Deep ocean mass fluxes in the coastal upwelling off Mauritania from 1988 to 2012: variability on seasonal to decadal timescales
Deep ocean mass fluxes in the coastal upwelling off Mauritania from 1988 to 2012: variability on seasonal to decadal timescales
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DOI：
10.5194/bg-13-3071-2016
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发表时间：
2016-01-01
期刊：
影响因子：
4.9
通讯作者：
Wefer, Gerold
中科院分区：
文献类型：
--
作者：
Fischer, Gerhard;Romero, Oscar;Wefer, Gerold
二十多年来的沉积物陷阱记录从东部边界上升流生态系统（EBUE）的白朗角，毛里塔尼亚，分析了深海质量通量，通量分量和它们的季节到十年的时间尺度上的变化。总质量通量揭示了年际波动叠加的年代际时间尺度上的波动。冬季高通量的生物硅（BSi），作为海洋生产的措施（主要是硅藻）在很大程度上对应于一个积极的北大西洋涛动（NAO）指数（12月至3月）。然而，这种关系是脆弱的。2004-2005年冬季北大西洋涛动处于中性状态时，BSi正距平最高。在2001年和2005年之间的几个夏季，当前一个冬季NAO是中性的，甚至是负的，发生了更多的情节BSi沉积事件。我们认为，不同的灰尘爆发和沉积在海洋表面在冬季和偶尔在夏季/秋季增强颗粒沉降和碳输出在短时间尺度上通过压载效应。沙尘爆发（例如2005年）对海洋碳循环的阶段性扰动可能削弱了通量与大规模气候振荡之间的关系。由于浮游植物的生物量是高的全年，任何干（冬季）或湿（夏季）沉积的细粒尘埃颗粒被认为是提高效率的生物泵，将灰尘到密集和快速沉降的有机丰富的聚集。在撒哈拉/萨赫勒地区经历了一段相当干燥的时期后，在2005年的多尘年份中观察到了BSi和灰尘通量之间的良好对应关系。在1997-1999年最强的厄尔尼诺-南方涛动（ENSO）期间，所有的总通量都发生了很大的变化，在暖厄尔尼诺期间获得了近1a的低通量，而在随后的冷拉尼娜阶段获得了高通量。对于十年的时间尺度，Bakun（1990）认为，由于风的增加，沿海上升流会加强（“Bakun上升流加强假说”; Cropper等人，2014年，全球气候变化。在过去的25年里，我们没有观察到任何通量分量的增加，这可能支持这一点。此外，矿物粉尘的通量并没有随着时间的推移显示出任何积极或消极的趋势，这可能表明在过去几十年中荒漠化或“撒哈拉绿化”的加剧。
A more than two-decadal sediment trap record from the Eastern Boundary Upwelling Ecosystem (EBUE) off Cape Blanc, Mauritania, is analysed with respect to deep ocean mass fluxes, flux components and their variability on seasonal to decadal timescales. The total mass flux revealed interannual fluctuations which were superimposed by fluctuations on decadal timescales. High winter fluxes of biogenic silica (BSi), used as a measure of marine production (mostly by diatoms) largely correspond to a positive North Atlantic Oscillation (NAO) index (December-March). However, this relationship is weak. The highest positive BSi anomaly was in winter 2004-2005 when the NAO was in a neutral state. More episodic BSi sedimentation events occurred in several summer seasons between 2001 and 2005, when the previous winter NAO was neutral or even negative. We suggest that distinct dust outbreaks and deposition in the surface ocean in winter and occasionally in summer/autumn enhanced particle sedimentation and carbon export on short timescales via the ballasting effect. Episodic perturbations of the marine carbon cycle by dust outbreaks (e.g. in 2005) might have weakened the relationships between fluxes and large-scale climatic oscillations. As phytoplankton biomass is high throughout the year, any dry (in winter) or wet (in summer) deposition of fine-grained dust particles is assumed to enhance the efficiency of the biological pump by incorporating dust into dense and fast settling organic-rich aggregates. A good correspondence between BSi and dust fluxes was observed for the dusty year 2005, following a period of rather dry conditions in the Sahara/Sahel region. Large changes of all bulk fluxes occurred during the strongest El Nino-Southern Oscillation (ENSO) in 1997-1999 where low fluxes were obtained for almost 1aEuro-year during the warm El Nino and high fluxes in the following cold La Nina phase. For decadal timescales, Bakun (1990) suggested an intensification of coastal upwelling due to increased winds (''Bakun upwelling intensification hypothesis''; Cropper et al., 2014) and global climate change. We did not observe an increase of any flux component off Cape Blanc during the past 2 and a half decades which might support this. Furthermore, fluxes of mineral dust did not show any positive or negative trends over time which might suggest enhanced desertification or ''Saharan greening'' during the last few decades.