Redox geochemistry of the red ‘orthoceratite limestone’ of Baltoscandia: Possible linkage to mid-Ordovician palaeoceanographic changes

The orthoceratite limestone of the Ordovician epicontinental sea of Baltoscandia is one of the oldest Phanerozoic examples of pelagic marine red beds (MRBs). Being enriched in authigenic hematite, MRBs are considered to be sensitive palaeoceanographic redox indicators. In this paper, the origin of the reddening of the Ordovician MRBs and its timing and redox conditions was addressed at two sections, and in a drill core in the Kinnekulle area, Sweden, through the application of diffuse reflectance spectroscopy, sedimentary petrology (microfacies and electron microprobe), bulk-rock and in-situ element geochemistry (laser-ablation ICP-MS) and molybdenum stable isotope systematics. Enrichment in hematite (up to ~0.05 wt%) occurred during very early diagenesis under low sedimentation rates (~5 mm/kyr), which is comparable to several examples of Phanerozoic MRBs. The reddening was associated with the mm-scale, in-situ mobility of Fe, Mn, As, Mo and U, due to FeMn redox cycling between primary and secondary minerals and pore water under oxic and suboxic to anoxic conditions; it was not related to significant changes in seawater chemistry. Stratigraphic red-to-grey transitions likely coincided with two mid-Darriwilian global regressions (the so-called Täljsten and at the base of the Gullhögen Formation), likely due to the effects of changing sedimentation rates and sub-bottom redox potentials. The Ordovician MRBs coincided with a positive δ¹³C excursion during the middle Darriwilian, which is different to MRB examples cited from the Devonian to the Cretaceous age, which frequently coincide with shifts to lower values of δ¹³C. Although MRBs are believed to show a time-specific occurrence in greenhouse or transitional greenhouse-to-icehouse climatic modes by several authors, our study suggests that causal links between MRBs and global carbon cycle remain unclear.