Monsoon control on channel avulsions in the Late Quaternary Congo Fan

Quaternary Science Reviews 204 (2019) 149e171

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Monsoon control on channel avulsions in the Late Quaternary Congo Fan M. Picot a, T. Marsset b, L. Droz a, *, B. Dennielou b, F. Baudin c, M. Hermoso d, 1, M. de Rafelis c, T. Sionneau b, M. Cremer e, D. Laurent b, M. Bez f IUEM, Laboratoire G
eosciences Oc
ean, UMR6538 (CNRS-UBO-UBS), Place N. Copernic, 29280, Plouzan
e, France Ifremer, REM-GM-LES, BP 70, 29290, Plouzan
e, France c Sorbonne Universit
e, CNRS-INSU, Institut des Sciences de la Terre de Paris, ISTeP UMR7193, 75005, Paris, France d University of Oxford, Department of Earth Sciences, South Parks Road, Oxford, OX1 3AN, United Kingdom e University Bordeaux 1, EPOC, UMR5805, 33615, Pessac, France f Centre Scientifique et Technique Jean Feger, Total, Avenue Larribau, 64018, Pau, France a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 June 2018 Received in revised form 22 October 2018 Accepted 29 November 2018 Available online 10 December 2018

The mechanisms governing the development of deep-sea fans is a matter of debate and their understanding at Milankovitch and millenial time-scales is challenged by complex architectures and the lack of material suitable for establishing reliable chronostratigraphies. Based on a detailed investigation of the emplacement of channel-levee-lobe systems and their successive bifurcations and seaward-landward migrations (Picot et al., 2016), we present for the last 210 ka a detailed chronostratigraphic framework of the migration pattern based mainly on radiocarcarbon dating of channels and lobes abandonment or initiation. The comparison of architectural cycles to proxies of external factors (sea-level and climate) suggest that sea-level changes have minor impacts on the architectural evolution of the Congo Fan. In contrast, comparison with climate and environmental proxies (West African monsoon, pollen grain assemblages, Kaolinite/Smectite) evidences a major impact on the timing of the development of the architectural pattern, at least for the last 38 kyr. A general scheme of the growth pattern of the Congo Fan in link with climate evolution is proposed: the stacking pattern of the Congo Fan responds to humidity/aridity cycles that generate successive progradation and retrogradation of avulsion points. These climate changes are under the control of the West African monsoon which, by controlling the rainfall and vegetation on the watershed, ultimately impacts the composition and volume of the sediment source and the transport capacity of gravity flow generated in the canyon and flowing in the turbiditic channels. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Quaternary Monsoon South atlantic Sedimentology-marine cores Congo Turbidite system Sub-bottom profiles

1. Introduction Submarine deep-sea fans (also called turbidite systems) are the largest depositional accumulations in the oceans. The occurrence of sand (porous material) in channels and lobes and mud (low permeability material) in levees explains that they are frequently surveyed by the oil industry and are among the biggest oil and gas plays in the world. They also represent precious archives for past

* Corresponding author. E-mail address: [email protected] (L. Droz). 1
, CNRS-INSU, Institut des Sciences de la Present address: Sorbonne Universite Terre de Paris, ISTeP UMR 7193, 75005, Paris, France. https://doi.org/10.1016/j.quascirev.2018.11.033 0277-3791/© 2018 Elsevier Ltd. All rights reserved.

continental and marine environments. Therefore, understanding the forcings that govern their emplacement and development is therefore a matter of high societal and scientific interest. In most cases, they are linked to major fluvial systems and are fed by terrigeneous inputs during low sea-levels when the canyons connect to the rivers. Additionally high sedimentation rates in these fans make them important archives of past environmental changes. The growth of turbidite systems is potentially governed by the interplay of numerous forcing factors, internal (topographic compensation, sinuosity of channels, dynamics of currents …) and external (tectonic, eustatism, climate). External controlling factors have been emphasized in numerous studies, including eustacy (Kolla and Macurda, 1988; Posamentier and James, 1991; Lopez, 2001;

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Posamentier and Kolla, 2003; Hodgson et al., 2006; Bourget et al., 2013), intra-basinal and extra-basinal tectonic movements (Hoorn et al., 1995, in the Amazon Fan; Prather et al., 1998, in the Gulf of Mexico; Broucke et al., 2004; Turakiewicz, 2004 and Anka et al., 2009, in the Congo Fan; Somme et al., 2009, in the Corsica Fans) and climatic changes in the drainage basin (e.g. Milliman and Syvitski, 1992; Zabel et al., 2001, in the Niger delta; Toucanne et al., 2008, in the Armorican Fan; Ducassou et al., 2009 in the Nile Fan) sometimes linked to orbital periodicities (Foucault and Fang, 1987; Weltje and de Boer, 1993; Schneider et al., 1997; Weber et al., 2003; Ducassou et al., 2009; Cantalejo and Pickering, 2015; Scotchman et al., 2015). External forcings work in combination with internal forcings to control fan growth (Hodgson et al., 2006), such as the position and timing of channel avulsion along the basin (Flood and Piper, 1997; Pirmez et al., 1997; Lopez, 2001; Kneller, 2003; Maslin et al., 2006; Kolla, 2007; Labourdette and Bez,
lat et al., 2010; Armitage et al., 2012; Ortiz-Karpf et al., 2010; Pre 2015; Morris et al., 2016). From architectural analysis, Marsset et al. (2009) and Picot et al. (2016) hypothesized that the architecture of the Congo turbidite system is controlled by a combination of internal and climatic factors. Indeed, climate (temperature and rainfall) controls numerous parameters in a watershed such as the riverine runoff, vegetal cover, rate and type of erosion (mechanical or chemical) and the transport capacity of rivers. Consequently, the climate controls the sediment supply (nature, quantity, frequency) transferred into the basin and therefore impacts the development of turbidite systems (e.g. Zabel et al., 2001, for the Niger delta; Ducassou et al., 2009, for the Nile Fan; Toucanne et al., 2012, for the Armorican Fan; Hessler et al., 2018, in the Gulf of Mexico). Variations in sediment supply determine transport efficiency of turbidity currents and sediment distribution in turbidite systems and consequently their growth pattern (Mutti and Normark, 1987; Reading and Richards, 1994). However, the precise forcing of climate on sediment supply remains poorly understood because of the difficulty to distinguish the direct control of the climate from the control of the climate-driven eustatic fluctuations as is debated for the Amazon Fan (Flood and Piper, 1997; Pirmez et al., 1997; Lopez, 2001; Maslin et al., 2006). Due to the incision of the canyon into the estuary (Moguedet, 1988), the Congo turbidite system has had a constant connection with the Congo River at least since the Pliocene (Ferry et al., 2004) and the turbidity current activity during the present-day high sea level is attested by ruptures of communication cables in the canyon (Heezen et al., 1964) and turbidity current detected by mooring lines (Khripounoff et al., 2003; Vangriesheim et al., 2009; AzpirozZabala et al., 2017). This suggests a permanent sediment supply, regardless of sea level changes. These characteristics make the Congo sedimentary system an ideal case study for trying to elucidate climate forcing on the sedimentation of a deep-sea fan. Based on a huge database including geophysical and core data, the Late Quaternary architecture of the Axial Fan of the Congo turbidite system (over ca. 210 kyr) was described as a succession of 4 prograding-retrograding architectural cycles (A-D, Fig. 1D) bounded by maximal retrogradation of channel-levee-lobe systems (Marsset et al., 2009; Picot et al., 2016). No chronostratigraphic framework was available so the forcing factors on these cycles remained hypothetical (Picot et al., 2016). The present contribution aims to provide a chronostratigraphic framework based on the lithostratigraphic and chronostratigraphic analyses of sediment cores collected at key locations within prograding-retrograding cycles and deals with the impact of climate and eustatic forcings on the Congo Fan growth pattern for the last 210 ka with a focus on the last 38 kyr.

2. General setting and previous work 2.1. The Congo turbidite system 2.1.1. Geological background The Congo turbidite system extends from 2000 to 5200 m water depth on the Congo-Angola margin (Fig. 1A). This margin formed from the opening of the South Atlantic Ocean in Early Cretaceous times and was affected by subsequent tectonic deformation partly caused by salt tectonics (Emery et al., 1975; Reyre, 1984; Jansen, 1985). After deposition of a first Albian-Eocene deep-sea fan (Anka, 2004; Anka et al., 2010), the combined effects of an uplift of the margin associated with a global sea level fall and high precipitation in the watershed during the Oligocene led to an increase in erosion rates and river load and thereby enhanced terrigenous supply into the Angola basin. These favorable conditions to sediment deposition led to the initiation of a Cenozoic turbidite system
ranne, (Reyre, 1984; Droz et al., 1996; Lavier et al., 2001; Anka and Se 2004). Since that time, the Congo River mouth was stabilized (Karner and Driscoll, 1999) and this point source has permanently fed the turbidite system (Droz et al., 2003). 2.1.2. Architecture and age of the Congo turbidite system since 780 ka The Quaternary Congo turbidite system includes three individual fans (Fig. 1B): Northern Fan (780-540 ka), Southern Fan (540210 ka) and Axial Fan (210 ka to present) (Droz et al., 2003). Due to the connection between the river and the canyon head, a permanent fluvial supply fed the turbidite system, throughout the Late Quaternary, even during sea-level high stand conditions (Heezen et al., 1964). The development of the Northern, Southern and Axial Fans resulted from the stacking of more than 100 channel-levee-lobe systems (Picot et al., 2016), including 52 channels belonging to the Axial Fan (Fig. 1A), the most recent channel of the Axial Fan (number Ax52) being the only one currently active with a currently developing channel-mouth lobe (e.g. Khripounoff et al., 2003; Dennielou et al., 2017). Throughout the Late Quaternary, channellevee-lobe systems have repeatedly shifted lateraly in response to channel avulsions and show prograding/retrograding architectural cycles (Marsset et al., 2009; Picot et al., 2016) corresponding to periods of increasing/decreasing channel lengths and basinward/ landward migrations of avulsion points. In the Axial Fan, four architectural cycles were identified (A, B, C and D from the oldest to the youngest) despite two zones corresponding to undifferentiated units (UP1 and UP2) of amalgamated channels and lobes without clear connecting channels (Picot et al., 2016) (Fig. 1C). The architectural parameters (Distance to Avulsion, DA and Avulsion Length, AL) were interpreted respectively as proxies of the capacity of the turbidity currents to trigger channel avulsions (DA) and of the transport capacity of the turbidity currents (AL) (Picot et al., 2016). The morphological parameters of lobes and channels outline the major control of local topography (compensation) on the architecture of the Congo Axial Fan, mainly downfan. In addition, this approach revealed exceptional upfan channel avulsions suggesting a possible trigger by external forcing factors such as sea-level and climate that control the composition and volume of the sediment source and the type of gravity flow in the turbiditic channels (Picot et al., 2016). 2.2. The Congo fluvial system The Congo fluvial system is the most important hydrographic network of West Africa, with the 4370 km long stream of the Congo River itself and a huge watershed of 3.7
106 km2, the so-called

