Composition of the sand fraction of sediment cores from the West-African continental margin

Seven cores from the West African continental margin in 12-18° N have been investigated by means of a coarse fraction analysis.
Four of the seven cores contain allochthonous material: turbidites and debris flow deposits. The source of the allochthonous material is in about 300-600 m water depth. The age of the slide induced debris flow deposits is at the end of oxygen isotope stage 2. One debris flow deposit is covered by a turbidite (core GIK13211-1).
The turbidites in the deep-sea core GIK13207-3 originate from river-influenced sediments from the West-African continental margin, whereas the autochthonous sequences are influenced by volcanic material from the Cape Verde Islands.
Particle by particle supply from upper slope areas has been found in all four cores from the continental slope.
Current sorting occurs on the submarine diapir (core GIK13289-3), whereas core GIK13291-1 on the NW-flanc, 200 m below core GIK13289-3, has no current sorting, except for stage 1 and parts of stage 5. The current sorting is reflected by parallel variations of median diameters of whole tests and of fragments of planktonic foraminifers, by higher median diameters of foraminifers on top of the diapir, by reduced accumulation rates and increased sand fraction percentages in core GIK13289-3 compared to core GIK13291-1.
The Late Quarternary climatic history of the West-African near coastal area (12-18° N) has been redrawn:
- in oxygen isotope stage 1 a humid climate is found in 12-18° N (This "humid impression" in 18° N, which is actually an arid area, is due to the poleward directed undercurrent, which transports Senegal river material to the north).
- in oxygen isotope stage 2 an arid climate existed in 14-18° N, whereas in 12° N river discharfe persisted. But within stage 2 dune formation occured in 12° N on the (dry) shelf, additionally to fluviatile sediment input.
- Older periods are preserved in autochthonous sediments of core GIK13289-3 and GIK13291-1, where oxygen stage 3,5 and 7 (the latter only in core GIK13289-3 present) show a humid climate (as well as in stage 5 of core GIK13255-3), interrupted by short arid intervals in core GIK12389-3, and stage 4 and 6 show an arid climate, interrupted by short humid periods The allochthonous stage 5 sediment in core GIK13211-1 also reflects a humid climate.
The dissolution of planktonic foraminifers is strongest in th eLate Holocene and shows a minimum in the early Holocene, where also pteropods are preserved. The degree of carbonate dissolution is related mainly to the fine matter content (< 63 µm) whereas water depth is a less decisvive factor.

针对西非大陆边缘12°N至18°N海域的7个沉积物岩芯，采用粗组分分析（coarse fraction analysis）方法开展了研究。7个岩芯中有4个含有异地物质（allochthonous material）：浊积岩（turbidites）与碎屑流沉积（debris flow deposits）。该异地物质的源区水深约为300~600米。由滑坡诱发的碎屑流沉积形成于氧同位素阶段（oxygen isotope stage）2末期。其中1个碎屑流沉积被一层浊积岩覆盖（岩芯GIK13211-1）。
深海岩芯GIK13207-3中的浊积岩源自西非大陆边缘受河流影响的沉积物，而其原地沉积序列则受佛得角群岛（Cape Verde Islands）火山物质的影响。
在大陆坡的全部4个岩芯中，均发现了来自上坡区域的逐粒级供给沉积物。
海底底辟（submarine diapir）附近的岩芯GIK13289-3存在底流分选作用；而位于其西北侧翼、水深比GIK13289-3低200米的岩芯GIK13291-1则未出现底流分选现象，仅在第1阶段及第5阶段的部分层位存在例外。底流分选作用体现为：与岩芯GIK13291-1相比，岩芯GIK13289-3中浮游有孔虫（planktonic foraminifers）全壳及其碎片的中值粒径呈现同步变化、底辟顶部有孔虫的中值粒径更高、沉积速率更低且砂级组分占比更高。
研究重新厘定了西非近岸海域（12°N~18°N）的晚第四纪（Late Quaternary）气候历史：
- 氧同位素第1阶段，12°N~18°N海域呈现湿润气候（18°N海域本为干旱区，却呈现"湿润特征"，这是因为北向沿岸逆流将塞内加尔河沉积物搬运至北部所致）。
- 氧同位素第2阶段，14°N~18°N海域气候干旱，而12°N海域仍有河流径流持续输入。但在第2阶段内，12°N海域的干陆架上还发育了沙丘沉积，同时伴随河流沉积物输入。
- 较老的气候记录保存在岩芯GIK13289-3与GIK13291-1的原地沉积物中：其中氧同位素第3、5、7阶段（第7阶段仅在GIK13289-3中被检出）均对应湿润气候（岩芯GIK13255-3的第5阶段同样为湿润气候），其间被短期干旱时段打断；而氧同位素第4、6阶段则对应干旱气候，其间被短期湿润时段打断。岩芯GIK13211-1中的异地第5阶段沉积物同样反映了湿润气候。
浮游有孔虫的溶解作用在全新世晚期（Late Holocene）最为强烈，在全新世早期（early Holocene）达到最弱，该时期翼足类（pteropods）也得以保存。碳酸盐溶解（carbonate dissolution）程度主要与细粒物质（<63μm）含量相关，而水深的影响相对较弱。