Vincent Hanquiez

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.

Past deep-ocean circulation and the paleoclimate record-Gulf of Cadiz

Deep marine currents are strongly influenced by climatic changes. They also deposit, rework, and sort sediment, and can generate kilometer-scale sedimentary bodies (drifts). These drifts are made of thoroughly bioturbated, stacked sedimentary sequences called contourites [Gonthier et al., 1984]. As a consequence, change in the direction or intensity of currents can be recorded in the sediments.

Contourites in the Gulf of Cadiz: a cautionary note on potentially ambiguous indicators of bottom current velocity

Facies associations in cores collected in the deep part of the Gulf of Cadiz, which is under the influence of the lower branch of the Mediterranean Outflow Water, are investigated in terms of the classical contourite model using grain-size analyses and thin sections of indurated sediment. Cores include both low-energy (contourite drift) and high-energy (channel) environments. The thin sections and grain-size distributions show that clayey fine silts and sandy coarse silts are the most common facies associations in the studied contourite sequences, while coarse-grained, gravelly contourites are less common. Grain-size distributions are unimodal in the fine-grained and bi- or trimodal in the coarser-grained contourites. This change in grain-size composition is related both to the partial removal of the fine-grained fraction and to the replenishment of the coarser-grained one. In addition, most of the contacts between individual facies are sharp rather than transitional. This suggests that the contourite sequence is only in part related to changes in bottom current velocity and flow competency, but may also be related to the supply of a coarser terrigeneous particle stock, provided by either increased erosion of indurated mud along the flanks of confined contourite channels (mud clasts), or by increased sediment supply by rivers (quartz grains) and downslope mass transport on the continental shelf and upper slope. The classical contourite facies association may therefore not be solely controlled by current velocity, but may be the product of a variety of depositional histories. The classical contourite depositional sequence should therefore be interpreted with greater care and in the light of the regional sedimentological background. In addition, the wisdom of exclusively using mean or modal particle size for the interpretation of depositional contourite processes is questioned. Instead, it is proposed that the vertical evolution of grain-size populations in the facies successions forming contourite sequences be assessed.

Holocene land–sea climatic links on the equatorial Pacific coast (Bay of Guayaquil, Ecuador)

We analyzed the pollen content of a marine core located near the Bay of Guayaquil in Ecuador to document the link between sea surface temperatures (SSTs) and changes in rainfall regimes on the adjacent continent during the Holocene. Based on the expansion/regression of five vegetation types, we observe three successive climatic patterns. In the first phase, between 11,700 and 7700 cal. yr BP, the presence of a cloud (Andean) forest in the mid altitudes and mangroves in the estuary of the Guayas basin, were associated with a maximum in boreal summer insolation, a northernmost position of the Intertropical Convergence Zone (ITCZ), a land–sea thermal contrast, cloud dripping, and dry edaphic conditions. Between 7700 and 2850 cal. yr BP, the expansion of the coastal vegetation and the regression of the mangrove indicate a drier climate with weak ITCZ and low El Niño Southern Oscillation (ENSO) variability while austral summer insolation gradually increased. The interval between 4200 and 2850 cal. yr BP was marked by the coolest and driest climatic conditions of the Holocene because of the weak influence of the ITCZ and a strengthening of the Humboldt Current. After 2850 cal. yr BP, high variability and amplitude of the Andean forest changes occurred when ENSO frequency and amplitude increased, indicating high variability in land–sea connections. The ITCZ reached the latitude of Guayaquil only after 2500 cal. yr BP inducing the bimodal precipitation regime we observe today. Our study shows that besides insolation, the ITCZ position, and ENSO frequency, changes in eastern equatorial Pacific SSTs play a major role in determining the composition of the ecosystems and the hydrological cycle of the Ecuadorian Pacific coast and the Western Cordillera in Ecuador.

