Simiyu Samuel Mwanga

Dr Benoit Gaudin

Dr Felix Kioli

Ngunzo

Family and its Influence on Kenyan Athletes' Performance: the Role of Social Environment

The purpose of this study was to investigate the impact of family on Kenyan runners' performance. These family attributes included: the influence of significant others such as parents, brothers, sisters, uncles, grandparents and others; reasons for specializing in distance running; motives for participation; and the type of family support in promoting athletes' talent development. Questionnaires were administered to 246 runners drawn from Iten in Elkeyo Marakwet County, Eldoret in Uasin Gishu County and Nandi Hills in Nandi County between April and May 2015. Data were analysed using descriptive statistics, i.e. frequencies and percentages. Results indicated that the family play an important role in the development of athletic talent as well as influencing their performance. The study recommends that sports personnel involved in the nurturing of local talent rethink, design and implement talent identification and nurturing programmes with the involvement of family for the successful development of sports population.

Introduction

Sports are now a multi-billion dollar business. 3 With the promise of lucrative careers looming large, parents often encourage young children who have some athletic promise to hone their skills early on. In communities all across the country, parents encourage and sometimes force their children to specialize in one sport. Furthermore, it has become rather common for children to be turned over to professionals for training for future athletic careers. 4 Distance running has always served as a source of tremendous pride in the Kalenjin community. Victory is especially sweet because it represents something larger than the athlete herself or himself; it symbolizes the values and aspirations of the entire people. 5 Sports often heighten parent-child involvements, they may lead to parental expectations for participation and sometimes to unrealistic hopes of success. Sports bring together members of a community (supporting local athletes and teams) or even a nation (as seen during World Cup matches and the Olympics) and promote an overall feeling of unity and social solidarity. 6 There are many factors that are associated with successful athletic performance in different athletic contests. These factors include the genetic constitution of the players, technical and tactical preparation, physical and psychological attributes. The career orientations of elite athletes are constructed by the socialization process in which socialization agents (significant 2 others), socio-cultural influences, the self and the environment all have significant roles to play. 7 The interaction between an athlete and socializing agents' "changes over time as is evident in the different phases of becoming involved and committed in a quest for sporting excellence". 8 Athletic identity could be developed and maintained by the influence of others as well as the athlete himself/herself and it shows how one's athletic involvement and experience can affect the person psychologically and cognitively. 9 The role of significant others (kin, teachers, coaches and peers) in sport socialization has been a topical issue in sport sociology. Numerous research studies have alluded to the fact that for athletic ability to be translated into good performance, the athlete must get support and encouragement from significant others. 10 According to Rameshon, 11 poverty in Kenya has driven some to succeed in running and to be successful in life. For a Kenyan, it is an avenue to travel and seize opportunities. Poverty makes the runners to work very hard to do well in running. Both propositions point towards an assumption that most people, particularly the Kalenjin, engage in sports to improve their socioeconomic status. However if poverty was the only factor then why is it that the poorest Asian and African nations are not producing Olympic champions? Why are the marginalized people in northern Kenya not producing Olympic champions? This shows that other social conditions in the family play a role. These factors could involve family support and goal setting, as well as values that determine the physical activities that children engage in.

In view of the relationship raised above, the family should be seen as playing two main functions; primary socialization and personality stabilization. Primary socialization is the process by which children learn the cultural norms of the society into which they are born. Personality stabilization refers to the role that the family plays in assisting adult family members emotionally. 12 Though the family is a pool for genetic inheritance, continued focus on genetic and physiological endowment by western scientists ignores the role of family in nurturing talents through powerful role models, social and physical support. One sociological explanation is that such domination results not from innate physical superiority alone but from a complex set of social conditions that channels a disproportionate number of physically talented young Kenyans into athletic careers. 13 As far as nutrition and diet are concerned, several scientific studies have shown that Kenyan runners simply rely on food to fuel their work outs and races. However this is true in most parts of Kenya and these communities do not produce elite athletes. It is also true that many parts of 3 the world and Kenya's not-so-great health record suggests the country has not discovered the secret to great nutrition. This focus fails to account for the role of family in developing and maintaining athletic identity through socialization process.

Rintaugu et al. 14 investigated psychosocial attributes of Kenyan university athletes and their findings indicated that the student athletes came from families of higher socio-economic status. That is, parents have university education and are involved in good or white-collar jobs. The study further indicated that university athletes are influenced by themselves, team mates and coaches. In general parents transmit attitudes and values about physical activity by paying activity fees, are powerful role-models and provide physical and emotional support to their offspring. 15 This study sought to establish if the same attributes hold for elite Kenyan runners or if the conclusion of this study is only valid for university athletes who are not the best runners in the nation.

Therefore the purpose of this study was to find out the influence of family on athletes' performance. Understanding the role of the family in promoting Kenya distance running culture is critical to the development of runners' sporting careers and will enable stakeholders to provide meaningful assistance as well as create an appropriate nurturing environment. This is consistent with the need to study Kenyan runners and unearth such factors as family and other complex social conditions that influence performance. Thus, the study is guided by the objectives of finding out the family support systems that maintain and promote athletic identity.

The study employed cross-sectional survey research design and collected data from 246 Kenyan elite and upcoming runners, ten families, six county sports officials and seven coaches. Questionnaires were administered to 246 runners drawn from Iten in Elgeyo Marakwet County, Eldoret in Uasin Gishu County and Nandi Hills in Nandi County between April and May 2015.Data were analysed using descriptive statistics, i.e. frequencies and percentages.

A self-administered questionnaire was utilized for data collection. The questionnaire sections sought from participants' parents' education level, type of parental support given to athletes, running experience, family members engagement in running and athletes' opinions of whether family support played a key role in their success. Interviews sought the same information from coaches, family members, retired athletes and sports officials due to their prior experiences and expertise in management of running activities. 14 Rintaugu et al, op. cit., 2012. 15 This study show that 31.1% of the parents had primary school education, 30.3% secondary school education, 22.8% some college education, 9.5% university education and 6.2% did not attend school. Out of 246 respondents 5 did not respond to this question. Most of the Kenyan runners come from the families that had basic education. The table shows that two third of the Kenyan runners one family member who had participated in running or was currently taking part (32.7% had no family members who had been runners). 11% had their fathers being a runner, 13.9% their mothers, 24.1% their brothers and sisters, and 9% their grandparents. Out of 246 respondents one person did not respond to this question. These findings further show that most of the Kenyan runners were of opinion that their family encouraged them to participate in athletics. 74.2% asserted that their continued participation was largely due to parental encouragement and support accorded to them in the course of training. 25.8% did not agree to this and said that their participation is influenced by their friends, selfmotivation and role models.

Presentation of results

Parents education level

Family involvement in running

Results of family encouragement

Opinions on whether family support plays a key role in the runner's success

My family provides support that enables me to succeed The results show that two thirds (66.3%) of Kenyan athletes agree that the family provides support that enables them to succeed as runners. Another 19.7% disagree while 13.5% did not express any opinion.

Other findings of the study show that in the early stages of athletic development in Kenya the family did not play any significant role in the development of the running culture. The first child exposure was in school through their teachers and physical education program. It was here that pupils' talents were identified and nurtured through a series of interschool competitions.

Many years back the family did not have exposure to sports and running was only considered as a hobby, a physical fitness program, recreational program or an academic program at school. It is only in the last 25 years when sport became professional that families started to show interests because they now considered sport as a career choice and therefore begun to see their children as fantastic sources of income. It is during this time that the family begun to realise that their sons and daughters are financial assets as a result of the achievements of the elite runners who had gone to become professional runners.

The findings of this study are aligned with previous studies that show that athletes are significantly motivated by their parents and peers to become involved in sports. 16 For many years running was not seen as important apart from a few cases where young men and women (and especially men) left schools to join programs that had athletic influence such as the armed forces, the police, the Kenyan prison system, and government parastatals such as the Kenya railway, the Kenya port authority and Kenya power and lighting.

8

The only way the family has come in is if another member of the family happens to have been a runner or a sports person. The family generally began to show interest in the last 20 years. Some athletes such as David Rudisha, Mark Keino, Billy Konchellah and others started following the footsteps of their families. It has been reported that a significant number of successful athletes come from families where family members are involved in sport. 17 This study established that their family provided them with financial support to buy training equipment such as shoes and clothing, and to pay for gym training, provide food, and pay for rent when athletes stayed in camps. Parents also exempt them from duties at home so that they can concentrate on training, provide medication in cases of injury during training, instilled discipline, provided encouragement, moral support and social support where athlete faced stiff competition, acted as role models and coaches by providing advice on the best way to succeed in sports through hard work and perseverance and persistency, provide psychological support through motivation and spiritual support through prayers towards success and achievement. According to Brother Colm and Muche Richard those who succeed in their career by winning, the family came in to seek investment advisors to help their children invest their earnings because now they see their running as a career.

These findings agree with other research studies that have shown that parents transmit attitudes and values about physical activity in general and sport in particular, pay activity fees, are powerful role models and provide physical and emotional support to their offspring. 18 Indeed Côté 19 found that families, particularly parents, play an important role in elite athlete development as athletes progress through what were identified as sampling, specializing, and investment years providing important sources of social support, especially when their child faced adversity or had to deal with setbacks.

