Abstract

The Ediacaran–Cambrian transition was a time of profound reorganisation of the biosphere, coinciding with the assembly of Gondwana and increasing atmospheric and oceanic oxygenation. The lower Cambrian marine sediments of the Stansbury Basin, South Australia, were deposited at low northern paleolatitudes on the western margin of the emerging Pacific Ocean. Here we report results of a multi-pronged investigation employing trace and rare earth element (REE) abundances, total organic carbon (TOC) contents and pyritic sulfur isotopic compositions (δ³⁴S_py) in carbonaceous shales from three formations within the Normanville and Kanmantoo groups (sequences Є1.2 to Є2.2). TOC ranges from ≤0.5 wt% in the Emu Bay Shale, through 0.4–1.9 wt% in the Talisker Formation, to 0.2–2.6 wt% in the Heatherdale Shale. Covariance is demonstrated between trace elements and organic matter, with comparative uranium enrichment in the Heatherdale Shale likely linked to increased primary productivity. Heavy REEs and yttrium are typically more depleted than light REEs. Provenance appears to evolve through the upper Normanville sediments into the Kanmantoo Group, corroborating published detrital zircon interpretations that suggest an increasing dominance of southerly derived sources. The prevailing paleoredox regime for the Heatherdale Shale and the Talisker Formation was dysoxic, evolving into progressively more reducing conditions up section, the latter exhibiting a secular decline in δ³⁴S_py (from +10 to −11‰ VCDT). Conversely, redox proxies concur in demonstrating that the basal fossiliferous Emu Bay Shale accumulated beneath an oxic water column. Comparison of trace element and REE distributions with those of slightly older sequences on the Yangtze Platform, South China, reveals striking similarities, implying that the trace element chemistry of the Paleo-Pacific and Asian oceans was homogeneous.

1. KEY POINTS

2. Trace and rare earth element, total organic carbon and sulfur isotopic data provide a multi-faceted profile of shales from three formations in the Stansbury Basin.

3. The prevailing paleoredox regime for the Heatherdale Shale and Talisker Formation was dysoxic, whereas the basal Emu Bay Shale accumulated beneath an oxic water column.

4. The strong covariance between TOC and normalised Mo, V and U in these shales highlights the evolving bioproductivity and oxygenation of the early Cambrian ocean.

5. Similar element signatures in lower Cambrian black shales, South China, reflect the homogeneous biogeochemistry of the Paleo-Pacific and Asian oceans.

摘要

埃迪卡拉纪-寒武纪过渡是生物圈深刻重组的时期，恰逢冈瓦纳纪的集合以及大气和海洋氧合的增加。南澳大利亚斯坦斯伯里盆地的下寒武统海洋沉积物沉积在新兴太平洋西缘的低北古纬度地区。在这里，我们报告了采用痕量和稀土元素 (REE) 丰度、总有机碳 (TOC) 含量和黄铁矿硫同位素组成 (δ ³⁴ S _py) 来自 Normanville 和 Kanmantoo 组内三个地层的碳质页岩（序列 Є1.2 至 Є2.2）。TOC 范围从鸸鹋湾页岩的 ≤ 0.5 wt%，Talisker 地层的 0.4-1.9 wt%，到 Heatherdale 页岩的 0.2-2.6 wt%。微量元素和有机物质之间存在协方差，Heatherdale 页岩中的相对铀富集可能与初级生产力的提高有关。重 REE 和钇通常比轻 REE 消耗得更多。物源似乎通过上诺曼维尔沉积物演变成 Kanmantoo 群，证实了已发表的碎屑锆石解释，表明南风来源的主导地位日益增加。Heatherdale 页岩和 Talisker 组的主要古氧化还原状态是​​缺氧的，³⁴小号_PY（从10至-11‰VCDT）。相反，氧化还原代理也证明了含化石的鸸鹋湾页岩沉积在含氧水柱下。将微量元素和稀土元素分布与华南扬子地台稍老的序列进行比较，揭示了惊人的相似性，这意味着古太平洋和亚洲海洋的微量元素化学是同质的。

1. 关键点

2. 痕量和稀土元素、总有机碳和硫同位素数据提供了来自斯坦斯伯里盆地三个地层的页岩的多方面剖面图。

3. Heatherdale 页岩和 Talisker 地层的主要古氧化还原体系是缺氧的，而基底鸸鹋湾页岩在含氧水柱下积累。

4. 这些页岩中 TOC 与归一化 Mo、V 和 U 之间的强协方差突出了早期寒武纪海洋不断发展的生物生产力和氧化作用。

5. 华南下寒武统黑色页岩中的类似元素特征反映了古太平洋和亚洲海洋的均质生物地球化学。