A climate archive underfoot
In the flat landscape of the Netherlands, near the city of Amersfoort, lies a reference point for Earth's climate history. It is not a dramatic cliff face or a deep canyon, but a sequence of clays and sands buried underground. This is the type section for the Eemian Interglacial, a warm period that lasted from approximately 130,000 to 115,000 years ago. A geological "type section" is the specific location that provides the official definition for a particular rock layer or time period. This one is named after the nearby River Eem.
The site's significance was first identified in the 1870s by Dutch geologist Pieter Harting. He was studying sediments from boreholes and noticed that the clays contained fossil molluscs completely different from those in the modern North Sea. Species like the gastropod Bittium reticulatum and the bivalve Venerupis senescens were present, which today live in warmer waters off the coasts of Portugal and in the Mediterranean. This discovery was the first strong evidence that this part of Europe was once submerged under a much warmer sea.
During the Eemian, global average temperatures were about 1 to 2 degrees Celsius warmer than pre-industrial times. This seemingly small difference had enormous consequences. A significant portion of the Greenland and West Antarctic ice sheets melted, causing global sea levels to rise to a height of at least 6.6 meters, and possibly over 8 meters, higher than they are today. The sediments at Amersfoort, deposited at the bottom of that ancient sea, provide a direct physical record of this warmer world.
Reading the Eemian record
Microscopic fossils tell the story of the Eemian environment the microscopic fossils preserved in the clay. After Harting's initial work with molluscs, scientists in the 20th century, W.H. Zagwijn, applied pollen analysis to the Amersfoort cores. This technique showed the changing vegetation over thousands of years. The pollen record records a succession of forests. As the climate warmed after the preceding Saalian ice age, birch and pine forests first colonized the landscape. These were replaced by dense, mixed-oak forests with hazel (Corylus), oak (Quercus), and elm (Ulmus). At the peak of the interglacial warmth, the hornbeam (Carpinus) became common. As the climate began to cool again towards the next ice age, fir (Abies) and pine forests returned.
The animal life was also different. While no large mammal fossils have been found at the Amersfoort type section itself, Eemian deposits elsewhere in Europe show a diverse fauna. Straight-tusked elephants (Palaeoloxodon antiquus), Merck's rhinoceros (Stephanorhinus kirchbergensis), and even hippos (Hippopotamus amphibius) inhabited the river valleys and forests of temperate Europe.
The Amersfoort site itself is not a single, continuous record but has several breaks, or unconformities. These gaps in the sediment layers show changes in the environment, such as shifts in sea level and the local hydrodynamics of the shallow Eemian sea. By studying these layers, scientists can reconstruct the advance and retreat of the sea that once covered much of the Netherlands.