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1.
Our understanding of the ancient ocean-atmosphere system has focused on oceanic proxies. However, the study of terrestrial proxies is equally necessary to constrain our understanding of ancient climates and linkages between the terrestrial and oceanic carbon reservoirs. We have analyzed carbon-isotope ratios from fossil plant material through the Valanginian and Lower Hauterivian from a shallow-marine, ammonite-constrained succession in the Crimean Peninsula of the southern Ukraine in order to determine if the Upper Valanginian positive carbon-isotope excursion is expressed in the atmosphere. δ13Cplant values fluctuate around − 23‰ to − 22‰ for the Valanginian-Hauterivian, except during the Upper Valanginian where δ13Cplant values record a positive excursion to ∼− 18‰. Based upon ammonite biostratigraphy from Crimea, and in conjunction with a composite Tethyan marine δ13Ccarb curve, several conclusions can be drawn: (1) the δ13Cplant record indicates that the atmospheric carbon reservoir was affected; (2) the defined ammonite correlations between Europe and Crimea are synchronous; and (3) a change in photosynthetic carbon-isotope fractionation, caused by a decrease in atmospheric pCO2, occurred during the Upper Valanginian positive δ13C excursion. Our new data, combined with other paleoenvironmental and paleoclimatic information, indicate that the Upper Valanginian was a cool period (icehouse) and highlights that the Cretaceous period was interrupted by periods of cooling and was not an equable climate as previously thought.  相似文献   

2.
We report new data on oxygen isotopes in marine sulfate (δ18OSO4), measured in marine barite (BaSO4), over the Cenozoic. The δ18OSO4 varies by 6‰ over the Cenozoic, with major peaks 3, 15, 30 and 55 Ma. The δ18OSO4 does not co-vary with the δ34SSO4, emphasizing that different processes control the oxygen and sulfur isotopic composition of sulfate. This indicates that temporal changes in the δ18OSO4 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 δ18OSO4 also does not co-vary with the δ18O 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 δ18OSO4 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 δ18OSO4. Our qualitative interpretation of the record suggests that sulfate concentrations were probably lower earlier in the Cenozoic.  相似文献   

3.
We present a time series of carbon and oxygen stable isotope records of the last 30?000 14C years throughout the last glacial-postglacial cycle from western Qinghai-Xizhang (Tibet) Plateau. A 20-m core drilled in the south basin of Zabuye Salt Lake was analyzed for inorganic and organic carbon and total sulfur contents, δ13C and δ18O values of carbonates. Our results indicate that climatic changes have led to a drastic negative shift of stable isotope ratios at the transition between the Last Full Glacial and the postglacial phase during Later Pleistocene times (∼16.2 kyr BP), and a rapid positive shift at the transition from Pleistocene to Holocene (∼10.6 kyr BP). The first shift is marked by the drop of δ18Ocarb values of about 10‰ (from +2 to −8‰) and δ13Ccarb values of about 3‰ (from 5 to 2‰). The second shift which occurred at the transition from Pleistocene to Holocene was of similar magnitude but in the opposite direction. Isotope data, combined with total organic and inorganic carbon contents and the lithological composition of the core, suggest this lake was an alluvial pre-lake environment prior to ca. 28 14C kyr BP. During ca. 28-16.2 14C kyr BP, Zabuye Lake was likely a moderately deep lake with limited outflow. The cool and arid glacial climate led the lake level to drop drastically. Extended residence time overwhelmed the lower temperature and caused a steady increase of δ13Ccarb and δ18Ocarb values and total inorganic carbon content in the sediments. During ca. 16.2-10.6 14C kyr BP, this lake probably overflowed and received abundant recharge from melting glaciers when the deglaciation was in its full speed. A spike of markedly enhanced δ13Ccarb and δ18Ocarb is seen at ∼11.5 kyr BP, probably due to the isotopic effects left behind by the short but severe Younger Dryas (YD) event. After ca. 10.6 14C kyr BP, Zabuye Lake probably closed its surface outflow, due to strong desiccation and drastic climate warming. The Early and Middle Holocene were characterized by unstable climatic conditions with alternating warmer/cooler episodes as indicated by the severe fluctuations of total organic carbon, δ13C and δ18O values. A hypersaline salt lake environment was finally formed at Zabuye after ∼5 14C kyr BP when the mirabilite and halite concentrations steadily increased and became the dominant minerals in the sediments. Severe imbalance of inflow/outflow resulted in the drastic increase of total sulfur, δ13Ccarb and δ18Ocarb values and dominance of halite in the lake since ca. 3.8 kyr BP to present.  相似文献   

