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Rustam Rams
Rustam Rams

Todays Sea-Floor Sunday Images Are From An Active Submarine Sedimentary System In The Mediterrane

The sparsity of permanent seismic instrumentation in marine environments often limits the availability of subsea information on geohazards, including active fault systems, in both time and space. One sensing resource that provides observational access to the seafloor environment are existing networks of ocean bottom fiber optic cables; these cables, coupled to modern distributed acoustic sensing (DAS) systems, can provide dense arrays of broadband seismic observations capable of recording both seismic events and the ambient noise wavefield. Here, we report a marine DAS application which demonstrates the strength and limitation of this new technique on submarine structural characterization. Based on ambient noise DAS records on a 20 km section of a fiber optic cable offshore of Moss Landing, CA, in Monterey Bay, we extract Scholte waves from DAS ambient noise records using interferometry techniques and invert the resulting multimodal dispersion curves to recover a high resolution 2D shear-wave velocity image of the near seafloor sediments. We show for the first time that the migration of coherently scattered Scholte waves observed on DAS records can provide an approach for resolving sharp lateral contrasts in subsurface properties, particularly shallow faults and depositional features near the seafloor. Our results provide improved constraints on shallow submarine features in Monterey Bay, including fault zones and paleo-channel deposits, thus highlighting one of many possible geophysical uses of the marine cable network.

Today’s Sea-Floor Sunday Images Are From An Active Submarine Sedimentary System In The Mediterrane

Less is known about the expressions of activity (quiescent or eruptive) of submarine MVs although estimates of their number (up to 105) exceed that of onshore MVs by up to two orders of magnitude5,10. In the Black Sea, seepage from active MVs is associated with warm sediments ascending from depth and upward flow of aqueous fluids and free gas11,12. At the Håkon Mosby MV (Norway), variations in fluid flow have been found to trigger mud movements and outflows that changed the seafloor morphology and released up to 43000 m3 of methane13,14. Deep-sea MVs are often associated with the presence of shallow gas hydrates12,15 and are known to sustain chemosynthesis-based seafloor ecosystems referred to as cold seeps16. However, the current understanding of the mechanisms driving fluid flow, the fluid sources (i.e. deep vs. shallow), the discharge volumes of volatiles and the impact of mud volcanism on seafloor ecosystems is poorly constrained (e.g. Bohrmann et al.12). To move forward in the understanding of these interrelated issues requires systematic investigations of submarine MV systems to detect the sites of seepage relative to MV morphologies, quantify the fluid discharge, and characterize the composition of fluids emitted at and in the vicinity of the locations of active mud extrusion.

Considering the presence of thermogenic hydrocarbons in both fluid domains, it is inferred that seepage at Venere MV involves an upward-branching plumbing system and that overpressured gas diverted laterally from the main MV conduit migrates upward and mixes with gas of shallow origin. As a result, chemosynthesis-based communities that rely on persistent fluid supply and thick authigenic carbonates are limited to the peripheral seeps along the ring faults of the caldera. An upward-branching plumbing system that experiences high subsurface pore pressures can explain how both mud breccia discharge and quiescent-type seepage co-exist at an individual MV. Such a mechanism may allow for mud breccia discharge to be sustained over timescales exceeding the short-lived eruptions typically associated with active mud volcanism. Furthermore, the upward-branching fluid migration pathways exert an important control on the distribution of seepage-dependent chemosynthesis-based oases of life around submarine MVs.

Submarine channels are spectacular features that can extend for thousands of kilometres across the seafloor, are often kilometres wide and up to hundreds of metres deep. They are formed by density currents; underwater flows of sand, mud and water that are denser than sea water and therefore flow along the seafloor. These channels are very important as they are the major transport pathway for moving sediments to the deep sea and form the largest sedimentary deposits on Earth. These deposits are significant hosts for gas and oil reserves and hold key information on past climate change and mountain building episodes. Such flows are difficult to study, typically being infrequent and highly destructive; they pose a major hazard to sea-floor engineering such as cables and pipelines, and have often destroyed scientific measurement equipment. Consequently,our knowledge of such flows comes mainly from laboratory experiments, and understanding of their deposits from studies of ancient examples now exposed on land. As a consequence there are no detailed studies of these flows in natural channels, and no studies that link flow measurements to the deposits that are produced. There is almost no otherenvironment on Earth where we do not have any knowledge of how flow processes are linked to their sedimentary deposits, and this in the largest deposits on Earth! Consequently, there is an urgent need to improve our understanding of the interactions between flow, morphology and deposits within an active submarine channel. However, in addition to the technical problems associated with monitoring these density-driven flows, most submarine channels actually formed when sea-levels were much lower than today; present-day flows are typically much smaller than those that formed the channels/deposits, making study of interactions impossible. Furthermore, innovative techniques are required to measure etailed flow patterns within these channels.

