Elsevier

Marine Geology

Volume 427, September 2020, 106217
Marine Geology

Invited research article
Sedimentary evolution of the Le Danois contourite drift systems (southern Bay of Biscay, NE Atlantic): A reconstruction of the Atlantic Mediterranean Water circulation since the Pliocene

https://doi.org/10.1016/j.margeo.2020.106217Get rights and content

Highlights

  • The MOW/AMW dominated SW European margins from the late Pliocene onwards.

  • The Le Danois Drift was generated in both interglacial and glacial periods.

  • Cyclic features of the Le Danois Drift lost their expression after the late Quaternary.

  • Modern water masses may not be involved in the built-up of contourite drifts.

Abstract

The evolution of the Le Danois contourite depositional systems (CDS) during the Pliocene and Quaternary was investigated based on high-resolution seismic reflection data. From old to young, six seismic units (U1–U6) bounded by major discontinuities (H1–H6) were identified. Regarding variations of the bottom-current circulation, four evolution stages of the Le Danois CDS were identified, including onset (~5.3 to 3.5–3.0 Ma), initial (3.5–3.0 to 2.5–2.1 Ma), intermediate (2.5–2.1 to 0.9–0.7 Ma) and drift-growth (0.9–0.7 Ma to present day) stages. The CDS associated with the Atlantic Mediterranean Water (AMW) along the middle continental slope initiated at ~3.5–3 Ma and was widely built after the Mid-Pleistocene Transition (MPT; 0.9–07 Ma). At a shallower water depth, a second CDS associated with the Eastern North Atlantic Central Water (ENACW) started to develop from the late Quaternary (~0.47 Ma) onwards. In the AMW-related drift system, the Le Danois Drift was generated both under glacial and interglacial climatic oscilations. Repeated internal structures in unit 5 that consist of acoustically transparent lower parts, moderate amplitude upper parts and high amplitude erosional surfaces at the top, are compared with interglacial/glacial cycles since the middle Pleistocene to the present day. These cyclic features suggest coarsening-upward sequences of the Le Danois Drift and processes related to enhanced AMW during glacial stages. The estimated sedimentation rate of the Le Danois CDS reached a maximum during the MPT (at least ~27 cm/ky) and then decreased until present-day (~5 cm/ky). Variations of sedimentary stacking patterns and processes of the Le Danois CDS imply full domination of the intermediate water mass along the central Atlantic and southwest European continental slopes from the late Pliocene (~3.5–3.0 Ma) onwards.

Introduction

Contourite drifts are extensive sediment bodies built up through the action of alongslope bottom currents (Rebesco et al., 2008). When contour-following alongslope processes dominate over gravitational downslope processes along continental margins, Contourite Depositional Systems (CDS) may develop as various depositional (drifts) and erosive features (Hernández-Molina et al., 2008). In the NE Atlantic, circulation of the Mediterranean Outflow Water (MOW) leads to the generation of persistent bottom currents along the southwest European continental margin (van Aken, 2000b; Rogerson et al., 2012). In order to be able to distinguish pure thermohaline outflow (MOW) in the most proximal site to the source (the Strait of Gibraltar at the Gulf of Cádiz), the modified waters in the NE Atlantic are referred to as the Atlantic Mediterranean Water (AMW) (Rogerson et al., 2012; Flecker et al., 2015; Liu et al., 2019). Domination of Mediterranean water along the North Atlantic margins has resulted in the development of various contourite drifts and CDS (Collart et al., 2018; Hernández-Molina et al., 2011; Llave et al., 2006; Mena et al., 2018; Van Rooij et al., 2007, Van Rooij et al., 2010; Liu et al., 2019). These contourites display large variations in size, shape and vertical position along MOW/AMW pathways, thus provide valuable sedimentary records of the margin evolution and paleoceanography (Van Rooij et al., 2007; Ercilla et al., 2011; García et al., 2016; Hernández-Molina et al., 2016).

In the Gulf of Cádiz, the generation of the Cádiz CDS coincided with the first MOW enhanced stage during the early Pliocene (Llave et al., 2011; Roque et al., 2012; van der Schee et al., 2016). From the early Quaternary (~2 Ma) onwards, the Cádiz CDS experienced a major growth stage (Hernández-Molina et al., 2016). At the distal AMW site, sediment records from Goban Spur and Porcupine Seabight document extreme erosive events resulted from the AMW since the late Pliocene (Van Rooij et al., 2007; Huvenne et al., 2009; Thierens et al., 2013; Delivet et al., 2016). The ages of erosional events are difficult to reconstruct from sediment thicknesses and stacking pattern data due to erosional activities (Raddatz et al., 2011). Fast accumulation rates of the Porcupine CDS have only been recorded after the middle Pleistocene (0.725–0.65 Ma), which is more than ~1 My late compared to the Cádiz CDS (Huvenne et al., 2009; Hebbeln et al., 2016). There are still temporal gaps of MOW/AMW variability from proximal to distal sites. As such, a linkage between the Gulf of Cádiz and Porcupine Seabight is needed to document regional paleocirculation and associated sedimentary influences along the continental margins.

