A geochemical revisit of the expanded Cretaceous-Paleogene (K/Pg) boundary section in the Maastrichtian type area

The most recent of the ‘Big Five’ mass extinctions of Phanerozoic life occurred at the Cretaceous-Paleogene (K-Pg) boundary, ~66 million years (Myr) ago. The global K-Pg boundary interval is correlated with the Chicxulub meteorite impact event on the Yucatán Peninsula in México, dated at 66.043 ± 0.011 Myr ago. However, deciphering the exact timing and mechanisms of the processes that were responsible for the abrupt extinction of 75% of species on Earth including the iconic non-avian dinosaurs, remains a field that needs more detailed study. A small number of expanded K-Pg boundary sites, both within the Chicxulub impact crater and in terrestrial and marine environments at more distal locations, have recently revealed unprecedented new insights in the direct aftermath following the Chicxulub impact event (e.g., Gulick et al., 2019).

More high-resolution sedimentological, geochemical and palaeontological analyses of expanded K-Pg sites around the globe might further unravel the effects of the dynamic impact cratering processes on the short-term paleoenvironmental and climatic change in the earliest Paleogene. An interesting candidate for this is the Type Maastrichtian area, in the border region of northeastern Belgium and the southeastern part of the Netherlands, that yields a >100 m thick sequence of shallow marine sediments. We revisited and resampled the Geulhemmerberg K-Pg site, which preserves a unique series of calcarenites, shell hashes and clays in the underground galleries near Valkenburg, the Netherlands (e.g., Smit & Brinkhuis, 1996).

The aims of this study are twofold. Firstly, we aim to place the Geulhemmerberg K-Pg section in a long-term palaeoenvironmental proxy record of the Type Maastrichtian, covering a (near-) continuous marine sedimentation over the last c. 1 Myr of the Cretaceous. This timescale covers the onset of Deccan Traps volcanism in India and therefore, we will be able to differentiate the environmental effects of Deccan volcanic activity and of the Chicxulub impact on this shallow marine ecosystem across the K-Pg boundary. Secondly, we apply high-resolution geochemical techniques on the Geulhemmerberg K-Pg stratigraphy to obtain novel insights in the distribution of potential impact ejecta, the response of the marine biosphere to the climatic perturbations and the overall timing of this likely storm-driven deposit (Smit & Brinkhuis, 1996).
Regarding the first aim, this study represents the top section of a c. 115 m thick composite stratigraphic framework that has been constructed as part of the Maastrichtian Geoheritage Project (Vellekoop et al., 2022) and covering as well the Hallembaye and former ENCI quarries (Fig. 1). Revisiting the Geulhemmerberg K-Pg site resulted in a sample set obtained at 5 cm stratigraphic resolution for the 7.5 meters below the basal K-Pg shell hash, yielding the local stratigraphic units IVf3 to IVf7 of the Meerssen member of the Maastricht Formation. At Geulhemmerberg, the base of the hardground at -4 meter (relative to the K-Pg boundary) is linked to the base of the IVf5 unit, which can be correlated to a similar interval in the nearby Curfs and former ENCI quarries. We performed stable carbon (δ13C) and oxygen (δ18O) isotope analyses on carbonate powders, together with bulk micro-X-ray fluorescence (µXRF) elemental analyses (following Vellekoop et al., 2022). The µXRF data reveal an abrupt drop in CaCO3 content at the K-Pg boundary interval compared to the rest of the >100 m thick sequence. In addition, the δ13C signal shows a slight decreasing trend towards the top, but a clear negative anomaly at the boundary seems to be missing (Fig. 1).

The second aim of this study centers around high-resolution analyses of the base of the c. 1 m thick IVf7 interval, based on 25 µm resolution µXRF element mappings and point-linescans of continuous sediment blocks, following the methodology described in Kaskes et al. (2021). The profiles based on this first pilot-study show distinct variations in e.g. Fe content across the various clay and shell hash horizons (Fig. 2), but no clear enrichments are seen in e.g. Cr and Ni. The absence of a clear Cr signal is in line with the absence of an iridium anomaly at this shallow marine locality, in contrast to distal deep marine K-Pg boundary sites (Smit & Brinkhuis, 1996). Future work will focus on the geochemical mapping of burrow structures to identify potential impact ejecta components, and on a more thorough major and trace element comparison between the multiple clay layers in the Geulhemmerberg section, to better understand the sequence of events in the aftermath of the Chicxulub impact for this shallow marine palaeoenvironment.