Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, Beijing, 100871, China.
School of Earth and Space Science, Peking University, No. 5 Yiheyuan Road, Haidian District, Beijing, 100871, China.
Sci Rep. 2019 Sep 20;9(1):13628. doi: 10.1038/s41598-019-49993-2.
The earliest unambiguous evidence for animals is represented by various trace fossils in the latest Ediacaran Period (550-541 Ma), suggesting that the earliest animals lived on or even penetrated into the seafloor. Yet, the O fugacity at the sediment-water interface (SWI) for the earliest animal proliferation is poorly defined. The preferential colonization of seafloor as a first step in animal evolution is also unusual. In order to understand the environmental background, we employed a new proxy, carbonate associated ferrous iron (Fe), to quantify the seafloor oxygenation. Fe of the latest Ediacaran Shibantan limestone in South China, which yields abundant animal traces, ranges from 2.27 to 85.43 ppm, corresponding to the seafloor O fugacity of 162 μmol/L to 297 μmol/L. These values are significantly higher than the oxygen saturation in seawater at the contemporary atmospheric pO levels. The highly oxygenated seafloor might be attributed to O production of the microbial mats. Despite the moderate atmospheric pO level, microbial mats possibly provided highly oxygenated niches for the evolution of benthic metazoans. Our model suggests that the O barrier could be locally overcome in the mat ground, questioning the long-held belief that atmospheric oxygenation was the key control of animal evolution.
最早的明确动物证据是在埃迪卡拉纪末期（550-541Ma）的各种痕迹化石中发现的，这表明最早的动物生活在海底或甚至渗透到海底。然而，最早的动物大量繁殖时海底的氧逸度（O fugacity）还没有明确的定义。动物进化中，优先在海底殖民也是不寻常的。为了了解环境背景，我们采用了一种新的示踪剂，即与碳酸盐结合的亚铁（Fe），来定量海底的氧化程度。中国南方晚埃迪卡拉纪石板滩灰岩中含有丰富的动物痕迹，其 Fe 含量范围为 2.27 到 85.43ppm，对应于海底的氧逸度为 162μmol/L 到 297μmol/L。这些值明显高于当代大气 pO 水平下海水的氧饱和度。高度氧化的海底可能归因于微生物垫的 O 生成。尽管大气 pO 水平适中，但微生物垫可能为底栖后生动物的演化提供了高度氧化的小生境。我们的模型表明，O 障碍可以在垫底局部克服，这质疑了长期以来认为大气氧化是动物进化关键控制因素的观点。
