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1.
The timing of eustatic sea level fluctuations over the vertical range + 15 to ?11 m has been deduced from 230Th234U dating of Bermudan corals and speleothems. On this tectonically stable carbonate island, interglacial periods are characterized by platform submergence, development of patch reefs, and the deposition of littoral and eolian carbonates, whereas glacial periods are times of platform emergence, carbonate diagenesis, soil development, and the deposition of speleothems in caves extending below present sea level. Interglacial periods are observed at about 200,000, 130,000 to 90,000, and 10,000 yr BP to present. The sea level history of the last interglacial period (130,000 to 90,000 yr BP) is complex, consisting of at least two short, distinct episodes of high sea stand (at 125,000 and 97,000 yr BP) superimposed on a longer period of general platform submergence. The sea level data derived from this study are compatible with those from other stable areas such as the Bahamas, but in addition suggest that eustatic sea level changes can be rapid, on the order of 5 to 10 m/1000 yr.  相似文献   

2.
The isotopic ratios 18O16O and 13C12C show an annual periodicity in the coral Montastrea annularis from Bermuda, Jamaica and Barbados. The abundances of 18O and 13C are positively correlated in the Jamaica and Barbados samples, but inversely related in the Bermuda sample. Annual high density growth bands are formed during the season of warmest water temperature at all 3 sites and are enriched in 16O. M. Annularis has a constant displacement from oxygen isotopic equilibrium and accurately records seasonal temperature variations via the temperature-dependent aragonite-water fractionation factor. Light intensity, through the activity of the coral's endosymbiotic algae, regulates the depth-dependent and seasonal variations in the skeletal carbon isotopic composition.  相似文献   

3.
Thirty-nine species of scleractinian corals have been recovered from under a high dune on the western (mainland) side of North Stradbroke Island, eastern Australia. The corals are associated with thin intertidal sediments and their good condition implies burial in situ and preservation in a saturated zone. Most likely this occurred as the coast prograded and a large dune advanced into the littoral zone, burying intertidal sediments and coral. The species assemblage indicates a sheltered environment but one open to the ocean without wide fluctuations in salinity. Three species yielded a mean 230Th234U age of 105,000 yr B.P. which is significantly younger than the nearest Pleistocene corals at Evans Head, New South Wales. The corals provide evidence of a sea stand near present sea level during isotope Stage 5c, which is considerably higher than previously suggested for this period. Their good condition implies that the overlying parabolic dune is of comparable age and formed during that high stand of sea level. Also, the isotope age provides a maximum period for the development of giant podzols in the podzol chronosequences on coastal dunes in southern Queensland.  相似文献   

4.
Uplift rates of the Loyalty Islands (S.W. Pacific) have been determined from 230Th234U dating of raised coral terraces standing 2 to 7.5 m above sea level. The ages of the terraces on Ouvéa and Lifou correspond closely to previously documented periods of high sea levels at 120,000 and 180,000 yr BP. A +2-m terrace on Beautemps-Beaupré is considered to be beyond the range of the dating technique. The uplift rates of the various islands show a decrease toward the west. This lends support to the hypothesis of a lithospheric bulge of the leading edge of the Australian plate prior to its subduction at the level of the New Hebrides trench.  相似文献   

5.
Drill cores of Enewetak Atoll, Marshall Islands, reveal six stratigraphic intervals, numbered in downward sequence, which represent vertical coral growth during Quaternary interglaciations. Radiocarbon dates indicate that the Holocene sea transgressed the emergent reef platform by about 8000 yr B.P. The reef grew rapidly upward (about 5 to 10 mm/yr) until about 6500 yr B.P. Afterward vertical growth slowed to about 0.5 mm/yr, then lateral development became dominant during the last several thousand years. The second interval is dated at 131,000 ± 3000 yr B.P. by uranium series. This unit correlates with oxygen-isotope substage 5e and with terrace VIIa of Huon Peninsula, New Guinea, and of Main Reef-2 terrace at Atauro Island. The third interval is not dated because corals were recrystallized and it is tentatively correlated with either oxygen-isotope stages 7 or 9. The age of the fourth interval is estimated at 454,000 ± 100,000 yr B.P. from measured 234U238U activity ratios. This unit is correlated with either oxygen-isotope stage 9, 11, or 13.  相似文献   

