The Significance of Diagenesis versus Riverine Input in Contributing to the Sediment Geochemical Matrix of Iron and Manganese in an Intertidal Region

Holocene deposits of the NW German coastal plain consist of many different lithological facies, for example tidal flats, brackish water sediments and peat beds. The effects of the Holocene sea-level rise on palaeoenvironmental conditions of this coastal sequence were studied by inorganic geochemical methods in conjunction with diatom analyses. Three hundred samples from two parallel sediment cores which cover the entire NW German Holocene were taken at high resolution and were examined for major and minor elements and bulk parameters. Selected samples were analysed for redox-sensitive trace elements and REE distribution, reactive iron and bulk sediment34S/32S-ratios. Chemical parameters, for example Si/Al and Ca/Sr-ratios and total organic carbon (TOC) contents correlate with depositional factors such as wave-energy and lithofacies changes, which allow a detailed reconstruction of the palaeoenvironment. Diatom analyses reveal information about changes between marine, brackish and limnic conditions and serve to reconstruct palaeosalinity. Early diagenetic effects are evident in the TOC-rich intervals. Most peat layers are affected by sulphate reduction and resulting pyrite formation as well as by enrichments in redox-sensitive trace elements. The highest enrichments are seen for As, Mo, Re and U, indicating a distinct seawater influence. S-isotope ratios of peat samples are compatible with pyrite formation under both open- and closedsystem conditions, depending on exposition to seawater. The inorganic geochemical and diatom data suggest limnic conditions at the base of the sequence and repeated changes towards marine conditions within the overlying clastic units. On the other hand, data obtained from the clastic units yield evidence of a recurrent succession from open to restricted marine, brackishlagoonal, and finally fen environments. Clastic sediments overlying peat layers, correlate with the increase of marine-derived geochemical signatures and pelagic diatoms attest transgressive overlaps. The analyses suggest that major controls on the palaeoenvironments were (i) climate-related oscillations of the coast line and (ii) the morphology of the coastal region allowing marine incursions even into distal semi-terrestrial lowlands.