Paleoceanography of the South China Sea since the middle Miocene: evidence from planktonic foraminifera

Abstract

Late middle Miocene to Pleistocene planktonic foraminifera (PF) in fairly continuous hemipelagic sediments recovered at Ocean Drilling Program (ODP) Site 1143 from the southern South China Sea (SCS) were studied with sampling intervals of 10 to 150 cm. A total of 16 PF events are revised for the 516-m long section, providing an age model for the last 12 Ma at this site. On the basis of benthic foraminiferal δ 18O stratigraphy and the smoothed planktonic age model, new ages are assigned to three PF events (last occurrence [LO] Globorotalia multicamerata sensu lato [s.l.] at 2.18 Ma, first occurrence [FO] Sphaeroidinella dehiscens sensu stricto [s.s.] at 3.6 Ma, and FO Globigerinoides conglobatus at 6.6 Ma) in the South China Sea. Distinct PF faunal transition from Globorotalia siakensis– G. mayeri group to mixed-layer species occurred during the early late Miocene (∼9.6 Ma). The transition reflects a deepened upper water thermocline, possibly corresponding to the closure of the Indonesian seaway and intensification of equatorial Pacific warm currents. Abundance variations of Neogloboquadrina spp. indicate a marked shoaling of the thermocline in the northern South China Sea after 3–2.5 Ma affected possibly by a strengthened Asian winter monsoon, while a deeper thermocline existed in the southern South China Sea since the late Miocene, characterizing the west Pacific “warm pool” (WPWP). A deepening thermocline influenced by consistently warm surface water since the late Miocene in the southern South China Sea is also evidenced by a two-stepwise increase of Pulleniatina group, by a gradually decreasing Sphaeroidinella– Sphaeroidinellopsis spp. and abundant Globorotalia menardii and related species. The FO Globigerinoides ruber (pink) at approximately 1 Ma and an abrupt increase in the pink-to-all ratio of G. ruber at 0.4 Ma at Site 1143 provide two additional PF events for deciphering Pleistocene biostratigraphy and paleoceanography.