Climate-Induced Turbidity Current Activity
in NW-African Canyon Systems
R. Henrich, T.J.J. Hanebuth, Y. Cherubini, S. Krastel, R. Pierau,
and C. Zühlsdorff
Abstract A major outcome from studying the sedimentary archives of the Timiris
Canyon off Mauritania and the Dakar Canyon off Senegal was a clear climatically
controlled triggering of turbidite events. The records of both canyons display a high
frequency of siliciclastic turbidite activity during deglacial sea level rise, obviously
being induced by remobilisation of huge eolian dune fields that had expanded close
to the shelf edge during glacial exposure. Further in glacial periods, frequent turbidite beds are recorded in the Timiris Canyon and sporadic turbidites occur also
during intermediate sea level rises in the late phase of Heinrich events. The latter
seem also to be related to a general increased dust supply and remobilisation of
local dunes.
Keywords Submarine canyons • turbidite frequencies • hyperarid continental
margins • climate archives in canyons
R. Henrich ()
University of Bremen, Faculty of Geosciences and MARUM, PO Box 330 440, 28334
Bremen, Germany
e-mail: henrich@uni-bremen.de
T.J.J. Hanebuth and S. Krastel
MARUM — Center for Marine Environmental Sciences, and Faculty of Geosciences,
University of Bremen, Klagenfurter Strasse, 28359 Bremen, Germany
C. Zühlsdorff
Department of Earth Science, University of Bergen, Allégaten 41, 5007 Bergen, Norway
S. Krastel
Christian-Albrechts-Universität zu Kiel, Leibniz-Institut für Meereswissenschaften,
IFM-GEOMAR, Gebäude 4, Wischhofstrasse 1-3, 24148 Kiel, Germany
Y. Cherubini
Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences,
Telegrafenberg, B 452, D-14473 Potsdam, Germany
D.C. Mosher et al. (eds.), Submarine Mass Movements and Their Consequences,
Advances in Natural and Technological Hazards Research, Vol 28,
© Springer Science + Business Media B.V. 2010
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Introduction
The Atlantic continental margin off Saharan Africa hosts one of the largest dust
sources on Earth (Goudie and Middleton 2001) and today, hemipelagic particle flux
from coastal upwelling and dust supply are the dominant sediment sources along
the NW-African margin; subordinate fluvial input is restricted to the northernmost
sector of Morocco (Holz et al. 2004). During glacial times, the hyper-arid core
region of the Sahara was considerably expanded due to increased aridity and overall
higher wind speeds. As a consequence, dust export to the ocean increased
immensely (Sarnthein 1978; Sarnthein et al. 1982), and huge erg systems took
place on the shelves (Lancaster et al. 2002). In addition, deposition of eolian sand
turbidites occured on the slope (Sarnthein and Diester-Hass 1977).
Between 18°N and 20°N along the NW-African margin, the slope seafloor is characterised by channelled sediment transport within numerous well-developed canyon
systems and a series of shelf-cutting gullies (Fig. 1). The most noticeable system is the
meandering Timiris Canyon running westwards from the shelf break for at least
450 km down to a water depth greater than 4,000 m (Krastel et al. 2004; Antobreh and
Krastel 2006). South of this area, the margin hosts a huge young slide complex, i.e. the
Mauritania Slide Complex (Henrich et al. 2008) and no larger canyons are found until
the more humid Senegalese margin off Dakar; e.g., Dakar Canyon, Cayar Canyon
(Kounkou and Barusseau 1988). Previous studies clearly demonstrated that these canyons strongly influence the evolution of sedimentary regimes on the continental slope
(Zühlsdorff et al. 2007, 2008; Hanebuth and Henrich 2009). Since they serve as major
conduits for turbidity currents transporting shelf and upper slope material, continuous
sedimentary records inside such a system may be used as important climate archives.
This article intends to summarize our present knowledge on how these canyon systems, the Timiris Canyon off Mauritania and the Dakar Canyon off Senegal, dominate
with general sediment dynamics on these particular margins, and how their sedimentary archives document past climatic changes and sea-level oscillations.
