Rapid soil formation and carbon accumulation along a Little Ice Age soil chronosequence in southeast Alaska

This study aims to assess the co-evolution of soil development and soil organic carbon (SOC) accumulation along a postglacial soil chronosequence in southeast Alaska. We hypothesize that in the early stages of soil development, larger SOC stocks are primarily found in surficial soil horizons and gradually become more abundant in subsurface horizons over time. Seven moraines were dated using dendrochronology, yielding ages of 73, 82, 89, 128, 155, 207, and 247 years. SOC and pedogenic Fe oxyhydroxide values from mature Spodosols, sampled outside the chronosequence, were used as a reference for the steady state of Spodosols in the region. From 73 to 128 years, soils were classified as Typic Cryorthents; by 155 years, as Spodic Dystrocryepts; and as Typic Haplocryods in the older moraines. A shift in SOC accumulation rates and depth distribution occurred once spodic properties developed. During the Entisol phase, most SOC stocks were concentrated in surficial horizons. After meeting spodic criteria, there was a shift toward subsurface mineral horizons, hosting over 50 % of SOC stocks. This shift in SOC depth distribution was supported by a significant positive relationship between subsurface SOC stocks and age, but not with surficial SOC stocks. Similarly, surficial SOC accumulation rates were elevated during early pedogenesis (e.g., 0.26 Mg C ha^-1 yr⁻¹), while subsurface SOC accumulation was lower (0.04 Mg C ha^-1 yr⁻¹), increasing to 0.19 Mg C ha^-1 yr⁻¹ once spodic properties were met. A significant positive correlation between pedogenic Fe and age highlights the role of SOC in enhancing the weathering of Fe-bearing minerals. Our findings support the hypothesis that subsurface SOC stocks rapidly become dominant over time with Spodosol development. This study underscores the consequences of glacier retreat on a portion of the terrestrial ecosystem, with direct impacts on carbon cycling.