Oxygen Isotope Composition Of Carbonates Research Articles

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.

Carbonates in CM2 chondrites: constraints on alteration conditions from oxygen isotopic compositions and petrographic observations

High-precision measurements of the oxygen isotopic compositions of carbonates (calcite and dolomite) from five CM2 chondrites are presented and put into context of the previously determined mineralogic alteration index (MAI), which places these meteorites into an alteration sequence. The carbonate oxygen isotopic compositions range from +20.0 to +35.7‰ for δ 18O, +8.0 to +17.7‰ for δ 17O, and −0.7 to −2.7‰ for Δ 17O. Carbonate Δ 17O values are inversely correlated with MAI and track the evolution of fluid composition from higher to lower Δ 17O values with increasing alteration on the CM parent body. Similar Δ 17O values for calcite and dolomite fractions from the same splits of the same meteorites indicate that calcite and dolomite in each split precipitated from a single fluid reservoir. However, reversed calcite dolomite fractionations (δ 18O dol − δ 18O cc) indicate that the fluid was subject to processes, such as freeze–thaw or evaporation, that fractionated isotopes in a mass-dependent way. Consideration of the carbonate isotopic data in the context of previously proposed models for aqueous alteration of carbonaceous chondrites has provided important insights into both the evolving alteration conditions and the utility of the models themselves. The data as a whole indicate that the isotopic evolution of the fluid was similar to that predicted by the closed-system, two-reservoir models, but that a slightly larger matrix–water fractionation factor may apply. In the context of this model, more altered samples largely reflect greater reaction progress and thus probably indicate more extended times of fluid exposure. Petrographic observations of carbonates reveal a trend of variable carbonate morphology correlated with alteration that is also consistent with changes in the duration of fluid–rock interaction. The data can also be reconciled with fluid-flow models in a restricted region of the parent body, which is consistent with assertions that the different types of carbonaceous chondrites derive from different regions of their parent bodies. In this case, the model results for a 9-km-radius body, and our data place the location of the CM chondrite formation in a 100-m-thick zone 1 km from the surface. The size of this zone could be increased if the model parameters were adjusted.

Fluid Chemical Evolution as a Factor in Controlling the Distribution of Gold at the Archean Golden Crown Lode Gold Deposit, Murchison Province, Western Australia

The Golden Crown lode gold deposit is one of a number of northwest-southeast– to north-south–trending quartz veins hosted by a steeply dipping, 500-m–thick, differentiated dolerite sill (the Great Fingall sill) and adjacent graphitic shale in the Day Dawn district of the Murchison province in the Archean Yilgarn craton, Western Australia. Quartz veins of the Golden Crown deposit are shear zone-hosted, crosscut the dominant host-rock stratigraphy at a high angle, and are laminated, partly boudinaged, and generally bifurcate at their extremities. Veins are dominated by dark gray or milky white quartz, with subordinate calcite and ankerite, and, in order of abundance, arsenopyrite, pyrrhotite, pyrite, galena, chalcopyrite, and minor sphalerite. The vein sulfides coexist with native gold and are most abundant at vein margins. Textural evidence indicates that precipitation of the majority of these minerals overlapped, and that most phases can be considered to have been in equilibrium during vein genesis. Mineral chemistry, fluid inclusions, alteration, and ore assemblages show that there are significant systematic gradients in intensive variables within the Golden Crown deposit. These are recorded over the approximately 450 m of vertical exposure of the vein system, and over up to 30 m into the wall rocks. Based on proximal alteration assemblages and zoning patterns, zoned wall-rock alteration haloes are divisible into four sequences, three of which are closely related to gold mineralization. Specific alteration sequences occur within different parts of the vein system. Biotite alteration predominates in the deeper portions, whereas muscovite alteration characterizes the upper and southern portions of the system. Higher-grade gold zones within the Golden Crown deposit are limited to the northern end of the system in the deeper portion of the veins, and expand along strike to the south in the upper portions. Fluid inclusions, interpreted to be trapped during gold mineralization, are low-salinity (<10 wt % NaCl equiv) H2O-CO2 ± CH4 inclusions with variable carbonic to aqueous contents that are interpreted to be due to fluid immiscibility. Inferred methane concentrations in the carbonic phase vary significantly from 19 ± 7 mole percent in the vicinity of the shale contact to 5 ± 2 mole percent in the shallower, southern end of the deposit. Carbon and oxygen isotope compositions of carbonates ( δ 13Ccalcite = –10.5 to –1.9‰; δ 13Cankerite = –5.3 to –4.1‰; δ 18Ocalcite = 7.0–18.3‰; and δ 18Oankerite = 7.7–9.2‰) and oxygen isotope compositions of vein quartz ( δ 18Oquartz = 10.2–12.5‰) are well within the range of known Archean orogenic lode gold deposits. Combining the results of all geothermometers and geobarometers, a temperature of 300° to 400°C and pressure of 2.5 ± 0.5 kbars are determined, with temperature variations over the exposed quartz vein of less than 40°C. The As content (at. %) of arsenopyrite and Au/Ag ratios of native gold show significant compositional variation, which appears to be controlled by parameters such as a H2S(aq) and f O2, not temperature gradients. On the basis of the variations in fluid composition determined in the reefs, the two major processes responsible for high-grade gold mineralization are interpreted to have been phase separation in the deeper portions of the ore-forming system (approximately below –400-m level) and, above these levels, mixing between two modified ore fluids, one reduced fluid that had reacted with graphitic shale and a relatively oxidized fluid that had partly equilibrated with dolerite.

