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Kerogen extracted from rock samples for palynological studies frequently requires oxidative treatment in order to liberate palynomorphs from amorphous organic matter (AOM). Treatment with nitric acid (70% HNO3) is sometimes adequate though many Mesozoic organic-rich samples require more severe oxidation to break up clumped AOM. A widely used method involves the repeated oxidation of a preparation in Schulze’s solution (70% HNO3 supersaturated with KClO3), followed each time by rinsing with 2% potassium hydroxide solution (KOH) (Jones, 1994). The mid-Cretaceous Greenhorn Formation, exposed near Pueblo, Colorado, USA, is composed of interbedded bioturbated limestones and laminated calcareous shales (Cobban & Scott, 1972). In the middle part of the Bridge Creek Limestone Member, kerogen extracted from these two lithologies is markedly different. Diverse, predominantly gonyaulacineaen dinocyst assemblages are found in kerogen from the bioturbated limestone samples. These either require five minutes of nitric acid treatment or no oxidation at all. Most of the AOM is finely disseminated and passes through a standard 10 μm sieve mesh. Dinocysts in the >10μm kerogen fraction from the laminated calcareous shale samples are outnumbered by clumped AOM at a ratio of between 10 and 1000:1. Prior to oxidation, gonyaulacineaen and peridiniineaen dinocysts are seen to be present. However, if Schulze’s solution and KOH are applied until most of the clumped AOM has disintegrated (a process which takes from 1 to 48 hours), Gonyaulacineae are rare (<5%) in resulting dinocyst assemblages (which are dominated by well-preserved Peridiniineae). . . .

A note of caution concerning the application of quantitative palynological data from oxidized preparations