Hydrodynamics of pollutant and organic carbon settling: implications for dispersal and concentration in oceans

了解人为污染物（如微塑料和纳米塑料）和有机碳的运输、沉积和掩埋过程，是预测海洋污染热点和量化蓝碳储量的主要挑战。海洋沉积物中陆地有机碳颗粒的埋藏可以去除大气中的二氧化碳，这有助于在地质时间尺度上调节气候。沉积的时空变异性意味着这些蓝碳储量的量化仍然高度不确定。有机碳的埋藏效率取决于暴露于氧气的时间，这与悬浮沉降率有关，而悬浮沉降率仍然没有得到很好的限制。同样，微塑料颗粒的流体力学和沉降速率，其形状和组成的范围，还没有研究。海洋表面堆积的塑料约占全球海洋塑料总量的1%，其余99%最终进入深海海底。这些材料大部分以微塑料的形式出现：小（<1毫米）碎片和纤维，但人们对这些颗粒在海洋环境中的分布知之甚少。因此，迫切需要了解有机碳和污染物在开放海洋环境中是如何分散和隔离的。对其流体动力学行为的理解的进步，为旨在预测这些颗粒的扩散和储存的模型的发展提供了信息。由于对微塑料和其他具有复杂形态的低密度颗粒（如粘土絮凝体和有机碳颗粒）的沉降知之甚少，学生将在Sorby实验室套件中使用最先进的沉降池和颗粒成像测速仪（PIV）设备设计新颖的物理实验。现有的测量系统已经开发出来，用来测量冰粒下落的动力学。Sorby实验室套件具有独特的能力，可以使用现有的经过验证的实验系统来测量这些粒子动力学。物理实验将用于指导和发展直接数值模拟（DNS）实验，其中Navier-Stokes方程在没有任何湍流模型的情况下进行数值求解，以研究更长时间尺度和更大空间尺度上的扩散模式。

Understanding the transport, deposition, and burial processes of anthropogenic pollutants (e.g. micro- and nanoplastics), and organic carbon, are major challenges to predicting marine pollution hotspots and quantifying blue carbon stocks. Burial of particulate terrestrial organic carbon in marine sediments removes CO2 from the atmosphere, which helps to regulating climate over geologic time scales. Spatial and temporal variability in deposition means that quantification of these blue carbon stocks remain highly uncertain. Burial efficiency of organic carbon depends on the exposure time to oxygen, which is tied to suspension settling rates that remain poorly constrained. Similarly, the hydrodynamics and settling rates of microplastic particles, which range in shape and composition, have not been studied. Sea surface accumulations of plastics account for ~1% of the estimated global marine plastic budget, and the remaining 99% ends up in the deep seafloor. Much of this material occurs as microplastics: small (<1 mm) fragments and fibres, yet the distribution of these particles in the marine environment is poorly understood.Therefore, there is an urgent need to understand how organic carbon and pollutants are dispersed and sequestered in the open marine environment. Advances in understanding of their hydrodynamic behaviour have the scope to inform the development of models that aim to forecast the dispersal and storage of these particles. Because very little is known about the settling of microplastics and other low-density particles with complex morphologies, such as clay flocs and organic carbon particles, the student will design novel physical experiments using state-of-the-art settling tanks and particle imaging velocimetry (PIV) equipment in the Sorby Lab Suite. An existing measurement system that has been developed to measure the dynamics of falling ice particles will be adapted. The Sorby Lab Suite has a unique capability to measure these particle dynamics using an existing proven experiment system. The physical experiments will be used to inform and develop direct numerical simulation (DNS) experiments, in which the Navier-Stokes equations are numerically solved without any turbulence model, to investigate dispersal patterns over longer timescales and greater spatial scales.