Abstract

Radiolarians and diatoms are documented for the first time from the mid-Cretaceous succession of the Sergipe Basin, a passive marginal basin in northeastern Brazil. Prevailing palaeoceanographic conditions are inferred for the episodes of siliceous radiolarian and diatom biomineralization/preservation. Radiolarian faunas are first recorded in the middle to upper Albian, from scattered occurrences, and subsequently throughout most of the Cenomanian-Turonian succession. Spumellarian forms are dominant in all the sections. Nassellarian forms seem to have thrived in relatively deep-water environments, in middle neritic to upper bathyal pelagic biotopes, and have been recovered from upper Albian and uppermost Cenomanian to middle Turonian sediments. Diatom frustules are only recorded from upper Cenomanian and lower Turonian deposits. These seem to have been more abundant in shallower neritic environments. The onset of the radiolarian assemblages in middle-late Albian times (with waning low-oxygen pelagic conditions) is thought to be a response to better developed oceanic circulation patterns and to a water mass saturated in dissolved silica, perhaps generated by deep-sea volcanic processes in the formation of early oceanic crust and the mid-oceanic ridge in the northern South Atlantic. On the other hand, the record of radiolarian and diatom tests throughout the Cenomanian-Turonian succession is commonly associated with dysaerobic to quasi-anaerobic bottom conditions. This is not only in keeping with high epipelagic primary productivity in well-oxygenated surface waters and that the sea water apparently contained a high level of dissolved silica, but also suggests that the bottom and interstitial waters were enriched in carbon dioxide, had a low pH and slightly negative redox-potential (Eh). The overall conditions would have favoured the biomineralization and post-mortem preservation of siliceous organisms increasing, therefore, the radiolaria+diatom/foraminifera ratio in the sediments, which supports the conclusions of several previous authors.