Paleoceanography and marine ecosystem over 250 ky offshore Southwest Western Australia; Insight from nannofossil assemblages

Abstract

Nannofossil fluctuations during glacial-interglacial cycles reveal paleo-ecosystem responses to climatic and oceanic changes. This study examines nannofossil assemblages from IODP Hole U1516B in the Mentelle Basin (offshore southwest Western Australia (SWWA)), a region influenced by the Western Australian Current (WAC), Leeuwin Undercurrent (LUC), Leeuwin Current (LC), and its eddies. Using nannofossil counts, diversity indices, and organic geochemistry over 250 ky, this study assesses how circulation shifts impacted communities and productivity. Results reveal LC dominance through its eddies, extending >200 km westward offshore at Cape Leeuwin, unlike the 100 km width in offshore Western Australia. During glacials, LC weakened, assessed by small placoliths blooms, although persistence of Florisphaera profunda indicates weak LC influence. Total organic carbon (TOC) and nitrogen (TN) exhibit a moderate correlation, with slightly higher glacials TOC values reflecting enhanced productivity. Carbon‑nitrogen ratios (C/N > 12) suggest mixed marine-terrestrial organic matter. Nannofossil abundance declines during glacials, likely due to aeolian or high terrigenous input, while diversity rises, indicating evenly distributed communities. Interglacials show high abundance but low diversity, dominated by F. profunda. These trends link LC strength to ecosystem structure, where during interglacial LC suppresses upwelling, favoring oligotrophic taxa like F. profunda, whereas during glacials, LC weakening permits WAC and LUC-driven mixing, boosting productivity and diversity. These findings emphasize how oceanic circulation and productivity shifts, driven by glacial-interglacial cycles, structured historical ecosystems. Similar dynamics may influence future ecological responses to climate change, particularly as warming alters current systems.