Lithogeochemical and sulfide trace-element systematics across the Permian–Triassic boundary, Perth Basin, Western Australia: constraints on the shallow marine environment during the end-Permian mass extinction

Sedimentary pyrite trace-element composition is an established proxy for determining paleo-ocean geochemistry and atmospheric oxygen concentrations through deep time. However, its applicability over shorter time-scales (<i>i.e.</i> &lt;20 Ma) is not well known. To test this, we targeted fine-grained pyrite in the Hovea Member of the Kockatea Shale (Perth Basin, Western Australia), which encompasses the late Permian inertinitic interval and the end-Permian to Early Triassic sapropel, and spans approximately 10 million years. The end-Permian mass extinction (EPME) was the largest extinction event in Earth history, and its greatest effect is documented in the marine environment. Samples were collected from two oil exploration wells—Redback-2 and Hovea-3—spaced ∼20 km apart. In the two boreholes, a change in depositional facies (<i>i.e.</i> between the inertinite and sapropel) occurs below the Permian–Triassic boundary and records the transition from a marginal marine to a shelf environment. This transition is highlighted by several lithogeochemical indicators (<i>e.g.</i> negative shift δ¹³C values and C_org reduction; increases in Ca, Fe and P), which are themselves tied to fundamental changes in modal mineralogy between the two zones. Importantly, the sapropel also records a major increase in iron sulfide burial over that in the inertinite. LA-ICPMS analyses of pyrite demonstrate that trace-element abundance is highest in samples below the facies transition, and in places reaches a few percent, particularly of Ni (4 wt%), Co (1.5 wt%) and As (2.8 wt%). Moreover, these and other trace elements decrease by an order of magnitude in concert with the negative shift in δ¹³C values in the sapropel zone. Various whole-rock based paleosalinity indicator ratios (<i>e.g.</i> B/Ga) indicate that the areas of the Perth Basin intersected by Redback-2 and Hovea-3 were not fully connected to the open ocean at the time of the EPME, which leads us to conclude that the very high trace-element values in the sedimentary sulfides are reflective of regional environmental shifts rather than a global signal. Nonetheless, a geochemical contribution from a distant igneous province, such as the Siberian Traps Large Igneous Province, cannot be ruled out. Our work underscores the strength of sedimentary pyrite as a robust paleoenvironmental proxy in the marine environment and highlights the need for further investigation of pyrite trace-element profiles across the mass extinction interval in other sedimentary sequences around the globe.KEY POINTSLA-ICPMS-based geochemistry of sedimentary pyrite from the Hovea Member of the Kockatea Shale is considered within a lithochemostratigraphic context.The overall interpretation of the results involves a change in depositional setting from the marginal in the late Permian brackish waters to shelfal marine and loss of oxygen in the Early Triassic Perth Basin. LA-ICPMS-based geochemistry of sedimentary pyrite from the Hovea Member of the Kockatea Shale is considered within a lithochemostratigraphic context. The overall interpretation of the results involves a change in depositional setting from the marginal in the late Permian brackish waters to shelfal marine and loss of oxygen in the Early Triassic Perth Basin.

沉积黄铁矿的微量元素组成是确定地质历史时期古海洋地球化学特征与大气氧浓度的成熟代用指标。然而，其在较短时间尺度（即小于20百万年）内的适用性尚不明确。为验证这一问题，我们选取西澳大利亚珀斯盆地（Perth Basin）科卡塔页岩（Kockatea Shale）霍维亚段的细粒黄铁矿作为研究对象，该段涵盖晚二叠世惰质组段以及二叠纪末至早三叠世腐泥层，时间跨度约10百万年。

二叠纪末生物大灭绝（end-Permian mass extinction, EPME）是地球历史上规模最大的生物灭绝事件，其对海洋环境的影响最为显著。我们从两口间距约20千米的石油勘探井——Redback-2与Hovea-3中采集了样品。在这两个钻孔中，沉积相的转变（即惰质组段与腐泥层之间）发生于二叠纪-三叠纪界线之下，记录了从陆缘海环境向陆棚环境的过渡。这一转变可通过多项岩石地球化学指标加以体现（例如δ¹³C值负偏移与有机碳（Corg）含量降低；钙、铁、磷元素含量升高），而这些指标本身也与两个层位间的矿物组成模式的根本性变化相关。

重要的是，相较于惰质组段，腐泥层的硫化铁埋藏量也出现了显著升高。对黄铁矿开展的激光剥蚀电感耦合等离子体质谱（LA-ICPMS）分析显示，沉积相转变界面之下的样品微量元素丰度最高，局部可达数重量百分比，尤以镍（4 wt%）、钴（1.5 wt%）与砷（2.8 wt%）最为突出。此外，这些元素与其他微量元素的含量随腐泥层中δ¹³C值的负偏移降低了一个数量级。

多项基于全岩的古盐度指标比值（例如硼镓比B/Ga）表明，在二叠纪末生物大灭绝发生时期，Redback-2与Hovea-3所揭露的珀斯盆地区域并未与开阔大洋完全连通。据此我们认为，沉积硫化物中极高的微量元素含量反映的是区域环境变化，而非全球信号。不过，来自远端火成岩省（如西伯利亚暗色岩大火成岩省（Siberian Traps Large Igneous Province））的地球化学贡献仍不能排除。

本研究证实了沉积黄铁矿作为海洋环境中可靠古环境代用指标的有效性，并强调需要对全球其他沉积序列中的生物大灭绝界线层位开展黄铁矿微量元素剖面的进一步研究。

核心要点
基于激光剥蚀电感耦合等离子体质谱（LA-ICPMS）的科卡塔页岩（Kockatea Shale）霍维亚段沉积黄铁矿地球化学研究，被置于岩石化学地层学框架下开展分析。
研究结果的整体解释表明，沉积环境从晚二叠世半咸水的陆缘环境，转变为早三叠世珀斯盆地的陆棚海洋环境，并伴随水体缺氧。
基于激光剥蚀电感耦合等离子体质谱（LA-ICPMS）的科卡塔页岩（Kockatea Shale）霍维亚段沉积黄铁矿地球化学研究，被置于岩石化学地层学框架下开展分析。
研究结果的整体解释表明，沉积环境从晚二叠世半咸水的陆缘环境，转变为早三叠世珀斯盆地的陆棚海洋环境，并伴随水体缺氧。