Processes and characteristics of sedimentary carbon in Loch Etive, Scotland

Climate change is one of the most severe global challenges, requiring innovative solutions to mitigate its effects. Conserving and managing natural long-term carbon sinks are amongst the most important climate mitigation and adaptation strategies and often represent a low-cost, low-risk, nature-positive strategy. Fjordic systems are increasingly recognised as important long-term carbon stores, sequestering carbon on geological timescales within their sediments. They have distinct processes and characteristics which make them ideal sedimentary carbon sinks. By investigating these in Loch Etive, a mid-latitude fjord in Scotland, this thesis contributes to the necessary knowledge base to understand and protect these long-term carbon stores. This study employs an interdisciplinary approach, combining in situ sediment sampling with hydrodynamic and particle tracking modelling for the first time within Loch Etive, to resolve the processes relating to sedimentary carbon (Figure 1). Analysis of the sediment within Loch Etive shows clear along-fjord gradients in sediment properties, highlighting processing of sedimentary organic carbon throughout the fjord. The results demonstrate that the terrestrial environment is a vital contributor to the sedimentary carbon sink in Loch Etive with bulk organic carbon ages of fjordic sediments indicating a redistribution of aged organic carbon from land to fjord. The dominant hydrographical features, simulated through three-dimensional numerical modelling, show that the fjord is governed by a two-layer estuarine circulation. In the upper basin, freshwater flux modulates the circulation, while tides generate highly energetic exchange in the lower basin. Application of a random-walk particle tracking model illustrates that this particular fjordic hydrography results in depositional hotspots in the deepest part of the upper basin, while the energetic lower basin is less conducive to sediment accumulation. After six weeks, particle retention in the upper fjord exceeds 40%, confirming that the upper basin in Loch Etive is an effective sediment trap at the land-ocean interface.