1,721,084 research outputs found
Stable isotope study of water, gypsum and carbonate samples from the Bannock and Tyro Basins, Eastern Mediterranean.
No full textSeawater and brine samples collected along vertical profiles above the Bannock and Tyro Basins (eastern Mediterranean), and gypsum samples collected by dredging and coring the anoxic section of the Bannock Basin, have been studied for their δ18O values. The following conclusions may be drawn from these data: (1) The water of the deep brines is isotopically slightly different from modern Mediterranean bottom water and might be a 'fossil water' possibly formed during a recent, climatically cooler stage or when the evaporation was slightly higher than nowadays. (2) The similarity between the δ18O values of the brine samples from the Bannock and Tyro Basins supports the possibility that the ages of the two basins are very close to each other. (3) Gypsum crystals on the bottom of the Bannock Basin are probably being formed by the dissolution and reprecipitation of sub-bottom (or outcropping) Messinian evaporites. This hypothesis is supported by the oxygen isotopic values exhibited by the gypsum crystallization water and by the oxygen and sulphur isotopic composition of gypsum. (4) The shells of pelagic organisms included in gypsum are probably very recent, but not contemporary, and may be referred to cooler environmental conditions. © 1990
δ18O of carbonate, quartz and phosphate from belemnite guards: Implications for the isotopic record of old fossils and the isotopic composition of ancient seawater
No full textBelemnite guards of Cretaceous and Jurassic age were found to contain varying amounts of quartz deposited both on the external surface and inside the rostrum. The oxygen isotopic composition of coexisting carbonate, quartz and phosphate from the same rostrum was measured according to well-established techniques. None of these compounds showed isotopic values in equilibrium with one another. Assuming δ18O values of the diagenetic water within the range of meteoric waters, the δ18O(SiO2) yield temperatures in agreement with the apparent secondary origin of this phase. The δ18O(CO2-3) range, with a certain continuity, between -10.8 and +0.97 PDB-1 with most of the intermediate values being within the range of the carbonate isotopic values of Mesozoic fossils. The most positive isotopic results obtained from phosphate are close to +23/+24‰ (V-SMOW). They can hardly be related to a secondary origin of the phosphate, or to the presence of diagenetic effects, since these results are among the most positive ever measured on phosphate. As far as we know there is no widespread diagenetic process determining an 18O enrichment of phosphate. The very low concentration of phosphate did not allow the determination of its mineralogical composition. All the available δ18O(PO43-) values from belemnite and non-belemnite fossils of marine origin of Tertiary and Mesozoic age are reported along with the newly measured belemnites. The following conclusions may be drawn from the data reported: (1) the pristine oxygen isotope composition of fossil marine organisms (either carbonate or phosphate) may easily undergo fairly large changes because of oxygen isotope exchange processes with diagenetic water; this process is apparent even in the case of geologically recent fossils; (2) the δ18O(PO43-) of belemnite rostra seems to be, at least in the case of the most positive results, in isotopic equilibrium with environmental water because of the similarity between the results from Cretaceous belemnites and the results from Cretaceous and Lower Tertiary pelecypods and fish teeth; 3) if so, the only feasible interpretation that can be suggested for the 18O enriched data is the possibility of a relatively large variation of the oxygen isotopic composition of ocean paleowater from Jurassic to recent time. © 2002 Elsevier Science B.V. All rights reserved
Fossil hydrothermal systems tracking Eocene climate change in Antarctica: Reply
No full textDuring the Cenozoic, alkaline magmatism related to rifting in the Ross Sea embayment affected the basement of northern Victoria Land, Antarctica. Shallow intrusions supplied the necessary heat to cause groundwater circulation through permeable rocks, producing local hydrothermal systems. In the granitoid country rocks of the two adjacent areas of Mt. Monteagle and Mt. McGee, the biotite deltaD values range from -69% to -141% and from -66% to -183%, respectively, and K-feldspars show delta O-18 values as low as 3.7% and 4.4%. These values result from isotope exchange between igneous minerals and hydrothermal fluids with a dominant meteoric water component. Geological and tectonic histories of the two areas show marked similarities; therefore, different delta O-18 and deltaD values of hydrothermally altered rocks may be interpreted in terms of different isotopic composition of the waters feeding the hydrothermal systems. The isotopic shift of these meteoric waters indicates that climate change occurred in the time span between the 42 Ma intrusion of Mt. Monteagle and the 38 Ma intrusion of Mt. McGee, thus suggesting a cooling episode in the Ross Sea region during late Eocene time
Oxygen and hydrogen isotopic composition and zircon morphology of some Pan-African granites
No full textThe Pan-African orogeny took place throughout the Late Proterozoic and Cambrian time. Its final stage (c.500 Ma) is evidenced by radiometric age determinations. A set of samples from Pan-African granites collected from the Gondwana supercontinent (West Africa, East Africa, Brazil and Antarctica) were studied for their oxygen and hydrogen isotopic composition and zircon morphology. The purpose of this study was to obtain further information on the genesis and geochemical history of these granites. As regards the isotopic composition of single phases, the results generally point to equilibrium conditions. The temperatures obtained from phase-phase equilibrium equations are close to 500°C. The zircon morphology, on the other hand, points to a major role of water activity in the evolution of these granites which, in the light of isotope data, must have taken place at T above 500°C. -from Author
Isotopic composition, growth rates and biological behaviourof Chamelea gallina and Callista chione from the Gulf of Trieste (Italy)
No full textIsotopic composition of Holocene shells from raised beaches and ice shelves at Terra Nova Bay (Northern Victoria Land, Antarctica).