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Fig. 1. Results from previous work on the Congo sedimentary system (modified from Picot et al., 2016) and location of studied cores. (A) The Congo sedimentary system from source (Congo River and its catchment) to sink (Quaternary turbidite system). (B) Channel-lobes map of the Late Quaternary Axial Congo Fan (210 ka to present) with location of Figs. 5 and 7 to 10 (boxes) and cores (red stars) used in this study. Blue stars: reference cores KZaï-02 and GeoB1008-3 (used for time calibration of climate and environmental proxies) and KZaï-01 used to refine the age model of GeoB1008-3 and KZaï-02 (Supplementary Material A). (C) Progradation-retrogradation cycles evidenced by the evolution of channels lengths (CL), position of avulsion points (DA) through time and avulsion length (AL ¼ CL-DA) (Picot et al., 2016). RZCS cores used in this study (and corresponding figure numbers) are positioned on the CL diagram. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Congo Basin (Van Weering and Van Iperen, 1984) (Fig. 1A). With a discharge of about 40 000 m3/s the Congo River is ranked the second largest river in the world after the Amazon River (Mouzeo, 1986; Laraque et al., 2001). However, the suspended load at the Congo River mouth, which largely consists of kaolinite, quartz, organic matter and iron hydroxides (Eisma et al., 1978) is estimated to be only 55 Mt/yr, i.e. about 20 times lower than the Amazon River discharge (Wetzel, 1993). The suspended load represents 38% of the total matter being transported by the Congo River the other

62% being transported as dissolved elements (Laraque et al., 2013) and bedload, which remains unknown. Coarse-grained sediments are trapped along the fluvial system because of the low watershed gradients (mean of 0.33‰, according to Moguedet (1988)) and because of numerous lakes (Moguedet, 1988; Wefer et al., 1998; Turakiewicz, 2004), making the Congo turbidite system a mud-rich fan. The low suspended load of the Congo River is attributed to an important chemical alteration of rocks, favored by the present-day warm and humid tropical climate (Summerfield and Hulton, 1994)

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and by the extensive vegetation cover (Seyler et al., 2006). According to Sionneau et al. (2010), the absence of chlorite and the dominance of aluminum in illite composition in continental slope sediments of the Congo-Angola margin confirms the high contribution of the Congo River sediments and the very low contribution of south-eastern trade winds in the sediments supplied onto the Congo-Angola margin and deep basin.

of kaolinite, the most abundant clay mineral, which accumulates in thick lateritic deposits. These deposits protected by a widespread vegetation cover are mainly eroded by the Congo River and transported from the continent to the ocean (Eisma et al., 1978; Pastouret et al., 1978).

2.3. Climatic and environmental conditions in the Congo watershed

This work is based on previous work that established the prograding/retrograding diagram for the period ~200 ka to present (Picot et al., 2016) (see section 2.1.2). The assumption that only one channel is active at any time is based after the analysis of seismic profiles where geometrical organization shows that channel-levees systems do not operate synchronously (Picot et al., 2016). This is also commonly admitted for many submarine fans (e.g. Curray and Moore, 1971; Damuth et al., 1983; Flood et al., 1991; Droz et al., 2003). Channel avulsion results in the abandonment of the ancient channel down-dip the avulsion point and generates a disruption of the equilibrium profile. Consequently the abandoned portion of the initial channel can no longer serve as a conduit for turbidity currents (Flood et al., 1991; Lopez, 2001; Pirmez et al., 1997). In order to provide a chronostratigraphic framework to the architectural cycles, during the Reprezaï 1 cruise (Marsset and Droz, 2010), coring targets were selected on sub-bottom profiler (SBP) lines at key locations in the avulsion network for their potentiality to provide age constraints on channel-levee-lobe systems (Fig. 1B and C, Table 1). Depth correlation of sediment cores with SBP was made in a Kingdom Suite project by converting depth in cores into ms twt using a velocity of 1480 m/s as measured at site 1077 of ODP Leg 175 (Shipboard-Scientific-Party, 1998) for depths < 35 mbsf.

High resolution climatic and paleoenvironmental reconstitutions of the last 200 kyr from marine sediments show that in the Congo watershed, rainfalls (and therefore vegetation, erosion and sediment discharge) are dominantly controlled by the western and eastern African monsoon driven by the insolation and tuned on precession cycles of 23 kyr (Schneider et al., 1994, 1995; Jahns, 1996; Gingele et al., 1998; Dupont et al., 2001; Gobet, 2008; Sionneau et al., 2010; Caley et al., 2011; Pilarczyk, 2011; Bayon et al., 2012; Dalibard et al., 2014; Hardy et al., 2016; Hatin et al., 2017). Distribution and intensity of precipitations, to some extent, are also controlled by the variations of the trade winds regime from the continent (Flores et al., 2000; Zabel et al., 2001), which induces a latitudinal migration of the precipitation front, known as the Inter Tropical Convergence Zone (ITCZ) (Jahns, 1996). This phenomenon is under the influence of the development of the high latitude polar icecap tuned on eccentricity cycles of 100 kyr (Leroux, 1993). In this respect, strengthening of the northeast trade winds during glacial periods (Marine Isotopic Stages MIS 6, 4, 2) induced a migration of the ITCZ towards the south (Jahns, 1996) and thus weak monsoons and rainfall, leading to enhance aridity. Moreover, because of its latitudinal position astride the equator, the Congo watershed is also exposed to a double maximum of insolation and of precipitation each year, and is under the influence of the sub-Milankovitch periodicities of 11.5 kyr and 5.5 kyr (Berger and Loutre, 1997; Berger et al., 2006), which could allow more precipitations even during glacial periods. However, these modulations of 11.5 kyr and 5.5 kyr are not clearly highlighted in marine sediments (Dalibard, 2011). A dominantly arid climate on the watershed favored a low river discharge (Gingele et al., 1998), development of savannah vegetation (Dupont et al., 2001; Dalibard et al., 2014), enhanced mechanical erosion (Schneider et al., 1997; Gingele et al., 1998) and consequently a high sediment production (mainly coarse grains, i.e. quartz and feldspar). Clay minerals provided by the trade winds consist of kaolinite (Kalu, 1979; Balsam et al., 1995) smectites and chlorites from old rocks (Aston et al., 1973) and illites (Bornhold and Summerhayes, 1977). A dominantly humid climate favored an increase of river discharge (Mikkelsen, 1984; Gingele et al., 1998; Abrantes, 2003; Holtvoeth et al., 2005), the development of evergreen rain forest and enhanced chemical erosion (Schneider et al., 1997; Durham et al., 2001). These environmental conditions favored the genesis

3. Material and methods

3.1. Sub-bottom profiler (SBP) SBP was operated in a chirp configuration (1.8e5.2 kHz) that offers a vertical resolution of 0.25 m and a maximal penetration of 100 m. The chirp profiles (see A in Figs. 5, 7e10) for the localization of profiles) were used to precisely describe the organization of sedimentary bodies in the area of cores and to identify the channellevee systems or lobes that were sampled and potentially dated using the cores. 3.2. Sediment cores The geographical positions, depths, lengths, and dating objectives of the seven selected Calypso cores (Fig. 1B and C) are summarized in Table 1. As sediment distortion commonly occurs during piston coring, the corer was equipped with accelerometers in order to record the relative movements of the corer and of the piston for estimating sediment over-sampling or under-sampling (Skinner and McCave, 2003; Bourillet et al., 2007). The accelerometer data

Table 1 List of sediment cores used in this study. Name RZCS-01 RZCS-06 RZCS-07 RZCS-15 RZCS-21 RZCS-25 Geo 1008e3 KZaï-01 KZaï-02

Latitude S S S S S S S



6 6 6 6 7 5 6

Longitude 0

34.175 6.2450 7.1130 58.4370 23.2360 47.7170 350

S 5 420 S 6 24.200

E E E E E E E



0

8 46.091 7 50.3820 7 50.5960 5 39.6060 4 37.8060 5 43.8400 10 190

E 11 140 E 98 4.100

Depth (m)

Length (cm)

Location

IGSN

4020 4166 4163 4973 5178 4843 3124

2193 1071 996 2192 1942 2033 1204

Channel Ax02 levee and lobe Ax19 fringe Channel Ax21 levee and lobe Ax45 Channel Ax21 levee and lobe Ax45 Channel Ax09-12 levee Lobe Ax10 and channel Ax11-12 levee Edge of the superficial lobe area fed by Ax50 channel Southern Fan hemipelagic cover