Tin–polymetallic sulfide deposits in the eastern part of the Dachang tin field (South China) and the role of black shales in their origin

The Dafulou and Huile vein and stratabound cassiterite–sulfide deposits and sheeted ore veins at the Kangma cassiterite–sulfide deposit are located in the eastern part of the Dachang tin field. These deposits are hosted in a sedimentary sequence containing significant concentrations of organic matter in the form of Lower Devonian calcareous black shales and hornfels. These rocks together with the younger intrusion of Longxianggai granite (91±2 Ma) actively participated in the formation of Sn-polymetallic deposits. The following three major stages have been distinguished in stratiform and vein-type orebodies at Dafulou, Huile and Kangma: stage I (cassiterite, pyrrhotite, arsenopyrite, tourmaline, carbonate), stage II – main sulfide stage (quartz, cassiterite, arsenopyrite, pyrrhotite, sphalerite, stannite, pyrite, carbonates) and stage III (native Bi, galena, electrum, sulfosalts). Stage IV (post-ore), recognized at Huile is represented by barren carbonates and zeolites. Whole rock geochemistry has revealed that at Dafulou, Bi and Cu correlate strongly with S, whereas V and Pb correlate well with Corg (organic carbon). The similar distribution patterns of selected elements in average slightly mineralized low-Ca black shales indicate a fluid composition similar for all deposits studied. Studies of graphitization of the organic matter in black shales adjacent to orebodies indicate that d(002) and FWHM (full width in half maximum)/peak height values gradually decrease in the following sequence: Dafulou deposit → Kangma deposit → Huile deposit. The pyrolysate of wall rocks at the Dafulou deposit is relatively enriched in asphaltenes and maltenes (55.6–72.0% of the pyrolysate) comparable with pyrolysate obtained from more distal black shales (19.2–28.5%). Typical GC-MS spectra of pyrolysate from distal black shales are dominated by alkanes in the n-C15 to n-C25 range, aromatic molecules being represented mostly by alkyl-naphthalenes. In contrast, only traces of aliphatic hydrocarbons in the n-C14 to n-C18 range and elemental sulfur were identified in pyrolysates from pyrrhotitized wall rocks. The earliest fluid inclusions of the studied system occur in the quartz–tourmaline–cassiterite assemblage of stage I at Dafulou. These inclusions are H2O–CO2–CH4-rich, with 10 to 20 vol% of aqueous phase. P–T conditions of the trapping of inclusions are estimated to be up to 400 °C and 1.3 to 2.0 kbar (between 5.0 and 7.5 km under lithostatic pressure). In contrast, the presence of a low density gaseous CO2–CH4 phase indicates relatively low pressures during the formation of the breccia-type quartz–calcite–cassiterite–sulfide mineralization (stage II), when P–T conditions probably reached approx. 380 to 400 °C and 0.6 kbar (up to 6 km under hydrostatic pressure). Fluid inclusion data and oxygen isotope thermometry indicate that cassiterite–sulfide ores of the main sulfide stage (stage II) formed from aqueous-carbonic fluid (CO2/CH4 =≈10) at temperatures of up to 390 °C at Dafulou and in a temperature range of 250 to 360 °C at Huile and 260 to 370 °C at Kangma. The δ34S values of sulfides from Dafulou range mostly between –1 and –6‰, whereas sulfides from the Kangma and Huile deposits are characterized by more negative δ34S values (between –8 and –11‰, and between –9 and –12‰, respectively). These data suggest that bacteriogenic sulfides of black shales were a dominant source of reduced sulfur for epigenetic (vein and replacement) mineralization. Oxygen isotopic compositions of five quartz-cassiterite pairs from Dafulou and Huile show a relatively narrow range of calculated oxygen isotope temperatures (250–320 °C, using the equation of Alderton 1989) and high δ18Ofluid values between +8 and +10‰ (SMOW), which are in agreement with fluid derivation from and/or high temperature equilibration with the Longxianggai granite. The carbon and oxygen isotope composition of carbonates reflects variable carbon sources. Stage I calcite is characterized by narrow ranges of δ13C (–7.0 to –9.5‰ PDB) and δ18O (+15.0 to +17.5‰ SMOW). This calcite shows ubiquitous deformation, evidenced by intense development of twins. Fluid compositions calculated at 330 °C for the Dafulou and Huile deposits and at 270–300 °C for the Kangma deposit (δ18Ofluid between +10.0 and +11.5‰ SMOW, δ13Cfluid between –5.5 and –7.5‰ PDB), agree with fluid derivation from and/or equilibration with the peraluminous, high-δ18O Longxianggai granite and suggest a significant influence of contact metasedimentary sequences (carbon derived from decomposition and/or alteration of organic matter of calcareous black shales). The δ13 C values of organic matter from the Lower to Upper Devonian host rocks at the Dafulou deposit (–24.0 and –28.0‰) fit with a marine origin from algae. However, organic matter adjacent to the host rock-ore contact displays a slight enrichment in 13C. The organic carbon from the Huile and Kangma deposits is even more 13C enriched (-24.6 to –23.5‰). The most heavy δ13 C values (–16.5‰) were detected in hornfels sampled at the contact of the Upper Devonian sediments with the Longxianggai granite. The δ13C data broadly correlate with the degree of structural ordering (degree of graphitization) of organic matter, which indicates that both variables are related to thermal overprint.