4.
We provide two new determinations of the oxygen isotopic composition of seawater during the last glacial maximum (LGM). High-resolution oxygen isotopic measurements were made on interstitial waters from Ocean Drilling Program (ODP) Sites 1168 and 1170 in the southeast Indian Ocean sector of the Southern Ocean. We use a diffusion-advection numerical model to calculate the glacial-interglacial change in bottom-water δ18Osw from the pore water δ18O profiles; the first such determinations from this part of the oceans. Statistical analyses of the model runs indicate that Circumpolar Deep Water (CDW) δ18Osw changed by 1.0-1.1±0.15‰ since the last glacial maximum (LGM). Our results are consistent with a previous calculation from a South Atlantic Southern Ocean location (ODP Site 1093) also situated within CDW. The new values determined in this study, together with previous estimates, are converging on a global average Δδ18Osw of 1.0-1.1‰.Using the calculated bottom-water δ18Osw, we have extracted the temperature component from the benthic foraminiferal δ18O record at Sites 1168 and 1170. Since the LGM, bottom waters at these two sites warmed by 2.6 and 1.9°C, respectively. The absolute temperature estimates for the LGM (−0.5°C [Θ=−0.6°C] at Site 1168 and −0.2°C [Θ=−0.4°C] at Site 1170) are slightly warmer than those reported from previous studies using the same technique, but are consistent with more homogenous deep-ocean temperatures during the LGM relative to the modern.  相似文献   

5.
Authigenic carbonates were sampled in methane-enriched piston core sediments collected from gas venting sites on the western continental slope of the Ulleung Basin, East Sea of Korea. Multidisciplinary investigations on these carbonates, including the scanning electronic microscope (SEM) observations and mineralogical-geochemical compositions, were carried out to identify the carbon and oxygen sources and the forming mechanism of these carbonates. The authigenic carbonates from the study area correspond to semi-consolidated, compact concretions or nodules ranging from 2 to 9 cm in size. X-ray diffraction and electron microprobe analyses showed that most of the sampled carbonate concretions were composed of almost purely authigenic high-Mg calcite (10.7–14.3 mol% MgCO3). Characteristically, microbial structures such as filaments and rods, which were probably associated with the authigenic minerals, were abundantly observed within the carbonate matrix. The carbonates were strongly depleted in δ13C (−33.85‰ to −39.53‰ Peedee Belemnite (PDB)) and were enriched in δ18O (5.16–5.60‰ PDB), indicating that the primary source of carbon is mainly derived from the anaerobic oxidation of methane. Such methane probably originated from the destabilization of the underlying gas hydrates as strongly supporting from the enriched 18O levels. Furthermore, the strongly depleted δ13C values (−60.7‰ to −61.6‰ PDB) of the sediment void gases demonstrate that the majority of the gas venting at the Ulleung Basin is microbial methane by CO2 reduction. This study provides another example for the formation mechanism of methane-derived authigenic carbonates associated with gas-hydrate decomposition in gas-seeping pockmark environments.  相似文献   

