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Age (1000 14C yr.)

in Argentina. Note variable age scale on abscis-

FIGURE 22 Summary of lacustrine changes along the Pole-Equator-Pole: Americas (PEP 1) transect. Black histograms represent generic lacustrine conditions (lake level, salinity, etc.) expressed in dimensionless terms of effective water balance as compared to modern conditions. Unfilled histograms represent full glacial (FG) lake conditions for comparison. Shaded histograms represent questionable interpretations based on data that are lacking. Red dots on histogram abscissas identify the early Holocene period (10,000 14C B.P.). Dashed horizontal lines reflect major zones of contrasting full to late glacial and Holocene lake conditions. Locations of sites are given in Fig. 1. (From Markgraf et al., 2000. With permission.)

FIGURE 22 Summary of lacustrine changes along the Pole-Equator-Pole: Americas (PEP 1) transect. Black histograms represent generic lacustrine conditions (lake level, salinity, etc.) expressed in dimensionless terms of effective water balance as compared to modern conditions. Unfilled histograms represent full glacial (FG) lake conditions for comparison. Shaded histograms represent questionable interpretations based on data that are lacking. Red dots on histogram abscissas identify the early Holocene period (10,000 14C B.P.). Dashed horizontal lines reflect major zones of contrasting full to late glacial and Holocene lake conditions. Locations of sites are given in Fig. 1. (From Markgraf et al., 2000. With permission.)

gressive development of marsh seral stage communities around the lake. A midlake core, however, indicates that climate was the principal driver of limnological change at this site.

To the south, lakes in the Great Basin and southwestern United States were high and fresh during all or part of the full and late glacial periods as a consequence of displaced westerly storm tracks and cooler mean an nual temperatures related to the residence of polar and arctic air masses in this region. The early Holocene witnessed desiccation in these basins as the storm tracks and air masses retreated northward following the progressive melting and retreat of the continental ice sheets. Chronologies require further refinement to demonstrate and track this progressive displacement, if indeed it happened. As the records now stand—in cluding those from northern and central México, which also apparently received westerly moisture during the late glacial period—the retreat of this moisture source may have been very rapid. High lake levels at all sites between 13,000 and 12,000 14C B.P. fall by 10,000 14C B.P., although the southernmost site in this Northern Hemisphere grouping, Lago de Pátzcuaro, seems to have received increasing amounts of easterly moisture as the westerly sources declined. This decline resulted in little change in the lake level, but a clear change in the seasonality of diatom productivity. At Owens Lake, the diatom and ostracode records show that this basin fluctuated between full fresh and shallow saline conditions during the late glacial-Holocene transition (-11,000-9000 14C B.P.), with the changes taking place in less than a century. Such high-resolution records illustrate that the progressive retreat of climate systems responsible for maintaining large lakes in the Great Basin was not unidirectional, but involved at least as much variability as is seen today in the climate of this region.

Low-elevation Mesoamerican and northern South American lakes were all low or dry during the full glacial period and most of the late glacial period. Lake levels began to rise as easterly precipitation arrived or developed in this region. Dry conditions at these sites during the late glacial period are consistent with other Caribbean records, which suggest a south or southwest displacement of the Azores High and a consequent displacement or reduction of the ITCZ and tropical marine moisture sources (Bradbury, 1997). Hadley cell circulation may have been reduced by tropical cooling and a reduction of the latitudinal temperature gradient between the tropics and subtropics (Rind, 1998).

The filling of the Caribbean lake basins after 10,000 14C B.P. may represent a retreat of the Azores High and reestablishment of the northeasterly trade winds, which bring moisture to the region.

At the latitude of Central America and northern South America, there seems to be a west-to-east gradient in the timing of the transition to high lake conditions. La Yeguada appears to have risen, warmed, and freshened earlier than Laguna Los Lirios and Lago de Valencia at approximately the same latitude. The potential significance of this gradient could relate to the way in which the ITCZ was spatially manifested and strengthened by the end of the late glacial period. More well-dated sites in this region will help define the longitudinal gradients of climate change suggested by these data.

High-elevation fens and lakes in equatorial South America probably never suffered severe moisture deficits, but the abundance of Isoetes in late glacial and early Holocene records suggests cold temperatures and reduced productivity, perhaps associated with greater and more persistent lake ice cover. The arrival of warmer temperatures that included easterly precipitation in the latest glacial and early Holocene periods had important limnological effects on lake productivity, sediment chemistry, and mineralogy that reflected increased weathering and nutrient delivery to these lacustrine systems. In general, these systems became more productive in climatic regimes of increased warmth and precipitation that sponsored greater coverage of Andean forests.