Stable Isotope Ecology of a Late Miocene Population of Gomphotherium productus (Mammalia, Proboscidea) from Port of Entry Pit, Oklahoma, USA

Abstract The stable carbon and oxygen isotope compositions of serial samples of enamel from tusks of Gomphotherium productus (Mammalia, Proboscidea) from Port of Entry Pit, Oklahoma (early Hemphillian North American Land Mammal Age, ca. 7.5 Ma), were measured to examine intra-annual and interannual variation. Sample series from each of six tusks spanned approximately one year of tusk growth. Carbon isotope compositions range from −11.3 to −9.2‰ (VPDB) and exhibit no pattern of seasonal variation, indicating the diet of gomphotheres at Port of Entry Pit was dominated by C3 vegetation throughout the year. Phosphate oxygen isotope composition (δ18Op) ranges from 18.9 to 22.2‰ (VSMOW); carbonate oxygen isotope composition (δ18Oc) ranges from 26.1 to 30.1‰ (VSMOW). None of the tusks exhibit seasonal variation in δ18O, and the average within-tusk range in δ18Op is 1.7‰. Neither the fluorine composition of the specimens nor the relationship between δ18Op and δ18Oc values from splits of the same samples suggest s...

Stable Isotope Ecology of a Late Miocene Population of Gomphotherium productus (Mammalia, Proboscidea) from Port of Entry Pit, Oklahoma, USA

