Coral Reefs
Coral reefs occur almost everywhere in the shallow waters of the tropical ocean. They are complex communities and offer excellent archives for reconstructing past environmental conditions. They often reach an age of several hundred years. Their annual growth is documented in the density bands and thus enables the investigation of geochemical parameters in seasonal resolution going back to times before the industrial revolution of the 19th century. Photo: Anna Roik/GEOMAR

Cold-water corals
The distribution of coral reefs is not limited to the warm tropics. The so-called cold-water corals of the higher latitudes do not live in symbiosis with green algae like their tropical relatives and are thus largely independent of light and especially depth. It has only been known for two decades that these cold-water corals also form mighty reefs and are found on almost all continental margins of the world's ocean in water depths between 1000 and 40 metres. Cold-water corals are also excellent natural archives. Photo: JAGO Team/GEOMAR

Ice cores
Ice cores from the mighty ice sheets in Greenland and Antarctica, resulting from annual deposits of snow, are excellent climate archives. The deeper an annual layer lies in the ice, the older and thinner it is, as the weight of the overlying layers compresses it. So far, the longest boreholes in the ice are just over three kilometres deep. By counting the layers of ice, it became clear that the ice cores in Greenland go back 123,000 years into Earth's history and in Antarctica even more than 800,000 years. Photo: Eli Duke, WAIS Divide Field Camp, CC-BY 2.0

Ice cores
When analysing ice layers, scientists are particularly interested in the small air bubbles trapped in the ice. These "fossil" air bubbles are reliable archives that reflect the composition of the atmosphere of past times. It became clear from the measured carbon dioxide and methane concentrations that the global atmosphere was subject to very strong and rapid fluctuations, especially during the ice age cycles with its cold and warm phases. In addition, dust particles trapped in the ice are valuable indicators for reconstructing changes in atmospheric circulation and their influence on the ocean. Photo: CSIRO, CC BY 3.0

Sediment cores
The seafloor represents the largest and most extensive climate archive on Earth, as sediments are deposited slowly but continuously on the seafloor over many millions of years. In drilling cores, the individual sediment layers can be read like pages of a book that records the climate history of the Earth. The shells of microfossils they contain provide important information about the ocean and climate conditions of past times. Photo: Jan Steffen/GEOMAR