These findings also agree with the findings of Gould, Dieffenbach and Moffett 20 that the ways in which parents and coaches influenced athletes were both direct, such as teaching or emphasizing certain psychological lessons and indirect, involving modelling or unintentionally creating certain psychological environments. These results also support Bloom's 21 , Côté's 22 and Csikzentmihalyi et al.'s 23 talent development research, demonstrating the important roles parents play in athletic talent development.

A good deal of consistency was found across domains in terms of the investments of tangible and intangible resources found to be essential in nurturing promising individuals with talent. In addition to financial support and transportation to numerous competitions and performances, parents found ways to provide social-emotional support (e.g., facilitating a disciplined involvement while avoiding excessive expectations and pressure). The parents also served as 17 Ibid.

18 Grevis, op. cit.,1991;Wasonga, op. cit.,1996. 19 Côté, J., The influence of the family in the development of talent in sport, The Sport Psychologist, 13, 1999, pp.395-417. 20 models for disciplined independence and fostered disciplined independence in their talented children. That is, parents often modelled hard work while supporting their children; they reinforced their children's tendency to work independently and expected those behaviours from them. These findings were in agreement with Bloom's 24 results that clearly showed that talent development is a long-term process that involves more than just the talented person, but also a strong support system with parents playing a primary role.

Results also revealed that a number of individuals and institutions influenced the athletes' psychological development including the community, family, the individual him or herself, nonsport personnel and sport environment personnel. Coach and family influences were perceived to be particularly important. Those that don't have family influence were encouraged to participate in athletics by their teachers, friends, coaches, community, sponsors and were self-motivated due to a love of running. These findings were consistent with Ericsson's 25 concern that without developing the love of the game children will not have the motivation to sustain the effort needed to pursue excellence that has been found to take as many as 10,000 hours or 10 years of deliberate practice to develop. Indeed Rintaugu's 26 findings showed that university athletes are influenced by themselves, team mates and coaches. The same scenario seems to be replicated in Kenyan elite runners who are not supported by the family, where some of them especially in private sponsored training camps, and church sponsored institutions such as St Patrick high school use excellence in sports as a way of life to fuel further achievements both academically and economically. However as Gould, D, & Carson, S. 27 put it, the most interesting finding was that parental support and commitment was critical to athlete development motivation, love of the game and ability to succeed.

The findings also revealed that culturally, social life in Iten allowed women to participate in sports. For example, before 1990s daughters only sought education and got married; their family received dowry as the only source of wealth from their daughters. This attitude changed when in 1991-1997 girls made a breakthrough in the professional ranks with financial rewards. The parents realised that their daughters could become significant sources of income. Susan Sirma was the first Kenyan woman to win a medal in 1992 Barcelona Olympics and Sally Barsosio became the first Kenyan woman to win a world title when she won the 10,000m at the World Championships of 1997. Parents then started to encourage their daughters to become athletes so that they could make money. As a result subsequent women such as Pauline Konga, Tagla Lorupe, Catherine Ndereba, Pamela Chelimo, Janeth Jepkosegei, Nancy Lagat, Edna Kiplagat, Prisca Jeptoo etc. have been produced. Parents now see athletic as a career for their daughters. These findings are in agreement with Akpata and Gitonga's 28 findings that parental social economic status plays a key role in children's socialization into sport.

24 Bloom, op. cit., 1985. 25 The results revealed that the parents of most of Kenya's elite athletes and upcoming athletes had basic education of primary, secondary level or they are not educated at all. These results contradict findings of Rintaugu 29 which indicated that the student athletes came from families which can be labelled as upper social economic status i.e. parents have university education and are involved in "good"/white collar jobs. But these results showed that that most parents were of middle and low socio-economic status involved mainly in farming and small-scale business and a few were employed as civil servants.

The study also revealed that beside positively contributing to childhood athletic development and success, parents also at times contribute to the downfall and eventual disappearance of athletes from the world of running. This is particularly true when a child fails to make a breakthrough in running after years of training. Overemphasis on winning, holding unrealistic expectations, criticizing or pampering their child too much and withdrawing support were attributes considered to hurt the athletes and contribute to their downfall. This in one way leads to feelings of inferiority, failure or at times pride, which can make the child develop arrogance, and even failure to attend training. These findings were consistent with Gould, L. Rolo,Jannes&Sie-Pennisiv 30 and Hellstedt 31 that a young athlete's emotional response was shown to be related to his or her perceptions of parental pressure. Parents can influence their children's emotional responses through evaluation, particularly unfavourable evaluations, and the expectations they have for their children. Indeed Hellstedt 32 found that the "degree of parental pressure is related to the type of affective reaction from the young athlete", with high levels of parental pressure related to negative athlete response. The athletes were also shown to be apprehensive about how their parents would react emotionally, such as with disappointment or disapproval, when they did not perform well. In addition, according to these young athletes, continued sport participation was due, in part, to the desire to please their parents, further emphasizing the strong influence parents have on their children's sport participation as well as their emotional responses to such participation.

Conclusion

The findings of this study revealed that the career orientations and paths of elite Kenyan runners are constructed by the socialization process in which socialization agents (significant others), socio-cultural influences, the self and the environment have significant roles to play. Interactions between an athlete and socializing agents change over time, as is evident in the different phases of becoming involved and committed in a quest for sporting excellence. Athletic identity could be developed and maintained by the influence of others as well as the athlete him/herself. This contributes to the view that socialization into sport is rather a continuous exercise. From the sociological point of view the self emerges as a product of society through the process of socialization, so that the relationship between groups lead to the fusion of individualities into a feeling of wholeness so that one's self becomes identified with the beliefs, values and purpose of 29 Rintaugu, op. cit., 2012. 30 Gould, D., Lauer, L. Rolo, S., Jannes, C., & Sie-Pennisi, N., The parents' role in junior tennis success and failure, Final Report, U.S. Tennis High Performance Sport Science Grant, 2004. 31 Hellstedt, J. C., Early adolescent perceptions of parental pressure in the sport environment,Journal of Sport Behavior, 13, 1988, pp.135-144. 32 Ibid.

the society. Man is a group-related being from the very beginning of life. Indeed man is not born human; he becomes human through the process of involvement in social life. The aspects of the self are developed in us as children through our relationships with others. They are developed as we come to perceive ourselves as the objects of actions towards us, and others as the objects of our actions towards them. 33 The career choice of an individual is largely influenced by the family from which s/he comes. Then for proper talent development of an individual, there is a need to involve the family as the primary agent of socialization into a sports career. Factors such as the family's socio-economic status, opportunities for success and inequalities propel individuals to choose a career that will help them overcome these hurdles, and this is largely influenced by the ability of people to make use of the social environment to achieve the mentioned success. It is also evidenced that children often follow the career paths of their parents, especially when it comes to sports and therefore parents love and participation in certain sports is essential for the sports socialization of their children.

Frequency

  Grevis, M., Children in sport, in Bull J.J. (ed.), Sport Psychology, A Self-help Guide,London: The Crowood, 1991; Wasonga, T.A., Childhood sport socialization in development of competitive sports and talent in Kenya, in AmusaL.O., Wekesa M. & Toriola A.L. (eds), The Making of An African Athlete: A Multidimensional Approach,Gaborone: AFAHPER-SD, 1996.

  Ericsson, K. A. (ed.), The road to excellence: the acquisition of expert performance in the arts,sciences, sports, and games,Mahwah, NJ: Erlbaum, 1996. 26 Rintaugu, op. cit., 2012. 27 Gould, D, & Carson, S., Myths surrounding the role of youth sports in developing Olympic champions, Youth StudiesAustralia, 23(1),2004, pp.19-26. 28 Akpata, D.O., & Gitonga, E.R., Social Economic status of secondary school interscholastic athletes in Nairobi province, Kenya,Journal of International council for physicalEducation, Recreation sports and Dance, 353, (2), 2002, pp.28-30.

Rintaugu, op. cit., 2005.

Lowry R.P. and Rankin R.P., Sociology: The Science of Society, Charles Scribner'sSons, 1969.


Claire Croguennec

Livio Ruffine

Bernard Dennielou

François Baudin

Jean-Claude Caprais

Vivien Guyader

Germain Bayon

Christophe Brandily

Julie Le Bruchec

Claire Bollinger

Yoan Germain

Laurence Droz

Nathalie Babonneau

Christophe Rabouille

Evidence and age estimation of mass wasting at the distal lobe of the Congo deep-sea fan

Introduction

Sequential early diagenetic processes take place within shallow continental margin sediments.

