Quaternary Interglacials

Within the dating resolution available to us, Neotropical interglacials appear to coincide in timing and general character with those documented elsewhere. The interglacials are known by their marine isotope stages (MIS): 5e (c. 130— 116 kyr bp), MIS 7 (c. 240—200 kyr bp), MIS 9 (330—300 kyr bp) and MIS 11 (425— 390 kyr bp). They generally last about 15—40 kyr. While a 100-kyr cycle appears to underlie the glaciations of the last half million years, the intensity of interglacial periods appears to be related to precessional amplitude (Broecker, 2006).

Three records exist that provide insights into multiple glacial cycles in the Andes. The High Plain of Bogota, Lake Titicaca (Hanselman et al., 2005) and the Salar de Uyuni, although only the MIS 6 to MIS 1 portion of this record has been published so far (Fritz et al., 2004). The amount of ecological change in three fossil pollen records that cover either or both of MIS 1 and MIS 5e from Lake Titicaca reveals that—true to the precessional prediction—MIS 5e is the most extreme of these interglacials (Figure 2.3). The onset of MIS 5e is taken to be at 136kyr bp based on the chronology used in Hanselman et al. (2005) and 11 kcal. yr bp is taken as the start of the Holocene.

Years into interglacial

Mis 5e

Years into interglacial

Mis 5e

- Titicaca main lake LT01-2B

Huinaymarca LT01-3B Holocene

- - - ■ Titicaca main lake LT01-2B Titicaca main lake NES8-1 PC

Figure 2.3. A comparison of MIS 5e and the Holocene based on insolation and changes in community composition revealed through DCA. Data are from Hanselman et al. (2005) and insolation curves from Analyseries 1.2 (Berger, 1992; Paillard et al., 1996)}.

The data are drawn from a deep water core from Lake Titicaca LT01-2B (240-m water depth), a shallower water core (40-m water depth) from the Huinaymarka sub-basin (Core LT01-3B), and a piston core from 130-m water depth that provides a detailed Holocene record from the main basin (core NE98-1PC; Paduano et al., 2003). The fossil pollen data were combined into a single matrix and ordinated using detrended correspondence analysis (DCA) (Hill, 1979; McCune and Mefford, 1999). The scores for Axis 1 are plotted against time since the start of the relevant interglacial, and therefore provide a comparative trajectory of the amount of ecological change that took place. Core LT01-3B has a hiatus in the middle of the interglacial, but shows a very similar pattern of community change leading into and out of the event as found in the deep water core LT01-2B. That hiatus is entirely consistent with our hypothesis of increased evaporation and reduced precipitation compared with that of the Holocene that marked the peak of MIS 5e.

During 5e, and to a slightly lesser extent in previous interglacials, warming caused the upper Andean forest limit to move about 200 m above its modern elevation. Both the Colombian and Bolivian records indicate that the peak of MIS 5e may have been relatively dry. This drying is especially evident in Lake Titicaca, where the abundance of benthic and saline-tolerant diatoms, and peak abundances of pollen of Chenopo-diaceae/Amaranthaceae suggest the lowest lake levels of the last 340,000 years. Chenopodiaceae/Amaranthaceae pollen types are commonly derived from salttolerant plants, or from plants that grow in areas subject to irregular inundation (Marchant et al., 2002). Parsing out the effects of warming versus reduction in precipitation is not easy, as both would lower lake level. In Colombia the estimate of warming based on migration of tree line appears to have been about 1°C (Van der Hammen and Hooghiemstra, 2003), and this is consistent with the tentative estimate of 1-2°C for Titicaca (Hanselman et al., 2005).

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