Other| June 01, 2001 Stable Isotope Ecology of a Late Miocene Population of Gomphotherium productus (Mammalia, Proboscidea) from Port of Entry Pit, Oklahoma, USA DAVID L. FOX; DAVID L. FOX 1Department of Geological Sciences and Museum of Paleontology, University of Michigan, Ann Arbor, MI 48109 *Current address: Department of Earth Sciences, University of California, Santa Cruz, CA 95064 Search for other works by this author on: GSW Google Scholar DANIEL C. FISHER DANIEL C. FISHER 1Department of Geological Sciences and Museum of Paleontology, University of Michigan, Ann Arbor, MI 48109 Search for other works by this author on: GSW Google Scholar Author and Article Information DAVID L. FOX *Current address: Department of Earth Sciences, University of California, Santa Cruz, CA 95064 1Department of Geological Sciences and Museum of Paleontology, University of Michigan, Ann Arbor, MI 48109 DANIEL C. FISHER 1Department of Geological Sciences and Museum of Paleontology, University of Michigan, Ann Arbor, MI 48109 Publisher: SEPM Society for Sedimentary Geology Accepted: 22 Dec 2000 First Online: 03 Mar 2017 Online ISSN: 1938-5323 Print ISSN: 0883-1351 Society for Sedimentary Geology PALAIOS (2001) 16 (3): 279–293. https://doi.org/10.1669/0883-1351(2001)016<0279:SIEOAL>2.0.CO;2 Article history Accepted: 22 Dec 2000 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation DAVID L. FOX, DANIEL C. FISHER; Stable Isotope Ecology of a Late Miocene Population of Gomphotherium productus (Mammalia, Proboscidea) from Port of Entry Pit, Oklahoma, USA. PALAIOS 2001;; 16 (3): 279–293. doi: https://doi.org/10.1669/0883-1351(2001)016<0279:SIEOAL>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract The stable carbon and oxygen isotope compositions of serial samples of enamel from tusks of Gomphotherium productus (Mammalia, Proboscidea) from Port of Entry Pit, Oklahoma (early Hemphillian North American Land Mammal Age, ca. 7.5 Ma), were measured to examine intra-annual and interannual variation. Sample series from each of six tusks spanned approximately one year of tusk growth. Carbon isotope compositions range from −11.3 to −9.2‰ (VPDB) and exhibit no pattern of seasonal variation, indicating the diet of gomphotheres at Port of Entry Pit was dominated by C3 vegetation throughout the year. Phosphate oxygen isotope composition (δ18Op) ranges from 18.9 to 22.2‰ (VSMOW); carbonate oxygen isotope composition (δ18Oc) ranges from 26.1 to 30.1‰ (VSMOW). None of the tusks exhibit seasonal variation in δ18O, and the average within-tusk range in δ18Op is 1.7‰. Neither the fluorine composition of the specimens nor the relationship between δ18Op and δ18Oc values from splits of the same samples suggest significant post-depositional alteration. The oxygen isotope data imply that Hemphillian meteoric water had δ18O values that are indistinguishable from modern values in the region today. However, because polar ice sheets were smaller and mean ocean water δ18O lower than present, the estimates of meteoric water composition from gomphothere tusk δ18Op are consistent with warmer mean annual temperatures during the Hemphillian. The within-tusk variations in δ18Op are consistent with similar or much reduced patterns of seasonal temperature variation in comparison to today, depending on air mass flow during the late Miocene. Alternative explanations include seasonal migratory behavior and reliance by the gomphotheres at Port of Entry Pit on sources of drinking water with relatively constant δ18O values. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Isotopic tracers of the origin of Ca in a carbonate crust from the Middle Atlas, Morocco

Carbonate crusts developed on basalt were studied at Tazouta in the Moroccan Middle Atlas. The chosen representative weathering profile is 2.8 m thick and is composed from base to top of a fresh basalt overlain by a coherent saprolite, a friable saprolite and a surface soil. Carbonate crust genesis occurs by replacement of primary and secondary clay minerals by low Mg calcite. The 14C dates of the calcite range from 15 to 14 ka in the coherent saprolite, and from 12 to 8.5 ka in the friable saprolite. Smectitic weathering and replacement of Si-bearing minerals by carbonate are coupled processes leading to a strong calcium net gain (300 kg/m 3) within the friable saprolite. The low weathering rate of basalt compared to the calcrete genesis rate, and the high mobility of calcium in supergene weathering mantles, limit the basaltic Ca contribution to calcrete genesis. The contribution of the Triassic sediment is also limited by the mineralogical unavailability of Ca. The 87Sr/ 86Sr ratios confirm the low contribution of basalt and indicate an external origin of Ca. The Sr isotope ratios of calcrete are different from those of the adjacent Liassic carbonates and are close to rainfall values. The carbon isotope compositions of carbonates (−8‰ to −9‰ vs. PDB) imply a significant contribution of C3-dominated organic carbon during carbonate precipitation. Oxygen isotopic compositions of carbonates (average −5.24‰ vs. PDB), show that carbonates are 18O-enriched compared to meteoric waters (−4‰ to −3.5‰ vs. SMOW).

Isotopic composition of recent shark teeth as a proxy for environmental conditions

