Benthic foraminifera assemblages of the Gulf of Aden

Benthic and selected planktic foraminifera and stable isotope records were determined in a piston core from the Gulf of Aden, NW Arabian Sea that spans the last 530 ka. The benthic foraminifera were grouped into four principal assemblages using Q-mode Principal Component Analyses. Comparison of each of these assemblages with the fauna of the nearby regions enabled us to identify their specific environmental requirements as a function of variability in food supply and strength of the oxygen minimum zone and by that to use them as indicators of surface water productivity. The benthic foraminiferal productivity indicators coupled with the record of Globigerina bulloides, a planktic foraminifer known to be sensitive to productivity changes in the region, all indicate higher productivity during glacial intervals and productivity similar to present or even reduced during interglacial stages. This trend is opposite to the productivity pattern related to the SW summer monsoon of the Arabian Sea and indicates the role of the NE winter monsoon on the productivity of the Gulf of Aden. A period of exceptionally enhanced productivity is recognized in the Gulf of Aden region between ~60 and 13 kyr indicating the intensification of the NE winter monsoon to its maximal activity. Contemporaneous indication of increased productivity in other parts of the Arabian Sea, unexplained so far by the SW summer monsoon variability, might be related to the intensification of the NE winter monsoon. Another prominent event of high productivity, second in its extent to the last glacial productivity event is recognized between 430 and 460 kyr. These two events seem to correspond to periods of similar orbital positioning of rather low precession (and eccentricity) amplitude for a relatively long period. Glacial boundary conditions seem to control to a large extent the NE winter monsoon variability as also indicated by the dominance of the 100 ka cycle in the investigated time series. Secondary in their importance are the 23 and 41 ka cycles which seem also to contribute to the NE monsoonal variability. Following the identification of productivity events related to the NE winter monsoon in the Gulf of Aden, it is possible now to extend this observation to other parts of the Arabian Sea and consider the contribution of this monsoonal system to the productivity fluctuations preserved in the sedimentary records.

本研究对采自阿拉伯海西北部亚丁湾、时间跨度覆盖过去53万年的活塞岩芯（piston core），开展了底栖有孔虫（benthic foraminifera）、筛选浮游有孔虫（planktic foraminifera）及稳定同位素记录的测定分析。研究采用Q型主成分分析（Q-mode Principal Component Analyses），将底栖有孔虫划分为4个主要组合；通过将上述有孔虫组合与邻近区域的动物群进行比对，我们明确了各类组合的特定环境需求——其与食物供给变化、缺氧层（oxygen minimum zone）强度密切相关，并据此将其作为表层水生产力的代用指标。底栖有孔虫生产力代用指标，结合对该区域生产力变化敏感的浮游有孔虫Globigerina bulloides的记录，均显示冰期时段生产力更高，而间冰期阶段生产力与现代相当甚至有所降低。该趋势与阿拉伯海西南夏季风相关的生产力模式截然相反，表明东北冬季风对亚丁湾生产力具有调控作用。在约60至13 kyr时段内，亚丁湾区域出现了异常显著的生产力升高时段，指示东北冬季风强度达到峰值；阿拉伯海其他区域同期出现的生产力升高信号，目前尚无法用西南夏季风变化解释，其可能与东北冬季风的增强有关。在约430至460 kyr时段内，还识别出另一显著的高生产力事件，其规模仅次于末次冰期的高生产力事件。这两次事件似乎对应于一段相对较长的时期，彼时轨道参数处于类似位置，岁差（precession）与偏心率（eccentricity）的振幅均较低。冰期边界条件似乎在很大程度上调控了东北冬季风的变化，这一点也通过研究时间序列中占主导的100 ka周期得到印证；次要的周期信号为23 ka与41 ka周期，它们同样对东北季风变化具有一定贡献。在明确亚丁湾区域与东北冬季风相关的生产力事件后，我们如今可将该观测结果推广至阿拉伯海其他区域，并探讨该季风系统对沉积记录中保存的生产力波动的调控作用。