Age determination, hemipelagic intervals and turbidite analysis from different sediment cores of the Nazare Canyon, offshore Portugal

Submarine canyons are one of the most important pathways for sediment transport into ocean basins. For this reason, understanding canyon architecture and sedimentary processes has importance for sediment budgets, carbon cycling, and geohazard assessment. Despite increasing knowledge of turbidity current triggers, the down-canyon variability in turbidity current frequency within most canyon systems is not well constrained. New AMS radiocarbon chronologies from canyon sediment cores illustrate significant variability in turbidity current frequency within Nazaré Canyon through time. Generalised linear models and Cox proportional hazards models indicate a strong influence of global sea level on the frequency of turbidity currents that fill the canyon. Radiocarbon ages from basin sediment cores indicate that larger, canyon-flushing turbidity currents reaching the Iberian Abyssal Plain have a significantly longer average recurrence interval than turbidity currents that fill the canyon. The recurrence intervals of these canyon-flushing turbidity currents also appear to be unaffected by long-term changes in global sea level. Furthermore, canyon-flushing and canyon-filling have very different statistical distributions of recurrence intervals. This indicates that the factors triggering, and thus controlling the frequency of canyon-flushing and canyon-filling events are very different. Canyon-filling appears to be predominantly triggered by sediment instability during sea level lowstand, and by storm and nepheloid transport during the present day highstand. Canyon-flushing exhibits time-independent behaviour. This indicates that a temporally random process, signal shredding, or summation of non-random processes that cannot be discerned from a random signal, are triggering canyon flushing events.

海底峡谷是沉积物向海盆输运的最重要通道之一。正因如此，明晰峡谷构造与沉积过程，对于沉积物收支、碳循环以及地质灾害评估均具有重要意义。尽管学界对浊流触发机制的认知不断加深，但多数峡谷系统内沿峡谷向下的浊流频率变化特征仍未得到明确约束。来自峡谷沉积岩芯的新型加速器质谱（AMS）放射性碳年代序列显示，纳扎雷峡谷内部的浊流频率随时间呈现显著变化。广义线性模型与考克斯比例风险模型分析结果表明，全球海平面对充填峡谷的浊流频率具有显著影响。海盆沉积岩芯的放射性碳测年结果显示，抵达伊比利亚深海平原的大型峡谷冲刷型浊流，其平均重现间隔显著长于仅充填峡谷的浊流。此外，这类峡谷冲刷型浊流的重现间隔似乎不受全球海平面长期变化的影响。进一步而言，峡谷冲刷型与峡谷充填型浊流的重现间隔统计分布存在显著差异，这表明触发并控制峡谷冲刷型与充填型浊流事件频率的因素存在显著差异。峡谷充填型浊流主要由海平面低位期的沉积物失稳触发，而在当前的海平面高位期，则主要由风暴作用与雾状层输运触发。峡谷冲刷型浊流则表现出时间无关性特征，这意味着触发峡谷冲刷事件的机制可能为时间随机过程、信号破碎效应，或是无法从随机信号中辨识的非随机过程的叠加效应。