Fossil Coral in the Amazon Basin Confirms Ancient Seaway During the Miocene

Deep beneath the Amazon rainforest, scientists have uncovered fossil coral and other marine remains that confirm parts of the basin were once covered by shallow ocean waters. Sediment cores extracted from Colombia and Brazil show that during the Miocene epoch, roughly 18 to 12 million years ago, marine waters advanced far inland from the Caribbean before retreating. The findings provide strong evidence that the Amazon Basin experienced repeated episodes of ocean flooding long before it became the world’s largest rainforest.

The most comprehensive study was published in Science Advances in 2017 by a team led by Carlos Jaramillo of the Smithsonian Tropical Research Institute. Researchers analyzed sediment cores drilled at sites including Saltarín in Colombia and 105 AM in Brazil. Within these cores, they identified marine microfossils, coastal pollen, shark teeth, mantis shrimp remains, and chemical signatures consistent with saltwater conditions.


“Our results indicate that shallow marine waters covered the region at least twice during the Miocene, but the events were short lived rather than a continuous full marine occupancy,” the authors wrote.

Drilling Into the Amazon’s Geological Record

The Amazon Basin today is dominated by freshwater rivers and dense rainforest, but its deep subsurface tells a different story. By drilling tens of meters below ground, scientists accessed sediment layers that preserve environmental conditions from millions of years ago.

These layers alternate between freshwater and marine indicators. In marine intervals, researchers found fossils of organisms that can only survive in saltwater. Shark teeth and mantis shrimp are among the clearest biological markers. Marine coral fragments and invertebrate remains provide additional confirmation that parts of the basin were once submerged.

The presence of marine pollen and specific chemical signatures in the sediments further supports the interpretation of shallow sea conditions. These markers cannot form in purely freshwater environments.

Tectonics and Rising Seas

Geologists attribute these marine incursions to a combination of tectonic activity and higher global sea levels during the Miocene. At the time, the Andes Mountains were actively rising. This uplift altered drainage patterns and caused parts of the Amazon Basin to subside.

When regional subsidence coincided with elevated global sea levels, ocean waters from the Caribbean were able to flood hundreds of kilometers inland. These incursions were temporary. As tectonic forces reshaped the landscape and sea levels fluctuated, marine waters retreated, leaving behind layers of sediment that recorded the transition.

The Miocene landscape of northern South America was therefore dynamic. Instead of a stable rainforest, the region shifted between marine embayments, wetlands, and freshwater systems over millions of years.

Implications for Amazon Evolution

The discovery of fossil coral and marine sediments deep in the basin has significant implications for understanding the evolution of the Amazon River system. Many researchers believe that these marine phases influenced how the basin eventually drained eastward into the Atlantic.

Marine incursions may have reshaped river networks, redistributed sediments, and affected early ecosystems. Over time, as the Andes continued to rise and marine connections closed, freshwater environments expanded and the modern Amazon River system took shape.

The findings also help explain patterns of biodiversity in the region. Shifts between marine and freshwater conditions would have created opportunities for species dispersal and adaptation. Transitional environments may have contributed to the extraordinary ecological complexity seen in the Amazon today.

Scientific Debate and Ongoing Research

While evidence for Miocene marine incursions is strong, researchers continue to study the extent and ecological impact of these events. Some scientists suggest that parts of the basin experienced mixed coastal and freshwater environments rather than full marine coverage at all times.

Christopher Dick of the University of Michigan has noted that sediment and fossil data clearly show marine influence, but questions remain about how widespread the shallow seas were and how long they persisted in different areas.

Despite these ongoing discussions, the broader scientific consensus has shifted. The Amazon Basin was not exclusively terrestrial throughout its history. Instead, it experienced episodes of ocean flooding that left behind unmistakable marine signatures.

A Changing View of the Amazon

Today, the Amazon Basin is defined by vast rivers, tropical rainforest, and unparalleled biodiversity. Yet beneath its soils lie fossil coral, marine sediments, and saltwater organisms that testify to a very different past.

The sediment cores analyzed by Jaramillo and colleagues provide direct geological evidence that shallow seas once occupied parts of the basin during the Miocene. These discoveries have reshaped scientific understanding of South America’s geological evolution and the processes that shaped one of Earth’s most important ecosystems.

The Amazon’s history is not a simple progression from dry land to rainforest. It includes marine episodes, tectonic shifts, and environmental transitions spanning millions of years. Fossil coral deep beneath the forest floor confirms that even the world’s largest tropical rainforest was once touched by the sea.