6.
The concentrations of 238Pu, 239 + 240Pu, 241Am and 137Cs were determined in rain samples collected at Monaco in the course of 1978–1979. Based on these data, the annual deliveries of these radionuclides to the Mediterranean by rain are computed to be 0.18 ± 0.01 pCim?2 for 238Pu, 8.1 ± 0.1 pCim?2 for 239 + 240Pu, 0.58 ± 0.02 pCim?2 for 241Am and 351 ± 4 pCim?2 for 137Cs.Comparing the delivery data with the mixed layer inventories of 239 + 240Pu and 241Am in the Mediterranean, the upper limits of the mean residence time of these radionuclides in the mixed layer were estimated to be 12.3 yr for 239 + 240pu and 2.9 yr for 241Am. These values are consistent with the conclusion deduced from the vertical distribution pattern of these transuranic elements in the Mediterranean.Based on delivery values, the annual activity ratios for 238pu239 + 240Pu, 241Am239 + 240Pu and 239 + 240pu137Cs are found to be 0,022, 0.072 and 0.023 respectively. The 238pu239 + 240pu and 239 + 240Pu137Cs activity ratios vary within relatively narrow ranges with time, while a much wider variation was observed for the 241Am239 + 240Pu activity ratio. The cause of the wider variation of the 241Am239 + 240Pu ratio may be related to the difference in the mean age of fallout brought down in different seasons.  相似文献   

7.
Eleven samples of crude oil from the Molasse Basin of Southern Germany were fractionated and their contents of sulfur and nitrogen as well as the stable isotope compositions of these elements (34S32S and 15N14N, resp.) investigated.According to the δ34S determinations, all crude oils from the Tertiary base of the Western and Eastern Molasse belong to one oil family and differ significantly from the Triassic and Liassic oils in the Western Molasse.An enrichment of 34S was observed with increasing polarity of crude oil fractions. The isotope distributions of sulfur in the polar constituents of the biodegraded oils from the sandstones of Ampfing, however, approach a homogeneous distribution.The nitrogen isotope distribution is rather uniform in Southern German oils. A regional differentiation can be recognized, although the overall isotopic variation is small. The δ15N values of the crudes and asphaltenes do not correlate.  相似文献   

8.
Emerged coral reef terraces on the Huon Peninsula in New Guinea were reported in a reconnaissance dating study by Veeh and Chappell 1970. Age definition achieved was not good for several important terraces, and we report here a series of new 230Th234U dates, which further clarify the history of late Quaternary eustatic sea level fluctuations. More than 20 reef complexes are present, ranging well beyond 250,000 yr old: we are concerned with the seven lowest complexes. Major reef-building episodes dated by 30Th234U are reef complex I at 5–9 ka (kilo anno = 1000 yr), r.c. IIIb at 41 ka (four dates), r.c. IV at 61 ka (four dates), r.c. V at 85 ka (two dates), r.c. VI at 107 ka (two dates), and r.c. VII at 118–142 ka. Complex II was previously dated by 14C at 29 ka: this age has not yet been confirmed, and may be only a lower limit. The reef crests were built during or immediately before intervals of sea level maxima, when rates of rising sea level and tectonic uplift briefly coincided. The culmination of each reef-building episode was only a few thousand years in duration, and multiple dates from the same reef complex generally group within the statistical errors of the individual dates.Several methods can be used to estimate the altitude of each sea level maximum relative to present sea level. The least complicated is to calculate mean tectonic uplift rate for each profile of the terraces, and use the mean rate to calculate the tectonic displacement of each dated reef complex on that profile. The difference between the present altitude of a reef complex and its calculated tectonic uplift gives the paleosea level at the time the reef grew. We estimate uplift rates for six surveyed sections by calibrating against published paleosea level estimates from Barbados and elsewhere, viz 125 ka, paleosea at +6 m; 103 ka, ?15 m; 82 ka, ?13 m. For each section the individual uplift rates for reefs V, VI, and VIIb are within 5% of their section means. Using the mean rates. paleosea level estimates for reef crests II, IIIB, and IV are made for each section. Consistency of estimates between sections is good, giving ?28 m for the 60 ka paleosea level, around ?38 m for the 42 ka level and ?41 m for the 28 ka level (if the age is older the paleosea level would be lower. Using the mean uplift rates, the 82 ka and 103 ka paleosea levels are also estimated for each section: all individual estimates are plotted graphically, and a sea level curve drawn. The reef stratigraphy indicates sea level lowerings between each dated reef crest: the crests probably represent the interstadials of the Wisconsin (Würm, Weichsel) Glaciation, and intervening lower levels correspond to stadials. Since the last time of eustatic sea level higher than the present (about 125 ka), five sea level maxima occurred at roughly 20-ka intervals, none being as high as the present.  相似文献   