1.1
Regional Settings
The study area off Mauritania is located close to the northernmost boundary
between the modern northern hemisphere Trade wind system and the African monsoonal system in southern direction (Nicholson 2000). Both atmospheric systems
interact with each other by shifting seasonally northward due to summer Trade
wind weakening and southward by Trade winds strengthening in winter. Similar
climatic shifts are also recognised on longer time scales, with the effect of humidification when monsoonal patterns prevail and aridification when Trade winds
become stronger. In response to these changes, strong seasonal upwelling centers
are located on the outer shelf (Wefer and Fischer 1993) which are traced by high
concentrations of organic matter in the sediments (Fütterer 1983). In glacial times,
these upwelling centres shifted towards the upper slope due to sea level lowering
(Matthewson et al. 1995; Martinez et al. 1999).
Climate-Induced Turbidity Current Activity in NW-African Canyon Systems
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Fig. 1 (a) Distribution of canyons along the NW-African margin in the sector off Mauritania and
Senegal. Locations of studied cores indicated by red dots. (b) Bathymetric map of the Timiris
Canyon compiled from Hydrosweep mapping during Cruise M58/1
North of Cap Timiris, the generally narrow Mauritanian shelf expands significantly
and forms the shallow marine Gulf d’Arguin. The continental slope off the Gulf
d’Arguin is steep with a gradient of up to 6° in its upper part (Antobreh and Krastel
2006), and the shelf edge is incised by numerous channels, especially in the south (Fig.
1). A series of ~15 tributary gullies cuts into the shelf edge merging together further
downslope to form the Timiris Canyon (Fig. 1). These tributaries have the capacity to
collect current-driven material from the shelf. In its upper course, the canyon cuts
deeply into the basement whilst a channel-levee system developed further downslope
(Antobreh and Krastel 2006). In contrast to this, the Dakar Canyon runs straight downslope incising up to 700 m in its upper course (Fig. 1). In the distal part the incising depth
decreases to less than 20 m at the continental rise and the canyon splits into a main active
channel and a channel remnant filled with stratified sediments.
1.2
Methods
Shallow-acoustic and seismic profiling and sediment coring were carried out during
Cruises M58/1 (Schulz et al. 2003) and M65/2 (Krastel et al. 2006) with the German
research vessel Meteor. In this study, we use published data from sediment cores from
the Dakar Canyon (Fig. 1, Cores 9612–9615) and from the upper Timiris Canyon (Fig. 1,
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Fig. 2 Parasound profile crossing the lowermost part of the Timiris Canyon in NE-SW direction.
Core locations 9624 on the north-western levee, 9,623 on a terrace inside the channel, and 9,626
in a small side-channel incising the south-eastern levee are indicated. Inlay displays Hydrosweep
bathymetry with core positions
Core 8509; Zühlsdorff et al. 2007, 2008) as well as new data from the lower Timiris
Canyon (Fig. 2). A bathymetric map of the distal Timiris Canyon is displayed in Fig. 2.
In the upper course of this sector the canyon runs SE-NW incising 90 m deep into the
seafloor, whereas in the lower course the canyon turns westward and becomes much
shallower towards its end with an incision depth of less than 20 m. Locations of cores
comprise (1) an NE-SW core traverse in the upper sector: Core 9624 recovered in a
northeastern levee position, Core 9623 recovered from a 40–50 m high elevated terrace
inside the canyon, and Core 9626 taken from a accessory, shallow channel incising the
southwestern levee. Further, (2) Core 9627 was recovered a few km north of the distalmost, shallow exit of the canyon and (3) Core 9628 about 20 km northeast of the upper
course. The main box in Fig. 2 presents a Parasound profile across the sediment traverse
of Cores 9624–9623–9626 displaying a well-developed horizontal reflection pattern for
all core locations which is typical for hemipelagic background sedimentation with
intercalation of thin turbidite beds. In the thalweg, a sharp dense reflection pattern is
recognised indicative of either consolidated seafloor sediments or thick and coarse sand
beds. Here, a coring attempt was not successful. The overall distribution of sediment
cores over different elevations offers the opportunity to estimate volumes of the suspensions clouds running through the lower Timiris Canyon by detecting the level of overspill, e.g. over the 40-m elevated terrace in the canyon, within the small channel on the
shallow elevated SW-levee as well as on the highly elevated NE-levee. In addition, Core
9627 provides as an excellent control position for over-spilling suspension clouds discharging at the end of the canyon into the abyssal plain.
