Agnès Baltzer

In situ geotechnical characterization of sediments on the Nova Scotian Slope, eastern Canadian continental margin

Abstract A seabed 2-m-long cone penetrometer and coring system (Geotechnical Module) has been used at 17 stations in four transects on the Scotian Slope to characterise in situ shear strength and induced pore pressure on several different types of late Pleistocene and early Holocene failure. Study sites were selected using the SAR high-resolution deep-towed acoustic system equipped with a digital 160–190 kHz sidescan sonar and a 3.5 kHz subbottom profiler. Several distinctive types of “geotechnical signature” were recognised from plots of cone resistance and induced pore pressure with depth in the sediment. Normally consolidated sediments show a progressive increase in cone resistance with depth (to about 75 kPa at 2 m subbottom). Holocene surficial muds show spectacular apparent overconsolidation, reaching a peak of 250 kPa at about 50 cm subbottom and then decreasing down to 1.5 m. This overconsolidation is associated with Zoophycos burrows. Late Pleistocene sediments exhumed by bedding plane slides show strong true overconsolidation consistent with the original depth of burial inferred from high-resolution seismic stratigraphy. Debris flows show only a slight shear stress gradient with depth (40–45 kPa over 0.5–1 m subbottom) with under-consolidation due to remoulding of sediment.

Deep structure of the Santos basin-São Paulo plateau system, SE Brazil

The structure and nature of the crust underlying the Santos Basin-Sao Paulo Plateau System (SSPS), in the SE Brazilian margin, are discussed based on five wide-angle seismic profiles acquired during the Santos Basin (SanBa) experiment in 2011. Velocity models allow us to precisely divide the SSPS in six domains from unthinned continental crust (Domain CC) to normal oceanic crust (Domain OC). A seventh domain (Domain D), a triangular shape region in the SE of the SSPS, is discussed by Klingelhoefer et al. (2014). Beneath the continental shelf, a ~100 km wide necking zone (Domain N) is imaged where the continental crust thins abruptly from ~40 km to less than 15 km. Toward the ocean, most of the SSPS (Domains A and C) shows velocity ranges, velocity gradients, and a Moho interface characteristic of the thinned continental crust. The central domain (Domain B) has, however, a very heterogeneous structure. While its southwestern part still exhibits extremely thinned (7 km) continental crust, its northeastern part depicts a 2–4 km thick upper layer (6.0–6.5 km/s) overlying an anomalous velocity layer (7.0–7.8 km/s) and no evidence of a Moho interface. This structure is interpreted as atypical oceanic crust, exhumed lower crust, or upper continental crust intruded by mafic material, overlying either altered mantle in the first two cases or intruded lower continental crust in the last case. The deep structure and v-shaped segmentation of the SSPS confirm that an initial episode of rifting occurred there obliquely to the general opening direction of the South Atlantic Central Segment.

Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders

About a decade ago, a large field of pockmarks (individual features up to 30 m in diameter and <2 m deep) was discovered in water depths of 15–40 m in the Bay of Concarneau in southern Brittany along the French Atlantic coast, covering an overall area of 36 km2 and characterised by unusually high pockmark densities in places reaching 2,500 per square kilometre. As revealed by geophysical swath and subbottom profile data ground-truthed by sediment cores collected during two campaigns in 2005 and 2009, the confines of the pockmark field show a spectacular spatial association with those of a vast expanse of tube mats formed by a benthic community of the suspension-feeding amphipod Haploops nirae. The present study complements those findings with subbottom chirp profiles, seabed sonar imagery and ultrasonic backscatter data from the water column acquired in April 2011. Results show that pockmark distribution is influenced by the thickness of Holocene deposits covering an Oligocene palaeo-valley system. Two groups of pockmarks were identified: (1) a group of large (>10 m diameter), more widely scattered pockmarks deeply rooted (up to 8 ms two-way travel time, TWTT) in the Holocene palaeo-valley infills, and (2) a group of smaller, more densely spaced pockmarks shallowly rooted (up to 2 ms TWTT) in interfluve deposits. Pockmark pore water analyses revealed high methane concentrations peaking at ca. 400 μl/l at 22 and 30 cm core depth in silty sediments immediately above Haploops-bearing layers. Water column data indicate acoustic plumes above pockmarks, implying ongoing pockmark activity. Pockmark gas and/or fluid expulsion resulting in increased turbidity (resuspension of, amongst others, freshly settled phytoplankton) could at least partly account for the strong spatial association with the phytoplankton-feeding H. nirae in the Bay of Concarneau, exacerbating impacts of anthropogenically induced eutrophication and growing offshore trawling activities. Tidally driven hydraulic pumping in gas-charged pockmarks represents a good candidate as large-scale short-term triggering mechanism of pockmark activation, in addition to episodic regional seismic activity.

