Highly Variable Organic Carbon Burial in Thermokarst Lake Sediments on the Qinghai‐Tibet Plateau

Abstract Thermokarst lakes are dynamic carbon sinks and sources in permafrost landscapes, but their organic carbon (OC) burial remains poorly constrained. We analyzed 210 Pb‐dated sediment cores from 12 thermokarst lakes on the Qinghai–Tibet Plateau to quantify sedimentary OC, total nitrogen (TN), C:N stoichiometry, and recent sediment mass accumulation rates (MAR) and OC burial rates (OCBR). Chronology‐dependent comparisons were restricted to sediment intervals with CRS‐derived calendar years of 1900 or later. Across all sediment layers, OC concentrations (1.12–250 g/kg), TN (0.094–26.1 g/kg), and C:N ratios (1.5–188) exhibited pronounced within‐ and among‐lake variability but with no consistent vertical trends shared across lakes. In post‐1900 sediments, MAR (113–21,964 g/m 2 /yr) and OCBR (6.1–1,167 gC/m 2 /yr) also showed substantial within‐core and among‐lake variability. Lake‐level median MAR and OCBR varied from 496 to 5,831 g/m 2 /yr and from 17.8 to 191 gC/m 2 /yr, respectively. MAR and OCBR were positively related in most lakes, but the slopes differed markedly, suggesting that similar increases in sediment accumulation translated into different increments of OC burial among lakes. Across lakes, median MAR decreased with mean annual precipitation (MAP), whereas median OCBR increased with MAP; no significant relationships were detected with mean annual temperature, elevation, lake area, or lake depth. These results show that recent OC burial cannot be inferred from sediment accumulation alone, but reflects differences in both sediment supply and the OC content of deposited material, with postdepositional preservation representing an additional potential influence.