Geochemical and microfacies characterization of a Holocene depositional sequence in northwest Germany

Holocene depositional sequences in the coastal zone of the North Sea reflect climate-driven sea-level changes that led to lateral interfingering of marine, brackish, lagoonal and limnic sediments and peat layers. This succession is reflected in an 18 m sediment core drilled in the marsh lands of the river Weser. The core spanning the entire Holocene was analyzed at high stratigraphic resolution for major and minor elements, bulk and molecular constituents and microfacies characteristics. Abundant triterpenoids in sediments of the basal organic unit indicate nutrient-rich limnic paleoconditions not yet influenced by the rising sea level. Lithological description of several sedimentary units overlying the basal sequence initially suggested a mainly lagoonal setting with occasional influence by tidal channel activities; geochemical and microfacies analyses confirmed repeated marine ingression. Marine conditions are indicated by pyrite enrichment, the presence of sterathiols formed by the reaction of sterols with reduced sulfur species, and the occurrence of marine pelagic diatoms. Towards the top of the sedimentary sequence, estuarine conditions become more and more pronounced as the river Weser established its modern course whereas it had been quite variable previously. Estuarine conditions are reflected by sterol and amino acid distributions and by limnic diatom species that are mixed with marine pelagic forms. Although the majority of the clastic units consist of silty to clayey sediments, some intercalated quartz sand layers reflect the increase of hydrodynamic energy characteristic of lower tidal flats and channels. Lumps of reworked peat within fine clastic deposits also reflect high energy events. Even a dark layer within the upper clastic unit, formerly considered a fossil soil horizon, is not the direct result of regression-induced soil development, but originates mainly from eroded fossil organic material from the basal unit. This interpretation was established by biomarker analysis and radiocarbon dating.