Volcanogenic CO2 emissions affect radiocarbon dating in a case study from the Laacher See crater lake, Germany

Lake sediments form a valuable and often continuous record for reconstructing past climate and the occurrence and impact of natural hazards. The interpretation of this record, however, relies heavily on a robust chronology formed by age-dating the sediments. For recent (i.e. Quaternary) lake sediments, radiocarbon dating of organic material is a fundamental dating technique. However, constructing a lake sediment chronology can be challenging, since the use of radiocarbon dating is dependent on many factors, including the type of material to be dated, depositional circumstances and possible contamination of ¹⁴C. Volcanogenic CO₂, for instance, is depleted in ¹⁴C. This implies that in regions with surface exhalations of volcanic CO₂ the concentration of ¹⁴C in the surrounding atmosphere is diluted. For this study, the effect of volcanogenic CO₂ gas emissions on the use of radiocarbon dating was investigated in the Laacher See volcanic crater in western Germany. This crater was formed after the eruption of the Laacher See Volcano around 13 ka BP. It contains multiple degassing vents emitting CO₂ of magmatic origin, in the form of underwater bubble seeps in the lake (“wet mofettes”) and onshore soil degassing (“dry mofettes”). Living plant material, i.e. leaves of Taraxacum genus plants, were sampled in several locations in the crater and dated to examine their range in radiocarbon ages and spatial variability. Additionally, a > 4 m long sediment core taken in the lake was sampled for organic material and bulk sediment to assess the offset of radiocarbon ages to their true or expected ages. Our results show that all dated samples exceed their true or expected ages, with the Taraxacum samples giving variable radiocarbon ages of up to 9000 a BP. Along a transect of sampled Taraxacum plants, the radiocarbon ages decrease with an increasing distance from the degassing vents along the lake shore. The radiocarbon ages of the sediment core samples show that organic material deposited in the lake is also affected by volcanogenic CO₂ emissions, with some radiocarbon ages exceeding the age of the Laacher See eruption that formed the crater, although no regular offset could be determined for these samples with regard to their depth in the core. Furthermore, the radiocarbon ages do not correspond to a ²¹⁰Pb/¹³⁷Cs age-depth model that was established for the top of the core. Radiocarbon dating is shown to not provide reliable results for establishing a chronology for the sedimentary infill of Laacher See. Further research is required to better understand the influence of volcanogenic CO₂ on organic material, such as effects of temporal and spatial variations in CO₂ flux. In the case of Laacher See, other age-dating techniques should be considered to establish an age-depth model with reliable, non-¹⁴C dependent ages.