Climate-Induced Turbidity Current Activity in NW-African Canyon Systems
451
Sedimentary facies were characterised by visual core description and by structural
and textural analysis of x-ray radiographs. Standard sedimentological and geochemical
parameters were routinely determined on all cores in 5- to 10-cm resolution. Carbonate
and total organic carbon contents were measured and the distribution of major elements
of terrigenous and marine origin was analysed in 1-cm resolution with a XRF core scanner. The stratigraphy of all cores relies on (1) numerous radiocarbon samples converted
into calibrated ages using CALIB in the latest version 5.0.1 (Stuiver et al. 1998; all ages
shown here are given in cal ka BP) and (2) by determination of established marine
oxygen isotope events taken from the oxygen isotope record measured on the planktonic foraminifer Globigerinoides ruber (Figs. 3 and 5).
Fig. 3 Sedimentary records of Cores 9624, 9623 and 9626 displaying distribution of turbidite
types and variations in composition of the hemipelagic background sediments. AMS-14C dates are
indicated by red arrows; blue arrows mark age determinations from Marine Isotope Stage (MIS)
events in the d18O record
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2.1
R. Henrich et al.
Results and Discussion
Characterization of Turbidites
Figure 3 displays the sedimentary records of cores along the NE-SW traverse across
the lower Timiris Canyon. Hemipelagic background sediments comprise pale brownish to pale greenish foram-nanno and mixed siliceous-carbonaceous muddy hemipelagites. In addition, occasionally grey siliceous silty terrigenous mud layers are
observed. A prominent structural feature in these sediments is moderate bioturbation
(Fig. 4). Within these hemipelagic sediments numerous turbidite layers are intercalated. There are no turbidites during the last 10 ka within the interglacial sediment
sections and only a few thin layers during MIS-5 stadials. In contrast, the glacial and
deglacial sections bear numerous turbidite layers of different textural and structural
composition and of various thicknesses. All turbidites reveal a sharp but non-erosive
base and internal structural variations of Bouma C-D-E and D-E sequences. Several
thick and coarse-grained turbidites show a typical fining-upward sequence with intercalated coarse to medium to fine greyish quartz sand, such as in Core 9623 from the
terrace and 9626 from the small channel incised on the SW levee (Fig. 4a).
In Core 9623 an amalgamated bed occurs evidencing that two turbidity currents
interacted with each other. Admixture of fine biogenic shell debris comprising fragments of molluscs, bryozoans and echinoids clearly reveals the outer shelf/uppermost continental slope as source area for these turbidites. Core 9624 on the NE
levee bears numerous thin (often only few centimetres thick) turbidite layers composed of olive grey homogenous mud. Occasionally, a very thin (milliimetres to a
few centimetres) layer of dark grey very fine sand occurs at their base. Based on
structural and textural evidence in radiographs these mud turbidites can be classified as typical overspill deposits. These overspill turbidites also occur in the basal
sections of Cores 9623 and 9626, and in particular in the lower section of Core
9627 recovered at the exit of the canyon. Fig. 4b displays a sediment section from
Core 9627 which nicely illustrates variations in flow dynamics of these overspill
deposits. The standard beds consist of: (1) a basal parallel-laminated and microcross-bedded very fine sand, (2) a middle part of alternations of parallel-laminated
silt and mud layers which are occasionally modified by convolute lamination, and
(3) a top layer of homogenous mud. Beside these standard beds some other beds
show a repetition and a variable combination of these characteristics, likely reflecting interaction of different suspension clouds.
