An Ancient Climate Record
Along the wild coastline of Mallorca's Serra de Tramuntana, large banks of what looks like dark, fibrous peat line the beaches. These mounds, known as banquettes, are not soil or seaweed; they are composed entirely of the dead leaves of Posidonia oceanica, a species of seagrass endemic to the Mediterranean. Storms and currents wash the shed leaves ashore, where they accumulate in thick, compact layers that can be several meters high.
These unassuming piles of dead leaves are a scientific record. Researchers have dug into these banquettes, treating them like geological strata. By applying radiocarbon dating to the compressed leaf material at different depths, they have created a timeline of deposition. The deepest layers of some banquettes have been dated to 4,000 years old, representing a continuous, undisturbed record of the local marine environment. The composition and thickness of the layers provide data on past storm intensity and frequency, while chemical analysis of the leaves records historical sea-level changes and water quality.
The living Posidonia oceanica meadows offshore are also ancient. Through genetic testing, scientists have discovered vast clonal colonies that are effectively single organisms. One such colony stretching between the islands of Ibiza and Formentera is estimated to be up to 100,000 years old, making it one of the oldest known living organisms on Earth.
An Unsung Ecosystem Engineer
The underwater meadows of Posidonia oceanica, sometimes called Neptune Grass, are significant to the Mediterranean ecosystem. These seagrass beds are highly effective at carbon sequestration, with some estimates suggesting they can capture and store carbon up to 15 times more efficiently per year than a comparable area of the Amazon rainforest. The dense network of roots and rhizomes, called a matte, traps sediment and builds up the seabed over millennia, locking away huge amounts of "blue carbon."
The banquettes on the shore continue this ecological work. The dense, entangled masses of leaves are remarkably resilient and absorb the force of storm waves, protecting beaches and dunes from erosion. Without them, the coastline would be far more vulnerable to being washed away.
As the leaves slowly decompose, they release important nutrients into the sandy coastal soil, supporting dune vegetation. The banquettes also provide food and shelter for a complex web of invertebrates, which in turn are a food source for coastal birds. For many years, these important structures were removed from tourist beaches, but a growing awareness of their ecological and protective role is leading to better management practices.