(Fig. 9) Pollen analysis of sediment core GIK16017-2

The improved understanding of the pollen signal in the marine sediments offshore of northwest Africa is applied to deep-sea core M 16017-2 at 21°N. Downcore fluctuations in the percentage, concentration and influx diagrams record latitudinal shifts of the main northwest African vegetation zones and characteristics of the trade winds and the African Easterly Jet. Time control is provided by 14C ages and 180 records. During the period 19,000-14,000 yr B.P. a compressed savanna belt extended between about 12 ° and 14-15°N. The Sahara had maximally expanded northward and southward under hyperarid climatic conditions. The belt with trade winds and dominant African Easterly Jet transport had not shifted latitudinally. The trade winds were strong as compared to the modern situation but around 13,000 yr B.P. the trade winds weakened. After 14,000 yr B.P. the climate became less arid south of the Sahara and a first spike of fluvial runoff is registered around 13,000 yr B.P. Fluvial runoff increased strongly around 11,000 yr B.P. and maximum runoff is recorded from about 9000-7800 yr B.P. Around 12,500 yr B.P. the savanna belt started to shift northward and became richer in woody species: it shifted about 6° of latitude, reached its northernmost position during the period of 9200-7800 yr B.P. and extended between about 16° and 24°N at that time. Tropical forest had reached its maximum expansion and the Guinea zone reached as far north as about 15°N, reflecting very humid climatic conditions south of the Sahara. North of the Sahara the climate also became more humid and Mediterranean vegetation developed rapidly. The Sahara had maximally contracted and the trade winds were weak and comparable with the present day intensity. After about 7800 yr B.P. the southern fringe of the Sahara and accordingly the savanna belt, shifted rapidly southward again.对西北非洲近海海洋沉积物中花粉信号（pollen signal）的深入认知，被应用于北纬21°处的深海岩芯（deep-sea core）M 16017-2。岩芯垂向的百分比、浓度与通量（influx）波动记录了西北非洲主要植被带的纬度迁移，以及信风（trade winds）与非洲东风急流（African Easterly Jet）的活动特征。本研究的年代约束由¹⁴C测年数据与180记录提供。 在距今19000至14000年期间，一条压缩的稀树草原带分布于约12°N至14°-15°N之间。在极端干旱的气候条件下，撒哈拉沙漠达到南北向扩张的最大范围。以信风与非洲东风急流主导输送为特征的风带未发生纬度迁移。相较于现代，彼时信风强度较强，但在距今约13000年时，信风开始减弱。 距今14000年之后，撒哈拉以南区域的气候干旱程度降低，距今约13000年时出现首次河流径流峰值。距今约11000年时，河流径流显著增强，并在距今9000至7800年期间达到峰值。距今约12500年时，稀树草原带开始向北迁移，木本物种占比提升：该带向北迁移约6个纬度，在距今9200至7800年期间达到最北位置，彼时分布范围为约16°N至24°N。热带森林达到最大扩张范围，几内亚植被带（Guinea zone）最北可达约15°N，这反映了撒哈拉以南区域极为湿润的气候条件。撒哈拉以北区域的气候也变得更为湿润，地中海型植被（Mediterranean vegetation）快速发育。此时撒哈拉沙漠达到最大收缩范围，信风强度减弱，与现代强度相当。 距今约7800年之后，撒哈拉沙漠的南缘以及相应的稀树草原带再次快速向南迁移。