Pteropod preservation on the Pakistan shelf and continental slope

Fifteen surface sediment samples from the Pakistan shelf and upper continental slope and a Late Quaternary high-sedimentation rate core (573 m water depth, Pakistan continental margin) have been analysed to improve the understanding of the factors influencing pteropod preservation. The aragonite compensation depth (ACD) is located at 250-400 m water depth, which corroborates previous observations of a very shallow ACD in the northern Arabian Sea. With the exception of the Hab transect off Karachi, the ACD coincides with the upper boundary of the OMZ located at 250 m water depth. The shell preservation index of the pteropod Limacina inflata (LDX) was applied on six surface sediment samples showing good to very good preservation (LDX: 2.2 to 1.3).
The 30 000 yr long record of sediment core SO90 137KA is characterized by alternations between bioturbated and laminated sediments. Bioturbated sediments occurring in the Early Holocene, Younger Dryas and time-equivalents of Heinrich events contain well to perfectly preserved tests of L. inflata (LDX: 2.1-0.2), whereas only traces of pteropods are found in laminated intervals.
The close linkage of pteropod preservation in the surface sediments and in core 137KA to well-oxygenated conditions can be explained by repetitive intermediate water formation in the Arabian Sea down to at least 600 m water depth in times of enhanced NE monsoons during stadials and H-equivalents. Low amounts of pteropods in laminated sediments (interstadials, Late Holocene) and in the present-day oxygen minimum zone (OMZ) indicate a weak NE monsoon, stable OMZ and shallow ACD.

本研究对取自巴基斯坦陆架与上大陆坡的15个表层沉积物样品，以及位于巴基斯坦大陆边缘、水深573米的晚第四纪高沉积速率岩芯开展分析，旨在深化翼足类（pteropod）保存影响因素的认知。文石补偿深度（aragonite compensation depth, ACD）的分布区间为250~400米水深，该结果佐证了此前阿拉伯海北部文石补偿深度极浅的观测结论。除卡拉奇外海的Hab断面外，文石补偿深度与250米水深处的氧最小带（oxygen minimum zone, OMZ）上界完全重合。本研究针对6个保存状况良好至极佳的表层沉积物样品，采用翼足类物种*Limacina inflata*的壳体保存指数（shell preservation index, LDX）进行评估，其LDX值范围为2.2至1.3。沉积岩芯SO90 137KA的3万年沉积记录以生物扰动层与纹层状沉积物交替发育为典型特征。早全新世、新仙女木期以及与海因里希事件（Heinrich events）同期的沉积层段中，生物扰动发育的沉积物内保存有完好至近乎完美的*Limacina inflata*壳体（LDX值介于0.2~2.1之间）；而纹层状沉积段中仅能发现极少量翼足类痕迹。表层沉积物与岩芯SO90 137KA中翼足类保存状况与富氧环境的紧密关联，可通过冰期及海因里希事件同期阶段东北季风增强时，阿拉伯海至少600米水深以浅区域反复形成中层水来解释。纹层状沉积段（对应间冰阶、晚全新世）以及现代氧最小带中翼足类丰度极低，这指示当时东北季风偏弱、氧最小带处于稳定状态且文石补偿深度较浅。