Physical and chemical depositional processes when volcanoes meet lacustrine environments: the Cretaceous Imjado Volcanics, Jeungdo, southwestern Korea

A Cretaceous volcano-sedimentary succession (Imjado Volcanics, Jeungdo, SW Korea) was analysed to understand volcanic influences on physical and chemical depositional processes of a shallow alkaline lake during and after explosive eruptions. The succession is composed of primary and resedimented volcaniclastic deposits interbedded with fine-grained sediments and a bedded chert. The primary volcaniclastic deposits are characterized by two end-members: thick (20 m) welded lapilli tuff and thin (0.9 m) planar stratified tuff deposits. The first member deposits were accumulated by steady pyroclastic density currents (PDCs) that displaced the lake water from the shoreline. The second end-member deposits were accumulated by unsteady PDCs rapidly disintegrated at the shoreline and transformed into turbidity currents. Reworked volcaniclastic deposits are constituted of reverse to normally graded volcaniclastic sandstone, accumulated by hyperpycnal flows originating from subaerial discharge. On top of this deposit, a bedded chert is exposed and composed of microcrystalline texture without biogenic remains. The microscopic analysis and stratigraphic relationship suggest that the chert bed is formed by chemical precipitation as a result of changes in hydrochemistry of lake water by inflowing of fresh water (hyperpycnal flows) in the alkaline bottom water of the lake.

Development of Deep-Sea Anoxia in Panthalassa During the Lopingian (Late Permian): Insights From Redox-Sensitive Elements and Multivariate Analysis

The end-Permian mass extinction (EPME) was the most severe mass extinction event of the Phanerozoic, and was associated with the development of global oceanic anoxia. The intensification of ocean anoxia preceded the EPME, but the degree of intensity and timing of oceanic redox changes in the mid-Panthalassa Ocean remain debated. Here we present the results of geochemical and multivariate statistical analyses of a late Guadalupian to Lopingian (middle–late Permian) bedded chert succession from the Iwaidani section, Japan, which preserves pelagic deep-sea facies from the ocean floor to the lower flank of a mid-Panthalassan seamount. The entire section yields a low manganese-enrichment factor (MnEF <1), suggesting that suboxic conditions has appeared in the depositional environment already in the late Guadalupian. Enrichment factors of other redox-sensitive trace-elements (e.g., vanadium and uranium) and principle component analysis (PCA) of major element data show the development of suboxic to weakly anoxic conditions across the Guadalupian/Lopingian boundary. Subsequently, anoxic conditions, as inferred from enrichments in U, Mo, Ni, Cu, Zn, and Tl, were developed during the middle Lopingian. Extremely high concentrations of U and Mo (enrichment factors of ∼6 and ∼5,500, respectively) indicate that H2S-rich euxinic conditions developed during the latest Lopingian and around the time of the EPME. The cause of the shift toward more reducing conditions in the early–middle Lopingian is unknown, but PCA results suggest that the euxinic conditions occurred in association with intensified continental weathering in response to a temperature rise during the ca. 200 kyr before the EPME.

Biotic and environmental changes in the Panthalassa Ocean across the Norian (Late Triassic) impact event

Stratigraphic records of impact ejecta preserved in a pelagic deep-sea setting occur within Upper Triassic successions of the subduction-generated accretionary complexes of central Japan. A significant biotic turnover in radiolarians occurred during the ~ 300 kyr time interval after the impact event, which is characterized by a remarkable reduction in the burial flux of radiolarian silica. However, the nature of the environmental conditions at this time remains unclear. To investigate the environmental changes that triggered a decline in radiolarian burial flux after the impact event, geochemical proxies (major, trace, and rare earth elements) were applied to the middle–upper Norian (Upper Triassic) bedded chert succession of the Mino Belt, central Japan. A progressive environmental deterioration is evident from (1) a post-impact shutdown of burial flux of primary and silica- and apatite-secreting organisms; and (2) a subsequent abrupt increase in chemical weathering associated with a sustained reduction in the burial flux of radiolarian silica. No significant redox changes were observed across the impact event. The continental weathering proxies suggest a transient increase in weathering intensity occurred during the decline of radiolarian burial flux, likely in response to a short-term warm and humid period. Our results delineate a remarkable record of progressive environmental changes in the Panthalassa Ocean after this large impact event.

Biogenically induced bedded chert formation in the alkaline palaeo-lake of the Green River Formation

Rhythmically bedded cherts are observed in both pelagic marine and lacustrine deposits, but the formation mechanism in the latter remains highly uncertain. Our study of alternating chert–dolomite beds in the Eocene Green River Formation, Utah, USA reveals dense accumulations of organic-matter spheres (30–50 μm diameter) of probable algal cyst origin in the chert layers, and centennial- to millennial-scale periodicities in chert layer deposition. A positive correlation between the degree of degradation of the organic spheres and Si distribution implies decomposition of algal organic matter lead to precipitation of lacustrine chert. As both alkalinity and dissolved silica were likely high in the palaeo-lake waters of the Green River Formation, we hypothesize that decomposition of algal organic matter lowered the pH of sediment pore waters and caused silica precipitation. We propose a formation model in which the initial abundance of algal organic matter in sediment varies with productivity at the lake surface, and the decomposition of this algal matter controls the extent of silica precipitation in sediment. The formation of rhythmically bedded chert–dolomite may be linked to centennial- to millennial-scale climatic/environmental factors that modulate algal productivity, which are possibly tied to solar activity cycles known to have similar periodicities.

Geochemical characteristics and geological significance of the bedded chert during the Ordovician and Silurian transition in the Shizhu area, Chongqing, South China

A large amount of bedded chert widely deposited during the Ordovician and Silurian transition in South China. In this study, analyses of the petrographic characteristics, the major elements and rare earth elements (REEs), were conducted on 31 bedded chert samples obtained from the Wufeng and Longmaxi Formations at the Qiliao section in the Shizhu area of the Sichuan Basin to determine the sedimentary environment and the origin of the bedded chert during the Ordovician and Silurian transition. The following conclusions were obtained: (i) the bedded chert in the Wufeng and Longmaxi Formations at the Qiliao Section originated mainly from terrigenous input and siliceous organisms and was slightly influenced by hydrothermal fluid, (ii) siliceous organisms were a key factor controlling the differences in the SiO2 content of the bedded chert in the Wufeng and Longmaxi Formations at the Qiliao Section, and (iii) the bedded chert in the Wufeng and Longmaxi Formations deposited in a continental margin environment during the Ordovician and Silurian transition.