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

Holocene depositional sequences in the coastal zone of the North Sea reflec t climate-driven sea-level changes that led to lateral interfingering of ma rine, brackish, lagoonal and limnic sediments and peat layers. This success ion 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 st ratigraphic resolution for major and minor elements, bulk and molecular con stituents and microfacies characteristics. Abundant triterpenoids in sedime nts of the basal organic unit indicate nutrient-rich limnic paleoconditions not yet influenced by the rising sea level. Lithological description of se veral sedimentary units overlying the basal sequence initially suggested a mainly lagoonal setting with occasional influence by tidal channel activiti es; geochemical and microfacies analyses confirmed repeated marine ingressi on. Marine conditions are indicated by pyrite enrichment, the presence of s terathiols formed by the reaction of sterols with reduced sulfur species, a nd the occurrence of marine pelagic diatoms. Towards the top of the sedimen tary sequence, estuarine conditions become more and more pronounced as the river Weser established its modern course whereas it had been quite variabl e previously. Estuarine conditions are reflected by sterol and amino acid d istributions and by limnic diatom species that are mixed with marine pelagi c forms. Although the majority of the clastic units consist of silty to cla yey sediments, some intercalated quartz sand layers reflect the increase of hydrodynamic energy characteristic of lower tidal flats and channels. Lump s of reworked peat within fine elastic deposits also reflect high energy ev ents. Even a dark layer within the upper elastic 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. (C) 1998 Elsevier Science Ltd. All rights reserved.