Description: 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.

Description: Dissolved chloride concentrations higher than seawater were observed over a broad depth range in pore water profiles from International Ocean Discovery Program Site U1517 on the Hikurangi Margin. This Cl maximum is not associated with an 87Sr/86Sr anomaly, indicating that it is not caused by hydration reactions during ash alteration. We use a numerical modeling approach to examine possible causes for recent gas hydrate formation that can result in the observed Cl high. Our approach considers sedimentation, sea level, and bottom water temperature (BWT) changes due to glaciation as drivers for the downward migration of the base of gas hydrate stability and gas hydrate formation. The modeling results reveal that lowering of sea level during glaciation can allow methane hydrate dissociation followed by postglacial hydrate formation as sea level rises. However, BWT cooling of 2 °C during glaciation followed by warming during deglaciation would mostly counteract the impacts of sea level change. Bottom water cooling during glaciation is expected in this region and many locations worldwide. As a result, our simulations do not support the previous hypotheses of large‐scale gas hydrate dissociation due to sea level drop during glaciation, which have been proposed as triggers for widespread gas release and slope failure. Such a mechanism is only possible where BWT remains constant or increases during glaciation. Our simulations indicate that sedimentation constitutes the largest factor driving recent methane hydrate formation at Site U1517, and we suggest that sedimentation may play a larger role in gas hydrate dynamics along margins than previously recognized.

Description: Screaton et al. (2019, https://doi.org/10.1029/2019GC008603) examined the role of sedimentation, sea level, and bottom water temperature (BWT) changes due to glaciation as drivers for the downward migration of the base of gas hydrate stability and gas hydrate formation. International Ocean Discovery Program (IODP) Site U1517 in the Hikurangi margin was used as a case study because data at this site document a marked increase in chloride over a broad depth range, which was attributed to recent gas hydrate formation. In a comment on Screaton et al. (2019, https://doi.org/10.1029/2019GC008603), Sultan (2020, https://doi.org/10.1029/2019gc008846) used a linear thermal profile to argue that inferences and characterization of methane hydrate at IODP Site U1517 were incorrect because some occur below his estimated base of gas hydrate stability (BGHS). Based on this apparent discrepancy, Sultan (2020, https://doi.org/10.1029/2019gc008846) further stated that low‐chloride spikes may be unreliable indicators of methane hydrate occurrence. In this reply, we emphasize that unsteady‐state, and thus nonlinear, thermal profiles are likely in areas experiencing active sedimentation and bottom‐water temperature (BWT) changes. The resulting deviation from steady‐state temperature profile shifts the BGHS downward. In addition, sedimentation has the potential to bury methane hydrate more rapidly than it dissociates, helping to explain how methane hydrate could be observed below the BGHS. We also review the supporting evidence for gas‐hydrate occurrence at Site U1517 and the criteria used for Site U1517 site selection.