In view of advancing global warming, understanding the variability and timing of past climate changes remains crucial. A look into the geological past can provide insight into how the Earth has responded to rapid climate change in the past and help us predict how our climate might develop in the future. The Baltic Sea region is an important starting point for understanding climate dynamics in Northern Europe. However, precise age dating is essential for comparing various climate archives in the Baltic Sea region.
Scientists from the GFZ and the Leibniz Institute for Baltic Sea Research Warnemünde (IOW) have now been able to contribute significantly to the more precise dating of a climate archive (marine sediment) thanks to the discovery of tiny volcanic ash particles in the Baltic Sea sediment. This was the first systematic search for volcanic ash particles in the Baltic Sea. The site of the investigation was the western part of the Gotland Basin, one of the deepest areas of the Baltic Sea. In the study recently published in the journal Geology, the scientists calculated, among other things, the age differences between tephra ages (volcanic ash) and radiocarbon ages, thereby improving the sediment age accuracy for the last approximately 7,000 years. According to their results and dating points, the Holocene Thermal Maximum (period of higher temperatures) ended around 3,900 years ago, i.e. about two centuries earlier than previously assumed in the region.
Valuable climate archive marine sediment: Age of layers difficult to assess
Tools are needed to reconstruct past climate conditions: Deep layers of lake and marine sediments deposited long ago are particularly valuable geological archives. They serve as powerful ‘climate time machines’. The pollen or benthic foraminifera (single-celled organisms) contained in a fine layer of marine sediment, for example, provide clues about the prevailing climate conditions at the time, such as how dry or humid the climate once was.
However, in order to exploit the full potential of sediment layers for climate reconstruction, one factor is crucial: the reliability and accuracy of the age models used to date them. In short: how old is each thin sediment layer, and when was it deposited? The organic material still present in the sediments is often dated using the carbon isotope 14C, whose radioactive decay can be traced over time. However, 14C dating in Baltic Sea sediments remains a major challenge, as phases in which the organic material, which was originally plant or animal matter, was relocated (homogeneous sediments) alternate with phases in which it remained virtually unchanged and chronologically in place (laminated sediment). Samples from homogeneous redeposited sediments are currently impossible or difficult to assess in terms of their age. This means that there can still be considerable uncertainty in the dating of organic material in the Baltic Sea, with margins of error of several hundred years.
Volcanic ash as distinctly datable isochronous anchor points
A very important approach to better assessing the age of sediments is the primary use of well-dated tiny volcanic glass particles called tephra. These particles are transported over long distances by volcanic ash clouds and therefore settle over wide areas and also on the sea floor after volcanic eruptions (in Northern Europe especially in Iceland), so that they appear as natural time markers in a wide variety of climate archives and are therefore extremely useful. Their specific chemistry links them precisely to individual volcanic eruptions and at the same time enables researchers to compare different geological archives on land and in the sea.
Palaeoclimatologists Daniela Müller, Achim Brauer and other colleagues from the GFZ and the Leibniz Institute for Baltic Sea Research in Warnemünde have now succeeded in this difficult search with the discovery of tiny volcanic glass particles directly in the marine sediments of the Baltic Sea. The discovery of tephra from five different Icelandic volcanic eruptions in the Baltic Sea and the comparison with radiocarbon dates sheds light on the previous overestimation of age when using radiocarbon dating.
Radiocarbon age of the Baltic Sea overestimated
The sediments of the Baltic Sea indicate major climate-induced changes in deep water formation during the Holocene (the past 11,700 years), but the exact dating of these changes has been difficult due to unreliable radiocarbon ages.
Over the last c. 7,000 years, radiocarbon dating of organic material from the Baltic Sea sediments has often been significantly overestimated, sometimes by centuries or even millennia. These discrepancies vary greatly and are related to climate fluctuations. During cold phases, such as the Little Ice Age, strong deep water currents transport old sediments and carbon into deep basins such as the western Gotland Basin, leading to an overestimation of the age of sediments.
During warm periods, such as the Holocene Thermal Maximum, this mechanism is insignificant due to weak deep water currents. Here, only the so-called marine reservoir age (MRA) has an effect, which describes the period of time during which the carbon absorbed from the atmosphere remains in the seawater before it is deposited in the sediment or incorporated into marine organisms. This period is corrected using radiocarbon ages from land-based organisms, which obtain the constantly regenerating radiocarbon directly from the atmosphere and therefore reflect the current age without any time lag. On a global average, radiocarbon ages from the sea are approximately 400 years older than those in the atmosphere.
