Www Winds South Chile Holocene

Viewed from this very general, low-resolution perspective, full glacial-early Holocene lake records along the PEP 1 transect (Fig. 22) document only six major interacting climate scenarios (Fig. 23):

1. Unglaciated Arctic lake systems witness progressive warming related to deglaciation by a reduction in the thickness and seasonal extent of ice cover and a concomitant increase in productivity. Additional records could certainly show other climate/lake interactions related to changing moisture sources and winds, changing seasonality of insolation, and varying styles of land/lake interactions mediated by vegetation and erosion changes under climate control.

2. Midlatitude, western North American late glacial lakes responded to an intensification and greater persistence of westerly moisture delivered by storm tracks forced south by climate changes that encouraged the growth of ice sheets and southerly displacement of polar air masses.

3. South of central México and north of Lago Titicaca in southern Bolivia (16.0°S), all lacustrine systems seem to have been low or dry in the full glacial period, apparently as a consequence of substantially reduced ITCZ / trade wind circulation systems that today provide moisture to these basins. Late glacial moisture increases characterized the western sites, whereas the timing of the arrival of easterly moisture approximately coincided with the beginning of the Holocene at the eastern lake sites (Lago de Valencia and Laguna Los Lirios). The spatial coherence and climatic significance of this apparent gradient must await additional study of well-dated records.

4. Lake records from southern South America at lat itudes 16°-23.5°S were also low or dry during the full glacial period, but record important high stands during the late glacial period. The most notable of these (the

Tauca lake stage) occurred at Lago Titicaca and Salar de

Uyuni, but apparently not at Laguna Lejía (23.5°S), which rose to higher than modern levels only during the latest glacial and early Holocene. Barring unfore seen chronological problems at these sites, the reasons for this rise are not understood at present. They may re late to cool ocean temperatures coupled with the location of the northern, moisture-bearing limbs of high-pressure systems generated by polar outbreaks or other mechanisms. The oceans may have been too cold and

Www Winds South Chile Holocene

FIGURE 23 Summary of hypothesized climate causes of full to late glacial lake status changes compared to the Holocene along the Pole-Equator-Pole: Americas (PEP 1) transect. The numbers that identify roughly coherent paleoclimatic scenarios correspond to the numbered conclusions in Section 16.4. ka = thousands of radiocarbon years before present. Locations of sites are given in Fig. 1.

FIGURE 23 Summary of hypothesized climate causes of full to late glacial lake status changes compared to the Holocene along the Pole-Equator-Pole: Americas (PEP 1) transect. The numbers that identify roughly coherent paleoclimatic scenarios correspond to the numbered conclusions in Section 16.4. ka = thousands of radiocarbon years before present. Locations of sites are given in Fig. 1.

(or) the appropriate wind systems too far equatorward to provide moisture to Laguna Lejía until the early Holocene.

5. Between 30° and 41°S latitude, two lake systems, Salinas del Bebedero and Laguna Cari Laufquen, document high lake stages during the full and early late glacial periods and lower levels thereafter. These changes may also reflect moisture sources related to outbreaks of polar air and the interaction with low-pressure systems in the western Atlantic. Such pro cesses would have been most active and intense in the winter, and precipitation derived from them could have maintained lakes in otherwise arid environments. Cooler overall temperatures clearly contributed to a positive moisture balance. Low temperatures may have been more important than precipitation per se in reducing evaporation and allowing high lake stages.

6. South of latitude 45°S, Lago Cardiel is the only studied representative of lake systems with sediment and geomorphic evidence of past lake stages higher than modern levels. Lago Cardiel has no record of full or late glacial stages higher than its present, comparatively low levels. If it was indeed low during the full and late glacial periods, the reason may be ascribed to a cold South Pacific Ocean and a northward displacement of westerly storm tracks beyond the latitude of Lago Cardiel. If so, higher than modern levels in the early Holocene period must reflect warm ocean sources of moisture finally reaching relevant latitudes to the south and the poleward retreat of the westerlies to deliver the moisture to the lake as happens today.

At this scale of resolution, it appears that past inter-hemispheric climate interactions in the Western Hemisphere are synchronous, but not necessarily of the same sign, and are therefore linked, at least indirectly. The massive Laurentide/Cordilleran ice sheet or those processes that sustained it may have forced climate changes throughout the western Northern Hemisphere and perhaps even as far south as central South America. The southerly displacement of westerly storm tracks in North America has been modeled (Kutzbach, 1987), and western North America lake records register this displacement. Easterly precipitation sources associated with the ITCZ diminished in importance or disappeared altogether, perhaps as a consequence of general cooling in tropical and subtropical regions. At least no records in northern and central South America studied so far appear to document increased easterly (ITCZ) precipitation during full or early late glacial times. To account for dry conditions during the full and late glacial periods, the ITCZ is supposed to have resided farther north by South American workers (e.g., Martin et al., 1997) and farther south by North American workers (e.g., Bradbury, 1997). Perhaps the resolution to this problem lies in the weakening, restriction, and redirection of easterly moisture sources by the effects of mobile polar highs as has been suggested by Leroux (1993).

A late glacial expansion of southern South American lakes as a result of intensified and more northerly displaced westerly moisture sources conflicts with pollen evidence at the same latitude west of the Andes. As a consequence, precipitation of moisture from the southwest Atlantic, related to outbreaks of polar air originating over a very cold Antarctica, may have provided a positive hydrologic balance for these lacustrine systems. The synchroneity of midlatitude lake-level records in both hemispheres probably relates to much colder polar climates ultimately thought to be caused by reduced heat transport to the north and south related to reduced thermohaline circulation controlled by glacial meltwater processes in the North Atlantic Ocean (e.g., Alley, 1995).

To provide relevant input to interhemispheric pale-oclimate interactions and histories, future paleolimno-logical studies must concentrate on high-resolution, multiproxy analyses, including coherent, fine-scale sediment facies analyses with chronological control at millennial scales. Most important, paleolimnological records must be convincingly related to local and regional climate controls through both neolimnological and climate monitoring.


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