Geochemical characteristics of sediments in the southwestern part of central oceanic basin of the East Vietnam Sea

Le Duc Anh1,*, Nguyen Nhu Trung1, Nguyen Hoang3, Ngo Xuan Thanh2, Ngo Bich Huong1,
Mai Duc Dong1, Renat Shakirov3, Trinh Hoa Thu1, Bui Van Nam1
Received: 3 September 2020; Accepted: 26 November 2020

Abstract
The Southwestern part of the Central Oceanic Basin of the East Vietnam Sea is the transition zone between Southeast continental shelf and the East Sea oceanic crust. The topography of the seabed in the study area is characterized by a bottom divergence zone, subsidence along the Northeast - Southwest spreading ridge axis with the depth ranging from 300 m to 4200 m. The results of factor analysis according to the chemical composition of sediment samples show seven separate correlation groups. At the continental shelf and continental slope, sediments are characterized as terrigenous sediments rich in carbonate materials and organism body fragments. On the top, bottom and slopes of seamounts, the sources of sedimentary materials is mainly weathered products from terrigenous rock. The enrichment coefficient of main elements in sediments is similar to that of terrigenous sediments. However, the enrichment coefficient of trace elements is quite complicated. The increment and decrement of the enrichment coefficient of trace elements result from chemical reaction of exchanging material composition between marine sediments and the surrounding environment. The top and bottom of seamounts indicate the source of terrigenous materials, the sedimentary environment less affected by carbonate accumulation, and the decrement of trace element composition, which are favorable prerequisites for the formation of ferromanganese crusts and nodules.The Southwestern part of the Central Oceanic Basin of the East Vietnam Sea is the transition zone between Southeast continental shelf and the East Sea oceanic crust. The topography of the seabed in the study area is characterized by a bottom divergence zone, subsidence along the Northeast - Southwest spreading ridge axis with the depth ranging from 300 m to 4200 m. The results of factor analysis according to the chemical composition of sediment samples show seven separate correlation groups. At the continental shelf and continental slope, sediments are characterized as terrigenous sediments rich in carbonate materials and organism body fragments. On the top, bottom and slopes of seamounts, the sources of sedimentary materials is mainly weathered products from terrigenous rock. The enrichment coefficient of main elements in sediments is similar to that of terrigenous sediments. However, the enrichment coefficient of trace elements is quite complicated. The increment and decrement of the enrichment coefficient of trace elements result from chemical reaction of exchanging material composition between marine sediments and the surrounding environment. The top and bottom of seamounts indicate the source of terrigenous materials, the sedimentary environment less affected by carbonate accumulation, and the decrement of trace element composition, which are favorable prerequisites for the formation of ferromanganese crusts and nodules.
Keywords: East Vietnam Sea, Southwest sub-basin, sediment, ferromanganese crusts and nodules.