Synopsis Of Site Records

16.2.1. Zagoskin Lake: Norton Sound, Alaska (63.40N, 162.08oW; 15 m)

Zagoskin Lake occupies a small, circular maar crater (500 m diameter) without significant fluvial input from the surrounding low, peat-mantled, volcanic surface (Ager, 1982). Overall, the geochemical record (e.g., Al) of Zagoskin Lake indicates a complacent lacustrine record with major sediment input from eolian sources (Fig. 2). However, biogenic silica and organic matter that relate to the productivity of the lake system increased after 15,000 14C B.P., approximately coincident with the pollen transition from herbaceous (tundra) vegetation to Betula. Larger increases in organic matter and biogenic silica occurred in the mid- and late Holocene as still warmer climates allowed alder and spruce vegetation to characterize the region (Ager and Brubaker, 1985). Warmer summers and seasonally open lake conditions probably caused this increased productivity. The limnological effects of increasing warmth in Arctic environments are of increasing effective moisture, nutrient fluxes, and more effective illumination as lakes remain free of ice and snow for greater periods of time.

16.2.2. Klamath Lake: Klamath Basin,

Oregon (42.3110N, 121.9150W; 1263 m)

The diatom stratigraphy of the Caledonia Marsh core from the margin of Klamath Lake documents the effect of wind on a lacustrine system. During the full glacial period, arid climate and strong easterly wind regimes (Barnosky et al., 1987) allowed the transportation and redeposition of Pliocene diatoms (Aulacoseira paucistriata) from outcrops of diatomaceous sediment in the Klamath Basin (e.g., Bradbury, 1991) to the shallow and turbid Klamath Lake (Fig. 3). By 15,000 14C B.P., the increased effective moisture and reduced wind stress produced an open, but still shallow, eutrophic lake as shown by the stratigraphic distribution of Stephanodiscus niagarae. Between 15,000 and 10,000 14C

Weight % Loss on ignition (%) analyst Weight %

FIGURE 2 Selected stratigraphic profiles from Zagoskin Lake in Alaska. Pollen zonation from T.A. Ager; biogenic silica data courtesy of S.M. Colman; geochemical data from J.P. Bradbury—all of the U.S. Geological Survey.

Weight % Loss on ignition (%) analyst Weight %

FIGURE 2 Selected stratigraphic profiles from Zagoskin Lake in Alaska. Pollen zonation from T.A. Ager; biogenic silica data courtesy of S.M. Colman; geochemical data from J.P. Bradbury—all of the U.S. Geological Survey.

FIGURE 3 Selected diatom profiles from core C-2, Caledonia Marsh, Klamath Lake, Oregon. (Data from J.P. Bradbury.)

B.P., open-water diatoms decreased in favor of marsh epiphytic and benthic species such as Staurosira con-struens, indicating an encroachment of vascular emergent vegetation. Marsh conditions persisted and expanded in the early Holocene, probably as a result of lower water levels and somewhat drier climates than previously existed.

16.2.3. Lake Lahontan: West-Central Great Basin, Nevada (center = 40.0oN, 119.0oW; High Stand = -1330 m)

The lake level chronology (Fig. 4) of Lake Lahontan is based on >100 14C dates on shoreline tufa, gastropods, soils, and plant and animal remains (Benson et al., 1990). This large data set indicates intermediate lake levels during the full glacial period (-18,000 14C B.P.) for Pyramid Lake, the lowest elevation basin in the system. Lake levels fell moderately 17,000-15,000 14C B.P.

and then rose rapidly to a high stand of 1330 m between 14,000 and 13,000 14C B.P. Levels fell equally rapidly (<500 years) after 12,500 14C B.P. to a low stand that separated the major basins in the system (1207 m). Levels again rose minimally (10 m) between 10,500 and 10,000 14C B.P. and then fell to modern lake elevations by 9500 14C B.P. (Benson et al., 1992).

The Lahontan and other Great Basin records record high stands that reflect abundant precipitation in the Sierra Nevada to the west from westerly storm tracks displaced to the south (Thompson et al., 1993). Large, shallow, and variably interconnected basins make lake levels of the Lahontan system sensitive to control by in-terbasin spilling and evaporation that complicate the lake-level history. The highest stand at 14,000-13,000 14C B.P. may document the passage of the storm track system across the latitude of the Lahontan basin on its way north as the ice sheet became smaller and lower (Thompson et al., 1993), coupled with the rapid melting of Sierra Nevada alpine glaciers.

FIGURE 4 Lake stage elevation versus age of the Lahontan basin from data gathered in the area of Pyramid Lake, Nevada. (Compiled from Benson et al., 1990, 1992.)

16.2.4. Owens Lake: Southern Great Basin, California (36.380N, 118.0oW; 1085 m)

Diatom and ostracode assemblages from cored lake sediments deposited between 25,000 and 14,500 14C B.P. document a full, turbid, overflowing lake during and after the full glacial period. The boreal ostracode Cytherissa lacustris indicates cold, stable conditions at 24,000-21,000 14C B.P., perhaps associated with the permanent residence of the Polar Front at the latitude of Owens Lake at that time. The stratigraphy of Asterionella formosa, a planktonic diatom blooming in the summer and fall, after 19,000 14C B.P. suggests that Owens Lake became less turbid, but remained fresh following the full glacial period (Fig. 5). Pollen, diatom, and ostracode evidence indicates that precipitation increased at least 1.5-2 times and that summers and winters were at times as much as 10°C colder than they are now (Bradbury and Forester, in press).

