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The fate of carbon in upland floodplain sediments: a combined geomorphological and organic geochemical approach
[Thesis]. Manchester, UK: The University of Manchester; 2017.
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Abstract
As conveyors of water and sediment, rivers play an instrumental role in landscape evolution (Turner et al., 2015). River systems were traditionally considered as passive pipes of terrestrial organic carbon (OC), but are now viewed as active sites of OC processing, redistribution and storage (Aufdenkampe et al., 2011). Floodplains are an important part of this system and have the capacity to act as sources or sinks of carbon (Zehetner et al., 2009), but most importantly active hotspots of organic matter (OM) transformation (Hoffmann et al., 2009; Zocatelli et al., 2013). POC eroded from highly-organic peat soils, may be interrupted in its transport through the fluvial system, by temporary storage on floodplain landforms (Evans and Warburton, 2005; Evans et al., 2006). It is important to investigate the fate of fluvial peatland POC, in order to fully close the terrestrial peatland carbon budget, to account for subsequent mineralisation and explore the processes that lead to redistribution and storage. The River Ashop in the southern Pennines, UK, drains the slopes of both Bleaklow and Kinder Scout which are upland plateaux, which support an extensive cover of blanket peat (Evans and Lindsay, 2010). These peatlands have been severely eroded and are vulnerable to future erosion as they are marginal to the climatic space suitable for growth of peat bogs in the UK (Clark et al., 2010). The wider peatland catchment features cohesive, organic-rich floodplains, which are atypical in an upland landscape, and thus suitable for investigation in their role in the fate of eroded carbon. OM quality was an important focus of this research into the redistribution of terrestrial organic carbon. As such, OM quality was conceptualised, by identifying how different research disciplines identify with the term, and ultimately providing a classification scheme to assist individuals in their exploration of OM character. A novel approach using ITRAX core scanning data was used to establish carbon stocks of floodplains in the River Ashop catchment. Sedimentological characteristics were insufficient to distinguish between allochthonous and autochthonous organic matter storage. However, dating was used as a rapid and accurate tool to assess carbon source on the 'off-site' floodplain, and could perhaps be used in the wider environment where there are large discrepancies between the ages of source materials. Investigations into the OM quality, provided suitably convincing evidence that substantial overbank deposition of eroded 'old' peat had occurred. Despite this, viewing these sites simply as areas of carbon storage is misleading. In fact, these systems have potentially been turning over substantial quantities of carbon to the atmosphere. Contextual information from modern-day fluvial POC fluxes showed that both 'off-site' and 'on-site' floodplains only play a minor role in storing carbon. Geomorphological events, particularly the substantial sediment flux generated from peatland erosion were critical in the formation of these floodplains. In these peatland systems, erosion, deposition and turnover of carbon are intimately linked at the landscape scale, and floodplains are a dynamic component of this system.