BFBGX-86912 BFBGX-86917 BFBGX-86918 BFBGX-86926 BFBGX-86932 BFBGX-86936

914 3417

1000 1820

Northern slope of the Congo deep-sea fan Southern Fan hemipelagic cover

BFBGX-86532 BFBGX-86531

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were processed with CINEMA software (Bourillet et al., 2007) and allowed better constraints to be placed on the correlation between litho- and echo-facies and to convert depths in the sediment core into depth below sea floor (cf details on the method in Toucanne et al., 2009). The cores revealed to be of good quality, with no deformation at all from the top to 10 m and only slight deformation downwards. The cores (10 me22 m long) were split lengthwise, photographed with a digital camera and a detailed description of the lithofacies was made. Lithofacies were determined on the basis of sediment composition, sedimentary structures, grain size and ichnofacies with the main aim to identify turbiditic and hemipelagic deposits. Grain size analysis was performed by a Malvern Hydro 2000G laser granulometer. Several methods were applied to obtain climatic and environmental signals and to constrain the age model of KZaï-02 reference core (see Supplementary Material A): - X-rays were acquired on 1 cm thick slabs with the Scopix system (Migeon et al., 1999). - Logging of physical properties was achieved at one-centimeter intervals with the Geotek-MSCL (Multi-Sensor Core Logger) of Ifremer to acquire gamma-density, p-wave velocities, magnetic susceptibility, spectrocolorimetry and lightness (L*). L*, in the Congo turbidite system, is related to the amount of organic matter (Nederbragt et al., 2006). - The organic carbon (Corg) and total organic carbon (TOC) were
. Pyrolysis measured at ISTeP Laboratory of Sorbonne Universite analysis was carried out to determine the TOC and the carbonate fraction (Eq-CaCO3) using a Rock Eval 6 Turbo device® (Vinci Technologies). The analysis followed the procedure described by Baudin et al. (2015) for recent marine sediments. Total carbon (Ctot) was measured using a LECO elemental analyzer. Corg was calculated as the difference between Ctot and mineral carbon assuming that carbonate is the only form of mineral carbon. - XRD analyses were carried out at Ifremer following the precise protocol of sample preparation and processing described in Holtzapffel (1985) and Bout-Roumazeilles et al. (1999). A Brüker AXS e D8 ADVANCE diffractometer (Cu anti cathode; 40 KV; 25 mA) has been used and the results have been analysed by using the Macintosh MacDiff® 4.2.5 software developed by Petschick (2000). From Gingele et al. (1998) and Sionneau et al. (2010) the Kaolinite/Smectite ratio (K/S) can be used as a proxy for the extension and intensity of freshwater plume and therefore the discharge of the Congo River.

3.3. Chronostratigraphic approach and proxies The abandonment and definitive inactivity of a channel-leveelobe system is the response to an avulsion that generates the shift of turbidite depocenters (e.g. Armitage et al., 2012; Ortiz-Karpf et al., 2015) and initiates a new system. It was identified by the occurrence of a hemipelagic deposit draping abandoned levees, both in cores and on sub-bottom profiles (when hemipelagic thickness was sufficient to be resolved, i.e. > 25 cm which is the vertical resolution of the SBP, e.g. Figs. 5 and 7 to 10). The bulk sediment composition of turbidites is dominantly controlled by the source of the sediment (i.e. the drainage basin) but also by depositional and erosional processes in turbidity currents. Therefore drawing a chronostratigraphic framework in turbiditic environments relies on the occurrence of hemipelagic interval preserved between turbidites (e.g. Hoogakker et al., 2004; Toucanne et al., 2008; Lombo-Tombo et al., 2015). The strategy developed to determine the chronostratigraphic framework of the

153

progradation-retrogradation diagram (Fig. 1C) was to ascribe ages for the hemipelagic intervals in sediment cores and, by correlation, on the SBP lines. This aim was achieved by correlating selected climatic and oceanographic proxies with those from chronostratigraphic reference sediments cores and by AMS radiocarbon dating. Piston cores KZaï-02 (from ZaïAngo 2 cruise, Cochonat, 1998), GeoB1008-3 (Schneider et al., 1997) and KZaï-01 (Bayon et al., 2012) retrieved in hemipelagic deposits of the Congo slope and base of slope were selected as chronostratigraphic references. Core KZaï-02 is located outside the Axial Fan (Fig. 1B) and is exclusively composed of hemipelagic sediments coeval with the turbiditic Axial Fan and spanning ca. 190 ka. Nearby cores GeoB1008-3 and KZaï-01 (Fig. 1B, Table 1) were used to better constrain the age model of core KZaï-02 (cf. details in Supplementary Material A). When possible, dating of terrestrial organic matter embedded in turbidites was carried out. Baudin et al. (2010) and Baudin et al. (2017) showed that organic matter in the channel-levees and lobes of the youngest channel-levee system is at least at 70e80% of terrestrial origin and we assume that plant debris in the >63 mm fraction must be close to 100% of terrestrial origin which is confirmed by a recent palynofacies study on the material of the present-day terminal lobe (Schnyder et al., 2017). Terrestrial organic debris have been transported and, therefore, have intrinsic ages older than their stratigraphic interval. However, previous studies showed an average age of ca. 300 years only for the coarse particle organic matter in the Congo River near Kinshasa (Spencer et al., 2012), and that radiocarbon dating of terrestrial organic matter can give reliable ages for turbiditic levees (Migeon et al., 2004; Savoye et al., 2009), showing that the residence time of organic debris in the Congo River is negligible for our purpose. Nevertheless, whenever possible, two neighboring levels (of about tens of centimeters) were dated to verify the consistency of the dates. The scarcity of carbonates in pelagic sediments, probably because cores are located at water depths close to lysocline or the CCD, but also because of dilution by terrigenous material, did not allow a continuous foraminiferal oxygen isotope record. Radiocarbon dating was also limited for the same reason but also because the stratigraphic records frequently extend beyond the range of radiocarbon dating (ca. 50 ka BP). Radiocarbon dating was carried out both on planktic foraminifera shells and on plant debris (Table 2). Seven dates were obtained with the Artemis program at Laboratoire de Mesure du Carbone 14 (LMC14) of CEA-Saclay (Gif-sur-Yvette, France) with the AMS (Accelerator Mass Spectrometry) and 14 dates were obtained from the Poznan Radiocarbon Laboratory (Poland). About 10 mg of planktic foraminifera were picked out of the >200 mm fraction. Most samples consist in Globigerinoides ruber specimens. When G. ruber was too scarce, other planktic species (Globigerinoides trilobus, Globorotalia menardii, Neogloboquadrina dutertrei) were also picked to fulfill the weight necessary for analysis. For plant debris, sediment was washed in order to obtain up to 30 mg of plant debris >63 mm. Calibration of all dates was made with Calib 7.0 software (Stuiver et al., 2005). The calibration curves MARINE13 and SHCal13 have been selected respectively for marine samples (foraminifera) and continental samples (plant debris) (Hogg et al., 2013; Reimer et al., 2013). Confidence interval of 95.4% (2 sigma) has been used. Oxygen isotope-based stratigraphy of RZCS-01 has been established on batches of 3e20 planktic foraminifera in the 200e400 mm fractions range. The samples consist in Globigerinoides ruber pink. Sample spacing is 10 cm in hemipelagic intervals. In turbidite sequences, the top of the sequences has been sampled and corresponds to a spacing of about 20 cm d18O was measured using a VG Isogas Prism II mass spectrometer with an on-line VG Isocarb at
. Results are expressed Oxford University or at Sorbonne Universite

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Table 2 Radiocarbon dates obtained on cores RZCS-01, RZCS-06, RZCS-07, RZCS-15, RZCS-21 and RZCS-25. Italic lines indicate samples that provided an age reversal not retained to establish the age models; ff: foraminifera. Core

Depth (cm bsf)

Material

14

Calendar age (cal yr BP)

Lab Code

RZCS-01 RZCS-01 RZCS-01 RZCS-01

1e2 192e193 401e404 432e434

bulk ff G. ruber G. ruber G. ruber

4225 ± 35 12 370 ± 60 37 050 ± 360 35 570 ± 400

4335 ± 58 13 822 ± 74 41 641 ± 281 39 703 ± 476

Poz-49624 Poz-49625 SacA 32736 SacA36245

RZCS-06 RZCS-06 RZCS-06 RZCS-06 RZCS-06 RZCS-06

1e2 278.5e280 304.5e305.5 425e429 509.5e510.5 526e527

G. ruber G. ruber G. ruber bulk ff Plant debris Plant debris

4075 ± 35 10 030 ± 50 12 010 ± 50 26 000 ± 120 41 000 ± 3000 37 200 ± 1600

4119 ± 64 10 961 ± 24/11 067 ± 66 13 426 ± 82 30 427 ± 135 44 403 ± 2597 41 107 ± 1412

Poz-49627 Poz-49628 Poz-49629 SacA 32737 Poz-64488 Poz-64489

RZCS-07 RZCS-07 RZCS-07

11e12 275e276 540.5e542

bulk ff G. ruber bulk ff

3960 ± 40 12 470 ± 60 33 640 ± 260

3949 ± 40 13 910 ± 79 38 044 ± 474

Poz-49544 Poz-49545 SacA 32738

RZCS-15 RZCS-15

1e2 83e84

N. dutertrei G. trilobus þ G. sacculiferus

10 490 ± 40 37 950 ± 530

11 484 ± 5/11 684 ± 144 41 935 ± 377

SacA36249 SacA36250

RZCS-21 RZCS-21 RZCS-21 RZCS-21 RZCS-21

6e7 49e52 105e106 159e160 299.5e300.5

bulk ff G. ruber Plant debris Plant debris G. menardii

4450 ± 30 11 775 ± 45 25 200 ± 730 27 440 ± 550 48 000 ± 2000

4576 ± 68 13 243 ± 59 28 820 ± 775 31 061 ± 430 ~53 000

Poz-49632 SacA 32741 Poz-64655 Poz-64493 Poz-49597

RZCS-25 RZCS-25

1e2 934e937

Plant debris G. ruber

4415 ± 35 5590 ± 30

4572 ± 65 5961 ± 46

Poz-64499 SacA 32742

C age (yr)

against the international VPDB reference. Due to the scarcity of foraminifera, with numerous barren levels, the results are discontinuous. However, d18O values and their variations can be compared to those of GeoB1008-3. Values lower than or close to 1.5‰ are indicative of interglacial periods, while values higher than or close to 0.5‰ are indicative of glacial periods. Sediment lightness (L*) is controlled by the sediment mineralogy and composition such as carbonates and clay, opal or organic carbon content (Balsam et al., 1999; Nederbragt et al., 2006; Debret et al., 2010). L* in core KZAI-02 shows a good correlation with Kaolinite/Smectite ratio (see Fig. 2). Total Organic Carbon (TOC) in sediments of the Congo-Angola margin is related to the precessiondriven record of insolation, and thus, to variations in upwelling intensity and fluvial runoff (Holtvoeth et al., 2001). TOC marked cyclic fluctuations are therefore a reliable chronostratigraphic proxy in the Gulf of Guinea (Gingele et al., 1998; Dalibard et al., 2014; Hatin et al., 2017). When possible these two proxies were used for the chronostratigraphic correlations (Figs. 2 and 5). 4. Results 4.1. Sub-bottom echo-facies The analysis of very high-resolution seismic data (SBP) resulted in the identification of four main echo-facies, the main characteristics of which are synthesized in Table 3 and Fig. 3. SBP echo-facies are those classically encountered in similar turbidite environments (e.g. Damuth, 1975; Damuth and Kumar, 1975; Damuth and Hayes, 1977; Droz et al., 2001; Mouchot et al., 2010; Biscara, 2011). Most of the echo-facies are stratified (Fig. 3a-c) with varying vertical and lateral amplitude of reflectors. Two main configurations are identified: (i) wedge-shaped configuration characterizes the levees of channels (Fig. 3a) and the lobes fringes (Fig. 3b); (ii) flat-topped lens-shaped configuration corresponds to small ponded interchannel basins (Fig. 3c). Nonpenetrative echo-facies characterize the channel fills (Fig. 3d) and lens-shaped lobe body (Fig. 3e). Hyperbolic echo-facies are artifacts due to the roughness of the seafloor or to high slope gradients such

