The application of microfaunal and sedimentological data for Holocene landscape and sea-level reconstruction on the German North Sea coast

Detailed reconstructions of palaeo-landscape and sea-level changes on the German North Sea coast are crucial for the understanding of coastal evolution, dynamic forming pro-cesses and local controlling mechanisms of coastal and sea-level changes. This research con¬tributes to reconstructing Holocene coastal changes in the German Bight and provides a high-resolution relative sea-level (RSL) curve for the tidal basin of Norderney by applying a novel approach of a transfer function (TF) based on microfaunal data. Based on this new knowledge, future projections of RSL rise can be supported and, thus, future coastal-protection measures can be better adjusted. This study focuses on sediment archives from two different study areas: (1) the Jade-Weser region and (2) the East Frisian islands, which were both investigated for their sedi-mento¬logical, geochemical and microfaunal characteristics. This leads to the reconstruc-tion of phases of coastal pro- and retrogradation, which can be related to existing RSL curves. However, uncertainties due to peat compaction, indicative meaning of peats and missing reliable quantitative data remain. Therefore, in a new approach, modern associa-tions of fora¬mini¬fers and ostracods were investigated at the southern coast of Spiekeroog. These exhibit a vertical zonation, which is mainly influenced by the elevation relative to the mean sea level and therefore suitable for the development of a TF for RSL change. Such a TF relates the modern elevation to the relative species frequency of foraminifer and os-tracod associations and enables the determination of elevations of fossil samples relative to the palaeo sea level from fossil microfaunal assemblages. For the first time, a combined dataset of foraminifers and ostracods is used for TF development, showing a clear im-provement in terms of vertical error (RMSEP) and correlation (R²boot) compared to using exclusively foraminifers. The final TF provides a vertical error of ±29.7 cm, which is a sig-nificant im¬prove¬ment compared to the error ranges of solely peat-based RSL reconstruc-tions. This improved TF was applied to Holocene samples from the tidal basin of Nor-derney. Due to the intense sediment reworking of the tidal-flat deposits, as revealed by the chrono-logical data (14C dating), the resulting RSL curve reconstructs the RSL evolution until ~2500 cal BP. In this time range, the deceleration of the RSL rise is clearly evident, and the smaller error range compared to that of existing peat-based RSL curves from the southern North Sea region especially highlights the potential of fossil salt-marsh deposits for TF applications. Comparison to the available sea-level related peat data from the same cores suggests a compaction of at least ~0.7–0.9 m, whereas some deposits seem to have under¬gone double compaction effects due to the existence of an intercalated peat. The latter led to uncertainties concerning the third and last phase (~3700–2500 cal BP) of the TF-based RSL curve. Moreover, comparison to a local peat-based RSL curve for the tidal basin of Langeoog reveals poten¬tial changes in palaeo-tidal range that occurred after ~6100 cal BP. Here, the TF data may indicate a stronger rise of mean high water com-pared to mean low water, since the data plot below the base of basal peat data points from Langeoog, which suggests a course too low for the curve. This could also be explained by a combination of the indicated compaction effects and palaeo-tidal changes. Finally, by comparing it with existing peat-based curves from Belgium and the Netherlands, the RSL curve for Norderney confirms regional differ¬en¬ces in glacial isostatic adjustment (GIA). In sum, this study provides valuable insights into the Holocene coastal and RSL evolution in the German Bight, where reconstructions have so far been hampered by less precise RSL index points. TF development based on microfaunal data offers new perspectives for understanding the RSL evolution at a much higher resolution. Consequently, it can be con-sidered a useful tool also for other coastal areas in that it allows a more precise differen-tiation and quantification of all underlying processes, both at a local and regional scale