Brief Review Of Paleotemperature Signals From Selected Lowland Records

The first record of glacial age sediment from Central America was from beneath Lake Gatun (ca. —30 m elevation compared with modern sea level) in Panama (Bartlett and Barghoorn, 1973) (Fig. 2). Lake Gatun was formed when the Panama Canal was constructed and the Chagres River was dammed. Marshes that once flanked the Chagres and Trinidad Rivers are now submerged by Lake Gatun. Cores raised from the flooded marshes provided sediments that were riverine in origin. Because of the potential inclusion of pollen that had been reworked or transported long distances, Flen-ley (1979) urged caution in their interpretation. However, the source rivers drain low-elevation basins that presently would not support montane forest elements.

26U ¿/U zm 290" 300" 310" 320 330"

26U ¿/U zm 290" 300" 310" 320 330"

FIGURE 2 Locations and cooling estimates for sites in the lowland neotropics and subtropics at the last glacial maximum. Numbers indicate degrees Celsius (°C) of cooling compared with modern temperatures. Where the cooling is not quantified, it is indicated by "c." Sites marked with a circle are based on pollen records; those marked with a star are based on groundwater isotopic data.

FIGURE 2 Locations and cooling estimates for sites in the lowland neotropics and subtropics at the last glacial maximum. Numbers indicate degrees Celsius (°C) of cooling compared with modern temperatures. Where the cooling is not quantified, it is indicated by "c." Sites marked with a circle are based on pollen records; those marked with a star are based on groundwater isotopic data.

Indeed, the modern pollen rain throughout their catchment areas would be remarkably uniform (Bush, personal observation). The largest changes in pollen abundance in this record reflect the migration of mangrove in response to changes in sea level. However, the presence of Iriartea, Ericaceae, Ilex, and Podocarpus pollen in late glacial assemblages indicates a downslope migration of these taxa of between 500 and 1000 m. Bartlett and Barghoorn suggested that this downslope shift represented a minimum temperature depression of 2.5°C. In an extensive study of modern pollen rain in Panama, Bush et al. (personal observation) did not find Iriartea pollen in any assemblage at less than 1000 m. Assuming a lapse rate of 5.5°C/1000 m, it would appear likely that the Gatun record documents at least a 5°C cooling. The complete lack of Quercus pollen in this record is striking, as pollen of this genus is abundant at two central Panamanian sites of glacial age (El Valle, 500 m: Bush and Colinvaux, 1990; and La Yeguada, 650 m: Bush et al., 1992). The modern pollen rain study shows Quercus to be ubiquitous above 1700 m elevation, and so we may infer that the temperature depression was not sufficient to bring Quercus down to sea level. Thus, the cooling would not have exceeded 8°C.

At Lake Quexil in Guatemala (Leyden et al., 1993, 1994), the coolest time on record was between 24,000 and 14,000 14C B.P., when temperatures were between 6° and 8°C lower than present values. As there is clear evidence of drying during the glacial maximum at this site, it is possible that there would have been some steepening of the lapse rate. For this reason, Leyden considers the lower of these estimates to be more realistic.

The first compelling evidence that glacial cooling affected the Amazon basin came with the discovery of Podocarpus timber at 1100 m elevation at Mera, Ecuador (Liu and Colinvaux, 1985). Equatorial Podocarpus species are almost exclusively montane, seldom living at elevations lower than 1800 m. Liu and Colinvaux (1985) inferred a temperature depression of ca. 4°C for the period between 33,000 and 30,000 14C B.P. on the basis of this evidence. A more detailed analysis of the sediments and the discovery of a second site at San Juan Bosco (970 m elevation) in Ecuador widened the list of cool indicator taxa to include Magnolia, Drimys, Alnus, Hedyosmum, Weinmannia, and grasses of the three-carbon (C3) photosynthetic pathway. With further dating, a cooling of 7.5°C was suggested for the period from 33,000-30,000 14C B.P. and 4°C for the period from 30,000-26,000 14C B.P. (Bush et al., 1990).

Since 1990, new data sets for different lowland ecosystems have produced further evidence of a substantial temperature depression during the glacial period. Lagoa Crominia in the Cerrado of central Brazil suggested a cooling of 5°C at 18,000 14C B.P. (Ferraz-Vi-centini and Salgado-Labouriau, 1996). Lagoas dos Ol-hos and Serra Negra (De Oliveira, 1992) and the swamp of Salitre (Ledru, 1993) all document the Pleistocene expansion of Araucaria forests. Records for southern Brazil indicate the expansion of subtropical grasslands (Behling and Lichte, 1997; Behling et al., 1998). In each case, a 5°C lowering of temperature during glacial times is inferred. In the lowland Amazon, lakes perched atop massifs, such as the Serra dos Carajas (Absy et al., 1991) and the Hill of Six Lakes (Colinvaux et al., 1996), have provided long records of the lowland forest environment. The Carajas record is interpreted by Absy et al. (1991) primarily in terms of wet and dry events, but the pollen record is consistent with a glacial cooling (Absy, personal communication). Two records from the Hill of Six Lakes contain significant amounts of Podocarpus, Hedyosmum, and Weinmannia pollen, leading to the suggestion of a 5°C glacial cooling (Col-invaux et al. 1996).

Table 1 presents a summary of available data documenting paleotemperature at 18,000 14C B.P. for the tropical and extratropical regions of the New World.

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