The research team, led by Andrea Grottoli of The Ohio State University together with colleagues from Sorbonne Université, Arizona State University’s Bermuda Institute of Ocean Sciences, and the Stazione Zoologica Anton Dohrn, set out to test a long-standing hypothesis: that environmental conditions in the Mediterranean are simply too variable or too limited for reef-building corals to re-establish. To do this, they compiled seawater temperature, salinity, pH, alkalinity, chlorophyll, and photosynthetically available radiation, or PAR, from 22 tropical and subtropical reef sites around the world and compared them with 10 Mediterranean sites. The reef sites were deliberately chosen to span the full range of environments where reef-building corals thrive today, from mangroves and seagrass beds to volcanic vents, tide pools, upwelling zones, and high-latitude reefs.

The historical backdrop is essential to understanding the mystery. Reef-building, symbiotic corals flourished across the Mediterranean region until the late Miocene, between roughly 11.65 and 5.33 million years ago, building reefs that included Acropora species whose relatives still live in the Caribbean today. Then the region’s fortunes collapsed in stages. Around 13 million years ago, the open marine seaway connecting the Mediterranean to Middle Eastern seas through the Tethys closed, cutting off the westward supply of tropical coral larvae. At the same time, tectonic drift pushed the Mediterranean northward out of the subtropics, cooling its waters. The final blow came with the Messinian Salinity Crisis about 5.95 million years ago, when the basin nearly dried out and eliminated virtually all remaining reef-building corals.

Two survivors linger as relics of that lost world. The endangered Cladocora caespitosa is the only potential reef-builder left in the basin, and it still forms substantial carbonate banks at the Columbretes Islands in the western Mediterranean and in Croatia’s Mljet National Park in the Adriatic, making it a living link to the Miocene reefs. Oculina patagonica persists at low densities throughout much of the sea. Both species tolerate seawater temperatures as low as 10 degrees Celsius, and O. patagonica has actually shifted into deeper, cooler waters as the surface has warmed. A single Indo-Pacific coral, Oulastrea crispata, was documented once in 2012, but none of these hardy stragglers has managed to build anything resembling a tropical reef.

The new analysis produced a strikingly split verdict. Using nonparametric multivariate statistics, including multidimensional scaling and analysis of similarity, the researchers found that the environmental conditions of the southern Mediterranean are statistically indistinguishable from those of high-latitude coral reef sites such as Bermuda and Japan. Even the carbonate chemistry works in the corals’ favor: the Mediterranean’s high alkalinity and pH keep seawater strongly supersaturated with aragonite, the calcium carbonate mineral corals use to build their skeletons, with saturation states of 3.0 to 3.8 in the western basin and 3.6 to 4.2 in the eastern basin. In an era when ocean acidification is eroding reef-building capacity elsewhere, the Mediterranean’s chemistry would actively encourage coral calcification. Chemistry, in other words, is not the culprit.

If the southern sea is habitable, why have no reef corals arrived? The team examined four possible introduction pathways. From the Atlantic through the Strait of Gibraltar, the prevailing currents are cool, nutrient-rich extensions of the North Atlantic Current, and notably, the subtropical Azores, which sit directly in their path, host no reef-building corals at all. The Canary Islands do have tropical corals but lie too far south to serve as a natural larval source. The Suez Canal, opened in 1869, offers the most direct route from the Red Sea, yet only two soft coral species have completed the crossing. The canal’s flow dynamics may be the bottleneck: north of the hypersaline Great Bitter Lake, which averages a salinity of 41, water flows southward in summer, precisely when Red Sea corals spawn in spring, pushing larvae away from the Mediterranean rather than toward it.

Intriguingly, the authors argue that the salinity barrier itself may not be insurmountable. Red Sea corals routinely live at salinities of 38 to 41 and have been found at 45; the coral Fungia granulosa tolerates up to 50 for nearly a month, and Acropora and Stylophora pistillata showed no significant physiological harm at 44 in a week-long experiment. Modeling suggests a Red Sea to Mediterranean transit takes 11 to 39 days, well within the 40-to-70-day larval competency documented for many Australian corals. The seasonal timing of canal flow, rather than the brine of the Bitter Lakes, appears to be the decisive obstacle, though hitchhiking on ship hulls, rafting on debris, or aquarium cast-offs remain possible routes for future introductions.