2.2
Terrigenous Sediment Supply to the Hemipelagic
Background Sediments and Turbidite Occurrence
Carbonate and TOC records as well as records of the terrigenous portion (e.g. XRF
counts of Fe and Ti) in hemipelagic sediments of the past 140 ka in Core 9624,
Climate-Induced Turbidity Current Activity in NW-African Canyon Systems
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Fig. 4 (a: left) Photo of Core 9623 illustrating the high variety of turbidite types. From base to
top: mud spill-over turbidites, a amalgamated siliciclastic turbidite bed, a siliciclastic turbidite
with fining upward quartz sand and shell hash. (b: right): Radiographs and drawing of sedimentary structures and lithology of Core 9627 (162 to 237 cm) displaying the flow dynamics of seven
spill-over turbidites and hemipelagic background sediments. Abbreviations: HPaf,mb – hemipelagite
with abundant foraminifers, moderate bioturbation; TSmc – siliciclastic spill-over turbidite with
micro-cross bedding; TMfl,cl – silty muddy spill-over turbidite with faint sub-mm-sized parallel
lamination (fl), convolute lamination(cl)
from the NE levee, reveal pronounced shifts in the sedimentary regime on the lower
continental slope (Fig. 5). Periods of increased terrigenous supply indicated by
maximum values in Fe and Ti (Fig. 6) correlate with intervals of elevated TOC
values (Fig. 5), i.e. at 70–15 ka and 140–129 ka. Sedimentation rates increase from
3–5 cm/ka during interglacial conditions to 20–40 cm/ka during glacial and early
deglacial periods. This result suggests that organic matter and terrigenous material
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Fig. 5 Oxygen isotope, carbonate and TOC record of hemipelagic sediments in Core 9624 from
the north-western levee of the distal Timiris Canyon. Stratigraphic model displaying AMS-14C
ages (red arrows) and ages points deduced from MIS events (blue arrows) according to the time
scale of Martinson et al. (1987)
was supplied by the same mechanism, e.g. by scavenging the water column of
organic matter and terrigenous particles. Previous studies have shown that Fe and
Ti are good proxies for dust input to the ocean in this region (Hanebuth and
Lantzsch 2009; Hanebuth and Henrich 2009). This observation is corroborated by
results from grain-size analysis of contemporaneous deposits from the upper
Timiris Canyon, revealing typical silt-size spectra of dust (Zühlsdorff et al. 2007).
Hence, during these periods much stronger Trade winds increased offshore dust
supply, strengthened upwelling intensity at the uppermost continental slope and
increased sediment accumulation and subsequent downslope re-suspension over
wide areas.
With regard to these aspects, the pronounced cyclic patterns of Fe and Ti during
MIS 5 (Fig. 6) appear to be of particular interest since they monitor climatic
Climate-Induced Turbidity Current Activity in NW-African Canyon Systems
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Fig. 6 Record of the portion of terrigenous material in the hemipelagic background sediments of
Core 9624 as evidenced by elemental Fe and Ti intensities. Intervals of low, intermediate and high
contents are marked by yellowish bars. Time intervals are indicated over the red arrows. In addition in the lower panel the number of spill-over turbidite events is indicated by olive bars and
below their occurrence time is displayed
oscillations with maximum values during the cold MIS events 5.4 and 5.2 and
minima during the warm intervals 5.5, 5.3 and 5.1. Turbidite over-spilling the
NE-levee of the lower Timiris Canyon is observed during the following periods:
139–132 ka (5 events), 98 ka (1 event), 42 ka (1 event), 37–32 ka (6 events), and
26–19 ka (23 events). Obviously, re-suspension processes on the continental slope
are coincident with these climatic peaks suggesting a trigger closely linked to climatic forcing. Release of turbidity currents always took place during these periods
of high terrigenous supply (Fig. 6).
As a conclusion, these turbidites were triggered mainly by variations in overall
sediment supply. During high-glacial and early deglacial time, tributary gullies on
the uppermost slope were rapidly and repetitively filled with subsequent release of
turbidity currents and large volumes suspension clouds passed through the lower
Timiris Canyon leading to over-spilling on the NE levee. These turbidites were
induced by remobilisation of huge eolian dune fields that expanded close to the
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shelf edge during glacial exposure (Lancaster et al. 2002). In addition to this scenario, partial flooding of the outer shelf during successive sea-level rises in MIS 3
may have caused re-mobilization of eolian dune material.
2.3
History of Turbidite Frequency: Comparing the Timiris
and Dakar Canyon Systems
Figure 7 summarizes the history of turbidite activity in the Timiris Canyon. The
NE-SW sampling traverse allows monitoring of the surface level of suspensions
clouds. The highest level and, thus, largest-volume suspension clouds are indicated
by over-spilling turbidites detected in Core 9624 on the NE levee. A slightly lower
level is reached by turbidites observed in Core 9626 but did not extend to the level
of Core 9624. The smallest-volume turbidites are found on the terrace within the
canyon (Core 9623). An additional monitoring position is provided by Core 8509
from a terrace within a wide area in the upper Timiris Canyon (Zühlsdorff et al.
