Particle fluxes and in-situ particle properties measured with sediment traps and a moored camera system from 2008 - 2010 in region off Cape Blanc / NW-Africa

We compared particle data from a moored video camera system with sediment trap derived fluxes at ~1100 m depth in the highly dynamic coastal upwelling system off Cape Blanc, Mauritania. Between spring 2008 and winter 2010 the trap collected settling particles in 9-day intervals, while the camera recorded in-situ particle abundance and size-distribution every third day. Particle fluxes were highly variable (40-1200 mg m**-2 d**-1) and followed distinct seasonal patterns with peaks during spring, summer and fall. The particle flux patterns from the sediment traps correlated to the total particle volume captured by the video camera, which ranged from1 to 22 mm**3 l**-1. The measured increase in total particle volume during periods of high mass flux appeared to be better related to increases in the particle concentrations, rather than to increased average particle size. We observed events that had similar particle fluxes, but showed clear differences in particle abundance and size-distribution, and vice versa. Such observations can only be explained by shifts in the composition of the settling material, with changes both in particle density and chemical composition. For example, the input of wind-blown dust from the Sahara during September 2009 led to the formation of high numbers of comparably small particles in the water column. This suggests that, besides seasonal changes, the composition of marine particles in one region underlies episodical changes. The time between the appearance of high dust concentrations in the atmosphere and the increase lithogenic flux in the 1100 m deep trap suggested an average settling rate of 200 m d**-1, indicating a close and fast coupling between dust input and sedimentation of the material.

本研究将锚定摄像系统（moored video camera system）获取的颗粒数据，与毛里塔尼亚布兰克角附近高度动态的沿岸上升流系统（coastal upwelling system）中约1100米水深处由沉积物捕集器（sediment trap）得到的颗粒通量进行对比分析。2008年春季至2010年冬季期间，沉积物捕集器以9天为间隔收集沉降颗粒，而摄像系统则每3天记录一次原位（in-situ）颗粒丰度与粒径分布（size-distribution）。颗粒通量波动幅度极大（40~1200 mg·m⁻²·d⁻¹），且呈现显著的季节变化模式，峰值出现在春、夏、秋三季。沉积物捕集器得到的颗粒通量模式，与摄像系统捕获的总颗粒体积呈显著相关，该体积范围为1~22 mm³·L⁻¹。在高质量通量时段，观测到的总颗粒体积增幅，与颗粒浓度提升的相关性似乎优于与平均粒径增大的相关性。我们观测到部分事件的颗粒通量相近，但颗粒丰度与粒径分布存在显著差异，反之亦然。此类观测结果仅能通过沉降物质的组成变化加以解释，该变化同时涉及颗粒密度与化学成分的改变。例如，2009年9月来自撒哈拉的风成粉尘输入，导致水柱中形成了大量粒径相对较小的颗粒。这表明，除季节变化外，某一海域的海洋颗粒组成还存在间歇性变化。大气中高粉尘浓度出现的时间，与1100米水深捕集器内岩源通量（lithogenic flux）升高的时间间隔，推算出平均沉降速率为200 m·d⁻¹，这表明粉尘输入与物质沉积之间存在紧密且快速的耦合关系。