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Abstract The Sambosan accretionary complex of southwest Japan was formed during the uppermost Jurassic to lowermost Cretaceous and consists of basaltic rocks, carbonates and siliceous rocks. The Sambosan oceanic rocks were grouped into four stratigraphic successions: (i) Middle Upper Triassic basaltic rock; (ii) Upper Triassic shallow-water limestone; (iii) limestone breccia; and (iv) Middle Middle Triassic to lower Upper Jurassic siliceous rock successions. The basaltic rocks have a geochemical affinity with oceanic island basalt of a normal hotspot origin. The shallow-water limestone, limestone breccia, and siliceous rock successions are interpreted to be sediments on the seamount-top, upper seamount-flank and surrounding ocean floor, respectively. Deposition of the radiolarian chert of the siliceous rock succession took place on the ocean floor in Late Anisian and continued until Middle Jurassic. Oceanic island basalt was erupted to form a seamount by an intraplate volcanism in Late Carnian. Late Triassic shallow-water carbonate sedimentation occurred at the top of this seamount. Accumulation of the radiolarian chert was temporally replaced by Late Carnian to Early Norian deep-water pelagic carbonate sedimentation. Biotic association and lithologic properties of the pelagic carbonates suggest that an enormous production and accumulation of calcareous planktonic biotas occurred in an open-ocean realm of the Panthalassa Ocean in Late Carnian through Early Norian. Upper Norian ribbon chert of the siliceous rock succession contains thin beds of limestone breccia displaced from the shallow-water buildup resting upon the seamount. The shallow-water limestone and siliceous rock successions are nearly coeval with one another and are laterally linked by displaced carbonates in the siliceous rock succession. 相似文献
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Immediately before the extinction of the end‐Guadalupian (Middle Permian; ca 260 Ma), a significant change to the global carbon cycle occurred in the superocean Panthalassa, as indicated by a prominent positive δ13C excursion called the Kamura event. However, the causes of this event and its connection to the major extinction of marine invertebrates remain unclear. To understand the mutual relationships between these changes, we analyzed the sulfur isotope ratio of the carbonate‐associated sulfate (CAS) and HCl‐insoluble residue, as well as the carbon isotope ratio of bulk organic matter, for the Middle‐Upper Permian carbonates of an accreted mid‐oceanic paleo‐atoll complex from Japan, where the Kamura event was first documented. We detected the following unique aspects of the stable carbon and sulfur isotope records. First, the extremely high δ13C values of carbonate (δ13Ccarb) over +5 ‰ during the Capitanian (late Guadalupian) were associated with large isotopic differences between carbonate and organic matter (Δ13C = δ13Ccarb ? δ13Corg). We infer that the Capitanian Kamura event reflected an unusually large amount of dissolved organic matter in the expanded oxygen minimum zone at mid‐depth. Second, the δ34S values of CAS (δ34SCAS) were inversely correlated with the δ13Ccarb values during the Capitanian to early Wuchiapingian (early Late Permian) interval. The Capitanian trend may have appeared under increased oceanic sulfate conditions, which were accelerated by intense volcanic outgassing. Bacterial sulfate reduction with increased sulfate concentrations in seawater may have stimulated the production of pyrite that may have incorporated iron in pre‐existing iron hydroxide/oxide. This stimulated phosphorus release, which enhanced organic matter production and resulted in high δ13Ccarb. Low δ34SCAS values under high sulfate concentrations were maintained and the continuous supply of sulfate cannot by explained only by the volcanic eruption of the Emeishan Trap, which has been proposed as a cause of the extinction. The Wuchiapingian δ34SCAS–δ13Ccarb correlation, likely related to low sulfate concentration, may have been caused by the removal of oceanic sulfate through the massive evaporite deposition. 相似文献
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The Capitanian (Upper Guadalupian) to Wuchiapingian (Lower Lopingian) shallow-marine limestones at Akasaka and Ishiyama in central Japan record unique aspects of the extinction-related Guadalupian–Lopingian boundary (G-LB) interval. The ca. 140 m-thick Akasaka Limestone consists of the Capitanian black limestone (Unit B; 112 m) and the Wuchiapingian light gray dolomitic limestone (Unit W; 21 m), with a black/white striped limestone (Unit S; 9 m) between them. The G-LB horizon is assigned at the base of Unit W, on the basis of the first occurrence of the Wuchiapingian fusulines. The Capitanian Unit B and the Wuchiapingian Unit W were deposited mostly in the subtidal zone of a lagoon, whereas the intervened Unit S and the lowermost Unit W were in the intertidal zone. A hiatus with a remarkable erosional feature was newly identified at the top of Unit S. These records indicate that the sea-level has dropped significantly around the G-LB to have exposed the top of the atoll complex above the sea-level. The Ishiyama Limestone, located ca. 10 km to the north of the Akasaka limestone, retains almost the same depositional records. The extinction of large-tested fusuline (Yabeina) and large bivalves (Alatoconchidae) occurred in the upper part of Unit B, and the overlying 20 m-thick limestone (the uppermost Unit B and Unit S) below the hiatus represents a unique barren interval. The upper half of the barren interval is more depleted in fossils than the lower half, and this likely represents a duration of the severest environmental stress(es) for the