6.
A comparison of the oxygen isotope signal in deep-sea benthic foraminifera with the record of glacio-eustatic sea level for the last 160,000 years reveals that the amplitude of the benthic δ18O records predicts more continental ice volume than appears to be reflected in lowered sea level stands. These differences between the benthic δ18O ice volume estimates and radiometrically-dated records of eustatic sea level are consistent with the presence of a large floating Arctic Ocean ice mass during glacial intervals. The presence of an Arctic Ocean ice sheet during glacial intervals may account for the two climatic modes observed in oxygen isotope records which span the entire Pleistocene. The early Pleistocene (1.8 to 0.9 Myr B.P.) interval is characterized by low-amplitude, high-frequency δ18O fluctuations between glacial and interglacial periods, while the late Pleistocene (0.9 Myr B.P. to present) is characterized by large-amplitude, low-frequency δ18O changes. These two climatic modes can be explained by the initiation of earth orbital conditions favoring the co-occurrence of glacial period Arctic Ocean ice sheets and large continental ice sheets approximately 900,000 years before present.  相似文献   

7.
The biogeochemistry of methane in the sediments of Lake Caviahue was examined by geochemical analysis, microbial activity assays and isotopic analysis. The pH in the water column was 2.6 and increased up to a pH of 6 in the deeper sediment pore waters. The carbon isotope composition of CH4 was between − 65 and − 70‰ which is indicative for the biological origin of the methane. The enrichment factor ε increased from − 46‰ in the upper sediment column to more than − 80 in the deeper sediment section suggesting a transition from acetoclastic methanogenesis to CO2 reduction with depth. In the most acidic surface layer of the sediment (pH < 4) methanogenesis is inhibited as suggested by a linear CH4 concentration profile, activity assays and MPN analysis. The CH4 activity assays and the CH4 profile indicate that methanogenesis in the sediment of Lake Caviahue was active below 40 cm depth. At that depth the pH was above 4 and sulfate reduction was sulfate limited. Methane was diffusing with a flux of 0.9 mmol m− 2 d− 1 to the sediment surface where it was probably oxidized. Methanogenesis contributed little to the sediments carbon budget and had no significant impact on lake water quality. The high biomass content of the sediment, which was probably caused by the last eruption of Copahue Volcano, supported high rates of sulfate reduction which probably raised the pH and created favorable conditions for methanogens in deeper sediment layers.  相似文献   

8.
To improve our knowledge about the geochemical and environmental aftermath of Neoproterozoic global glaciations, we analyzed stable isotopes (δ13C, δ18O, δ34S) and elemental concentrations (Ca, Mg, S, Sr, Fe, and Mn) of the ～ 10-m-thick Zhamoketi cap dolostone atop the Tereeken diamictite in the Quruqtagh area, eastern Chinese Tianshan. Available chemostratigraphic data suggest that the Tereeken diamictite is probably equivalent to the Marinoan glaciation. Our new data indicate that organic and carbonate carbon isotopes of the Zhamoketi cap dolostone show little stratigraphic variations, averaging ? 28.2‰ and ? 4.6‰, respectively. In contrast, sulfur isotopes show significant stratigraphic variations. Carbonate associated sulfate (CAS) abundance decreases rapidly in the basal cap dolostone and δ34SCAS composition varies between + 9‰ and + 15‰ in the lower 2.5 m. In the overlying interval, CAS abundance remains low while δ34SCAS rises ～ 5‰ and varies more widely between + 10‰ and + 21‰. The range of δ34Spy of the cap dolostone overlaps with that of δ34SCAS, but direct comparison shows that δ34Spy is typically greater than δ34SCAS measured from the same samples. Hypotheses to explain the observations must account for both the remarkable sulfur isotope enrichment of pyrites and the inverse fractionation. We propose that CAS and pyrite were derived from two isotopically distinct reservoirs in a chemically stratified basin or a basin with a sulfate minimum zone. In this model, CAS was derived from shallow, oxic surface waters with moderate sulfate concentration and depleted in 34S due to the post-glacial influx of sulfur from continental weathering. In contrast, pyrite was derived from anoxic bottom waters (or a sulfate minimum zone) with low sulfate concentration and 34S enrichment due to long-term syn-glacial sulfate reduction. The rapid shift in CAS abundance and sulfur isotope composition within the cap dolostone is interpreted to reflect the mixing of the two reservoirs after initial deglaciation. Comparison with other post-Marinoan cap carbonates shows significant spatial heterogeneity in δ34SCAS, which together with strong temporal variation in δ34SCAS, points to generally low sulfate concentrations in post-Marinoan oceans.  相似文献   