Climatically-driven impacts on sedimentation processes in the Bay of Quiberon (south Brittany, France) over the last 10,000 years

The Bay of Quiberon (NW France), which is protected by the Quiberon Peninsula, provides well-preserved sedimentary archives for high-resolution reconstruction of Holocene climate variability. Very-high-resolution seismic profiles were obtained from two Seistec boomer seismic campaigns conducted in the bay. The seismic surveys reveal the deposition of five distinct sedimentary units. This succession is correlated with radiocarbon-dated sediments sampled in a long CASQ core (MD08-3204CQ). The analysis of the sediments shows significant changes in grain size, indicating a complex series of palaeo-climate events dated at 8800, 8600, 4270, 3860, 2470 and 1060 cal. yr BP. These may be correlated with millennial timescale climate variability corresponding to the phases of Holocene rapid climate changes (RCCs), associated with negative North Atlantic Oscillation (NAO) index and usually characterized by weaker winter storms. On the contrary, periods between RCCs are characterized by the predominance of westerly winds and stormy conditions for Brittany (i.e. positive NAO). However, only storm events occurring during the RCCs have been preserved in the Bay of Quiberon. This paper aims to reconstruct the Holocene sedimentary infill of the bay, highlighting the role of episodic acceleration phases of the sea-level rise on the preservation of sedimentary archives. Thus, the Bay of Quiberon provides substantially a complete sedimentation record of the last 10,000 years characterized by a series of complex palaeo-environmental changes.

The Mont-Saint-Michel Bay: An Exceptional Megatidal Environment Influenced by Natural Evolution and Man-Made Modifications

Due to its exceptional tidal range and the immensity of its tidal flat, the Mont-Saint-Michel bay is a favored model for research on coastal hydro-sedimentary processes, reconstruction of the depositional wedge during the Holocene, and landscape evolution. The western tidal flat consists of a large muddy to sandy embayment, whereas the eastern intertidal domain consists of estuarine deposits characterized by finely laminated muddy sand, enriched in biogenic carbonate. The present-day morphology and sedimentary dynamics are the result of a complex balance between tide-dominated sediment supply and erosive processes under wave action and tidal channels migration. The bay has been filled for thousands of years to the rhythm of sea level oscillations, leading to the gradual progradation of the shoreline. In addition to this natural evolution, human activities have developed in the bay, both on the western tidal flat (fixed fisheries and shellfish farming) and on the eastern estuarine side (polder formation, dikes, and dam construction), and have progressively changed the landscapes. In 1884, Victor Hugo issued an appeal for the protection of the Mont-Saint-Michel in these terms: “The Mont-Saint-Michel is to France what the Great Pyramid is to Egypt. It must be protected from any damage. The Mont-Saint-Michel must remain an island. This combined work of nature and art must be conserved at all costs.” Since that time, salt marshes have progressed, the land reclamation and man-made developments to facilitate the access to the mount also continued, so the maritime character of Mont is still endangered.