Southern Hemisphere

Few detailed Holocene records exist at present for the Southern Hemisphere (Fig. 8). For the northern Altiplano of Bolivia and Peru, dating of paleolake shorelines (Servant and Fontes, 1978; Servant et al., 1995) and sedimentological and ostracode data from cores in the small basin of Lake Titicaca indicate high lake stands during late glacial times and a subsequent decline (Wirrmann and DeOliveira Almeida, 1987; Wirrmann and Mourguiart, 1995), although the timing of the regression is not very well constrained. Clearly, the interval from 8000-3600 B.P. was one of extreme aridity in both the northern and southern Altiplano, and many lakes went dry or dropped significantly below modern levels. The small basin of Lake Titicaca desiccated except in a single, small, deep hole (Wirrmann and De-Oliveira Almeida, 1987; Abbott et al., 1997a), and seismic studies in the large basin suggest a lake-level decline of nearly 100 m (Seltzer et al., 1998). Sedimen-tological and geochemical records for small, glacier-fed lakes in the northern Altiplano suggest the onset of glacial retreat between 10,700 and 9700 B.P., the absence of glaciers in the watershed of the lakes after 8800 B.P., and episodic drought throughout the mid-Holocene (Abbott et al., 1997b). Similarly, in Lake Miscanti in

Totem Northern Southern Hemisphere
FIGURE 8 Map of South America showing locations of sites mentioned in the text and the positions of the Intertropical Convergence Zone (ITCZ) at present.

northern Chile at 23oS, a transition from finely laminated carbonate mud to banded carbonate sediments with allochthonous debris and sand suggests a decline in lake levels after 8000 B.P., following early Holocene high stands (Grosjean et al., 1995; Valero-Garces et al., 1996). Sedimentological, mineralogical, and geochemi-cal data for Laguna del Negro Francisco at 27oS in southern Chile show that desiccation occurred prior to 6000 B.P., and frequent subaerial exposure of the lake bottom occurred between 6000 and 3800 B.P. Evidence from the latter site indicates that the dry mid-Holocene was punctuated by humid phases of relatively short duration (Grosjean et al., 1997).

Lake levels began to rise throughout the Altiplano at ca. 3600 B.P. and attained modern levels at ca. 3000 B.P. in the southern Altiplano (Valero-Garces et al., 1996; Grosjean et al., 1997; Nunez et al., 2000) and by 2100 B.P. in the northern Altiplano, as evidenced by lithological, biological, and geochemical data for both Titicaca and the smaller lakes (Wirrmann and Mourguiart, 1995; Abbott et al., 1997a,b; Cross et al., 2000). However, clearly the climate of the last 2000 years was variable, punctuated by intervals of both high effective moisture (Grosjean et al., 1997) and drought (Abbott et al., 1997a; Bin-ford et al., 1997).

In northern Patagonia (41°S), lake shorelines show an early Holocene decline in lake levels, following late glacial and glacial high stands (Galloway et al., 1988). In southern Patagonia, geomorphic evidence from Lago Cardiel (Stine and Stine, 1990) similarly suggests a decline in lake levels from an early Holocene high stand after 7700 B.P., followed by persistent low levels until ca. 5100 B.P. A brief lake-level rise at 5100 B.P. is followed by several thousand years of dry conditions, with a series of century-scale episodes of moderate lake-level rise during the last 2000 years (Stine, 1994).

15.4.1. South American Climate Patterns

Most of the precipitation in the South American Altiplano falls during the austral summer and is highly correlated with convection in the Amazon basin. Precipitation variability is linked to shifts in the position and intensity of the Bolivian High, an upper troposphere high-pressure system, which controls the upper atmospheric winds and advection of moisture from the Amazon (Lenters and Cook, 1997). In the contemporary climate system, a northward shift in the position of the Bolivian High is associated with reduced precipitation on the Altiplano, and this scenario may provide an analog for mid-Holocene circulation patterns and the resultant mid-Holocene aridity (Fig. 9b). These pos-


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  • carmen
    What hemisphere is lake titicaca in?
    1 year ago

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