Grain-scale iron isotopic distribution of pyrite from Precambrian shallow marine carbonate revealed by a femtosecond laser ablation multicollector ICP-MS technique: Possible proxy for the redox state of ancient seawater     

Manabu Nishizawa  Hiroki Yamamoto  Subaru Tsuruoka  Yusuke Sawaki  Yoshiaki Kon  Tsuyoshi Komiya  Takafumi Hirata 《Geochimica et cosmochimica acta》2010,74(9):2760-8899

The redox state of Precambrian shallow seas has been linked with material cycle and evolution of the photosynthesis-based ecosystem. Iron is a redox-sensitive element and exists as a soluble Fe(II) species or insoluble Fe(III) species on Earth’s surface. Previous studies have shown that the iron isotopic ratio of marine sedimentary minerals is useful for understanding the ocean redox state, although the redox state of the Archean shallow sea is poorly known. This is partly because the conventional bulk isotope analytical technique has often been used, wherein the iron isotopic record may be dampened by the presence of isotopically different iron-bearing minerals within the same sample. Here we report a microscale iron isotopic ratio of individual pyrite grains in shallow marine stromatolitic carbonates over geological time using a newly developed, near-infrared femtosecond laser ablation multicollector ICP-MS technique (NIR-fs-LA-MC-ICP-MS).We have determined that the grain-scale iron isotopic distribution of pyrite from coeval samples shows a bimodal (2.7 and 2.3 Ga) or unimodal pattern (2.9, 2.6, and 0.7 Ga). In particular, pyrite from the 2.7 Ga Fortescue Group shows a unique bimodal distribution with highly positive (+1.0‰ defined as Type 1) and negative δ⁵⁶Fe values (−1.8‰ defined as Type 2). Type 1 and 2 pyrites occasionally occur within different siliceous layers in the same rock specimen. Layer-scale iron isotopic heterogeneity indicates that the iron isotopic ratios of the two types of pyrite are not homogenized by diagenesis after deposition. Some cubic pyrites have a core with a positive δ⁵⁶Fe value (1‰) and a rim with a crustal δ⁵⁶Fe value (0‰). The observed isotopic zoning suggests that the positive δ⁵⁶Fe value is a primary signature at the time of stromatolite formation, while secondary pyrite precipitated during diagenesis.The positive δ⁵⁶Fe value of Type 1 and the large iron isotopic difference between Type 1 and 2 (2.8‰.) suggest partial Fe(II) oxidation in the 2.7-Ga shallow sea, i.e., pyritization of ⁵⁶Fe-enriched ferric oxyhydroxide (Type 1) and ⁵⁶Fe depleted Fe²⁺aq in seawater (Type 2). Type 2 pyrite was probably not produced by microbial iron redox cycling during diagenesis because this scenario requires a higher abundance of pyrite with δ⁵⁶Fe of 0‰ than of −1.8‰. Consequently, the degree of Fe(II) oxidation in the 2.7-Ga shallow sea can be estimated by a Fe²⁺aq steady-state model. The model calculation shows that half the Fe²⁺aq influx was oxidized in the seawater. This implies that O₂ produced by photosynthesis would have been completely consumed by oxidation of the Fe²⁺aq influx. Grain-scale iron isotopic distribution of pyrite could be a useful index for reconstructing the redox state of the Archean shallow sea.  相似文献   

Variability in the uranium isotopic composition of the oceans over glacial-interglacial timescales     

Tezer M. Esat  Yusuke Yokoyama 《Geochimica et cosmochimica acta》2006,70(16):4140-4150

