Authigenic carbonates of the South China Sea Archives for geochemical cycling and geodynamic activity

Abstract

Cold seeps are areas where hydrogen sulphide, methane and other hydrocarbon-rich fluids occur at the ocean floor and produce specific seafloor features including chemosynthetic communities and authigenic minerals precipitation. Mostly present at continental margins, seepages characterise with various features responding to the influence of tectonic, climate and gas hydrate dynamics at different locations. Therefore, the evolution of the seep area and its influence of diagenesis provide archives of geochemical cycling and geodynamic activities. The formation of gas hydrates sequesters methane gas in the subsurface of seep areas under critical low temperature and high pressure conditions. Authigenic carbonate is one of the most common manifestations at seep sites, in which the precipitates provide the chronology of seepages and environmental archives. Therefore, it is ideal for inferring evidence to resolving geochemical cycling and geodynamic activities at cold seep. In deep water setting (>1000 mbsl), gas hydrates are predominantly stable, active seepages are contributing the critical amount of methane. Active seepages in deep water are under presumably less climatic influence such as sea level and temperature variations. However, factors that control seepages and the evolution of long-term seepages under such environmental conditions are poorly constrained. This study aims at investigating the processes that govern the activity and evolution of cold seeps on active margin and their potential as environmental archives by studying the morphology of Yam Seep and its authigenic carbonates. The seep area is located at Four-Way Closure Ridge, offshore southwestern Taiwan, which is an accretionary ridge at the active margin between the Eurasian Plate and the Philippine Sea Plate. This study is based on data from two research cruises (RV SONNE SO266 and RV Ocean Researcher I OR1-1163). They were obtained using inter-disciplinary investigations include: (1) shipborne and autonomous underwater vehicle hydroacoustic seafloor and water column echosounder (2) visual observations system, (3) coring and drilling samplers e.g. gravity core and drill rig MeBo200 and (4) water sampling. In addition, various analyses such as U-Th radiometric dating, stable isotope analyses, minerology analyses, microscopic petrography analyses, gas analyses were carried out after the cruise on selected samples. Results of this study are presented in three manuscripts and a report. In the first manuscript, high-resolution hydroacoustic bathymetry, backscatter map and TV-sled observations revealed a 49,000 m2 carbonate paved area called Yam Seep. The entire area is covered by a heavily fractured methane-derived carbonate crusts and build-ups indicating long-term influence of seepages on seafloor morphology. Representing current active seepage, hydroacoustic anomalies so-called “flares” that indicate gas bubbles in the water column can be compiled with high methane concentration (up to 18 nM) in the water column and abundant chemosynthetic communities within carbonate fractures. The inter-disciplinary investigation of the Yam Seep demonstrated that its discharging at the seafloor and seabed properties can be attributed to continuous uplifting of Four-Way Closure Ridge. The second manuscript presents the petrography and stratigraphy of a 5-meter-long carbonate core that distinguishes three distinct microfacies in which the sediment consolidated 1) micrites are intercalated by 2) clear aragonites and 3) grey aragonites (intercrystal of different aragonite forms, not discussed in the second manuscript) in the entire core. Uranium thorium (U-Th) dating on clear aragonites and shells reveals a persistent seepage over a time-span ranging from ~40 to 2 ka. Intercalated clear aragonites also present a general precipitation progress of cementing the fractured micrites, in which the oldest aragonites occur at fractured boundaries or unconformities. Dating on the fractured boundaries and unconformities suggests a cycle of tectonic and seepage activities that consists of an active period from ~37 to 27 ka, a relatively quiet period from ~27 to 16 ka and another active period from ~16 to 12 ka. The third manuscript aims at confining the characteristic of seep aragonite isotopic archives that revealed the interplay between seawater, gas hydrate and seepage methane using δ13C, δ18O and δ234U isotopes. Seep aragonite δ234Ui is a back calculated value that represents the seawater δ234U during seep aragonite precipitation. Seep aragonite δ234Ui archives represent the paleo seawater indicating the sea-water like archive in seep pure aragonites. Seep aragonites δ18O values present a trend following a paleo seawater profile with a generally depleted variability. Depleted values suggest the influence of gas hydrate formation environment and fractionation differences between different aragonite forms. Seep aragonites δ13C variations represent (1) the mixing of δ13C of seawater dissolved inorganic carbon (DIC) and seepage methane and (2) a decoupling of isotopic archives during aragonite precipitation. The last part of the thesis is a report presenting significant results of computer tomography scanning on the 5-meter carbonate core revealing the porosity variations within the consolidated rock. Pores within MeBo-24 present two distinct intensities which represent open pores and filled pore in which lateral precipitation grows within open pores without completely cementing the pore space. The scanning reconstructing porosity of carbonate presents a systematic trend correlating to the microfacies observed in the second manuscript. Therefore, this study provides useful evidence for reconstructing gas bubbles migration and gas hydrate formation in the sediments in the earliest stage. Yam Seep’s morphology, authigenic carbonates chronology, and the seep aragonites archives provide an phenomenal record of ~40,000-year-old long-term seepage under tectonic influenced in the deep water area and a new insight of seep aragonites environmental archives.