Fig. 2. Age model of KZaï-02 with main proxies used in this study. d18O in ‰ VPDB (orange), lightness (L*, light blue), TOC (black), pollens (green) (Dalibard et al., 2014), Kaolinite/Smectite ratio (blue) (Sionneau et al., 2010). The age model was modified from (Hatin et al., 2017) mainly by using constraints from sharply constrained KZaï-01 (see Supplementary Material A). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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Table 3 Description of echo-facies encountered in the study area, interpretation and corresponding lithofacies (see Fig. 4 and Table 4). See Fig. 3 for illustrated examples. SBP Chirp Echo-facies

Description

Context

Damuth and Olson (2015) 3.5 kHz echo-facies possible equivalence

lithofacies

Interpretation

Stratified Fig. 3a-c

Wedge-shaped configuration. Continuous and high frequency reflectors. Vertically variable amplitude Downlapping or onlapping depending on the slope of the top of the underlying unit Lenticular shape with flat top. Continuous reflectors with vertically variable amplitude. Onlapping. Fig. 3a Chaotic facies with high amplitude reflectors Fig. 3d Lenticular shape. Chaotic facies with reflectors of variable amplitude Fig. 3e Hyperbolae

Lateral to channel axes Fig. 3c Distal termination of lobes Fig. 3b

Type 1A or Type 4

Turbidite-types 1, 2 and occasionally 3 and 4 Turbidite-types 1, 2

Levees

Isopach, very low amplitude parallel reflectors

Non-penetrative Fig. 3d-e

Hyperbolic Fig. 3f

Transparent Fig. 3g

Type 2

Lobe fringes

Inter-channel fills

Type 1AC with lenticular configuration

Turbidite-types 1, 2

patially (?) ponded turbidites

Underneath or just near a recent channel axis

Type 3

Turbidite-types 3, 4

Channel fill

Thickest part of a lobate sediment accumulation

Type 3

Turbidite types 3, 4 and occasionally 1, 2,

Lobe body

Channel flanks, lobe top

Type 8

Turbidite-types 3, 4

Draping the underlying topography

Type 1B but more transparent

Hemipelagic

Artifacts related to roughness of the seafloor or high slope gradients Draping, turbiditic quiescence

as on channel flanks (Fig. 3f). Very low amplitude parallel reflectors to transparent echo-facies with draping isopach configuration (Fig. 3g) characterizes the hemipelagic cover. 4.2. Lithofacies and deposit types

Fig. 3. Main groups of echo-facies encountered in the study area. See Table 3 for description of facies.

Lithofacies in the Congo Axial Fan are mainly fine-grained (silt and clay size) turbidites already described by Van Weering and Van Iperen (1984), Gervais et al. (2001) and Migeon et al. (2004) and commonly observed on other deep-sea fans (Piper, 1978). Four types of turbiditic facies and hemipelagic intervals were identified (Fig. 4, Table 4). The turbiditic facies correspond dominantly to Td and Te divisions of the Bouma sequence showing a normal grading from coarse silts to clay. Ta divisions are less common and made up of fine sand with common or abundant plant debris. Turbidite-Type 1 and Type 2 facies were found in the distal part of levee deposits and in fringes of terminal lobes. Type 1 was also observed in turbidites infilling an interchannel low. Type 3 was found in the proximal part of levees and in the lobe body Ax10. Hemipelagic deposits are dominantly clayey and contain abundant biogenic debris, mostly siliceous (diatoms, radiolaria, spicules) but also carbonated (coccoliths and foraminifera). Thick (up to 2.8 m) hemipelagic facies was found at the top of channel-levee systems and is interpreted as a quiescence of the turbidity current activity. Since no shut off of the turbiditic activity was described at the fan scale, because of the permanent connection of the canyon with the Congo River (Droz et al., 2003; Picot et al., 2016), this quiescence is interpreted as a shift in the turbiditic depocenter after a channel avulsion upstream. Slumps and debrites are not present in the studied cores that were mainly retrieved in levees and lobe fringes. In the Congo Fan thick mass-transport deposits are less abundant than in other fans such as the Amazon Fan (e.g. Pirmez and Flood, 1995) and are restricted to upfan locations or near the base of the continental slope, where they rework several channel/levees systems (Droz et al., 2003). Mass transport deposits such as debrites or slumps

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Fig. 4. Lithological facies defined from RX scopix (left) and grain size analysis (red line) and corresponding photos (right) (modified from Marsset et al., 2013). Turbidite facies: Type 1: Homogeneous clayey-silty turbidite facies; Type 2: Laminated clayey-silty turbidite faces; Type 3: Silt based turbidite facies; Type 4: Sand bases turbidite facies. Hemipelagic facies: clays. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

are, however, frequent in the youngest channel-mouth lobes (Ax 52) (Dennielou et al., 2017). 4.3. Seismostratigraphic, lithostratigraphic and chronostratigraphic frameworks at the coring sites The seismostratigraphic and lithostratigraphic frameworks at

the coring site of cores RZCS-01, RZCS-06, RZCS-07, RZCS-15, RZCS21 and RZCS-25 are based on the definition of seismic units from their geometries and echo-facies (Table 3, Fig. 3) and their associated lithofacies (Table 4, Fig. 4). Note that seismic units U and T are numbered locally in a stratigraphic order, from U1 to Un and from T1 to Tn from the oldest to the youngest. Therefore they do not correspond to same units in Figs. 5 and 7e10.

Table 4 Different types of lithofacies identified in this study. See Fig. 4. Deposit types

Lithofacies association *(Bouma, 1962) **(Stow and Piper, 1984)

Characteristics

Interpretation

Turbidite-type 1

Te*

Dilute and low-velocity spillovers from the uppermost part or tail of channelized turbidity currents

Turbidite-type 2

Td and Te*

Turbidite-type 3

Tc, Td, Te*

Turbidite-type 4

Ta, Td, Te*

Hemipelagic

Te* or F**

Up to 15 cm thick. Te made up of homogeneous silty-clay with no sedimentary structures. Slight normal grading Up to 15 cm thick. Td made up of laminated silty-clay alternating infra-millimetric to millimetric silty laminae and millimetric to centimetric clayey laminae. Normal grading Up to 12 cm thick. Tc made up of silts with some cross-bedding and frequent macro wood and leaf debris. Normal grading. Up to 16 cm thick. Ta made up of fine to very fine sand with frequent macro wood and leaf debris. Normal grading. Up to several meters thick. Dominantly clayey with abundant biogenic debris, mostly siliceous (diatoms, radiolaria, spicules), but also carbonated (coccoliths, foraminifera). Large burrows.

Spillover of the body of a single turbidity current where silt and clay laminae are produced by burst and sweep processes (Chough and Hesse, 1980)

Spillover of upper and deeper parts of turbidity currents (Migeon et al., 2004) Spillover of the head (coarse basal lithofacies) and the body (upper fine lithofacies) of turbidity currents (Migeon et al., 2004) Slow and low energy sedimentation

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Fig. 5. Context and lithology of core RZCS-01. (A) Plan view from Picot et al. (2016) showing the extension of lobe Ax19 (light grey), the surrounding channels Ax02, and the location of the core and the SBP profile illustrated in B and C. (B) SBP profile (location in A) illustrating the large-scale architecture of the sampled area and referring to channel numbers of Picot et al. (2016). C) Zoom of SBP profile on the area of RZCS-01 (vertical red bar) and observed seismic units, either turbiditic (U1 to U4) or hemipelagic (T1 and T2). (D) Synthetic lithological log with correspondences to SBP units and their interpretations. Note that numbering of seismic units U and T is local for Figs. 5 and 7 to 10. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

4.3.1. Characteristics and age of lithostratigraphic units in the area of core RZCS-01 (abandonment of channels Ax02 and Ax19) Core RZCS-01 (Table 1 and Fig. 5) was collected in order to sample sediments deposited after a major upstream bifurcation of the channel and identified as a retrogradation peak between cycles B and C (Fig. 1C) in an area where lobe Ax19 rests on channel-levee systems Ax02-Ax18 (Fig. 5A and B). Six units (U1, T1, U2, U3, T2 and U4 in stratigraphic order) were identified on the SBP profile (Fig. 5C) among which only the 5 uppermost were sampled in core RZCS-01 (Fig. 5D). The wedgeshaped well-stratified unit U1 thins towards the SE and consists of turbidite-type 1 and 2 lithofacies (Table 4). It is interpreted as stacked levee deposits of channels Ax02 to Ax18 (Picot, 2015; Picot et al., 2016). The overlying T1 unit is made of hemipelagic lithofacies (Table 4) and is interpreted as a quiescence in turbidity current activity after the abandonment of channel Ax18. Unit U2.1 is highly reflective and chaotic and generates acoustic masking and correlative loss of underlying reflectors (U1). The overlying unit U2.2 is transparent to chaotic, with few discontinuous reflectors evolving toward the top to a more stratified facies. Unit U2.2 is made up of turbidite-type 2 lithofacies (Table 4). Unit U2.1 was not sampled. Unit 2 shows a lobate planform and is located at the end of channel Ax19 and is interpreted as the terminal lobe of channel Ax19 (Picot et al., 2016). The turbidite-type 2 lithofacies at the

fringe of the lobe (Unit 2.2) are similar to those found on levees in the presently active lobe at the mouth of channel Ax58 (Dennielou et al., 2017). Unit U3 covers units U2.2 and T1 and shows continuous and high-amplitude reflectors with onlapping geometries that evoke terrigenous deposits infilling an inter-channel basin. The sediment core has sampled the fringe of this unit that corresponds to turbidite-type 1 lithofacies (Table 4). Unit T2 covers the whole area where it drapes Units T1, U2.2 and U3. It consists of hemipelagic lithofacies. It is interpreted as the quiescence in turbidity current activity after the abandonment of channel Ax19. Unit U4 is characterized by high-amplitude, relatively continuous and onlapping reflectors corresponding to the infill of an inter-channel low. The fringe of this infill consists of turbidite-type 1 lithofacies. The area is finally covered by 20 cm of hemipelagic lithofacies, under sub-bottom profiler resolution. The age model of RZCS-01 is based on dating of planktonic foraminifera oxygen isotopes, Corg and spectrocolorimetry (lightness L*) data obtained in the hemipelagic intervals in units T1 and T2 and in units U1 and U4 that may contain few hemipelagic intervals. The correlation of these proxies with those from TOC and L* of core KZaï-02 and oxygen isotopes from core GeoB1008-3 allowed identifying isotopic stages 1e7 (Fig. 6). This correlation is corroborated by AMS radiocarbon dating of bulk or monospecific planktonic foraminifera on the first 5 m of the sediment core (Table 2).

