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

Abstract

Abstract The accuracy of 210 Pb-based sediment chronology relies on determining atmospheric 210 Pb inventory by subtracting 226 Ra-supported 210 Pb. This standard analytical approach assumes a closed system where the gaseous intermediate, 222 Rn, remains in secular equilibrium with 226 Ra. However, in lacustrine systems characterized by slow sedimentation and high porosity, 222 Rn escape can invalidate this assumption, significantly distorting age models. This study investigated 222 Rn mobility across three distinct systems: Lake Balaton (Hungary), Lake St. Anna, and Red Lake (Romania). In Balaton, high 226 Ra (25–55 Bq/kg) and porosity (70–90%) induced significant 222 Rn loss (3 ± 0.2–37 ± 4 Bq/l) within the upper 30 cm, leading to in situ 210 Pb 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 222 Rn 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 222 Rn escape (2 ± 0.2–28 ± 4 Bq/l) relative to its 226 Ra 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 210 Pb if equilibrium is assumed. We conclude that pore water analysis and subsequent correction for radon diffusion are critical for reliable 210 Pb chronologies, particularly in slow-accumulating, porous sediments spanning the last 150 years.