14C and δ13C characteristics of organic matter and carbonate in saltmarsh sediments from south west Scotland

Stable isotope mass balances versus concentration differences of dissolved inorganic carbon - implications for tracing carbon turnover in reservoirs

The aim of this study was to identify sources of carbon turnover using stable isotope mass balances. For this purpose, two pre-reservoirs in the Harz Mountains (Germany) were investigated for their dissolved and particulate carbon contents (dissolved inorganic carbon (DIC), dissolved organic carbon, particulate organic carbon) together with their stable carbon isotope ratios. DIC concentration depth profiles from March 2012 had an average of 0.33 mmol L^-1. Increases in DIC concentrations later on in the year often corresponded with decreases in its carbon isotope composition (δ¹³C_DIC) with the most negative value of -18.4 ‰ in September. This led to a carbon isotope mass balance with carbon isotope inputs of -28.5 ‰ from DOC and -23.4, -31.8 and -30.7 ‰ from algae, terrestrial and sedimentary matter, respectively. Best matches between calculated and measured DIC gains were achieved when using the isotope composition of algae. This shows that this type of organic material is most likely responsible for carbon additions to the DIC pool when its concentrations and δ¹³C_DIC values correlate negatively. The presented isotope mass balance is transferable to other surface water and groundwater systems for quantification of organic matter turnover.

Ecosystem uptake and transfer of Sellafield-derived radiocarbon (14C). Part 1. The Irish Sea

Ecosystem uptake and transfer processes of Sellafield-derived radiocarbon (¹⁴C) within the Irish Sea were examined. Highly variable activities in sediment, seawater and biota indicate complex ¹⁴C dispersal and uptake dynamics. All east basin biota exhibited ¹⁴C enrichments above ambient background while most west basin biota had ¹⁴C activities close to background, although four organisms including two slow-moving species were significantly enriched. The western Irish Sea gyre is a suggested pathway for transfer of ¹⁴C to the west basin and retention therein. Despite ongoing Sellafield ¹⁴C discharges, organic sediments near Sellafield were significantly less enriched than associated benthic organisms. Rapid scavenging of labile, ¹⁴C-enriched organic material by organisms and mixing to depth of ¹⁴C-enriched detritus arriving at the sediment/water interface are proposed mechanisms to explain this. All commercially important fish, crustaceans and molluscs showed ¹⁴C enrichments above background; however, the radiation dose from their consumption is extremely low and radiologically insignificant.

Constraints on the use of anthropogenic radionuclide-derived chronologies for saltmarsh sediments

Numerous studies have employed anthropogenic radionuclides deposited in accumulating sediments to derive chronologies for use in investigations of geomorphological processes and in reconstructing temporal trends in contaminant deposition. However, relatively few have interrogated their use in systems that erode as well as accrete sediment, or have addressed limitations in their applicability in systems that experience variable rates of accumulation. This paper examines the utility of Sellafield-derived radionuclides for reconstructing sedimentary processes in two contrasting saltmarsh systems in the south-west of Scotland. Sedimentation rates and patterns derived from the radionuclide chronologies are outlined and compared to results established through conventional geomorphological methods. The results confirm that the vertical distribution of anthropogenic radionuclides can be useful in determining sedimentation rates, but that these cannot always be used as indicators of contemporary processes, particularly where erosion is occurring. Their use is also limited unless profiles are obtained from spatially diverse geomorphological units. Integrating the use of chronologies with other independent methods secures more robust data for assessing both marsh sustainability and their longevity as radionuclide sinks.