Sulphur and iron geochemistry of Holocene coastal peats (NW Germany): a tool for palaeoenvironmental reconstruction

A drill core from the marshlands of NW Germany covering the entire Holocene was analysed at high-resolution by geochemical (XRF, Cirm-MS) and microfac ies methods (diatoms, thin sections, SEM) in order to provide information a bout the palaeoenvironmental development. In addition, microbial experiment s were carried out with sulphate-reducing bacteria (SRB) and cellulose ferm enting fungi in seawater using pear as substrate. The core contains an inte rcalated reed peat layer (2.73-3.02 m) and a basal peat (5.78-6.42 m). The basal peat consists in the lowest part of fen woodland and raised bog pear, while the upper part is formed by a reed pear bed. The peat layers are cha racterised by a distinct enrichment of pyrite with negative bulk sediment d elta S-34 values (av. -11.5%, range -2.6 to -26.7%) reflecting microbial su lphate reduction in some peat intervals under almost open system conditions with respect to the input of seawater sulphate. Pyrite occurs exclusively with framboidal texture in close vicinity to the organic matter which indic ates localised pyrite formation at the decay site in microenvironments with high concentrations of dissolved Fe. Microbial experiments reveal that pea t does not represent an appropriate substrate fur SRB and that SRB have to rely on the preceding decomposition of peat by other microorganisms in micr oenvironments. The total sulphur content of the peat layers averages to 7.8 wt% with a maximum value of 28.2 wt% unusually round within the basal peat at the transition of fen woodland and raised bog peat. The pyrite enrichme nts can be explained by two different scenarios which are related to the in tensity of the sea-level rise. The first scenario applies to a moderate sea -level rise and is decisive for the reed peats where pyrite formation coinc ides with peat growth. A brackish zone is assumed for the pear-forming envi ronment which is influenced by iron-rich freshwater and sulphate-rich seawa ter. As the freshwater forms the major Fe source the amounts of pyrite incr ease to a certain degree with decreasing salinity in this environment. The palaeosalinity is estimated by thr investigation of littoral and pelagic di atoms. The second scenario applies to the extensive pyrite formation within the basal peat at the transition of fen and bog peat. This pyrite was like ly formed after pear growth favoured by a steep sea-level rise. Thin sectio ns of this pear interval reveal elastic layers (1-3 mm) with completely pyr itised interfaces. As these elastic layers contain marine pelagic diatoms ( e.g. Paralia sulcata) we propose that they are caused by tidal channel acti vities during a phase of a steep sea-level rise. The marine incursion led t o a partial buoyancy of the basal pear at weak transition zones, i.e, the t ransition from fen to bog peat. As a result of the bouyancy, marine suspend ed particulate matter can be introduced laterally in-between the peat secti ons. When the peat settles down a elastic layer remains whose interface wit h the peat may have favoured the inflow of waters of higher salinity. There fore, an enhanced formation of pyrite was possible due to the combination o f sulphate-rich ground water and iron-rich peatland waters. (C) 2001 Elsevi er Science B.V. All rights reserved.