Significance of Late Cenozoic Zoophycos in Antarctica
The first occurrence of the trace fossil Zoophycos from marine deposits offshore of Antarctica is in the ANDRILL 1-B (AND1B) core, which penetrated 1285m of sediment beneath the Ross Ice Shelf. Its presence both constrains Pliocene (~3.3 Ma) climate and depositional environments and elucidates the much-debated history of the Antarctic benthic fauna. At two depths separated by ~70 cm, the core penetrated portions of the central shaft as well as several horizontal spreite. All components may not be part of the same structure. Spreite are 4mm thick, lack lamellae, and contain particles from underlying layers, implying upward transport. The Zoophycos occur in a sequence of mudstone, sandstone, and diamictite; within this they are in claystone and in finely laminated sandy mudstone reflecting suspension settling, but they are absent from diamictite that records debris-flow deposition. Because Zoophycos producers (ZooPro) penetrate decimeters beneath the sediment water interface that they originally colonized, the distribution of spreite within this core interval may not reflect their fine-scale habitat preferences. The larger heterolithic interval in AND1B in which Zoophycos occurs comprises a transition from warm Early Pliocene conditions with open productive waters (diatomite deposited) to glacial conditions in the Late Pliocene (diamictite deposited; Naish et al., 2008). Worldwide Late Quaternary Zoophycos are most abundant at depths >1000m, where rates of sedimentation and nutrient flux are low, and where sediment is fine-grained and IRD not abundant (Lwemark and Schfer, 2001). By analogy, the ZooPro colonized the seafloor at AND1B during a window of reduced sedimentation between deluges of diatoms and dumps of diamictites. The foray of ZooPro into the relatively shallow AND1B depths (200-1000m) during the Pliocene documents "emergence" of benthic animals, supporting suggestions that the unique modern Antarctic and Southern Ocean faunas result from both "emergence" and "submergence" during the Cenozoic (Brandt et al., 2007).