Oxygen isotope composition of clinoptilolites

Oxygen isotope ratios were obtained from authigenic clinoptilolites from Barbados Accretionary Complex, Yamato Basin, and Exmouth Plateau sediments (ODP Sites 672, 797, and 762) in order to investigate the isotopic fractionation between clinoptilolite and pore water at early diagenetic stages and low temperatures. Dehydrated clinoptilolites display isotopic ratios for the zeolite framework (delta 18Of) that extend from +18.7? to +32.8? (vs. SMOW). In combination with associated pore water isotope data, the oxygen isotopic fractionation between clinoptilolite and pore fluids could be assessed in the temperature range from 25ºC to 40ºC. The resulting fractionation factors of 1.032 at 25ºC and 1.027 at 40ºC are in good agreement with the theoretically determined oxygen isotope fractionation between clinoptilolite and water. Calculations of isotopic temperatures illustrate that clinoptilolite formation occurred at relatively low temperatures of 17ºC to 29ºC in Barbados Ridge sediments and at 33ºC to 62ºC in the Yamato Basin. These data support a low-temperature origin of clinoptilolite and contradict the assumption that elevated temperatures are the main controlling factor for authigenic clinoptilolite formation. Increasing clinoptilolite delta18Of values with depth indicate that clinoptilolites which are now in the deeper parts of the zeolite-bearing intervals had either formed at lower temperatures (17-20ºC) or under closed system conditions.

Helium and oxygen isotope ratios of ODP Sites 193-1188 and 193-1189

Fluid mixing processes and thermal regimes within the Snowcap and Roman Ruins vent sites of the PACMANUS hydrothermal system, Papua New Guinea, were investigated using 3He/4He ratios from fluid inclusions within pyrite and anhydrite and the d18O signature of anhydrite. Depressed 3He/4He ratios of 0.2-6.91RA appear to be caused by significant atmospheric diffusive exchange, whilst He-Ne diffusive fractionation precludes correction using 20Ne. 40Ar/36Ar ratios of 295-310 are elevated above seawater, indicating the majority of argon is seawater derived but with a magmatic component. d18O anhydrite ratios are 6.5 per mil to 11 per mil for Snowcap and 6.4 per mil to 11.9 per mil for Roman Ruins. Using oxygen isotope fractionation factors for the anhydrite-water system, the temperatures calculated assuming isotopic equilibrium at depth are up to 100 °C cooler than fluid inclusion trapping temperatures. It is likely that anhydrite is precipitated rapidly, preventing d18O equilibration. By comparing new d18O values for anhydrite with corresponding published 87Sr/86Sr ratios, seawater is inferred to penetrate deep into the Snowcap system with little conductive heating. A simple fluid mixing model has been constructed whereby the differing venting styles can be explained by a plumbing system at depth which favors delivery of end-member hydrothermal fluid to the high temperature sites.

Pliocene-Pleistocene oxygen isotope record DSDP Site 586, Ontong Java Plateau, Pacific Ocean i

Oceanographic changes in the western equatorial Pacific during the past 6 Ma are inferred from oxygen isotopic analyses of planktic and benthic foraminifera from Ontong Java Plateau (DSDP Site 586). The taxa are Globigerinoides sacculifer, Pulleniatina, Cibicidoides wuellerstorfi, and Oridorsalis umbonatus. Cooling and ice buildup are indicated by an 18O enrichment of 0.3 per mil in the planktic species near 3.4 Ma. This shift apparently is compensated in the benthic data by a warming of the deep waters by between 1° and 2° C. We suggest that the dominant source of upper deep water supply to the Pacific changed from Antarctic to North Atlantic at that time, the North Atlantic-derived water being warmer. Near 2.8 Ma (approximately) the planktic foraminifera again record an enrichment in 18O (Delta delta18O=0.25 per mil). We suggest ice buildup in the northern hemisphere as the cause, because of subsequent sharp increase in fluctuations of the delta18O signal, that is, instability. The enrichment is magnified in the benthic foraminifera (Delta delta18O = 0.5 per mil) by a cooling of the deep water by 1.5° at the time, presumably signalling a glacial-type reduction of North Atlantic Deep Water (NADW) production. Episodic divergence between the signals of G. sacculifer and Pulleniatina in the Pleistocene apparently reflects periods of increased upwelling in the western equatorial Pacific. The amplitude of ice volume fluctuations cannot be reconstructed from delta18O data alone, unless there are constraints on temperature variations. The increase in amplitude of fluctuation of the benthic and planktic signals during the Pleistocene may be attributed either to an increase in maximum ice volume, or to an increase in the fractionation of continental ice, or a combination of both causes.

