Biogenic silica diagenesis under early burial in hemipelagic marine sediments

Four aspects of biogenic silica diagenesis are researched under this project: signatures of ongoing silica diagenesis in pore water, drivers leading to anomalous compaction (abrupt petrophysical changes), bottom-water temperature controls on silica dissolution and recycling, and temperature-time relationships in silica later diagenesis. Results of this project are discussed in Chapters 4–7. Chapter 4 demonstrates, based on pore chemistry, mineralogy, and thermodynamic analyses, that solubility equilibrium is reached for diagenetic silica in pore water at opal-A to opal-CT transition zone captured by Ocean Drilling Program Sites 794 and 795 in Japan Sea; thus opal-CT is precipitating across the transition. It is also argued that ion transport mechanisms have failed to erase signatures of ongoing diagenesis. Chapter 5 utilises texture and mineralogical analyses of hemipelagic sediments from Sites 794 and 795 to place anomalous compaction across the transition zone in a diagenetic context. A sharp decrease in opal-A content in the transition under dissolution causes a significant decrease in sediment stability which results in a porosity drop and other petrophysical variations. Subsequent precipitation of opal- CT has, however, had far less effects on porosity. Chapter 6 models variations in silica dissolution and recycling with bottom-water temperatures using data from 22 representative Ocean Drilling Program sites. Though model expression of early diagenesis confirms a relationship between temperature and opal-CT formation in young sediments (L 4 Ma) near the seabed in the Antarctic, the established inverse temperature-time correlation in silica diagenesis contradicts main controls from low bottom temperature over a short geological time. Chapter 7 investigates temperature-time relationships in silica diagenesis by examining 67 scientific drill sites where the opal-A to opal-CT transition zone lies in Cenozoic sediments. Based on these constraints, the reference opal-CT stability field is modified. The chapter also applies a model that formulates time-temperature dependence of silica diagenesis in representative Sites 794 and 795.