Quantifying in-situ gas hydrates at active seep sites in the eastern Black Sea using pressure coring technique

Abstract

Abstract. In the eastern Black Sea, we determined methane (CH 4 ) concentrations, gas hydrate volumes, and their vertical distribution from combined gas and chloride (Cl − ) measurements within pressurized sediment cores. The total gas volume collected from the cores corresponded to concentrations of 1.2-1.4 mol CH 4 kg −1 porewater at in-situ pressure, which is equivalent to a gas hydrate saturation of 15-18 % of pore volume and amongst the highest values detected in shallow seep sediments. At the central seep site, a highresolution Cl − profile resolved the upper boundary of gas hydrate occurrence and a continuous layer of hydrates in a sediment column of 120 cm thickness. Including this information, a more precise gas hydrate saturation of 22-24 % pore volume could be calculated. This volume was higher in comparison to a saturation calculated from the Cl − profile alone, resulting in only 14.4 %. The likely explanation is an active gas hydrate formation from CH 4 gas ebullition. The hydrocarbons at Batumi Seep are of shallow biogenic origin (CH 4 > 99.6 %), at Pechori Mound they originate from deeper thermocatalytic processes as indicated by the lower ratios of C 1 to C 2 -C 3 and the presence of C 5 .

Effects of postglacial seawater intrusion on sediment geochemical characteristics in the Romanian sector of the Black Sea

“…These seeps play a pivotal role in the Black Sea methane cycling (Pape et al, 2008;Reeburgh et al, 1991;Schmale et al, 2010a) and are the major methane source responsible for the high concentrations measured in the water column (Greinert et al, 2006;Greinert et al, 2010;Reeburgh, 2007). However, a significant amount of methane generated in the sediment does not reach the water column (Wallmann et al, 2006) as it is partly stored as gas hydrates (Blinova et al, 2003;Bohrmann et al, 2003;Heeschen et al, 2011;Ker et al, 2019;Popescu et al, 2007;Zander et al, 2017) or oxidized within the anoxic sediment (Holmkvist et al, 2011;Jorgensen et al, 2004;Jorgensen et al, 2001;Meister et al, 2013;Niewohner et al, 1998;Reeburgh et al, 1991;Regnier et al, 2011;Reitz et al, 2011;Wallmann et al, 2006;Zander et al, 2020).…”

Characterizing the variability of natural gas hydrate composition from a selected site of the Western Black Sea, off Romania

“…In the present study, we combined two different techniques (gas chromatography and micro-Raman spectroscopic imaging) to characterize samples of natural hydrates collected during the Ghass cruise (2015) in the Romanian water sector of the Western Black Sea, which is known as the largest anoxic and methane-rich water mass on Earth. The Black sea contains several identified and inferred NGH deposits along its margin (Colin et al, 2020;Haeckel et al, 2015;Heeschen et al, 2011;Minshull et al, 2020;Nasif et al, 2020;Pape et al, 2011;Popescu et al, 2007;Zander et al, 2020). Our molecular-scale approach allowed to investigate the variability in cage occupancy and the impact of the presence of minor guest components on the overall methane content.…”

Joint interpretation of geophysical field experiments in the danube deep-sea fan, Black Sea

“…Gas hydrates in the Black Sea were first documented in 1972 (Yefremova and Zhizchenko 1974) and interpreted from seismic data in subsequent investigations (e.g. Vassilev and Dimitrov 2002;Bohrmann et al 2003;Popescu et al 2006;Heeschen et al 2011;Riboulot et al 2018;Ker et al 2019). Reeburgh et al (1991) and others estimated a total volume of about 1 to 5 * 10 12 m 3 of hydrate in the Black Sea sediments.…”