The expansion of Central and Northern European Neolithic populations was associated with a multi-century warm winter and wetter climate

It is still debated whether climate changes had an impact on the emergence, spread, and disappearance of early production-based (Neolithic) adaptations. To date, and despite the incorporation of various paleoclimatic proxies, there exists no spatial reconstruction of the regional impact of the North Atlantic cooling events on Central–Western European climate and environments during the early Holocene. In order to address these two issues, we estimated seasonal and annual temperature and precipitation from a marine pollen record from Trondheimsfjord (central Norway) along with 68 pollen records distributed across Central–Western Europe for the time period associated with the Linearbandkeramik (LBK) cultural tradition, 7600–6900 yr cal. BP. Two distinct vegetation-derived rapid, <100 years, climate changes, contemporaneous with reduced warm Atlantic water (AW) inflow and winter storminess in the northern North Atlantic, bracket the expansion of the LBK. The geographic expansion of LBK populations appears to coincide with winter warming by ca. 2.5°C on average, and an increase in summer and winter precipitation, while its decline is associated with decreases in winter temperature, by ~1.5°C on average, and summer rainfall. Our results confirm that LBK subsistence practices were well-adapted to wet and relatively warm winters and cool summers, which are favorable to some cultigens, such as einkorn. This is in contrast to the hypothesis that cooler and wetter climatic conditions would induce increased instability of agricultural communities leading to the decline of LBK populations.

Monitoring Sea Level and Topography of Coastal Lagoons Using Satellite Radar Altimetry: The Example of the Arcachon Bay in the Bay of Biscay

Radar altimetry was initially designed to measure the marine geoid. Thanks to the improvement in the orbit determination from the meter to the centimeter level, this technique has been providing accurate measurements of the sea surface topography over the open ocean since the launch of Topex/Poseidon in 1992. In spite of a decrease in the performance over land and coastal areas, it is now commonly used over these surfaces. This study presents a semi-automatic method that allows us to discriminate between acquisitions performed at high tides and low tides. The performances of four radar altimetry missions (ERS-2, ENVISAT, SARAL, and CryoSat-2) were analyzed for the retrieval of sea surface height and, for the very first time, of the intertidal zone topography in a coastal lagoon. The study area is the Arcachon Bay located in the Bay of Biscay. The sea level variability of the Arcachon Bay is characterized by a standard deviation of 1.05 m for the records used in this study (2001–2017). Sea surface heights are very well retrieved for SARAL (R~0.99 and RMSE 0.93 and RMSE 0.82 but with a higher RMSE >0.92 m). For the topography of the intertidal zone, very good estimates were also obtained using SARAL (R~0.71) and CryoSat-2 (R~0.79) with RMSE lower than 0.44 m for both missions.

To What Extent Multidecadal Changes in Morphology and Fluvial Discharge Impact Tide in a Convergent (Turbid) Tidal River

Understanding nonstationary tides in tidal rivers is a major contemporary challenge. In particular, the response of river tides to natural developments in the estuary remains poorly investigated. This study analyzes the evolution of tidal characteristics over the last six decades in the Garonne Tidal River (GTR, SW France), in order to explore the effect of natural and human-induced morphological and hydrological changes on river tides. The tidal Garonne is an excellent example, as it has been subject to decreasing river discharges, natural morphological changes, and gravel extraction. Tidal range (TR), distortion (AM4/ AM2), and asymmetry direction (2/M2-/M4) were calculated at four locations from the water level time series of 1953, 1971, 1982, 1994, 2005, and 2014. The annual time series of M2 and M4 amplitudes and phases were obtained through complex demodulation. Results reveal that both TR and 2/M2-/M4 have increased since the 1950s. River flow modulates TR and AM4/AM2 significantly. A long-term decrease in summer discharges from 200 6 50 to 100 6 50 m s would increase TR by 111.5% in the upper GTR. Natural morphological changes amplified TR and 2/M2-/M4 by up to 112–15% between 1953 and 2014. TR and 2/M2-/M4 doubled in the regions affected by gravel extraction between 1953 and 1971. Further, the persistence of mobile mud in the GTR increased TR seasonally but also interannually (by up to 116% in winter and spring of dry years). The potential impact of these changes on suspended sediments is discussed, revealing complex feedback between the evolution of hydrology, morphology, tides, and sediment trapping.

Biostratigraphy of the Holocene and of the Main Cold Events of the Late Quaternary in the Gulf of Cadiz

The Gulf of Cadiz, west of the Strait of Gibraltar, is the site of water exchange between the Atlantic Ocean and the Mediterranean where many palaeoceanographic studies have taken place over the last several decades. Based on 21 cores from three cruises, oxygen isotope curves, 139 radiocarbon dates, and microfaunal analyses, this work presents and discusses the main bioevents used as detailed biostratigraphic points in this area for the late Quaternary. Those bioevents, such as well-known cold events (Younger Dryas, Heinrich events) or bioevents occurring during the Holocene, are essentially based on planktonic foraminifer species and/or coiling ratio and the point occurrences of pteropod species. The large and extensive data set allows us to discuss ages and the spatial validity of such bioevents.

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.