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

Abstract

The distribution of contaminant radionuclides from the Sellafield nuclear fuel reprocessing plant was used to establish chronologies for three saltmarsh sediment cores from south west Scotland. δ¹³C and ¹⁴C analyses indicated that the cores provided a useful archive record of variations in input of organic matter and carbonate. The results imply that prior to major releases of contaminant ¹⁴C from Sellafield, the ¹⁴C specific activity of organic matter in Irish Sea offshore sediments was about 24 Bq kg⁻¹ C, while that of the carbonate component was below the limit of detection. These results provide baseline data for modelling the uptake of contaminant ¹⁴C by the Irish Sea sediment system. The study confirmed that δ¹³C analyses provide a sensitive means of apportioning the origin of saltmarsh organic matter between C₃ terrigenous plants, C₄ terrigenous plants and suspended particulate marine organic matter. For the <2 mm fraction of sediment, a clear pattern of decreasing marine organic input was observed in response to increasing elevation of the marsh surface as a result of sediment accumulation. Bulk sediment, including detrital vegetation, had a dominant input from terrigenous plants. The combined use of δ¹³C and ¹⁴C data revealed that organic matter in the marine organic component of the <2 mm fraction of contemporary surface sediments of the saltmarshes is dominated by recycled old organic material.