9.
The present work aimed at studying the origin of particulate organic matter in Guanabara Bay and in some rivers of the Guanabara basin by using elemental composition, isotopic ratios (δ13C and δ15N) and molecular markers (sterols) in samples collected in two periods (winter and summer). Elemental and isotopic compositions were determined by dry combustion and mass spectrometry, respectively, while sterols were investigated by GC–FID and GC–MS. Higher sterol concentrations were present in the north-western part of the bay in winter (5.10–23.5 μg L–1). The high abundance of algal sterols (26–57% of total sterols), the elemental composition (C/N=6–8) and the isotopic signatures (δ13C=−21.3‰ to −15.1‰ and δ15N=+7.3‰ to +11.1‰) suggested the predominance of autochthonous organic matter, as expected for an eutrophic bay, although seasonal variation in phytoplankton activity was observed. Coprostanol concentration (fecal sterol) was at least one order of magnitude higher in the particulate material from fluvial samples (4.65–55.98 μg L–1) than in the bay waters (<0.33 μg L–1). This could be ascribed to a combination of factors including efficient particle removal to sediments in the estuarine transition zone, dilution with bay water and bacterial degradation during particle transport in the water column.  相似文献   

10.
The Miocene Tejeda caldera on Gran Canaria erupted ~ 20 rhyolite–trachyte ignimbrites (Mogán Group 14–13.3 Ma), followed by ~ 20 phonolitic lava flows and ignimbrites (Fataga Group 13–8.5 Ma). Upper-Mogán tuffs have been severely altered immediately within the caldera margin, whereas extra-caldera Mogán ignimbrites, and overlying Fataga units, are apparently unaltered. The altered intra-caldera samples contain minerals characteristic of secondary fluid–rock interaction (clays, zeolites, adularia), and relics of the primary mineral assemblage identified in unaltered ignimbrites (K-feldspar, plagioclase, pyroxene, amphibole, and groundmass quartz). Major and trace-element data indicate that Si, Na, K, Pb, Sr, and Rb, were strongly mobilized during fluid–rock interaction, whereas Ti, Zr, and Nb behaved in a more refractory manner, experiencing only minor mobilization. The δ18O values of the altered intra-caldera tuffs are significantly higher than in unaltered extra-caldera ignimbrites, consistent with an overall low-temperature alteration environment. Unaltered extra-caldera ignimbrites have δD values between − 110‰ and − 173‰, which may reflect Rayleigh-type magma degassing and/or post-depositional vapour release. The δD values of the altered intra-caldera tuffs range from − 52‰ to − 131‰, with ambient meteoric water at the alteration site estimated at ca. − 15‰. Interaction and equilibration of the intra-caldera tuffs with ambient meteoric water at low temperature can only account for whole-rock δD values of around − 45‰, given that ?Dclay–water is ca. − 30‰ at 100 °C, and decreases in magnitude at higher temperatures. All altered tuff samples have δD values that are substantially lower than − 45‰, indicating interaction with a meteoric water source with a δD value more negative than − 15‰, which may have been produced in low-temperature steam fumaroles. Supported by numerical modeling, our Gran Canaria data reflect the near-surface, epithermal part of a larger, fault-controlled hydrothermal system associated with the emplacement of the high-level Fataga magma chamber system. In this near-surface environment, fluid temperatures probably did not exceed 200–250 °C.  相似文献   

