The late Neogene to Quaternary surface water changes as responses from planktonic foraminifera at the transitional Subantarctic Zone

Abstract

A well preserved and continuous fossil record of planktonic foraminifera could provide insights into global paleoceanographic and surface water changes. We utilize a ~10-million-year record of planktonic foraminifera along with biogenic proxies and stable isotopes from the Ocean Drilling Program (ODP) Site 1088 Hole B, to understand the late Neogene to Quaternary faunal evolution and surface water changes happening in the Subantarctic Zone. Organic carbon and CaCO3 proxy records suggest high productive surface waters linked to the biogenic productivity high in the Indian Ocean between ~9.4 and 8.1 Ma. The planktonic foraminiferal fossil record between ~7.1 and 6.2 Ma shows high relative abundance of surface-dwelling faunas Globigerina bulloides, Globigerinita glutinata and Neogloboquadrina pachyderma simultaneously with high CaCO3 content. The evolution of deep dwelling faunas especially the globoconellids can be tracked between the late Miocene to middle Pliocene as a consequence of deeper export in nutrients and organic matter. At site 1088 the evolution within the Globoconella inflata lineage coincides with changes in hydrographic conditions. The Miocene – Pliocene transition (~6 to 4.5 Ma) marks the appearance of Globorotalia crassaformis and Neogloboquadrina dutertrei at the site. The warmer middle Pliocene assemblage between 4.6 and 3 Ma shows higher abundance of Globoconella inflata and Globoconella puncticulata. An inverse relationship between the polar fauna Neogloboquadrina pachyderma and Globoconella inflata, typical of low to mid lattitudes, suggest a southward position of the subtropical and subantarctic fronts during the middle Pliocene. Enriched organic carbon and ẟ13C values at Site 1088 indicate the presence of nutrient rich surface waters between 4.2 and 4.0 Ma and 3.4 to 3.1 Ma. Transition to a much dynamic Pleistocene saw glacial interglacial variability among the dominant planktonic foraminifera at the site indicating varying surface water conditions controlled by the movement of hydrographic fronts. Our study shows how the large changes in global climate, ocean circulation, nutrient enrichment and frontal movements influenced the abundance, occurrence and evolution of surface and deep dwelling planktonic foraminifera over the last 10 Ma.