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Fig. 6. RZCS-01 14C dates (red dots, Table 2) and tie points (black lines) resulting from the multi-proxies (d18O, L*, Corg) analyses and correlations with the curves of the reference cores GeoB1008-3 (d18O) and KZaï-02 (L*, TOC). Light blue areas highlight the proposed correlations. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Therefore, the age model allows the estimation of the abandonment of channel Ax19 terminal lobe at ca. 70 ka BP. 4.3.2. Characteristics and age of lithostratigraphic units in the area of cores RZCS-06 and RZCS-07 (abandonments of channel Ax21 and Ax45) Cores RZCS-06 and RZCS-07 (Table 1) were sampled on both sides of channel Ax21 (Fig. 7A and B) in order to date the abandonment of this channel and the beginning of the architectural cycles C and D (Fig. 1C). Despite the occurrence of hyperbolae on the SBP line around the channel it was possible to identify six main seismic units (Fig. 7C) which were sampled in cores RZCS-06 (Fig. 7D) and RZCS-07 (Fig. 7E). Unit U1 lies at the base of the channel-levee system and, shows a chaotic echo-facies. It corresponds to the upper part of the undifferentiated package UP1 (see Section 2.1.2 and Picot et al. (2016) for a detailed description of the undifferentiated packages, UP, observed in the Congo Axial Fan). Unit U1 is made up of turbidite-types 1 and 3 at the base, evolving to coarser turbiditetype 4 to the top (Table 4). Unit U2 is a wedge-shaped unit composed of bedded and continuous reflectors, thinning outwards from Ax21 channel axis and corresponding to the right levee of channel Ax21. It is dominantly made up of turbidite-type 3 lithofacies (Table 4). Unit U20 is obliterated by hyperbolae but seems characterized by high amplitude chaotic facies (Fig. 7C). It is dominantly composed of turbidite-type 3 lithofacies with several occurrence of coarser turbidite-type 4 at the base. Unit U20 occurs at the same stratigraphic position as unit U2 but is interpreted either as a channel infill or as inner levee deposits of channel Ax21.

Unit U3 is isopach to slightly wedge-shaped, thinning northwestwards. It is made of bedded echo-facies showing amplitudes evolving from high at the SE part of the line, to very low at the NW end of the profile. It is made up of alternations of turbidite-type 1 and hemipelagic lithofacies (Table 4). Unit U3 is interpreted as distal levee deposits of channel Ax22 and possibly of distal nearby channels with alternating periods of turbiditic quiescence. Unit T is isopach and drapes the underlying units. It consists of hemipelagic lithofacies and is interpreted as turbiditic quiescence after the abandonment of channel Ax22. Unit U4 is characterized by a slightly bedded facies with faint continuous reflectors and lateral amplitude variations. It is made up of dominantly turbidite-type 1 and 2 (Table 4). It is interpreted as the fringe deposits of lobe Ax45 (Fig. 7B and C). Unit U5 with a comparable seismic facies, also consists of turbidite-type 1 and 2 and is observed at the same stratigraphic level as U4. Despite the physical discontinuity represented by channel Ax21, U4 and U5 could correspond to the same unit, i.e. the Ax45 lobe fringe which could extend southwards, beyond Ax21. A last hemipelagic cover is not visible on the SBP line but was found on both cores as a 10 cm thick interval of hemipelagic lithofacies. It is interpreted as the abandonment of the distal lobe of channel Ax45. The age models on these sediment cores are based on AMS radiocarbon dating of bulk or monospecific planktonic foraminifera (Table 2) in the hemipelagic units T1, U3 and T2. Another radiocarbon dating was made on a plant debris sample collected in unit U2. Therefore, the age models allow estimation of the abandonment of channel Ax21 between 38 and 41.1 cal ka BP, the end of deposits

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Fig. 7. Context and lithology of cores RZCS-06 and RZCS-07. (A) Plan view from Picot et al. (2016) showing the geographical context of cores (B) SBP profile (location in A) illustrating the large-scale architecture of the sampled area and referring to channel numbers of Picot et al. (2016). C) Zoom of RZCS-06 and RZCS-07 (vertical red bars) sampled area and observed seismic units, either turbiditic (U1 to U5) or hemipelagic (T). (D) and (E) Synthetic lithological logs with correspondences to SBP units and their interpretations. Note that numbering of seismic units U and T is local for Figs. 5 and 7e10. See Fig. 5 for key to colors of the lithologic log. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

linked to channels Ax22 and younger channels between 30.4 and 13.9 cal ka BP, the onset of the fringe of the distal lobe of channel Ax45 at ca. 11 cal ka BP and its abandonment at ca. 4.1 cal ka BP (Fig. 7).

4.3.3. Characteristics and age of lithostratigraphic units in the area of core RZCS-15 (abandonment of channel Ax06, estimation of initiation and abandonment of group of channels Ax09 to Ax12) Core RZCS-15 (Table 1) was collected in order to date the abandonment of channel Ax09 (Table 2, Fig. 8A and B) and the maximum progradation of cycle A (Fig. 1C). It was also expected to constrain the abandonment of channel Ax06 (Fig. 8A and B). Six units resting on a transparent to high-amplitude chaotic basal unit were identified (Fig. 8C), among which the 5 uppermost were sampled in core RZCS-15 (Fig. 8D). Unit U1, not sampled in the sediment core, thins towards the SW and is characterized by moderate amplitude and continuous reflectors affected by sediment waves to the NE. It is interpreted as levee deposits from a channel older than the Axial Fan. Unit T1 is a draping isopach unit with vertically varying amplitude from high at the base to low at the top and continuous reflectors. It is composed of hemipelagic lithofacies with 3 cm-thick turbidite-type 3 beds (Table 4) and is interpreted as a hemipelagic interval marking the abandonment of the Northern Fan (Picot et al., 2016). The turbiditic lithofacies intercalated in this thick hemipelagic interval suggests ongoing sporadic activity of the former channels. Unit U2 is lens-shaped and shows relatively continuous and low amplitude reflectors. It fills a small depression and is made up of turbidite-type 2, which is

interpreted as the fringes of lobe Ax06 that lies to the north (Fig. 8A and B). Unit U3 consists of relatively continuous reflectors with variable amplitude pinching out to the SW and is composed of turbidite-type 2 lithofacies (Table 4). It corresponds to the right levee of channels Ax09 to Ax12 (Fig. 8A and B). Unit U4 is characterized by continuous reflectors onlapping unit U3 and comprises turbidite-type 2 with few turbidite-type 4 lithofacies (Table 4). The origin of U4 is not clear, but could correspond to an interchannel infilling unit, as already observed by Babonneau (2002). Thin (10e20 cm) successions of hemipelagic lithofacies are intercalated between U2, U3 and U4 (not visible on the chirp profile) suggesting a significant turbiditic quiescence between these units. Unit T2 is a thin (80 cm) transparent layer consisting of hemipelagic lithofacies that drape the turbiditic units and is interpreted as marking the abandonment of channel Ax12. Because of the deep water depth (4925 m), close to the carbonate compensation depth, calcareous microfossils are poorly preserved, and it was not possible to produce a continuous oxygen isotope. The age model is therefore based on two AMS radiocarbon on foraminifera at the top of the core (at 80 cm), and on the occurrence of the foraminifera Globorotaloides hexagona until 9.30 m, indicative of the isotopic stage 4e5 transition dated at 71 ka (Zachariasse et al., 1984; Lisiecki and Raymo, 2005). Therefore, the age model (Fig. 8D) allows estimation for the initiation and the abandonment of the lobe of channel Ax06 before 71 cal ka BP. The initiation of channel Ax09 and the abandonment of channel Ax12 would be older than 41.9 cal ka BP but possibly younger than 71 ka.

