Trace element geochemistry of pyrite in sediments across the permian triassic boundary

Sedimentary pyrite trace element composition is an established proxy for assessing palaeo-ocean geochemistry and atmospheric oxygen concentrations over billions of years. The proxy is based on results from several experimental studies, and also on in situ analyses of pyrite from ancient and modern sediments via Laser Ablation Inductively-Coupled Plasma Mass Spectrometry (LA-ICPMS).<br>The experiments which simulated pyrite FeS2 formation at low temperatures in seafloor muds have shown that the incorporation of trace elements dissolved in seawater occurs in the earliest stages of nucleation of an unstable monosulfide (i.e., the pyrite precursor). As sulfide growth continues in the mud, the trace elemental signature is inherited by the subsequent pyrite. The proxy assumes that the pyrite structure has retained its original trace element composition through geological time, and furthermore, that this composition mirrors the palaeo sea- and pore water chemistry at the time of pyrite formation. Due to the common lack of preservation of original compositions that result from diagenesis, deformation, and metamorphism, only the earliest pyrite forms - framboids and microcrystalline disseminated euhedral grains - have been considered for palaeo environment interpretation.<br>The first order assessment of deep-time trends in sedimentary pyrite trace element contents revealed temporal correlation between trace element proportions in pyrite from worldwide ancient sediments (mostly black shales) and globally recognized events. For example, elevated gold in Archean sedimentary pyrite has been linked to the progressive oxygenation of ferruginous Au and Fe-rich waters; cyclical trends of nutrient elements have been observed in Phanerozoic sedimentary pyrite; and the Se/Co maximum has been temporally linked with the first global oxygenation event, whereas Se depletion and Se/Co minimums coincided with three major mass extinctions. However, the applicability of the proxy over shorter timescales (i.e., &lt; 20 Ma) is not well constrained and is potentially problematic even over longer timescales, consider that high natural variability of trace elements (i.e., multiple orders of magnitude) can occur even within the same sample.<br>Therefore, this thesis’ working hypothesis states that pyrite geochemistry should reflect changes in shales across the Permian-Triassic boundary because 1) abundant pyrite in black shales from several Permian-Triassic localities around the world has been regarded as evidence of oceanic anoxia during the End-Permian Mass Extinction, and 2) this extinction was the largest such in Earth history, and was driven by global changes in palaeo environmental settings likely associated with the emplacement of the Siberian Traps Large Igneous province formation.<br>Pyrite-bearing samples have been selected from sedimentary sequences straddling the end-Permian mass extinction in different depositional settings. Shallow nearshore and shallow marine environments are recorded in the Hovea Member, the base of the Kockatea Shale Formation in the Perth Basin in Western Australia, sampled in two oil boreholes. The Meishan section in China has also been sampled, as it represents the Global Stratotype Section and Point (GSSP) of the PTB, with excellent quarry exposures of the Permian-Triassic sediments from a shallow continental shelf setting of the ancient Tethys Ocean. In addition, the abyssal plains of the end-Permian Panthalassic Ocean are represented in this study by the Opal Creek section in Canada and the Ubara section in Japan.<br>Stratigraphic level and the PTB position in the studied sequences have been previously constrained by palaeontologists, indicating the first appearance datum of Triassic conodonts H. parvus or, in the absence of conodonts (as in the WA Hovea member), by the last record of the P. microcorpus palynofacies. The δ13C excursion or shift, globally associated with the end-Permian mass extinction, as determined by this and other works, is also present within all the studied sediments, allowing for better cross-correlations.