Abstract
Cold seep carbonates are important archives of pore water chemistry and ancient methane seepage activity. They also provide an important contribution to the global carbon sink. Seep carbonates at three sediment layers (3.0, 52.1, and 53.6 mbsf) were collected at site W08B in the Qiongdongnan Basin of the South China Sea. This study investigated the mineralogy, microstructure, stable carbon and oxygen isotopes, trace elements, and U-Th dates of these carbonates to identify the relationship between methane flux and authigenic carbonate precipitation. The results showed that the δ13C and δ18O values of all carbonates are similar, indicating that the carbon source for shallow carbonates and deep carbonates has remained constant over time and included biogenic and thermogenic methane. Although carbonates were found in three sediment layers, the two main stages of methane seepage events were discernible, which was likely caused by the dissociation of gas hydrates. The first methane seep took place at 131.1–136.3 ka BP. During a dramatic drop in the sea level, the seep carbonate at 52.1 mbsf formed at 136.3 ka BP through the anaerobic oxidation of methane (AOM). The carbonate at 53.6 mbsf resulted from the vertical downward movement of the sulfate-methane transition zone with decreasing methane flux at 131.1 ka BP. This is the reason for the age of carbonates at 52.1 mbsf being older than the age of carbonates at 53.6 mbsf. The second methane seep took place at 12.2 ka BP. Shallow carbonate formed at that time via AOM and is now located at 3 mbsf. Moreover, thin-section photomicrographs of deep carbonate mainly consisted of matrix micrite and biological debris and acicular aragonite occurred as vein cement filling the pore spaces between the matrix micrite. The acicular aragonite was mainly influenced by the timing of the carbonate precipitation of minerals. This research identified a long history of methane seep activity reflected by the vertical distribution of carbonates.
Highlights
IntroductionA deep-sea cold seep is a unique environment where deeply sourced fluids (containing methane and other hydrocarbon gases) migrate upward and seep from the seafloor [1]
A deep-sea cold seep is a unique environment where deeply sourced fluids migrate upward and seep from the seafloor [1]
The thin-section photomicrographs of deep carbonate mainly consist of matrix micrite and biological debris (Figure 3)
Summary
A deep-sea cold seep is a unique environment where deeply sourced fluids (containing methane and other hydrocarbon gases) migrate upward and seep from the seafloor [1]. Unlike hydrothermal systems, which are mainly distributed in mid-ocean ridges [4,5], cold seeps are widely dispersed throughout the active and passive continental margins. A special chemosynthetic ecosystem [13], widely distributed carbonate on the seafloor [14], and gas bubble emission from the seafloor [15,16] are important indicators for the identification of marine cold seeps. It is of great significance to study the distribution and activity of these cold seeps because they are used as indicators for deep-water petroleum exploration at the early stage [17], but are closely related to global climate change [18], ocean acidification [19], and the global carbon cycle [20,21]
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call