The Late Miocene-Early Pliocene Biogenic Bloom: Duration, Causes and Paleoceanographic Implications

The Late Miocene-Early Pliocene Biogenic Bloom (ca. 9.0-3.5 Ma) is a paleoceanographic phenomenon marked by increased marine biological productivity documented in numerous locations, from the Atlantic, to the Pacific and Indian oceans. In deep-sea sediments, the Biogenic Bloom is characterised by a marked increase in biogenic CaCO3 (mainly from calcareous nannofossils and planktonic foraminifera) and SiO2 accumulation rates (mainly from radiolarians and diatoms). Supporting evidence includes the presence of diatom assemblages indicative of elevated productivity, the abundance of suboxic and dysoxic benthic foraminifera, and changes in sediment geochemistry (e.g. reduced Mn/Sc ratio), which point to low dissolved oxygen levels at intermediate water depths. This phenomenon has remained a mystery in the geological record, as the several-million-years high productivity conditions documented for the Biogenic Bloom necessitate a profound alteration in the global nutrient cycling of the oceans to be explained. Despite its widespread occurrence, fundamental questions regarding the temporal and spatial extent, as well as the driving mechanisms behind this phenomenon remain unanswered. In this context, this Ph.D. thesis aims to comprehensively document the Biogenic Bloom through an integrated quantitative approach. We produced comparable datasets from diverse areas worldwide, from the Pacific (IODP Site U1506 and U1488) to the Atlantic (ODP Site 1085) oceans. The first fundamental step of the project consists of developing a highly resolved calcareous nannofossil biostratigraphic classification for all study sites, aiming to establish reliable chronological frameworks. The age models were employed to calculate the linear sedimentation and derive the carbonate mass accumulation rates, a proxy used to identify the Biogenic Bloom on a global scale. The second phase of the project involves an in-depth benthic foraminiferal investigation. Taxonomic and quantitative studies of the benthic foraminiferal assemblages, as well as morphogroup and statistical analyses were carried out for each study site to infer the environmental conditions in the deep-sea in terms of nutrient availability, trophic conditions, and oxygen concentrations during the Biogenic Bloom. These results allow for a thoughtful discussion on the local, regional and global mechanisms and processes that may have triggered the different expressions of the Biogenic Bloom. One of the main advances of this Ph.D. thesis is certainly the discussion on the timing of the Biogenic Bloom, and the conclusion that the onset was synchronous at ca. 8.0 Ma, and the ending appears to be diachronous, with the Biogenic Bloom lasting longer in the Atlantic Ocean, until around 3.0 Ma, compared with the Pacific Ocean (until ca. 4.5-4.0 Ma). The main result obtained through the analysis of benthic foraminiferal assemblages indicates that this prolonged phase of high productivity did not persist consistently for millions of years, but it was marked by high variability and different phases, most likely related to local-to-regional processes. Furthermore, all study sites document an increase in seasonal phytoplankton blooms in surface waters and the consequent thriving of phytodetritus-exploiting taxa (PET) at the seafloor. The PET dominance phase represents a global feature during the Biogenic Bloom, which was most likely related to the local or regional paleoenviromental and paleoceanographic evolution at each particular site, rather than a synchronous response.