The O, C, and Sr isotope compositions of teeth from ten species, belonging to five families, and three orders of sharks were measured to determine the influence of habitat, diet, and possible species-specific fractionation effects on the isotopic composition of biogenic phosphate from fish. The sharks were recently caught in subtropical waters off the KwaZulu-Natal (KZN) coast of South Africa, as well as from cold waters in Prince William Sound (PWS), Alaska, and Victor Bay (VB), Nunavut, Canada. δ 18O values of tooth phosphate (δ 18O P) range from 20.9 to 23.5‰ for the KZN sharks. For most species the range in measured δ 18O P values is about 0.6‰, but it may be as high as 1.1‰ for different teeth from a single shark. Dentine and enameloid within individual teeth have no apparent differences in δ 18O P values. The δ 18O P values of the KZN shark teeth reflect the typical habitat of the studied species, primarily the thermal structure of the water column off KZN at depths between 20 and 280 m. The δ 18O P values of teeth from different Greenland sharks from VB and Pacific sleeper sharks from PWS are very homogeneous, averaging 25.8 and 24.7‰, respectively. These values appear to be in equilibrium with deep (>500 m) ocean waters in each case at temperatures of about −0.3°C or less. There is little discernable evidence for species-specific fractionation effects for the oxygen isotope composition of phosphate in the studied marine fish. The oxygen isotope composition of carbonate in apatite averages about 9.1‰ higher than corresponding δ 18O P values, in agreement with equilibrium fractionation between carbonate and phosphate, but with a large variance (1σ = ±1.5‰). δ 18O C values also vary by up to 1‰ between enameloid and dentine within single teeth, but in a non-systematic way. Differences in δ 13C values between carbonate in enameloid and dentine is also large (up to 8‰) but the δ 13C values vary systematically. Enameloid is always enriched in 13C compared to dentine and the 13C content increases with developmental stage of the teeth. δ 13C values measured for enameloid (1.6 to 4.8‰) appear to approach equilibrium with dissolved inorganic carbon in seawater. In contrast, δ 13C values for dentine range from −6.4 to −2.3‰ for KZN sharks, and −9.0 to −10.8‰ for the cold-water sleeper sharks, and are compatible with a predominantly dietary carbon source. The 87Sr/ 86Sr ratios of teeth from KZN sharks as well as those from PWS and VB are uniform, averaging 0.709167. Sr content varies from 1270 to 2100 ppm, a range that is similar to that in well preserved fossilized teeth. Seawater Sr is thus clearly incorporated in vivo. Concentrations of Sm and Nd are in the ppb range and contrast the ppm range in fossilized teeth, indicating a postmortem incorporation of rare earth elements in apatite of the teeth.

No vital effect on δ 18O and δ 13C values of fossil brachiopod shells, Middle Devonian of China

Biogenic fractionation (vital effect) of carbon and oxygen isotopes during metabolism is ubiquitous among marine organisms. Whether there is vital effect on the isotopic compositions of fossil brachiopods represents one of the most controversial issues in the carbonate stable isotope geochemistry. Furthermore, the problem involves not only the feasibility of the utilization of fossil brachiopod shells as analysis samples for obtaining the original carbon and oxygen isotopic compositions of carbonates, but also the credibility of the data and conclusions already achieved in the study field. With fossil brachiopods from Middle Devonian of China, this study investigates the vital effect by comparisons of δ 18O and δ 13C between the fossils of the same original living environment and the shell’s preservation at levels of species, genus, and family. No systematic differences of δ 18O and δ 13C between fossils were found, which led to the conclusion that no detectable vital effect occurred in the fossil brachiopod shells. The argument that the fossil brachiopod possibly has biogenic fractionation with the stable isotopes of carbon and oxygen resulted likely from studies of fossils that are different in the original living environment or the state of preservation.

Oxygen isotopic composition of carbonate in the nakhla meteorite: implications for the hydrosphere and atmosphere of mars

We have located small areas of siderite within the mesostasis of the Nakhla meteorite. High concentrations of Mn (up to ∼50% rhodochrosite) and elevated D/H ratios indicate that the siderite is not a terrestrial alteration product. The isotopic composition of oxygen in the siderite has been determined with the Manchester ISOLAB 54 ion microprobe. The siderite has δ18O = +34 ± 1‰, which is higher than any other martian carbonate yet reported. If the δ18O value is the result of equilibration with water at <60°C, then the carbonate could have formed from CO2/H2O produced during the degassing of Mars, and not modified subsequently isotopically. Formation from a water rich fluid at >60°C requires that the fluid was heavy isotope enriched relative to fluids produced during planetary degassing. An enrichment of 8–15‰ is consistent with theoretical outgassing models that are able to account for enhancements of Δ17O in martian alteration products. Estimated deposition temperatures would be raised to 80–170°C. The effect of a global scale fixation of martian CO2 as carbonate operates in the opposite direction and could lead to a reduction in δ18O of the martian hydrosphere of a few permil.

Stable carbon and oxygen isotopic compositions of carbonates in middle Mesoproterozoic Wumishan Formation and sea-level change

δ13 C of dolostones from the Wumishan Formation (1 310–1 207 Ma) in the Ming Tombs area near Beijing ranges from −1.5 to 1.5‰ (PDB), and generally, the δ18O ranges from −4 to −5‰ (PDB). The secular variation of stable carbon isotopic compositions is cyclic. δ13C displays positive shifts from a sequence boundary to 1.5 per mil (PDB), then a negative shifts to −1.5 per mil (PDB). In some unit, where δ13C values of the rocks are negative, they have relatively high δ18O values close to −4.0‰ (PDB). These characteristics of the carbon and oxygen isotopic compositions may be associated with sea-level changes.

