-caps were not totally developed during this time interval, marine palaeotemperatures ranging from 13.0 ± 1.3 °C to 17.2 ± 1.3 °C were estimated. Comparison of our results with those obtained in other World's areas suggests that the oxygen isotope ratios of Pisco vertebrates reflect the influence of both global and local events, such as the setting of the Atacama Desert, the cold Humboldt Current or the global phases of ice-cap growth and decay.

Oxygen isotope fractionation between crocodilian phosphate and water

Oxygen isotope compositions of phosphate (δ 18O p) were measured in tooth enamel from captive and wild individuals of 8 crocodilian species. A rough linear correlation is observed between the δ 18O p of all the studied species and the oxygen isotope composition of ambient water (δ 18O w). Differences in mean air temperature, diet and physiology could contribute significantly to the large scatter of δ 18O p values. The combination of these parameters results in a fractionation equation for which the slope (0.82) is lower than that expected (≥ 1) from predictive model equations that assume temperature and diet as fixed parameters. Taking into account large uncertainties, the observed oxygen isotope fractionation between phosphate and ambient water does not statistically differ from that formerly established for aquatic turtles. Case studies show that δ 18O p values of fossil crocodile tooth enamel can be used to discriminate between marine and freshwater living environments within a precision of about ± 2‰ only.

Oxygen isotope composition of phosphate in organic compounds: Isotope effects of extraction methods

Organophosphorus (P org) is an important phosphorus (P) source in aquatic environments and P org cycling is an integral part of the overall biogeochemical cycle of P. It has been recently demonstrated that the 18O/ 16O ratio of PO 4 3 - ( δ 18O P) is a useful tracer of reactions and processes controlling the biogeochemical cycling of inorganic phosphate (P i) in natural waters. The goal of this work was to develop methods that can recover the δ 18O value of organically bound PO 4 3 - moiety groups in P org ( δ 18 O P org ). This work compares the oxygen (O) isotope effects of four different P i extraction methods: dry combustion, acid hydrolysis, UV radiation (UVR) and enzyme-catalyzed hydrolysis. The dry combustion method for releasing PO 4 3 - from P org promotes extensive oxygen isotope exchange between phosphate and atmospheric O 2 and thus, erases the original δ 18 O P org value. Both acid and enzyme-catalyzed hydrolysis of P org incorporate oxygen atoms from water into extracted P i and thus δ 18O values of P i extracted using these two methods only partially retain the original δ 18 O P org value. The oxygen isotope effect on P i extracted from P org using UVR methods shows that the original δ 18 O P org values of phosphomonoesters are retained in extracted PO 4 3 - moieties, and the oxygen isotope effects on P i extracted from phosphodiesters (e.g., due to water incorporation) are small. Thus, δ 18O values of PO 4 3 - moieties extracted from P org may be applied to trace P org sources and also to study water-mass sources and mixing processes.

The oxygen isotopic composition of phosphate in Elkhorn Slough, California: A tracer for phosphate sources

Elkhorn Slough, a small seasonal estuary in central California, has been subjected to increased nutrient loading from agricultural and other non-point sources. However, because nutrients do not behave conservatively, tracing nutrient sources and cycling in ecosystems like Elkhorn Slough has been difficult to assess. This is particularly true of phosphorus (P), which has only one stable isotope and cannot be used as an isotopic tracer. However, isotopic fractionation of oxygen in phosphate at surface water temperatures only occurs as a result of enzyme-mediated, biochemical reactions. Thus, if phosphate demand is low relative to input and is not heavily cycled within the ecosystem, the δ 18O of phosphate will reflect the isotopic composition of phosphate sources to the system. We utilized the δ 18O of dissolved inorganic phosphate (DIP) within the main channel of the slough and nearby Moss Landing Harbor and the δ 18O of reactive phosphate from sediment and soil samples collected within the watershed to understand phosphate sources and cycling within Elkhorn Slough. Trends in the δ 18O of DIP were seasonally consistent with high values near the mouth reflecting oceanic phosphate (19.1‰–20.3‰), dropping to a minimum value near Hummingbird Island in the central slough (point source, 14.1‰–14.4‰), and increasing again near the head of the slough, reflecting fertilizer input (18.9‰–19.3‰). Reactive phosphate δ 18O values extracted from sediments and soils in the watershed range from 10.6‰ in a drainage ditch to 22.3‰ in creek sediments near agriculture fields. The wide range in phosphate δ 18O values reflects the variations in land use and application of different fertilizers in this agriculturally dominated landscape. These data suggest that phosphate δ 18O can be an effective tool for identifying P sources and understanding phosphate dynamics in estuarine ecosystems.

