Temperature reconstruction of sediment cores from the northeastern Arabian Sea

In order to reconstruct the monsoonal variability during the late Holocene we investigated a complete, annually laminated sediment record from the oxygen minimum zone (OMZ) off Pakistan for oxygen isotopes of planktic foraminifera and alkenone-derived sea surface temperatures (SST). Significant SST changes of up to 3°C which cannot be explained by changes in the alkenone-producing coccolithophorid species (inferred from the Gephyrocapsa oceanica / Emiliania huxleyi ratio) suggest that SST changes are driven by changes in the monsoon strength. Our high-(decadal)-resolution data indicate that the late Holocene in the northeastern Arabian Sea was not characterized by a stable uniform climate, as inferred from the Greenland ice cores, but by variations in the dominance of the SW monsoon conditions with significant effects on temperatures. Highest SST fluctuations of up to 3.0°C and 2.5°C were observed for the time interval from 4600 to 3300 years B.P. and during the past 500 years. The significant, short-term SST changes during the past 500 years might be related to climatic instabilities known from the northern latitudes ("Little Ice Age") and confirm global effects. Surface salinity values, reconstructed from delta18O records after correction for temperature-related oxygen isotope fractionation, suggest that in general, the past 5000 years were characterized by higher-than-recent evaporation and more intense SW monsoon conditions. However, between 4600 and 3700 years B.P., evaporation dropped, SW monsoon weakened, and NE monsoon conditions were comparatively enhanced. For the past 1500 years we infer strongly fluctuating monsoon conditions. Comparisons of reconstructed salinity records with ice accumulation data from published Tibetan ice core and Tibetan tree ring width data reveal that during the past 2000 years, enhanced evaporation in the northeastern Arabian Sea correlates with periods of increased ice accumulation in Tibet, and vice versa. This suggests a strong climatic relationship between both monsoon-controlled areas.为重建全新世晚期的季风变化特征，我们针对巴基斯坦外海氧最小值带（OMZ）处一套完整的年层理沉积物记录开展研究，分析了浮游有孔虫的氧同位素组成，以及基于烯酮反演得到的海表温度（SST）。高达3℃的显著海表温度变化无法通过产烯酮颗石藻物种的种群变化（通过大洋桥石藻/赫氏颗石藻比值推断）解释，这表明海表温度变化由季风强度变化驱动。我们的高（年代际）分辨率数据显示，阿拉伯海东北部的全新世晚期气候并非如格陵兰冰芯所推断的那般稳定均一，而是存在西南季风主导状态的周期性波动，并对区域温度产生显著影响。在距今4600至3300年以及过去500年这两个时段，观测到了最高达3.0℃和2.5℃的海表温度波动。过去500年间发生的显著短期海表温度变化，可能与北半球已知的气候不稳定事件——“小冰期”——相关，印证了全球气候响应的一致性。通过校正温度相关的氧同位素分馏效应后，基于δ¹⁸O记录重建的表层盐度数据显示，总体而言，过去5000年的蒸发量高于现代，西南季风活动更为强盛。但在距今4600至3700年期间，区域蒸发量下降，西南季风减弱，东北季风状态相对增强。我们推断，过去1500年的季风系统存在强烈的状态波动。将重建的盐度记录与已发表的青藏高原冰芯冰积累数据以及青藏高原树木年轮宽度数据进行对比后发现，在过去2000年中，阿拉伯海东北部的蒸发增强时段与青藏高原冰积累增加的时段高度吻合，反之亦然。这表明两个受季风调控的区域之间存在紧密的气候关联。