Evidence for rapid climate change in North America during the latest Paleocene thermal maximum: oxygen isotope compositions of biogenic phosphate from the Bighorn Basin (Wyoming)

Oxygen isotope records of Cenozoic sea water temperatures indicate that a rapid warming event known as the Latest Paleocene Thermal Maximum (LPTM) occurred during the otherwise gradual increase in world temperatures during the Late Paleocene and Early Eocene. Oxygen isotope analysis of the carbonate and phosphate components of hydroxyapatite found in mammalian tooth enamel and body scales of river-dwelling fish from the Bighorn Basin in Wyoming were made to investigate corresponding changes in the terrestrial climate. A comparison of carbonate and phosphate isotope data from modern and fossil material indicates that some diagenetic alteration of the fossil material has occurred, although systematically larger intra-tooth ranges in the oxygen isotope composition of carbonate indicate that it is more likely to have been affected than phosphate. Carbonate and phosphate from the ecologically diverse mammals and fishes both record a shift to higher oxygen isotope ratios at the same time and of the same duration as the LPTM. These shifts reflect a change in the isotopic composition of regional precipitation, which in turn provides the first evidence for continental climate change during the LPTM. Assuming the present-day relation between the oxygen isotope composition of precipitation and temperature applies to conditions in the past, and that animal physiology and behavior is relatively invariant over time, the isotopic shift is equivalent to an increase of surface temperature in western North America of several degrees. This result is consistent with the magnitude of high-latitude ocean warming, and provides a basis for relating marine and terrestrial oxygen isotope records to records of terrestrial biotic change.

ORIENTATION OF CARBONATE LAMINATIONS IN GRAVELLY SOILS ALONG A WINTER/SUMMER PRECIPITATION GRADIENT IN BAJA CALIFORNIA, MEXICO

We observed, along a north-to-south transect in central Baja California, that the positioning(top vs. bottom) of pedogenic carbonate on gravel in Holocene soils was related to the proportion of winter versus summer/early fall precipitation. Carbonate coatings were found on gravel bottoms at sites where precipitation occurs predominantly in winter but on gravel tops at sites where summer/fall precipitation is dominant. The positioning of carbonate coatings co-varied with other ecosystem properties such as floral distribution, oxygen isotope composition of carbonate, and depth to the top of the Bk horizon in response to precipitation patterns. The processes responsible for the orientation of carbonate on gravels are unknown. We proposed that the nature of soil thermal gradients during major precipitation events or periods may influence carbonate solubility and, therefore, the loci of deposition. While the deposition of carbonate on gravel tops is apparently unique and restricted to areas of summer/fall precipitation in Baja California, the positioning of carbonate coatings in gravelly soils in this region may also provide some insight into long-term climate patterns in the perarid regions of the Sonoran Desert.

Isotope Data on Barite and Celestite Deposits Related to Three Mesozoic Evaporitic Sequences of the Neuquén Basin, Argentina

Stratabound barite and celestite deposits, related mainly to three evaporitic sequences, occur in the Mesozoic Neuquen Retroarc Basin, developed to the east of the Andean Cordillera of western Argentina. This basin is filled with Jurassic and Cretaceous marine to continental sediments that unconformably overlie basement rocks of Paleozoic to Triassic age. Celestite deposits formed by initial precipitation from seawater, with later crystallization during diagenesis and recrystallization related to Eocene intrusive activity. This is supported by evidence of evaporitic associations, textures, and Sr and S-isotope data. The barite deposits were deposited in a near-shore environment and could have formed as a result of interaction between barium absorbed in clay minerals (derived from weathering of basement rocks) and hypersaline seawater. This genetic model is supported by evidence such as the stratabound setting, textures, and Sr and S-isotope data. Carbon and oxygen-isotopic compositions of carbonates, which are interbedded with celestites, are in the range expected for marine carbonates. Lead-isotopic compositions of galenas from bedded and vein barite deposits of Upper Jurassic and Lower Cretaceous ages are consistent with Pb remobilization from a source located at high levels of the upper crust, possibly the basement rocks.

