currents on sediments is twofold: they move sediments prior to and after deposition.
In the first scenario (prior to deposition) sediment is transported and deposited by
bottom currents favouring the formation of large-scale, plastered, and sheeted drifts,
which contribute to the building of the open continental slope, base of slope, and the
infilling of the basins (Fig. 6.6); in this scenario sediment deposition occurs far from
the sources of continental sediments, i.e. rivers and streams, coastal erosion, and
including Saharan aeolian input (e.g. Jiménez-Espejo et al. 2008; Moreno et al.
2002; López-Gonzalez et al. 2019). This activity has been occurring at least since the
opening of the Strait of Gibraltar, at around 5.33 Ma (Ercilla et al. 2016; Juan et al.
2016). When sediment reaches the sea, the AW, LMW, and DMW drive the
alongslope transport that provokes the dispersion and distribution of sediment
(mostly the fine sediment) over large areas of the Alboran Sea (Fig. 6.15). In addition
to this alongslope component of sediment transport, there is a downslope component, also exerted by the water masses, specifically by the water mass interfaces.
These interfaces favour the occurrence of nepheloid layers with a relatively high
content of fine sediments (e.g. McCave and Tucholke 1986) that move sediment
downslope (Fig. 6.15). Deposition occurs when sediment motion velocity decays,
producing the rapid settling of the suspended sediment, and thus drift formation. The
sediment motion is related to large-scale velocity variations in the AW, LMW, and
DMW pathways, which are themselves mostly governed by the interplay between
the trend and morphology of the Iberian and African margins and basins (Fig. 6.1a),
together with the Coriolis Force. The predominance of fine contourites in the
sampled cores (Figs. 6.13 and 6.14) points to the fact they were deposited under
generally low-energy conditions, at least in the case of the most recent (Holocene)
contourite deposits. The vertical grain-size variations observed in those facies may
be the result of changes in bottom current velocity (e.g. Toucanne et al. 2007; Stow
and Faugères 2008) and sediment provenance (e.g. Brackenridge 2014; Rebesco
et al. 2014).
When bottom currents move sediment after deposition, they contribute to
reworking the seafloor. The effects of this reworking activity are seen clearly in
erosive (moats, channels) and mixed (terrace) contourite features. Reworking occurs
when the bottom current velocity increases enough to erode the seafloor
(e.g. Nittrouer et al. 2007) (Figs. 6.6 and 6.15) and it occurs at two different scales
in the Alboran Sea: (1) local; and (2) regional. Local accelerations of the DMW
occur when this water mass is confined by the Alboran Trough and Al-Hoceima
Valley (Fig. 6.1a). Along the Alboran Trough, the seafloor is mostly eroded,
although channel-related drifts do form when the velocity decreases. Conversely,
in the Al-Hoceima Valley there is only erosion, and the DMW transports the eroded
sediment along the African margin, towards the Strait of Gibraltar. Locally, the
LMW and DMW also accelerate when they interact with seamounts and steep
scarps, producing seafloor reworking at the break of the steep slopes (Figs. 6.6 and
6.15). This reworking contributes to excavating erosive moats that constitute striking
depressions at the feet of the seamounts (Palomino et al. 2011; Ercilla et al. 2016).
This reworking/erosion contributes to the relatively high concentrations of
suspended sediment at the benthic boundary layer that is deposited close to the
190
G. Ercilla et al.
In the first scenario (prior to deposition) sediment is transported and deposited by
bottom currents favouring the formation of large-scale, plastered, and sheeted drifts,
which contribute to the building of the open continental slope, base of slope, and the
infilling of the basins (Fig. 6.6); in this scenario sediment deposition occurs far from
the sources of continental sediments, i.e. rivers and streams, coastal erosion, and
including Saharan aeolian input (e.g. Jiménez-Espejo et al. 2008; Moreno et al.
2002; López-Gonzalez et al. 2019). This activity has been occurring at least since the
opening of the Strait of Gibraltar, at around 5.33 Ma (Ercilla et al. 2016; Juan et al.
2016). When sediment reaches the sea, the AW, LMW, and DMW drive the
alongslope transport that provokes the dispersion and distribution of sediment
(mostly the fine sediment) over large areas of the Alboran Sea (Fig. 6.15). In addition
to this alongslope component of sediment transport, there is a downslope component, also exerted by the water masses, specifically by the water mass interfaces.
These interfaces favour the occurrence of nepheloid layers with a relatively high
content of fine sediments (e.g. McCave and Tucholke 1986) that move sediment
downslope (Fig. 6.15). Deposition occurs when sediment motion velocity decays,
producing the rapid settling of the suspended sediment, and thus drift formation. The
sediment motion is related to large-scale velocity variations in the AW, LMW, and
DMW pathways, which are themselves mostly governed by the interplay between
the trend and morphology of the Iberian and African margins and basins (Fig. 6.1a),
together with the Coriolis Force. The predominance of fine contourites in the
sampled cores (Figs. 6.13 and 6.14) points to the fact they were deposited under
generally low-energy conditions, at least in the case of the most recent (Holocene)
contourite deposits. The vertical grain-size variations observed in those facies may
be the result of changes in bottom current velocity (e.g. Toucanne et al. 2007; Stow
and Faugères 2008) and sediment provenance (e.g. Brackenridge 2014; Rebesco
et al. 2014).
When bottom currents move sediment after deposition, they contribute to
reworking the seafloor. The effects of this reworking activity are seen clearly in
erosive (moats, channels) and mixed (terrace) contourite features. Reworking occurs
when the bottom current velocity increases enough to erode the seafloor
(e.g. Nittrouer et al. 2007) (Figs. 6.6 and 6.15) and it occurs at two different scales
in the Alboran Sea: (1) local; and (2) regional. Local accelerations of the DMW
occur when this water mass is confined by the Alboran Trough and Al-Hoceima
Valley (Fig. 6.1a). Along the Alboran Trough, the seafloor is mostly eroded,
although channel-related drifts do form when the velocity decreases. Conversely,
in the Al-Hoceima Valley there is only erosion, and the DMW transports the eroded
sediment along the African margin, towards the Strait of Gibraltar. Locally, the
LMW and DMW also accelerate when they interact with seamounts and steep
scarps, producing seafloor reworking at the break of the steep slopes (Figs. 6.6 and
6.15). This reworking contributes to excavating erosive moats that constitute striking
depressions at the feet of the seamounts (Palomino et al. 2011; Ercilla et al. 2016).
This reworking/erosion contributes to the relatively high concentrations of
suspended sediment at the benthic boundary layer that is deposited close to the
190
G. Ercilla et al.