shallow-marine protists/animals on the mid-oceanic paleo-atoll complex. Small-tested fusulines re-appeared at the base of Unit W above the hiatus. These facts prove that the elimination of shallow-marine biota occurred during the Capitanian shallowing of Akasaka paleo-atoll before the subaerial exposure/erosion across the G-LB. The overall shallowing and the development of such a clear hiatus at the top of a mid-oceanic seamount, in accordance with the contemporary sea-level curve based on continental shelf records, indicates that a remarkable sea-level drop has occurred globally during the latest Capitanian. This further suggests that a cool climate likely has appeared even in the low-latitude domains in Panthalassa to cause the decline of the Middle Permian shallow-water protists/animals that adapted to warmer seawater. The Wuchiapingian biota first appeared immediately after this erosional episode, i.e., during the onset of warming after the G-LB. 相似文献
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δ13C data from Tethyan sections provide evidence of profound changes in the carbon cycle during the Lower Triassic. Sections from the Panthalassa realm were investigated to establish whether these variations are also present there. In the Jurassic accretionary wedges in Japan, exotic blocks having a Panthalassan affinity, have been incorporated. The majority of the blocks are pelagic cherts but rare shallow-water carbonates are also present. We present a δ13C study on the Lower Triassic of a shallow-water carbonate succession deposited on a mid-oceanic seamount and accreted to the Chichibu Belt, Japan. Two sections have been measured at Kamura, central Kyushu Island. The carbon isotope curve shows depleted values across the Permian–Triassic boundary (PTB), subsequently followed by an increase to heavier values into the Dienerian, culminating in a maximum of almost +4‰ V-PDB, before a steep drop at a stratigraphic gap. Low values are recorded in the Smithian, but rise to enriched δ13C values > +3.5‰ near the Smithian–Spathian boundary. The observed trend of the stable carbon isotope curve from Japanese sediments mirrors the curves derived from sections in the Tethys (e.g. Italy, Iran, Turkey, Oman and the South China Nanpanjing Basin). Our results support the interpretation of this curve as representing a global trend across the PTB and in the Lower Triassic, although some distinct features are absent around the Dienerian/Smithian boundary. Profound variations of the carbon isotope curve in the Lower Triassic are presented for the first time from a marine section outside of the Tethys. They indicate severe, global changes in the Lower Triassic carbon cycle, and the causative processes must have significantly contributed to the delayed biotic recovery after the PTB. Large amounts of carbon were shifted between carbon reservoirs, most probably between shallow- and deep-ocean waters, and/or ocean and sediment. Anoxia followed by overturn of the ocean water masses may have been the mechanism which quickly altered ecological conditions in the ocean leading to variable availability of nutrients and oxygen, and changes in isotope composition of the available carbon in the surface waters that was incorporated in the precipitated carbonate. 相似文献
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The Permian planktonic distribution is not well known, in contrast to other fossil distributions of nekton and benthos. Radiolaria are representative microfossils of plankton in the Permian palaeocean. This study compares both correlated radiolarian occurrences (Pseudotormentus De Wever et Caridroit and Quadriremis Nazarov et Ormiston) in literature data from the Pacific Rim and quantitative data from the Liuhuang and Gujingling sections in South China. Pseudotormentus distributions are concentrated in the Panthalassa, whereas Quadriremis occurrences are distributed over both the Panthalassa and the Palaeotethys. The uneven distribution of Pseudotormentus seems to have been controlled by a difference in the oceanic basins, indicating the presence of faunal differences in Permian planktonic microorganisms between the Panthalassa and the Palaeotethys. In other words, this study infers the provincialism of Permian planktonic microorganisms. The uneven distribution is explainable by a hypothesis that Pseudotormentus was strongly affected by a Panthalassan equatorial current. 相似文献
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Palaeobiogeography of Early Jurassic Lithiotis-type bivalve buildups as recovery effect after Triassic/Jurassic mass extinction and
their connections with Asian palaeogeograph 
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下载免费PDF全文 The huge, up to 40-50 cm long bivalves, Lithiotis, Cochlearites and Lithioperna, which dominated within “Lithiotis” facies (sensu – Fraser et al., 2004 with lit-erature cited therein), are most significant representa-tives of buildup-maker of shallow marine/lagoonal bivalve mounds (reefs) in numerous places of Tethyan- Panthalassa margins during Pliensbachian-Early Toar-cian times. The distribution of Lithiotis-facies bivalves from Western (Spain, Italy) and Middle Europe (Slo-venia, Croatia, Albania) trough north Africa (Morocco) and Arabian Peninsula (Oman, Arabian Emirates) up to Timor Island, Himalaya Mts (Nepal, China) and west-ern margin of both Americas (USA, Peru) indicates world-wide, rapid expansion of such Lithiotis-type bivalves (Leinfelder et al., 2002; Fraser et al., 2004; Krobicki et al., 2008). The Early Jurassic migration routes were connected both with break-up of Pangea and oceanic circulation, which facilitated high speed of distribution of larva’s of such oyster-like bivalves. 相似文献
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