<br>This study relies on hand specimen petrographic inspection and LA-ICPMS as the primary tools, which are complimented by bulk sediment geochemistry determined via four-acid digestion and ICPMS analysis, optical and scanning electron microscopy, organic carbon and total sulfur content analysis using combustion and multichannel infrared spectrometry, carbon and sulfur isotopic ratios determination via stable isotopes mass spectrometry in bulk (IRMS) and in situ (SHRIMP), Raman spectrometry for pyrite-marcasite discrimination, and analysis of palynofacies. LA-ICPMS analyses of pyrite have revealed microscopic distribution and correlation of trace elements in sedimentary pyrite, allowing for the domaining of the data into several groups. Co, Ni, As, Se, and Sb closely follow Fe and S, and are likely incorporated into the pyrite structure by substitution. Other elements such as Mn, Cu, Zn, Pb, Bi, Tl, Mo, Ag, Cd, Hg, and Te could be in the structure or present as unevenly distributed nano-inclusions of other sulphides, whereas Ti, Sn, W, Cr, V, U and REE, once thought to be part of pyrite composition, are exclusively associated with diverse mineral inclusions derived from the surrounding siliciclastic clay-rich matrix, which include silicates (recognised by Si and Al), carbonates and sulphates (recognised by Ca, Mg, Ba, and Sr), as well as some refractory minerals (e.g. titanite and monazite). Quantification of trace elements contents via LA-ICPMS has improved over the last decade, involving the development and characterization of matrix -matched reference materials and algorithms for matrix-pyrite signal deconvolution. Four reference materials and three algorithms of data reduction have been used during this thesis development:<br>In the WA Permian-Triassic Hovea Member sediments (Ch2) from two boreholes, Redback-2 and Hovea-3, a change in depositional facies from a marginal marine Permian inertinite to a shelf environment Triassic sapropel is recorded. This transition is highlighted by several lithogeochemical indicators (e.g., negative δ13C shift and reduction in Corg contents; increases in Ca, Fe, and P), and separates the fundamental changes in modal mineralogy between the two zones. Importantly, the sapropel records a major increase in iron sulphide burial over that in the inertinite. LA-ICPMS analyses of pyrite demonstrates that trace element enrichment is chiefly highest in the Permian horizons, and the elements with greatest affinity for substitution into the pyrite structure are particularly enriched, i.e., 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 δ13C shift in the earliest Triassic sapropel. Various whole-rock based paleosalinity indicator ratios (e.g., 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 sulphides are reflective of shifts in the regional depositional environment rather than a global signal. Nonetheless, a geochemical contribution from seawater or a distant igneous province, such as the Siberian Traps Large Igneous Province (STLIP), cannot be ruled out.<br>This work revealed the presence not only of pyrite but also marcasite (the orthorhombic FeS2 polymorph) intergrown with marcasite, which is stable under low pH and high ∫O₂. Furthermore, marcasite occurrences in this study are unique to the mass extinction intervals in all four sequences, this mineral received particular attention (Ch3). Marcasite from the Kockatea Shale is characterised by low concentrations of all trace elements except Mn, as well as a distinctly heavier (³⁴S-enriched) δ³⁴S signature (up to +11‰), coupled with small positive and negative nonzero Δ³³S anomalies -0.04 to +0.06 ‰). This combination of ³⁴S enrichment and variable ³³S suggests an additional source of sulphur that could be either volcanic or riverine input, rather than localized extreme sulfate reduction in porewaters. The variety of textures, trace element geochemistry, and sulphur isotopic signature of these sulfides from the P-T transition favour an early diagenetic environment of formation, rather than late diagenesis as was previously thought. The ubiquitous presence of marcasite around the δ¹³C excursion in the end -Permian sediments further supports the notion that the EPME was at least partially caused by a large-scale pH drop in the global hydrosphere, which, in turn, could be a result of acidic rains combined with CO₂ and SO₂ released by the STLIP igneous activity. An additional literature review of other sedimentary sequences around the globe indicates that sedimentary-diagenetic marcasite may be more abundant in the rock record than previously recognized, which carries implications for the geochemistry of the marine environment through geologic time.<br>The Permian-Triassic Ubara section (Ch4), south-eastern Japan, exposes part of the Mino-Tamba metamorphic belt. The Permian-Triassic sediments that accumulated in the abyssal peri-equatorial Panthalassa are now part of the accretionary prism. These rocks experienced significant transport from their depositional site due to seafloor spreading, and furthermore, the oceanic plate subduction and accretion starting at approximately 250 Ma. The compositional difference of fine-grained pyrite from the Permian cherts (Unit III) and Triassic black shales (Uni II) is evident across all redox-sensitive elements. Through 80 cm of the Permian chert facies, Se, As, and Mo increase steadily but gradually, whereas all other elements remain relatively flat and elevated. However, approxi

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