Uranium-series mass spectrometric analyses of corals from the uplifted last glacial terraces at Huon Peninsula, Papua New Guinea, that grew from 50,000 years ago to 30,000 years ago show systematically low values of ²³⁴U/²³⁸U, at the time of coral growth, compared with modern corals. When combined with coral data from other studies a systematic trend emerges indicating shifts in the ²³⁴U/²³⁸U ratio at times of major glacial-interglacial transitions that involve large variations in sea-levels. From last glacial to Holocene, the rate of change in δ²³⁴U is approximately 1‰ per thousand years. The variations in the U budget of the oceans appear to be due to accumulation of excess ²³⁴U in near shore areas in anoxic and suboxic sediments, in salt marshes and mangroves, in estuaries, and in continental margins during periods of warm climate and high sea-levels. These near-shore areas are exposed during periods of low sea level resulting in rapid oxidation of U into highly soluble phases. The subsequent release of ²³⁴U-enriched uranium into the oceans occurs over a sustained period, in step with rising sea-levels.  相似文献   

The distribution of chiral asymmetry in meteorites: An investigation using asymmetric autocatalytic chiral sensors     

Tsuneomi Kawasaki  Yusuke Fujii  Kenso Soai  Sandra Pizzarello 《Geochimica et cosmochimica acta》2006,70(21):5395-5402

We separated and analyzed several organic and inorganic phases of the carbonaceous chondrite matrix to determine whether they contained any inherent asymmetry. Our intent was to determine any possible foci of asymmetry besides the one determined for meteoritic amino acids. As a probe, we employed a very sensitive asymmetric autocatalytic reaction. We were able to determine that asymmetry still resides in powders after extraction with water and solvents as well as in the insoluble organic material (IOM) obtained after demineralization. Asymmetry is not found any longer in the IOM after hydrothermal treatment and in meteorite powders from which all organics had been removed by O₂ plasma at low temperature. The data are interpreted to indicate a diverse molecular asymmetry residing in yet unknown meteorite organics; these organics might have had an inductive effect on organic molecular evolution upon exogenous delivery to the early Earth.  相似文献   

Cumulative Benioff strain-release, modified Omori's law and transient behaviour of rocks   总被引：1，自引：1，他引：1  

Yusuke Kawada  Hiroyuki Nagahama 《Tectonophysics》2006,424(3-4):157

Irreversible thermodynamic theories with internal state variables can be used to derive a general constitutive law for both transient and steady-state behaviours of rocks. This constitutive law can represent the concepts of damage and damage evolution in either the fibre-bundle model or continuum damage mechanics. We have previously proposed an empirically based constitutive law for both the transient and steady-state behaviours of rocks ultimately derived from laboratory experimental data. We show here that this law is concordant with the general constitutive law derived from irreversible thermodynamic theories, and that the relaxation modulus has a temporal power–law that depends on a structural fractal property of rocks. Our constitutive law predicts forms for the cumulative Benioff strain-release for precursory seismic activations and the modified Omori's laws of aftershocks, both aspects of the temporal fractal properties of seismicity. These seismic properties can also be derived by the fibre-bundle model or continuum damage mechanics. Our model suggests that these time-scale invariant processes of seismicity may be regulated by the fractal structures of crustal rocks.  相似文献   

Short-period noncontact binaries     

Atsuma Yamasaki  Akira Okazaki  Masatoshi Kitamura 《Astrophysics and Space Science》1986,118(1-2):279-282

Five short-period noncontact binaries (GR Tau, UU Lyn, BV Eri, RU Eri, and BW Eri) are studied. It is found from photometric and spectroscopic observations that (1) both components are noncontact to each other but are very close to the respective Roche lobes; (2) period change does not appear or it is very small; and (3) component stars of UU Lyn and RU Eri are normal main sequence stars while those of GR Tau and BV Eri are smaller and less massive components indicating in an advanced evolutionary state having experienced mass reversal.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.  相似文献   

A new type of primitive baleen whale (Cetacea; Mysticeti) from Kyushu, Japan     

Yoshihko  Okazaki 《Island Arc》1994,3(4):432-435

Abstract An important and relatively complete fossil mysticete cetacean specimen was collected from the Oligocene age Ashiya Group in Kyushu, Japan. The specimen includes a complete skull, with the anterior part of the vertebral column, several ribs, and the left pectoral limb. It provides significant information about the early evolution of cetotheres and transitional steps from the Archaeoceti to Mysticeti. Several features of the specimen, such as the anterior position of the nasal opening and large mandibular foramen, are primitive, but others are derived, such as the wide maxilla, broad supraorbital process of the frontal, occipital shield overhanging the temporal fossae, and the flat articulation of the distal end of the humerus. This animal, therefore, has a mixture of primitive and derived characters. It represents a stage of evolution in mysticetes that has been previously unrecognized.  相似文献   

