Characteristics and Significance of Trace Fossils in Late Miocene Deep-Sea Volcaniclastic Sediments in the Central Basin of South China Sea

Volcaniclastics is one of the important compositions of deep-sea sediments produced by volcanic eruptions. It is with great research values for studies on paleoenvironmental evolution in the deep-sea volcanic environment. The purpose of this work is to reveal the pattern of the Late Miocene benthic environment revolution in the central basin of the South China Sea (SCS) by analyzing the trace fossil assemblages in the volcaniclastic sequence drilled at site U1431 during the International Ocean Discovery Program (IODP) expedition leg 349. The study shows that trace fossils were developed well in siltstone and claystone during the Middle-Late Miocene (12.5–7.4 Ma, mainly in Late Miocene), and were dominated by Zoophycos,Thalassinoides,Chondrites, and Planolites.The volcanoes in the central basin of the SCS experienced repeated alternations of eruption and quietness during Late Miocene. The trace fossil assemblage in site U1431 implies that the basin is a quiet deep-sea environment with sedimentary rate varying between 5–15 cm/ka during volcanic activity quiet periods. By analyzing the associations between the trace fossil assemblages and the lithologic stratigraphic features, the study shows that the composition of the trace fossil assemblage is limited by substrate lithology and the dissolved oxygen concentration in the water masses at the sediment/water interface. The study also suggests that during eruptions, submarine volcanic activities can potentially provide nutrients for surface seawaters, and these nutrients can promote phytoplankton growth and subsequently increase the supply of organic carbon to the seafloor. The organic carbon on the sea floor then may consume the dissolved oxygen in the sediment pore water, making it unfavorable to the activities of the benthic organisms. In contrast, during volcanic eruption quiet periods, flux of organic carbon to the seafloor possibly reduced. Consumption of the dissolved oxygen in the pore water thus reduced, and the growth of the benthic organisms subsequently developed.