The former presence of organic matter caused its later absence: Burn‐down of organic matter in oceanic red beds enhanced by bioturbation (Eocene Variegated Shale, Carpathians)

Eocene oceanic red beds that formed in a well-oxygenated setting at low sedimentation rates below the calcite compensation depth are effectively barren of organic carbon in the present state. Recurrent distal low-erosive turbidites preserve the bioturbated zone underneath that documents seasonal and long-term fluctuating accumulation of considerable amounts of organic matter on the sea floor as evidenced by Scolicia; the producers of this trace fossil exploited nutritious organic matter conserved in turbidite-buried sea floor deposits. Over the long term, slow average sedimentation of (hemi)pelagic oxic (red) mud led to long oxygen exposure times and low burial of organic matter. Consequently, trace fossils representing persistent sediment-feeding modes are of small size. Although the food-limited setting appears appropriate for producers of graphoglyptids, such ‘stationary’ burrows have not been encountered because seasonal deposition of organic matter fostered at least temporary surface-layer feeding organisms, for instance producers of Nereites irregularis that intensively reworked the sediment and hence, hindered graphoglyptid production. These findings confirm palaeoceanographic modelling results that suggest upwelling in the study area during the Eocene.