A library of ancient climate
Along the coastal cliffs of the Tjörnes peninsula in northern Iceland, a thick sequence of sedimentary rock records deep global change. Known as the Pliocene Tjörnes Beds, this geological formation is over 500 meters thick and preserves a detailed record of climate, vegetation, and marine life stretching from the late Miocene to the early Pleistocene. Unlike most of Iceland, which is composed of volcanic rock, the Tjörnes peninsula contains these rare sedimentary layers—sandstones, siltstones, and deposits of lignite (brown coal)—interbedded with basalt lava flows.
These layers document a dynamic environment. The sediments were deposited in settings that shifted between shallow seas, estuaries, and swamps. The lignite seams contain fossilized plant remains from species like spruce, oak, and beech, indicating that forests once grew in a climate warmer than today's. The entire sequence sits within the Tjörnes Fracture Zone, a tectonically active area that connects Iceland's onshore and offshore volcanic rift zones. This tectonic activity has lifted and exposed the ancient seabed, making its fossil record accessible.
The Pacific invasion
the significant event recorded by the Tjörnes Beds is of a massive biological migration event. The layers of rock are divided into three main biozones, named for the dominant mollusks found within them: the Tapes zone, the Mactra zone, and the Serripes zone. While the lower zones contain fossils of Atlantic species, the Serripes zone, dated to about 3.6 million years ago, shows the sudden appearance of numerous Pacific mollusk species.
This faunal interchange shows when the Bering Strait opened sufficiently to allow a continuous flow of water and life from the Pacific Ocean into the Arctic, and subsequently into the North Atlantic. The arrival of species such as Serripes groenlandicus and Macoma calcarea in the North Atlantic for the first time is recorded directly in these Icelandic cliffs. This "Trans-Arctic Interchange" was not limited to shellfish; the passages also allowed for the migration of larger marine animals. Fossils of seals, walruses, and even a large right whale skull have been discovered within the formation, recording the ancient marine ecosystem. The layers also track climate oscillations, with warmer-climate species in the lower Pliocene beds giving way to cold-adapted Arctic species and the first appearance of glacial deposits, called tillites, in the upper layers, heralding the beginning of the Ice Ages.