11.
An area of massive barite precipitations was studied at a tectonic horst in 1500 m water depth in the Derugin Basin, Sea of Okhotsk. Seafloor observations and dredge samples showed irregular, block- to column-shaped barite build-ups up to 10 m high which were scattered over the seafloor along an observation track 3.5 km long. High methane concentrations in the water column show that methane expulsion and probably carbonate precipitation is a recently active process. Small fields of chemoautotrophic clams (Calyptogena sp., Acharax sp.) at the seafloor provide additional evidence for active fluid venting. The white to yellow barites show a very porous and often layered internal fabric, and are typically covered by dark-brown Mn-rich sediment; electron microprobe spectroscopy measurements of barite sub-samples show a Ba substitution of up to 10.5 mol% of Sr. Rare idiomorphic pyrite crystals (∼1%) in the barite fabric imply the presence of H2S. This was confirmed by clusters of living chemoautotrophic tube worms (1 mm in diameter) found in pores and channels within the barite. Microscopic examination showed that micritic aragonite and Mg-calcite aggregates or crusts are common authigenic precipitations within the barite fabric. Equivalent micritic carbonates and barite carbonate cemented worm tubes were recovered from sediment cores taken in the vicinity of the barite build-up area. Negative δ13C values of these carbonates (>−43.5‰ PDB) indicate methane as major carbon source; δ18O values between 4.04 and 5.88‰ PDB correspond to formation temperatures, which are certainly below 5°C. One core also contained shells of Calyptogena sp. at different core depths with 14C-ages ranging from 20?680 to >49?080 yr. Pore water analyses revealed that fluids also contain high amounts of Ba; they also show decreasing SO42- concentrations and a parallel increase of H2S with depth. Additionally, S and O isotope data of barite sulfate (δ34S: 21.0-38.6‰ CDT; δ18O: 9.0-17.6‰ SMOW) strongly point to biological sulfate reduction processes. The isotope ranges of both S and O can be exclusively explained as the result of a mixture of residual sulfate after a biological sulfate reduction and isotopic fractionation with ‘normal’ seawater sulfate. While massive barite deposits are commonly assumed to be of hydrothermal origin, the assemblage of cheomautotrophic clams, methane-derived carbonates, and non-thermally equilibrated barite sulfate strongly implies that these barites have formed at ambient bottom water temperatures and form the features of a Giant Cold Seep setting that has been active for at least 49?000 yr.  相似文献   

12.
The present study employs a method for analysis of the sulfur isotopic composition of trace sulfate extracted from carbonates collected in Namibia in order to document secular variations in the sulfur isotopic composition of Neoproterozoic oceanic sulfate and to assess variations in the sulfur cycle that may have accompanied profound climatic events that have been described as the snowball Earth hypothesis. The carbonates in the Otavi Group of Northwest Namibia contain 3-295 ppm sulfate. Positive excursions, to a high of 40‰ (CDT), occur above the lower (Chuos Formation) and upper (Ghaub Formation) glacial intervals in the Rasthof and Maieberg cap carbonates, respectively. Positive excursions at the top of the Rasthof Formation (reaching 51‰) and within the overlying Gruis Formation (34‰) do not appear to correspond to glaciation. The δ34Ssulfate values within the Ombaatjie Formation exhibit shifts over relatively short stratigraphic intervals (tens of meters), varying between ∼15 and 25‰. Cap carbonates from Australia exhibit positive δ34Spyrite trends with amplitudes similar to those of Namibian δ34Ssulfate, although, more data are necessary to firmly establish these δ34S trends as global in nature. δ34Ssulfate excursions found in Namibian cap carbonates are consistent with the snowball Earth hypothesis in that they appear to reflect nearly complete reduction of sulfate in an isolated, anoxic global ocean, although, there are other mechanisms that may have facilitated these large shifts in δ34Ssulfate. Regardless, the low sulfate concentrations in Otavi carbonates, the high amplitude variability of the δ34Ssulfate curve, and the apparently full reduction of sulfate (as implied from δ34Spyrite data), even in strata low in Corg, suggest that Neoproterozoic oceanic sulfate concentrations were much lower than modern values. Additionally, the buildup of ferrous iron and banded-iron formations during the Sturtian glacial event would indicate that Fe supply exceeded sulfide availability during the glacials and/or that all sulfide was fixed and buried. This could be construed as further evidence in support of low oceanic sulfate (and sulfide) at this time.  相似文献   

