Modern-day coccolithophorids are known to be the most significant producers of dimethylsulphide (DMS) in the marine phytoplankton. Part of this DMS passes from the seawater into the atmosphere, where it oxidizes to form H2SO4 and falls as acid rain. During the Late Cretaceous coccolithophorids were superabundant, their remains now forming the Cretaceous Chalk.
The Late Cretaceous transgression gave rise to extensive shelf seas and progressively increased the areal extent of the coccolithophorid’s habitat. Assuming that coccolithophorids produced DMS during the Late Cretaceous, it is likely that acid rainfall was common and that the level of acidification of the atmosphere increased progressively, in parallel with the progress of the transgression. The increasing rate of extinctions documented by the Late Cretaceous fossil record may therefore be a reflection of an increasing level of acidification of the atmosphere during this period.