Deglaciation

The timing and rate of Andean deglaciation is somewhat contentious, as it has been suggested that the southern Andes mirrors the Vostok record from Antarctica, while the northern Andes mirrors the GISP record from Greenland (Seltzer et al., 2002). It appears that the more southern tropical Andes entered a deglacial phase between 21 kcal. yr bp and 19 kcal. yr bp (within the classic LGM of the northern hemisphere), while the northern Andes may not have warmed until c. 16 kcal. yr bp. This relatively early deglaciation is manifested in most Central Andean records (Figure 2.4).

The deglaciational path had some bumps in it, though most of the apparent hiccups were probably abrupt changes in precipitation rather than deviations from a steady change in temperature (Bush et al., 2004). A dry event at c. 16.5 kcal. yr bp is recorded strongly in Lake Titicaca, and was followed by a cool, wet event that centers on c. 15.1 kcal. yr bp. This latter event is consistently manifested in Central Andean records, though it is more evident in the records between c. 11°S and 7°S rather than those at higher latitudes.

Thereafter, the trend out of the last ice age is relatively constant in the southern tropical Andes, whereas the northern Andes appears to reflect the Caribbean and northern hemispheric episodes of abrupt warming and cooling. Consequently, in Peru

1000

Huascaran 10

Trend Magnetic Susceptibility

5 460

6 450

5 440

Figure 2.4. Central Andean insolation, and the extent of physical and community change during deglaciation and the Holocene. Datasets are Lake Chochos magnetic susceptibility (note inverted log scale; Bush et al., 2005); Huascaran SlsO ice core (Thompson et al., 1995); Lake Junin Sl8O calcite (Seltzer et al., 2000); Lake Caserococha fossil pollen DCA Axis 1 (Paduano, 2001); Lake Consuelo fossil pollen DCA Axis 1 (Bush et al., 2004 and new data from D. Urrego); Lake Titicaca fossil pollen DCA Axis 1 (Paduano et al., 2003); insolation (DJF) for 10°S from Analyseries 1.2 (Berger, 1992; Paillard et al., 1996). VSMOW = Vienna standard marine ocean water; VPDB = Vienna peedee belemnite.

Huascaran 10

1000

5 460

6 450

5 440

Figure 2.4. Central Andean insolation, and the extent of physical and community change during deglaciation and the Holocene. Datasets are Lake Chochos magnetic susceptibility (note inverted log scale; Bush et al., 2005); Huascaran SlsO ice core (Thompson et al., 1995); Lake Junin Sl8O calcite (Seltzer et al., 2000); Lake Caserococha fossil pollen DCA Axis 1 (Paduano, 2001); Lake Consuelo fossil pollen DCA Axis 1 (Bush et al., 2004 and new data from D. Urrego); Lake Titicaca fossil pollen DCA Axis 1 (Paduano et al., 2003); insolation (DJF) for 10°S from Analyseries 1.2 (Berger, 1992; Paillard et al., 1996). VSMOW = Vienna standard marine ocean water; VPDB = Vienna peedee belemnite.

and Bolivia the deglaciational warming appears to have been on average <1°C per millennium, whereas in the northern Andes a relatively large jump in temperatures at the onset of the Holocene, perhaps 4°C within the space of a few hundred years, is thought to have occurred. Thus, these systems have responded to warming events whose rates differed by about an order of magnitude.

Evidence for the presence, or absence, of the Younger Dryas event in South America has engendered considerable debate (Heine, 1993; Hansen, 1995; Van der Hammen and Hooghiemstra, 1995; Rodbell and Seltzer, 2000; Van't Veer et al., 2000; Bush et al., 2005). Some records reveal an oscillation that fits well with the chronology of the North Atlantic event (e.g., Van't Veer et al., 2000); however, other records— such as that of Titicaca and glacial advances in Ecuador and Peru—reveal an oscillation that pre-dates the Younger Dryas by 500 years (Rodbell and Seltzer, 2000; Paduano et al., 2003). In summary, it appears that the Younger Dryas is better represented in the northern section of the Neotropics than south of the equator. Furthermore, in most settings if a change is contemporaneous with that of the North Atlantic it is manifested in precipitation change rather than in temperature shifts resulting in some glacial re-advance or retreat (Clapperton, 1993; Rodbell and Seltzer, 2000; Smith et al., 2005).

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