These processes are biogeochemical redox reactions mediated by microbes and involve a large variety of chemical species [START_REF] Schulz | Marine Geochemistry[END_REF]. The nature of the dominant oxidant leads to a stratification of the sedimentary column in three main zones: the oxic, suboxic and anoxic zones [START_REF] Froelich | Early oxidation of organicmatter in Pelagic sediments of the Eastern Equatorial Atlantic -Suboxic Diagenesis[END_REF]. The suboxic zone can have a significant impact on carbon cycling [START_REF] Beckler | Importance of microbial iron reduction in deep sediments of river-dominated continental-margins[END_REF], but the anoxic zone generally represents the largest one, and can be subdivided in two layers: the sulfate reduction layer where this compound is the main electron acceptor, and the methanogenic layer characterized by methane generation. Within the upper layer, sulfate reduction can be coupled with two different oxidation reactions. The first one is the degradation of organic matter [START_REF] Berner | Early diagenesis: A theoretical approach[END_REF][START_REF] Claypool | The origin and distribution of methane in marine sediments[END_REF]. This reaction requires that part of the organic matter deposit bypasses both the oxic and suboxic zones, and reaches the anoxic zone to finally react with the sulfate. It is commonly called the Organoclastic Sulfate Reduction (OSR) [START_REF] Malinverno | Modeling sulfate reduction in methane hydrate-bearing continental margin sediments: Does a sulfate-methane transition require anaerobic oxidation of methane[END_REF]Meister et al., 2013a), and leads to the production of bicarbonate and hydrogen sulfide as expressed below:

SO 4 2-+ 2 CH 2 O => 2 HCO 3 -+ H 2 S (1)
The second sulfate reduction-coupled reaction is the Anaerobic Oxidation of Methane (AOM) (Barnes and Goldberg, 1976a, b;[START_REF] Knittel | Anaerobic oxidation of methane: progress with an unknown process[END_REF][START_REF] Reeburgh | Methane consumption in Cariaco Trench waters and sediments[END_REF], where methane reacts with the sulfate to produce bicarbonate, hydrogen sulfide ions and water. This reaction is formulated as follows:

CH 4 + SO 4 2-=> HCO 3 -+ HS -+ H 2 O (2)

It is governed by a consortium of methanogenic archaea and sulfate-reducing bacteria [START_REF] Boetius | A marine microbial consortium apparently mediating anaerobic oxidation of methane[END_REF][START_REF] Hoehler | Field and Laboratory Studies of Methane Oxidation in an Anoxic Marine Sediment -Evidence for a Methanogen-Sulfate Reducer Consortium[END_REF][START_REF] Valentine | New perspectives on anaerobic methane oxidation[END_REF] and is restricted to a narrow interval within the sediment called the Sulfate Methane Transition Zone (SMTZ).

This interval also corresponds to the boundary between the sulfate reduction and the methanogenic zones. The methane generated within the methanogenic layer, in turn, migrates upward to further supply the AOM process at the SMTZ. Thus, the redox reactions (1) and ( 2) are generally in competition for sulfate consumption on continental margins, and the prevalence of one over the other strongly depends on the nature and the availability of the electron donor; namely organic matter vs. methane.

The occurrence of these geochemical processes is particularly pronounced in areas characterized by a high content of labile organic matter (Meister et al., 2013b;[START_REF] Pastor | Modeling biogeochemical processes in sediments from the Rhone River prodelta area (NW Mediterranean Sea)[END_REF][START_REF] Wijsman | A model for early diagenetic processes in sediments of the continental shelf of the Black Sea[END_REF], such as the distal lobe complex of the Congo deep-sea fan (Pozzato et al., This issue; Schnyder et al., This issue). This is a remarkable geological structure [START_REF] Babonneau | Morphology and architecture of the present canyon and channel system of the Zaire deep-sea fan[END_REF][START_REF] Savoye | Geological overview of the Angola-Congo margin, the Congo deep-sea fan and its submarine valleys[END_REF][START_REF] Savoye | Structure et évolution récente de l'éventail turbiditique du Zaïre: premiers résultats scientifiques des missions d'exploration Zaïango1 & 2 (marge Congo-Angola)[END_REF] which is unique for such a geochemical study as it still experiences a continuing input of organic matter (Baudin et al., This issue;[START_REF] Rabouille | Organic matter budget in the Southeast Atlantic continental margin close to the Congo Canyon: In situ measurements of sediment oxygen consumption[END_REF][START_REF] Stetten | Organic matter characterization and distribution in sediments of the terminal lobes of the Congo deep-sea fan: evidence for the direct influence of the Congo River[END_REF], transported by strong turbidity currents from the Congo River through its permanently connected submarine canyon [START_REF] Khripounoff | Direct observation of intense turbidity current activity in the Zaire submarine valley at 4000 m water depth[END_REF][START_REF] Savoye | Geological overview of the Angola-Congo margin, the Congo deep-sea fan and its submarine valleys[END_REF]. Indeed, such turbidity currents have been recorded in the canyon by the break of communication cables during the 19 th century [START_REF] Heezen | Congo submarine canyon[END_REF] and in the deep-sea channel-levee system by mooring lines in 2001 [START_REF] Khripounoff | Direct observation of intense turbidity current activity in the Zaire submarine valley at 4000 m water depth[END_REF] and 2004 [START_REF] Vangriesheim | Turbidity events observed in situ along the Congo submarine channel[END_REF]. Thus, the lobes are recognized as being modern dynamic structures characterized by a changing morphology and are potentially prone to instabilities [START_REF] Vangriesheim | Turbidity events observed in situ along the Congo submarine channel[END_REF].

In such a geological environment, pore water elements involved in early diagenetic processes may hold a historical record of sedimentary instabilities as they move from steady to transient state due to the instability event. For instance, researchers have started to use sulfate profiles in order to evidence the past occurrence of tectonic events and sedimentary instabilities [START_REF] Fischer | Subduction zone earthquake as potential trigger of submarine hydrocarbon seepage[END_REF][START_REF] Halbach | Migration of the sulphate-methane reaction zone in marine sediments of the Sea of Marmara -can this mechanism be tectonically induced?[END_REF][START_REF] Henkel | An interdisciplinary investigation of a recent submarine mass transport deposit at the continental margin off Uruguay[END_REF][START_REF] Hensen | Control of sulfate pore-water profiles by sedimentary events and the significance of anaerobic oxidation of methane for the burial of sulfur in marine sediments[END_REF][START_REF] Hong | A kinetic-model approach to quantify the effect of mass transport deposits on pore water profiles in the Krishna-Godavari Basin, Bay of Bengal[END_REF][START_REF] Zabel | Importance of submarine landslides for non-steady state conditions in pore water systems -lower Zaire (Congo) deep-sea fan[END_REF]. In this regard, [START_REF] Zabel | Importance of submarine landslides for non-steady state conditions in pore water systems -lower Zaire (Congo) deep-sea fan[END_REF] have exploited the pore water profiles of sediment cores from the Congo deep-sea fan in order to reconstruct the scenario of geological events that occurred in the area. They hypothesized that submarine landslides may disrupt the steady-state conditions of the system and then affect the shape of sulfate profiles. Similarly, [START_REF] Hensen | Control of sulfate pore-water profiles by sedimentary events and the significance of anaerobic oxidation of methane for the burial of sulfur in marine sediments[END_REF]

Geological settings

2.1.The Congo deep-sea fan: morphology and evolution

The Congo deep-sea fan is a very large mud-rich turbidite system located on the Congo-Angola margin (Fig. 1). It is an upper Pleistocene fan lying at the base of slope where it has developed a network of channel-levees and lobes [START_REF] Droz | Architecture of an active mud-rich turbidite system: The Zaire Fan (Congo-Angola margin southeast Atlantic): Results from ZaiAngo 1 and 2 cruises[END_REF][START_REF] Savoye | Structure et évolution récente de l'éventail turbiditique du Zaïre: premiers résultats scientifiques des missions d'exploration Zaïango1 & 2 (marge Congo-Angola)[END_REF].

Among all the channel-levees which compose the system, only one is presently active as it is still connected upstream to the submarine Congo canyon [START_REF] Babonneau | Morphology and architecture of the present canyon and channel system of the Zaire deep-sea fan[END_REF][START_REF] Khripounoff | Direct observation of intense turbidity current activity in the Zaire submarine valley at 4000 m water depth[END_REF][START_REF] Savoye | Geological overview of the Angola-Congo margin, the Congo deep-sea fan and its submarine valleys[END_REF]. One distinctive feature of this canyon lies in the fact that it enters into the Congo River estuary, and such a coupling enables the direct transfer of terrestrial materials to the canyon and the channel-levees. Thus, the Congo deepsea fan remains active because energetic turbidity currents transport a large volume of organic matter-rich sediments originating from the river through the canyon and the active channel to the terminal lobe complex at abyssal depths [START_REF] Khripounoff | Direct observation of intense turbidity current activity in the Zaire submarine valley at 4000 m water depth[END_REF]Rabouille et al.;[START_REF] Vangriesheim | Turbidity events observed in situ along the Congo submarine channel[END_REF].

2.2.The terminal lobe complex

The study area is located at the extremity of the terminal lobe complex where the morphology is rather flat, with small slope of 0.2° on average (Rabouille et al.;[START_REF] Savoye | Structure et évolution récente de l'éventail turbiditique du Zaïre: premiers résultats scientifiques des missions d'exploration Zaïango1 & 2 (marge Congo-Angola)[END_REF] (Fig. 2). The currently active lobe complex covers an area of ~3,000 km 2 , and is located at ~760 km from the shore and water depths up to 5100 m [START_REF] Babonneau | Morphology and architecture of the present canyon and channel system of the Zaire deep-sea fan[END_REF][START_REF] Savoye | Structure et évolution récente de l'éventail turbiditique du Zaïre: premiers résultats scientifiques des missions d'exploration Zaïango1 & 2 (marge Congo-Angola)[END_REF].