Oxygen Isotope Composition Of Human Teeth And The Record Of Climate Changes In France (Lorraine) During The Last 1700 Years

In order to study climate variations during the last 1700 years in eastern France, fifty-eight oxygen isotope compositions of phosphate were measured in human tooth enamel. The individuals, who lived in Lorraine, are assumed to have drunk local water derived directly from rainfall. According to previous work, drinking water is the main source of oxygen that sets the isotopic composition of phosphatic tissues in humans. The empirical fractionation equation determined from our data combined with those of Longinelli's one [Geochim. Cosmochim. Acta 48 (1984) 385] was used to calculate the oxygen isotope composition of meteoric waters. The mean air temperature was inferred from these isotope ratios and the Von Grafenstein et al.'s [Geochim. Cosmochim. Acta 60 (1996) 4025] relationship between δ18O and air temperature. Oxygen isotope composition of present-day individuals yields a mean air temperature of 9.9 ± 1.7 ◦C which is consistent with meteorological data. Application of this method to historical individuals results in mean air temperatures estimates 0 to 3 ◦C higher than present-day air temperature. These warm air temperatures are not realistic during the so-called Little Ice Age for which an air-cooling of about 0.5 to 2 ◦C has been documented. We propose that these relatively high δ18O values of human tooth enamel reflect higher mean δ18O values of meteoric water which can be attributed to an increased proportion of summer rainfall during the “Little Ice Age” time frame in Lorraine

Oxygen isotope compositions of Late Jurassic vertebrate remains from lithographic limestones of western Europe: implications for the ecology of fish, turtles, and crocodilians

Oxygen isotopes can be used to reconstruct the palaeoenvironmental conditions in which vertebrates lived, and thereby give an indication of their ecology. This has been done for faunas from the famous Late Jurassic lithographic limestones of Western Europe. Oxygen isotope compositions of phosphate from apatite were measured in aquatic or semiaquatic vertebrate remains (fish, turtles, and crocodilians) deposited in these protected coastal marine environments. The unknown water composition in the oxygen isotope fractionation equations between phosphate and water was solved for turtles by assuming that modern relationship between environmental water and turtle bone isotope compositions applies to the past. The results show that: (1) coastal marine waters were thermally homogenous at the regional scale of Western Europe; (2) oxygen isotope data discriminate between coastal marine and freshwater to brackish water inhabitants; (3) the plesiochelyid turtles were coastal marine inhabitants—their δ18O values allow the calculation of the oxygen isotope compositions of marine waters, thus refining the estimate of water temperatures by combining δ18O values from fish; and (4) the marine isotopic signature and the cranial anatomy of plesiochelyid turtles lead us to propose that they had a salt-excreting system (i.e., the fundamental physiological feature that controls osmoregulation in the marine environment). These Late Jurassic turtles are proposed to be the first known marine turtles, several tens of million years before the chelonioids or ‘sea turtles.’

Oxygen isotope compositions of phosphate from arvicoline teeth and Quaternary climatic changes, Gigny, French Jura

Oxygen isotope compositions of biogenic phosphates from mammals are widely used as proxies of the isotopic compositions of meteoric waters that are roughly linearly related to the air temperature at high- and mid-latitudes. An oxygen isotope fractionation equation was determined by using present-day European arvicoline (rodents) tooth phosphate: δ18Op = 20.98(±0.59) + 0.572(±0.065) δ18Ow. This fractionation equation was applied to the Late Pleistocene karstic sequence of Gigny, French Jura. Comparison between the oxygen isotope compositions of arvicoline tooth phosphate and Greenland ice core records suggests to reconsider the previously established hypothetical chronology of the sequence. According to the δ18O value of meteoric water–mean air temperature relationships, the δ18O value of arvicoline teeth records variations in mean air temperatures that range from 0° to 15°C.