An isotopic palaeoenvironmental study of human skeletal remains from the Nile Valley

Stable isotope measurements were carried out on human skeletal remains from the Nile Valley, ranging in age from about 7000 to 1750 yr B.P. and on a set of other samples among which are archaeological food remains. Bone samples were analyzed for δ18O(PO3−4), δ18O(CO2−3) and δ13C(CO−3) of carbonate hydroxylapatite and δ13C and δ15N of collagen. Food remains and tissue samples were analyzed for their δ13C and δ15N values. The isotopic composition of bone collagen, apatite and food remains (partially representative of the diet in ancient Egypt) suggest a mixed diet including C3 plant food and, probably, animal resources (both fresh-water fish and C3 animals). A rather surprising observation is the lack of differences between isotopic composition of remains of different social classes spanning from the very poor village of Gebelein to the middle class of the rich town of Asyut to the distinguished people who underwent mummification processes after their death. From the palaeoclimatological point of view, the δ18O(PO3−4) values, ranging from 20.6 to 24.5‰, seem to reflect the isotopic composition of Nile river water (also related to relative humidity) rather than real climatic (temperature) variations through time. The oxygen isotope compositions of carbonate and phosphate of the same samples (from 30.2 to 32.5 and from 20.6 to 22.2, respectively) suggest isotopic equilibrium conditions, thus confirming the possibility of also using the carbonate measurements for palaeoclimatological studies, at least in the case of recent and well preserved samples.

Isotopic Biogeochemistry ( 13 C, 18 O) of Mammalian Enamel from African Pleistocene Hominid Sites

The carbon and oxygen isotope composition of carbonate in enamel hydroxylapatite can provide information on photosynthetic pathways of plants at the base of food webs, and on hydrological conditions. Retrieval ofpalaeoenvironmental information from isotopic composition of vertebrate fossils is complicated by potential diagenetic overprinting. In this study, alteration has been assessed by examining the extent to which expected biological carbon and oxygen isotope patterns are disrupted in fossils of species whose diets can be independently predicted by other criteria. The biological patterns used are 1) the differences in carbon isotope composition between grazers and browsers, and 2) the differences in oxygen isotope composition between hippopotamus and terrestrial herbivores. Results obtained on enamel samples from Tighenif (Algeria, =700,000 yr), Melka-Kunture (Ethiopia, 0.7-1.5 myr), and Anabo Koma (Djibouti, -1.6 myr) suggest that in vivo carbon and oxygen isotope compositions are preserved in most cases. Moreover, in all three regions, modern patterns of C3 versus C4 grass dominance were present within the Pleistocene.

Proposed in situ secondary ion mass spectrometry on Mars

Secondary ion mass spectrometry is a powerful analytical tool, which has the potentiality, through molecular ion emission, of detecting minor phases, as well as the unique capability of directly measuring isotope abundances in mineral or organic phases without any prior physical, chemical or thermal processing. Applied to the in situ analysis of the Martian regolith, it can provide evidence of the presence of carbonates and, by inference (if carbonates constitute significant deposits), of past liquid water—a necessary condition for the development of life. In addition, oxygen isotopic composition of carbonates preserves a record of the temperature at which this phase precipitated and may therefore help decipher the past climatology of Mars. Detection of a carbon isotopic composition shift between carbonates and organic matter (on Earth, the result of a kinetic fractionation effect during photosynthesis) would provide a definite clue regarding the existence of a past biochemical activity on Mars.

The relationship of precipitation seasonality to the flora and stable isotope chemistry of soils in the Vizcaíno desert, Baja California, México

The proportion of winter to summer rainfall decreases with decreasing latitude in the Vizcaino desert of Baja California, Mexico. Vegetation characteristics, soil carbon contents, the stable carbon isotope composition of soil organic matter and pedogenic carbonate, and the stable oxygen isotope composition of carbonate were measured at four locations along this precipitation transect. The number of species (≈ 20), the ground cover (≈ 30%), and the proportion of succulents to total species (≈ 60%) were similar at all sites. The northenmost site possessed species (e.g. Ephedra californica, Yucca schidigera) peculiar to winter rainfall zones while the southern site had several species typically common only to summer rainfall zones (e.g. Cercidium praecox, Lemaiveocereus thurberi). Organic carbon contents of the soils were low and varied little with rainfall distribution. σ13C values of organic carbon were approximately 2 to 3‰ more positive at the southernmost site than at the other locations, possibly reflecting a greater proportion of biomass from either succulents or C4 summer grasses. The σ13C value of tissue from a representative CAM plant (Opuntia echinocarpa) did not vary with precipitation patterns suggesting that the σ13C values of CAM species are insensitive to the climate variations in this area. σ13C value of pedogenic carbonate varied little with location (−4·1 to −5·8‰), reflecting small differences in soil respiration rates with latitude. σ18O values of pedogenic carbonate varied significantly from north (−3·6‰) to south (−7·3‰), and are interpreted as reflecting the differences in the relative contribution of isotopically ‘heavy’ winter storms from the Pacific to the isotopically ‘light’ summer and fall storms from the southern Pacific and the Gulf of Mexico. The results of the study reinforce the observation that subtle climatic differences in arid regions are reflected in the floral composition and in the chemistry of the soils. The Holocene data obtained in this study provide a basis for interpreting the climatic significance of isotopic data from paleosols in the Vizcaino Desert region.