A particular emphasis has been given to that region during two recent scientific cruises, WACS in 2011 [START_REF] Olu | WACS cruise. RV Pourquoi pas?[END_REF] and Congolobe in 2012 [START_REF] Pastor | Modeling biogeochemical processes in sediments from the Rhone River prodelta area (NW Mediterranean Sea)[END_REF](Rabouille, -2012;;[START_REF] Rabouille | CONGOLOBE cruise. RV Pourquoi pas? Rabouille, C[END_REF].

The lobe complex shows a prograding pattern that has developed by the successive abandonment of an ongoing lobe and the leftward lateral migration of sediment supply from the channel to build the next lobe (Figs. 2 and3). Five lobes have been identified, based on both the hierarchical order of the distributaries and on the seismic characteristics of the sedimentary column [START_REF] Babonneau | Morphology and architecture of the present canyon and channel system of the Zaire deep-sea fan[END_REF]. The oldest lobe (#1) is the most proximal and the youngest (#5) the most distal (Fig. 2). Older abandoned lobes are also visible on the surface around the studied lobe (Fig. 2).

The extremity of lobe number 5 corresponds to a 20 thick bulge that progrades onto the abyssal plain sediments (Fig. 3). Here, the lobe is better defined on the multibeam acoustic backscatter than on the bathymetry data (Fig. 3A) as on the former it shows high backscatter with a width of ca. 10 km, narrowing to ca. 4 km downstream at the core location (Fig. 3B and C). However, although the multibeam bathymetry does not show evidence of channelization, it reveals a certain roughness of the seabed, which likely corresponds to abundant slides, head-scars and sediment blocks (Fig. 3C). This is further supported by the seismic high amplitude on the seabed and the lack of internal structure in the lobe (Fig. 2B).

Other studies have already revealed that the lobe complex is characterized by abundant distributaries where sediment instabilities have been described in the form of slides and blocks of meter scale that can be partly buried by turbidites (Fig. 4) [START_REF] Babonneau | Mode de fonctionnement d'un chenal turbiditique méandriforme : Cas du système turbiditique du Zaïre[END_REF][START_REF] Bonnel | Mise en place des lobes distaux dans les systèmes turbiditiques actuels : analyse comparée des systèmes du Zaïre, Var, et Rhône[END_REF][START_REF] Savoye | Geological overview of the Angola-Congo margin, the Congo deep-sea fan and its submarine valleys[END_REF]. The connection of the lobe complex with the Congo The age of the lobe complex is still elusive because of the difficulty to date terrigeneous material. However extrapolations based on radiocarbon dating of organic carbon as well as 210 Pb xs and 137 Cs analyses [START_REF] Stetten | Organic matter characterization and distribution in sediments of the terminal lobes of the Congo deep-sea fan: evidence for the direct influence of the Congo River[END_REF] suggest that the current lobe complex may have started to develop during the Holocene, probably around 6 kyr BP [START_REF] Savoye | Geological overview of the Angola-Congo margin, the Congo deep-sea fan and its submarine valleys[END_REF]).

Based on these statements and assuming that the successive build-up of lobes 1 to 5 was homogeneous in time, the inception of lobe number 5 may have started no later than 1 kyr ago.

Sampling and analytical methods

3.1.Core location

The study core, named WACS-06, is a 19.80m length Calypso piston core. It was retrieved at 4996 m water depth at the extremity of lobe number 5. Although the base of the lobe is not clearly visible on the sub-bottom profile, it is likely that the core has sampled the whole thickness of the lobe (Fig. 2). After recovery, the core was directly cut into 1m-length sections for sampling and analysis, followed by description and photography.

3.2.Water sampling and analyses

Both bottom seawater and pore waters were collected for geochemical analyses of dissolved elements. The former was sampled few centimeters above the seafloor using the PEP (Préleveur d'Eau par Pompage) water multi-sampler operated by the ROV Victor 6000, whereas the latter was extracted directly from core WACS-06. Each core section was transported to the onboard laboratory for pore water extraction at 4 °C by means of Rhizon  soil moisture samplers (Rhizosphere Research Products, Netherlands) [START_REF] Seeberg-Elverfeldt | Rhizon sampling of pore waters near the sediment/water interface of aquatic systems[END_REF]. The sampler consists of a hydrophilic and porous polymer tube (2.5 mm x 50 mm), which is introduced into the sediment at regular intervals, and connected to a 10mL preevacuated syringe for pore water collection, followed by subsampling. Plasma Mass Spectrometry (HR-ICP-MS Element2, Thermo Scientific ® ). The calibration was based on the standard addition method of known amounts of each element in the NASS-5 standard seawater to avoid matrix effects. The estimated accuracy on the measurements of major and minor elements was better than ±3%. For strontium isotope-ratio measurements, 125 µL of sample was loaded on a column containing a crown ether cation exchange resin (Sr Spec, Eichrom) for separation from the other dissolved elements. The collected strontium fraction was then analyzed on a Multicollector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS Neptune, Thermo Scientific ® ). 87 Sr/ 86 Sr ratios were corrected from mass discrimination using exponential normalization with the NIST SRM-987 standard ( 86 Sr/ 88 Sr=0.1194) and the resulting values are given with an accuracy better than 0.00004.

The analyses of bottom seawater samples were limited to chloride and sulfate concentrations, while all the aforementioned elements were measured for pore water samples.

3.3.Sediment core description, sampling and analyses

After pore-water sampling, the whole-round core was analyzed using a Multi-Sensor Core Logger (MSCL) from Geotek ® to determine both gamma-density and magnetic susceptibility profiles. Thereafter, each section was split lengthwise in two halves, a working one and an archive. The archive half was photographed with a digital camera and detailed visual descriptions of the lithofacies were performed. The split core was analyzed at 1 cm intervals with an Avaatech core scanner in order to determine the relative abundance of elements (counts) and elements ratios [START_REF] Richter | The Avaatech XRF Core Scanner: technical description and applications to NE Atlantic sediments[END_REF]. Ninety-six sediment samples of 1g were taken along the working half at a sampling resolution of 20 cm for organic matter analysis.

Pyrolytic analysis was carried out to measure the carbonate fraction (Eq-CaCO 3 ), and then determine the total organic carbon (TOC) together with the hydrogen index (HI) using a Rock Eval 6 Turbo device ® (Vinci Technologies). The analysis is based on the pyrolysis of organic matter-bearing sediment followed by flame ionization and infrared detections. A complete description of the analytical protocol can be found elsewhere [START_REF] Baudin | Guidelines for Rock-Eval analysis of recent marine sediments[END_REF].

Concretely, the salts contained in the sediment samples were washed away to avoid interference with the flame ionization detector, and the pyrolysis was started at temperature lower than for classical pyrolysis (180 °C instead of 300 °C) to avoid early cracking of labile organic molecules. The hydrogen index was calculated from the Rock-Eval data using the following formula: HI=S2*100/TOC. Here, S2 is the amount of hydrocarbons generated through thermal cracking of nonvolatile organic matter in mg/g of rock and TOC is total organic carbon in wt % [START_REF] Espitalié | Méthode rapide de caractérisation des roches mètres, de leur potentiel pétrolier et de leur degré d'évolution[END_REF].

3.4.Modeling

A numerical model was used to evaluate how the anaerobic oxidation of methane affects the sulfate profile. A transport-reaction equation has been solved to simulate the evolution of the sulfate profile over time [START_REF] Berner | Early diagenesis: A theoretical approach[END_REF][START_REF] Boudreau | Diagenetic Models and their Implementation[END_REF][START_REF] Chuang | Relating sulfate and methane dynamics to geology: Accretionary prism offshore SW Taiwan[END_REF]. The model is based on the mass balance equation:

[ ] [ [ ] ] [ ] (eq. 1)
Where t is time (yr), is the sediment porosity, x is the depth within the sedimentary column (m), is the sediment tortuosity, C i and D i are the concentration (mM) and diffusion coefficient (m 2 yr) of the dissolved species "i", respectively,  is the upward fluid velocity and R AOM is the AOM reaction rate. Tortuosity within the sedimentary package was used to correct the diffusion coefficients in free solutions. Boudreau's correlation was used to calculate the sediment tortuosity:

( )

(eq. 2)
The AOM rate was expressed by a bimolecular kinetic equation:

[ ][ ] (eq. 3)
With k AOM the associated constant rate.

All parameters used in the model are listed in Table 1. Both terms, transport and AOM reaction, were solved simultaneously using a centered finite difference method with a uniform grid. The discretization over the length domain was defined with a grid of 1000 points.

Dirichlet condition (constant concentrations) was applied at the core boundaries, excepted for the sulfate downcore where the Neumann condition (open flux) was applied. The upward fluid velocity range was adjusted from the chloride profile.

