2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 3
Presentation Time: 8:30 AM


LANE, Chad S.1, HORN, Sally P.1 and MORA, Claudia I.2, (1)Dept. Geography, Univ of Tennessee, Knoxville, TN 37996, (2)Dept. Geological Sciences, Univ of Tennessee, Knoxville, TN 37996-1410, clane6@utk.edu

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.