9.
Radiocarbon and 230Th-234U dates of calcic horizons from calciorthid soil profiles in the Mojave Desert were used to calculate the rate of deposition of pedogenic CaCO3. A major period of CaCO3 deposition appears to have occurred about 20000 yBP forming calcic horizons below 100-cm depth during a climatic regime with greater effective rainfall than in the present. The overall rate of deposition has been 1.0 to 3.5 g CaCO3/m2/yr during soil formation. This rate is consistent with present-day rates, assuming that the atmospheric deposition of Ca limits the process. Stable isotope ratios in calcic horizons indicate that CaCO3 precipitated from a soil environment with CO2 of ? 15.5%. 13C12C (vs. PDB) and H2O of + 2.0%. 18O16O (vs. SMOW). These values suggest that CaCO3 precipitates when seasonal drought simultaneously lowers soil pore pCO2 and enriches soil water 18O by evaporation. The role of soil calcic horizons in the global geochemical cycle of carbon is discussed.  相似文献   

10.
Numerical modeling of the terrestrial oxygen budget based on the revised δ13Ccarb record by Veizeret al. (1980) has shown that total photosynthetic oxygen has varied between ±7% and ±10% of its average reservoir size (~3.2 × 1022 g) during the last 800 myr as a result of oscillations of the sedimentary reservoir of organic carbon. Calculated curves of oxygen evolution display a distinct minimum in the Early Paleozoic framed by two maxima in the Latest Proterozoic and the Mesozoic. The sympathetic relationship observed between the curves of total oxygen evolution and respective functions for the partial reservoir of sulfate-bound oxygen suggests that the O2 required for an additional conversion of sulfide to sulfate was most probably provided by excess burial of organic carbon, the results of the modeling thus adding credence to current interpretations proposed for the negative correlation between the secular 13C12C and 34S32S trends.  相似文献   

11.
Diffusion of ions in sea water and in deep-sea sediments   总被引:3,自引:0,他引:3  
The tracer-diffusion coefficient of ions in water, Dj0, and in sea water, Dj1, differ by no more than zero to 8 per cent. When sea water diffuses into a dilute solution of water, in order to maintain the electro-neutrality, the average diffusion coefficients of major cations become greater but of major anions smaller than their respective Dj1 or Dj0 values. The tracer diffusion coefficients of ions in deep-sea sediments, Dj,sed., can be related to Dj1 by Dj,sed. = Dj1 · αθ2, where θ is the tortuosity of the bulk sediment and a a constant close to one.  相似文献   

