Microplastic occurrence in the deep-sea sediment of the Indonesian Throughflow, Banggai Sea

• This study represents the first investigation of microplastic pollution in deep-sea sediments within Indonesian waters. • Strong negative correlation was found between microplastic abundance and distance from the coast ( r = −0.88845, p = 0.0219). • Polymer analysis identified PET, PE, LDPE, and PE. • Spatial analyses using Principal Component Analysis (PCA) and Agglomerative Hierarchical Clustering (AHC) identified potential pollution sources and suggests ITF role in the distribution and transport of microplastics into the Banggai Sea. The potential for floating microplastics to sink and accumulate in marine sediments makes deep seabed as the final deposition sites. While deep-sea microplastic studies have been conducted globally, none have previously focused in Indonesian dee-sea, particularly along the Indonesian Throughflow (ITF) pathways. This study investigated the abundance and characteristics of microplastics in deep-sea sediments of the Banggai Sea, one of the key ITF routes. Sediment samples were collected using a box corer, followed by organic matter digestion and density separation for microplastic extraction. Visual identification was conducted under a microscope, assisted by the hot needle test, and polymer types were confirmed using Raman spectroscopy. Microplastic abundance ranged from 200 to 866.67 particles/Kg (dry weight), with an average of 533.33 ± 355.28 particles/Kg. Although there were no significant differences in abundance among stations (Kruskal-Wallis, p = 0.05027), a strong negative correlation was found between microplastic abundance and distance from the coast ( r = -0.88845, p = 0.0219). Fiber-shaped and transparent microplastics were dominant, and the polymers identified were PET, PP, LDPE, and PE. These characteristics are consistent with those reported in other Pacific Ocean regions, suggesting that oceanic circulation, including the Pacific-ITF inflow, may contribute to the distribution and transport of microplastics into the Banggai Sea.