Chert–clastic sequence in the Cretaceous Shimanto Accretionary Complex on the central Kii Peninsula,SW Japan     

Yusuke Shimura  Makoto Takeuchi  Tetsuya Tokiwa 《Island Arc》2020,29(1)

Ocean plate stratigraphy (OPS) within an ancient accretionary complex provides important information for understanding the history of an oceanic plate from its origin at a mid‐ocean ridge to its subduction at a trench. Here, we report a recently discovered chert–clastic sequence (CCS) that comprises a continuous succession from pelagic sediments to terrigenous clastics and which constitutes part of the OPS in the Akataki Complex within the Cretaceous Shimanto Accretionary Complex on the central Kii Peninsula, SW Japan. As well as describing this sequence, we present U–Pb ages of detrital zircons from terrigenous clastic rocks in the CCS, results for which show that the youngest single grain and youngest cluster ages belong to the Santonian–Campanian and are younger than the radiolarian age from the underlying pelagic sedimentary rock (late Albian–Cenomanian). Thus, the CCS records the movement history of the oceanic plate from pelagic sedimentation (until the late Albian–Cenomanian) to a terrigenous sediment supply (Santonian–Campanian).  相似文献   

Fate of carbonates within oceanic plates subducted to the lower mantle, and a possible mechanism of diamond formation     

Yusuke Seto  Daisuke Hamane  Takaya Nagai  Kiyoshi Fujino 《Physics and Chemistry of Minerals》2008,35(4):223-229

We report on high-pressure and high-temperature experiments involving carbonates and silicates at 30–80 GPa and 1,600–3,200 K, corresponding to depths within the Earth of approximately 800–2,200 km. The experiments are intended to represent the decomposition process of carbonates contained within oceanic plates subducted into the lower mantle. In basaltic composition, CaCO₃ (calcite and aragonite), the major carbonate phase in marine sediments, is altered into MgCO₃ (magnesite) via reactions with Mg-bearing silicates under conditions that are 200–300°C colder than the mantle geotherm. With increasing temperature and pressure, the magnesite decomposes into an assemblage of CO₂ + perovskite via reactions with SiO₂. Magnesite is not the only host phase for subducted carbon—solid CO₂ also carries carbon in the lower mantle. Furthermore, CO₂ itself breaks down to diamond and oxygen under geotherm conditions over 70 GPa, which might imply a possible mechanism for diamond formation in the lower mantle.  相似文献   

Carbon isotope chemostratigraphy of a Precambrian/Cambrian boundary section in the Three Gorge area, South China: Prominent global-scale isotope excursions just before the Cambrian Explosion     

Tomoko Ishikawa  Yuichiro Ueno  Tsuyoshi Komiya  Yusuke Sawaki  Jian Han  Degan Shu  Yong Li  Shigenori Maruyama  Naohiro Yoshida   《Gondwana Research》2008,14(1-2):193

