Geochemical analysis of nine sections from the Nama Group, Namibia

The first appearance of skeletal metazoans in the late Ediacaran (~550 million years ago; Ma) has been linked to the widespread development of oxygenated oceanic conditions, but a precise spatial and temporal reconstruction of their evolution has not been resolved. Here we consider the evolution of ocean chemistry from ~550 to ~541 Ma across shelf-to-basin transects in the Zaris and Witputs Sub-Basins of the Nama Group, Namibia. New carbon isotope data capture the final stages of the Shuram/Wonoka deep negative C-isotope excursion, and these are complemented with a reconstruction of water column redox dynamics utilising Fe-S-C systematics and the distribution of skeletal and soft-bodied metazoans. Combined, these inter-basinal datasets provide insight into the potential role of ocean redox chemistry during this pivotal interval of major biological innovation. The strongly negative d13C values in the lower parts of the sections reflect both a secular, global change in the C-isotopic composition of Ediacaran seawater, as well as the influence of 'local' basinal effects as shown by the most negative d13C values occurring in the transition from distal to proximal ramp settings. Critical, though, is that the transition to positive d13C values postdates the appearance of calcified metazoans, indicating that the onset of biomineralization did not occur under post-excursion conditions. Significantly, we find that anoxic and ferruginous deeper water column conditions were prevalent during and after the transition to positive d13C that marks the end of the Shuram/Wonoka excursion. Thus, if the C isotope trend reflects the transition to global-scale oxygenation in the aftermath of the oxidation of a large-scale, isotopically light organic carbon pool, it was not sufficient to fully oxygenate the deep ocean. Both sub-basins reveal highly dynamic redox structures, where shallow, inner ramp settings experienced transient oxygenation. Anoxic conditions were caused either by episodic upwelling of deeper anoxic waters or higher rates of productivity. These settings supported short-lived and monospecific skeletal metazoan communities. By contrast, microbial (thrombolite) reefs, found in deeper inner- and mid-ramp settings, supported more biodiverse communities with complex ecologies and large skeletal metazoans. These long-lived reef communities, as well as Ediacaran soft-bodied biotas, are found particularly within transgressive systems, where oxygenation was persistent. We suggest that a mid-ramp position enabled physical ventilation mechanisms for shallow water column oxygenation to operate during flooding and transgressive sea-level rise. Our data support a prominent role for oxygen, and for stable oxygenated conditions in particular, in controlling both the distribution and ecology of Ediacaran skeletal metazoan communities.骨骼动物（skeletal metazoans）在晚埃迪卡拉纪（Ediacaran，约5.5亿年前，即Ma）的首次出现，曾被关联至氧化海洋环境的广泛发育，但目前学界尚未就其演化的精确时空重建达成共识。本研究聚焦纳米比亚纳马群（Nama Group）扎里斯与维普茨次盆地内的陆棚-盆地剖面（shelf-to-basin transects），重建约550 Ma至541 Ma期间的海洋化学演化过程。本研究的新碳同位素数据捕捉到了舒拉姆-沃诺卡（Shuram/Wonoka）大型负碳同位素偏移的最终阶段；同时结合铁-硫-碳系统学（Fe-S-C systematics）分析与骨骼、软躯体动物（soft-bodied metazoans）的分布特征，重建了水体柱的氧化还原动力学过程。上述跨盆地数据集的综合分析，为揭示这一重大生物创新关键时段内海洋氧化还原化学的潜在作用提供了新视角。 剖面下部显著负偏的δ¹³C值，既反映了埃迪卡拉纪海水碳同位素组成的全球性长期变化，也体现了“局地”盆地效应的影响——远岸向近岸缓坡环境过渡区域出现的δ¹³C值负偏程度最高，正是该效应的直观体现。但关键的是，δ¹³C值由负转正的转变发生在钙化动物（calcified metazoans）出现之后，这表明生物矿化作用（biomineralization）的起始并未发生在同位素偏移后的环境中。 值得注意的是，本研究发现，在标志舒拉姆-沃诺卡偏移结束的δ¹³C值由负转正的时段及其后，深水水体柱普遍处于缺氧且亚铁型的环境中。因此，若碳同位素趋势反映了大规模轻同位素有机碳库氧化后全球尺度海洋氧化作用的转变，那么该氧化作用并不足以完全充氧深海环境。 两个次盆地均展现出高度动态的氧化还原结构：浅水环境的内缓坡区域经历了短暂的充氧过程。缺氧环境的形成，要么源于深层缺氧水体的间歇性上升流，要么缘于更高的生产力水平。这类环境仅能支撑短命且单优种的骨骼动物群落。与之形成对比的是，发育于较深内缓坡与中缓坡环境的微生物凝块石（thrombolite）礁体，所支撑的群落生物多样性更高、生态关系更复杂，且包含大型骨骼动物。这些长寿的礁体群落以及埃迪卡拉纪软躯体生物群，尤其富集于持续充氧的海侵系统（transgressive systems）中。本研究认为，中缓坡的位置使得物理通风机制得以在海侵淹没与海平面上升期间运作，为浅水环境水体柱充氧提供了条件。本研究数据证实，氧气尤其是稳定的氧化环境，在调控埃迪卡拉纪骨骼动物群落的分布与生态特征方面发挥了关键作用。