Middle Famennian (Late Devonian) interval with pyritized fauna from the Holy Cross Mountains (Poland): Organic geochemistry and pyrite framboid diameter study

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The pyrite framboid diameter and the extractable organic matter from the Middle Famennian rhythmic succession of limestones, nodular limestones and marly shales with pyritized fauna (KPG horizon) from the Holy Cross Mountains (Poland) were analyzed in order to constrain the water column structure during their deposition. In all samples investigated, small framboids (<5 μm) that may have formed in the water column occur but the majority of the framboids have larger diameters, characteristic for their formation within the sediments, below an oxic or dysoxic water column. This suggests that during deposition of the KPG horizon, the depositional environment was highly dysoxic, but not anoxic. Low concentration of the aryl isoprenoids and isorenieratane, and very high values of the AIR parameter point to the episodic photic zone euxinia and later post-depositional, partial degradation of the OM in suboxic conditions. Intermittent, but certainly short-lasting, anoxic conditions on the sea-floor, may also be supported by the lamination of the rock matrix and increased mortality of the juvenile ammonoids. The results of this study indicate that anoxic conditions prevailed in the shallow depths below the water-sediment interface, where pyritization following by dissolution of aragonitic shells of fauna occurred.

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