Subtropical Northeastern Atlantic intermediate water variability duirng 490-427 ka (late MIS 13 to MIS 12): benthic foraminiferal evidence

High-resolution foraminiferal stable isotopes and benthic foraminiferal faunal records of IODP Site U1391 were adopted to reconstruct the subtropical Northeastern Atlantic intermediate water variability from the late Marine Isotope Stage (MIS) 13 to MIS 12. Five faunal turnovers were recognized based on multivariate statistical analyses of benthic foraminiferal census data from the size fraction >250 μm. The dominance of Uvigerina peregrina parva coincides with high benthic foraminiferal accumulation rates (BFAR), high benthic δ13C and the presence of dark-colour sediments during the final stage of MIS 13, also accompanied by frequent occurrences of Planulina ariminensis, an indicator of Mediterranean Outflow Water (MOW), which indicates MOW-related high oxic and mesotrophic to slightly eutrophic bottom water environments. MIS 12c and MIS 12b are characterized by the miliolids - Bulimina mexicana assemblage, together with low BFAR, high benthic δ13C and the presence of light-colour sediments, revealing oligotrophic and well-oxygenated seafloor conditions associated with the possible advection of nutrient-poor Glacial North Atlantic Intermediate Water (GNAIW) to the studied site. A prominent increase in organic matter supply and a slight decrease in dissolved oxygen concentration during MIS 12a were reflected by the dominance of Bulimina aculeata and miliolids, high BFAR, low benthic δ13C and the dark-colour sediments. A shift in foraminiferal composition from the dominance of miliolids and Bulimina mexicana to the dominance of Bulimina aculeata and miliolids, combined with significantly decreased benthic δ13C, suggests an increased influence of southern-sourced waters (SSW) and a decreased influence of GNAIW. At the beginning of Termination V (TV), infaunal taxa mainly composed of Uvigerina proboscidea, B. aculeata and Globobulimina pacifica dominate the benthic foraminiferal population, which may be attributed to eutrophic and poorly-oxygenated bottom water environment strongly influenced by SSW. Afterwards, miliolids rapidly became the predominant taxa. The predominance of miliolids was probably the result of selective carbonate dissolution caused by the corrosiveness of pore water related to decomposition of organic matter.

本研究采用国际大洋发现计划（International Ocean Discovery Program, IODP）U1391站位的高分辨率有孔虫稳定同位素与底栖有孔虫群落记录，重建了海洋同位素阶段（Marine Isotope Stage, MIS）13晚期至MIS 12期间北大西洋亚热带中层水的演化历史。基于粒径>250μm级分的底栖有孔虫计数数据的多元统计分析，本研究识别出5次群落更替事件。在MIS 13末期，远游优游虫小型亚种（Uvigerina peregrina parva）占优势，伴随底栖有孔虫堆积速率（benthic foraminiferal accumulation rates, BFAR）升高、底栖δ¹³C值偏高，且沉积物颜色偏深；该阶段同时频繁出现指示地中海流出水（Mediterranean Outflow Water, MOW）的阿里米扁平卷虫（Planulina ariminensis），表明当时底层水体处于与MOW相关的高氧化、中营养至轻度富营养环境。MIS 12c与MIS 12b以粟米虫类-墨西哥卷转虫（Bulimina mexicana）组合为特征，同时伴随低BFAR、高底栖δ¹³C值与浅色沉积物，揭示当时海底环境为贫营养且氧化条件良好，推测贫营养的冰川北大西洋中层水（Glacial North Atlantic Intermediate Water, GNAIW）向研究站位发生了平流输送。在MIS 12a阶段，有机质供给量显著升高、溶解氧浓度小幅下降，这一变化反映在以刺卷转虫（Bulimina aculeata）与粟米虫类占优势、高BFAR、低底栖δ¹³C值以及深色沉积物的特征上。有孔虫群落从粟米虫类与墨西哥卷转虫占优，转变为刺卷转虫与粟米虫类占优，且底栖δ¹³C值显著降低，表明南部源水（southern-sourced waters, SSW）的影响增强，而冰川北大西洋中层水的影响减弱。在终止期V（Termination V, TV）初期，以内栖类群（infaunal taxa）为主的群落以长吻优游虫（Uvigerina proboscidea）、刺卷转虫与太平洋球拟单格虫（Globobulimina pacifica）占优势，这可能归因于受南部源水强烈影响的富营养、低氧化底层水环境。此后，粟米虫类迅速成为优势类群。粟米虫类占优的现象可能是由有机质分解相关的孔隙水腐蚀性引发的选择性碳酸盐溶解所导致的。