If the ITCZ and its associated easterly trade wind precipitation were forced south by full glacial conditions in North America (e.g., the Laurentide ice sheet), which thereby deprived low-latitude lakes of moisture, its residence at higher latitudes in South America should be registered in lake records south of Lago de Junín (11°S); this lake is the southernmost one with a lake-level history similar to that of the low-latitude basins in Mesoamerica and northern South America. Whereas the records from central South America (16°-23.5°S) show variable latest glacial and early Holocene high stands, none of the basins in this zone appear to have been high or particularly fresh during the full glacial period—although according to the available chronology, Lake Titicaca rose to a high stand shortly thereafter. Although high-elevation sites near or north of the equator remained relatively high during the mid-Holocene, high-elevation sites farther south (Titicaca, Uyuni, and Lejía) desiccated or at least shrank by the mid-Holocene. Presumably, the high Holocene levels of lakes north of 16°S latitude reflect an intensification or concentration of easterly moisture sources related to the release of latent heat at high latitudes. There is no convincing evidence that the easterly (trade wind) moisture source existed at higher latitudes in South America during full and late glacial times in anything resembling its present form.

Lake Titicaca and Salar de Uyuni have comparable records of a major late glacial high lake stage, called the Tauca stage, a dry (Ticaña) event, and a minor resurgence in the early Holocene period, the Coipasa event. The major high stand at Laguna Lejía, in contrast, seems to correlate more closely to the Coipasa event according to reliable radiocarbon dates. Its magnitude was considerably greater than the Coipasa event farther north. Although Lake Titicaca subsequently rose to comparatively high levels in the late Holocene, the Salar de Uyuni remained low (Sylvestre, 1997).

The source of moisture for these lake basins appears to have come from the east, probably as a result of moisture advected by the South Atlantic subtropical high-pressure zone. The South Atlantic Ocean was warm by the time of the Tauca high stage event and could serve as a moisture source for lakes at the latitude of Titicaca and Salar de Uyuni. The comparatively equatorial position of the South Atlantic subtropical high may reflect the diminishing effect of the Laurentide ice sheet at that time and the continuing role of Antarctic sea ice in determining gradients of atmospheric circulation. Possibly the large Coipasa-age lake stand at Laguna Lejía (11,000-9000 14C B.P.) records the diminishing effect of Antarctica on the latitude of the South Atlantic subtropical high and its subsequent migration farther poleward at the beginning of the Holocene—coincident with the development and strengthening of the easterly trade wind circulation patterns that characterize the region today.

The two Patagonian sites in southern South America (Salinas del Bebedero [33°S] and Laguna Cari Laufquen [41°S]) have similar lake-level histories reminiscent of the North American Great Basin pattern— high levels during the full and late glacial periods and lower levels by the early Holocene. These lakes are fed by rivers from the Andes. Today, this segment of the Andes receives westerly moisture primarily during winter, when storm tracks shift toward the equator (Markgraf et al., 1992). Consequently, higher lake levels at Bebedero in the past could have related to a more persistent location, and (or) intensification, of storm tracks at these latitudes. The extensive evidence of full and late glacial moraines (>13,90014C B.P.) west of the Andes (Lowell et al., 1995) may testify to the importance of westerly precipitation in the Andes at that time. However, pollen records on the west side of the Andes (La Serena: 32°S; Puerto Octay: 40°S) and immediately east of the Andes (Gruta del Indio: 34.75°S; Aguado: 40°S) do not indicate increased moisture during the full and late glacial periods (Villagran and Varela, 1990; Moreno, 1997; D'Antoni, 1983; Markgraf and Bianchi, 1999). If westerly moisture sources were not responsible for high full and late glacial levels at Salinas del Bebedero and Laguna Cari Laufquen, it may become necessary to invoke southerly or easterly moisture associated with polar outbreaks to accomplish this (Leroux, 1993; Marengo and Rogers, 2000). Evidence of lower glacial snow lines and, hence, of more intensive glaciation on the east side of the Andes at latitudes 24°-28°S (Fox and Strecker, 1991) also suggest the possibility of easterly moisture sources at times in the past.

A single date on lithoid tufa at the highest latitude site, Lago Cardiel (49°S), indicates a full glacial lake stage marginally higher than average Holocene levels. No record of late glacial high stands exists for this site, but very high (+55 m) lake levels after 10,000 14C B.P. at Lago de Cardiel are consistent with a moisture increase due to lowered seasonality and a wide amplitude of storm tracks that reached to 50°S in response to warmer southern oceans, reduced sea ice, and a shallow thermal gradient between Antarctica and Patagonia.

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