Benthic foraminifera of surface sediments from the continental slope off Sierra Leone, North-West Africa

On "Meteor" cruise 30 (1973) 22 piston-cores were collected off Sierra Leone from water-depths between about 5000 m (Sierra Leone Basin) and 500 m (upper continental slope) with the objective to study the sediment composition and age as well as processes of sedimentation on the continental slope in a tropical humid region. Granulometric analysis and determinations of the carbonate contents of the sediment samples were carried out, as well as qualitative and quantitative analysis of the components of the grain size fractions > 63 µm and of the planktonic and benthonic foraminifera > 160 µm.
Presently, the cold Canary Current influences the composition of the planktonic foraminifera within the northwestern area of investigation (profile A), whereas the planktonic fauna of the eastern area (profile C) seems to be truly tropical. In all Quaternary sediments from the continental slope off Sierra Leone, species of Globorotalia are less abundant than in truly pelagic sediments. For that reason, the zonation of the Pleistocene sediments based on the presence or absence of Globorotalia cultrata does not always agree with the climatic changes reflected in the sediments. Concerning past climates better results can be obtained by using the changes in percentage abundances of Globigerina sp. sp. and Globigerinoides sp. sp. as indicators for cool and warm temperatures. The Tertiary sediments contain a pelagic foraminiferal assemblage.
In the Holocene sediments the benthonic foraminifera do not only serve as good paleodepth indicators, but their communities are also restricted to defined water masses, which change their positions in accordance with climatic changes. Thus, Cassidulina carinata in the area of investigation is an excellent indicator for sediments deposited during times, which were cooler than today; this is true for all cores from the continental slope off Sierra Leone independent of water-depth although this species presently abounds at water-depths around 600 m.
The cores from the continental rise and from the Sierra Leone Basin (M30-261, M30-146, M30-147) were deposited below the calcium carbonate compensation depth. Only small sections of the cores consist of the original carbonate-free sediments, whereas the main part of the sediment column is redeposited material, rich in foraminifera, which normally live on the upper continental slope, or even on the shelf. From these cores only M30-261 can be subdivided into biostratigraphic zones ranging from zone V to zone Y.
In all cores from the middle and upper continental slope of the eastern area of investigation (profile C; KL 230, 209-204) and in cores KL 183 and KL 184 from the northwestern area (profile A) we observed an undisturbed succession of sediments from the biostratigraphic zones X (partly), Y and Z. All cores from the central area (M30-181, M30-182, M30-262 to 264) and M30-187 from the upper slope of profile A show variable hiatuses in the sedimentary record. Locally, high velocity bottom currents were probably responsible for erosion, nondeposition or minimal sedimentation rates. These currents might have been initiated partly by the somewhat exposed position of this part of the continental slope, where the shelf edge bends from a northwest towards an eastern direction, and partly by very young tectonic movements. Fracture zones with vertically displaced fault blocs are frequent at Sierra Leone continental margin. According to seismic measurements by McMaster et al. (1975) the sites of the central area are located on an uplifted fault bloc explaining the reduced sediment rates or erosion. Unlike the central area, the eastern area (profile C) is situated on a downfaulted bloc with high sediment rates.
The sediments from the cores of profile B as well as the turbiditic deep-sea sediments were deposited under a higher flow regime; therefore they are coarser than the extremely fine-grained sediments of the cores from profile C. Since the sand fraction (> 63 µm) is mainly composed of foraminifera, besides pteropods and light-colored fecal pellets, the carbonate content increases with the increasing percentage of the coarse grain fraction. Higher concentrations of quartz were only observed in core sections with considerable carbonate dissolution (mainly in the X-Zone), and, in general, in all sediments from the eastern area with higher terrigenous input including larger concentration of mica. Especially during times transitional between glacials and interglacials (or interstadials) the bottom currents were intensified. The percentages of coarse fraction and carbonate increase with increasing current velocities. Calcium carbonate dissolution becomes important in water depths > 3500 m. During cooler times the lysokline is depressed.
Light-colored fecal pellets were redeposited from Late Neogene sediments (M30-187, M30-181). In the area of investigation they occur in the Holocene and mainly the Pleistocene sediments of the cores from the northwestern and central area because only here Tertiary sediments have been eroded at the uppermost continental slope.
In the central area there are at least two periods of non-sedimentation and/or erosion which can be confined as being (1) not older than middle Pliocene and not younger than zone V and (2) younger than zone W. The local character of the erosion is documented by the fact that a complete Late Quaternary section is present in the cores of the northwestern and eastern area, each within less than 100 km from incomplete cores from the central area.