Results

4.1.Core description

The combination of visual description of core WACS-06 (Fig. 5) with its physical properties and XRF chemical composition (Fig. 6) allows distinguishing five lithofacies.

- This facies is interpreted as pelagic deposits.

-Lithofacies 3 corresponds to laminated brown to dark stiff clay. It is characterized by a high gamma-density (1.4 g.cm -3 ) that is interpreted as muddy turbidites.

-Lithofacies 4 corresponds to fine to medium sand. It is characterized by a high gammadensity (≥ 1.6 g.cm -3 ).

-Lithofacies 5 corresponds to mudclasts embedded in a clayey matrix and is interpreted as a debrite.

Furthermore, the whole sediment core is characterized by abundant deformations, which are indicative of mass wasting. An index with a scale ranging between 0-1 was proposed in order to visualize the degree of deformation. Index zero indicates that the sediment is not deformed, whereas index one indicates that the original strata are still visible but have been tilted (up to 90°) and present numerous folds and faults (Fig. 5).

The combination of lithofacies and deformation-evidencing features results in the identification of eight units (Fig. 5):

-Unit 1 (0-38 cm) corresponds to muddy turbidites (lithofacies 1) with clearly visible horizontal laminations typically observed on undeformed sediments.

-

4.2.Carbonate and organic matter distribution

Carbonate and TOC contents, together with the HI, are plotted as a function of sediment depth (Fig. 6). Carbonate content is relatively low (3-4 wt.%) over the first 10 meters of the core. The sedimentary section located below that depth exhibits several peaks where the Eq-CaCO 3 reach values as high as 25 wt.%. Overall, TOC contents are very high as expected in the area [START_REF] Baudin | Distribution of the organic matter in the channel-levees systems of the Congo mud-rich deep-sea fan (West Africa). Implication for deep offshore petroleum source rocks and global carbon cycle[END_REF]Baudin et al., This issue) and range between 1 and 4 wt.%. As can be seen in Fig. 6, its variation with depth is correlated with the demarcation of the sedimentary units. TOC contents decrease steadily with depth in the turbiditic lithofacies all along Unit 1 and 2 from 4 wt.% at the top core to 1 wt.% at the base and then the values remain low and uniform in the pelagic lithofacies within Unit 3; and even within Unit 4.

Interestingly, Unit 5 is characterized by a slightly parabolic (leftward concave) TOC profile with the highest values measured for core WACS-06, ranging between 3 and 4.3 wt.% and with a maximum at 840 cmbsf. The two lower units exhibit TOC contents around 2 wt.%, with significant positive and negative fluctuations from values encountered in Unit 3 and 4, in relation with the alternation of pelagic and turbiditic lithofacies. the upper part of Unit 6. Thus, as the initial stratification of the sediment is still visible (Fig. 5) and due to the fact that this area is located at the distal part of the lobe where the turbidity current eventually ended up, the deformations from Unit 2 to 7 likely result from slides, with the exception of Unit 4 that is interpreted as a debrite [START_REF] Mulder | Classification of offshore mass movements[END_REF][START_REF] Tripsanas | Submarine mass-transport facies: new perspectives on flow processes from cores on the eastern North American margin[END_REF]. Nevertheless, both slide and debrite evidence the occurrences of sedimentary instabilities [START_REF] Henkel | An interdisciplinary investigation of a recent submarine mass transport deposit at the continental margin off Uruguay[END_REF]. Moreover, as mentioned previously, there is a clear correlation between lithofacies, deformation and unit demarcation, and one of the questions which arise when examining those data is whether they were deposited during one or several mass movements. In fact, many submarine instabilities and deformations develop in multiple phases, and multi-disciplinary investigations are often needed to fully unravel these processes and estimate their timing [START_REF] Vanneste | Seafloor instabilities and sediment deformation processes: The need for integrated, multi-disciplinary investigations[END_REF]. The numerous multidisciplinary cruises carried out to intensively investigate the Congo deepsea fan have led to the conclusion that there is no petroleum system beneath in this area [START_REF] Rabouille | CONGOLOBE cruise. RV Pourquoi pas? Rabouille, C[END_REF][START_REF] Savoye | Geological overview of the Angola-Congo margin, the Congo deep-sea fan and its submarine valleys[END_REF][START_REF] Sibuet | Deep-sea environment and biodiversity of the West African Equatorial margin[END_REF]. Accordingly, and because the geological setting is also not suitable for abiogenic methane generation

4.3.Geochemistry of pore waters

Unusual location of the methane source and the consequence of the on the pore-water profiles

As mentioned earlier, with the exception of chloride and strontium to a lesser extent, all pore water profiles exhibit clearly visible unusual features (Fig. 7). For instance, the measured sulfate profile is very different from profiles commonly encountered in most marine sedimentary settings [START_REF] Borowski | Global and local variations of interstitial sulfate gradients in deep-water, continental margin sediments: Sensitivity to underlying methane and gas hydrates[END_REF][START_REF] Chuang | Relating sulfate and methane dynamics to geology: Accretionary prism offshore SW Taiwan[END_REF][START_REF] Hensen | Control of sulfate pore-water profiles by sedimentary events and the significance of anaerobic oxidation of methane for the burial of sulfur in marine sediments[END_REF][START_REF] Martens | Biogeochemical cycling in an organic-rich coastal marine basin 4. An organic carbon budget for sediments dominated by sulfate reduction and methanogenesis[END_REF][START_REF] Pohlman | Assessing sulfate reduction and methane cycling in a high salinity pore water system in the northern Gulf of Mexico[END_REF] in at least two respects: The first one is the unexpected low sulfate concentrations (12-13 mM against 28.3 mM for the bottom water) measured at the upper part of the core over meter scale, and this will be discussed in the following section. The second one consists on the presence of a depleted sulfate interval located between 400 and 950 cmbsf, far from the core ends. This depletion is caused by anaerobic oxidation of methane from the source located within Unit 3, leading to the occurrence of two sulfate methane transition zones at 400 and 950 cmbsf. Evidence of AOM is supported by the barium profile which displays a maximum in pore water concentrations at the sulfate-depleted interval bounded by the two SMTZs as barite dissolution frequently occurs beneath the SMTZ and releases barium ions into the pore water [START_REF] Noethen | Reconstructing changes in seep activity by means of pore water and solid phase Sr/Ca and Mg/Ca ratios in pockmark sediments of the Northern Congo Fan[END_REF]. The alkalinity peak just beneath the shallowest SMTZ is also consistent with the AOM-related activity [START_REF] Kastner | Geochemical constraints on the origin of the pore fluids and gas hydrate distribution at Atwater Valley and Keathley Canyon, northern Gulf of Mexico[END_REF][START_REF] Noethen | Reconstructing changes in seep activity by means of pore water and solid phase Sr/Ca and Mg/Ca ratios in pockmark sediments of the Northern Congo Fan[END_REF].

Such an occurrence of two SMTZs has already been reported in the literature [START_REF] D'hondt | Distributions of microbial activities in deep subseafloor sediments[END_REF][START_REF] Mogollon | Timescales for the development of methanogenesis and free gas layers in recently-deposited sediments of Arkona Basin (Baltic Sea)[END_REF][START_REF] Parkes | Deep sub-seafloor prokaryotes stimulated at interfaces over geological time[END_REF][START_REF] Torres | Crustal fluid and ash alteration impacts on the biosphere of Shikoku Basin sediments, Nankai Trough, Japan[END_REF]. also presented results from the IODP Site C0012 drillings in the Shikoku basin where two SMTZs were found at 19800 and 43300 cmbsf. At Arkona basin in the Baltic Sea, they were identified at 180 and 500 cmbsf by [START_REF] Mogollon | Timescales for the development of methanogenesis and free gas layers in recently-deposited sediments of Arkona Basin (Baltic Sea)[END_REF]. For the two first studies, the development of two SMTZs was ascribed to the existence of two different sulfate sources supplying each SMTZ [START_REF] D'hondt | Distributions of microbial activities in deep subseafloor sediments[END_REF][START_REF] Parkes | Deep sub-seafloor prokaryotes stimulated at interfaces over geological time[END_REF][START_REF] Torres | Crustal fluid and ash alteration impacts on the biosphere of Shikoku Basin sediments, Nankai Trough, Japan[END_REF]. Sulfate was supplied from depth by the upward migration of brine. The depths of the SMTZs are also deeper, tens to hundreds of meters below seafloor. In the Arkona basin, the development of two SMTZs was attributed to the presence of residual sulfate in the Pleistocene marine clays underlying an organic-rich unit [START_REF] Mogollon | Timescales for the development of methanogenesis and free gas layers in recently-deposited sediments of Arkona Basin (Baltic Sea)[END_REF]. For core WACS-06, the chloride profile shows no evidence of brine supplied beneath by upward migration. Thus, the sulfate present at the bottom of the core also originates from the seawater, and therefore, there is two possible explanations for the occurrence of the two SMTZs. The first one would require a lateral migration of methane. Thus, methane is likely transported through the sandy layers observed in Unit 3. This unit exhibits low values of gamma-density and Zr/total counts with several peaks at the sandy intervals, whereas the K/ Ti ratio profile shows an inverse trend (Fig. 6), which is indicative of coarse grained sediment. Methane can be easily transported laterally through these layers [START_REF] Sultan | New insights into the transport processes controlling the sulfate-methane-transition-zone near methane vents[END_REF], which act as drain channels. Alternatively, the occurrence of the two SMTZs may directly result from the youngest sediment slide that brought the methane source (i.e. Unit 3) to its current location (~400 cmbsf). Unit 6, 7 and 8, which have not undergone mass wasting, have likely preserved sulfate in concentrations close to that observed within the units 1 and 2 (Fig. 7). Thus, methane would react with sulfate from both the upper and the lower units. In either case proposed here, the establishment of the AOM results from the sedimentary event involving Unit 3 as it is not co-genetic with the underlying units. 5.2.3. AOM vs. OSR: What is the dominant process responsible for the sulfate reduction at the investigated area?

