Submarine-hydrothermal exhalative ore layers in black shales from South China and associated fossils — insights into a Lower Cambrian facies and bio-evolution

Marine redox variations and nitrogen cycle of the early Cambrian southern margin of the Yangtze Platform, South China: Evidence from nitrogen and organic carbon isotopes

“…At the Yuanjia section, the analyzed data exhibit a similar pattern (␦ 15 N bulk values near zero with low ␦ 13 C org values around −32.5‰) (Fig. 2b), but these data may have not fully recorded the biogeochemical signature of the deep water realm because of the extremely condensed nature of sedimentary deposition due to a rapid rise in sea level (Goldberg et al, 2007;Schütter, 1998;Steiner et al, 2001Steiner et al, , 2007. The ␦ 15 N bulk signature which is close to the ␦ 15 N value of air (0‰) indicates that biologically available nitrogen mainly came from N 2 -fixation by diazotrophic cyanobacteria accompanied by little or no isotopic fractionation (Sigman et al, 2009).…”

“…6b). A Ni-Mo enrichment layer along the transitional zone between the platform and basin was deposited under these euxinic conditions (Lehmann et al, 2007;Steiner et al, 2001). Previous studies argued that the Ni-Mo enrichments in the sediments could originate from hydrothermal input (Lott et al, 1999;Steiner et al, 2001), enrichment from seawater (Lehmann et al, 2007;Mao et al, 2002), or more likely the combination of both processes (Pašava et al, 2008).…”

“…These 'black shales' are suggested to have been deposited under widespread anoxic, or even euxinic conditions based on numerous lines of geochemical evidence (Canfield et al, 2008;Goldberg et al, 2005Goldberg et al, , 2007Guo et al, 2007a;Kimura and Watanabe, 2001;Lehmann et al, 2007;Och et al, 2013;Wang et al, 2012a;Wille et al, 2008;Xu et al, 2012). Presumably, because of the rapidity of this sea level rise, lower Cambrian units across much of South China are condensed with only a few centimeters thickness in the deeper water realms (Goldberg et al, 2007;Schütter, 1998;Steiner et al, 2001Steiner et al, , 2007. Related to this condensation, a polymetallic enrichment layer occurs sporadically in the lower portion of this transgressive black shale along a NE-striking 1600 km belt, which can be used as a correlation marker Och et al, 2013;Zhu et al, 2003), but mineable ore layer is usually less than half meters in thickness and exposed only within Guizhou and Hunan provinces (Jiang et al, 2006(Jiang et al, , 2007.…”

“…A Ni-Mo enrichment layer along the transitional zone between the platform and basin was deposited under these euxinic conditions (Lehmann et al, 2007;Steiner et al, 2001). Previous studies argued that the Ni-Mo enrichments in the sediments could originate from hydrothermal input (Lott et al, 1999;Steiner et al, 2001), enrichment from seawater (Lehmann et al, 2007;Mao et al, 2002), or more likely the combination of both processes (Pašava et al, 2008). Recent studies demonstrated that the Ni-Mo sulfide ore layer, which is usually less than half meters in thickness (Jiang et al, 2006), has relatively lower ␦ 98 Mo values (<1.6‰) (Lehmann et al, 2007;Xu et al, 2012) (low-temperature hydrothermal fluids ␦ 98 Mo = +0.8‰, McManus et al, 2002), while black shales above and below this Ni-Mo ore layer have high ␦ 98 Mo values (∼1.9-2.4‰) (Chen et al, 2015) (modern seawater ␦ 98 Mo = +2.34‰, Goldberg et al, 2013), suggesting that hydrothermal input of Mo may have contributed only to the Ni-Mo ore layer, but not to the adjacent black shales (Chen et al, 2015).…”

Microscopic characterization and formation mechanisms of deep-water sandy-debris-flow and turbidity-current sandstones in a lacustrine basin: a case study in the Yanchang Formation of the Ordos Basin, China

“…Previous studies established the parameter dU = 2U total /(U total ? Th/3) using U -Th/3 and total uranium: dU [ 1 implies an anoxic environment and dU \ 1 suggests oxic water (Wignall 1994;Steiner et al 2001;Wignall and Twitchett 1996;Jones and Manning 1994).…”