12.
Oxygen isotope exchange between BaSO4 and H2O from 110 to 350°C was studied using 1 m H2SO4-1 m NaCl and 1 m NaCl solutions to recrystallize the barite. The slow exchange rate (only 7% exchange after 1 yr at 110°C and 91% exchange after 22 days at 350°C in 1 m NaCl solution) prompted the use of the partial equilibrium technique. However, runs at 300 and 350°C were checked by complete exchange experiments. The temperature calibration curve for the isotope exchange is calculated giving most weight to the high temperature runs where the partial equilibrium technique can be tested. Oxygen isotope fractionation factors (α) in 1 m NaCl solution (110–350°C), assuming a value of 1.0407 for αCO2H2O at 25°C, are:
1031nαBaSO4?1 m NaCl = 2.64 (106T2) ? 5.3 ± 0.3
.These data, when corrected for ion hydration effects in solution (Truesdell, 1974), give the fractionation factors in pure water:
1031nαBaSO4H2O = 3.01 (106/T2) ?7.3 ± 0.1
.In the 1 m H2SO4-1 m NaCl runs, sulfur isotope fractionation between HSO?4 and BaSO4 is less than the detection limit of 0.4%. A barite-sulfide geothermometer is obtained by combining HSO?4H2S and sulfide-H2S calibration data.Barite in the Derbyshire ore field, U.K., appears to have precipitated in isotopic equilibrium with water and sulfur in the ore fluid at temperatures less than 150°C. At the Tui Mine, New Zealand, the barite-water geothermometer indicates temperatures of late stage mineralization in the range 100–200°C. A temperature of 350 ± 20°C is obtained from the barite-pyrite geothermometer at the Yauricocha copper deposit, Peru, and oxygen isotope analyses of the barite are consistent with a magmatic origin for the ore fluids.  相似文献   

13.
Gabbro and diorite from the Skaergaard layered igneous intrusion contain noble gases which are mixtures of atmospheric and juvenile components. Atmospheric noble gases predominate in samples that have undergone extensive oxygen isotope exchange with meteoric-hydrothermal water. The source of the atmospheric noble gas component is inferred to be the hydrothermal circulation system. A juvenile component with 40Ar36Ar ≥ 6100 and containing fission xenon is also present This component predominates in samples showing unaltered magmatic oxygen isotope compositions. Neon of atmospheric isotopic composition is associated with the juvenile radiogenic 40Ar and fission xenon. The source of this second noble gas component may be either the crustal country rock or the upper mantle. If the neon is juvenile primordial neon from a mantle source region, terrestrial primordial 20Ne22Ne is the same as atmospheric to within 4%. However, subduction of atmospheric noble gases into the upper mantle may provide an alternate source of neon and other noble gases in the mantle.  相似文献   

14.
The inert gases have been measured in six size fractions covering the range below 500 μm, in a single feldspathic fragment weighing 523 μg, and in an agglutinate particle weighing 465 μg. The two size fractions between 125 and 250 μm as well as 250 and 500 μm were separated into magnetic and non-magnetic portions, which were measured separately. Like the Apollo and Luna 16 fines, the terra fines represented by Luna 20 are very rich in trapped solar-wind gases, but they contain relatively less He4 and Ne20, which is revealed by their average He4Ne20 ratio of 35 and Ne20Ar36 ratio of 2.9. Obviously the terra materials are less retentive for solar-wind He and Ne than typical mare fines such as 10084. Whether this is due to the relatively small TiO2 or the relatively large plagioclase content of the former is not resolved. (Ar36Kr84)trapped and (Ar36Xe132)trapped ratios are relatively large; the average values are 2800 and 14400, respectively. The apparent Ne21 radiation ages of all the size fractions are in the range 209–286 × 106 yr; the average is 260 × 106 yr. This is in the range of values known for the Apollo and Luna 16 fines. The feldspathic fragment has a much greater apparent Nec21 age of 780 × 106 yr. The Ar40-Ar36 systematic reveals the presence of two Ar40 components, because Ar40 = (1.41 ± 0.076)Ar36 + (0.490 ± 0.130) × 10?4 (cm3 STP/g). The Ar40Ar36 slope of 1.41 is not inconsistent with an origin of the sample from a relatively old terra region.  相似文献   

