Past and present potential of the Adriatic deep sea sediments to produce methane hydrates

There is a growing understanding that methane hydrates (MHs) distributed globally in permafrost and deep sea sediments present an enormous unconventional reservoir of methane (CH4); however, there is also increasing concern about their role in the global climate change. The study focuses on the evaluation of the environmental conditions in the deep Adriatic Sea during the Last Glacial Maximum (LGM, 21.5–18.3 ka BP) and presently with respect to MHs potential occurrence. The MHs phase stability diagram was calculated in order to evaluate the methane hydrate stability zone (MHSZ) by using the Croatian Legacy Data and the digital bathymetry map of the Adriatic Sea obtained from the Croatian Hydrocarbon Agency (CHA). Environmental data from different surveys published in the scientific literature were used to assess the environmental conditions in the deep Adriatic Sea during the LGM and present. The sea level rise of 100 m since the end of the LGM was taken into consideration. The volume of methane in place (MIP) as an estimation of the amount of CH4 stored in MHs deposits at standard conditions of pressure and temperature (SPT, T0 = 273.15 K, P0 = 0.101325 MPa) was calculated by using combined gas law VSPT = (P×V/T) × (TSPT/PSPT). Evaluation of the MHs phase stability diagram for the Adriatic Sea in present environmental conditions has revealed that MHs are exactly at the boundary of stability. This has been calculated for the potential temperature of 13 °C, the salinity of 3.87% (data measured at the E2-M3A deep ocean observatory of the Southern Adriatic), and the average geothermal gradient of 17 °C km−1 reported in the literature and verified by the Croatian Legacy Data of CHA. According to the published literature, LGM deep sea temperature was 2–4 ° C lower and seawater was saltier. Consequently, the estimation of MHSZ during the LGM taking into consideration the temperature of 10 °C and salinity of 3.98% revealed a potential deposit of methane in place (MIP) of more than 415 × 109 m3, the majority of which probably dissociated in the sea/atmosphere system in the last 18 ka. The results have shown that MHs reservoir in the deep sea Adriatic basin shows boundary instability for MHs occurrence which might be of importance for studying the role of MHs in climate change. Further research is needed as follows: (1) thermodynamic modeling in order to understand if the MHs dissociation is concluded; and (2) in the case of the transient condition, seismic data analysis in order to reveal the presence of a relic bottom simulating reflection.