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Fig. 8. Context and lithology of core RZCS-15. (A) Plan view from Picot et al. (2016) showing the geographical context (B) SBP profile (location in A) illustrating the large-scale architecture of the sampled area and referring to channel numbers of Picot et al. (2016). C) Zoom of SBP profile on the area of RZCS-15 (vertical red bar) and observed seismic units, either turbiditic (U1 to U4) or hemipelagic (T1 and T2). (D) Synthetic lithological log with correspondences to SBP units and their interpretations. Note that numbering of seismic units U and T is local for Figs. 5 and 7e10. See Fig. 5 for key to colors of the lithologic log. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

4.3.4. Characteristics and age of lithostratigraphic units in the area of core RZCS-21 (abandonment of channel Ax12) Core RZCS-21 was collected on the superposed levees of channels Ax11 to Ax12, in order to date the abandonment of channel Ax12 and the maximum progradation of cycle A. (Table 1, Figs. 1C and 9A and B). Four units were identified (Fig. 9C) and were sampled in core RZCS-21 (Fig. 9D). Unit U1 is characterized by a dominantly transparent facies with few faint or discontinuous high-amplitude reflectors and an irregular top (Fig. 9C) and comprises ca. 9 m of turbidite-type 3 lithofacies overlain by 5 m of turbidite-type 2 lithofacies (Table 4). Unit U1 is interpreted as fringe deposits of the lobe at the termination of channel-levee system Ax10 (Picot et al., 2016) (Fig. 9A and B). Unit U2 is wedge-shaped and composed of relatively continuous reflectors with variable amplitude that pinch out laterally away from channel Ax11-12 and comprises turbiditetype 1 lithofacies (Table 4). Unit U2 is interpreted as stacked levee deposits from channels Ax11 and Ax12 (Picot et al., 2016). The unit located between U2 and U3 is not resolved on the SBP line but was sampled in the sediment core, and is ca. 30-cm-thick and composed of hemipelagic lithofacies with few silt laminae. This unit is interpreted as a turbiditic quiescence after the abandonment of channel Ax12. Unit U3 is flat-topped and lens-shaped with more or less continuous reflectors onlapping unit U2, characteristic of inter-channel infilling units, similar to that described for U2 of RZCS-15. It consists of turbidite-type 2 lithofacies. The seismically transparent unit T drapes units U3 and U2, made up of hemipelagic lihofacies, is interpreted as a quiescence in turbidity current activity after the end of the inter-channel infill and marks the abandonment of the area by turbidite sedimentation. Due to the wide dominance of turbidite deposits it was not possible to analyze foraminifera oxygen isotopes. The age model (Fig. 9D) is based on 5 AMS radiocarbon dating obtained on bulk and monospecific foraminifera in hemipelagic deposits and on

plant debris collected in turbidites (Table 2). These show that channel Ax12 was abandoned around 53 cal ka BP and that the definitive turbiditic quiescence occurred at around 29 cal ka BP.

4.3.5. Characteristics and age of lithostratigraphic units in the area of core RZCS-25 (abandonment of channels Ax50, Ax51 and Ax52) Core RZCS-25 (Table 1) was collected in order to constrain the age of abandonment of channels Ax50, Ax51 and Ax52 that correspond to the maximum progradation of cycle D (Fig. 1C). The core was retrieved on the edge of the lobe area fed by channel Ax50 and deposited on the levee of a channel attributed to the Northern Fan (Picot et al., 2016) (Fig. 10A and B). Three units were identified (Fig. 10C) but only two were sampled in core RZCS-25 (Fig. 10D). Unit U1 is made of bedded and continuous reflectors laterally passing to a lenticular and transparent unit corresponding to the fringe of a lobe possibly belonging to the Northern Fan (Droz et al., 2003; Picot et al., 2016). This unit U1 was not intersected in the sediment core. Unit T is thick (about 10 m), isopach and drapes unit U1. It is seismically transparent to stratified, with low amplitude and very continuous reflectors. It is made up of hemipelagic lithofacies and is interpreted as the quiescence in turbidity current activity after the abandonment of the Northern Fan. It is therefore, contemporaneous to the deposition of the Southern and Axial Fans, i.e. a minimum time span approaching 500 ka (Droz et al., 2003). Unit U2 is characterized by reflectors of laterally and vertically variable amplitude onlapping unit T. U2 is composed of two sub-units (U2.1 and U2.2) separated by an angular unconformity. U2.1 has the typical lenticular geometry of lobes and was not sampled in the sediment core. Unit U2.2 shows an infilling geometry and is made up of turbidite-type 2 lithofacies (Table 4). These two sub-units of unit U2 are interpreted as deposits of two lobes constituting the superficial lobe area fed by channel Ax50 (Picot et al., 2016). A last unit not resolved on the SBP line but sampled in the sediment core corresponds to 10 cm of hemipelagic

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Fig. 9. Context and lithology of core RZCS-21. (A) Plan view from Picot et al. (2016) showing the geographical context (B) SBP profile (location in A) illustrating the large-scale architecture of the sampled area and referring to channel numbers of Picot et al. (2016). C) Zoom of SBP profile on the area of RZCS-21 (vertical red bar) and observed seismic units, either turbiditic (U1 to U3) or hemipelagic (T). (D) Synthetic lithological log with correspondences to SBP units and their interpretations. Note that numbering of seismic units U and T is local for Figs. 5 and 7e10. See Fig. 5 for key to colors of the lithologic log. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

lithofacies (Table 4). It is interpreted as the turbiditic quiescence after the abandonment of channel Ax50, contemporary with Ax51 and Ax52 channels.

The age model of core RZCS-25 is based on two AMS radiocarbon dates on plant debris and on monospecific foraminifera (Table 2) at the top and base of the turbidite lithofacies. The onset of

Fig. 10. Context and lithology of core RZCS-25. (A) Plan view from Picot et al. (2016) showing the geographical context (B) SBP profile (location in A) illustrating the large-scale architecture of the sampled area and referring to channel numbers of Picot et al. (2016). C) Zoom of SBP profile on the area of RZCS-25 (vertical red bar) and observed seismic units, either turbiditic (U1 to U2.2) or hemipelagic (T). (D) Synthetic lithological log with correspondences to SBP units and their interpretations. Note that numbering of seismic units U and T is local for Figs. 5 and 7e10. See Fig. 5 for key to colors of the lithologic log. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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channel Ax50 is dated at ca. 6 cal ka BP and the abandonment occurred before 4.6 cal ka BP for a maximal duration of 1.4 kyr. 4.4. Chronostratigraphic framework of the architectural diagrams The age models established from chronostratigraphic constraints obtained from cores RZCS-01, 06, 07, 15, 21 and 25 provide a temporal framework for the initiation and/or abandonment of the key channels or lobes that characterize the architectural evolution of the Axial Fan (Table 5). The tie points obtained are reported on the diagrams of architectural parameters (Fig. 11), with two other control points at the base of the Axial Fan (210 ka from Droz et al., 2003) and at its top (current activity of the last channel Ax52). Time constraints allow evaluation of the total duration of cycles and the mean channels duration for each cycle (Fig. 10, Table 6). Because of the lack of an accurate age model for cycles A and B, their exact respective duration cannot be calculated. However, mean channel duration of 7.4 kyr for theses cycles allows a tentative estimate for durations of 96 kyr and 44 kyr respectively. The most important implication of this chronostratigraphic framework is a strong disequilibrium between the ante- and post38 ka Axial Fan, with more than half of the channels (Ax22 to Ax52, i.e. 30 channel-levee systems) being emplaced during the last 38 kyr (end of MIS 3 to MIS1), i.e. during one fifth of the total duration of the Axial Fan (210 kyr). However the architecture analysis (Picot et al., 2016) showed that all channels have not been recovered especially distal avulsions of Ax12 (not covered by data) and possibly channels older than Ax01 (not identified in data) that would increase the number of channels of cycle A. Architectural cycles appear to have variable time spans. However, cycle D, the most short-lived cycle (10 kyr), is known to be currently active and therefore incomplete (as indicated by the lack of the retrogradational part of the cycle). If we do not consider this abnormally short duration, cycles A to C have durations of the same order (59e96 kyr). Considering the last 38 kyr, the mean channel durations of channels Ax22 to Ax44 (cycle C) and channels Ax45 to Ax52 (cycle D) are quite consistent (2.2 and 1.4 kyr respectively). However, the constraints provided for the abandonment of the channel-levee system Ax50 (4.6 ka) indicate a duration variability of cycle D channels: the recent channels Ax51 and Ax52 could have been active during about 2.3 kyr each (i.e. similar to mean duration of channels in cycle C). This duration is consistent with the duration of the Amazon Fan channels (2e3 kyr) during periods of sea level fall (Flood and Piper, 1997). Considering the five lobes recognized in the most recent lobe complex of channel Ax52, each lobe appears to have been deposited within a relatively small duration, i.e. about 460 years which is consistent with the very high sedimentation rates recovered in the

lobes (0.5e10 cm/year according to Rabouille et al. (2017). This lobe duration is also in tie with the most recent Amazon Fan lobes (17 lobes during 9.3 kyr, i.e. 550 years/lobe from 19.7 cal ka BP to
gou, 2008). 10.4 cal ka BP (Maslin et al., 2006; Je 5. Discussion Channel avulsion is the main process that controls the stratigraphic architecture and channel-levees stacking patterns of deepsea fans. It can be triggered by many factors that can be internal to the fan, such as channel development processes (flow stripping, spillover, sharp bends) (e.g. Piper and Normark, 1983; Hiscott et al., 1997) or levee failures or breaching (e.g. Ortiz-Karpf et al., 2015) and accommodation (e.g. Armitage et al., 2012; Morris et al., 2016), or can be external, such as changes in the sediment sources forced by climate and sea level fluctuations (e.g. Zabel et al., 2001; Ducassou et al., 2009). Picot et al. (2016) concluded that the development of the Late Quaternary Congo Fan resulted from a combined influence of internal and external forcing factors. The local topography (internal forcing factor) controls the pattern of channel networks (either prograding or prograding/expanding). Major upfan channel avulsions (marking major retrogradations of the fan) are suspected to be triggered by fluctuations in the sediment source composition (external forcing factor). Picot et al. (2016) did not consider the forcing by tectonics since no major tectonic event is known to have occurred over the last 200 kyr on the Congo margin. The chronostratigraphic framework of the architectural diagrams determined in this study, albeit a relatively coarse resolution, allows for examination of the impact of sea-level and climate forcing factors. 5.1. Correlation wirth sea-level fluctuations Sea-level fluctuations change the base level of rivers (Wheeler, 1964; Schumm, 1993). Cross (1994) defined a composite (stratigraphic) base level, which is the surface of equilibrium between erosion and deposition within both marine and continental areas. Therefore, sea-level changes modify the equilibrium profile from source to sink by adjustment of current dynamics (erosion or deposition) to the new slopes. Consequently, sea-level changes can significantly modify the location of the sediment depocenter on continental margins, and therefore impact the growth pattern of deep-sea fans. Generally, the influence of sea-level fluctuations is major on margins with large continental shelves where they lead to a binary pattern with sediment supplied to the fan during glacial low sea levels (when rivers incise the shelf and connect to the canyons), and sediment starvation during interglacial high sea ^ne Fans, among others). For fan systems levels (e.g. Amazon and Rho located on narrow continental shelves, sea-level fluctuations have a

