TY - JOUR A1 - Zhang, Liyu A1 - Chen, Daizhao A1 - Kuang, Guodun A1 - Guo, Zenghui A1 - Zhang, Gongjing A1 - Wang, Xia T1 - Persistent oxic deep ocean conditions and frequent volcanic activities during the Frasnian-Famennian transition recorded in South China JF - Global and planetary change N2 - The Frasnian-Famennian (F-F) transition of Late Devonian was a critical episode in geological history, recording a major mass extinction event. In this study, we focus on an F-F succession from a deep marine context in Bancheng, southern Guangxi, South China, to investigate coeval changes in pelagic environments of the Paleo-Tethys Ocean. The studied succession is exclusively composed of bedded cherts intercalated with multiple siliceous volcanic ash beds. A SIMS zircon U-Pb Concordia age of 367.8 +/- 2.5 Ma is reported for a tuffaceous layer slightly above the F-F boundary. Geochemical ratios of Al/(Al + Fe + Mn), Ce/Ce*, Y/Ho, and Al, Fe contents in bedded cherts indicate that they are of predominantly biogenic/chemical origin with some terrigenous inputs. Negligible enrichment of redox sensitive elements (Mo, U, V) and low V/Cr ratios (<2) suggest persistently oxic conditions existed in the deep pelagic basin at Bancheng, South China during the F-F transition. These findings call into question the widely held hypothesis that marine anoxia was the primary killing mechanism for the F-F crisis. In contrast, multiple tuffaceous layers throughout the F-F boundary succession indicate frequent volcanic activity, which could have released massive amounts of greenhouse gases into the atmosphere, inducing climate warming. This scenario may have increased continental weathering and riverine fluxes into the ocean, reconciling the increases in Al2O3 content and Al/(Al + Fe + Mn) ratio across the F-F boundary. Documentation of persistently oxic conditions and frequent volcanic activitiy provides new perspectives on the inter-relationship between volcanism, climate, and oceanic redox fluctuation during the F-F biotic crisis. KW - Late Devonian KW - Bedded chert KW - Major and trace elements KW - Deep ocean redox condition KW - Volcanic activity KW - Zircon U-Pb dating Y1 - 2020 U6 - https://doi.org/10.1016/j.gloplacha.2020.103350 SN - 0921-8181 VL - 195 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Zhou, Xiqiang A1 - Chen, Daizhao A1 - Dong, Shaofeng A1 - Zhang, Yanqiu A1 - Guo, Zenghui A1 - Wei, Hengye A1 - Yu, Hao T1 - Diagenetic barite deposits in the Yurtus Formation in Tarim Basin, NW China: Implications for barium and sulfur cycling in the earliest Cambrian JF - Precambrian research N2 - Barite concretions and bands are widely distributed in black shale-chert horizons in the Yurtus Formation of Lower Cambrian in Aksu area, northwestern Tarim Basin, NW China. They mainly consist of coarse-grained anhedral to euhedral barite crystals with minor dolomites and pyrites. Petrological features indicate these concretions grew from the porewater in unconsolidated sediments at shallow burial below sediment-water interface. The slight deviation of Sr-87/Sr-86 ratios (0.7083 to 0.7090) and significant elevated delta S-34 values (56.8-76.4 parts per thousand CDT) of barite samples with respect to those of the Early Cambrian seawater further support that barite deposits precipitated from the enclosed porewater in sediment column, which evolved from the penecontemporaneous seawater with weak interaction with the host fine-grained siliciclastic sediments and highly-depleted sulfate in response to prolonged strong bacterial sulfate reduction without necessary renewal. The abundant organic matters in the basal Yurtus Formation should have facilitated developing sulfate-depleted methanogenesis zone and sulfate-methane transition zone (SMTZ) slightly after deposition. Therefore, barite deposits in the Yurtus Formation most likely resulted from diagenetic barium cycling and persistently grew from the porewater in the static SMTZ with a low sedimentation rate in the Early Cambrian. In comparison with the