Carbon and oxygen isotopic composition of carbonates from saliferous deposits of the Pripyat Trough, Belarus

δ 18O and δ 13C in accessory calcite and dolomite from salt rock and in intrasalt carbonate beds of the Upper Frasnian and Upper Famennian evaporite formations of the Pripyat Trough vary over a wide range. Within all the intrasalt carbonate interbeds four distinct groups of rocks have been recognised on the basis of their isotopic composition. The ranges of variation of δ 13C(PDB) and δ 18O(SMOW) within these groups are respectively: (a) +0.7 to +0.8 and +35.4 to +36.4%; (b) −4.5 to −0.9 and +30.0 to +32.2%; (c) −7.0 to −2.8 and +19.8 to +25.5%; (d) −17.5 to −9.7 and +20.9 to +26.4%. The isotopic composition of accessory carbonates is more uniform. The main body of values falls within the range −11 to −3% for δ 13C and +18 to +25% for δ 18O. The carbonate matter with the observed combinations of δ 13C and δ 18O could have been formed during sedimentation and diagenesis in sabkhas, on subaetially exposed islands, on evaporite depression slopes washed by sea water and freshwater and in stagnant zones with hot brines. The carbon isotopic composition of intrasalt carbonate rocks is, on the average, lighter than that of accessory carbonates from the salts. This shows that the organic matter was most readily accumulated in evaporite basins during non-salt sedimentation.

The origin of fluids and the effects of metamorphism on the primary chemical compositions of Barberton komatiites: New evidence from geochemical (REE) and isotopic (Nd, O, H, [formula omitted]) data

Numerous greenstone relics, all containing the two lowermost formations of the Onverwacht Group, occur in the Archean trondhjemitic/tonalitic gneiss terrains south of the Barberton Greenstone Belt. In this study, we report detailed petrological, geochemical and isotopic (Nd, O, H, 40Ar 39Ar ) data obtained on komatiites from the Schapenburg Greenstone Remnant (SGR), the largest and best-preserved greenstone relic. The main goals are 1. (1) to date the metamorphism affecting the SGR using the 40Ar 39Ar dating method on amphiboles, 2. (2) to evaluate the effect of metamorphism on the preservation of primary isotopic and chemical signatures, 3. (3) to estimate the temperature and water/rock ratios that prevailed during metamorphic recrystallization in order to constrain the composition and origin of the reacting fluid phase. 40Ar 39Ar ages of 2.9 Ga obtained on two amphibole separates from the Schapenburg metavolcanics reveal the existence of a metamorphic event younger than the emplacement age (3.5 Ga). This metamorphic event belongs to a series of discrete periods of thermal activity from 3.4 to 2 Ga, each of which coincides with a major episode of magmatic activity. The ultrabasic lava flows acquired their δ 18O values (from +3.2 to +5%.) at high temperature (≈450°C) under high water/rock ratios. The reacting water had initial isotopic values typical of metamorphic fluids ( δ 18 O = +5 to +7%.; δD = −65 to −50%.). REE patterns were not disturbed by metamorphic recrystallization. Despite the long time interval between emplacement and metamorphism (≈600 Ma), ϵ Nd ( T) values are uniform throughout the whole magmatic suite, indicating that the Sm-Nd system was closed on the sample scale during metamorphism. The mantle source of these greenstones was depleted in LREE as evidence by ϵ Nd ( T) ≈ +2.5. Chemical fluxes during metamorphism were calculated for elements unfractionated by olivine removal (e.g. Na, Ca, Ti, Al, and Sr), by normalizing to Nd. They suggest a significant mobility of most major elements in the cumulate zones of the lava flows. By contrast, spinifex zones appear to have preserved most of their primary chemical signatures during metamorphic recrystallization. Their CaO Al 2O 3 and Al 2O 3 TiO 2 ratios can be used with confidence to determine the PT conditions of melting in the mantle source. A general model of water-rock interactions applied to the sedimentary and magmatic rocks of the Onverwacht Group is also presented. The model involves conditions of metamorphism deduced from this study and available data from the literature. In addition to metavolcanic rocks, most of the oxygen isotope compositions of carbonates and cherts (except the highest values) can be explained by reequilibration with metamorphic fluids under greenschist-amphibolite facies conditions.