Turbidity currents triggered at river mouths form an important highway for sediment, organic carbon, and nutrients to the deep sea. Consequently, it has been proposed that the deposits of these flood-triggered turbidity currents provide important long-term records of past river floods, continental erosion, and climate. Various depositional models have been suggested to identify river-flood-triggered turbidite deposits, which are largely based on the assumption that a characteristic velocity structure of the flood-triggered turbidity current is preserved as a recognizable vertical grain size trend in their deposits. Four criteria have been proposed for the velocity structure of flood-triggered turbidity currents: prolonged flow duration; a gradual increase in velocity; cyclicity of velocity magnitude; and a low peak velocity. However, very few direct observations of flood-triggered turbidity currents exist to test these proposed velocity structures. Here we present direct measurements from the Var Canyon, offshore Nice in the Mediterranean Sea. An acoustic Doppler current profiler was located 6 km offshore from the river mouth, and provided detailed velocity measurements that can be directly linked to the state of the river. Another mooring, positioned 16 km offshore, showed how this velocity structure evolved down-canyon. Three turbidity currents were measured at these moorings, two of which are associated with river floods. The third event was not linked to a river flood and was most likely triggered by a seabed slope failure. The multi-pulsed and prolonged velocity structure of all three (flood- and landslide-triggered) events is similar at the first mooring, suggesting that it may not be diagnostic of flood triggering. Indeed, the event that was most likely triggered by a slope failure matched the four flood-triggered criteria best, as it had prolonged duration, cyclicity, low velocity, and a gradual onset. Hence, previously assumed velocity-structure criteria used to identify flood-triggered turbidity currents may be produced by other triggers. Next, this study shows how the proximal multi-pulsed velocity structure reorganizes down-canyon to produce a single velocity pulse. Such rapid-onset, single-pulse velocity structure has previously been linked to landslide-triggered events. Flows recorded in this study show amalgamation of multiple velocity pulses leading to shredding of the flood signal, so that the original initiation mechanism is no longer discernible at just 16 km from the river mouth. Recognizing flood-triggered turbidity currents and their deposits may thus be challenging, as similar velocity structures can be formed by different triggers, and this proximal velocity structure can rapidly be lost due to self-organization of the turbidity current.

中文翻译：

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洪水引发的浊流有多独特？

在河口引发的浊流形成了一条重要的通道，将沉积物、有机碳和营养物质输送到深海。因此，有人提出，这些洪水引发的浊流沉积物提供了过去河流洪水、大陆侵蚀和气候的重要长期记录。已经提出了各种沉积模型来识别河流洪水引发的浊流沉积物，这些模型主要基于这样的假设，即洪水引发的浊流的特征速度结构被保留为沉积物中可识别的垂直粒度趋势。洪水触发浊流的速度结构提出了四个标准：延长流动持续时间；逐渐增加速度；速度大小的周期性；和低峰值速度。然而，很少有对洪水触发的浊流的直接观测来测试这些提议的速度结构。在这里，我们展示了来自地中海尼斯近海瓦尔峡谷的直接测量数据。声学多普勒电流剖面仪位于离河口 6 公里处，并提供了可以直接与河流状况相关的详细速度测量值。另一个停泊在离岸 16 公里处，展示了这种速度结构是如何沿着峡谷向下演化的。在这些系泊处测量了三个浊流，其中两个与河流洪水有关。第三个事件与河流洪水无关，很可能是由海底斜坡破坏引发的。所有三个（洪水和滑坡触发）事件的多脉冲和延长速度结构在第一次系泊时是相似的，表明它可能不是洪水触发的诊断。事实上，最有可能由斜坡破坏触发的事件最符合四个洪水触发标准，因为它具有持续时间长、周期性、低速和渐进性。因此，以前假设的用于识别洪水触发的浊流的速度结构标准可能由其他触发因素产生。接下来，本研究展示了近端多脉冲速度结构如何重组峡谷下方以产生单个速度脉冲。这种快速发生的单脉冲速度结构以前与滑坡触发事件有关。本研究中记录的流量显示多个速度脉冲的合并导致洪水信号被切碎，因此在距离河口仅 16 公里处不再可辨别原始引发机制。