Submillennium-scale migrations of the rainforest-savanna boundary in Colombia: ~(14)C wiggle-matching and pollen analysis of core Las Margaritas

摘要

We present the 11 150-cal-yr-long pollen record Laguna Las Margaritas (3°23'N, 73°26'W; 290 m altitude), located at a site sensitive to climatic change near the transition from the Amazonian rainforest to the savanna of the Llanos Orientales in Colombia. In the 10-m-long core nine AMS ~(14)C bulk samples show ages from 9760±60 to 854±36 BP and provide initial time control. Thirty-one additional AMS ~(14)C samples of selected macrofossils provide time control from 6250 to 4050 call BP; for this interval precise time control was obtained by ~(14)C wiggle-match dating. From 11 150 to 9100 call BP, grass savanna dominated the landscape while gallery forest along the drainage system was poorly developed. Water availability was lower than today and the length of the dry season longer. From 9100 to 7330 call BP gallery forest expanded pointing to wetter conditions. From 7330 to 5430 cal BP savanna was increasingly replaced by forest, but with alternating abundance of both vegetation types. From 5430 to 2500 cal BP forest and wooded savanna dominated the western Llanos Orientales suggesting high precipitation rates. Expansion of Mauritia palm forest at 2500 cal BP reflects increasing water availability and stagnant water environments. Increasing savanna between 2500 and 1000 cal BP may represent a combined natural and anthropogenic signal. Development from open savanna to forest during the Middle Holocene is synchronous with a decreasing caloric seasonality, and a southward migration of the Intertropical Convergence Zone, suggesting that the large-scale climatic and vegetational change in the Colombian savannas is precession-forced. High-frequent migration of the savanna-rainforest boundary started around 6000 cal BP and continued at least to 3000 cal BP. Precipitation regimes in northwest and northeast South America seem opposite: dry conditions in the Colombian savanna area seem to reflect a La Nina setting of the climate system; wet conditions reflect an El Nino setting. Δ~(14)C fluctuations reflect changes of solar activity and we tested the hypothesis that Δ~(14)C fluctuations correspond to climatic and vegetational change. For the interval 6250 to 4050 cal BP, we applied ~(14)C wiggle-match dating (WMD), i.e. matching a series of radiocarbon ages in ~(14)C BP from the sediment, with the dendrochronology-based calibration curve, and we also wiggle-matched the pollen record vs. the Δ~(14)C record. In this first attempt in a neotropical ecosystem, we could not find unambiguous support that changes in solar activity did trigger climatic and vegetational change in the savannas of the Llanos Orientales.