Seafloor diagenesis and carbonate–volcanic–iron–phosphate interactions on the Rio Grande Rise substrate

Sediment diagenesis is a key paleoceanographic process for understanding the geological history of the oceans. Submarine plateaus and seamounts host distinctive lithification processes and are sensitive to sea-level and atmospheric changes due to their relatively shallow bathymetry. In this context, the Rio Grande Rise (SW Atlantic Ocean) is well-suited for the investigation of early diagenetic processes on summit carbonates. We analyzed 30 dredged samples collected by the Geological Survey of Brazil in 2011, using optical petrography, scanning electron microscopy, electron probe microanalysis, bulk-rock major elements geochemistry, C and O stable isotopes, and 87Sr/86Sr geochronology. Sample lithologies range from mud- to grain-supported low Mg-calcite carbonates, mainly composed of bioclasts of red algae, foraminifera, bryozoans, echinoderms, and corals, in addition to volcanic sandstone and siltstone, recording the occurrence of an Oligo-Miocene isolated carbonate platform. Diagenetic features included micritization, cementation of fibrous to bladed isopachous low Mg-calcite cementation, euhedral and mosaic calcite, syntaxial overgrowths and recrystallization. δ18O and δ13C values vary between -1.68% to +4.18% and -1.35% to +2.62% (V-PDB), respectively. Volcanic-fluid interaction is indicated by Mg- and Fe-smectite (saponite, nontronite) formation under alkaline conditions, and Fe-Mn oxide cementation since the early Cenozoic. Phosphatization is also a prevalent process that took place during Miocene acidic conditions, promoting cement dissolution and phosphatization under cold, oligotrophic water masses. Our results reveal a complex diagenetic history, which shaped the seafloor of the Rio Grande Rise over the last 25 million years, providing insights into the paleoceanographic changes in the South Atlantic Ocean during the Paleogene-Neogene.