As mentioned earlier, sulfate concentrations much lower than that of seawater were measured at the uppermost part of core WACS-06. Our uppermost pore-water sample was collected at 62 cmbsf. Such low values close to the sediment-water interface are often encountered at sites characterized by methane emissions or shallow gas hydrates [START_REF] Chuang | Relating sulfate and methane dynamics to geology: Accretionary prism offshore SW Taiwan[END_REF][START_REF] Ruffine | Pore water geochemistry at two seismogenic areas in the Sea of Marmara[END_REF][START_REF] Tryon | Pore fluid chemistry of the North Anatolian Fault Zone in the Sea of Marmara: A diversity of sources and processes[END_REF][START_REF] Wilson | Subsurface methane sources and migration pathways within a gas hydrate mound system, Gulf of Mexico[END_REF], and usually results from the anaerobic oxidation of methane (AOM) coupled with sulfate reduction near this interface. However, we have demonstrated in the previous section that the shallowest SMTZ is located at around 400 cmbsf, therefore quite far from the sediment-water interface. In addition, evidence of neither gas hydrates nor methane emission has been observed from the ROV dives, tough methane fluxes ranging between 1.8 to 139 mmol.m -2 .day -1 have been measured from Calmar benthic chambers in vesicomyid habitats [START_REF] Khripounoff | Variability in gas and solute fluxes through deep-sea chemosynthetic ecosystems inhabited by vesicomyid bivalves in the Gulf of Guinea[END_REF]. After splitting, core WACS-06 presented no evidence of vesicomyids at the collected site. Thus, AOM is not responsible for the abnormally low sulfate concentrations observed at the core top; this is rather due to organoclastic sulfate reduction as it has been shown that the TOC contents of the surficial sediment are very high [START_REF] Baudin | Distribution of the organic matter in the channel-levees systems of the Congo mud-rich deep-sea fan (West Africa). Implication for deep offshore petroleum source rocks and global carbon cycle[END_REF][START_REF] Stetten | Organic matter characterization and distribution in sediments of the terminal lobes of the Congo deep-sea fan: evidence for the direct influence of the Congo River[END_REF]. Moreover, in situ measurements have shown that the oxic layer is very thin in this area, with a thickness of a few centimeters at most [START_REF] Beckler | Importance of microbial iron reduction in deep sediments of river-dominated continental-margins[END_REF]Pozzato et al., This issue;[START_REF] Rabouille | Organic matter budget in the Southeast Atlantic continental margin close to the Congo Canyon: In situ measurements of sediment oxygen consumption[END_REF]. Thus, it is likely that the oxygen supply is not sufficient enough to oxidize the whole labile organic-matter fraction available; part of it bypasses the oxic and suboxic layers [START_REF] Beckler | Importance of microbial iron reduction in deep sediments of river-dominated continental-margins[END_REF] and enters into the anoxic one for being oxidized by sulfate. All this evidence indicates that OSR is active within the first ~60 cm of sedimentary column, and likely the main process responsible for the sulfate depletion at this sedimentary interval. This is strengthened by the high values of alkalinity measured from our upper samples (~30 mM) as OSR produces twice more carbonate than it consumes sulfate.

On the other hand, one can clearly see from Fig. 7 that sulfate concentrations remain constant, with value of ~12-15 mM over meter scale at both the uppermost and lowermost units while the TOC contents are still high. This suggests that sulfate does not significantly react further with the organic matter. The labile part was quickly consumed at the uppermost part of the core, leaving a refractory material for deposition at the lobes [START_REF] Pastor | Modeling biogeochemical processes in sediments from the Rhone River prodelta area (NW Mediterranean Sea)[END_REF](Rabouille, -2012;;[START_REF] Stetten | Organic matter characterization and distribution in sediments of the terminal lobes of the Congo deep-sea fan: evidence for the direct influence of the Congo River[END_REF]. Therefore, the OSR is hampered and sulfate at concentrations lower than that of seawater is allowed to diffuse downward within the sediment. Based on these results, we postulated that OSR is the dominant process responsible for sulfate reduction over the first ~60 cm upper sedimentary column, and then AOM takes over and depletes the sulfate in Unit 3 to 5.

5.3.Constraining the age of the youngest sedimentary event

As mentioned earlier, the investigated area continuously receives sediments transported through the channel by turbidity currents, and therefore it may be constantly subject to sedimentary instabilities. In our sampled core, it has been demonstrated that these turbidity currents play a structuring role in the methane flow path. The transport-reaction model used here considered that AOM prevails over OSR since it has been shown that the latter reaction is restricted within the first ~60 cm depth less affected by instabilities, even though methane concentrations are below the pore-water saturation level. These low measured values are likely the result of methane exsolution during the recovery of the none-pressurized core from 4996 m water depth to the sea surface. The extraction process by Rhizon ® involves the creation of a small vacuum inside the syringe that could affected methane concentration.

However, that would be of minor effect compared to core recovery as the measured methane concentration is near saturation at atmospheric pressure. Moreover, consideration of OSR would require the knowledge of parameters like apparent initial age and the initial concentration of the POC during sedimentation [START_REF] Luo | Estimating the time of pockmark formation in the SW Xisha Uplift (South China Sea) using, reaction-transport modeling[END_REF]Meister et al., 2013b;[START_REF] Stolpovsky | Toward a parameterization of global-scale organic carbon mineralization kinetics in surface marine sediments[END_REF][START_REF] Wallmann | Kinetics of organic matter degradation, microbial methane generation, and gas hydrate formation in anoxic marine sediments[END_REF], which were not measured here and are unavailable. Nevertheless, using this AOM-based approach allows simulating its importance in the sulfate reduction, and any deviation from the measured profile will be due to the contribution of OSR. The initial conditions for the simulation is taking such as methane is supplied from Unit 3; and sulfate concentration varies slightly and linearly from 12.10 to 14.70 mM as seen in Unit 1 and 2, and Unit 6 to 8, respectively. The of the simulation give an AOM incipient of 96 ±10 years (Fig. 9). The best fit is obtained for a methane-source depth located at 560 cmbsf instead of the 420 cmbsf measured from pore water analysis. This depth is still located within Unit 3, however it is very close to the boundary with the muddy debrite. One possible explanation of this difference is the omission of OSR within the model. Though, good agreement between the measured profile and the simulation results is indicative of the prevalence of AOM as the main process affecting the sulfate consumption within Unit 3 to 5.

Conclusion

Occurrence of sedimentary events at the more distal part of the Congo deep-sea fan have been revealed by combining pore-water and sediment geochemistry, together with the sedimentary details observed from the visual description of core WACS-06. Such a study on the identification of sediment instabilities is important if, ultimately and beyond this study , we aims to understand the impacts of these events on the distribution and reactivity of organic matter in the area as well as their consequences on the transport of redox species within the sedimentary column. From our investigated core, correlations are observed, both from the sediment and the pore waters, between the unit demarcation and the profiles of the chemical

  presented an interesting example from the western part of the Argentine Basin in which they further interrelated different shapes of sulfate profiles with sedimentary events. Their observations led to a classification in which they distinguished four types of transient profiles beside the linear steady-state one: kink, concave up, concave down and sigmoidal (s-type) profiles. More recently, Hong et al. (2014) investigated the occurrence of mass transport deposits in the Krishna-Godavari basin and their impact on pore-water profiles of species involved in the geochemical processes responsible of sulfate depletion. They showed that these sedimentary events brought the sulfate and ammonium profiles to a transient state characterized by an Sshape. In addition, they estimated the age of these events and the thickness of the transported sedimentary masses. Starting from steady-state conditions and using a mathematical expression of the sediment disturbances, they were able to model the evolution of dissolved-element profiles over time and estimate the elapsed time since the occurrence of the sedimentary events. Furthermore, they also predicted the fate of the dissolved species by considering an evolution to steady state.The goals of this study were to focus on sedimentary instability occurrences at the distal part of the most recent terminal lobe of the Congo deep-sea fan and their related effects on the transport of reactive species within the upper sedimentary column, with consequences on the sequential early diagenesis processes. Although previous studies have demonstrated the impacts of sedimentary instabilities on geochemical pore-water profiles, to our knowledge none of them has led to the type of profiles we have obtained from the analyses of methane, major and minor dissolved elements. The data set used here combines both geochemical and geological results from a long-piston core, and allowed us to identify sedimentary intervals that have experienced mass wasting. We further incorporated our data into a transportreaction model to simulate the evolution of the sulfate profile in response to sedimentary instabilities, and to discuss the timescale needed for the development of the current observed profile. Finally, our overall results provide new insights into the key role instabilities can play in the transport of chemical species within the sedimentary column by changing their migration paths.

