Rhodalgal-foramol facies in equatorial carbonates. Insights from Miocene tectonic islands of the central Atlantic

Shallow-water carbonates ranging in thickness from 250 to 500 m have been deposited during the Miocene on the summit of transverse ridges flanking the Romanche and Vema transforms that offset the equatorial Mid- Atlantic Ridge. These carbonate platforms are dominated by the same biota assemblages consisting mainly of perforate larger benthic foraminifera and red algae; corals are present occasionally as minor components. We discuss several factors (i.e. temperature of sea water, elevated CO2 levels, hydrothermal brine fluxes, trophic conditions, biological-induced precipitation processes, water energy, substratumstability or instability and platform geometry) that could have played a role in favoring specific features that characterize our Atlantic carbonates relative to other known examples fromthe equatorial realm.We use the knowledge on the biota association of St. Paul Rocks, small islets fromthe equatorial Atlantic, the onlymodern example of oceanic tectonic islands, to compare present day with ancient assemblages. We propose that equatorial upwelling and humid conditions, CO2 input and iron addition, related to the midocean ridge, water energy and substratum instability may have created the conditions suitable for the predominance of calcitic organisms over aragonitic corals in the Miocene tectonic islands of equatorial Atlantic. Moreover, we do not exclude for the Vema paleoislands the influence of the fresh-water plumes from South American rivers.