Ichnofabric analysis of deep-ocean sediment cores from the South Atlantic ocean: Eocene–Oligocene transition (IODP expeditions 390 and 393)

Ichnological changes characterize the Eocene-Oligocene Transition (EOT) recorded in deep-ocean sediments cored along the South Atlantic Transect at sites U1556, U1557, and U1558 during International Ocean Discovery Program (IODP) Expeditions 390 C, 395E, 390, and 393. Trace fossil diversity, intensity of visible bioturbation, maximum trace fossil diameter, and tiering of pre-EOT, transitional (EOT-1 to Oi-1 [Oligocene Isotope Event]), and post-EOT ichnofabrics document how benthic community and resulting trace fossil assemblages were affected by extreme climate changes from hothouse to coolhouse conditions. The infauna responded to fluctuating bottom and pore water oxygenation, organic matter availability, sedimentation rate, temperature changes, and changing deep-ocean circulation. Trace fossil parameters were collected onboard the JOIDES Resolution and from high-resolution images of retrieved and slabbed sediment cores. The images were treated using ImageJ with a FIJI add-on to improve trace fossil visibility. Ichnofabric data are supplemented by physical (e.g. reflectance, magnetic susceptibility, lithology) and geochemical sediment data (e.g. δ18O and δ13C isotope signature). The deeper-water site sediments (∼5000 m modern water depth and ∼4100 m syn-EOT; U1556–U1557) are characterized by Zoophycos, Chondrites, Phycosiphon, Planolites, and Palaeophycus, whereas the shallower-water site (∼4300 m modern water depth and ∼3200 m syn-EOT; U1558) is characterized by Planolites, Arenicolites, Skolithos, Zoophycos, and Palaeophycus. Modal bioturbation index of visible traces (BIt) is stable at deeper sites and declining at the shallower site from the pre-EOT to post-EOT intervals. Average maximum trace fossil diameters decreased in the deeper site U1556 and increased everywhere else across the EOT. Trace fossil diversity decreased slightly at most sites and locally increased during the transitional period. Tiering patterns shifted across the boundary, particularly for Chondrites and Zoophycos. Comparative analysis of cores taken meters apart reveals localized changes in seafloor topography and patchy distribution of food and benthos.