Stable carbon and oxygen isotope ratios for different test sizes of live benthic foraminifera from the Arabian Sea

We determined the stable oxygen and carbon isotopic composition of live (Rose Bengal stained) tests belonging to different size classes of two benthic foraminiferal species from the Pakistan continental margin. Samples were taken at 2 sites, with water depth of about 135 and 275 m, corresponding to the upper boundary and upper part of the core region of the oxygen minimum zone (OMZ). For Uvigerina ex gr. U. semiornata and Bolivina aff. B. dilatata, delta13C and delta18O values increased significantly with increasing test size. In the case of U. ex gr. U. semiornata, delta13C increased linearly by about 0.105 per mil for each 100-µm increment in test size, whereas delta18O increased by 0.02 to 0.06 per mil per 100 µm increment. For B. aff. B. dilatata the relationship between test size and stable isotopic composition is better described by logarithmic equations. A strong positive linear correlation is observed between delta18O and delta13C values of both taxa, with a constant ratio of delta18O and delta13C values close to 2:1. This suggests that the strong ontogenetic effect is mainly caused by kinetic isotope fractionation during CO2 uptake. Our data underline the necessity to base longer delta18O and delta13C isotope records derived from benthic foraminifera on size windows of 100 µm or less. This is already common practice in down-core isotopic studies of planktonic foraminifera.

Density and stable oxygen isotope profiles of four snow pits from Berkner Island, Filchner-Ronne Ice Shelf, Antarctica

The ice cap on Berkner Island is grounded on bedrock within the Filchner-Ronne Ice Shelf and is, therefore, expected to be a well-suited place to retrieve long-term ice-core records reflecting the environmental situation of the Weddell Sea region. Shallow firn cores were drilled to 11 m at the two main summits of Berkner Island and analysed in high depth resolution for electrical d.c. conductivity (ECM), stable isotopes, chloride, sulphate, nitrate and methane-sulphonate (MSA). From the annual layering of dD and non-sea-salt (nss) sulphate, a mean annual snow accumulation of 26.6 cm water at the north dome and 17.4 cm water at the south dome are obtained. As a result of ineffective wind scouring indicated by a relatively low near-surface snow density, regular annual cycles are found for all species at least in the upper 4-5 m. Post depositional changes are responsible for a substantial decrease of the seasonal dD and nitrate amplitude as well as for considerable migration of the MSA signal operating below a depth of 3-4 m. The mean chemical and isotopic firn properties at the south dome correspond to the situation on the Filchner-Ronne Ice shelf at a comparable distance to the coast, whereas the north dome is found to be more influenced by maritime air masses. Persistent high sea-salt levels in winter snow at Berkner Island heavily obscure the determination of nss sulphate probably due to sulphate fractionation in the Antartic sea-salt aerosols. Estimated time-scales predict ages at 400 m depth to be ca. 2000 years for the north and ca. 3000 years for the south dome. Pleistocene ice is expected in the bottom 200 and 300 m, respectively.

Stable oxygen isotope ratios of marine barite from Cenozoic sediments

We report new data on oxygen isotopes in marine sulfate (delta18O[SO4]), measured in marine barite (BaSO4), over the Cenozoic. The delta18O[SO4] varies by 6x over the Cenozoic, with major peaks 3, 15, 30 and 55 Ma. The delta18O[SO4] does not co-vary with the delta18O[SO4], emphasizing that different processes control the oxygen and sulfur isotopic composition of sulfate. This indicates that temporal changes in the delta18O[SO4] over the Cenozoic must reflect changes in the isotopic fractionation associated with the sulfide reoxidation pathway. This suggests that variations in the aerial extent of different types of organic-rich sediments may have a significant impact on the biogeochemical sulfur cycle and emphasizes that the sulfur cycle is less sensitive to net organic carbon burial than to changes in the conditions of that organic carbon burial. The delta18O[SO4] also does not co-vary with the d18O measured in benthic foraminifera, emphasizing that oxygen isotopes in water and sulfate remain out of equilibrium over the lifetime of sulfate in the ocean. A simple box model was used to explore dynamics of the marine sulfur cycle with respect to both oxygen and sulfur isotopes over the Cenozoic. We interpret variability in the delta18O[SO4] to reflect changes in the aerial distribution of conditions within organic-rich sediments, from periods with more localized, organic-rich sediments, to periods with more diffuse organic carbon burial. While these changes may not impact the net organic carbon burial, they will greatly affect the way that sulfur is processed within organic-rich sediments, impacting the sulfide reoxidation pathway and thus the delta18O[SO4]. Our qualitative interpretation of the record suggests that sulfate concentrations were probably lower earlier in the Cenozoic.

Late Pleistocene oxygen isotope record of biognic silica

We determined the isotopic composition of oxygen in marine diatoms in eight deep-sea cores recovered from the Atlantic sector of the Southern Ocean. The analytical reproducibility and core-to-core consistency of the isotopic signal suggests that diatom delta18O can be used as a new paleocenographic tool to reconstruct past variations in surface water characteristics and to generate 18O -isotope-based stratigraphy for the Southern Ocean. The data indicate that diatom delta18O reflects sea surface temperature and seawater isotopic composition and that diatoms retain their isotopic signal on timescales of a least 430 ka. The delta18O analyses of different diatom assemblages reveal that the isotopic signal is free of species effects and that the common Antarctic species have the same water-opal fractionation. The transition from the last glacial maximum (LGM) to the Holocene is fully recorded in high sedimentation rate cores. An 18O enrichment during the LGM, a post-LGM meltwater spike and an input of meltwater during the late Holocene are the main isotopic features observed in down core records. The origin of this meltwater was very likely melting icebergs and/or continental ice or by melting sea ice that had accumulated snow. The most pronounced meltwater effects are recorded in cores that are associated with the Weddel gyre. Our results provide the basis for extending isotope studies to oceanic regions devoid of carbonate; further, isotopic stratigraphies may be constructed for records and regions where they were previously not possible.

