The NW Madagascar continental margin receives high loads of terrigenous particulate organic matter during the wet season and especially linked to extreme events, originating from two major rivers, the Betsiboka and the Mahavavy Rivers. This particulate matter contains a high content of iron minerals from the weathering of red ferruginous/ferralitic soils of Madagascar. The presence of pockmarks, i.e. gas or fluid expulsion features on the continental slope, testifies to past/present methane migration through the sedimentary column, associated with early diagenetic processes. This study globally aims at deciphering the interactions between episodic sedimentation and geochemical processes influenced by fluids upward migration, using a sediment trap mooring and interface sediment cores at two sites on the continental slope. The present-day sedimentation along this margin undergoes two patterns. During the wet season, high continental fluxes generally result in an increase in particle fluxes on the slope. Longshore currents may deflect river plumes alongslope resulting in some periods of low particle flux on the slope during the wet season. During the dry season, the particles collected in the water column are probably originated from sediment remobilization from the shelf and slope. The observed progradation of the Mahavavy River delta and prodelta between 1984 and 2016 argues for an increase in temporary connections of the river with the canyon head during extreme events, inducing pulsed sedimentation offshore. This pulsed sedimentation could be responsible for enhancing pyritization on surface sediments, due to higher inputs of terrigenous organic matter and iron oxides. Finally, methane upward migration also influences the pyritization process through anaerobic oxidation of methane using sulfate as an electron acceptor and methanotrophs associated with this reaction significantly impact the δ¹³C of organic carbon towards more negative values.

马达加斯加西北部大陆边缘在雨季收到大量陆源颗粒有机物，特别是与极端事件有关，源于两条主要河流，贝齐博卡河和马哈维河。这种颗粒物质含有来自马达加斯加的红色铁质/铁铝土风化的高含量铁矿物质。斑点的存在，即大陆斜坡上的气体或流体排泄特征，证明了过去/现在的甲烷通过沉积柱的迁移，这与早期成岩作用有关。这项全球研究的目的是利用大陆坡上两个位置的沉积物捕集器系泊系统和界面沉积物芯，来解释受流体向上迁移影响的间歇性沉积和地球化学过程之间的相互作用。目前沿该边缘的沉积有两种模式。在雨季，高大陆通量通常会导致斜坡上的粒子通量增加。在湿季，长岸流可能会使河道羽流偏向斜坡，从而导致斜坡上某些时期的颗粒通量较低。在干旱季节，收集在水柱中的颗粒可能源自架子和斜坡上的沉积物迁移。在1984年至2016年期间观察到的马哈瓦维河三角洲和三角洲的逐渐发展，表明在极端事件期间河流与峡谷顶的临时连接增加，从而导致近海脉冲沉积。这种脉冲式沉降可能会增强表面沉积物的黄铁矿化，由于陆源有机质和氧化铁的投入较高。最后，甲烷的向上迁移还通过使用硫酸盐作为电子受体的甲烷的厌氧氧化来影响甲烷的热解过程，与此反应相关的甲烷营养生物显着影响δ¹³ C向更负值的有机碳。