Recreating atmospheric sulphur deposition histories : evidence from stable isotope investigations in ombrotrophic peat
Ombrotrophic peat deposits are important archives of palaeoenvironmental information. Potentially, their geochemistry may also reflect environmental change by recording alterations in natural and pollutant sulphur deposition. Over much of northwest Europe, the dominant sulphur source to natural, pre-anthropogenic atmospheric inputs will have been of maritime origin, which is characteristically enriched in the heavier 34S isotope of sulphur with 834S - +20 %0 CDT (Canyon Diablo Troilite). By contrast, fossil fuel burning produces S02, which is significantly 34S-depleted (0-5 %0 CDT). Changes in the source of atmospheric sulphur inputs to peatlands may be accompanied by a change in sulphur isotope composition of sulphur taken up by growing plants. The organic sulphur isotopic composition in ombrotrophic peat sequences could thus potentially provide a record of anthropogenic impacts through the assimilation of the sulphuric acid component of acid rain into surface vegetation and its subsequent preservation in the peat profile. This was investigated by measuring sulphur isotopic compositions through peat profiles from pristine situations and from a once heavily polluted site on the southern Pennines (UK) and also through a 34S tracer experiment at a waterlogged peat in northern England. Peat type was shown to be critical in determining whether a record of isotopic compositions of aerially deposited sulphur can be preserved in ombrotrophic peat deposits. Peats need to be anaerobic enough to prevent decomposition but not so waterlogged that the production of sulphides from bacterial sulphate reduction may obscure the organic sulphur signal. Even under these conditions a primary sulphur signal is unlikely to be preserved indefinitely as diagenetic sulphide incorporation in the long term (following burial below the water-table) may eventually obscure the organic sulphur signal. Isotopic analyses of pristine and polluted peats suggest that, in the drier upland peats sampled, recent changes (-600 y) in atmospheric sulphur chemistry are preserved within the organic sulphur fraction without significant masking by diagenetic processes.