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

Autor: F. Watermann, G. Gerdes, Hans-Jürgen Brumsack, Olaf Dellwig, Wolfgang E. Krumbein
Rok vydání: 2001
Předmět:
Zdroj: Palaeogeography, Palaeoclimatology, Palaeoecology. 167:359-379
ISSN: 0031-0182
DOI: 10.1016/s0031-0182(00)00247-9
Popis: A drill core from the marshlands of NW Germany covering the entire Holocene was analysed at high-resolution by geochemical (XRF, Cirm-MS) and microfacies methods (diatoms, thin sections, SEM) in order to provide information about the palaeoenvironmental development. In addition, microbial experiments were carried out with sulphate-reducing bacteria (SRB) and cellulose fermenting fungi in seawater using peat as substrate. The core contains an intercalated 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 peat, while the upper part is formed by a reed peat bed. The peat layers are characterised by a distinct enrichment of pyrite with negative bulk sediment δ 34 S values (av. −11.5‰, range −2.6 to −26.7‰) reflecting microbial sulphate 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 indicates localised pyrite formation at the decay site in microenvironments with high concentrations of dissolved Fe. Microbial experiments reveal that peat does not represent an appropriate substrate for SRB and that SRB have to rely on the preceding decomposition of peat by other microorganisms in microenvironments. The total sulphur content of the peat layers averages to 7.8 wt% with a maximum value of 28.2 wt% unusually found within the basal peat at the transition of fen woodland and raised bog peat. The pyrite enrichments can be explained by two different scenarios which are related to the intensity 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 coincides with peat growth. A brackish zone is assumed for the peat-forming environment which is influenced by iron-rich freshwater and sulphate-rich seawater. As the freshwater forms the major Fe source the amounts of pyrite increase to a certain degree with decreasing salinity in this environment. The palaeosalinity is estimated by the investigation of littoral and pelagic diatoms. The second scenario applies to the extensive pyrite formation within the basal peat at the transition of fen and bog peat. This pyrite was likely formed after peat growth favoured by a steep sea-level rise. Thin sections of this peat interval reveal clastic layers (1–3 mm) with completely pyritised interfaces. As these clastic layers contain marine pelagic diatoms (e.g. Paralia sulcata ) we propose that they are caused by tidal channel activities during a phase of a steep sea-level rise. The marine incursion led to a partial buoyancy of the basal peat at weak transition zones, i.e. the transition from fen to bog peat. As a result of the bouyancy, marine suspended particulate matter can be introduced laterally in-between the peat sections. When the peat settles down a clastic layer remains whose interface with the peat may have favoured the inflow of waters of higher salinity. Therefore, an enhanced formation of pyrite was possible due to the combination of sulphate-rich ground water and iron-rich peatland waters.
Databáze: OpenAIRE
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