Even if larvae did arrive in the southeastern Mediterranean, they might find the real estate wanting. Coral larvae need clean, hard substrate to settle on, but the major southern Mediterranean currents sweep past the Nile outflow, dragging a sediment plume along the Israeli coast and into the Levantine basin. High sedimentation blankets suitable surfaces and smothers settling larvae, and archaeological evidence suggests extreme sedimentation buried Cladocora corals in Israel’s Haifa Bay more than 2,000 years ago. Oceanographic fronts within the basin would then trap any newly established populations in the Levantine Sea, preventing them from spreading westward.

The northern Mediterranean faces a more fundamental constraint: winter. The analysis showed that northern sites are distinguished from all tropical and subtropical reef sites by cooler mean monthly maximum and minimum temperatures and lower surface PAR, the light that fuels the photosynthetic symbionts living inside coral tissue. Many northern sites sit at or above 40 degrees north latitude, and geological research has shown that even during past warm climates, reduced winter light intensity and day length stopped reef corals from expanding poleward beyond that line, regardless of how warm the water was. Sustained cool winters, possibly combined with winter light levels at the low end of what high-latitude reefs experience, make large tropical-style reef development in the north extremely unlikely, even under continued warming.

The net picture is one of a sea that is almost, but not quite, ready for reefs. In the south, the water chemistry, temperature, and light are all suitable, yet the delivery of coral larvae is throttled by canal hydrodynamics and their settlement is compromised by Nile sediment. In the north, the environment itself remains the barrier. The authors conclude that reef-building corals may yet establish populations in parts of the southern Mediterranean over time, whether through the Suez Canal, shipping, or the aquarium trade, but that dim winter light and long cool seasons will probably prevent the northern Mediterranean from tropicalizing with coral reefs in the near future. For now, the Mediterranean’s only reef-builder remains a small, endangered relic of a vanished Miocene world, waiting for conditions, and arrivals, that have not yet aligned.

Subject of Research: Environmental and historical factors limiting reef-building coral establishment in the Mediterranean Sea

Article Title: Why are coral reefs not found in the Mediterranean today?

Article References: Grottoli, A. G., Carbonne, C., Teixidó, N., Comeau, S., Smith, A., & Gattuso, J.-P. (2026). Why are coral reefs not found in the Mediterranean today?. Coral Reefs. https://doi.org/10.1007/s00338-026-02971-3

Image Credits: AI Generated

DOI: 10.1007/s00338-026-02971-3

Keywords: coral reefs, Mediterranean Sea, reef-building corals, Messinian Salinity Crisis, Suez Canal, photosynthetically available radiation, aragonite saturation, Cladocora caespitosa, species introductions, sedimentation, ocean warming, larval dispersal

The interdisciplinary approach employed by the KAUST team combined advanced seismic imaging, meticulous microfossil analyses, and precise geochemical dating methods to construct a detailed chronology of the desiccation and subsequent reflooding. Their data reveal a rapid transition, accomplished in less than 100,000 years—a geological instant—during which the Red Sea transformed from a shallow marine extension connected to the Mediterranean into an arid, hypersaline basin. This transformation culminated in the complete evaporation of seawater and the precipitation of extensive salt and gypsum layers, marking one of the most extreme environmental states recorded in Earth’s recent geological past.

Prior to drying out, the Red Sea maintained a connection to the Mediterranean Sea via a shallow sill to its north. This delicate hydrological linkage was severed, likely due to a combination of tectonic uplift and climatic aridity, which severely restricted water exchange and amplified evaporative loss. As the basin progressively concentrated salts, marine ecosystems collapsed, and the environment transitioned into a salt-encrusted desert. Simultaneously, at its southern end near the Hanish Islands, the Red Sea was isolated from the Indian Ocean by a volcanic ridge barrier that prevented direct inflow of seawater.

The dramatic finale of this desiccation phase was the sudden breaching of the volcanic dam separating the Red Sea from the Indian Ocean. Approximately 6.2 million years ago, a colossal marine flood surged through this barrier, carving a massive 320-kilometer-long submarine canyon that remains etched in the seafloor today. This inundation event rapidly restored marine conditions, flooding the basin and re-establishing its connection to global ocean systems within a remarkably brief geological timeframe. The scale and speed of this natural flood event are comparable to, yet distinct from, the later and more widely known Zanclean flood responsible for refilling the Mediterranean Sea nearly a million years afterwards.

