Reconstructing Paleo-ecosystems in Mesopotamia: Evidence from fauna in southern Iraqi sediments

To understand the history of a landscape, geologists often look at the layers of earth beneath our feet, much like reading the pages of a book written in stone and sand. These layers, known as sediments, accumulate over thousands of years, trapping clues about the climate, the water levels, and the life that once existed in a specific spot. In the region of southern Iraq, where the Tigris and Euphrates rivers meet the sea, the ground holds a complex record of how the coastline has shifted back and forth. This area, part of an ancient basin that once connected to a vast ocean, has seen its environment change dramatically from dry land to marshy wetlands and back to the sea. By studying the tiny shells and remains of animals preserved in these deep layers, scientists can reconstruct what the environment looked like at different times in the past, revealing how rising and falling sea levels have shaped the land we see today.

A team of researchers from the University of Basrah recently turned their attention to a strategic stretch of water known as Khor Abdullah, located between Iraq and Kuwait. This area is currently being developed with major ports, but beneath the modern construction lies a hidden archive of the past. The team drilled two deep holes, reaching down forty meters into the seabed, to extract long cylinders of sediment. Their goal was to examine the tiny animals living in these layers to piece together the story of the ancient ecosystems. They were not looking for large dinosaurs or massive fossils, but rather for the microscopic remains of creatures like foraminifera, which are single-celled organisms with tiny shells, and ostracods, which are small crustaceans resembling miniature clams. They also looked for the shells of snails and clams, as well as the skeletal structures of bryozoans, which are small, moss-like animals that live in colonies.

When the researchers analyzed the sediment from the two cores, they found a landscape that had changed frequently over time. In the upper sections of the cores, from the surface down to about eleven meters, the sediment was rich with life. They discovered a variety of tiny shells from creatures that thrive in shallow, warm waters, such as lagoons and the edges of the sea. These findings suggest that this part of the seabed was once a calm, shallow environment where the water was not too deep and the energy of the waves was moderate. The presence of certain types of shells also hinted at periods when the water became very salty, perhaps due to high evaporation in a lagoon setting, creating conditions where only the hardiest creatures could survive.

As the researchers moved deeper into the cores, between twelve and seventeen meters, the story changed dramatically. In this section, the sediment was almost completely devoid of animal life. The layers here were dominated by peat and organic material, indicating a swamp or a marshy plain where plants grew thickly but the conditions were too acidic or unstable for most marine animals to live. The few shells that did appear in this zone belonged to creatures that could tolerate fresh or slightly salty water, suggesting that the area was a closed wetland, cut off from the open sea. This silence in the fossil record tells a clear story of a time when the sea had retreated, leaving behind a vast, vegetated marshland.

The most striking discovery occurred deeper still, between twenty-eight and thirty meters. Here, the sediment suddenly contained large, colorful shells of marine clams and other sea creatures that are typically found in open, high-energy ocean environments. This layer represented a time when the sea had surged forward, covering the land in a process known as a marine transgression. The water was deep enough and the currents strong enough to support a thriving community of large marine animals. The researchers noted that the presence of these specific large shells, along with colorful gravel found in the other core at a similar depth, confirmed that this was a period when the coastline had moved significantly inland, turning what is now a port area into a shallow continental shelf.

Below this marine layer, from thirty-one meters down to the bottom of the forty-meter core, the animal life disappeared once again. The sediment in this deepest section consisted of silt and sand mixed with crystals of gypsum and salt. The total absence of fossils, combined with the presence of these evaporite minerals, indicates that the area had become a dry, harsh environment, likely a salt flat or a floodplain in a semi-arid climate. The conditions here were so extreme that no organisms could settle or preserve their remains. This suggests that the deepest layers of the core date back to a time before the current sea levels rose, possibly during a colder, drier period in the geological past.

By connecting these different layers, the researchers have mapped out a clear sequence of environmental changes. The ground beneath the Grand Faw Port has not been static; it has oscillated between dry land, freshwater marshes, shallow lagoons, and open marine environments. The study confirms that the region experienced a major advance of the sea, which brought large marine animals to the area, followed by a retreat that left behind salt flats and swamps. These findings provide a detailed picture of how the Mesopotamian ecosystem has responded to natural shifts in sea level and climate over thousands of years, offering a vital record of how this strategic coastal region has evolved long before human ports were built upon it.