Oxygen-isotope composition, temperature and depth of formation of chert at DSDP Leg 62 Holes

We measured oxygen-isotope compositions of 16 siliceous rocks from Deep Sea Drilling Project Sites 463, 464, 465, and 466 (Leg 62). Samples are from deposits that range in age from about 40 to 103 m.y. and that occur at sub-bottom depths of 9 to 461 meters. Mean d18O values range from 28.4 to 36.8 per mil and 36.0 ± 0.3 per mil for quartz-rich and opal-CTrich rocks, respectively. d18O values in chert decrease with increasing sub-bottom depth; the slope of the d18O/depth curve is less steep for Site 464 than for the other sites which indicates that chert at Site 464 formed at higher temperatures than chert at Sites 463, 465, and 466. Temperatures of formation of cherts were 7 to 42°C, using the silica-water fractionation factor of Knauth and Epstein (1976), or 19 to 56°C, using the equation of Clayton et al. (1972). Temperatures in the sediment where the cherts now occur are lower than their isotopically determined temperatures of formation, which means that the cherts record an earlier history when temperatures in the sediment section were greater. Estimated sediment temperatures when the cherts formed are comparable to, but generally slightly lower than, those calculated from Knauth and Epstein's equation. The isotopic composition of cherts is more closely related to environment of formation (diagenetic environment) or paleogeothermal gradients, than to paleoclimates (bottom-water temperatures). Opal-CT-rich rocks may better record paleo-bottom-water temperature. In Leg 62 cherts, better crystallinity of quartz corresponds to lower d18O values; this implies progressively higher temperatures of equilibration between quartz and water during maturation of quartz. The interrelationship of d18O and crystallinity is noted also in continental-margin deposits such as the Monterey Formation - but for higher temperatures. The apparent temperature difference between open-ocean and continental-margin deposits can be explained by the dominant control of temperature on silica transformation in the rapidly deposited continental-margin deposits, whereas time, as well as temperature, has a strong influence on the transformations in open-ocean deposits. Comparisons between the chemistry and d18O values of cherts reveal two apparent trends: both boron and SiO2 increase as d18O increases. However, the correspondence between SiO2 and d18O is only apparent, because the two cherts lowest in SiO2 are also the most deeply buried, so the trend actually reflects depth of burial. The correspondence between boron and d18O supports the conclusion that boron is incorporated in the quartz crystal structure during precipitation

Strontium, oxygen and hydrogen isotope ratios of basalts from DSDP Hole 504B

DSDP Hole 504B was drilled into 6 Ma crust, about 200 km south of the Costa Rica Rift, Galapagos Spreading Center, penetrating 1.35 km into a section that can be divided into four zones-Zone I: oxic submarine weathering; Zone II: anoxic alteration; Zones III and IV: hydrothermal alteration to greenschist facies. In Zone III there is intense veining of pillow basalts. Zone IV consists of altered sheeted dikes. Isotopic geochemical signatures in relation to the alteration zones are recorded in Hole 504B, as follows: Zone Depth(m) Average87Sr/86Sr Average delta18O (?) Average deltaD (?) I 275-550 0.7032 7.3 -63 II 550-890 0.7029 6.5 -45 III 890-1050 0.7035 5.6 -31 IV 1050-1350 0.7032 5.5 -36 Alteration temperatures are as low as 10°C in Zones I and II based on oxygen isotope fractionation. Strontium isotopic data indicate that a circulation of seawater is much more restricted in Zone II than in Zone I. Fluid inclusion measurements of vein quartz indicate the alteration temperature was mainly 300 +/- 20°C in Zones III and IV, which is consistent with secondary mineral assemblages. The strontium, oxygen, and hydrogen isotopic compositions of hydrothermal fluids which were responsible for the greenschist facies alteration in Zones III and IV are estimated to be 0.7037, 2?, and 3?, respectively. Strontium and oxygen isotope data indicate that completely altered portions of greenstones and vein minerals were in equilibrium with modified seawater under low water/rock ratios (in weight) of about 1.6. This value is close to that of the end-member hydrothermal fluids issuing at 21°N EPR. Basement rocks are not completely hydrothermally altered. About 32% of the greenstones in Zones III and IV have escaped alteration. Thus 1 g of fresh basalt including the 32% unaltered portion are required in order to make 1 g of end-member solution from fresh seawater in water-rock reactions.