XRF elemental contents, oxygen and carbon isotopes and biomarkers of sediment cores POS362-2_33, POS362-2_73 and POS362-2_99@en

Ocean deoxygenation is a rising threat to marine ecosystems and food resources under present climate warming conditions. Organic-rich sapropel layers deposited in the Mediterranean Sea provide a natural laboratory to study the processes that have controlled the changes in seawater oxygen levels in the recent geological past. Our study is based on three sediment cores spanning the last 10 thousand years (10 kyr BP) and located on a bathymetric transect offshore the western distributaries of the Nile delta. These cores are partly to continuously laminated in the sections recording sapropel S1, which is indicative of bottom-water anoxia above the western Nile deep-sea fan. We used a combination of microfacies analyses and inorganic and organic geochemical measurements to reconstruct changes in oxygenation conditions at seasonal to millennial time-scales. The regular alternations of detrital, biogenic and chemogenic sublayers in the laminated sequences are interpreted in terms of seasonal changes. Our microfacies analyses reveal distinct summer floods and subsequent plankton blooms preceding the deposition of inorganic carbonates formed in the water-column during spring-early summer. The isotopic signature of these carbonates suggests year-round anoxic to euxinic bottom waters resulting in high levels of anaerobic remineralisation of organic matter and highlights their potential to reconstruct seawater chemistry at times when benthic fauna was absent. Synchronous changes in terrigenous input, primary productivity and past oxygenation dynamics on millennial time-scales obtained by our multi-proxy study show that runoff-driven eutrophication played a central role in driving rapid changes in oxygenation state of the entire Levantine Basin. Rapid fluctuations of oxygenation conditions in the upper 700 m water depth occurred above the Nile deep-sea fan between 10 and 6.5 ka BP while deeper cores recorded more stable anoxic conditions. These findings are further supported by other regional records and reveal time-transgressive changes in oxygenation state driven by rapid changes in primary productivity during a period of long-term deep-water stagnation.在当前气候变暖背景下，海洋脱氧（Ocean deoxygenation）正对海洋生态系统与渔业资源构成日益严峻的威胁。沉积于地中海的富有机质腐泥层（sapropel）为研究近代地质历史时期海水含氧量变化的控制过程提供了天然实验场所。本研究依托3根沉积物岩芯展开，这些岩芯覆盖了距今1万年（10 kyr BP）以来的沉积记录，布设位置位于尼罗河三角洲西部分流河道近海的水深断面上。在记录腐泥层S1的沉积段中，这些岩芯呈现出部分至连续纹层状结构，指示西尼罗河深海扇上方曾存在底层水缺氧环境。我们结合微相分析、无机与有机地球化学测试，重建了季节至千年尺度下的海水氧含量环境变化。纹层序列中碎屑亚层、生物成因亚层与化学成因亚层的规律性交替，被解释为季节周期变化的反映。微相分析结果显示，在春末至初夏水体中形成的无机碳酸盐沉积之前，存在显著的夏季洪水事件与后续的浮游生物勃发过程。这些碳酸盐的同位素特征表明，当时底层水体全年处于缺氧至硫化缺氧（euxinic）状态，引发了高强度的有机质厌氧矿化作用，同时也凸显了在底栖生物缺失时期，这类碳酸盐具备重建海水化学特征的潜力。我们通过多代用指标研究得到的千年尺度陆源输入、初级生产力与古氧合动态的同步变化表明，径流驱动的富营养化在驱动整个黎凡特盆地氧含量环境的快速变化中发挥了核心作用。在距今10~6.5 ka BP期间，尼罗河深海扇上方水深700米以浅的水体氧含量环境发生了快速波动，而较深水处的岩芯则记录了更为稳定的缺氧环境。上述发现得到了其他区域记录的进一步佐证，并揭示了在长期深水停滞期内，初级生产力的快速变化驱动了氧含量环境的穿时变化。