Active sediment transport along trench axis: insights from X-ray fluorescence core scanning and magnetic analysis of marine sediments in the southwestern Ryukyu Trench

Abstract The southwestern Ryukyu Trench is an ideal place for investigating sediment transport from the Taiwan mountain belt to the Ryukyu Trench floor. To study the characteristics of trench turbidites and sediment transport along the trench, we utilize two piston cores: KR1518-PC04/PL04 at the southwestern end of Ryukyu Trench and YK1501-PC14/PL14 in the trench floor. We also collect two push cores, YK1611-6K1467MG and 6K1467MR, from undisturbed seabed surface to observe lithology. Our analysis reveals that the lithology of the cores shows homogeneous gray mud layers intercalated with very-thin fine sand layers, which we interpret as hemipelagites intercalated with very-thin turbidites. We identify 60 turbidites (0.3–4.0 cm thick) from YK1501-PC14 and 36 turbidites (0.9–4.2 cm thick) from KR1518-PC04. Based on observation from YK1501-PC14, most turbidites, which were less than 1 cm in thickness, displayed planar and sharp bottom contacts. The turbidite thickness of YK1501-PC14 (6448 m water depth) is thinner than that of KR1518-PC04 (6147 m water depth) in general. Through X-ray fluorescence core scans (ITRAX), we distinguished trench turbidites by elemental patterns and accurately determined the depths of every very-thin turbidite. Most hemipelagites from YK1501-PC14 and KR1518-PC04 show similar ITRAX profiles, indicating a similar source of the sediments. ITRAX analysis also revealed five intervals in high-Ca mud in YK1501-PC14, suggesting another potential source from the Ryukyu forearc. Anisotropy of Magnetic Susceptibility (AMS) results show that both YK1501-PC14/PL14 and KR1518-PC04/PL04 exhibit an oblate fabric indicating a normal sedimentary condition. Magnetic signatures showed the presence of pyrrhotite, supporting evidence that Taiwan-sourced sediments can be transported up to ~ 250 km by turbidity currents and spread from the Taiwan Island to the Ryukyu Trench floor. This study improves our understanding of Ryukyu Trench turbidites and provides valuable insights into active sediment transport along the southwestern Ryukyu Trench.