This global-average of marine reservoir age is generally used to correct radiocarbon ages in the Baltic Sea, although the MRA varies regionally and temporally. "This is where volcanic ash becomes the decisive factor. By identifying well-dated tephra layers in the sediments, we can not only overcome the uncertainties in radiocarbon dating, but also determine a more accurate regional marine reservoir age. This allows us to improve the age models we rely on to understand the past," says Daniela Müller, lead author of the study and former postdoctoral researcher in the Terrestrial Climate Archives working group of the Geomorphology Section at the GFZ Helmholtz Centre for Geosciences.
Results: Deviations from previous radiocarbon dating
The researchers determined the chemical composition of the tephra particles using electron probe microanalysis (EPMA), which enabled them to identify Icelandic tephra from a total of five different volcanic eruptions dating back approximately 4,500 to 2,000 years. Three tephra findings originate from the Hekla volcano – one of Iceland's most active volcanoes – and are named Hekla-4, Hekla-S and Hekla-3. Ash deposits from the Glen Garry tephra (Askja volcano) and Grákolla (Torfajökull volcano) were found in the same sediment layer and are of the same age. ‘The discovery of the Grákolla tephra in the sediments of the Baltic Sea is particularly exciting, as this is the first finding of this ash outside of Iceland,’ says Markus Schwab, head of the Laboratory for Tephra Analysis in Section 4.6 at GFZ.
Based on the age of the volcanic ash (tephra) as fixed points, it was thus possible to determine a regional marine reservoir age specific to the western Gotland Basin in the Baltic Sea. Using this reservoir age and the volcanic ash ages, the researchers calculated a revised age model for the western Gotland Basin with reduced dating uncertainties, which shifts earlier chronologies by about 200 years towards older dates. The precise dating in this study showed, among other things, that in homogeneous sediments containing old organic carbon, there were discrepancies between radiocarbon and volcanic ash ages of between 400 and 1200 years. In time intervals of laminated sediments that are not influenced by the inflow of old organic carbon, the radiocarbon ages consistently deviate from the volcanic ash ages by about 200 years, allowing the scientists to define a regional marine reservoir age of 250 ± 50 years for the West Gotland Basin.
According to the researchers' refined age model, the laminated sediment section deposited during the Holocene Thermal Maximum terminated earlier than previously assumed, namely around 3,900 years ago (cal. 1950 AD). The scientists' study allows to improve maps showing the distribution of volcanic ash plumes in the Baltic Sea region and to reduce dating uncertainties in other sediment cores from the Baltic Sea, enabling a precise link between paleoclimatic and environmental archives in the Baltic Sea region. Changes and different regional impacts of, for example, cold and warm phases can thus be compared much more accurately.
Based on these findings, the researchers refined the age model of the western Gotland Basin for the last approximately 7,000 years, shifting the age of key events such as the end of the Holocene Thermal Maximum back by about two centuries. ‘Tiny volcanic ash particles have made it possible to obtain more precise ages for the western Gotland Basin in the Baltic Sea, which is crucial for comparing the temporal and spatial effects of abrupt climate changes throughout the Baltic Sea region,’ says Daniela Müller.
Further information about the project:
The sediment core used was taken from a water depth of 193 metres and was examined as part of the BaltRap project (‘The Baltic Sea and its southern Lowlands: proxy – environment interactions in times of Rapid change’). The study area in the Baltic Sea for this study was the western part of the Gotland Basin, a sea basin east of Sweden. It also contains the deepest point in the Baltic Sea, the Landsort Deep, which lies 456 metres below sea level. Sediments accumulate particularly in such depressions, creating thick layers of sediment.
Original study: Daniela J.M. Müller, Ina Neugebauer, Rebecca J. Kearney, Markus J. Schwab, Oona Appelt, Markus Czymzik, Jérôme Kaiser, Helge W. Arz & Achim Brauer (2025): Constraining the Baltic Sea sediment chronology using tephrochronology. Geology, Vol. 53 (9), p. 743–747, https://doi.org/10.1130/G53346.1
