High-resolution reconstruction of a holocene coastal sequence (NW Germany)using inorganic geochemical data and diatom inventories

Holocene deposits of the NW German coastal plain consist of many different lithological facies, for example tidal flats, brackish water sediments and pear beds. The effects of the Holocene sea-level rise on palaeoenvironmenta l conditions of this coastal sequence were studied by inorganic geochemical methods in conjunction with diatom analyses. Three hundred samples from tw o parallel sediment cores which cover the entire NW German Holocene were ta ken at high resolution and were examined for major and minor elements and b ulk parameters. Selected samples were analysed for redox-sensitive trace el ements and REE distribution, reactive iron and bulk sediment (SiS)-Si-34-S- 32-ratios. Chemical parameters, for example Si/Al and Ca/Sr-ratios and tota l 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 pa laeosalinity. Early diagenetic effects are evident in the TOC-rich interval s. 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 s eawater influence. S-isotope ratios of peat samples are compatible with pyr ite formation under both open- and closed-system conditions, depending on e xposition to Seawater. The inorganic geochemical and diatom data suggest li mnic conditions at the base of the sequence and repeated changes towards ma rine conditions within the overlying elastic units. On the other hand, data obtained from the elastic units yield evidence of a recurrent succession f rom open to restricted marine, brackish-lagoonal, and finally fen environme nts. Clastic sediments overlying peat layers, correlate with the increase o f marine-derived geochemical signatures and pelagic diatoms attest transgre ssive overlaps. The analyses suggest that major controls on the palaeoenvir onments were (i) climate-related oscillations of the coast line and (ii) th e morphology of the coastal region allowing marine incursions even into dis tal semi-terrestrial lowlands. (C) 1999 Academic Press.