[1] Among the more durable puzzles of sedimentary geology is the so-called dolomite problem: the mineral dolomite, a calcium-magnesium carbonate, occurs abundantly in carbonate rocks of Precambrian and early Paleozoic age, yet it precipitates only rarely from modern seawater, even though the contemporary oceans are chemically supersaturated with respect to it. [2] The paradox is essentially kinetic rather than thermodynamic: although dolomite ought, on energetic grounds, to form readily, the strong hydration shell surrounding dissolved magnesium ions resists the dehydration necessary for those ions to be incorporated into a growing crystal lattice. [3] For much of the twentieth century, geologists therefore inferred that most ancient dolomite was not primary—not precipitated directly from seawater—but secondary, produced when magnesium-bearing fluids percolated through preexisting limestone and replaced its calcium, a process termed dolomitization.
[4] This replacement model accounted neatly for many dolomite bodies, particularly those bearing textural evidence of having once been limestone, but it struggled to explain the sheer volume of dolomite in the oldest carbonate successions, where no plausible reservoir of magnesium-rich fluid could be identified. [5] An alternative tradition held that at least some dolomite must be primary, and that the kinetic barrier had somehow been lowered in the ancient oceans, allowing magnesium to enter a growing lattice directly rather than only by later substitution. [6] Laboratory work over the past several decades has lent this view unexpected support: cultures of certain sulfate-reducing microorganisms have been shown to promote dolomite precipitation at ordinary temperatures, apparently because their metabolism removes sulfate, itself an inhibitor of dolomite nucleation, and because their cell surfaces furnish templates that ease the shedding of magnesium's hydration shell.
[7] These findings have not so much resolved the dolomite problem as reframed it. [8] The decline in dolomite abundance from the Precambrian to the present, once read as evidence that ancient conditions favored a single special mechanism, now appears to track the long-term rise in oceanic sulfate concentration, which would have progressively suppressed the microbial pathway. [9] Most researchers accordingly no longer seek one dominant explanation; they concede that dolomite has formed by several routes—microbial mediation, direct precipitation under particular chemistries, and later replacement—whose relative importance has shifted with the evolving composition of seawater. [10] What began as a search for a missing mechanism has become an inquiry into how the ocean's chemistry has changed through geologic time.
