Mass fluxes and organic carbon fluxes at four stations off Cape Blanc, NW Africa (Mauritania)

Vertical carbon fluxes between the surface and 2500 m depth were estimated from in situ profiles of particle size distributions and abundances me/asured off Cape Blanc (Mauritania) related to deep ocean sediment traps. Vertical mass fluxes off Cape Blanc were significantly higher than recent global estimates in the open ocean. The aggregates off Cape Blanc contained high amounts of ballast material due to the presence of coccoliths and fine-grained dust from the Sahara desert, leading to a dominance of small and fast-settling aggregates. The largest changes in vertical fluxes were observed in the surface waters (<250 m), and, thus, showing this site to be the most important zone for aggregate formation and degradation. The degradation length scale (L), i.e. the fractional degradation of aggregates per meter settled, was estimated from vertical fluxes derived from the particle size distribution through the water column. This was compared with fractional remineralization rate of aggregates per meter settled derived from direct ship-board measurements of sinking velocity and small-scale O2 fluxes to aggregates measured by micro-sensors. Microbial respiration by attached bacteria alone could not explain the degradation of organic matter in the upper ocean. Instead, flux feeding from zooplankton organisms was indicated as the dominant degradation process of aggregated carbon in the surface ocean. Below the surface ocean, microbes became more important for the degradation as zooplankton was rare at these depths.本数据集基于毛里塔尼亚布兰科角外海与深海沉积物捕集器相关的原位颗粒尺寸分布及丰度剖面，估算了海面至2500米水深的垂直碳通量。布兰科角外海的垂直物质通量显著高于当前公开海域的全球平均估算值。布兰科角外海的聚合体含有大量压载物质——其来源为颗石藻（coccoliths）与撒哈拉沙漠的细粒粉尘，因此该区域以小型且沉降速度快的聚合体为主。垂直通量的最大变化出现在表层海水（<250米）中，表明该区域是聚合体形成与降解的关键海域。降解长度尺度（degradation length scale, L）指每沉降1米时聚合体的降解比例，本数据集通过水柱中基于颗粒尺寸分布推导得到的垂直通量估算该参数。将该参数与通过船载直接测量沉降速度，以及利用微传感器测得的聚合体周边小型氧气通量所得出的每沉降1米聚合体再矿化比例进行了对比。仅依靠附着细菌的微生物呼吸作用，无法解释上层海洋中的有机质降解过程。研究结果表明，浮游动物的通量摄食才是表层海洋聚合态碳降解的主导过程。在表层海洋以下的水深环境中，由于浮游动物较为稀少，微生物在有机质降解过程中的作用愈发显著。