Hervé Gillet

Multiple bottom-simulating reflections in the Black Sea: Potential proxies of past climate conditions

Abstract A previously unknown pattern of multiple bottom-simulating reflections (BSRs) occurs on high-resolution reflection seismic data in the Danube deep-sea fan, associated with acoustic features indicating free gas. Our study provides evidence that this pattern is developed in relation with the architecture of distinct channel–levee systems of the Danube fan. Channel–levee systems hosting multiple BSRs act as relatively sealed gas-bearing systems whose top is situated above the base of the gas hydrate stability zone (BGHSZ). Inside these systems, free gas accumulates below the BGHSZ under a combined lithological, structural and stratigraphical control. The uppermost BSR marks the current equilibrium BGHSZ, for a gas composition of more than 99% methane. Model-derived depths of the BGHSZ for different gas compositions and pressure–temperature conditions show that multiple BSRs would correspond to the BGHSZ either for (1) layers of gas hydrates with high contents of heavy hydrocarbons or hydrogen sulphide, or (2) stable climatic episodes with temperatures between glacial values and the present-day conditions. As the gas hydrate compositions required by hypothesis (1) are in sharp contradiction with the general background of the gas composition in the study area, we suggest that multiple BSRs are most probably relics of former positions of the BGHSZ, corresponding to successive steps of climate warming. In this case, they can provide sea-bottom paleotemperature values for these episodes, and hence they are potential new proxies for deciphering past climate conditions.

Canyon morphology on a modern carbonate slope of the Bahamas: Evidence of regional tectonic tilting

New high-quality multibeam data presented here depict the northern slope of the Little Bahama Bank (Bahamas). The survey reveals the details of large- and small-scale morphologies that look like siliciclastic systems at a smaller scale, including large-scale slope failure scars and canyon morphologies, previously interpreted as gullies and creep lobes. The slope exhibits mature turbidite systems built by mass-fl ow events and turbidity currents. The sediment transport processes are probably more complex than expected. Slope failures show sinuous head scarps with various sizes, and most of the scars are fi lled with recent sediment. Canyons have amphitheater-shaped heads resulting from coalescing slump scars, and are fl oored by terraces that are interpreted as slump deposits. Canyons rapidly open on a short channel and a depositional fan-shaped lobe. The entire system extends for ~40 km. The development of these small turbidite systems, similar to siliciclastic systems, is due to the lack of cementation related to alongshore current energy forcing the transport of fi ne particles and fl ow differentiation. Detailed analyses of bathymetric data show that the canyon and failurescar morphology and geometry vary following a west-east trend along the bank slope. The changing parameters are canyon length and width, depth of incision, and canyon and channel sinuosity. Accordingly, failure scars are larger and deeper eastward. These observations are consistent with a westward tectonic tilt of the bank during the Cenozoic.

Carbonate slope morphology revealing a giant submarine canyon (Little Bahama Bank, Bahamas)

New high-quality multibeam data detail the morphology of the giant 135-km-long Great Abaco Canyon (GAC) located between Little Bahama Bank (LBB, Bahamas) and Blake Plateau. Knickpoints, chutes, and plunge pools mark the canyon main axis, which is parallel to the LBB margin. The canyon head covers a large area but does not represent the main source of the modern sediments. The material supplied through the LBB canyon systems originates below this head, which only shows erosive lineaments related to the pathway of currents running along the seafloor and restricted failure scars. Most of the sediment supply originates from the canyon sides. The northern canyon flank incises the Blake Plateau, which comprises contourites on top of a drowned Cretaceous carbonate platform. These deposits are susceptible to translational slides and form dissymmetric debris accumulations along the northern edge of the canyon. A large tributary drains the Blake Plateau. Two large tributaries connecting the southern flank of the GAC directly to the LBB upper slope form additional sources of sediments. Subbottom profiles suggest the presence of a sedimentary levee on the tributary canyon and of sediment gravity flow deposits. The GAC has been a permanent structure since the drowning of the Cretaceous platform, and its size and morphology are comparable to those of canyons in siliciclastic environments. The orientation of the GAC parallel to large-scale regional tectonic structures suggests a structural control. The size of the observed structures, especially plunge pools at the base of gigantic chutes, is unusual on Earth. The presence of deposits downflow of the pools suggests that the GAC results from or at least is maintained by persistent and sustained submarine gravity flows rather than by retrogressive erosion.