distribution of sedimentary barites in geological records, we tentatively proposed that a transition in diagenetic barium cycling and associated mineralization may have occurred from the Precambrian to Cambrian periods; this scenario may be causally linked to the changes in marine ecology (the advent of mesozooplankton and associated faecal pellet) and geochemistry (the increase of seawater sulfate concentration). Thus, the occurrence of diagenetic barite deposits in the Yurtus Formation implies that diagenetic barium cycling and more effective scavenging of barium from CH4- and Ba-rich porewaters within sediments might have become an nonnegligible process in continental margin areas, at least, since the earliest Cambrian, which could have significantly impacted the marine barium cycling. (C) 2015 Elsevier B.V. All rights reserved. KW - Barite concretion KW - Diagenetic barium cycling KW - Earliest Cambrian KW - Yurtus Formation KW - Tarim Basin, NW China Y1 - 2015 U6 - https://doi.org/10.1016/j.precamres.2015.03.006 SN - 0301-9268 SN - 1872-7433 VL - 263 SP - 79 EP - 87 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Zhang, Yanqiu A1 - Chen, Daizhao A1 - Zhou, Xiqiang A1 - Guo, Zenghui A1 - Wei, Wenwen A1 - Mutti, Maria T1 - Depositional facies and stratal cyclicity of dolomites in the Lower Qiulitag Group (Upper Cambrian) in northwestern Tarim Basin, NW China JF - Facies : an international journal of palaeontology, sedimentology, geology N2 - The Upper Cambrian Lower Qiulitag Group in the Tarim Basin, NW China, is overwhelmingly composed of cyclic dolomites. Based on extensive field investigations and facies analysis from four outcrop sections in the Bachu-Keping area, northwestern Tarim Basin, four main types of facies are recognized: open-marine subtidal, restricted shallow subtidal, intertidal, and supratidal facies, and these are further subdivided into ten lithofacies. In general, these facies are vertically arranged into shallowing-upward, metre-scale cycles. These cycles are commonly composed of a thin basal horizon reflecting abrupt deepening, and a thicker upper succession showing gradual shallowing upwards. Based on the vertical facies arrangements and changes across boundary surfaces, two types of cycle: peritidal and shallow subtidal cycle, are further identified. The peritidal cycles, predominating over the lower-middle Lower Qiulitag Group, commence with shallow subtidal to lower intertidal facies and are capped by inter-supratidal facies. In contrast, the shallow subtidal cycles, dominating the upper Lower Qiulitag Group, are capped by shallow-subtidal facies. Based on vertical lithofacies variations, cycle stacking patterns, and accommodation variations revealed by Fischer plots, six larger-scale third-order depositional sequences (Sq1-Sq6) are recognized. These sequences generally consist of a lower transgressive and an upper regressive systems tract. The transgressive tracts are dominated by thicker-than-average cycles, indicating an overall accommodation increase, whereas the regressive tracts are characterized by thinner-than-average peritidal cycles, indicating an overall accommodation decrease. The sequence boundaries are characterized by transitional zones of stacked thinner-than-average cycles, rather than by a single surface. These sequences can further be grouped into lower-order sequence sets: the lower and upper sequence sets. The lower sequence set, including Sq1-Sq3, is characterized by peritidal facies-dominated sequences and a progressive decrease in accommodation space, indicating a longer-term fall in sea level. In contrast, the upper sequence set (Sq4-Sq6) is characterized by subtidal facies-dominated sequences and a progressive increase in accommodation space, indicating a longer-term rise in sea level. KW - Dolomites KW - Facies KW - Stratal cyclicity KW - Sequences KW - Upper Cambrian KW - Tarim Basin KW - China Y1 - 2015 U6 - https://doi.org/10.1007/s10347-014-0417-1 SN - 0172-9179 SN - 1612-4820 VL - 61 IS - 1 PB - Springer CY - New York ER -