Recreating atmospheric sulphur deposition histories from peat stratigraphy: Diagenetic conditions required for signal preservation and reconstruction of past sulphur deposition in the Derbyshire Peak District, UK

Abstract

Three modern pristine and one previously polluted ombrotrophic peat were sampled and analysed to investigate whether the isotopic composition of atmospheric sulphur inputs could be preserved in the organic fraction of ombrotrophic peats without undue alteration through syn-depositional or diagenetic effects. Peat type (reflecting the environment of deposition and diagenesis) was shown to be critical in determining whether an environmental archive of sulphur deposition could be preserved within the profile. Of the pristine peats sampled, hummock peat from Shetland proved to be the most suitable exhibiting a near-constant δ 34S characteristic of the growing vegetation in the organic fraction of peat in the upper 30 cm of the profile. However, only short to medium term changes (<600 years) in input are likely to be preserved due to the masking effect produced by incorporation of sulphide, derived from bacterial sulphate reduction (BSR), into the organic fraction at depth. The organic fraction of peat sampled from waterlogged pools at Forsinard (NE Scotland) was masked by sulphide incorporation throughout the profile, while peat from Abisko (N. Sweden) was exposed to annual freeze–thaw cycles, which appear to have produced significant decomposition of the organically bound sulphur and loss of the original depositional isotopic signal. Three peat types were sampled from the southern Pennines, an area with a gross history of sulphur pollution during the Industrial Revolution: high hummock peat, low hummock peat and waterlogged peat from a pool complex. In drier, high and low hummock peat types, diagenesis and decomposition were not found to be significant processes in the upper portions of the peat. Hence, alterations in the isotopic composition of the organic sulphur fraction in the profile reflect that of the depositional environment. In the hummock peats, organically bound sulphur becomes isotopically lighter at around 1400 AD, which may reflect alterations in local sulphur inputs at this time due to the small-scale smelting of lead in the area. Isotopic compositions become progressively lighter still towards the peat surface layers, which correlate well with the excesses of sulphur deposition, largely from coal burning, during the Industrial Revolution.