Table 5 Chronostratigraphic constrains of the channels activity in the Axial Fan. Channel-levees

Initiation or abandonment

Core

Age

Chronostratigraphic proxy

Ax50

Abandonment

RZCS-25

>4.6 cal ka BP

Ax21

Initiation Abandonment Initiation Abandonment

6.0 cal ka BP ~4.1 cal ka BP 11 cal ka BP Between 38.0 and 41.1 ka BP

Ax12 Ax12 Ax19 Lobe Ax06

Abandonment Abandonment Abandonment Abandonment

RZCS-25 RZCS-06 RZCS-06 RZCS-06 RZCS-07 RZCS-15 RZCS-21 RZCS-01 RZCS-15

Plant debris Radiocarbon Dating Monospecific Foraminifers Radiocarbon Dating Monospecific Foraminifers Radiocarbon Dating Monospecific Foraminifers Radiocarbon Dating Plant debris Radiocarbon Dating Two foraminifers species Radiocarbon Dating Monospecific Foraminifers Radiocarbon Dating Correlation with L* and TOC, of core KZaï-02 Last Occurrence of Globorotaloides hexagona (MIS 5/4 transition)

Ax45

>41.9 cal ka BP ~53 cal ka BP ~70 ka 71 ka

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Fig. 11. Chronostratigraphy of the architectural diagrams and comparison with sea-level changes over 210 kyr. (a): Architectural diagrams in relative time scale (modified from Picot et al., 2016). Blue: Avulsion length. Green: Distance of avulsion (see Picot et al., 2016, for meaning and measurements of these architectural parameters). (b): Diagrams in absolute time scale, based on ages established on cores (red lines). The distance of avulsion (DA) parameter is now represented as points, considering that avulsion processes are instantaneous at the time scale of our study. Avulsion length (AL) versus time is shown as histograms, the width of each class being proportional to mean channels duration. (c): Sea-level curve from Grant et al. (2014). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Table 6 Mean duration of channels in the 4 architectural cycles and avulsion frequencies calculated for the last 38 kyr of the Axial Congo Axial turbidite system. Bold lettering: calculations relating to the complete cycles. Italic lettering: calculation for specific intervals of the cycles. Cycles

Channels

Age (ka)

Duration (kyr)

Number of channels

Mean duration/channel (kyr)

Number of avulsions

Avulsion frequency /10 kyr (from 38 to 0 ka)

D

Ax45 to Ax52 Ax51 to Ax52 Ax45 to Ax50 Ax20 to Ax44 þ UPs Ax22 to Ax44 þ UP2 Ax20 þ UP1 þ Ax21 Ax14 to Ax19 Ax01 to Ax13

11 to 0 4.6 to 0 11 to 4.6 70 to 11

11 4.6 6.4 59

8 2 6 27

1.4 2.3 1 2.2

7 1 5 26

6 2 8

38 to 11

27

24

1.1

23

9

70 to 38

32

3

10.6

2

210 to 70

140

6 13

7.4

5 12

C

B A a

44a 96a

Durations calculated by applying the mean channel duration to the number of channels.

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lesser influence on fan feeding, especially if the feeding canyon is permanently connected to the river, as is the case for the Congo Fan (Droz et al., 1996) and the Var Fan (Bonneau et al., 2014). Our results show that, at the scale of 200 kyr (Fig. 11), the architectural cycles display wide variations, possibly of the same order as the major sea-level changes (at least the 100 kyr Milankovich excentricity variations). However, the lack of time constraints for cycles A and B (200e38 ka) prevents more precise matching with a sea-level curve. Concerning the last 38 kyr where time constraints are more numerous (Fig. 12), the comparison is easier. The sealevel curves (Medina-Elizalde, 2013; Grant et al., 2014) show three main periods, with a slow decrease from - 80 m at 40 ka to 120 m around 20 ka (lowstand), to an increase from 20 ka to around 8 ka, followed by a stationary to slowly increasing highstand during the Holocene. Comparison with the architectural diagram shows that the prograding phase of cycle C occurs during a slow decrease in sea-level and that the maximum retrogradation is dated at 11 ka, at the end of the maximum sea-level rise. The lack of chronostratigraphic constraint does not allow to firmly determine if the maximum progradation correlates to the maximum lowstand or if the retrograding phase is coeval with the next increase in sea level. The prograding phase of cycle D occurs during the Holocene highstand. Due to loose time constraints, it is impossible to match the higher order sea-level fluctuations (few kyr) (Fig. 12a) with architectural sub-cycles (r and p in Fig. 12c). The correlations with sea-level curves (Fig. 12) therefore indicate that prograding phases are not

related to a specific state of sea level (decreasing from 80 m to 120 m in cycle C and around 0 m in cycle D), suggesting that sea level is probably not the main factor controlling the progradation of avulsion points. This hypothezis is strengthened by constant avulsion frequencies (Table 6 and Fig. 12b) during both lowering and rising sea levels (around 9 per 10 kyr between 38 and 4.6 ka). The suggested low impact of sea-level variations on the architecture evolution of the Congo Fan is consistent with the fact that the Congo River is permanently connected to the canyon leading to maintain the supply to the deep basin whatever the sea level (highstand or lowstand). However, lowerings of sea level can modify the equilibrium profile of the Congo canyon and estuary and provoke erosion on the shelf. In order to quantify the potential additional input related to a lowering of sea level and subsequent retrogressive erosion, a rough estimation has been calculated. Based on the pedogenic origin of smectites in a core located on the Congo Fan, Gingele et al. (1998) consider that the erosion of the shelf during lowstands does not impact sedimentation in the fan. Thereby, the Congo River is considererd to be the only source of supply to the deep basin. We therefore consider that during sealevel lowering, erosion triggered in the canyon, estuary and watershed is potentially the main process impacting the growth pattern of the Congo Fan. The occurrence of 300 m high coastal reliefs, around 150 km upstream the Congo River mouth (Fig. 13A and B), is a morphostructural barrier supposed to block retrogressive erosion further upstream. Considering the part of watershed dowstream from coastal reliefs (including the canyon and the

Fig. 12. Comparison of Congo Fan architecture, sea-level, climate and environmental evolutions since 38 ka. (a): Sea-level curves from Medina-Elizalde (2013) and Grant et al. (2014). Black arrows schematize the main trend of sea-level evolution. (b): Architecture of the Congo Fan with calculated channel durations and avulsion frequencies (see Table 6) and the architectural diagram (cycles C and D) illustrating the evolution of distance from the reference point to avulsion points. Blue and red arrows are respectively the main prograding (P) and retrograding (R) phases of the cycles. UP1 and UP2 are undifferentiated units (see Picot et al., 2016). (c): Reference core KZaï-02 with Podocarpus/ (Podocarpusþrain forest) pollen ratio (Dalibard et al., 2014) and Log K/S ratio (Sionneau et al., 2010). (d): West African monsoon (Caley et al., 2011). Red dotted lines are the time constraints of the architectural diagram provided by cores (see Fig. 11). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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estuary), the amount of sediment possibly removed by retrogessive erosion during the 80 to 120 m sea-level lowering (40 ka and 20 ka BP), is estimated between 0.4 and 5 MT/year (Fig. 13E) depending on the considered width affected by erosion (Fig. 13C), either the canyon width at 120 m (4 km) or the mean width of the alluvial plain (25 km). This contribution appears relatively low as it represents 1e9% of the present-day suspended load of 55 MT/year. More generally, this could represent the yearly amount of sediment that can be either added to the watershed sediment yield by retrogressive erosion of the exposed canyon-alluvial plain during

165

sea-level fall or subtracted by deposition in the flooded canyonalluvial plain during sea-level rises. If this does not significantly change the amount of sediment exported offshore, it may on the other hand change the composition of sediment involved in turbidity currents and therefore their transport capacity. We hypothesize that, during sea-level falls, erosion mainly occur in the canyon head and river bed, where sediments are essentially coarsegrained. Erosion would therefore likely increase the amount of sand and decrease the transport capacity of turbidity currents. In contrast, deposition in these environments during sea level rises

Fig. 13. Impact of a base level fall on the amount of sediment produced by regressive erosion. A: Altitude of the Congo River and depth profile of the Congo canyon to channel (redrawn from Babonneau, 2002). B: Zoom of A from the coastal reliefs in the Congo River distal watershed to the upper part of the Congo canyon. The effect of sea-level falls to 80 m and to 120 m are schematically indicated. C: Map of the Congo Canyon and zoom on the canyon head and estuary (based on Moguedet, 1988), showing a minimum width of 4 km at the canyon head and a mean width of 25 km for the estuary. D: Approximation of the potential eroded volume during a sea-level fall from 80 m to 120 m, considering a pyramidal shape. E: Annual production of sediment during a 80 to 120 m lowering (about 20 kyr, see Fig. 12) considering the erosion of alluvial sediments. (is the volumetric mass and Vp is the volume of the pyramide (see D).

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would likely decrease the amount of sand and increase the transport capacity. Within the uncertainties of the chronostratigraphic framework between 210-40 ka channel-levees progradation-retrogradation pattern may suggest a positive correlation between fan progradation and the overall sea-level fall between 200 and 135 ka (Fig. 11). However, this is contradictory with the postulated increase in sand and lower transport capacity of turbidity currents. Similarly, the major retrogradation around 11 ka, coeval to the abrupt sea level rise after the last glacial maximum (Fig. 12), contradicts the expected increase in transport capacity of turbidity currents. However, the processes and impacts of sea-level fluctuation on sediment quantity and composition deserve further investigation. 5.2. Correlation of architectural cycles with climatic proxies Climatic and environmental evolutions in the Congo watershed are mainly controlled by the monsoon regime (Caley et al., 2011) that controls precipitations and therefore production and erosion of sediments, and the carrying capacity of rivers in the drainage basin. These parameters can be extrapolated from proxies recorded in sediment core KZaï-02 (Fig. 12) such as the pollen ratio Podocarpus/(Podocarpusþrainforest), which is a proxy of humidity and vegetation extension in the Congo watershed (Dalibard et al., 2014), and the Kaolinite/Smectite ratio (K/S), which is a proxy for the extension and intensity of freshwater plume and therefore the discharge of the Congo River (Gingele et al., 1998; Sionneau et al., 2010). The age model for core KZaï-02 allows discussion of the role of the Congo River freshwater discharge and sediment load composition on the prograding/retrograding cycles over the last 38 kyr (Fig. 12): (1) The main prograding periods (P in Fig. 12a) of cycles C and D correspond to an increasing humidity and river freshwater input as shown by pollen assemblages (development of rain forests) and an increasing trend of K/S ratio (increasing extension and intensity of freshwater plume), and also to a high monsoon index indicative of high precipitations. (2) Conversely, the retrograding period during cycle C (R in Fig. 12a) correlates with a period of lower intensity monsoon (Fig. 12d) (i.e. arid period indicative of low precipitations). This period is, however, marked by a high variability of humid/arid pollen grain assemblages (Fig. 12b) and K/S signal (Fig. 12c) that indicate climate variability. During the 27-11 ka period of time, humidity and freshwater discharge are also positively correlated and show high amplitude fluctuations of a few thousand year periodicities. This higher order of variability could also be in phase with low amplitude changes in the progradation-retrogradation pattern (cf. subcycles r and p in Fig. 12a). However, the age model of the progradation-retrogradation diagram is inaccurate at the millennial time-scale, which prevents us making a more precise correlation. (3) The most significant retrogradation (i.e. most upfan avulsion) ca. 500 km upstream occurs around 11 ka at the transition between the arid and humid monsoon periods (Fig. 12d). At this arid/humid transition, precipitations and river runoff were enhanced, but for a short period of time, the extension of rainforest remained limited (Fig. 12b).

