The impact of ²²²Radon escape on ²¹⁰Pb-based sediment chronology and dating accuracy in lacustrine systems

The accuracy of ²¹⁰Pb-based sediment chronology relies on determining atmospheric ²¹⁰Pb inventory by subtracting ²²⁶Ra-supported ²¹⁰Pb. This standard analytical approach assumes a closed system where the gaseous intermediate, ²²²Rn, remains in secular equilibrium with ²²⁶Ra. However, in lacustrine systems characterized by slow sedimentation and high porosity, ²²²Rn escape can invalidate this assumption, significantly distorting age models. This study investigated ²²²Rn mobility across three distinct systems: Lake Balaton (Hungary), Lake St. Anna, and Red Lake (Romania). In Balaton, high ²²⁶Ra (25–55 Bq/kg) and porosity (70–90%) induced significant ²²²Rn loss (3 ± 0.2–37 ± 4 Bq/l) within the upper 30 cm, leading to in situ ²¹⁰Pb deficits (14–56%) and chronological errors reaching 200%. Lake St. Anna, with slow sedimentation and high organic content, showed the highest discrepancies (8–87%) despite lower ²²²Rn levels (0.2 ± 0.01–3 ± 0.2 Bq/l). Conversely, Red Lake exhibited the smallest deviations (1–32%) due to faster accumulation and sandy sediment, notwithstanding measurable ²²²Rn escape (2 ± 0.2–28 ± 4 Bq/l) relative to its ²²⁶Ra content (35 Bq/kg). The results confirm that these sediments cannot be treated as closed systems regarding radon. Significant gas escape leads to an underestimation of supported ²¹⁰Pb if equilibrium is assumed. We conclude that pore water analysis and subsequent correction for radon diffusion are critical for reliable ²¹⁰Pb chronologies, particularly in slow-accumulating, porous sediments spanning the last 150 years.