13.
The Flin Flon Belt of Canada contains Paleoproterozoic volcanic–sedimentary sequences that are related to the Trans‐Hudson Orogeny. The sequences include island arc volcanic and volcaniclastic rocks (Amisk Group) that are unconformably overlain by subaerial sedimentary rocks (Missi Group), and younger deep facies sediments. In the Flin Flon area, several north–south trending faults divide the sequences into blocks and obscure the depositional environment of the deep facies sediments. Locally, within the Flin Flon area, the Embury Lake Formation is in fault contact with island arc volcanic–sedimentary sequences of the Amisk and Missi Groups. To identify the depositional environment of the Embury Lake Formation, we used lithologic and geochemical approaches. Here, we report carbon isotopic values in organic matter (δ13Corg) and sulfur isotopes (δ34S), as well as total organic carbon and total sulfur measurements for the black shale in the formation. Samples were taken from a drill core that contains alternating bands of sandstone and black shale. Pyrite in the black shale is divided into four textural types: euhedral, vein‐type, elliptical, and microcrystalline. Microcrystalline pyrite is typically generated by microbially mediated sulfate reduction. An extremely low S/C ratio (avg. = 0.04) is consistent with lacustrine deposition. The ranges of δ13Corg (?36 ‰ to ?27 ‰) and δ34S (+3.0 ‰ to +7.7 ‰) values can be explained by bacterial photosynthesis that involved Calvin cycle and acetyl CoA pathways, and sulfate reduction in a low‐sulfate environment. Considering the depositional age reported in a previous study of < 1.84 Ga, the Embury Lake Formation was likely emplaced in a lacustrine setting during the Trans‐Hudson Orogeny.  相似文献   

14.
Revealing of the sources and distributions of sedimentary organic matter in the East China Sea (ECS) is important for understanding its carbon cycle, which has significant temporal and spatial variability due to the influences of recent climate changes and anthropogenic activities. In this study, we report the contents of both terrestrial and marine biomarkers including ∑C27+C29+C31n-alkanes (38.6-580 ng/g), C37 alkenones (5.6-124.6 ng/g), brassicasterol (98-913 ng/g) and dinosterol (125-1521 ng/g) from the surface sediments in the Changjiang River Estuary (CRE) and shelf areas of the ECS. Several indices based on biomarker contents and ratios are calculated to assess the spatial distributions of both terrestrial and marine organic matter in the ECS surface sediments, and these results are compared with organic matter distribution patterns revealed by the δ13C (−20.1‰ to −22.7‰) and C/N ratio (5-7.5) of total organic matter. The contents of terrestrial biomarkers in the ECS surface sediments decrease seaward, controlled mostly by Changjiang River (CR) inputs and surface currents; while higher contents of the two marine biomarkers (brassicasterol and dinosterol) occur in upwelling areas outside the CRE and in the Zhejiang-Fujian coastal zone, controlled mostly by marine productivity. Four proxies, fTerr(δ13C) (the fraction of terrestrial organic matter in TOC estimated by TOC δ13C), odd-alkanes (∑C27+C29+C31n-alkanes), 1/Pmar-aq ((C23+C25+C29+C31)/(C23+C25) n-alkanes) and TMBR (terrestrial and marine biomarker ratio) (C27+C29+C31n-alkanes)/((C27+C29+C31) n-alkanes+(brassicasterol+dinosterol+alkenones)), reveal a consistent pattern showing the relative contribution of terrestrial organic matter (TOM) is higher in the CRE and along the Zhejiang-Fujian coastline, controlled mostly by CR inputs and currents, but the TOM contribution decreases seaward, as the influences of the CR discharge decrease.  相似文献   

