Coupling of the Central Equatorial Indian Ocean with Indonesian Throughflow and its impact on climatic conditions and productivity during the Early Pliocene

The Indian Ocean’s surface circulation undergoes strong seasonal reversals driven by monsoonal winds, producing alternating periods of nutrient-rich upwelling (southwest monsoon) and low-productivity stratified waters (northeast monsoon). This study reconstructs Early Pliocene (4.5–3.6 Ma) oceanographic conditions in the Central Equatorial Indian Ocean (CEIO) using planktic foraminiferal assemblages from ODP Hole 716A to investigate changes linked to Indonesian Throughflow (ITF) variability, monsoon strength, and productivity. Foraminiferal habitat groups (mixed-layer dwellers, Thermocline dwellers, Eutrophic and Oligotrophic Indicator) and Principal Component Analysis reveal repeated transitions between oligotrophic (nutrient-poor) and eutrophic (nutrient-rich) conditions. Warm mixed-layer species dominate during several intervals (3.66–3.72 Ma, 3.81–4.0 Ma, 4.15–4.24 Ma), indicating enhanced stratification, reduced nutrients, and strengthened northeast monsoon conditions. In contrast, peaks in thermocline dwellers and productivity-related species (3.72–3.78 Ma) reflect episodes of upwelling driven by Wyrtki Jet activity. The delayed first appearance (~3.6 Ma) of Pulleniatina obliquiloculata in the Indian Ocean and the absence of Pulleniatina spectabilis throughout 4.5–3.6 Ma indicate restricted intermediate-water exchange between the Pacific and Indian Oceans, consistent with a partially closed ITF. Overall, Early Pliocene ITF restriction likely reduced Pacific water inflow, shoaled the thermocline, lowered productivity, strengthened the northeast monsoon, and contributed to the decline of the regional biogenic bloom. These findings demonstrate that Early Pliocene ITF restriction played a key role in modulating Indian Ocean hydrography and monsoon variability during a globally warm climate regime.