RESPONSE OF DEEP SEA BENTHIC FORAMINIFERA ACROSS THE EOCENE - OLIGOCENE BOUNDARY OF THE SOUTHERN OCEAN

Abstract

The Cenozoic Era was an eventful interval as the earth experienced many episodes of tectonic, climatic, and oceanographic changes. The Eocene-Oligocene Transition (EOT) witnessed a significant changeover from greenhouse to icehouse climatic conditions. These climatic fluctuations are believed to be caused by the rearrangement of tectonic plates and the change in global ocean water circulation. The Southern Ocean (SO) is crucial to monitoring these changes in the deep ocean environment as it connects three main ocean basins (Pacific, Atlantic, and Indian Oceans) and, consequently, atmospheric pCO2 flux and ocean heat budget. The climatic changes are primarily caused by the opening of different gateways around the Antarctic (e.g., the Drake Passage and the Tasman Passage). The Drake Passage (DP) opening is widely considered to have led to the Antarctic Circumpolar Current (ACC) formation. Many proxies are used to decipher the exact timing of the opening of the Drake Passage, but micropaleontological evidence is vital. However, the timing and associated biotic effects of the opening of the DP remain poorly constrained. Understanding the impact of climate change on the evolution and ecology of organisms is essential for preventing future biodiversity extinction. However, establishing the relationship between climate to ecological and evolutionary patterns of organisms is challenging. So, studying biodiversity dynamics at micro-evolutionary changes through time provides information to understand past biotic responses to climate change and predict biotic responses to future climatic change. In this study, deep-sea benthic foraminifers are used to constrain the paleoceanographic change with the help of abundance and geochemical data. The author has selected the Kerguelen Plateau (KP), a perfect location in the Southern Ocean that comes on the way of the ACC along with the DP’s latitude. The Ocean Drilling Program (ODP) Hole 1138A contains a long and continuous archive of marine sediments from the late Cretaceous to the Recent age. This study uses an age range from the middle Eocene to the early Oligocene to understand the past oceanic and climatic changes based on the benthic foraminiferal response. The author has processed 266 core samples of 10 cubic centimeters (cc) volume from the core number 34-R1-(1-3cm) to 37- R7-(20-22cm), spanning a depth of ~38m. For the qualitative and quantitative analysis and to observe the species diversity patterns, the size fraction of 125μm was split and picked approximately 300 specimens of benthic foraminifera, while for geochemical studies, the author took a 250μm to 350μm sieve size fraction. Using morphological details of specimens, the author identified 143 species of deep-sea benthic foraminifera and a detailed systematic taxonomic representing 57 genera, 39 subfamilies, 31 families, 20 superfamilies, and 5 suborders. This study is the first comprehensive taxonomic illustration of the deep-sea benthic foraminifera with highresolution photographic (SEM) images from the ODP Hole 1138A. The recovered benthic foraminifera have been identified in different biozones and established biostratigraphy based on LO or FO of geographically widespread species of benthic foraminifera with a newly updated chronostratigraphic chart (GTS 2012). The author proposed three biostratigraphic Zones: Nuttallides truempyi interval zone (base age after Pandey, 2019, i.e., 40.5 Ma to 37.02 Ma); Cibicidoides truncanus interval zone (37.02 Ma to 33.9 Ma); and Astrononion echolsi interval zone (33.9 Ma to an undefined top). The author has calculated the relative abundance of dominated species and different diversity indices to observe the response of benthic foraminifera to climate change. A faunal turnover has been noticed from the middle Eocene (~39 Ma) to the Eocene-Oligocene Boundary (33.9 Ma). A faunal response also has been observed during the early Oligocene Glacial Maxima (EOGM), also called the Oi-1 event (33.7 Ma to 33.5 Ma), at Oi-1a (32.8 Ma), and Oi-1b (31.7 Ma). This faunal turnover from the middle Eocene to the early Oligocene is believed to be caused by the change in the deep-water masses (SCW/Proto-ACC) after the opening of the DP and the onset of the Antarctic glaciation. A Q-mode factor analysis of the benthic foraminiferal species data has been performed using Principal Component Analysis (PCA), followed by a VARIMAX rotation in the SPSS statistical package to group the faunal assemblage. The faunal assemblages give the impression of the different ecological preferences of the benthic foraminifera species. The following assemblages are recorded - Cibicidoides kullenbergi (Ck), Nuttallides truempyi (Nt), Astrononion echolsi (Ae), Oridorsalis umbonatus (Ou), and Nuttallides umbonifer (Nu) assemblages. These assemblages indicate different ecological preferences; for instance, Nuttalides truempyi, which dominated the assemblages of the middle Eocene, was allied to relatively warm oligotrophic bottom water conditions, while the late Eocene was allied to O. umbonatus assemblages suggesting the diverse deep environment to cold, carbonate corrosive deep water mass and oligo-mesotrophic conditions. Cibicidoides kullenbergi (Ck) assemblage indicates the oxygen-rich and high nutrients in deep marine environments and a low carbon influx to the seafloor. Astrononion echolsi (Ae) assemblage shows the cold and well-oxygenated environment. Nuttallides umbonifer (Nu) assemblage are allied with cold, carbonate corrosive water mass of Antarctic origin. The opening age range (50 to 23 Ma) of the DP coincides with our studied age interval. The stable isotope excursion shows the timing of the opening of the Drake Passage from the studied site at ~39 Ma, where the oxygen isotopic (δ18O) excursion of the benthic foraminifer is positively recorded. The response in the relative abundance of benthic foraminifera of some species also shows positive excursion at ~39 Ma and ~37 Ma. Isotopic values become more or less constant after ~39 Ma, interpreted here as evidence of surface water transfer by ~39 Ma and a fully open DP for the bathyal depths by ~39-37 Ma, as indicated by sharp changes in benthic foraminiferal diversity and assemblages. Change in the carbon isotopic (δ13C) values indicates the replacement of older warm bottom water by the cold bottom water in the Southern Ocean. So, from the results, it can interpret that the opening of the DP for the deeper depths starts at ~39 Ma, and fully open and intensified ACC is established at ~37 Ma. In contrast, the Oligocene excursions are related to the fluctuations in the atmospheric CO2 recorded by the deep-sea benthic foraminifera during this studied interval. So, from the results, it can interpret that the change in deep sea conditions and the formation of three different water masses formed due to the opening of the DP. 1) Warm water mass and low food supplies: Nuttallides truempyi assemblages indicate warm water and low food supply conditions. This condition is formed due to the arrival of a low latitude water resource known as Warm Saline Deep Water (WSDW) at the bathyal depth sites during the middle Eocene. So, the surface and local productivity also decrease as low food supply. 2) Cold, corrosive carbonate deep water mass: Oridorsalis umbonatus and Nuttallides umbonifer assemblage show the appearance of cold, carbonate deep water mass (i.e., proto-AABW) at the high latitudes due to the opening of different seaways (the DP and the TG). This cooling starts at ~39 Ma while fully opened DP and well-established the ACC are recognized at ~37 Ma. 3) Cold water mass and high surface productivity: Astrononion echolsi assemblage shows the intensified cold water and formation of large ice sheets around the Ataractic. The ephemeral ice formed at ~37 Ma, but an intensively large ice sheet formation started after EOT. This cold water brings high nutrients and oxygen-rich deep water mass, which increases surface productivity.