Carbon isotope chemostratigraphy has been used for worldwide correlation of Precambrian/Cambrian (Pc/C) boundary sections, and has elucidated significant change of the carbon cycle during the rapid diversification of skeletal metazoa (i.e. the Cambrian Explosion). Nevertheless, the standard δ¹³C curve of the Early Cambrian has been poorly established mainly due to the lack of a continuous stratigraphic record. Here we report high-resolution δ¹³C chemostratigraphy of a drill core sample across the Pc/C boundary in the Three Gorge area, South China. This section extends from an uppermost Ediacaran dolostone (Dengying Fm.), through a lowermost Early Cambrian muddy limestone (Yanjiahe Fm.) to a middle Early Cambrian calcareous black shale (Shuijingtuo Fm.). As a result, we have identified two positive and two negative isotope excursions within this interval. Near the Pc/C boundary, the δ¹³C_carb increases moderately from 0 to + 2‰ (positive excursion 1: P1), and then drops dramatically down to − 7‰ (negative excursion 1: N1). Subsequently, the δ¹³C_carb increases continuously up to about + 5‰ at the upper part of the Nemakit–Daldynian stage. After this positive excursion, δ¹³C_carb sharply decreases down to about − 9‰ (N2) just below the basal Tommotian unconformity. These continuous patterns of the δ¹³C shift are irrespective of lithotype, suggesting a primary origin of the record. Moreover, the obtained δ¹³C profile, except for the sharp excursion N2, is comparable to records of other sections within and outside of the Yangtze Platform. Hence, we conclude that the general feature of our δ¹³C profile best represents the global change in seawater chemistry. The minimum δ¹³C of the N1 (− 7‰) is slightly lower than carbon input from the mantle, thus implying an enhanced flux of ¹³C-depleted carbon just across the Pc/C boundary. Hence, the ocean at that time probably became anoxic, which may have affected the survival of sessile or benthic Ediacaran biota. The subsequent δ¹³C rise up to + 5‰ (P2) indicates an increase of primary productivity or an enhanced rate of organic carbon burial, which should have resulted in lowering pCO₂ and following global cooling. This scenario accounts for the cause of the global-scale sea-level fall at the base of the Tommotian stage. The subsequent, very short-term, and exceptionally low δ¹³C (− 9‰) in N2 could have been associated with the release of methane from gas hydrates due to the sea-level fall. The inferred dramatic environmental changes (i.e., ocean anoxia, increasing productivity, global cooling and subsequent sea-level fall with methane release) appear to coincide with or occur just before the Cambrian Explosion. This may indicate synchronism between the environmental changes and rapid diversification of skeletal metazoa.  相似文献   

Internal structures and U–Pb ages of zircons from a tuff layer in the Meishucunian formation, Yunnan Province, South China     

Yusuke Sawaki  Manabu Nishizawa  Takeshi Suo  Tsuyoshi Komiya  Takafumi Hirata  Naoto Takahata  Yuji Sano  Jian Han  Yoshiaki Kon  Shigenori Maruyama 《Gondwana Research》2008,14(1-2):148

The Precambrian/Cambrian (PC/C) boundary is one of the most important intervals for the evolution of life, represented by prominent biological evolution from the first appearance of soft-bodied animals from the late Neoproterozoic to the sudden diversification of animals with mineralized skeletons in the Cambrian. In South China several areas contain many fossils and are well exposed, suitable for the investigation of PC/C boundary. However, geochronological relationships are still poorly known because of lack of combined detailed investigations of internal structures of zircons and in-situ U–Pb dating.We focus on the internal structure of zircons from a tuff layer within Bed 5 in the Meishucun section on which we undertook cathodoluminescence (CL) imaging and in-situ U–Pb dating with LA-ICP-MS and nano-SIMS. Over 600 zircons from the tuff layer were classified into three types based on their CL images: oscillatory rims, inherited cores and dull structures. U–Pb dating of the internal structure of the zircons by LA-ICP-MS clearly shows a distinct unimodal age population dependent on the structure: 531 ± 17 Ma for the oscillatory rims and 515 Ma for the dull structures. The clear oscillatory zonation, the prismatic morphology, and their occurrence indicate that the oscillatory rims were formed from felsic magmatism, and that the U–Pb nano-SIMS age of 536.5 ± 2.5 Ma records the depositional age of the tuff. Our results indicate that the PC/C boundary is situated below Bed 5, and therefore the bottom of Zone 1 (Marker A) is more appropriate for the PC/C boundary than is the top of Zone 1 (Marker B). The age of a positive anomaly (P2) in the early Cambrian is estimated to be ca. 536 Ma.  相似文献