Compilation of organic carbon distribution and sedimentology in the surface sediments on the continental margin offshore southwestern Africa

In this study we demonstrate the relevance of lateral particle transport in nepheloid layers for organic carbon (OC) accumulation and burial across high-productive continental margins. We present geochemical data from surface sediments and suspended particles in the bottom nepheloid layer (BNL) from the most productive coastal upwelling area of the modern ocean, the Benguela upwelling system offshore southwest Africa. Interpretation of depositional patterns and comparison of downslope trends in OC content, organic matter composition, and 14C age between suspended particles and surface sediments indicate that lateral particle transport is the primary mechanism controlling supply and burial of OC. We propose that effective seaward particle transport primarily along the BNL is a key process that promotes and maintains local high sedimentation rates, ultimately causing high preservation of OC in a depocenter on the upper slope offshore Namibia. As lateral transport efficiently displaces areas of enhanced OC burial from maximum production at highly productive continental margins, vertical particle flux models do not sufficiently explain the relationship between primary production and shallow-marine OC burial. On geologic time scales, the widest distribution and strongest intensity of lateral particle transport is expected during periods of rapid sea-level change. At times in the geologic past, widespread downslope lateral transport of OC thus may have been a primary driver of enhanced OC burial at deeper continental slopes and abyssal basins.本研究阐明了雾状层内的颗粒物横向输运，对高生产力大陆边缘的有机碳（OC）累积与埋藏过程的重要意义。我们采集了现代海洋最高产的近岸上升流区——西南非洲外海本格拉上升流系统——的表层沉积物与底部雾状层（BNL）悬浮颗粒物样品，并获取了对应的地球化学数据。通过对沉积模式的解析，以及对比悬浮颗粒物与表层沉积物的有机碳含量、有机质组成和碳十四年龄的下坡变化趋势，本研究证实颗粒物横向输运是调控有机碳供给与埋藏的核心机制。我们提出，主要沿底部雾状层进行的高效向海颗粒物输运，是促进并维持区域高沉积速率的关键过程，最终使得纳米比亚外海上陆坡沉积中心的有机碳得以高度保存。由于横向输运会将有机碳埋藏富集区从大陆边缘的初级生产峰值区高效转移，因此垂直颗粒物通量模型无法充分解释初级生产与浅海有机碳埋藏之间的关联。在地质时间尺度下，海平面快速变化时期的颗粒物横向输运分布范围最广、强度最高。因此在地质历史时期，大范围的下坡有机碳横向输运，可能是驱动较深大陆坡与深海盆地有机碳埋藏量提升的核心因素。