The double tombolo of Orbetello has formed by flooding of the central part of a single tombolo.

What is it about?

Double tombolos are intriguing natural oddities. Most tombolos are single sandy isthmuses connecting a mainland to a coastal island, but sometimes, more than one single sand bridge connects the island to the mainland, and there is no clear explanation to it. Here we show a case where the centre of the isthmus has been flooded to form a lagoon, and only the outer fringes of the isthmus remain emerged. We combine many different techniques to arrive at that conclusion, including ground-breaking acoustic imaging in the very shallow lagoon, more traditional acoustic imaging at sea, deep coring of the sandy isthmuses onshore, and various dating techniques to constrain the age and growth pattern of the isthmus.

Featured Image

Photo by Wei Zeng on Unsplash

Photo by Wei Zeng on Unsplash

Why is it important?

Double tombolos are much less common than single tombolos. Earlier approaches only considered modern wave climate to explain their existence. By using a radically different approach that consider the deep-time evolution of such isthmuses, we find that.but we find that the double-isthmus configuration is stage likely to happen frequently during the growth a tombolo, although it is short-lived. In the case at hand, it helps understand the current eutrophication crisis that affects the central lagoon, which costs millions of euros each year in remediation. Some have been tempted to see in this crisis a natural phenomenon, rather than an effect of contamination by fertilisers. We show that eutrophication does not arise as a natural result of lagoon evolution.

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