Flow dynamics and sedimentation in an active submarine channel: a process-product approach

海底航道是一种壮观的特征，可以跨越海底绵延数千公里，往往有几公里宽，最深可达数百米。它们是由密度流形成的；水下的沙、泥和水的流动比海水密度更大，因此沿着海底流动。这些水道非常重要，因为它们是将沉积物输送到深海的主要通道，形成了地球上最大的沉积矿床。这些矿床是天然气和石油储量的重要宿主，并保存着有关过去气候变化和造山事件的关键信息。这种流动很难研究，通常很少见，破坏性很强；它们对电缆和管道等海底工程构成重大危险，并经常破坏科学测量设备。因此，我们对这种流动的了解主要来自实验室实验，以及通过对现在暴露在陆地上的古代样本的研究来了解它们的沉积。因此，没有对天然河道中这些水流的详细研究，也没有研究将水流测量与产生的沉积物联系起来。地球上几乎没有其他环境我们不知道水流过程是如何与它们的沉积沉积联系在一起的，而这发生在地球上最大的沉积中！因此，迫切需要提高我们对活跃的海底航道内水流、地形和沉积物之间的相互作用的了解。然而，除了与监测这些密度驱动的水流有关的技术问题外，大多数海底河道实际上是在海平面比今天低得多的时候形成的；今天的水流通常比形成河道/沉积物的水流小得多，使得研究相互作用变得不可能。此外，需要创新的技术来测量这些通道内的详细流动模式。大约6000年前，海平面接近现在的水平，来自地中海的稠密含盐流体(10-15米厚)开始流经博斯普鲁斯海峡(经过伊斯坦布尔)进入黑海，形成了一个几乎一直活跃的海底航道网。该系统及其沉积沉积物的第一批壮观图像直到2005年才获得。这为首次研究活跃的海底航道中的水流和沉积物并利用这一知识制定和测试预测模型提供了独特的机会。我们召集了一个由英国和国际科学家组成的世界领先的团队来应对这一挑战。我们将使用Autosub3 NERC最先进的新型自动潜水艇(当然是黄色的！)将我们的测量设备“飞行”在底部上方，使我们能够以前所未有的细节绘制航道地形和三维(3D)流动，这与传统的基于船舶的方法相比是可能的。流动数据将与海底性质的测量、较小的形态特征(如床形)和地震数据相关联，地震数据通过沉积物成像以揭示沉积物的内部结构。这些数据将被用作创新的海底航道流动和沉积计算机模拟模型的输入条件。数值模拟和现场数据将结合在一起，使我们能够：i)相对于形成它们的水流来评估河道中的河床形态，从而首次根据较老岩石中保存下来的河床形态重建过去的水流；ii)建立弯曲水流模型，以便能够预测沉积物中的沉积物模式；以及iii)对水流和地形与长期沉积物沉积的联系有一个新的理解。这些数据将彻底改变我们对海底环境中流动和沉积的理解，主要应用于：i)地质灾害分析，ii)海底工程设计标准，以及iii)沉积沉积类型和分布的预测。

Submarine channels are spectacular features that can extend for thousands of kilometres across the seafloor, are often kilometres wide and up to hundreds of metres deep. They are formed by density currents; underwater flows of sand, mud and water that are denser than sea water and therefore flow along the seafloor. These channels are very important as they are the major transport pathway for moving sediments to the deep sea and form the largest sedimentary deposits on Earth. These deposits are significant hosts for gas and oil reserves and hold key information on past climate change and mountain building episodes. Such flows are difficult to study, typically being infrequent and highly destructive; they pose a major hazard to sea-floor engineering such as cables and pipelines, and have often destroyed scientific measurement equipment. Consequently, our knowledge of such flows comes mainly from laboratory experiments, and understanding of their deposits from studies of ancient examples now exposed on land. As a consequence there are no detailed studies of these flows in natural channels, and no studies that link flow measurements to the deposits that are produced. There is almost no other environment on Earth where we do not have any knowledge of how flow processes are linked to their sedimentary deposits, and this in the largest deposits on Earth! Consequently, there is an urgent need to improve our understanding of the interactions between flow, morphology and deposits within an active submarine channel. However, in addition to the technical problems associated with monitoring these density-driven flows, most submarine channels actually formed when sea-levels were much lower than today; present-day flows are typically much smaller than those that formed the channels/deposits, making study of interactions impossible. Furthermore, innovative techniques are required to measure detailed flow patterns within these channels. Around 6,000 years ago sea-level approached its present level, and dense salty fluid (10-15 m thick) from the Mediterranean started flowing through the Bosphorus Strait (past Istanbul) into the Black Sea, forming an almost constantly active sea-floor channel network. The first spectacular images of this system and its sedimentary deposits were only obtained in 2005. This provides a unique opportunity to study both flows and deposits in an active sea-floor channel for the very first time and to use this knowledge to formulate and test predictive models. We have assembled a world-leading group of UK and international scientists to tackle this challenge. We will use Autosub3 NERC's new state-of-the-art autonomous submarine (yellow, of course!) to 'fly' our measurement equipment just above the bottom, allowing us to map channel morphology and the three dimensional (3D) flows in unprecedented detail compared to what is possible from traditional ship-based methods. The flow data will be linked to measurements of sea-bed properties, smaller morphological features such as bedforms, and seismic data that images through sediment to reveal the internal structure of the deposits. These data will be used as input conditions for an innovative computer simulation model of flow and deposition in submarine channels. The numerical modelling and field-data will be combined to enable us to: i) assess bedforms in the channels with respect to the flows forming them, allowing reconstruction of past flows from preserved bedforms in older rocks for the very first time; ii) model bend flow to enable sediment patterns in the deposits to be predicted, and, iii) develop a new understanding of how flow and morphology is linked to long-term sediment deposition. These data will revolutionise our understanding of both flows and deposits in submarine environments, with key applications to: i) geohazard analysis, ii) design criteria for seafloor engineering, and, iii) prediction of sedimentary deposit types and distributions.