LAUSR

Carbonate mud, despite a rather mundane definition as the fraction of detrital carbonate sediment that is <63 m in size, is oversized in its importance to the marine system. The deposition of carbonate mud in the oceans serves both as a major sink in the global carbon cycle (1) and as a primary archive of geochemical data of Earth’s geologic past (2). Yet, despite this importance, the origin of carbonate mud remains one of the quintessential questions in carbonate sedimentology. In PNAS, Geyman et al. (3) systematically reconstruct the recipe for carbonate mud, and, by doing so, highlight what may be the secret ingredient of carbonate formation through time and space. To search for the recipe for carbonate mud, Geyman et al. (3) traveled to the Great Bahama Bank (Fig. 1), an isolated marine carbonate platform that has been the center of investigation of carbonate production since the early the 1900s (4). Over the last century, carbonate mud has been hypothesized to originate from a wide variety of processes, including the erosion of ooids, intraclasts, and skeletal bioclasts (5, 6); the degradation of calcified marine algae and marine seagrasses (7, 8); the abiotic precipitation of aragonite during bank-top whiting events (9, 10); and even the release of calcite crystallites from the gut tract of teleost fish (11). Although it is likely that all of these processes contribute, to some extent, to the formation of carbonate mud, the individual explanations remain vaguely unsatisfactory. Not only does carbonate mud often contain a high proportion (up to 40%) of indecipherable materials (12), but the recrystallization of fine-grained components can obscure the primary phase as less stable aragonite and high-magnesium carbonate phases recrystallize to low-magnesium calcite. Furthermore, an emphasis on these modern environments permits only limited translation of these hypotheses to the rest of Earth’s history, much of which lacked skeletal organisms. Geyman et al. (3), however, provide a master class in the scientific reconstruction of a complex system. To begin the process of reconstructing the recipe for carbonate mud, Geyman et al. (3) collected bulk sediment samples from which they could both obtain a mud-sized fraction and isolate the individual macroscopic components (ooids, calcifying algae, and the skeletal elements of foraminifera, bivalves, gastropods, and coral) that have been hypothesized to play a role in the formation of carbonate mud. Measuring a suite of parameters in each of these isolates—including C and O isotopes, and a suite of major, minor, and trace elements— provides the geochemical data necessary to assess sediment composition. Using a series of two-component mixing models, Geyman et al. (3) readily demonstrate that the carbonate mud is both isotopically and elementally distinct from the hypothesized component suite. To explore whether these ingredients are even capable of providing a recipe for carbonate mud, Geyman et al. (3) create a series of probabilistic mixing models in which they solve for the combination of end-members that best reproduce the isotopic and elemental composition of the carbonate mud. This exploration results in a model that consists of 77% aragonitic ooids and 22% calcitic foraminifera, with essentially no contribution from the remaining skeletal components. Yet, whereas this model provides a good fit to the major and minor element (Ca, Mg, and Sr) composition of carbonate mud, it fails to reproduce either the Cor O-isotope composition or trace element composition of the carbonate mud. Clearly, there is a missing ingredient. To decipher what this missing ingredient might be, Geyman et al. (3) apply an additional series of probabilistic models that explore the inverse model of carbonate mud deriving from a two-phase mixture between foraminiferal calcite (the only abundant source of calcite on the Bahama Banks) and an unknown aragonitic end member. As with the previous component-driven model, the inverse model predicts a carbonate mud that is composed of ∼77% aragonite and 22% calcite, but which requires a compositional end-member of aragonite that is chemically distinct from Bahamian ooids in order to reproduce the isotopic and elemental compositions of the carbonate mud. Intuitively, this result is not particularly surprising. Even some of the earliest observations using Scanning Electron Microscopy (SEM) suggested that there must be a nonskeletal source of aragonite (13) within carbonate mud. Therefore, at first glance, this model appears to support carbonate mud forming by direct precipitation of aragonite from the water column, such as during whiting events, during which banktop waters become milky white in response to an elevated load of precipitated (9, 14) or possibly resuspended (15, 16) carbonate sediment. But knowledge of the basic components of a recipe is often insufficient to reproduce a great meal. Rather than ending the analysis here, Geyman et al. (3) utilize the richness of data available for the region to further explore the origin of this aragonite component. Specifically, they note that whitings do not occur uniformly across the Bahama Banks, but instead are restricted to a narrow region of the shallow bank west of Andros Island. In an elegant example of scientific thought, Geyman et al. (3) utilize satellitederived sea surface temperature estimates at the location