Carbonate mudstones are important petroleum source rocks that also serve as snapshots of ancient seawater chemistry. Yet the origins of the carbonate mud (defined as particles less than 62.5 micrometers in diameter) that comprises the mudstones remain unclear.
The current science identifies two physical mechanisms behind the formation of carbonate mud: direct precipitation in the water column and the accumulation of mud-sized pieces of algal skeletons. However, neither process jives with current geochemical observations. For instance, recent work in the Bahamas showed that the current paradigm requires favorable water chemistry and specific circulation patterns to hold across the bulk of Earth’s history. Additionally, both mechanisms prove problematic when looking back through the geologic record: Scientists struggle to reconcile the rates and mechanisms of mud production in modern settings with the amount of mud in ancient carbonate rocks.
In a recent study, Trower et al. proposed that the mechanical abrasion of carbonate sand may be a third possible means of carbonate mud production. The breakdown of carbonate sand as a source of mud was initially introduced in a research paper published in 1879, but the hypothesis remained untested until now.
The research team evaluated their hypothesis using an adapted bedrock erosion model and physical experiments with a wet abrasion mill. Each approach was designed to predict the rate of mud production that resulted from sand-sized particles bashing against each other in a marine environment. The mill experiments assessed mud production as a function of sand grain type, grain size, and bed shear velocity.
When compared to observations from modern carbonate platforms and ancient carbonate deposits, the results suggest that colliding sand particles produce carbonate mud at geologically significant rates and may account for a considerable proportion of existing mud. Furthermore, the mill experiments produced mud particles that were indistinguishable from mud collected at field sites in the Caribbean. The erosion model indicates that abrading sand produces carbonate mud significantly faster than algal or precipitative mud production. The modeled results also provided insight into the role of hurricanes and weather events in mud production rates.
The conclusion that sand abrasion has been a source of carbonate mud throughout Earth’s history affects both past and future research. The notion of carbonate mud as a snapshot of seawater chemistry likely needs to be reevaluated because mud produced by abrasion could differ geochemically from mud produced by algae or precipitation from seawater. As a result, geochemical records from carbonate mudstones may contain previously unknown biases. (Geophysical Research Letters, https://doi.org/10.1029/2018GL081620, 2019)
—Aaron Sidder, Freelance Writer