Wang W, Guan C, Zhou C, Peng Y, Pratt L M, Chen X, Chen L, Chen Z, Yuan X, Xiao S
Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China.
Department of Geosciences, Virginia Tech, Blacksburg, VA, USA.
Geobiology. 2017 Jul;15(4):552-571. doi: 10.1111/gbi.12226. Epub 2017 Jan 6.
The Ediacaran Doushantuo Formation in South China is a prime target for geobiological investigation because it offers opportunities to integrate chemostratigraphic and paleobiological data. Previous studies were mostly focused on successions in shallow-water shelf facies, but data from deep-water successions are needed to fully understand basinal redox structures. Here, we report δ C , δ C , δ S , δ S , and δ N data from a drill core of the fossiliferous Lantian Formation, which is a deep-water equivalent of the Doushantuo Formation. Our data confirm a large (>10‰) spatial gradient in δ C in the lower Doushantuo/Lantian formations, but this gradient is probably due to the greater sensitivity of carbonate-poor deep-water sediments to isotopic mixing with C-depleted carbonate cements. A pronounced negative δ C excursion (EN3) in the upper Doushantuo/Lantian formations, however, is spatially consistent and may be an equivalent of the Shuram excursion. δ S is more negative in deeper-water facies than in shallow-water facies, particularly in the lower Doushantuo/Lantian formations, and this spatial pattern is interpreted as evidence for ocean redox stratification: Pyrite precipitated in euxinic deep waters has lower δ S than that formed within shallow-water sediments. The Lantian Formation was probably deposited in oscillating oxic and euxinic conditions. Euxinic black shales have higher TOC and TN contents, but lower δ S and δ N values. In euxinic environments, pyrite was predominantly formed in the water column and organic nitrogen was predominantly derived from nitrogen fixation or NH assimilation because of quantitative denitrification, resulting in lower δ S and δ N values. Benthic macroalgae and putative animals occur exclusively in euxinic black shales. If preserved in situ, these organisms must have lived in brief oxic episodes punctuating largely euxinic intervals, only to be decimated and preserved when the local environment switched back to euxinia again. Thus, taphonomy and ecology were the primary factors controlling the stratigraphic distribution of macrofossils in the Lantian Formation.
中国南方的埃迪卡拉纪陡山沱组是地质生物学研究的主要目标，因为它为整合化学地层学和古生物学数据提供了契机。以往的研究大多集中在浅海陆架相的层序，但需要深水层序的数据才能全面了解盆地的氧化还原结构。在此，我们报告了来自含化石的蓝田组钻孔岩芯的δ¹³C、δ¹⁸C、δ³⁴S、δ³⁴S和δ¹⁵N数据，蓝田组是陡山沱组的深水对应层位。我们的数据证实了陡山沱组/蓝田组下部δ¹³C存在较大（>10‰）的空间梯度，但这种梯度可能是由于贫碳酸盐的深水沉积物对与¹³C贫化的碳酸盐胶结物发生同位素混合更为敏感所致。然而，陡山沱组/蓝田组上部明显的负δ¹³C偏移（EN3）在空间上是一致的，可能相当于舒拉姆偏移。δ³⁴S在深水相比浅水相更负，特别是在陡山沱组/蓝田组下部，这种空间模式被解释为海洋氧化还原分层的证据：在缺氧深水中沉淀的黄铁矿的δ³⁴S低于在浅水沉积物中形成的黄铁矿。蓝田组可能是在氧化和缺氧条件振荡的环境中沉积的。缺氧的黑色页岩具有较高的总有机碳（TOC）和总氮（TN）含量，但δ³⁴S和δ¹⁵N值较低。在缺氧环境中，由于定量反硝化作用，黄铁矿主要在水柱中形成，有机氮主要来源于固氮或铵同化作用，导致δ³⁴S和δ¹⁵N值较低。底栖大型藻类和疑似动物仅出现在缺氧的黑色页岩中。如果原地保存，这些生物必定生活在以缺氧期为主的短暂氧化事件中，只有当局部环境再次转变为缺氧状态时才会大量死亡并保存下来。因此，埋藏学和生态学是控制蓝田组宏观化石地层分布的主要因素。
