- Quantifying the mass transport through marine sediments, and the geochemical response to such flow with numerical models has become a common and powerful approach for geochemical data interpretation. In this dissertation, I developed and applied transport-reaction models to unravel complex and interdependent reactions involving carbon, sulfur and silica transformations in shallow marine sediments, and the impact of physical (mass transport deposits) and depositional events (volcanic ash input) on the overall geochemical state of the system.
Carbon cycling in the gas hydrate bearing sediments of the Ulleung Basin was quantified using both box and kinetic modeling approaches. The box model balances mass, flux, and carbon isotopes of carbon (Chapter 2), and led to a better understanding of how methane is cycled in the marine sediments of this area. This effort demonstrates the significance of CO₂ reduction, a previously overlooked reaction. The picture of reaction network derived from this work serves as the foundation for a transport-reaction model (Chapter 3). The kinetic model results revealed a very different biogeochemistry between two distinct fluid-flow environments. At sites where transport is predominantly diffusive (non-chimney environments), organic matter decomposition is the dominant process driving production of methane, dissolved inorganic carbon (DIC) and consumption of sulfate. In contrast, anaerobic oxidation of methane (AOM) drives both carbon and sulfur cycles in the advective settings characterized by acoustic chimneys indicative of gas transport. I show that methane produced within the model domain, through CO₂ reduction and methanogenesis, fuels AOM in the non-chimney sites while AOM is primarily induced by methane from external sources at the chimney sites. A simulation of the system evolution from a non-chimney to a chimney condition was developed by increasing the bottom methane supply to an originally diffusion-controlled site. Results from this exercise show that the higher methane flux leads to a higher AOM activity, and enhanced organic matter decomposition through methanogenesis.
Organic carbon cycling is also affected by changes in the depositional environment, as shown by application of the kinetic model to the sediments from the Krishna-Godavary (K-G) basin along the eastern Indian margin (Chapter 4). Proximity to large rivers results in the widespread occurrence of mass transport deposits (MTD) throughout the basin. In this work, MTD is defined as a fluidized sediment block whose pore water composition is identical to sea water value to reflect the homogenization process during sediment transport. The pore water sulfate and ammonium profiles measured at seven sites drilled in the K-G Basin during the NGHP-01 expedition were simulated to provide a quantitative description of how MTDs can affect geochemistry profiles, not only for sulfate and ammonium but potentially all pore water species. This model provides reliable estimates of the MTDs thickness, the time elapsed after the most recent event, and the organoclastic sulfate reduction rate at these seven sites.
A transport-reaction modeling approach was also applied to investigate the silica diagenetic reactions fueled by volcanic ash decomposition in Shikuko Basin, Nankai Trough (Chapter 5). The model developed for this setting reproduces a silica diagenetic boundary (SDB) at each site, which is defined by marked decreases in reactive volcanic ash, pore water silica and potassium. Volcanic ash alteration was constrained by modeling pore water ⁸⁷Sr/⁸⁶Sr profiles. Below the SDB, formation of clinoptilolite consumes potassium and regulates the extension of amorphous silica by consuming SiO₂(aq). The observed low SiO₂(aq) and dissolved potassium in these deep sequences require continuous precipitation of clinoptilolite; however in order to maintain oversaturation of this mineral at the low SiO₂(aq) in sediments below the SDB, an increase in pH is required, consistent with pore water observations. Thermal history, rather than temperature alone, controls the inferred reaction network as shown by the convergence of the thermal maturity of sediments at the SDB from all studied sites and is consistent with other locations documented onshore Japan. These results are valuable as we move forward in understanding the mechanisms and consequences of ash alteration in convergent margins worldwide.