Question

The discovery of massive evaporite deposits beneath the Mediterranean Sea floor in the 1970s forced geologists to formulate the Messinian Salinity Crisis (MSC) hypothesis. This theory posits that approximately 5.96 million years ago, tectonic uplift closed the marine corridors connecting the Mediterranean to the Atlantic Ocean, leading to profound environmental transformations.

Two competing frameworks emerged to explain the stratigraphy of the resulting salt formations: the “deep-basin, deep-water” model and the “deep-basin, shallow-water” model. The deep-water model argues that the evaporites precipitated continuously from a deep, hypersaline body of water. Its proponents emphasize that the Mediterranean’s sheer volume of salt—over a kilometer thick in some regions—necessitates a steady, albeit restricted, inflow of Atlantic water, because the complete evaporation of a single Mediterranean volume would yield an evaporite layer merely 20 meters thick.

Conversely, the shallow-water model suggests that the Mediterranean evaporated almost entirely, reducing the sea to a series of shallow, hyper-saline lakes situated kilometers below global sea level. Proponents of this model point to the discovery of stromatolites—fossilized microbial mats that require sunlight for photosynthesis—intercalated directly within the evaporite layers in the deepest regions of the Mediterranean basin. Because stromatolites cannot form in the aphotic zone (deep water where sunlight does not penetrate), their presence suggests the basin floor was covered by remarkably shallow water during deposition. However, to account for the massive thickness of the salt deposits, advocates of the shallow-water model must also conclude that the tectonic dam at the Strait of Gibraltar was not permanently impermeable. Instead, the basin must have experienced repeated cycles of partial refilling from the Atlantic followed by rapid desiccation.

Based on the passage, which of the following can most reasonably be inferred about the stromatolites discovered within the Mediterranean evaporite layers?