Tracking Environmental Change of the Hadal Zone of the Yap Trench: from Variations of Biogeochemical Parameters in the Sediment and Seawater

Series seawater and sediment samples in the hadal zone of the Yap Trench were collected by the Jiaolong Submersible. Various biogeochemical parameters, including pH, alkanility, concentration of dissolved inorganic carbon, dissolved organic carbon, methane, nutrients, organic acids, dimethysulfoniopropionate, carbohydrates, and amino acids in the seawater samples, and concentrations of total organic carbon, different elements, and lipids in the sediment samples were determined. The morphological characteristics of the sediment samples were detected by scanning electron microscope. Various microfossils were identified in different layers of the sediment samples. The results showed that various biogeochemial parameters had complex variation trends in the seawater with water depths. The concentrations of lipids in the surface sediment were the highest with carbon number between 12 to 27. The total concentration of fatty acids in the surface sediment was much higher than those in the sediment from the offshore and deep sea areas, attributed to the funnel effect caused by the “V” terrain of the trench. Fatty acid 18:0 was the most abundant lipids in the sediment sample. Abormal high concentrations of fatty acid 18:1ω7 and alkanes indicated existence of hydrothermal fluids in the study area. The major sources of alkanes and fatty acids was autochthounous input, and the neutral lipids had both marine and terrestrial origin. The sediment samples on the eastern side of the trench were siliceous mud mainly composed of diatoms, radiolarian and sponge needles from surface to deep layer. The water content, total organic carbon content and the concentrations of each element in the sediment samples on the eastern side of the trench had little variation with depth. Based on the correlation of concentrations of the elements, the morphological characteristics of the sediment and the ratios of Fe/Al and Ti/Al, the sediment in the hadal zone of the trench had terrestrial, volcanic, biological and authigenic sources. Major source of the sediment in the eastern side of the trench were terrestrial; whereas the sediment in the western side of the trench received more volcaniclastic input. The depositional environment of the hadal zone in the trench was oxidative.