The Cantabrian continental margin (Fig. 1), positioned between the Gulf of Cádiz and Porcupine Seabight, lies in an understudied region with regard to AMW paleoceanography. Extensive submarine canyon systems interrupted alongslope transport of sediments, suggesting that downslope processes dominated (Weaver et al., 2000; Toucanne et al., 2008; Mulder et al., 2012). However, contourite depositional systems are identifiable among these canyon systems (Ercilla et al., 2008; Iglesias, 2009; Van Rooij et al., 2010; Liu et al., 2019), providing a unique opportunity to exam the AMW paleoceanography at an intermediate site. In the Le Danois Bank region, there are three water masses; 1) Eastern North Atlantic Central Water (ENACW), 2) AMW and 3) Labrador Sea Water (LSW) (Liu et al., 2019). Topographically constrained morphologies strongly intensify bottom currents, resulting in more frequent spatial variations of the related contourite features (González-Pola et al., 2012; Liu et al., 2019). A regional seismic stratigraphy of the Le Danois CDS has been addressed by Van Rooij et al. (2010), albeit on a relatively small set of seismic profiles. The CDS initiated during the early Pliocene and underwent a major growth stage from the late Pliocene onwards (Van Rooij et al., 2010). However, detailed insight of Pliocene-Quaternary sedimentary processes and evolution of the Le Danois CDS, as well as past circulation patterns of the AMW, are not well understood.

This work describes the seismic stratigraphy and sedimentary stacking patterns of the Le Danois CDS. The main objectives of this study are: (1) to document the temporal variability of contourite features; (2) to discuss sedimentary processes and evolution of drift systems; (3) to improve the understanding of past dynamics of intermediate water masses through the late Pliocene to the present day along the Cantabrian continental slope. The location of the Le Danois CDS is key for understanding past AMW variability between its proximal and distal sites. Newly acquired seismic data allows the identification of Le Danois contourite features in a higher lateral and temporal resolution.

Section snippets

Geological setting

The Le Danois Bank region, located at the Cantabrian continental margin, consists of an intraslope basin, the Le Danois Bank, the Lastres Canyon and the Asturias continental shelf (Figs. 1a and 2c). Contourite drifts and associated moats are present in the intraslope basin (Van Rooij et al., 2010; Liu et al., 2019). Among them, the Le Danois Drift is located between 790 and 1080 m water depth, while the Gijón Drift is located between 320 m and 1060 m water depth (Fig. 2c). The small Asturias

Methodology

This study is based on single channel sparker and multi-channel airgun reflection seismic data (Fig. 2a). High-resolution single channel seismic data were acquired using a 500 J energy SIG sparker (120 electrodes) during the R/V Belgica cruise ST1118a in 2011. Penetration of the acoustic signal is around 500 ms TWT. The dominant frequency is around 800 Hz and vertical resolution is about 1.5 m. A total of 27 sparker seismic lines, with a NNE-SSW (12 lines) and W-E to WNW-ESE (15 lines)

Seismic stratigraphic framework

Based on seismic interpretation of the sparker and airgun seismic profiles, six seismic units (U1–U6) bounded by major discontinuities (H1–H6) have been identified from old to young, respectively (Fig. 3, Table 1). The penetration of the sparker source (~500 ms TWT, while ~1500 ms TWT for airgun source) inhibits observation of the lowermost unit 1 (Fig. 4, Fig. 5). Unit 2 can only be recognized within the northern part of the intraslope basin (Fig. 3, Fig. 4), whereas both units could be

Chronostratigraphic framework

The chronostratigraphic framework in this study is established through the correlation of the seismic stratigraphy with limited regional borehole information (Cadenas and Fernández-Viejo, 2017). The tectonic and sedimentary evolution of the Cantabrian continental margin was described from seismic reflection data and borehole logs, which allowed the construction of a tentative chronostratigraphic framework (Ercilla et al., 2008; Iglesias, 2009; Van Rooij et al., 2010; Cadenas and

Conclusion

The Pliocene-Quaternary variability of contourite features and depositional processes of the Le Danois CDS illustrate past bottom-current circulation patterns along the Cantabrian continental margin. During the early Pliocene the AMW was very weak and the effective interaction of it along the Cantabrian continental margin initiated during the late Pliocene (3.5–3.0 Ma). During the early Quaternary (2.5–0.9 Ma), internal waves resulted from turbulent mixing of water masses and induced

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was carried out within the framework of a Chinese Scholarship Council “CSC Grant” (201506410062). This study built on achievements of project ESF Euromargins MOUNDFORCE, EC FP5 RTN EURODOM and EC FP6 HERMES 626 (GOCE-CT-2005-511234-1). The research cruises namely framed within the ECOMARG (REN2002-624 00916/MAR) and MARCONI (REN2001-1734 C03-01/M) projects. Shiptime RV Belgica was provided by BELSPO and RBINS–OD Nature. This study was conducted in collaboration with “The Drifters”

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