The high elevation páramos of Costa Rica are dominated by C₃ grasses (primarily bamboo) and evergreen shrubs; however, the high altitude C₄ grass Muhlenbergia flabellata may dominate suitable microhabitats with coarse substrates, such as glacial till. To explore late Pleistocene and Holocene C₃ -C₄ vegetation dynamics we measured the stable carbon isotope compositions of total organic carbon ( d¹³ C TOC) in two parallel lake sediment cores (core 1: 6 m, core 2: 5.6 m) from Lago de las Morrenas 1 (3477 m) at the foot of Cerro Chirripó, Costa Rica's highest peak. Both sediment records begin in late Pleistocene (~ 11,700 cal yr BP) glacial silt deposited as the ice last retreated from the Chirripó massif. Average d¹³ C TOC values are more positive in the basal glacial silts of both cores (core 1: -16.2 ‰ V-PDB, core 2: -19.7) than in overlying organic-rich Holocene sediments (-19.6, -20.7), suggesting the greater importance of C₄ plants during the late Pleistocene. The apparent increase in the proportion of C₄ plants in the late Pleistocene may be the result of decreased atmospheric pCO₂, increased aridity (perhaps seasonally), and/or soil conditions on newly exposed bedrock and glacial till that favored the C₄ grass Muhlenbergia flabellata. The lake sediment d¹³ C TOC record shows a strong link to fire dynamics. Periods of increased macroscopic charcoal input to the lake sediments are characterized by more depleted d¹³ C TOC during the Holocene and more enriched d¹³ C TOC values during the late Pleistocene. These relationships support the apparent ecological shift and also suggest that fire-related vegetation succession around Lago de las Morrenas 1 can be detected using d¹³ C TOC. Holocene average d¹³ C TOC values of ~ -20 ‰ , despite C₃ plant dominance, may reflect the presence of the aquatic CAM plant Isoetes, the HCO₃ ^- utilizing algae Bottryococcus braunii, and/or the trophic level fractionations imparted by zooplankton within the lake. Future compound-specific isotopic analyses will assist in isolating the contribution of these various sources to the organic carbon pool.