Modelling the dynamic response of shallow methane hydrates to simultaneous sea level and bottom water temperatures variations since the last glacial maximum on the Amazon Deep-Sea Fan, Brazil

Abstract

In marine sediments, methane hydrate can occur within the methane hydrate stability zone (MHSZ), which extends from the seafloor to a certain depth below it, within which the conditions of pressure and temperature are such that allow their formation and stability. Variations in sea level and in bottom water temperatures (BWTs) have opposite effects on the stability zone: a sea level rise leads to higher hydrostatic pressure, which tends to enlarge the MHSZ, while an increase in temperature tends to shrink it, and vice-versa. When the MHSZ shrinks, methane hydrate dissociates, releasing water and gaseous methane into the sediment pores. If the gaseous methane reaches the seafloor, it will be released into the ocean. In this study, we use numerical modelling to investigate the dynamic response of shallow methane hydrates at 550, 575, 600, 625, 650, 700 and 750 m water depth (mwd), on the Amazon Deep-Sea Fan, Equatorial Atlantic Ocean (near the northern coast of Brazil), to simultaneous sea level and BWTs increases, since the Last Glacial Maximum (LGM) up to the present. These water depths lie on the feather edge of the MHSZ, where hydrates are most sensitive to pressure and temperature perturbations. The results suggest that the methane hydrate stability decrease caused by the BWTs increases has completely overcome the increase in stability owing to sea level increase, for 550 and 575 mwd. For these two water depths, the MHSZ disappeared in the models and all the hydrate initially present has dissociated. The modelling results indicate that gaseous methane started to be released into the ocean at ca. 17,500 y BP for 550 mwd and at ca. 16,000 y BP for 575 mwd. For the other water depths considered, hydrate stability decrease caused by the BWT increase has only partly overcome the increase in stability due to pressure increase, so that the MHSZs have shrank, but have not totally disappeared in the models. Gaseous methane was released into the ocean from ca. 14,000 y BP to 7000 y BP for 600 mwd. For 625, 650, 700 and 750 mwd, no gaseous methane has been released into the ocean. Methane release amounts and flowrates are maximum limits for each depth, as no methane consumption processes, such as anaerobic oxidation, are modeled in this study.