How are subaqueous sediment density flows triggered, what is their internal structure and how does it evolve? Direct observations from monitoring of active flows
How are subaqueous sediment density flows triggered, what is their internal structure and how does it evolve? Direct observations from monitoring of active flows
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DOI：
10.1016/j.earscirev.2013.07.005
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发表时间：
2013-10
影响因子：
12.1
通讯作者：
P. Talling;C. Paull;D. Piper
中科院分区：
文献类型：
--
作者：
P. Talling;C. Paull;D. Piper
水下沉积物密度流是在地球上移动沉积物的体积最重要的过程之一，并形成地球上最大的沉积物堆积(海底扇)。它们也可以说是地球上监测最少的主要泥沙输送过程。在记录它们的时间和触发因素方面取得了重大进展，特别是在海底峡谷和三角洲前缘以及淡水湖泊和水库内，但从来没有直接测量过流出大陆坡面的水流的含沙量。这一有限的监测数据与河流系统等其他主要类型的沉积物流动形成了鲜明的对比，并确保了解海底沉积物密度流动仍然是地球科学的一项重大挑战。现有的监测数据定义了一系列水流类型，它们的性质和沉积有很大的不同。大陆斜坡上的大断层可以产生快速移动的水流(高达19米/S)，到达深海，并在海底扇上沉积厚厚的沙层。即使是小体积(0.008立方米)的峡谷头部崩塌，有时也会产生沟道化的水流，以5米/S的速度行进数百公里。台湾东南部的一个单一事件表明，河流洪水可以产生强大的水流，到达深海，在这种情况下，触发的是最近在峡谷头沉积的沉积物的失败。尽管这一过程产生了更短和更弱的洋流，但缺乏直接监测证据，证明洪水骤降产生了强大的洋流。每年在河流引流的三角洲前缘可能会发生大量的水流，在那里它们可以产生上坡迁移的新月状河床。这些水流往往发生在汛期，但不一定与个别洪峰有关，这表明它们往往是由三角洲前缘斜坡失稳引发的。由大陆架上的沙子注入的峡谷中，每年都会发生几次强烈的水流，与强烈的波浪作用有关。这些水流还可以产生上坡迁移的新月状河床，这些河床最有可能是由于与致密的近床层沉积物相关的退行性决口而形成的。超临界和完全湍流的扩张稀释流也是由峡谷中的波浪作用触发的。对河流洪水骤降产生的湖泊和水库中的泥沙密度流进行了更详细的监测。它们通常非常稀薄(沉积物体积百分比为0.01%)，以50厘米/S的速度移动，容易在密度分层淡水中产生交汇。未来工作的一个关键目标是开发透过覆盖的稀释云的沉积物的测量技术，以确定是否存在致密的近床层。还需要将流量监测与流量沉积的详细分析结合起来，以便了解流量是如何记录在岩石记录中的。最后，需要一种从源到汇的方法，因为海底流动的特征可能会沿着其流动路径发生重大变化。
Subaqueous sediment density flows are one of the volumetrically most important processes for moving sediment across our planet, and form the largest sediment accumulations on Earth (submarine fans). They are also arguably the most sparely monitored major sediment transport processes on our planet. Significant advances have been made in documenting their timing and triggers, especially within submarine canyons and delta-fronts, and freshwater lakes and reservoirs, but the sediment concentration of flows that run out beyond the continental slope has never been measured directly. This limited amount of monitoring data contrasts sharply with other major types of sediment flow, such as river systems, and ensure that understanding submarine sediment density flows remains a major challenge for Earth science. The available monitoring data define a series of flow types whose character and deposits differ significantly. Large (> 100 km3) failures on the continental slope can generate fast-moving (up to 19 m/s) flows that reach the deep ocean, and deposit thick layers of sand across submarine fans. Even small volume (0.008 km3) canyon head failures can sometimes generate channelised flows that travel at > 5 m/s for several hundred kilometres. A single event off SE Taiwan shows that river floods can generate powerful flows that reach the deep ocean, in this case triggered by failure of recently deposited sediment in the canyon head. Direct monitoring evidence of powerful oceanic flows produced by plunging hyperpycnal flood water is lacking, although this process has produced shorter and weaker oceanic flows. Numerous flows can occur each year on river-fed delta fronts, where they can generate up-slope migrating crescentic bedforms. These flows tend to occur during the flood season, but are not necessarily associated with individual flood discharge peaks, suggesting that they are often triggered by delta-front slope failures. Powerful flows occur several times each year in canyons fed by sand from the shelf, associated with strong wave action. These flows can also generate up-slope migrating crescentic bedforms that most likely originate due to retrogressive breaching associated with a dense near-bed layer of sediment. Expanded dilute flows that are supercritical and fully turbulent are also triggered by wave action in canyons. Sediment density flows in lakes and reservoirs generated by plunging river flood water have been monitored in much greater detail. They are typically very dilute (< 0.01 vol.% sediment) and travel at < 50 cm/s, and are prone to generating interflows within the density stratified freshwater. A key objective for future work is to develop measurement techniques for seeing through overlying dilute clouds of sediment, to determine whether dense near-bed layers are present. There is also a need to combine monitoring of flows with detailed analyses of flow deposits, in order to understand how flows are recorded in the rock record. Finally, a source-to-sink approach is needed because the character of submarine flows can change significantly along their flow path.