15.
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16.
Iron isotope fractionation during planetary differentiation   总被引：4，自引：0，他引：4  
The Fe isotope composition of samples from the Moon, Mars (SNC meteorites), HED parent body (eucrites), pallasites (metal and silicate) and the Earth's mantle were measured using high mass resolution MC-ICP-MS. These high precision measurements (δ56Fe ≈ ± 0.04‰, 2 S.D.) place tight constraints on Fe isotope fractionation during planetary differentiation.Fractionation during planetary core formation is confined to < 0.1‰ for δ56Fe by the indistinguishable Fe isotope composition of pallasite bulk metal (including sulfides and phosphides) and olivine separates. However, large isotopic variations (≈ 0.5‰) were observed among pallasite metal separates, varying systematically with the amounts of troilite, schreibersite, kamacite and taenite. Troilite generally has the lightest (δ56Fe ≈ − 0.25‰) and schreibersite the heaviest (δ56Fe ≈ + 0.2‰) Fe isotope composition. Taenite is heavier then kamacite. Therefore, these variations probably reflect Fe isotope fractionation during the late stage evolution and differentiation of the S- and P-rich metal melts, and during low-temperature kamacite exsolution, rather than fractionation during silicate-metal separation.Differentiation of the silicate portion of planets also seems to fractionate Fe isotopes. Notably, magmatic rocks (partial melts) are systematically isotopically heavier than their mantle protoliths. This is indicated by the mean of 11 terrestrial peridotite samples from different tectonic settings (δ56Fe = + 0.015 ± 0.018‰), which is significantly lighter than the mean of terrestrial basalts (δ56Fe = + 0.076 ± 0.029‰). We consider the peridotite mean to be the best estimate for the Fe isotope composition of the bulk silicate Earth, and probably also of bulk Earth. The terrestrial basaltic mean is in good agreement with the mean of the lunar samples (δ56Fe = + 0.073 ± 0.019‰), excluding the high-Ti basalts. The high-Ti basalts display the heaviest Fe isotope composition of all rocks measured here (δ56Fe ≈ + 0.2‰). This is interpreted as a fingerprint of the lunar magma ocean, which produced a very heterogeneous mantle, including the ilmenite-rich source regions of these basalts.Within uncertainties, samples from Mars (SNC meteorites), HED (eucrites) and the pallasites (average olivine + metal) have the same Fe isotope compositions as the Earth's mantle. This indicates that the solar system is very homogeneous in Fe isotopes. Its average δ56Fe is very close to that of the IRMM-014 standard.  相似文献   

17.
Adsorption studies with sediment from the Bay of Quinte (Lake Ontario) indicate that the adsorption isotherm for sulfate concentration greater than 20 mg/l can be described by a Langmuir-type equation. The accompanying changes in δ34S of the solutions range from +0.9 to +6.0‰ indicating enrichment of32S in the sediments. The corresponding isotopic fractionation between the adsorbed sulfate and the sulfate remaining in solution ranges from ?10 to ?24‰, the larger fractionations being associated with low sulfate concentrations. The adsorbed sulfate is shown to significantly afftect the isotopic composition of the sediment in contact with the sulfate-bearing solution. The desorption of adsorbed sulfate is considered a possible mechanism for enriching natural freshwaters with32S.  相似文献   

