Climate and vegetation of the Yucatan Peninsula during the Late Pleistocene

The Yucatan Peninsula provides a sensitive setting to investigate past climatic oscillations in Central America. The climate of the area is closely associated with seasonal variations in sea surface temperature of the Caribbean Sea and displacements of the Intertropical Convergence Zone (ITCZ). In this study, palynological evidence from sediments of Lake Peten-Itza, Guatemala, provided insight into vegetation changes that took place on the Peninsula during the last 86,000 years. Modern mud-water interface samples were used to evaluate the strength of the relationship between modern pollen spectra and contemporary vegetation. A new technique to analyze modern pollen distribution patterns along temperature and precipitation gradients provided a means to construct reliable pollen-climate transfer functions. These functions, in conjunction with the fossil pollen record from Lake Peten-Itza, were used to estimate temperature and precipitation on the Yucatan lowlands from 86,000 years BP (ka) to present.Tropical and temperate taxa coexisted on the Yucatan Peninsula from 86 ka to present. However, different vegetation assemblages emerged as the result of alternation in their dominance produced by dynamic environmental conditions. Pollen-based climatic estimates revealed that both precipitation and temperature responded to the North Atlantic Ocean circulation cycles. Vegetation changes in Yucatan clearly reflected three main factors: (i) Greenland cold stadials and warm interstadials that took place during the last glacial, (ii) massive ice discharges into the North Atlantic, and (iii) changes in the mean position of the ITCZ throughout the late Quaternary. Warmings between 0.5 and 1°C were evident during Greenland warm interstadials. By contrast, the pollen record, and by inference the vegetation record, did not show a systematic response to cold stadials. Only cold stadials concurrent with Heinrich Events (HEs) were associated with &sim1.5-2.5°C coolings.During the HEs, Quercus and Pinus forests, which dominated the landscape through the last glacial, were replaced by xerophytic woodlands and thorn forests. Precipitation estimates were highly variable during the last glacial, and were closely associated with the stadial-interstadial cycles. Nevertheless, wetter-than-modern conditions were inferred for most of the last glacial period, suggesting that pollen changes reflected intra- rather than inter-annual precipitation changes.A cooling of &sim5°C during the Last Glacial Maximum (LGM) was inferred from Lake Peten-Itza's pollen record. These results suggest that previous interpretations of the LGM conditions in the Central American lowlands, which were largely based on the record of Lake Quexil, Guatemala, were incorrect. Chronological inconsistencies in the pollen record from Lake Quexil were identified by comparing its pollen record with that of Lake Peten-Itza. Consequently, the period of maximum drought during the last 86,000 yr, previously thought to be the LGM, was evident during the deglaciation. Cold conditions prevailed until &sim14.5 kyr BP, when a two-step warming period onset Holocene conditions.The pollen analysis from Lake Peten-Itza depicted dynamic vegetation changes in the lowlands of Central America during the last glacial. The proximity of the Peten area to the mountains of Guatemala and Mexico, and the topographic diversity of the area, played a major role in facilitating the vegetation response to climatic changes. Individualistic species responses, through altitudinal migration and growth of disjunct populations, were the most likely mechanisms of vegetation change. Global and local factors, such as fire, global climate, and anthropogenic influence, affected the vegetation, although the system showed considerable resilience. However, the modern concurrence of pervasive influence of humans and unprecedented rates of global climate change seems likely to pose unprecedented threats to the terrestrial ecosystems of the Yucatan Peninsula.