2008). Following this depositional pattern, it is possbile to reconstruct the temporal
variability of turbidite activity as follows: (1) in periods of maximum glacial lowstand (MIS 6, late MIS 3, LGM), large-volume turbidites frequently filled the entire
distal Timiris Canyon spilling over the NE levee. Additionally, single events of
large volume over-spilling were recorded at 98 and 64 ka; (2) during early
Termination I, over-spill turbidites frequently reached the small channel incised on
the SW levee, but are not recorded on the higher elevated NE levee; (3) within the
canyon (Core 9623), turbidite activity continued until 11ka, but without spilling
over the levees on either side. (4) The youngest turbidite activity is recorded in
Core 8509 from the upper Timiris Canyon, revealing 900-year cycle during the
entire Holocene (Zühlsdorff et al. 2008). These turbidites, however, attained much
lower volumes, since they did not reach the distal Timiris Canyon. This correlation
is, to our knowledge, the first documentation of a close coupling of short-term
climatic cycles and turbidity current activity. The suggested basic trigger is that the
Fig. 7 Turbidite recurrence pattern in the lower and upper Timiris Canyon
Climate-Induced Turbidity Current Activity in NW-African Canyon Systems
HE6
YD HE2 HE4
HE5
HE1 HE3
HE10
457
HE11HE12 HE13 HE14
Turbidite stack
Timiris Canyon
Turbidite stack
Dakar Canyon
0
Sea level –40
[mbms] –80
–120
0
20
40
60
80
100
120 140
Age [ka BP]
160
180
200
Fig. 8 Comparison of turbidite activity in the Timiris Canyon and the Dakar Canyon. Stack plot
of all turbidite events recorded in these canyon systems compared to a global sea-level curve
(Siddall et al. 2003). White space above and below the intervals with turbidite events represents
continuous hemipelagic background sedimentation. Olive bars: special Holocene record
rhythmically appearing turbidites were linked to dust events recorded in a finegrained shelf depocentre for the past 5 ka (Hanebuth and Henrich 2009).
Comparison of the evolution of turbidite activity in the Timiris Canyon off
Mauritania with that of the Dakar Canyon off Senegal shows that both areas are
today influenced by intensive dust input. Dust supply is much higher off Mauritania,
however, as it is situated offshore the Saharan desert. Figure 8 displays stacked
records of turbidite activity in both canyons over the last two glacial-interglacial
cycles together with the sea-level reconstruction of Siddall et al. (2003).
In both canyons, high turbidity current activity is observed during glacial sealevel lowstands. High turbidite frequency also appears in both canyons at their
terminations. In the Dakar Canyon, additional turbidites coincide with the timing
of Heinrich events in the North Atlantic. During times of Heinrich events, the continental climate changed rapidly with evidence for concurrent higher dust supply
provided by high Ti/Ca ratios (Fig. 6). Increased aridity, enhanced wind strength
and reduced vegetation cover in the region of the modern Saharan-Sahelian transition may have provided a considerable source for this dust. In the Dakar Canyon,
turbidity current activity ends within Termination I, whereas only in the Timiris
Canyon, do recurrent small-volume turbidites extend over the entire Holocene
(Zühlsdorff et al. 2008; Hanebuth and Henrich 2009).
3
Conclusions
As summary, we postulate the following evolutionary model for the activity of
both NW-African canyon systems. During general and sporadic (Heinrich) sealevel lowstands, voluminous turbidity currents passed frequently through the
Timiris Canyon. Subsequently, during deglacial sea-level rise, flooding of the
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shelves induced erosion of dune sands and resulted in high turbidity current activity in both canyons. A major regional contrast is observed for time intervals of
sea-level highstand when the hyperarid Timiris region reveals recurrent dust
turbidites, whereas the Dakar Canyon is completely quiet due to a humid and
vegetated hinterland.
Acknowledgements M. Gutsch, H. Heilmann and B. Kockisch are thanked for their technical
assistance in the laboratory and C. Henrich is thanked for compilation of the radiograph line
drawings. d18O analyses and XRF core scanning were carried out at the lab facilities of MARUM.
The constructive reviews by Anabela Oliveira and Henko de Stigter are gratefully acknowledged.
This work was funded through the DFG-Research Center/Excellence Cluster “The Ocean in the
Earth System”.
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