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.