15.
Eggwaters from the chambered cephalopod Nautilus are depleted in both 18O and deuterium relative to ambient seawater. Eggwaters from six other species, including the related chambered cephalopod Sepia, do not show such depletion. These observations indicate that the previously observed step towards more positive δ18O values in calcium carbonate laid down after Nautilus hatches, relative to carbonate precipitated prior to hatching, can be explained by equilibration of the carbonate with water in the egg before hatching and with seawater after hatching. The presence of an oxygen isotope difference between eggwater and seawater for Nautilus and its absence for Sepia suggest that hatching will be recorded in the δ18O values of shell carbonates for some but not all extinct and extant chambered cephalopods.The δ13C values of the organic fraction of the siphuncle in Nautilus do not show any consistent pattern with regard to the time of formation before or after hatching. This observation suggests that the minimum in δ13C values previously observed for calcium carbonate precipitated after Nautilus hatches is not caused by a change in food sources once the animal becomes free-swimming, as has been suggested.  相似文献   

16.
The two major river systems on the west coast of India, Narbada and Tapti, their estuaries and the coastal Arabian sea sediments have been extensively studied for their uranium concentrations and 238U238U activity ratios.The 238U concentrations in the aqueous phase of these river systems exhibit a strong positive correlation with the sum of the major cations, σ Na + K + Mg + Ca, and with the HCO3? ion contents. The abundance ratio of dissolved U to the sum of the major cations in these waters is similar to their ratio in typical crustal rocks. These findings lead us to conclude that 238U is brought into the aqueous phase along with major cations and bicarbonate. The strong positive correlation between 238U and total dissolved salts for selected rivers of the world yield an annual dissolved 238U flux of 0.88 × 1010g/yr to the oceans, a value very similar to its removal rate from the oceans, 1.05 × 1010g/yr, estimated based on its correlation with HCO3? contents of rivers.In the estuaries, both 238U and its great-grand daughter 234U behave conservatively beyond chlorosities 0.14 g/l. These data confirm our earlier findings in other Indian estuaries. The behavior of uranium isotopes in the chlorosity zone 0.02–0.14 g/l, was studied in the Narbada estuary in some detail. The results, though not conclusive, seem to indicate a minor removal of these isotopes in this region. Reexamination of the results for the Gironde and Zaire estuaries (Martin et al., 1978a and b) also appear to confirm the conservative behavior of U isotopes in unpolluted estuaries. It is borne out from all the available data that estuaries beyond 0.14 g/l chlorosities act neither as a sink nor as a source for uranium isotopes, the behavior in the low chlorosity zones warrants further detailed investigation.A review of the uranium isotope measurements in river waters yield a discharge weighted-average 238U concentration of 0.22 μg/l with a 234U238U activity ratio of 1.20 ± 0.06ismissing. The residence time of uranium isotopes in the oceans estimated from the 238U concentration and the 234U238U A. R. of the rivers yield conflicting results; the material balance of uranium isotopes in the marine environment still remains a paradox. If the disparity between the results is real, then an additional 234U flux of about 0.25 dpm/cm2·103 yr into the oceans (about 20% of its river supply) is necessitated.  相似文献   

17.
18.
Sulfur isotope investigations carried out on elemental sulfur and sulfates of the Nea Kameni solfataras, Santorini, Aegean Sea, Greece, show a clear enrichment in the heavy sulfur isotope 34S against the assumed primordial 32S34S ratio of 22,220. Within the same crater, different vents, only a few meters apart from each other, produced δ differences up to 10‰, which remained constant for several years. This enrichment is most probably due to contamination by heavy sulfur from a nonvolcanic source. An enrichment in the same order of magnitude was observed in sulfur of recent and older lavas (δ 34S = ?1 ? +11‰).Potential contaminants like sulfide sulfur in hydrothermal ore veins of Athinios has a δ 34S mean value close to 0‰, sulfide and sulfate in the sedimentary basement has a δ 34S mean value of +2.6‰. Seawater sulfate from the area gives a value of δ 34S = 20‰, while sulfide from bacterial reduction of pore-water sulfate in recent iron ore sediments has δ 34S values between ?8 and ?5‰. Sulfate remaining in the pore solutions gave δ 34S = +27‰.The most probable explanation for the observed high δ 34S values in the solfataric sulfur and in some of the lavas of the Santorini area is contamination of the volcanic vents by Mediterranean Sea water.  相似文献   

