In Upper Cretaceous chalk sequences, the widespread occurrence of flint, as well as a possible astronomical pacing of their often-encountered rhythmic distribution, remains poorly constrained. The Campanian-Maastrichtian Hallembaye chalk succession (Maastrichtian type area, northeast Belgium) is characterized by the gradual evolution from no flint bands at its base to the regular presence of well-developed flint bands at its top. Here, the Hallembaye section is investigated to gain more insights into the underlying processes behind flint inception. A relationship is found between the amount of detrital material (i.e., clays) present in the chalk and flints, and the extent of silicification and flint development. Several astronomical cycles are identified within the succession using both a lithology-based flint proxy and high-resolution μXRF-based element data from chalk samples. A combined imprint of precession and obliquity is documented in the chalk Ti/Al profile. The flint bands display a predominant obliquity imprint with an increasing contribution of precession and eccentricity up-section. Two consistent stratigraphically integrated astronomical age models are preferred. The first model is a floating age model that is based on the minimal tuning of the short obliquity cycle in the Ti/Al signal. The second model is a numerical age model that is based on flint occurrences, tuned to a combined tuning target consisting of both the inclination and long eccentricity metronomes. Temporal variations in the hydrological cycle and consequent changes in eolian, fluvial and dissolved Si input to the European Basin appear astronomically controlled. In addition, flint nodules and bands are paced by Milankovitch timescales, reflecting astronomical control on the Si cycle and paleoenvironmental conditions governing conditions favorable towards flint formation.
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RBINS Staff Publications 2025
Aim Species inhabiting fresh waters are severely affected by climate change and other anthropogenic stressors. Effective management and conservation plans require advances in the accuracy and reliability of species distribution forecasts. Here, we forecast distribution shifts of Salmo trutta based on environmental predictors and examine the effect of using different statistical techniques and varying geographical extents on the performance and extrapolation of the models obtained. Location Watercourses of Ebro, Elbe and Danube river basins (c. 1,041,000 km2; Mediterranean and temperate climates, Europe). Methods The occurrence of S. trutta and variables of climate, land cover and stream topography were assigned to stream reaches. Data obtained were used to build correlative species distribution models (SDMs) and forecasts for future decades (2020s, 2050s and 2080s) under the A1b emissions scenario, using four statistical techniques (generalised linear models, generalised additive models, random forest, and multivariate adaptive regression). Results The SDMs showed an excellent performance. Climate was a better predictor than stream topography, while land cover characteristics were not necessary to improve performance. Forecasts predict the distribution of S. trutta to become increasingly restricted over time. The geographical extent of data had a weak impact on model performance and gain/loss values, but better species response curves were generated using data from all three basins collectively. By 2080, 64% of the stream reaches sampled will be unsuitable habitats for S. trutta, with Elbe basin being the most affected, and virtually no new habitats will be gained in any basin. Main conclusions More reliable predictions are obtained when the geographical data used for modelling approximate the environmental range where the species is present. Future research incorporating both correlative and mechanistic approaches may increase robustness and accuracy of predictions.
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