Radiocarbon age anomalies in post-impact lake sediments of the Hapcheon impact crater, Korea and their implication for crater evolution

Impact craters caused by meteorite collisions are phenomena that dramatically alter the Earth's surface and disturb the natural carbon cycle in subsequent sedimentary environments. To determine the impact date and impact-driven old carbon effects on radiocarbon cycles recorded in post-impact lake sediments, we performed radiocarbon (¹⁴C) dating of various organic fractions (humic acids, humins, charcoal, and organic plant fragments of various sizes) in post-impact lake sediments of the Hapcheon impact crater, Korea. Charcoal and plant fragments larger than 500 μm in the post-impact lake sediments of core 20CR09 constituted the oldest fraction (>45,000 cal BP), indicating pre-impact ages. Compared to micro-organic fragments (10–100 μm) consisting of terrestrial plant tissues, pollen, and limited freshwater algae with a narrow age range of 39,300–42,300 cal BP, the 100–500-μm plant fragments showed a slightly older age range of 41,670–43,910 cal BP throughout the 33-m-long lake sediment core. This suggests significant influence of well-mixed syn-impact organic fragments transported from slope deposits consisting of impact breccias and fallback deposits. Humin ages in the lake sediment of core 20CR05 were similar to those of the micro-plant fragments in core 20CR09, indicating that the humin fraction is mainly allochthonous. Humic acids at a depth of ca. 71 m, corresponding to the lowermost post-impact lake sediments (Stage 1), were dated to ∼21,000 cal BP, showing strong influence of post-impact humic acids ages. Whereas those from mid-depth lake sediments (Stage 2) were dated to 35,000–43,000 cal BP. This age reversal indicates increased input of syn-impact allochthonous humic acids, supported by positive correlation with terrestrial mineral input. This study demonstrated significant syn-impact radiocarbon effects and resultant age reversals in post-impact early lake sediments. The syn-impact age may be estimated by comparing ages of different-sized plant fragments (e.g., 10–100, 100–500, and >500 μm), humin, and humic acids. These findings suggest that climatic and environmental interpretations of these post-impact early lake sediments should be conducted only after testing various organic materials, if radiocarbon dating is applied. Lastly, compared to surrounding areas following natural climate changes, post-impact early lake environments in impact craters must be considered as anomalies with different ecosystems due to hydrothermal activity.