From a geological standpoint, the Red Sea is a young ocean basin born from the divergent tectonic activity separating the Arabian and African plates beginning around 30 million years ago. Initially a narrow rift valley, this depression evolved from lacustrine environments into a marine gulf through episodic flooding events, with the Mediterranean connection established roughly 23 million years ago. This tectonic and hydrological evolution set the stage for frequent environmental fluctuations, including episodic hypersalinity and biotic stress that eventually led to the remarkable Late Miocene desiccation cycle extensively documented now by the KAUST team.

Evaporative concentration over millions of years created ultra-saline conditions that decimated native marine biota, as evidenced by the disappearance of fossil reefs along the northern Red Sea coastline. The onset of desiccation stimulated the deposition of thick evaporite sequences—salt and gypsum—that filled large portions of the basin. These sediments serve as critical archives delineating past ocean chemistry and climatic oscillations, offering a window into the environmental stresses experienced by marine ecosystems during the Messinian Salinity Crisis.

The renewed marine inundation following the Indian Ocean flood marked a pivotal reset in the Red Sea’s ecological trajectory. The sudden inflow of seawater restored salinity balance, oxygenation, and nutrient exchange, which facilitated the resurgence of coral reef ecosystems and other marine fauna. This rebirth set a foundation for the rich biodiversity observed in the modern Red Sea, validating the basin as a living natural laboratory for studying oceanographic and evolutionary responses to extreme environmental perturbations.

This study also contributes substantially to our understanding of how ocean basins form, expand, and evolve through the coupling of tectonic processes and extreme climatic events. The Red Sea’s unique geological narrative exemplifies the dynamic nature of ocean gateways and the interplay between crustal movements and global oceanographic circulation. Moreover, identifying the timing and impact of such a mega-flood enhances reconstructions of paleoclimate and paleoceanography during critical intervals of Earth history.

The implications of this research extend beyond regional geology, linking the Red Sea to broader patterns of Messinian environmental upheaval and ocean circulation changes. By detailing one of the most dramatic marine desiccation-reflooding cycles yet documented, the KAUST scientists have provided a crucial reference point for geoscientists investigating ancient flood events, sea level fluctuations, and their influence on sedimentation and marine biodiversity.

The novel submarine canyon carved by the flood demonstrates the immense erosive power of oceanic waters released under extreme conditions. Its preservation on the seafloor affords researchers a rare opportunity to analyze the geomorphological and sedimentological consequences of such high-magnitude hydrological events, which are analogues for other ancient and possibly future marine catastrophes under climate change scenarios.

Lead author, Dr. Tihana Pensa, emphasizes how “the Red Sea basin chronicles one of Earth’s most extreme environmental transformations—from a desiccated salt desert to a vibrant marine ecosystem—reshaping our understanding of ocean basin development and resilience in the face of climate-driven crises.” The research not only elevates the scientific profile of the Red Sea but also reinforces KAUST’s prominence in pioneering oceanographic and geological investigations in the region.

In conclusion, this seminal work on the Red Sea’s Messinian history underscores the intricate connections between plate tectonics, climatic shifts, oceanographic barriers, and ecosystem survival during Earth’s dynamic past. The insights gleaned pave the way for further multidisciplinary studies aimed at unraveling the complexities inherent in marine basin evolution and the environmental forces that have shaped our planet’s oceans through deep time.

Subject of Research: Not applicable
Article Title: Desiccation of the Red Sea basin at the start of the Messinian salinity crisis was followed by major erosion and reflooding from the Indian Ocean
News Publication Date: 9-Aug-2025
Web References: http://dx.doi.org/10.1038/s43247-025-02642-1
Keywords: Marine geology, Geologic history, Floods

At the heart of this investigation is the Red Sea basin, a unique geological feature shaped by tectonic activity and climate variations over millions of years. During the late Miocene epoch, approximately 5.96 to 5.33 million years ago, the earth entered a phase of considerable climatic fluctuation. Early in this period, significant evaporation processes led to the dramatic desiccation of the Red Sea. The researchers meticulously analyzed sediment core samples, revealing a profound decrease in water levels that ultimately transformed the Red Sea into a series of isolated basins.

Understanding the causes behind this desiccation is crucial for contextualizing the broader implications of the Messinian Salinity Crisis. The study illustrates that regional aridification, combined with the tectonic activity of the African and Arabian plates, led to reduced inflow from tributaries and rivers that typically replenished the basin. The arid climate in surrounding regions acted as a catalyst for evaporation, triggering an ecological and geological cascading effect that would unfold over millennia.

