The relationship of plant leaf δ13Cn-alkanes and salinity in coastal ecosystems applied to palaeobotany: Case study from the Cenomanian of the Bohemian Cretaceous Basin, Czechia

Abstract

Stable carbon isotopes in fossil leaf cuticles (n-alkanes, δ13Cn−alkanes) extracted from the sediment are widely used for palaeoenvironmental reconstructions. This approach relies on a series of assumptions (such as the plant-group-specific isotopic range, or the atmospheric CO2 being the major factor for the isotopic values) and leaves out the complexity of the carbon isotope fractionation within the plant through time, space, and different environments. The leaf cuticle is a unique archive of local environmental conditions, which has the potential to constrain individual plant habitat. To explore the applicability of the information gained from the δ13Cn−alkanes from fossil plants, a fossil coastal environment was studied.During the Cenomanian (100.5–93.9 Ma), the basal part of the Bohemian Cretaceous Basin, including locality Pecínov (Czechia), was formed as a result of sea transgression to the Palaeozoic Bohemian Massif triggered by the Alpine orogenesis. Coastal halophytic vegetation growing in this zone of prograding sea has no extant equivalent, and there are uncertainties associated with vegetation distribution or individual taxa habitat.Therefore, we examine the relationship between δ13Cn−alkanes extracted from individual leaves of coastal C3 plants (fossil and modern) and osmotic stress (salinity and drought) to reconstruct the individual plant habitat, thus, vegetation distribution in the coastal zone. We investigated modern mangrove and salt marsh vegetation in New Zealand and the United Kingdom, respectively, as well as transition zones (mangrove invading original salt marshes) for δ13C, soil moisture and salinity to build a calibration of the relationship, which is then applied to the Cenomanian fossil leaves from the locality of Pecínov.We found a positive correlation between water stress (caused by salinity or drought) and the δ13C values of n‐C25, n‐C27, n‐C29 and n‐C31 alkanes when combining all species from all modern localities. However, the absence of a strong correlation within individual species suggests a combination of several factors controlling the carbon isotopic composition. Nevertheless, the general response of δ13C to osmotic stress can be applied to compare the habitat of modern and fossil coastal plants. Thus, by using this relationship, we reconstructed the relative salinity and water stress of individual species of the Cenomanian plants by relating them to modern species-specific ranges of n‐C29 isotopic signatures.We showed that the δ13Cn−alkanes of fossil plant cuticles/leaves can be a valuable tool in the reconstruction of plant habitat and help us understand evolution of fossil environments in time and space. Additionally, our data further confirm that the δ13C acquired from plants should be used with caution when reconstructing global atmospheric CO2 because the osmotic stress can shift the δ13C in plants growing in terrestrial environments, particularly in coastal sea-influenced ecosystems.