Evolution of mid-Cretaceous radiolarians in response to oceanic anoxic events in the eastern Tethys (southern Tibet, China)

Abstract

Oceanic anoxic events (OAEs) are well-known from their widespread black shale and carbon isotopic excursions in the (western and eastern) Tethys and Atlantic Ocean. However, the weakest link in the studies of the OAEs is biological evolution. Sedimentological, biostratigraphical and geochemical data from Albian to Coniacian strata in southern Tibet recorded turnover events and fluctuations in diversity of the radiolarian fauna within the eastern Tethys during OAE 1d and OAE 2. Abundant radiolarian fossils were obtained from the Gyabula Formation, with 93 species from 43 genera identified and assigned to the mid to Upper Cretaceous Acaeniotyle umbilicata, Archaeospongoprunum tehamaensis, Crucella cachensis, Alievium superbum, and Dictyomitra formosa zones. The association of carbon isotopic excursions, black shale and radiolarian turnover indicates extensive changes in the ocean-climate system. Nutrients are made increasing available to the marine plankton through submarine volcanic activity and rising sea-level, which were a likely cause of radiolarian turnover at/or near the OAEs. Active submarine tectonism-volcanism leads to the expansion of the hypoxic zone and may cause many deeper dwelling forms to become extinct whereas most of the shallower dwelling radiolarians survive. Radiolarian evolution thus provides a useful record with which to seek understand relationships between climate, paleoceanographic processes and plankton evolution.