The secret ingredient of carbonate mud

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.

Fig. 1.

Carbonate sediment on the Great Bahama Bank consists of a variety of components that include skeletal bioclasts, algae, and bank-top whiting mud. Geyman et al. (3), however, demonstrate that most of the carbonate mud in the Bahamas likely reflects a newly recognized precipitation source.

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 C- or 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 bank-top 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 satellite-derived sea surface temperature estimates at the location and time of more than 4,000 identified whiting events, combined with the temperature of water delivery to the Bahama Banks from the Florida Straits. Geyman et al. (3) estimate a window of temperatures (between 26.3 °C and 27.9 °C) for bank-top whiting precipitation. To test the extent to which such bank-top whiting events are represented in the carbonate mud, they use clumped isotope thermometry (17). Clumped isotopes measure the abundance of the relatively rare ¹³C and ¹⁸O isotopes in the carbonate relative to a random distribution of isotopes to derive the seawater temperature at the time of carbonate formation. Whereas the clumped isotope composition of ooids (the most common nonskeletal component of bank-top environments) indicates formation temperatures of 27.6 °C, well within the estimates for bank-top precipitation, the carbonate mud samples provide a temperature estimate of 23.6 °C, far lower than that expected for bank-top precipitation during whiting events.

The conclusion that carbonate mud precipitated from waters colder than that represented by bank-top environments and is therefore unlikely to be supplied by whiting events is an important breakthrough, although perhaps not surprising when observed within the broader picture of Bahamian carbonate sedimentation. Thick accumulations of carbonate mud have long been observed in deep (and colder) environments surrounding the carbonate bank (10), often with no clear connectivity to regions where whitings occur. Furthermore, some of these muds appear to have accumulated during the Last Glacial Maximum, at which time the Great Bahama Bank would have been subaerially exposed and unable to support bank-top precipitation. Clearly, there is still something missing from our recipe. But Geyman et al. (3) recognize that the inconsistency in formation temperatures, combined with the spatial and temporal distribution of mud deposits, provides key constraints that suggest that the missing ingredient must be a fundamental parameter of the water bodies in the region of the Great Bahama Banks.

The chemistry of carbonate formation in marine systems is complex but relatively well understood (18). Atmospheric CO₂ dissolves in marine waters, where it reacts to form a series of interlocking equilibria involving aqueous carbon dioxide [CO₂], bicarbonate [HCO₃⁻] and carbonate ions [CO₃²⁻], and associated hydrogen ions [H⁺] which define the pH and the relative proportion of carbonate species. Two additional remaining parameters that define the carbonate system are the collective sum of all the aqueous carbon species (TCO₂) and the total alkalinity (TA), which is a measure of the charge difference between the ocean’s major conservative cations and anions and which provides a measure of the ocean’s ability to produce precipitated carbonate.

The Bahama banks show both remarkably high alkalinity and a clear pattern of alkalinity depletion from the shelf edge to the bank interior (19). Such depletion of alkalinity suggests a scenario wherein carbonate precipitation initiates offshore of the carbonate bank in alkaline-rich deep waters, and continues as waters reach the carbonate bank, progressively depleting the alkalinity. In this scenario, the bank-top regions that are the locus of whiting events would be one of the least favorable places to precipitate carbonate, suggesting that precipitation associated with whitings in these bank-top environments (cf. ref. 20) may, in fact, represent transient alkalinity events.

Alkaline-rich deep waters offshore from the Bahamian banks are certainly capable of precipitating carbonate; aragonite is oversaturated even at depths of >600 m. However, even elevated alkalinity and saturation state are insufficient to drive precipitation in deep waters. This is true through most of the surface oceans, which are oversaturated with respect to both calcite and aragonite. Traditional models for nucleation and growth record a rather steep energy barrier to nucleation, driven by the energy requirements to form an interfacial boundary between the fluid and the nucleating phase. This energy barrier can be overcome, however, either by further increasing saturation state or by having a readily available supply of suitable nucleation sites. Geyman et al. (3) suggest both these factors are in play, proposing a scenario in which upwelling of alkaline-rich deep waters along the edge of the Bahama banks results in a combination of depressurization and warming, both of which decrease the solubility of CO₂, locally reducing TCO₂ and increasing the saturation state of carbonate. This increased saturation state, in combination with wave-driven transport providing seed crystals for nucleation, drives both progressive precipitation and associated alkalinity depletion.

To further explore whether alkalinity is the secret ingredient in the production of Bahamian carbonate mud, Geyman et al. (3) then look more broadly at the potential sources of enhanced alkalinity. Ocean circulation models suggest that export to the Atlantic of evaporative enriched Mediterranean waters may be the primary source of Bahamian alkalinity. A Mediterranean alkalinity source is supported by evidence of a 3-million-year period in which carbonate production and accumulation were dramatically reduced along the margins of the Great Bahama Bank, concurrent with the well-known Messinian salinity crisis, at which time the Mediterranean was tectonically isolated from the Atlantic (21), effectively shutting down the alkalinity source.

For the connoisseur, there is much to enjoy in this article. In their search for the recipe for carbonate mud, Geyman et al. (3) offer a depth and breadth of the datasets that delivers a rich layering of flavors, and a systematic progression of arguments that results in an elegant presentation. But perhaps most satisfying is that Geyman et al. (3) allow the complexity of the carbonate system to remain at the forefront. They identify a recipe for carbonate mud, emphasize the potential role of the secret ingredient of alkalinity, and provide a hydrodynamic mechanism to bring the recipe together, but they also leave open the possibility that, in different places, and at different times in Earth’s history, these ingredients might come together by distinctly different mechanisms.