18.
The Milankovi theory stresses that the summer insolation in the high northern latitudes that is dominated by the precession cycle controls the glacial/interglacial cycles in global climate change.If the climate system responds linearly to the external insolation forcing,the precession cycle of 23 or 19 ka should dominate the variations in the climatic proxy records.I performed spectral and evolutive cross spectral analyses on the high resolution benthic 18O and 13C records from the South China Sea and the North Atlantic,the proxies of global ice volume and ocean carbon reservoir respectively.I found that the obliquity instead of the eccentricity or the precession is the most marked cycle in the global ice volume and ocean carbon reservoir variations over the past 5 Ma.The analysis further reveals that only at the obliquity band instead of the eccentricity or the precession band does the global ice volume and ocean carbon reservoir display consistently high coherency and stable phase relationship over the past 5 Ma.The consistently positive or near-zero phases of the benthic 18O relative to the benthic13C at the obliquity band suggest that the global carbon cycle is involved in the polar ice sheet growth as an important internal feedback,not a determinative driving factor.The obliquity instead of the precession or the eccentricity takes the dominant role of driving the global climate change during the Pliocene and Pleistocene.  相似文献   

19.
In the Jungwon area, South Korea, two contrasting types of deep thermal groundwater (around 20–33 °C) occur together in granite. Compared to shallow groundwater and surface water, thermal groundwaters have significantly lower δ18O and δD values (> 1‰ lower in δ18O) and negligible tritium content (mostly < 2 TU), suggesting a relatively high age of these waters (at least pre-thermonuclear period) and relatively long subsurface circulation. However, the hydrochemical evolution yielded two distinct water types. CO2-rich water (PCO2 = 0.1 to 2 atm) is characterized by lower pH (5.7–6.4) and higher TDS content (up to 3300 mg/L), whereas alkaline water (PCO2 = 10− 4.1–10− 4.6 atm) has higher pH (9.1–9.5) and lower TDS (< 254 mg/L). Carbon isotope data indicate that the CO2-rich water is influenced by a local supply of deep CO2 (potentially, magmatic), which enhanced dissolution of silicate minerals in surrounding rocks and resulted in elevated concentrations of Ca2+, Na+, Mg2+, K+, HCO3− and silica under lower pH conditions. In contrast, the evolution of the alkaline water was characterized by a lesser degree of water–rock (granite) interaction under the negligible inflow of CO2. The application of chemical thermometers indicates that the alkaline water represents partially equilibrated waters coming from a geothermal reservoir with a temperature of about 40 °C, while the immature characteristics of the CO2-rich water resulted from the input of CO2 in Na–HCO3 waters and subsequent rock leaching.  相似文献   

20.
A large sediment deposit known as the Meiji Drift, located in the northwestern Pacific Ocean, is thought to have formed from deep water exiting the Bering Sea, although no notable deep water forms there presently. We determine the terrigenous sources since 140 ka to the drift using bulk sediment 40Ar–39Ar and Nd isotopic analyses on the silt-sized (20–63 μm) terrigenous fraction from Ocean Drilling Program (ODP) Site 884 to reconstruct paleo-circulation patterns. There are large changes in both isotopic tracers, varying on glacial–interglacial cycles. During glacial intervals, bulk sediment 40Ar–39Ar ages range between 40 and 80 Ma, while Nd isotopic values range from εNd = ? 1 to + 2. During interglacial intervals, sediments become much younger and more radiogenic, with bulk sediment ages falling to 2–15 Ma and Nd isotopic values ranging between εNd = + 5 and + 9. These data and quantitative comparison to potential source rocks indicate that the young Kamchatkan and Aleutian Arcs, lying NW and NE of the Meiji Drift, contribute the majority of sediment during interglacials. Conversely, older source rocks, such as those drained by the Yukon River and northeast Russia are the dominant origin of sediments during glacials. Mixing model calculations suggest that as much as 35–45% of the sediment deposited in the Meiji Drift during glacials is from the Bering Sea. It remains unclear whether thermohaline-type circulation or focussing of Bering Sea flow lead to the glacial–interglacial sediment source changes observed here.  相似文献   

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