(

  Rabouille et al., 2009; Rabouille et al., 2016; Savoye et al., 2009). This wide range of sedimentation rates reflects the strong variations observed from one morphosedimentary environment, such as channels or levees, to another.

  For the measurement of dissolved-methane and sulfide, ~2 mL of subsamples were preserved in pre-weighted nitrogen-purged vials using 1 mL of aqueous solutions of saturated HgCl 2 and ZnCl 2 1 M, respectively. The subsamples were stored at 4 °C and protected from the light to preserve the sulfide from degradation. The vials were weighted in the laboratory prior to analysis to determine the exact amount of pore-water sample. Total alkalinity (Alk) measurement was performed on board directly after subsampling by titration of 1 mL of sample with 0.1N HCl using a potentiometric titrator 848 Tritrino Plus from Metrohm ® . The uncertainty in the measurement was <5%. The titrant was routinely checked with the standards IAPSO seawater and Batches 113. The remaining samplevolume was stored in pre-evacuated vials from Labco ® and kept refrigerated at 4 °C for onshore analyses. Methane concentrations were determined by gas chromatography, with an estimated accuracy of 4%. A chromatograph Pr2100 from Perichrom ® equipped with a flameionization detector and coupled with a static-headspace injector (DANI ® HSS 86.50) was used[START_REF] Sarradin | Analysis of dissolved gases by headspace sampling gas chromatography with column and detector switching. Preliminary results[END_REF]. Total sulfide analysis (H 2 S) was performed by UVvisible absorption spectrophotometry at 670 nm using an instrument CARY 1C from Agilent ® and according to the methylene-blue method[START_REF] Cline | Spectrophotometric determination of hydrogen sulfide in natural waters[END_REF][START_REF] Fonselius | Determination of hydrogen sulphide[END_REF]. The detection limit was of 1 M. Concentrations of major dissolved-element (chloride, sulfate, calcium) were measured with an ion-exchange chromatograph Dionex ICS-5000 from Thermo Scientific ® , equipped with an electrical conductivity detector. IAPSO standard seawater was used as certified reference material for calibration. Minor dissolvedelements (barium and strontium) were analyzed by High Resolution Inductively Coupled

  Lithofacies 1 consists of brown to black laminated silty-clay with sequences of faint normal grading. It is characterized by low carbonate content (1-3%) and a high density (1.3 g.cm -3) ). The rather high Zr/total counts (0.06) combined with a low K/Ti (0.9) are indicative of grain sizes coarser than these present in the adjacent lithofacies (Croudace et al., 2006; Richter et al., 2006; Rothwell et al., 2006) (Fig. 6). This facies is interpreted as centimeter thick muddy turbidites. -Lithofacies 2 corresponds to a bioturbated greenish-grey siliceous clay with few intervals containing abundant foraminifera. It is characterized by a low density (~1.1 g.cm -3 ) and carbonate content varies between 0-25%. Owing to the deep water depth (~5000 m), the alternation of carbonate-bearing and carbonate-free intervals is interpreted as being the fluctuations of the carbonate lysocline and compensation depth during glacial and interglacial periods (Jansen et al., 1984). The low Zr/total counts (0.04) and high K/Ti ratios (1.7) are indicative of low content of coarse detrital grains.

  Unit 2 (38-295 cm) also corresponds to muddy turbidites (lithofacies 1) with important plastic deformations. Faults are clearly visible and delineate by color laminations. The contact between Unit 1 and 2 is erosional and slightly angular. -Unit 3 (295-527 cm) corresponds to pelagic sediment (lithofacies 2) with very few intervals of muddy turbidites (lithofacies 1). It also contains four layers of sandy patches made of sand (lithofacies 4). The contact between Unit 2 and 3 is sharp and oblique. -Unit 4 (527 to 572 cm) corresponds to a clayey debrite made of angular to rounded mud clasts in a soft muddy matrix (lithofacies 5). This unit was disturbed during the coring, leaving a partial 30 cm length void in the liner. The contact between Unit 3 cm) is characterized by the prevalence of pelagic sediment (ca. 80% of lithofacies 2) with few intervals of muddy turbidites (ca. 20% of lithofacies 1). It shows obvious plastic deformation with oblique to sub-vertical strata. cm) consists of an alternating strata of pelagic sediment (ca. 40% of lithofacies 2) and muddy turbidites (ca. 60% of lithofacies 1). All strata are horizontal and they do not show evidence of deformation. The contact between Unit 7 and 8 is conformable.

Fig. 7

 7 Fig.7shows pore-water concentration-depth profiles of chloride, sulfate, methane, alkalinity, calcium, barium and strontium, as well as the strontium isotope ratios. Arrows on the horizontal axis indicate the bottom seawater concentrations. Chloride concentrations are constant along the core, with values close to that of seawater. Measured H 2 S concentrations were below the detection limit, therefore the profile is not presented here. The lack of dissolved sulfide was also observed in pore waters collected at the same area by[START_REF] Beckler | Importance of microbial iron reduction in deep sediments of river-dominated continental-margins[END_REF], and they postulated that this lack of H 2 S is due to its titration by reactive manganese and iron. The sulfate profile shows an unusual shape with constant concentration values of 12.5 mM for the first 180 cm of sediment, followed by a linear decrease bringing values down to 1 mM at 400 cmbsf. Such low values are maintained between 400 and 950 cmbsf, and then increase linearly up to ~16 mM at 1320 cmbsf, to finally remain constant downcore. In Fig.7, calcium concentrations exhibit a minimum within the sulfate depleted-interval while methane and barium concentrations exhibit a maximum at the same interval. As the opposite to sulfate, the alkalinity profile exhibits an increase in concentration over the first meters with a distinct peak of 48.2 m at around 480 cmbsf, followed by a steady decrease up to 1262 cmbsf and then the concentrations remain nearly constant downcore. Interestingly there are prominent similarities in trend for the barium and alkalinity profiles on one hand, and the calcium and sulfate profiles on the other hand. The highest barium concentration value is observed at ~690 cmbsf, and the lowest value for calcium is at 660 cmbsf. Methane concentrations increase with depth in the upper part of the core with a peak of 414 µM found within the sulfate-depleted zone at 420 cmbsf, followed by a decrease to 5 M at 1100

  Based on the characteristics of both the identified lithofacies and the sequence of the units, we proposed two possible scenarios to explain the deformations observed on core WACS-06. They involve either two successive slides or a single long-lasting one.In the scenario of two successive slides (Fig.8), the initial stage at the sediment core location (Fig.8A) corresponds to the abyssal plain where pelagic sediment has built up with insertion of turbidites (Unit 8 to 6). The turbidites were probably supplied by an adjacent lobe complex to the south (visible in Fig.2). A Sliding of a mass corresponding to Unit 5 and possibly originating from the adjacent lobe complex would cause the deformation of Unit 7 and 6 (Fig.8B). This hypothesis is supported by the overconsolidated state of Unit 5, which is visually far more important than that of the units beneath. The over-consolidated state of Unit 5 indicates that it has been exhumed. Thus, Unit 6 has accommodated most of the load of the slid mass. The deposit of Unit 5 on the top of Unit 6 has generated shear stress, and this explains the important deformations observed. Such an argument is in coherence with the gradual decrease of shear stress towards the base of the core. The debrite (Unit 4) above Unit 5 then belongs to the same mass movement and correspond to a residual debris-flow following the slide and involving the same lithofacies 4 as the one observed in the displaced block. Such a juxtaposition of facies is common in mass movement-evidenced deposits[START_REF] Tripsanas | Submarine mass-transport facies: new perspectives on flow processes from cores on the eastern North American margin[END_REF][START_REF] Van Der Merwe | Origin and terminal architecture of a submarine slide: a case study from the Permian Vischkuil Formation, Karoo Basin, South Africa[END_REF]. Unit 3 (Fig.8C) which is composed of pelagic clay represents the interval of pelagic sedimentation after the slide. It is followed by the build-up of Unit 2 (Fig.8D), indicative of the onset of turbiditic sedimentation in relation with the progradation of the present lobe over the abyssal plain. The intercalation of the over-compacted Unit 5 and the debrite Unit 4 between the pelagic clay Unit 3 and 6 indicate that the former was not cogenetic with the latter, and thus they were deposited under different paleoceanographic and paleoclimatic conditions. The second slide (Fig.8E) would then affect the units 2 and 3, possibly with a short displacement as suggested by the low deformation of the strata.Unit 1 (Fig.8F) is not distorted and corresponds to the onset of a new turbiditic activity after this second slide. A fundamental requirement for this scenario is a time interval in the order of 100 kyrs between the two slides to account for the deposition of 2 m of pelagic clay (Unit 3) at an abyssal sedimentation rate of 2 cm.kyr -1 (Giresse and Barusseau, 1989; Jansen et al., 1984; Savoye et al., 2009). Alternatively, one may also considers a scenario with a single and more recent mass movement involving Unit 2 to 7, possibly characterized by several phases, sheared intervals and discontinuities. The chaotic seismic facies and the seabed roughness observed on Fig. 3C are indicative of the high disorganization of the sediment throughout the lobe, but our data are not sufficient to further constrain the sliding mechanism, and therefore decide which one between the two proposed scenarios has really occurred. 5.2.Source, migration and reactivity of methane 5.2.1. Methane source

