The paleoecological significance of stable carbon isotopes in lake sediments from the Chirripo Paramo of Costa Rica

The high elevation paramos 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, I measured the stable carbon isotope compositions of total organic carbon (o1 3Croc) 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 Chirripo. Both sediment records begin in late Pleistocene ( ~ 11,700 cal. yr. BP) glacial silt deposited as the ice last retreated from the Chirrip6 massif. Average o13CTOc values are more positive in the basal glacial silts of both cores than in overlying organic-rich Holocene sediments, possibly suggesting the greater importance of C4 plants during the late Pleistocene. An increase in the proportion of C4 plants in the late Pleistocene may have been the result of decreased atmospheric pCO2, increased aridity (perhaps seasonally), and/or well-drained soil conditions on newly exposed bedrock and glacial till that favored the C4 grass Muhlenbergia flabellata. The lake sediment o13CTOcrecord shows a strong link to fire dynamics. Periods of increased macroscopic charcoal input to the lake sediments are characterized by more depleted o13Croc during the Holocene and more enriched o13Croc values during the late Plefatocene. Relatively enriched average o13CTOc values of~ -20%0 occur throughout the Holocene despite evidence for continued C3 plant dominance. These enriched o13Cmc values may reflect the utilization of HCO3- photosynthesis by aquatic plants and algae, such as Bottryococcus braunii, and/or 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