Oxygenated bottom water conditions on Broken Ridge (central Indian Ocean) in the last 9 million years

The late Miocene and early Pliocene were marked by widespread oxygen depletion in the Pacific and Indian Oceans in conjunction with the so-called "Late Miocene Biogenic Bloom" (LMBB). As the oxygen depletion progressed, it was accompanied by an oxygen minimum zone (OMZ) expansion, hypothesized to extend as far south as Broken Ridge (31°S), Indian Ocean. Here, we used benthic foraminiferal (BF) assemblage from Ocean Drilling Program (ODP) Site 752 (ODP Expedition 121) to reconstruct the bottom water dissolved oxygen concentration on Broken Ridge during the last 9 million years. Taxonomic analyses on the >63μm fraction allowed the identification of 93 BF species and reported as benthic foraminifera row counts. The relative frequencies derived from BF row counts were used to determine diversity using the Fisher Alpha index and dissolved oxygen concentration was reconstructed using the Enhanced Benthic Foraminifera Oxygen Index. Benthic foraminifera with abundances below 5% across all samples were removed before statistical analysis to retain a robust signal of BF distribution without single and scattered occurrences. High abundance BF was reported in the grouped benthic foraminifera counts. Our results indicate well-oxygenated (dissolved oxygen concentration >3ml/L) bottom water conditions in the last 9 million years.