Fig. 1: The properties of mineral-associated organic carbon (MAOC) in lake sediments.

a Distribution of Fe mineral content and forms in the lake sediments, AM: active Fe minerals, IM: inert Fe minerals. Box plots indicate median (middle line), mean (square symbol), 25th, 75th percentile (box), and 1.5 interquartile range (IQR, whiskers) as well as outliers (single points), n = 42 independent samples. b Proportion of OC–Fe in total sedimentary organic matter, as well as the proportions of OC–Fe associated with active and inert minerals. Box plots indicate median (middle line), mean (square symbol), 25th, 75th percentile (box), and 1.5 interquartile range (IQR, whiskers) as well as outliers (single points), n = 42 independent samples. c The characteristics of organic matter associated with inert and active Fe minerals by Fluorescence excitation-emission matrices analysis. d The scatter plots between the intensity of metal (oxyhydr)oxides (Fe and Al) and OC functional groups in the sediment samples of Lake LG, DH, and BD. Data are presented with points colored by OC functional groups and linear regression lines (solid) fit to the data. Pearson’s correlation coefficient (R²) and corresponding P-values were computed to assess the strength and significance of the linear relationship, with two-sided P values provided in Supplementary Table S4. The confidence bands are 95% sure to contain the best-fit regression line. The SR-FTIR microspectroscopy analysis revealed the presence of several OC functional groups, including phenolics (3379 cm⁻¹), aliphatics (2920 cm⁻¹, 2856 cm⁻¹ and 1419 cm⁻¹), carboxylics (1716 cm⁻¹ and 1252 cm⁻¹), peptides (1653 cm⁻¹), quinone (1650 cm⁻¹), aromatics (1618 cm⁻¹), lignin derivatives (1515–1513 cm⁻¹), polysaccharides (1036 cm⁻¹), Fe oxides (Fe, 690 cm⁻¹), and Al oxides (Al, 915 cm⁻¹) within the area.