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On the geology of islands: deep pasts and future fossils

We are nearing Rapa Nui. It is the most eastern part of the Polynesian triangle, and one of the more isolated islands in the Pacific. Its closest neighbor lies 931km to the east, Isla Salas y Gómez, and though it is somewhere on the horizon, we cannot see it.

The Rapa Nui name for this island is Motu Motiro Hiva, meaning ‘bird island’. This is because Isla Salas y Gómez is just a low-lying rock jutting out of the seafloor, with nothing but seabirds on it. Below the surface, it is an important marine biodiversity area, particularly for deep-sea corals, sharks, and shallow-water reefs.

Rapa Nui and Isla Salas y Gómez are the only above-sea-level remnants of a chain of underwater mountains that have formed from a hotspot in the earth’s crust, where magma finds its way to the surface from within the earth’s mantle, thus forming volcanos. Volcanic hotspots are the cause of most islands in the Pacific. These volcanos, when rising out of the sea, become exposed to wind, rain, and waves, and over time erode from the high islands they once were and eventually, over millennia, sink back below the waves.

Hawaii is a famous example of this. The Big Island Hawai’i, has high volcanic mountains, and is the only active volcanic island in the archipelago. The four main islands certainly look as though they were volcanoes at one point in time. And as you head further west, the Northwestern Hawaiian Islands are little more than sandy atolls in the sea.

When these volcanic islands form, the first to visit and call them home, are birds. There is a recent example of how volcanic islands spring to life, though this tale is told much further from where we are now.

In 1963, in the North Atlantic, a volcano erupted 10 miles off of the coast of Iceland. Surtsey Island was born. The first plants to arrive came in 1965 as sea rocket (Cakile maritima). The next arrivers were Black Guillemots, who used the harsh, sharp volcanic rocks to conceal their nests. Later Black-legged Kittiwakes and Northern Fulmars began to breed on the island, then, the arrival of gulls brought a boom in plant diversity, carried in the stomachs and droppings of the birds.

The bird’s nutrient-rich guano fertilized the topsoil with marine-derived nutrients, kick-starting life on land. In 2013, a lush green meadow had formed in the center of the island where the gulls nested that spread over 30 acres. Today, life on the island includes many terrestrial species of birds, plants, lizards, and insects. The island is booming.

Seabirds are sometimes called ecosystem engineers by scientists. They feed the land with marine-based nutrients in their droppings. Some seabirds aid the dispersal of plant and insect species. When nesting, burrowing seabirds turn over the soil, burying nutrients, aerating the soils, and determining which plants remain. These positive benefits are not only felt by life on land, but also in the surrounding seas. On islands where rats have wiped out seabird populations, adjacent coral

reef systems are less healthy and diverse as those on rat-free, seabird rich islands. This is because all of those nutrients that seabirds bring to the topsoil of islands gets washed into the sea with rain events, thus feeding local marine communities. Without the birds, this important land-sea cycle is broken.

My research and that of my colleagues have found that seabirds, integral to island ecosystems, are themselves inadvertently polluting the many ‘Gardens of Eden’ of which they have built. Famous images from artist Chris Jordan on Midway Atoll in the North Pacific show eerie skeletons of seabird chicks, their stomachs filled with plastics. I have seen the same harrowing scenes of shearwaters, storm petrels, and tropic bird chicks in the South Pacific too. Further research found that seabirds that ingest plastics also leave behind microplastics and associated chemicals in. their guano (droppings), which they further embed into soil when burrowing.

Plastics are now so common in the sedimentary layers of the Earth that they are one of the key determining characteristics of a geological marker in time called the Anthropocene: “the age of humans”. These geological markers are how we classify ‘deep time’. The many millennia in which large geological and climatic events took place, like the Jurassic, Triassic, and Cretaceous periods, in which dinosaurs walked the Earth.

Just like paleontologists uncovering these deep pasts, I wonder if future archeologists will find fossilized seabirds filled with plastics in the deep future? Certainty, they will find plastics in other forms too. On the same volcanic islands in the Pacific, Plasti-glomerates have been found as a new form of rock, where lava has cooled and fused with plastics on beaches. Other pseudo-rocks have formed where geological activity has heated fishing gear on beaches forming a green plastic lining across the coastline.

The remaining rock of Isla Salas y Gómez and its associated underwater mountain chain serve as a reminder; that in the colossal paths of deep time, nothing can really last forever. Perhaps these islands, their seabirds, our plastics, will sink back into the sea, gradually, and leaving little trace. The seabirds’ story never told.

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