Reply to Comments by N. Sultan on “Sedimentation Controls on Methane‐Hydrate Dynamics Across Glacial/Interglacial Stages: An Example From International Ocean Discovery Program Site U1517, Hikurangi Margin”

Abstract

AbstractScreaton 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.