Geomorphological controls on fluvial carbon storage in headwater peatlands

Abstract

Geomorphological controls and catchment sediment characteristics control the formation of floodplains and affect their capacity to sequester carbon. Organic carbon stored in floodplains is typically a product of pedogenic development between periods of mineral sediment deposition. However, in organically‐dominated upland catchments with a high sediment load, eroded particulate organics may also be fluvially deposited with potential for storage and/or oxidation. Understanding the redistribution of terrestrial carbon laterally, beyond the bounds of river channels is important, especially in eroding peatland systems where fluvial particulate organic carbon exports are often assumed to be oxidised. Floodplains have the potential to be both carbon cycling hotspots and areas of sequestration. Understanding of the interaction of carbon cycling and the sediment cascade through floodplain systems is limited. This paper examines the formation of highly organic floodplains downstream of heavily eroded peatlands in the Peak District, UK. Reconstruction of the history of the floodplains suggests that they have formed in response to periods of erosion of organic soils upstream. We present a novel approach to calculating a carbon stock within a floodplain, using XRF and radiograph data recorded during Itrax core scanning of sediment cores. This carbon stock is extrapolated to the catchment scale, to assess the importance of these floodplains in the storage and cycling of organic carbon in this area. The carbon stock estimate for the floodplains across the contributing catchments is between 3482‐13460 tonnes, equating on an annualised basis to 0.8‐4.5% of the modern‐day POC flux. Radiocarbon analyses of bulk organic matter in floodplain sediments revealed that a substantial proportion of organic carbon was associated with re‐deposited peat and has been used as a tool for organic matter source determination. The average age of these samples (3010 years BP) is substantially older than Infrared Stimulated Luminesence dating which demonstrated that the floodplains formed between 430 and 1060 years ago. Our data suggest that floodplains are an integral part of eroding peatland systems, acting as both significant stores of aged and eroded organic carbon and as hotspots of carbon turnover. © 2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

Geomorphic controls on floodplain sediment and soil organic carbon storage in a Scottish mountain river

“…Floodplains are large depositional landforms, acting as carbon stores but also as sites with potentially high carbon mineralisation rates. As a result, floodplains act as hotspots of carbon turnover (Hoffmann et al, 2009b;Evans et al, 2013;Alderson et al, 2019). Unfortunately, the limited amount of available studies does not allow to accurately assess the role of floodplains in the mineral sediment and organic carbon cascades (Alderson et al, 2019).…”

“…The wet and cool conditions, which dominate the climate of the Scottish Highlands, will likely lead to high organic carbon concentrations in floodplain sediment as the high moisture content favours vegetation growth but limits respiration in the soil profile (Falloon et al, 2011). As peatlands are present in the Scottish landscape, both on the hillslopes and in floodplains, they might constitute an important element of the floodplain soil organic carbon storage (Alderson et al, 2019).…”

“…As a result, floodplains act as hotspots of carbon turnover (Hoffmann et al, ; Evans et al, ; Alderson et al, ). Unfortunately, the limited amount of available studies does not allow to accurately assess the role of floodplains in the mineral sediment and organic carbon cascades (Alderson et al, ).…”

Spatial organisation and physical characteristics of large peat blocks in an upland fluvial peatland ecosystem

“…Where atmospheric interactions heighten CO 2 release through J o u r n a l P r e -p r o o f Journal Pre-proof oxidation (Pawson et al, 2012), incorporation of peat blocks into the floodplain stratigraphy allows carbon sequestration (Evans and Lindsay 2010;Warburton and Evans, 2011). Floodplains have been described as both hotspots for carbon cycling and as areas of sequestration in upland fluvial peatland ecosystems (Alderson et al, 2019). An improved process-understanding of peat block transfer, degradation and residence through fluvial systems is essential for establishing representative carbon budgets (Evans and Warburton, 2007).…”