1973年“流星”号（Meteor）科考航次30航次在塞拉利昂外海采集了22根活塞岩芯（piston-cores），采样水深范围约为5000米（塞拉利昂海盆）至500米（上陆坡），旨在研究热带湿润区陆坡的沉积物组成、年代及沉积作用过程。本次研究对沉积物样品开展了粒度分析（granulometric analysis）与碳酸盐含量测定，并对粒径大于63微米的组分以及粒径大于160微米的浮游有孔虫（planktonic foraminifera）、底栖有孔虫（benthonic foraminifera）进行了定性与定量分析。
当前，加那利寒流（cold Canary Current）对研究区西北部（测线A）的浮游有孔虫组合构成影响，而研究区东部（测线C）的浮游动物群则呈现典型的热带特征。相较于真正的远洋沉积物，塞拉利昂外海陆坡的所有第四纪沉积物（Quaternary sediments）中，圆辐虫属（Globorotalia）物种的丰度更低。因此，基于圆辐虫（Globorotalia cultrata）的有无划分更新世（Pleistocene）沉积物生物地层带，并不总能与沉积物所记录的气候变化相契合。若以抱球虫属（Globigerina）未定种与拟抱球虫属（Globigerinoides）未定种的百分含量变化作为冷暖温度的指示指标，则可获得更优的古气候重建结果。第三纪沉积物（Tertiary sediments）中含有远洋有孔虫组合（pelagic foraminiferal assemblage）。
在全新世（Holocene）沉积物中，底栖有孔虫不仅可作为良好的古水深指示因子（paleodepth indicators），其群落还局限于特定的水团（water masses）中，而水团的位置会随气候变化发生改变。因此，研究区中的盔形虫（Cassidulina carinata）是冷于现代的沉积时期的绝佳指示物种，这一结论适用于塞拉利昂外海所有陆坡岩芯，不受水深影响——尽管当前该物种在约600米水深处最为繁盛。
采自陆隆（continental rise）与塞拉利昂海盆的岩芯（M30-261、M30-146、M30-147）沉积于碳酸钙补偿深度（calcium carbonate compensation depth, CCD）之下。这些岩芯仅小部分为原始的无碳酸盐沉积物，而沉积物柱的主体为再沉积物质（redeposited material），其中富含通常栖息于上陆坡甚至陆架的有孔虫。上述岩芯中仅M30-261可被划分为从V带至Y带的生物地层带（biostratigraphic zones）。
在研究区东部（测线C）的中、上陆坡所有岩芯（KL 230、209-204）以及研究区西北部（测线A）的KL 183、KL 184岩芯中，我们观察到了生物地层带X（部分）、Y及Z的未扰动沉积序列。研究区中部的所有岩芯（M30-181、M30-182、M30-262至264）以及测线A上陆坡的M30-187岩芯，则存在不同程度的沉积间断。局部区域的高速底流可能是侵蚀、沉积缺失或极低沉积速率的成因。此类底流的形成部分源于该段陆坡的暴露位置——此处陆架边缘从西北向东方弯曲，部分则源于新近的构造运动（tectonic movements）。塞拉利昂大陆边缘分布着大量带有垂直位移断块（fault blocs）的断裂带（fracture zones）。根据麦克马斯特等人（McMaster et al., 1975）的地震测量结果，研究区中部的站位位于抬升断块（uplifted fault bloc）之上，这解释了其沉积速率降低或遭受侵蚀的现象。与研究区中部不同，东部区域（测线C）位于断陷断块（downfaulted bloc）之上，沉积速率较高。
测线B的岩芯沉积物与浊积深海沉积物（turbiditic deep-sea sediments）沉积于更高的水动力条件下，因此较测线C岩芯的极细粒沉积物更为粗粒。由于砂级组分（sand fraction，粒径>63μm）主要由有孔虫组成，此外还包含翼足类（pteropods）与浅色粪球粒（light-colored fecal pellets），因此碳酸盐含量随粗粒组分占比的升高而增加。较高浓度的石英仅见于发生显著碳酸盐溶解作用的岩芯段（主要为X带），总体而言也见于陆源输入较高、云母浓度较大的东部区域所有沉积物中。尤其在冰期与间冰期（或间冰阶，interstadials）的过渡时期，底流会得到增强。粗粒组分与碳酸盐的占比随流速升高而增加。当水深超过3500米时，碳酸盐溶解作用变得显著。冷期时溶跃面（lysokline）会下移。
浅色粪球粒是从晚新近纪（Late Neogene）沉积物中再沉积而来的（M30-187、M30-181）。在研究区中，此类粪球粒见于西北部与中部岩芯的全新世（Holocene）及主要为更新世的沉积物中，因为仅在此处的上陆坡顶部发生了第三纪沉积物的侵蚀。
研究区中部至少存在两期沉积缺失或侵蚀事件：其一，年代不早于上新世中期且不晚于V带；其二，年代晚于W带。侵蚀的局域性可由以下事实证实：在距中部不完整岩芯不足100公里的西北部与东部区域的岩芯中，均保存有完整的晚第四纪（Late Quaternary）沉积剖面。