Remarkably, this investigation does not merely describe a decrease in water levels; it also delves into the intricate interactions between terrestrial and marine ecosystems during this tumultuous time. Prior studies have established that a considerable biodiversity existed within the Red Sea in the early Miocene, with numerous marine species thriving in a rich and diverse environment. The desiccation, however, irreversibly altered these habitats, leading to ecosystem stress that would ultimately drive many species to extinction.

Following the desiccation phase, the narrative of the Red Sea takes another dramatic turn as the researchers reveal significant erosion processes. With the basin’s water levels receding, sediment accumulation on the basin floor became more pronounced, setting the stage for inevitable erosion. Evidence found in the sediment cores indicates that ancient river systems, now buried under layers of silt, contributed to this erosion, and the sediment record illustrates how the landscape transformed dramatically.

As the Red Sea basin continued to evolve, the study documents a transformative phase characterized by reflooding, driven by the re-establishment of circulation patterns and connections with the Indian Ocean. This inundation event is critical to understanding how the Red Sea basin reconnected with broader oceanic systems after prolonged isolation. The researchers detail how the Mediterranean Sea’s connection to the Atlantic Ocean was not only restored but was also revitalized through a series of complex oceanographic changes.

The implications of these hydrological changes extend beyond the geological realm. The study emphasizes how alterations in salinity and temperature during the Messinian Salinity Crisis significantly impacted marine species diversity in adjacent oceanic environments. Fisheries, ecosystems, and oceanic currents became interconnected elements affected by changes originating from the Red Sea basin. The impact, deeply felt in ecological networks, underscores the interconnectedness of marine environments and the potential for localized changes to have global repercussions.

Furthermore, this research contributes to our understanding of climate resilience and adaptation. Examining how ecosystems responded to rapid changes provides critical insights into contemporary environmental challenges exacerbated by climate change. With modern marine environments facing similar pressures due to anthropogenic influences, there are lessons to be gleaned from the ancient past concerning species resilience and adaptation.

Additionally, the authors explore sedimentary records as windows into ancient climates, offering insights into past river flows, oceanic currents, and climatic conditions. The sediment layers embedded in the Red Sea basin reveal a history of sedimentation that helps reconstruct ancient environmental conditions and can serve as valuable analogs for current climate scenarios. By piecing together the geological puzzle, researchers can better predict how modern ecosystems might respond to climate variability.

The research findings hold significant implications for future geological inquiries. By establishing a clearer timeline of events during the Messinian Salinity Crisis, the study creates a framework for researchers aiming to further explore ocean basin evolution and the interplay between tectonic forces and climate. This work is a critical stepping-stone for initiatives aiming to elucidate the Earth’s complex geological history and its intricate interactions with climatic conditions.

In summary, Pensa, Huertas, and Afifi’s research provides a substantial enhancement to our grasp of the Messinian Salinity Crisis, portraying it as a dynamic sequence of desiccation, erosion, and eventual reflooding of the Red Sea basin. The investigative approach employed, with its fusion of paleoclimate data and sediment analysis, offers a profound case study in the enduring impacts of geological and climatic processes. As we navigate contemporary ecological challenges, the lessons from such ancient crises resonate more than ever, illuminating paths toward future resilience.

This groundbreaking work illuminates not only the geological processes at play but also encompasses the ecological ramifications during a time when life on Earth was profoundly shaped by changes in environment and climate. The interplay between the terrestrial and marine realms during the late Miocene highlights the need for holistic approaches to understanding our planet’s complex evolutionary history.

The significance of this research extends beyond the boundaries of academic understanding. It serves as a poignant reminder of Earth’s enduring capacity for change, adaptation, and resilience. As humanity grapples with contemporary challenges, reflecting on the responses of ancient ecosystems to drastic changes might offer pathways to navigate our current and emerging realities. The implications stretch into the future, urging us to learn from the past as we strive for sustainability and harmony with nature.

Subject of Research: Desiccation of the Red Sea basin at the start of the Messinian salinity crisis, erosion, and reflooding from the Indian Ocean.

Article Title: Desiccation of the Red Sea basin at the start of the Messinian salinity crisis was followed by major erosion and reflooding from the Indian Ocean.