19.
DH, 18O16O and 13C12C analyses were made of 14 whole rock and 28 mineral samples of rodingites associated dominantly with lizardite-chrysotile serpentinites from the West Coast of the U.S.A., New Zealand, and the Northern Appalachian Mtns. The δD values of the rodingite minerals are in three groupings: 5 monomineralic veins of pectolite, ?281 to ?429; 8 monomineralic veins of xonotlite, ?112 to ?135; all other minerals, including hydrogarnet, idocrase, prehnite, actinolite, nephrite, and chlorite, ?34 to ?80. Most calcites in rodingites have δ18O (+9.3 to +14.4) and (δ13C (?6.7 to +0.9) values similar to calcites in other Franciscan rocks, but distinct from the very low temperature calcite veins in serpentinites. The DH data, combined with δ18O values of xonotlite (+5.7 to +10.9) and pectolite (+8.9 to +12.4) suggest formation from meteoric-type waters at low temperatures; the DH depletion of pectolite, however, is anomalous. Rodingite whole rock values range from δ18O = +4.1 to +11.5 and δD = ?50 to ?86; one sample containing minor amounts of lizardite-chrysotile serpentinite has δD = ?92, outside this range. However, most rodingites of basaltic or gabbroic parentage are more restricted in δ18O (+4.1 to +8.6). Such a wide range in δ18O is consistent with the idea that most rodingites form over a relatively broad range of hydrothermal temperatures. Hydrogen isotopic data for most rodingite minerals (except xonotlite and pectolite) and for whole rocks are suggestive of non-meteoric waters. These DH data overlap those observed for veins of hydrous minerals found in Franciscan igneous rocks studied by Margaritz and Taylor (1976, Geochim. Cosmochim. Acta40, 215–234), possibly suggesting evolved D-enriched, connate type metamorphic waters generated during high P, low T Franciscan-type metamorphism at temperatures (250–500°C) comparable to estimates based on mineral stabilities. Such an interpretation is supported by the 18O16O and 13C12C data for calcite in rodingites.The isotope data appear to contradict some of the conclusions derived from geologic and petrologic studies that indicate concomitant metasomatism and serpentinization of their presently observed host rock. These data appear most consistent with the interpretation that most rodingite minerals, with the exception of late-stage veins of xonotlite and possibly pectolite, may involve metasomatism in association with antigorite serpentinization of ultramafic rock. Subsequent upward tectonic transport in many instances may result in incorporation of the rodingites into their presently observed lizarditechrysotile host rock during or subsequent to pervasive shallow level serpentinization by meteoric waters.  相似文献   

20.
The extent of oxygen isotopic exchange between detrital clay minerals and sea water was investigated by analyzing O18O16 ratios of separated fine-grained size fractions of deep-sea sediments from three North Pacific ocean cores. Isotopic results were interpreted according to models based on the assumption that the extent of isotopic exchange should increase with decreasing particle size and increasing time of exchange between the sediment and sea water. The data indicate that information concerning the provenance and mode of formation of detrital clay minerals can be obtained from the O18O16 ratios of the coarser-than-0.1 μm fraction of deep-sea sediments younger than several million years and the finer-than-0.1 μm fraction of deep-sea sediments younger than several tens of thousands of years. Furthermore, if the extent of chemical reaction between detrital clays and sea water is similar to the extent of oxygen isotopic exchange, such reaction may be important in regulating the chemistry of sea water.  相似文献   

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