Around 6,000 years ago sea-level approached its present level, and dense salty fluid (10-15 m thick) from the Mediterranean started flowing through the Bosphorus Strait (past Istanbul) into the Black Sea, forming an almost constantly active sea-floor channel network. The first spectacular images of this system and its sedimentary depositswere only obtained in 2005. This provides a unique opportunity to study both flows and deposits in an active sea-floor channel for the very first time and to use this knowledge to formulate and test predictive models. We have assembled a world-leading group of UK and international scientists to tackle this challenge.

We will use Autosub3 NERC's new state-of-the-art autonomous submarine (yellow, of course!) to 'fly' our measurement equipment just above the bottom, allowing us to map channel morphology and the three dimensional (3D) flows in unprecedented detail compared to what is possible from traditional ship-based methods. The flow data will be linked tomeasurements of sea-bed properties, smaller morphological features such as bedforms, and seismic data that images through sediment to reveal the internal structure of the deposits. These data will be used as input conditions for an innovative computer simulation model of flow and deposition in submarine channels. The numerical modelling and field datawill be combined to enable us to: i) assess bedforms in the channels with respect to the flows forming them, allowing reconstruction of past flows from preserved bedforms in older rocks for the very first time; ii) model bend flow to enable sediment patterns in the deposits to be predicted, and, iii) develop a new understanding of how flow and morphology islinked to long-term sediment deposition. These data will revolutionise our understanding of both flows and deposits in submarine environments, with key applications to: i) geohazard analysis, ii) design criteria for seafloor engineering, and, iii) prediction of sedimentary deposit types and distributions.

Other earlier observations and important evidence for the ocean encompass the interpretation of ancient shorelines from Viking Orbiter images (Parker et al. 1993), the topography of deformed shorelines (Perron et al. 2007), global distribution of Martian deltas and valleys within and along the margins of the northern lowlands (Di Achille and Hynek 2010), a branching network of inverted channel and channel-lobe deposits at Aeolis Dorsa (DiBiase et al. 2013), and tsunami deposits with runout lobes and backwash erosion (Rodriguez et al. 2016; Costard et al. 2017). For years, the controversial paleo-shoreline interpretation (Parker et al. 1989, 1993) has driven research in a very specific direction of documenting terrestrial analogs for the deposits located on the northern plains, where the hypothetical ocean existed. Some of the most convincing evidence includes a resemblance between teardrop-shaped islands in Martian outflow channels and erosional shadow remnants documented in deep-water environments on Earth (Moscardelli and Wood 2011), terrestrial submarine analogs for streamlined forms on Mars (Burr 2011), similarities between large-scale polygonal terrains on Mars and deep-water polygonal fault systems on Earth (Cooke et al. 2011; Allen et al. 2013; Moscardelli et al. 2012), comparisons between high-albedo mounds in Acidalia Planitia and terrestrial submarine mud volcanoes (Allen et al. 2013; Oehler and Allen 2012), and resemblance between boulder-size rocks contained within the Vastitas Borealis Formation on the northern plains of Mars and blocks and boulders that have been documented in deep-water terrestrial environments and reported in outcrop (Moscardelli 2014).

In seeking cyclic steps in submarine deposits, we analyzed high resolution context imager (CTX, Malin et al. 2007) data over 17 open basin deltas recognized by Di Achille and Hynek (2010). CTX has approximately 6 m/pixel resolution and full coverage over these features. Additionally, data from the High Resolution Imaging Science Experiment (HiRISE, McEwen et al. 2007) was consulted when greater detail was necessary. HiRISE has a resolution of roughly 0.5 m/pixel, but lower coverage. The JMARS geospatial information system platform loads the georeferenced images automatically, and we used this tool to make the mosaics (Christensen et al. 2009). Once the data were loaded, we sought periodic structure in the visible imagery and analyzed CTX mosaics at ever site.


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