Biogeochemical status of the Paleo-Pacific Ocean: clues from the early Cambrian of South Australia

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 (δ34Spy) 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 δ34Spy (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.KEY POINTSTrace 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.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.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.Similar element signatures in lower Cambrian black shales, South China, reflect the homogeneous biogeochemistry of the Paleo-Pacific and Asian oceans. 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. 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. 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. Similar element signatures in lower Cambrian black shales, South China, reflect the homogeneous biogeochemistry of the Paleo-Pacific and Asian oceans.埃迪卡拉纪-寒武纪之交（Ediacaran–Cambrian transition）是生物圈发生深刻重构的时期，恰逢冈瓦纳大陆（Gondwana）聚合以及大气与海洋的氧合作用增强。澳大利亚南部斯坦斯伯里盆地的下寒武统海相沉积物，形成于新兴太平洋西缘的低北古纬度区域。本文报道了一项多维度研究的结果：针对诺曼维尔群与坎曼图群内3个组（层位Є1.2至Є2.2）的碳质页岩，分析了其微量元素与稀土元素（trace and rare earth element, REE）丰度、总有机碳（total organic carbon, TOC）含量以及黄铁矿硫同位素组成（δ34Spy）。 鸸鹋湾页岩的总有机碳含量≤0.5 wt%，塔利斯克组为0.4~1.9 wt%，希瑟代尔页岩则为0.2~2.6 wt%。研究证实微量元素与有机质之间存在协变关系，希瑟代尔页岩中相对显著的铀富集现象，可能与初级生产力提升有关。重稀土元素与钇通常较轻稀土元素更为亏损。物源特征在诺曼维尔群上部沉积物至坎曼图群的过渡过程中发生演化，这佐证了已发表的碎屑锆石（detrital zircon）研究结论——该区域沉积物的物源逐渐以南向来源为主导。 希瑟代尔页岩与塔利斯克组的主导古氧化还原环境为贫氧（dysoxic）环境，向上层位逐渐演变为更强烈的还原环境，后者的δ34Spy值呈现长期下降趋势（从+10‰至-11‰，VCDT标准）。与之相反，氧化还原代用指标一致表明，底部含化石的鸸鹋湾页岩形成于有氧水柱环境之下。 将该区域的微量元素与稀土元素分布特征与华南扬子地台（Yangtze Platform）年代稍早的地层进行对比，二者展现出显著的相似性，这意味着古太平洋与亚洲大洋的微量元素地球化学特征具有均一性。 关键要点： 1. 微量元素与稀土元素、总有机碳及硫同位素数据，为斯坦斯伯里盆地3个组的页岩提供了多维度的地球化学剖面。 2. 希瑟代尔页岩与塔利斯克组的主导古氧化还原环境为贫氧环境，而底部含化石的鸸鹋湾页岩形成于有氧水柱环境之下。 3. 这些页岩中总有机碳与标准化后的钼、钒、铀含量之间存在显著协变关系，反映了早寒武世海洋初级生产力与氧合作用的演化过程。 4. 中国南方下寒武统黑色页岩中相似的元素特征，揭示了古太平洋与亚洲大洋具有均一的生物地球化学环境。 5. 微量元素与稀土元素、总有机碳及硫同位素数据，为斯坦斯伯里盆地3个组的页岩提供了多维度的地球化学剖面。 6. 希瑟代尔页岩与塔利斯克组的主导古氧化还原环境为贫氧环境，而底部含化石的鸸鹋湾页岩形成于有氧水柱环境之下。 7. 这些页岩中总有机碳与标准化后的钼、钒、铀含量之间存在显著协变关系，反映了早寒武世海洋初级生产力与氧合作用的演化过程。 8. 中国南方下寒武统黑色页岩中相似的元素特征，揭示了古太平洋与亚洲大洋具有均一的生物地球化学环境。