Long chain n-alkanes deposited from borehole PRGL1P (ROMESS1) in the upper slope of the Gulf of Lions

During the PROMESS campaign (summer 2004) a borehole (PRGL1) was drilled in the upper slope of the Gulf of Lions. Previous studies showed that the deposition of the sedimentary units at borehole PRGL1 was determined by changes in the discharge of the Rhone river, induced by sea level and climate variability with a strong 100 ka imprint. However, there are no detailed studies that focus on the glacial/interglacial transitions (i.e. Terminations) that define 100 ka Quaternary ice-age cycles. Here we present a high-resolution record of terrestrial long chain n-alkanes (C21 to C33) from MIS 3 to MIS 11 in order to monitor changes in the sediments deposited on the upper slope of the Gulf of Lions with the aim to provide new insight into the climatic events occurring during Terminations in the western Mediterranean Sea.
Principal component analysis grouped n-alkane on the basis of the degree of maturation of organic matter (fresh/degraded) that allowed to monitor past variability in the source of the sediment deposited in the upper slope as a consequence of sea level and climate change. This information was used to estimate the impact that northern ice-sheet variability had on the western Mediterranean basin. As sea level started to fall, fresh terrestrial organic matter increased owing to the seaward migration of Rhone's mouth. Simultaneously, the decrease in sea level exposed old and reworked organic material from the Rhone's catchment, including the delta plain and the continental shelf, that was eroded and transported into our study site. This resulted in an increase in the proportion of degraded terrestrial organic matter together with sea level decrease. Aside from sea level changes, the proportion of fresh vs degraded organic matter seems to be affected by the amount of continental runoff, with an increasing proportion of fresh terrestrial organic matter occurring at times of reduced continental runoff. During the last glacial, the increase of fresh terrestrial organic matter was contemporaneous with Heinrich events (HEs), suggesting dry and cold conditions on the western Mediterranean basin. These dry and cold millennial-scale episodes were reproduced during Terminations II, III and IV preceding flood events, most probably due to weakening/reinforcement of Atlantic Meridional Overturning Circulation (AMOC) that caused dry/humid conditions in western Mediterranean.

2004年夏季开展的PROMESS科考作业（PROMESS campaign）期间，研究人员在利翁湾（Gulf of Lions）陆坡上段钻取了编号为PRGL1的钻孔。既往研究表明，PRGL1钻孔处的沉积单元堆积过程受控于罗纳河（Rhone River）径流量变化，而径流量变化由海平面波动与气候变率驱动，且该信号具有显著的10万年（100 ka）周期特征。然而目前尚无针对界定第四纪（Quaternary）10万年冰期旋回的冰期-间冰期转换（Terminations，即终结期）开展的详细研究。本研究针对海洋同位素阶段（Marine Isotope Stage，MIS）3至MIS11的样品，分析了陆生长链正构烷烃（terrestrial long chain n-alkanes，C21至C33）的高分辨率记录，以期追踪利翁湾陆坡上段的沉积变化，为解析西地中海地区终结期（Terminations）期间的气候事件提供新认知。

本研究通过主成分分析（Principal Component Analysis，PCA），依据有机质的成熟度（新鲜/降解）对正构烷烃进行分组，借此追踪因海平面变化与气候变化导致的陆坡上段沉积物源区的古演化过程。基于该结果，研究人员评估了北半球冰盖变率对西地中海盆地的影响。

当海平面开始下降时，罗纳河河口向海迁移，导致新鲜陆生有机质的占比升高。与此同时，海平面下降暴露了罗纳河流域（包括三角洲平原与大陆架）中老旧且经再改造的有机质，这些物质被侵蚀并搬运至本研究的采样区域。这一过程使得随着海平面下降，降解态陆生有机质的占比同步升高。

除海平面变化外，新鲜与降解态有机质的占比还受大陆径流量的影响：当大陆径流量减少时，新鲜陆生有机质的占比会升高。

在末次冰期，新鲜陆生有机质占比升高与海因里希事件（Heinrich Events，HEs）同步发生，表明西地中海盆地当时处于干冷气候状态。

在终结期II、III、IV期间的洪水事件之前，也重现了这类千年尺度的干冷气候事件，其成因大概率与大西洋经向翻转环流（Atlantic Meridional Overturning Circulation，AMOC）的减弱/增强有关——该环流变化会引发西地中海地区的干/湿气候转换。