Siliceous phytoplankton in Late Quaternary sediments of ODP Site 175-1077@en

Late Quaternary fluctuations in the intensity of Congo River freshwater load were reconstructed using three different proxies (marine and freshwater diatoms, and the delta18O record of Globigerinoides ruber) preserved in the sediments of Ocean Drilling Program (ODP) Site 1077, located at the northern rim of the Congo River fan (5°10'S, 10°26'E). An abrupt change in the diatom assemblage is evident at Termination II: a two- to four-fold increase in (a) the relative abundance of a marine planktonic diatom tolerant of low salinity conditions (Cyclotella litoralis), and (b) in the concentration of freshwater diatoms. The microfossil data suggest a change in the environmental conditions surrounding Site 1077 from predominantly marine to mixed marine/brackish/fresh. The delta18O record of the planktic foraminifera G. ruber (pink) revealed negative deviations from the global oxygen isotope signal since Termination II which occurred during warm stage 1 and substages 3.2, 5.1, 5.3, and 5.5. Comparison of the isotopic signal of ODP Site 1077 with the record from a pelagic location (core GeoB1041 at 3°48'S, 7°05'W) confirms these results. The construction of an artificial delta18O curve using alkenone-derived sea surface temperature (SST) data from a nearby core (GeoB1008 at 6°S, 10°E) allowed us to estimate salinity and temperature effects on the ODP Site 1077 isotopic signal. Although increased SSTs may account for lighter delta18O values during warmer periods, they do not explain the extremely light values documented in the sediments of Site 1077. We used the oxygen isotope difference (Delta delta18O) between our site and GeoB1041 as a proxy for freshwater input. A general trend in the Delta delta18O was observed, with more negative values since Termination II. In addition, conspicuous Delta delta18O negative pulses coincided with periods of northern hemisphere summer insolation maxima over the African continent, suggesting an increase in the freshwater discharge from the Congo River due to enhanced precipitation on the hinterland. Here we propose that the abrupt change in environmental conditions at Site 1077 since Termination II is a consequence of a major reorganization in the depositional environment of the Congo River delta. This reorganization involved sustained equatorward displacement of the Angola-Benguela Front causing a northward deflection of the Congo River plume thus moving plume waters further north than normal and over Site 1077.本研究依托位于刚果河扇北缘（南纬5°10′、东经10°26′）的大洋钻探计划（Ocean Drilling Program, ODP）1077孔沉积物，采用三种不同代用指标——海洋与淡水硅藻、红拟抱球虫（Globigerinoides ruber）的氧同位素δ¹⁸O记录，重建了晚第四纪刚果河淡水负载强度的波动历史。在冰期终止II（Termination II）处，硅藻组合出现显著突变：其一，耐受低盐度环境的海洋浮游硅藻近岸圆筛藻（Cyclotella litoralis）的相对丰度提升2至4倍；其二，淡水硅藻的浓度显著升高。微体化石数据表明，1077孔周边的环境条件已从以海洋环境为主转变为混合的海洋/咸淡水/淡水环境。对浮游有孔虫红拟抱球虫（粉色变种）的δ¹⁸O记录分析显示，自冰期终止II以来，该记录相对于全球氧同位素信号出现负偏，这一时期对应暖期1阶以及3.2、5.1、5.3、5.5亚阶。将ODP 1077孔的同位素信号与远洋站位（南纬3°48′、西经7°05′的GeoB1041岩芯）的记录对比，验证了上述结果。利用邻近站位（南纬6°、东经10°的GeoB1008岩芯）的烯酮（alkenone）衍生海表温度（sea surface temperature, SST）数据构建人工δ¹⁸O曲线，我们得以估算盐度与温度对ODP 1077孔同位素信号的影响。尽管暖期升高的SST可以解释较轻的δ¹⁸O值，但无法解释1077孔沉积物中记录的极端轻同位素值。我们以本研究站位与GeoB1041之间的氧同位素差值（Δδ¹⁸O）作为淡水输入的代用指标。Δδ¹⁸O整体呈现出负偏趋势增强的特征，自冰期终止II以来，其负值愈发显著。此外，显著的Δδ¹⁸O负脉冲事件与非洲大陆北半球夏季日照峰值时期高度吻合，表明刚果河内陆流域降水增强导致其入海淡水径流量增加。本研究提出，自冰期终止II以来，1077孔所处环境的突变是刚果河三角洲沉积环境发生重大重组的结果。这一重组表现为安哥拉-本格拉锋面向赤道方向持续位移，导致刚果河羽状流向北偏转，使得羽状流水体较常规情况更靠北，最终覆盖ODP 1077孔所在区域。