Abstract
Geochemical and palynological records are presented for an expanded Turonian–Coniacian hemipelagic succession in the central Bohemian Cretaceous Basin. A high-resolution stratigraphic framework is provided by biostratigraphy and organic carbon stable-isotope (δ13Corg) chemostratigraphy. A short-term (100 kyr) sea-level curve has been derived from high-resolution transgressive/regressive maxima / shore-proximity data established from basin-wide sediment geometries. The viability of geochemical and palynological parameters as potential sea-level proxies is tested against this independently derived sea-level record. Elemental chemostratigraphy is demonstrated to offer a reliable means of identifying medium- to long-term (0.4–2.4 Myr) sea-level trends. Manganese maxima are associated with periods of high sea level, and troughs with intervals of low sea level. Falling Mn contents accompany regression and rising values transgression. Major transgressive events associated with medium-term sea-level change are marked by sharp increases in Ti/Al ratios, but short-term (100 kyr) sea-level cycles are not consistently identified. Long-term δ13Corg variation and dinoflagellate cyst species richness are positively correlated and show similarities to the sea-level curve. Baseline trends have a cycle duration close to the 2.4 Myr long-eccentricity cycle. Dinocyst species richness closely follows short-term changes in sea level, with marked increases in dinocyst diversity coincident with most short-term flooding events. Periods of rapid sea-level rise caused an influx of a more diverse ‘outer shelf’ assemblage into the study area, together with the addition of shallower water species, some of which may have been transported into the central basin by hypopycnal flows. Changes in the proportion and abundance of peridinioid dinoflagellate cysts (principally Palaeohystrichophora infusorioides) were controlled principally by changing nutrient levels. Proximity proxies derived from geochemical and palynological data are not always consistent with the independent sea-level model. This exemplifies the need to understand all factors influencing elemental geochemical and palynological proxies before making simplistic sea level interpretations.
Highlights
Sea-level change represents a major driver of environmental change in the modern and ancient Earth system
The basin originated by the reactivation of a fault system in the Variscan basement of the Bohemian Massif, and combined features of an epeiric sea formed during global transgression with those of a tectonically active setting that contains probably the highest proportion of siliciclastics of all the European Cretaceous basins north of the Alps (Uličný et al, 2009)
Stratigraphic variation is dominated by three major constituents: SiO2, ranging from 14–63 wt%; Al2O3, ranging from 2–16 wt%; and CaCO3 ranging from 4–71 wt%
Summary
Sea-level change represents a major driver of environmental change in the modern and ancient Earth system. The Late Cretaceous was characterised by a long-term eustatic sea-level maximum in the earliest Turonian (~93 Ma, Fig. 1) that was one of the highest sea-level stands in the Phanerozoic (Hancock and Kauffman, 1979; Haq et al, 1987; Haq and Al-Qahtani, 2005; Haq, 2014 and references therein). Despite the long-term climate optimum, superimposed higherfrequency sea-level change during the Turonian occurred within Milankovitch timescales (Fig. 1) and, debatably, was caused by glacioeustacy (Miller et al, 2003, 2005, 2011; Moriya et al, 2007; Bornemann et al, 2008; Kominz et al, 2008; Ando et al, 2009; Floegel et al, 2011; Uličný et al, 2014). The Turonian, provides an excellent opportunity to evaluate interactions between sea-level change and a wide range of palaeoenvironmental proxies within the most extreme Late Cretaceous super-greenhouse. Olde et al / Palaeogeography, Palaeoclimatology, Palaeoecology 435 (2015) 222–243
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