No full textSeventy-six samples of calcium carbonate from shells
and skeletons of fossil marine. organisms collected in
the area of Terra Nova Bay (Victoria Land, Antarctica)
have been measured for their 13C/12C and 180 /160 ratios.
Solitary corals have been measured along with bryozoans,
cirripeds, serpulids and pelecypods coming from fossil
beaches, supraglacial debris and dirt cones. Age determinations
have been carried out by means of 14C measurements.
The isotopic results obtained may be summarized as
follows: the measurements carried out on corals agree well
with previous measurements clearly indicating that these
organisms do not precipitate the CaC03 of their skeleton
under isotopic equilibrium conditions with environmental
water. The 0 180 (COi-) are probably shifted by a constant
value when compared with the 0180(~Oi-) values at
equilibrium. The isotopic results obtained from bryozoans
do not agree with the data reported in the literature. Our
specimens show <S180 values away from isotopic equilibrium
condition and <S13Cvalues very close to those of "normal"
marine carbonate.
Four different species of pelecypods have been studied,
their age ranging from recent to about 7000 years B.P.
In the case of Adamussium colbecki, there seems to be: a;
relationship between age and 8180 (C0 32-), older specimens
yielding more negative < 5 180 values. The isotopic values
obtained from cirripeds suggest similar conclusions.
Finally, both the oxygen and, carbon isotopic values of
serpulids are not very far from equilibrium conditions.
The main interest of these preliminary data is represented
by the possibility of detecting relatively. small climatic
changes even under these extreme conditions and by the
possibility of obtaining information on the biological
behaviour of various marine organisms living in this peculiar
environment
Environmental isotope data: oxygen isotope concentration in precipitation in N-E Italy (Friuli Venezia Giulia)
No full textMineralogica et Petrografica Act
Ricostruzione paleoclimatica attraverso l'analisi di uno speleotema nella sardegna centro-orientale: risultati preliminari
No full textThis paper, carried out in the framework of a local project 60% «Pleistocene-Holocene palaeoclimate and palaeogeography reconstructions based on marine and continental deposits of Sardinia» (Resp. Prof. Sandro De Muro) and within a series of researches involving several bodies (Archaeological Superintendence of NU/SS, Universities of Parma and Cagliari) and caving associations (Gruppo Speleo Ambientale Sassari, Gruppo Speleo-Archeologico Giovanni Spano of Cagliari, Unione Speleologica Cagliaritana and Gruppo Archeo Speleo Ambientale Urzulei), reports the first results of a palaeo-climate study carried out on a stalagmite sampled in the Su Palu cave, Codula Ilune, Urzulei (Central-East Sardinia).
This stalagmite has been subjected to radiometric U/Th dating and stable isotope δ180 and δ 13C analysis. These isotopic data based on 139 samples showed a general increase in stable isotope ratios that would indicate a growth of the concretion in a period of global warming (Upper Pleistocene-Holocene?).
The chronological setting by means of U/Th datings, unfortunately, did not allow to define the exact timeframe because of the presence of detrital Thorium. C14 analysis are under way to try to obtain information on the time during which the stalagmite growed
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