Diatoms are a major primary producer in the modern oceans and play a critical role in the marine silica cycle. Their rise to dominance is recognized as one of the largest shifts in Cenozoic marine ecosystems, but the timing of this transition is debated. Here, we use a diagenetic model to examine the effect of sedimentation rate and temperature on the burial efficiency of biogenic silica over the past 66 million years (i.e., the Cenozoic). We find that the changing preservation potential of siliceous microfossils during that time would have overprinted the primary signal of diatom and radiolarian abundance. We generate a taphonomic null hypothesis of the diatom fossil record by assuming a constant flux of diatoms to the sea floor and having diagenetic conditions driven by observed shifts in temperature and sedimentation rate. This null hypothesis produces a late Cenozoic (∼5 Ma to 20 Ma) increase in the relative abundance of fossilized diatoms that is comparable to current empirical records. This suggests that the observed increase in diatom abundance in the sedimentary record may be driven by changing preservation potential. A late Cenozoic rise in diatoms has been causally tied to the rise of grasslands and baleen whales and to declining atmospheric CO2 levels. Here we suggest that the similarity among these records primarily arises from a common driver—the cooling climate system—that drove enhanced diatom preservation as well as the rise of grasslands and whales, rather than a causal link among them.
Silicification is an important mode of fossil preservation but the extent to which silicified material represents an unbiased sampling of the total fossil assemblage within a given rock sample remains poorly quantified. Here, we use paired analyses of thin sections and acid-extracted silicified specimens from the same samples to examine the biases introduced during silicification of Lower Triassic Virgin Limestone carbonates preserved in the Muddy Mountains of southern Nevada. Bivalves dominate most thin sections in the point count data, but rarely silicify completely enough to be recognized in residue. Echinoderms and gastropods are less abundant in thin section but dominate the residues. The abundances of these groups in thin section and residue are only weakly correlated. These findings suggest that although silicification generally captures relative trends in proportional abundance of higher taxa among samples, the silicification process can be taxonomically biased. Given the biases that can occur during silicification, it should not be assumed that silicified collections present a pristine picture of taxonomic or paleoecologic composition. Petrographic analysis has the potential to illuminate the reliability of paleontological data based on silicified collections.
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