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Abstract:
We present a simple evaluation of the changes in gas hydrate stability zones expected across the most recent glacial/interglacial to evaluate gas hydrate formation and dissociation across climatic conditions. We describe a numerical modeling approach which is used to test whether a broad peak in chloride concentrations observed in Site U1517 on the Hikurangi Margin could be due to recent downward migration of the base of gas hydrate stability (BGHS) following the last glacial maximum (LGM). These simulations of gas hydrate stability shifts consider sea-level changes, propagation of bottom-water temperature (BWT) changes into the sediment, and solute diffusion. Thermal and chloride diffusion are simulated using a one-dimensional fully-implicit finite-difference model. Our results indicate that although BWT changes affect the BGHS at Site U1517, simulations with and without BWT changes can create a broad chloride peak. If the Site U1517 chlorinity peak is due to recent methane-hydrate formation, this suggests that methane released by previous dissociation remains within the sediments to be re-sequestered as hydrate forms. In other words, there was no significant loss of methane to the bottom water. These results are in agreement with the majority of the literature based on analyses of sediment records, and consistent with observations that there is no methane signal in atmospheric records. The combined modeling and observations presented here suggest that continental margin systems are continuing to adapt to changing sea levels and bottom water temperatures. We indicate when chloride peaks can be expected to provide evidence about these changes and caution about the potential of misattribution to other processes. Interactions between methane hydrate dynamics and slope instability were also considered. Site U1517 was located to investigate the Tuaheni slide complex. P/T changes across glacial/interglacials likely resulted in generation of methane gas at the BGHS at some point. Even though methane accumulations may have impacted slope stability in some margins, that seems unlikely at the Tuaheni slide because gas hydrates at Site U1517 occur > 65 meters beneath the slide mass.
