Miocene paleoenvironmental evolution based on benthic foraminiferal assemblages in the Lufeng Sag, northern South China Sea

摘要

The Miocene epoch marks the most crucial period during the Cenozoic cooling trend, characterized by the Middle Miocene Climatic Optimum(MMCO) and a series of short–lived cooling events(Miocene isotope events).To understand the paleoenvironmental evolution along the shallow water shelf in the South China Sea during the Miocene, the benthic foraminiferal assemblage and total organic carbon content(TOC) were analyzed at Hole LF14 located in the Lufeng Sag, northern South China Sea. Three benthic foraminiferal assemblages(e.g., the Uvigerina spp. assemblage, the Cibicides spp. assemblage, and the Cibicidoides spp. assemblage), corresponding to different watermass conditions, were recognized based on Q–mode factor analysis. Early studies suggested that Hole LF14 was deposited under semienclosed bay, middle to outer shelf or even upper bathyal environment during ～18.7–4.53 Ma. The dominant Uvigerina spp. assemblage was characterized by low diversity and shallow infaunal to infaunal species, indicating a warm, low–oxygenation and eutrophic conditions since the Early Miocene to MMCO(～18.7–14.24 Ma). An abrupt sea level drop and significant faunal changes were recorded during 14.24–13.41 Ma, suggesting development of the East Antarctic Ice Sheets, which resulted in a drop of sea level and change in benthic foraminiferal assemblages along the shallow water shelf. Beyond the Uvigerina spp.assemblage, the Cibicides spp. assemblage became important during the middle–late Middle Miocene(14.24–11.54 Ma). This assemblage was dominated by epifaunal species with relative high diversity, suggesting high–energy, high–oxygenation and oligotrophic conditions with episodic supply of organic food. The dominant Cibicidoides spp. assemblage with high diversity, indicates a mesotrophic conditions with relative high–oxygen content during the Late Miocene to Pliocene(11.54–4.53 Ma). The appearance and continuous occurrence of Ammonia spp. and Pseudorotalia spp. since 10.02 Ma, may reflect the influence of the Kuroshio Current.