Article References:

Pensa, T., Huertas, A.D. & Afifi, A.M. Desiccation of the Red Sea basin at the start of the Messinian salinity crisis was followed by major erosion and reflooding from the Indian Ocean. Commun Earth Environ 6, 649 (2025). https://doi.org/10.1038/s43247-025-02642-1

Image Credits: AI Generated

DOI: 10.1038/s43247-025-02642-1

Keywords: Messinian Salinity Crisis, Red Sea, desiccation, erosion, reflooding, marine ecosystems, ancient climates, climate resilience.

During the Messinian Salinity Crisis, which lasted from 5.97 to 5.33 million years ago, the Mediterranean became effectively separated from the Atlantic Ocean. As a result, the region experienced drastic evaporation rates that led to significant deposits of salt, transforming vast areas into arid salt flats. This research not only highlights the geological transformations that occurred during this period but also compels a reevaluation of the sequence and mechanics of the sea’s refilling.

Dr. Aaron Micallef, the principal investigator of the study and a researcher at the Monterey Bay Aquarium Research Institute in California, expressed the monumental scale of the Zanclean Megaflood. According to him, the flow rates generated by this event were unparalleled in Earth’s history, dwarfing those of any previously recorded floods. The study suggests that the evidence collected from geological features surrounding Southeast Sicily provides the most robust proof available regarding this extraordinary natural event.

The discovery integrates various geological findings with advanced geophysical data and numerical modeling, resulting in what researchers deem the most comprehensive analysis of the Zanclean Megaflood to date. The study examined over 300 asymmetric, streamlined ridges located in a corridor across the Sicily Sill, which historically separated the eastern and western Mediterranean basins. These geological formations exhibit signs of erosion consistent with large-scale, turbulent water flow, predominantly directed northeastward.

Professor Paul Carling, who is an Emeritus professor in the School of Geography and Environmental Science at the University of Southampton and co-author of the research, emphasized the formidable strength of the megaflood. He noted that the morphology of the ridges is distinct evidence of the sweeping power the event had, significantly altering the landscape and leaving enduring records in the geological strata.

Sampling of these ridges revealed a layer of rocky debris, which comprised materials that had been swiftly deposited under high-energy conditions. This unique layer, positioned at the boundary between the Messinian and Zanclean epochs, aligns with the timeline believed to be associated with the megaflood. Thus, it serves as a crucial reference point for dating this historical geological phenomenon.

Furthermore, the research revealed astonishing details through seismic reflection data, a technique analogous to ultrasound for geological mapping. This method allowed scientists to visualize layers of sediment and rock hidden beneath the surface, unveiling a striking ‘W-shaped channel’ on the continental shelf east of the Sicily Sill. This channel is significant because it forms a conduit that connects the ridges to a deep underwater valley known as the Noto Canyon, located in the eastern Mediterranean.

The geometry and position of this channel indicate that it functioned as a huge funnel, directing the torrent of water from the megaflood as it cascaded over the Sicily Sill into the eastern Mediterranean. Researchers developed sophisticated computer models to simulate the megaflood’s operational dynamics, revealing that the massive influx of water not only increased in speed, reaching up to 32 meters per second, but also shifted direction as it advanced, eroding geological features and transporting vast quantities of material across extensive distances.

These revelations are monumental, offering insights into a pivotal moment in Earth’s geological history. They illustrate how landforms may persist for millions of years, contributing to an understanding of past environmental conditions and providing clues for future geological research along the Mediterranean margins.

The collaborative study was made possible through the support from the National Geographic Society, the Deutsche Forschungsgemeinschaft, and the David and Lucile Packard Foundation. As the climatic conditions of the Earth continue to evolve, further exploration into these geological phenomena may yield crucial findings regarding the planet’s history and its future trajectory.

As scientists delve deeper into understanding these ancient cataclysmic events, they invite ongoing research and exploration into the Mediterranean region’s complex geological tapestry, underscoring that our planet’s past holds essential lessons for its future.

Subject of Research: Zanclean Megaflood and its impact on the Mediterranean Sea
Article Title: Land-to-sea indicators of the Zanclean megaflood
News Publication Date: 28-Dec-2024
Web References: University of Southampton
References: DOI: 10.1038/s43247-024-01972-w
Image Credits: University of Southampton Media Resources

Keywords

Geologic history, Floods, Erosion, Seawater, Salts, Computer modeling.