| Located between the largest ocean(Pacific)and largest continent(Eurasia),the South China Sea(SCS)has accumulated the important Cenozoic deposit,which contains the information of evolution of regional tectonics,climate and sedimentary environment.The SCS are also the keys to understand the uplift of the Tibetan Plateau,the closure of the Indonesian Seaway and the Asian monsoon dynamics.Since the semi-pelagic deposits of the SCS have the characteristics of high sedimentarty rate and well-preserved carbonates,which are benefit the high-resolution paleoceanographic research,even on their glacial/interglacial fluctuations.The South China Sea has become one of the hotspots of paleoceanography and sedimentology in the world,with large amount of the cruise for scientific investigations.In 2017,International Ocean Discovery Program(IODP)Expedition 368 conducted a series of operations in the northern margin of the SCS.A alternation of brownish gray to greenish gray intervals were found at Site U1502 at the Middle-Late Miocene sections.In this paper,the Core 10R and Core 29R Section 5W(29R5W)to Core 30R Section 6W(30R6W)at Hole A of Site U1502 were adopted for benthic foraminiferal identification and assemblage analysis,to reconstructed the properties of bottom water mass and the mechanism of green-brown lithological cycles in the clay sediments during the middle-late Miocene.The System taxonomy of the Middle-Late Miocene benthic foraminifers is was studied at Site U1502.A total of 29,041 benthic foraminifera belonging to 225 species and 78 genera were identified from 134 samples of Hole U1502A.Benthic foraminifershave a high abundance in Section 10R1W,and a relatively lower abundance in Sections 29R5W to 30R6W.Epistominella exigua,Nuttallides umbonifera,Globocassidulina subglobosa,Gyroidinoides orbicularis,and Oridorsalis umbonata are dominant in these two sections,implying a deep-water(abyssal)environment.Shallow-water species appear occasionaly in the upper section representing a turbidite source,especially in the greenish layers,while their abundance is reduced or barren in the lower sections.The alteration of brownish gray and greenish gray sediments in Section 10R1W is characterized by a compative thickness of both components.While sediments in Section 29R5W-30R6W are dominated by the thicker brownish gray layers,compared with the thin greenish ones.Planktonic foraminiferal fragmentation and benthic foraminiferal relatively abundance,U+B index and contents of dysoxic/oxic benthich foraminiferal species are used to disclose the variations of carbonate dissolution and dissolved oxygen content of bottom water,and sea surface productivity in the northern SCS during the Middle-Late Miocene.The brownish gray sediment may represents a stronger carbonate dissolution and high content of dissolved oxygen of the bottom water or a relatively lower sedimentation rate,and a lower sea surface productivity.While,the greenish gray sediment might reflect a bottom water with comparatively well-preservation of carbonate,lower content of dissolved oxygen,and possibley from faster deposit,such as the turbidites. |
