In-situ borehole temperature measurements confirm dynamics of the gas hydrate stability zone at the upper Danube deep sea fan, Black Sea

Abstract

In der Tiefsee ist die letzte Eiszeit noch nicht vorbeiGashydratvorkommen im Schwarzen Meer reagieren auf postglaziale Klimaänderungen 30.03.2021/Kiel/Bremen. Bei der Untersuchung von Gashydratvorkommen im westlichen Schwarzen Meer machte ein Team von Forschenden vom GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel und dem MARUM -Zentrum für Marine Umweltwissenschaften der Universität Bremen überraschende Entdeckungen. Entgegen bisheriger Erkenntnisse und Theorien fanden die Wissenschaftler*innen freies Methangas in Schichten, wo diese eigentlich nicht auftauchen dürften. In der jetzt in der internationalen Fachzeitschrift Earth and Planetary Science Letters veröffentlichten Studie schließen sie, dass sich das Gashydratsystem im Tiefseefächer der Donau durch die Klimaänderungen seit dem letzten glazialen Maximums weiterhin verändert.

Heat Flow Measurements at the Danube Deep-Sea Fan, Western Black Sea

“…Drilling at three sites up to~148 mbsf across the S2 channel-levees (Figure 1) defined the geophysical and geochemical setting of the mostly fine-grained turbidite sediment system, but no gas hydrates were recovered [30]. New downhole in situ temperature data combined with detailed pore water chemistry from recovered sediments and the drilling-derived gas composition [31] confirmed initial models [21,32] that the current base of the GHSZ is shallower than expected from the prominent BSRs in this region [17].…”

“…The sediments found in the associated fan complex consist of fine-grained turbidites (clay to silt grainsize, [30]) and form stacks of alternating channel and levees deposits [13,57,58]. Mass-transport deposits (MTDs) are found pervasively on the slope, with head-scarps predominantly seen at the slope edge and along channel walls [17]. The Danube and Dniepr deep-sea fans started to develop in the Pleistocene around 900 ka ago [54].…”

“…This significant change in temperature, pressure, and salinity conditions strongly affected the gas hydrate regime of the Black Sea [14,21,32,65]. Despite the increase in pressure promoting gas hydrate formation, the overwhelming temperature increase at the sea floor resulted in an up-dip migration of the base of the GHSZ, which is one of the primary controls on the limits of gas venting occurrences and associated slope instability across the study region [17,20].…”

“…The shallow feather edge of the sI gas hydrate systems in~720 m water depth marks the onset of BSRs towards deeper water [19][20][21]. With very few exceptions, gas venting is seen only in water depths shallower than the 720 m feather edge of the current GHSZ [17,[19][20][21][22][23][24]. 1), defining the background sedimentological and pore water geochemical constraints within the upper 200 to 300 m of sediment below seafloor [25].…”

“…Several seismic studies [9][10][11][12][13][14][15] were conducted to map bottom-simulating reflectors (BSRs), which are used as an indicator for the base of the structure I (sI) gas hydrate stability zone (GHSZ). However, BSRs are not widely distributed in the Black Sea [2,8,10,16] and occur in patches (Figure 1, see also e.g., [6,17]). Across the thick sediment deposits of the Viteaz channel-levee system of the Danube deep-sea fan, a stack of up to five BSRs was identified and linked to former climate conditions of the lower seafloor temperature and sea levels during glacial times compared to inter-glacial climate epochs [9,18].…”