Thermal evolution of Tethyan surface waters during the Middle‐Late Jurassic: Evidence from δ18O values of marine fish teeth

Oxygen isotope compositions of phosphate from vertebrate tooth enamel were measured to determine the evolution of tropical sea surface (<∼200 m depth) temperatures in the western Tethys during the Middle‐Late Jurassic. On the basis of a high‐resolution stratigraphic framework with a 1 Myr time resolution, vertebrate teeth were sampled on Aalenian to Portlandian isochrons over the Anglo‐Paris Basin. Asteracanthus sharks and Pycnodontidae teleosteans, identified as sea surface dwellers, have enamel with δ18O values that range from 18.5 to 22.3‰. Thermal variations of tropical surface waters, with amplitudes of a few degrees per few million years, suggest that Middle to Late Jurassic climates were quite variable. Assuming a seawater δ18O value of 0‰ for surface tropical waters in the absence of polar ice caps, temperatures increased from 25 to 29°C from the mid‐Bajocian to mid‐Bathonian. During the middle to late Bathonian, a strong geographic zonation in isotopic compositions is observed between the eastern and western parts of the basin. High δ18O values of fish tooth enamel (up to 22.3‰) could reflect the arrival of a cold water current from the Arctic during the opening of the North Sea rift. An apparent large drop of temperatures from 28 to 21°C is identified at the Callovian‐Oxfordian boundary over no more than ≈2–3 Myr. This cooling is compatible with previous paleobotanical and geochemical studies and can be precisely correlated with the migration of boreal ammonites into the Tethyan domain. Because isotopic sea surface temperatures are probably too low to be compatible with tropical climatic conditions, the δ18O value of seawater could have been >0‰ owing to limited growth of continental ice during the early middle Oxfordian. The resulting sea level fall is estimated to be at least 50 m and is compatible with a global regression stage. The middle Oxfordian thermal minimum is followed by a new warming stage of 3–4°C from the middle to the late Oxfordian.

δ 18O(PO 43−) and δ 18O(CO 32−) from belemnite guards from Eastern Europe: implications for palaeoceanographic reconstructions and for the preservation of pristine isotopic values

The oxygen isotopic composition of coexisting carbonate and phosphate from belemnite rostra was measured according to well established techniques in 42 samples of Early and Middle Jurassic age and in five samples of oyster shells. Most of the samples come from various locations in the Western Carpathians of Slovakia and Ukraine, and from central Poland. Three samples come from the Isle of Skye. The phosphate content of belemnite rostra, though variable, is systematically very low: consistently lower than about 0.3%. However, this phosphate concentration is close to that found in shells of modern marine organisms including pelecypods, gastropods and Sepia cuttlebones which, in some way, could be considered the modern belemnite counterpart. The measured oxygen isotopic composition of carbonate is within the normal range of values obtained from these fossils ranging from about −1.3 to about +0.6‰ (PDB-1) with the exception of three samples; the δ 13C values range from about −0.8 to about +2.8‰ (PDB-1). With the single exception of one sample from the Isle of Skye, the oxygen isotopic composition of phosphate from belemnite rostra ranges from +19.8 to +24.9‰ (V-SMOW), 22 of the samples measured showing δ 18O values equal to or heavier than +23.0‰. In contrast, the oyster values are considerably lighter, in the case of both carbonate and phosphate. 18O-enriched values can hardly be related to diagenetic processes that normally cause an oxygen isotope shift towards light values. If deposition temperatures are calculated from the heavily enriched values by means of the equation of Longinelli and Nuti [Earth Planet. Sci. Lett. 19 (1973) 373–376] and assuming the δ 18O of Jurassic ocean water to be equal to −1‰ taking into account the lack of ice caps during the Jurassic, the obtained temperatures range from about 8°C to about zero. These temperatures are obviously unreliable when Mesozoic palaeoceanographic conditions and palaeoclimate are taken into account. Two different hypotheses are suggested to explain these results, other hypotheses being rejected as unreliable. (1) Phosphate derived from the decaying organic matter of belemnites might have been introduced into belemnite rostra by early diagenetic fluids. If the phosphate of belemnite organic matter was isotopically heavy as happens nowadays in the flesh of molluscs, the inflow of this phosphate into the rostra could be responsible for the very positive δ 18O values shown by many belemnite rostra (this hypothesis is suggested by H.W.); (2) previous oxygen isotope measurements on Upper Cretaceous belemnites yielded δ 18O values very close to the most positive values obtained from Lower Tertiary pelecypods and fish teeth which are known to precipitate their phosphate under isotopic equilibrium conditions with seawater. These data suggest the possibility that the phosphate in belemnite rostra is primary phosphate so that the very positive data reported here can be considered the result of good preservation of the pristine isotopic composition of primary phosphate. Consequently, the only way to explain the very positive δ 18O values is to consider the oxygen isotopic composition of Jurassic ocean water to be more positive than nowadays by at least 3‰. This hypothesis is suggested by A.L. and A.D.M.