5.3. Processes of climatic forcing on avulsions Fluctuation of sedimentation on the Congo Fan is determined by the Congo watershed sediment yield and also by marine carbonate

and opal productivity. Jansen et al. (1984) show that, despite the difficulty to distinguish the contributions of these three sources, non-biogenic sedimentation, i.e. terrigenous, is higher during glacial periods than during interglacial periods suggesting that climatic fluctuations have a significant impact on the Congo River sediment yield. Since our new results on the chronostratigraphic framework of the architectural diagram (see Section 5.2) suggest that the style of fan growth, retrogradation or progradation, is mainly correlated to climatic periods, it seems reasonable to propose that the fan growth pattern is controlled by the characteristics of the sediment source (yield, possible in-situ storage, composition). We may postulate that fan progradation, i.e. progressively more downstream avulsions, occurs when turbidity currents have sufficient competence and transport capacity (Mutti and Ricci Lucchi, 1972; Reading and Richards, 1994; Gladstone and Sparks, 1998; Salaheldin et al., 2000; Kneller, 2003; de Leeuw et al., 2018). The humid climatic conditions identified during the prograding periods are, indeed, favorable to generate such high transport capacity turbidity current because of reduced sediment yield (Jansen et al., 1984) producing clayey material related to enhanced chemical alteration (Fig. 14A). A more clayey sediment yield may generate a muddy supply to the river mouth and trigger turbidity currents with low sand/mud ratio. Such mud-rich turbidity currents are capable of maintaining a good transport capacity throughout the channel and reach the most distal parts of the fan, The growth of levees and flow confinement are thereby facilitated and maintain the momentum of turbidity currents even over long distances as observed in the presently active channel (Babonneau et al., 2002). On the other hand, we suggest that retrogradation, i.e. progressively more upstream avulsions, occurs when turbidity currents loose competence and/or transport capacity and deposit their sediment load in the channel that becomes shallower. Spillover and flow stripping is therefore enhanced and favor upstream channel bifurcation. The arid conditions that prevailed during the main retrograding trend of cycle C resulted in an increased sediment yield (Jansen et al., 1984) and production of coarser sediment related to enhanced mechanical erosion (Fig. 14B) favorable to a higher sand/mud ratio. The triggered sand-rich turbidity currents have a reduced transport capacity prone to deposit their load along the channel and therefore favor channel infill and upstream avulsions. An exceptional fan retrogradation, 500 km upfan, occurred at the climatic transition at 11 ka. Although it may appear far-fetched to make an interpretation from this mere example, this transition is characterized by the increase in precipitation and, possibly by enhanced mechanical erosion during a certain period of time corresponding to the time needed for humid-type vegetation to colonize the watershed (Fig. 14C). This burst of coarse sediment may have significantly favored the channel infill and triggered the 500 km upstream avulsion. As mentioned by Picot et al. (2016), avulsions occurring upfan, where slopes are higher than downfan, may also be favored by local topography. 5.4. Implications for the sequence stratigraphic-based models of fan growth pattern Conversely to the classical sequence stratigraphy model (see the synthesis in Catuneanu, 2002, and references herein) that considers sea level changes as the main forcing on sediment flux and fan growth (e.g. Kolla and Perlmutter, 1993; Weber et al., 1997; Prins and Postma, 2000; Hodgson et al., 2006), we show that sea level changes are not a significant forcing factor on channel avulsion in the Late Quaternary Congo Fan. Our results rather show that the Congo Fan is mainly under the influence of climate-driven changes

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Fig. 14. Schematic climate-driven evolution of Congo Axial Fan sedimentation. (1) progradation-retrogradation architectural cycles C and D (simplified from Fig. 12). (2) Humid period forcing the progradation of the turbidite system. (3) Arid period favoring the retrogradation of system. (4) Arid/humid transition controlling the maximal retrogradation of the fan.

in sediment flux and composition. This is a direct consequence of the permanent connection between the river and the canyon. Indeed, a monsoon control was also evidenced in turbidite systems not connected to their river during highstand such as the Nile Fan (Ducassou et al., 2009) or the Indus Fan (Prins et al., 2000) and for which the sediment supply is controlled by both sea-level and climate fluctuations. This confirms that the climate should be given greater consideration in models as an important additional parameter possibly controlling fan growth patterns. 6. Conclusions In order to further our understanding of the factors controlling the architectural cycles identified in the Late Quaternary Congo turbidite system by Marsset et al. (2009) and Picot et al. (2016), we have established the chronostratigraphic framework of the cycles. The chronology is based on the dating of sediment cores that were strategically positioned after interpretation of very high-resolution seismic (sub-bottom profiler data) to establish the age of abandonment or initiation of key channel-levees or lobes. The main results of this work are:

 The work, based on a detailed analysis of channel-levee and lobe architecture and of lithological facies, revealed geometries, echo-facies and Bouma-type sequences (Bouma, 1962) similar to those encountered in mud-rich turbidite environments.  Age models of cores established mainly from radiocarbon dating allowed the initiation or abandonment of seven channel-levee systems and one lobe to be constrained, providing a chronostratigraphy for the observed prograding/retrograding architectural cycles, for the last 38 kyr. From this chronostratigraphy, it is established that more than half of the channels of the Axial Fan (around 30) formed during the last 38 kyr, i.e. during one fifth of the total duration of the Axial Fan (210 kyr). Avulsion frequencies vary between 9 and 2 per 10 kyr.  The timely constrained architectural cycles compared with sealevel variations shows that prograding periods are not tuned with a specific sea level (falling or rising). Together with the constant avulsion frequencies from 38 ka to 4.6 ka (i.e. covering falling and rising sea-levels), it suggests that sea-level change is not the main factor controlling the Congo Fan growth pattern. This is consistent with the perennial connection of the canyon to the Congo River, which ensures permanent supply to the fan.

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 Compared with the environmental and climate proxies, the architectural cycles emphasize that prograding phases are associated with humid periods of high freshwater discharge and therefore potentially low and fine-grained sediment yield while retrograding phases occur during arid periods with low freshwater discharge and potentially high and coarser-grained sediment yield. A major retrogradation, 500 km upfan, occurred at 11 ka at the arid/humid climatic transition. At this transition, monsoon precipitations increased but rainforest was still limited in the watershed favoring runoff, mechanical erosion and coarse-grained supply. The cyclic evolution of the architecture is therefore correlated to climate (humidity/aridity variations in link with the West African Monsoon) and subsequent environmental variations (watershed vegetation, sediment yield and freshwater discharge).  Prograding phases are therefore interpreted as periods of high capacity turbidity currents and probably increased confinement by channel erosion and levee construction. Retrograding phases are interpreted as periods characterized by low transport capacity turbidity currents and channel infill. The major retrogradation at arid/humid transition is interpreted as a period with turbidity currents of low transport capacity with enhanced channel infill. This study, thereby, outlines the link between fan stacking and channel avulsion pattern and the Congo freshwater discharge and sediment yield enforced by the monsoon regime. However, additional time constraints should be brought to improve correlation between prograding-retrograding cycles and sub-cycles and the monsoon regime. Other parameters (e.g. volume and sediment rate of, ideally, each channel-levee-lobe system) should be added to the study to verify if their temporal evolution is also organized as cycles. To validate the monsoon-driven growth pattern scheme of the Congo Fan forward 3D stratigraphic modeling is in progress to test the forcing of climate and the impact of the sea level. Lastly, it is important to better constrain the parameters (sediment load in the river, role of vegetal cover extent, role of sediment storage, etc.) in the watershed by hydrogeological modeling (work in progress). An important correlative issue of this work is that, conversely to sequence stratigraphy models that consider sea-level changes as the main control factor of margin sedimentation, the climatedriven changes of sediment flux and composition should be given greater consideration as an important additional parameter possibly controlling fan growth patterns. Acknowledgements This work was financially supported by the French INSU programs MARGES, SYSTER and Artemis (LMC14) and was undertaken in the framework of the "Laboratoire d'Excellence" LabexMER (ANR-10-LABX-19). Marie Picot PhD was co-funded by IFREMER and Total. Thomas Sionneau post-doctoral fellowship was funded by Total. We thank the Captains and crew members of the research vessels from Ifremer fleet onboard which the data were acquired between 1998 and 2011 (ZaiAngo 1 and 2 surveys on R/V L'Atalante, Reprezaï 1 onboard Le Pourquoi Pas? and Reprezaï 2 onboard Le Suroît) and the technicians from Genavir that ensured acquisitions of the geophysical data. The onboard scientific and technical teams of Reprezaï 1 and 2 cruises and technicians of the PAS laboratory of Ifremer are greatly thanked for their contribution to this work. We thank Florence Savignac (Sorbonne University, Paris) for her help in LECO and Rock-Eval analyses. Seismic data were analyzed using Kingdom Suite software, kindly made available to UBO by IHS. The manuscipt greatly benefited from critical reviews from Dr. D.

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