(

  Etiope and Lollar, 2013), the methane measured in core WACS-06 derives from microbial degradation of organic matter. Fig. 7 represents the concentration depth profiles of sulfate and methane in combination with the lithological description of the core. The highest methane concentration values are found within Unit 3, suggesting that the methane source is located in the deformed marine pelagic clay. Unit 3 is characterized by the lowest TOC content (2% whereas TOC content can be as high as 4%), suggesting that a large fraction of the organic matter has been recycled. Besides, the significant decrease in both the TOC content and the HI observed from the core top to depth 400 cmbsf, very close to the methane concentration peak, is indicative of the degradation of labile OM fraction coupled with methane generation (Hatcher et al., 2014). On the other hand, the TOC content and HI profiles of Unit 5 exhibit the highest values and one may think that this unit is a better source rock than Unit 3 for methanogenesis. However, the primary constituents of biologically-derived sedimentary organic matter consist of proteins, carbohydrates, lipids and lignin. They all are characterized by contrasting reactivity with regard to microbial diagenesis (Parkes et al., 1993). Fundamentally, labile proteins and carbohydrates are rapidly degraded whereas more refractory lipids and lignin are selectively preserved. The protein-rich organic matter from marine algae (with a relatively high HI signature) is intensely degraded in the methanogenesis zone to produce methane as final product. Consequently, the HI value decreases rapidly (from 3). On the contrary, the lignin-rich constituents of land plants (with low initial HI values, around 200) are refractory and do not consist of a better substrate for methanogenic producers. Therefore, the woody fragments are less degraded during early diagenesis and undergo only a small decrease of their HI values. Thus, the TOC content is high in turbiditic layers as observed for Unit 5 because terrestrial organic matter is more refractory and better preserved than in hemipelagic layers. All these observations support for a methane source located in Unit 3.

  On the Peru continental margin, D'Hondt et al. (2004) and Parkes et al. (2005) studied a long drilled core characterized by two SMTZs located at 3500 and 9000 cmbsf. Recently, Torres et al. (2015)

  The decrease in calcium concentrations with a minimum beneath the upper SMTZ depth is indicative of current precipitation of carbonates at this interval. However, strontium concentrations are very scattered along the core, with no clear indication of strontium uptake by carbonate precipitation. It has been proved that strontium can also be released from barite dissolution (Noethen and Kasten, 2011). Fig.7shows that the two processes carbonate precipitation and barite dissolution take place at the same sedimentary interval, and their occurrence may explain the scattering of the measured strontium data. 87 Sr/ 86 Sr ratios do not evidence of recrystallization of old carbonates as their values are close to that of the modern seawater, and this is in coherence with the estimated age of the lobe complex.

  Sensitivity tests have been performed to evaluate the influence of the upward fluid velocity and methane concentration on the model-derived age (Fig.10). The model results fit well with the measured profile for upward fluid velocities ranging between 0.5 and 1 cm.yr -1 . The latter range is representative of values expected in the area where the core has been retrieved. The age of 96 ±10 years is obtained assuming methane saturation at the source. Because we have no real constraint on methane concentration, we ran the simulation for different methane concentrations in order to estimate the longest time needed to reach the observed profiles. Fig. 10b shows that it takes around 140 years to reach the present-day sulfate profile for a methane concentration of 50 mM; lower concentrations do not lead to a good fit. Accordingly, we can reasonably say that the establishment of the AOM has occurred around one century ago. Furthermore, we postulated that the establishment of the AOM results from the sliding of Unit 3. Thus, the estimated age also corresponds to the age of the youngest slide. Such a hypothesis is consistent with the much older age (ca. 1 kyr) suspected for the onset of lobe 5. The modelderived age corresponds to an upper estimate as the incorporation of OSR would faster the sulfate consumption, and then would shorten the time needed to fit the present-day profile.

  species. These correlations allowed identifying the occurrence of two slides: A first slide that has exhumed a sedimentary block characterized by well-preserved organic matter, and a second one involving the displacement of a methane-bearing sedimentary block. The second slide has allowed the development of two Sulfate Methane Transition Zones, possibly with the implication of a lateral flow of methane in the pelagic clay. Numerical simulation of the measured sulfate profile indicated that the development of the double SMTZs has started around a century ago, and this also corresponds to the age of the youngest slide. In summary, this study showed that turbidity currents and sedimentary instabilities in the Congo deep-sea fan are important drivers in the distribution of chemical species within the sedimentary column at the scale of several meters, and accordingly affect the depth zonation of the redox processes. Further multidisciplinary investigations are needed in order to better understand the triggering mechanism of sediment instabilities in this area.

Fig. 1 :

 1 Fig. 1: 3D view of the Congo deep-sea fan (After Savoye et al., 2009): From the canyon to the distal lobes. The study area is indicated by the red square.

Fig. 2 :

 2 Fig. 2: (A) Backscatter imagery of the distal area of the present Congo turbidite system (data acquired during the ZaïAngo project), showing the end of the active channel-levee and the distal lobe complex composed of five main lobe units. Yellow spot indicates location of core WACS-06. The thin black line indicates location of 3.5 kHz echosounder profile. The thick black line indicates location of cartoon cross section of Fig. 8. (B) 3.5 kHz echosounder profile located at the extremity of the lobe 5. Location and penetration of core WACS-06 is indicated by the vertical yellow line.

Fig. 3 :

 3 Fig. 3: Plan view of multibeam data on the Congo terminal lobe (see location in Fig. 2). The yellow dot indicates location of sediment core WACS-06. A) Shaded bathymetric map showing the feeding turbiditic channel. Water depths range between 4750 m (red) and 5000 m (blue). The white line outlines the limits of the lobe. The number "5" is the number of the lobe in the lobe complex (Dennielou et al., This issue). B) Acoustic backscatter of the terminal lobe. Dark tone represents high backscatter while light tone represents low backscatter. The terminal lobe is characterized by high backscatter that remarkably contrasts with the low backscatter of adjacent pelagic sediments. C) Zoom of the area where core WACS-06 was retrieved (See the rough morphology with slide head-scars and blocks).

Fig. 4 :

 4 Fig. 4: Evidence of sedimentary instabilities and block avalanche in the lobe complex area (cf. Ph D thesis of Nathalie Babonneau and Cédric Bonnel).

Fig. 5 :

 5 Fig. 5: Lithostratigraphic subdivisions of core WACS-06 with their relative deformation level.

Fig. 6 :

 6 Fig. 6: Lithofacies, physical (gamma density and magnetic susceptibility) and chemical (Ca/Fe, CaCO 3 , K/Ti, TOC and HI) properties of core WACS-06.

Fig. 7 :

 7 Fig. 7: Geochemical pore water profiles as a function of depth for core WACS-06 recovered at the distal lobe of the Congo deep sea fan (The gray intervals represent the two SMTZs).

Fig. 8 :Fig. 10 :

 810 Fig. 8: Cartoon describing the possible scenario for sliding on the active lobe complex at the core location. Location of these theoretical cross sections is shown on Fig. 2. (A) Schematic representation of Unit 8 to 6, with the fossil lobe on top. (B) Occurrence of slide 1 implying the displacement of sediment blocks from the fossil lobe. This is accompanied by a debris

Table 1 :

 1 Parameters used in the numerical model.

	Parameter	Symbol	Unit	Value
	Model domain	L			m	19.22
	Depth of the methane source	l			m	5.60
	Molecular diffusion coefficient, SO4	DSO4	m 2 sec. -1	5.51E-10
	Molecular diffusion coefficient, CH4	DCH4	m 2 sec. -1	8.62E-10
	SO4 concentration at the sediment surface	[SO4]	mM	12.10
	SO4 concentration at the bottom core	[SO4]	mM	14.70
	SO4 concentration at 5.7 m	[SO4]	mM	0
	CH4 concentration at both top and bottom core	[CH4]	mM	0
	CH4 concentration at 5.7 m	[CH4]	mM	207
	SO4 flux at lower boundary domain	[	]	)	mM m -1	0
	Upward fluid velocity				cm yr -1	1
	Sediment porosity				-	0.88
	AOM constant rate	kAOM	mM -1 yr -1	0.08

Acknowledgement

We would like to thank the captain, shipboard crew and scientific party of RV Pourquoi pas?

for their valuable supports during the WACS cruise (chief scientist: Dr. K. Olu). This study was conducted in the framework of the ANR-project Congolobe funded by French National Research Founding under the grant ANR-11-BS56-0030, and is part of the integrated Ifremer topic called "Biodiversité et Fonctionnement des Ecosystèmes Profonds, Impacts".