Millions of mysterious crater-like ocean pits are linked to porpoises

Scientists have unraveled the mystery behind millions of crater-like ocean pits, known as pockmarks, scattered across the ocean floor worldwide, particularly in the North Sea.

These formations were believed to be the result of fluid discharges such as methane or groundwater. The ocean pits have now been linked to the behavior of marine vertebrates like porpoises and sand eels.

Surprising connection

The study was led by Dr. Jens Schneider von Deimling of Kiel University in collaboration with researchers from various other institutions.

The team identified a surprising connection between these ocean pits and the hunting patterns of porpoises. 

“Our results show for the first time that these depressions occur in direct connection with the habitat and behavior of porpoises and sand eels and are not formed by rising fluids,” said Dr. von Deimling. 

“Our high-resolution data provide a new interpretation for the formation of tens of thousands of pits on the North Sea seafloor.”

Studying the ocean pits

The research team, which included experts from the Alfred Wegener Institute and the University of Veterinary Medicine Hannover, meticulously examined the North Sea floor near Heligoland. 

In the medical realm, pockmarks are well known as deep scars on the skin and are obviously of biogenic nature. In geoscience, pockmarks are referred to as cone-shaped depressions in the seabed.

“Geologic pockmarks are traditionally believed to result from fluid (i.e., gas or liquid) venting with erosive agents emerging from beneath the seabed,” wrote the researchers.

The high-resolution data not only provide a new interpretation for the formation of tens of thousands of pits on the floor of the North Sea, but also suggest that the underlying mechanisms might be a global phenomenon that has been overlooked until now.

They were first observed in the 1970s within clastic sediments offshore Nova Scotia, Canada. Now, about five decades after their initial discovery, it has become clear that pockmarks are among the most widespread morphologic features in the oceans.

Ocean pits and the role of sand eels

Dr. Anita Gilles from the TiHo-Institute for Terrestrial and Aquatic Wildlife Research highlighted the significance of sand eels in this context. Being a crucial food source for the North Sea porpoise population, the sand eels inadvertently contribute to the creation of the ocean pits. 

“From analyses of the stomach contents of stranded porpoises, we know that sand eels are an important food source for the North Sea population”, says Dr Gilles.

Porpoises, while hunting for eels buried in shallow sediments, leave behind pits. Although these depressions resemble pockmarks, they are much shallower.

The detection and analysis of these seabed depressions have been made possible through advanced multibeam echosounder technology. 

“The formation mechanism of these pits, as we call them, probably also explains the existence of numerous crater-like depressions on the seafloor worldwide, which have been misinterpreted as the result of methane gas leaks,” said Dr. von Deimling.

Study implications 

In the North Sea alone, researchers identified 42,458 of these uniquely shaped, shallow pits. This discovery contradicts the previous belief that the depressions were primarily caused by rising fluids.

“Our results have far-reaching implications from a geological and biological perspective. They can help to assess the ecological risks associated with the expansion of renewable energies in the offshore sector and thus improve marine environmental protection,” said Dr. von Deimling.

The study is published in the journal Communications Earth & Environment. 

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