A hiatus between 15,500 and 13,500 14C B.P.—possi-bly due to lake shoaling, wave erosion, and redeposition of sediment in deeper parts of the basin—interrupts the record. Between 13,500 and 11,000 14C B.P., diatoms indicate very wet conditions and high sedimentation rates, perhaps relating to flooding and out-wash from receding alpine glaciers. Saline diatoms and ostracodes indicating decreased precipitation and discharge down the Owens River first appeared at ca. 11,000 14C B.P. After that time, the record shows alternating freshwater conditions punctuated by short (200-year) saline intervals, which became more saline throughout the early Holocene. By the mid-Holocene, the already shallow Owens Lake desiccated, creating a second hiatus between 6000 and 4000 14C B.P. At the end of the nineteenth century and the beginning of the twentieth century, Owens Lake was described as being about 15 m deep and having a salinity of 100-200 g/L (Lee, 1906).

16.2.5. Lake Estancia: Estancia Valley, Central New Mexico (34.50oN, 106.00oW; 1875 m)

A long sequence of late Pleistocene and early Holo-cene lacustrine deposits exposed by late Holocene deflation outcrop in and around the Lake Estancia playa. The sequence contains ostracodes, diatoms, and pollen, along with geochemical and sedimentological proxies of past lake changes (Bachhuber, 1982; Allen and Anderson, 1993) that document rapid fluctuations between full and intermediate lake-level stages (Fig. 6). A trend toward progressively decreasing lake levels began after 14,000 14C B.P. Sediments record strong fluctuations of lake levels, including desiccation and a final brief rise to the approximate early late glacial level between 11,500 and 10,000 14C B.P. Lake levels fell thereafter until desiccation at 8000 14C B.P., when mid-Holocene deflation began to remove sediments from the basin.

Like the Great Basin records, full glacial moisture was probably associated with more persistent and strengthened westerly storm tracks forced south to the latitude of the Estancia basin. The presence of the os-tracode Cytherissa lacustris in these deposits at 20,000-19,00014C B.P. suggested exceptionally cold conditions at Lake Estancia at that time (e.g., Bradbury and Forester, in press). Subsequent late glacial moisture pulses lack C. lacustris, but contain ostracodes (Candona rawsoni and Limnocythere ceriotuberosa) that indicate greater seasonal limnoclimatic variability.

16.2.6. Cuenca de Babícora: Chihuahua, México (29.000N, 108.000W; 2200 m)

Lacustrine sediments beneath an intermittently dry part of the Alta Babícora basin were exposed by excavation. Apparent, very slow, or variable sediment ac-

New Hemisphere Lakes
FIGURE 5 Selected diatom and ostracode profiles from Owens Lake, California. (Data from Bradbury and Forester, in press.)
Estancia Basin
FIGURE 6 Lake stage elevation vs age of the Estancia basin in New Mexico. (Data from Allen and Anderson, 1993.)

cumulation rates make this record difficult to evaluate. The bottom of the section (143 cm) is undated (Metcalfe et al., 1997), but may begin at ca. 13,000 14C B.P. (estimated by extrapolation), with a diatom assemblage characterized by Synedra fasciculata, Stephanodiscus asteroides, and S. niagarae that persists with fluctuations into the early Holocene period (Fig. 7). Synedra fascicu-lata is an attached brackish water diatom that often blooms in the summer (Bahls et al. 1984), whereas the Stephanodiscus species are planktonic in fresh, open water and bloom in the fall or early winter (S. niagarae) and spring (S. asteroides) (Bradbury and Forester, in press). The mixed assemblage probably indicates strong sea-sonality in water depth and hydrochemistry. Winter precipitation could provide fresh, open-water conditions to support first S. niagarae and then S. asteroides later in the spring. Evaporation during the arid summer would have increased lake salinity and decreased water depth to provide suitable habitats for Synedra fas-ciculata.

The inadequate late glacial chronology does not al-

FIGURE 7 Selected diatom profiles from Cuenca de Babícora, Chihuahua, México. (Data from Metcalfe et al., 1997.)

low useful discussion of the relative changes of these diatom taxa, but the assemblage clearly indicates a winter precipitation climate regime compatible with packrat midden evidence (Van Devender, 1990). Somewhat shallower, more saline environments (S. fasciculata) may have predominated at ca. 13,000 and 10,000 14C B.P. (extrapolated), and the site seems to have desiccated shortly after 9500 14C B.P. The Stephanodiscus / Syne-dra diatom assemblage—accompanied by Aulacoseira granulata, a summer, freshwater planktonic species— reappeared afterward, apparently documenting an undated Holocene interval of increased effective moisture.

16.2.7. Lago de Pátzcuaro: Michoacán, México (19.50N, 101.5oW; 2040 m)

The full glacial diatom record from a Lago de Pátzcuaro core (Bradbury, in press) was dominated by a Stephanodiscus species presumed to have bloomed in the spring under cold, low-light regimes (Fig. 8). Its presence suggests lake circulation and (or) hydrologic input in the winter and spring. An Aulacoseira species, characteristic of taxa that live in large lakes with deep mixing regimes, and Stephanodiscus niagarae, a diatom that usually blooms in the fall, probably indicate maximum effective moisture at 19,000-13,000 14C B.P. The presence of these late summer and fall diatoms re quired sufficient precipitation and cool temperatures to maintain Lago de Pátzcuaro as a fresh and comparatively deep system throughout the year, even after the end of the winter-spring rain season. Perhaps this interval of low temperatures and abundant moisture related to the glacial episodes dated on Iztaccíhuatl at the same time (Vazquez-Selem 1998). Late glacial pollen of Juniperus, Artemisia, and Isoetes (Watts and Bradbury 1982) also imply cooler and moister conditions in the winter.

After 10,000 14C B.P., Aulacoseira ambigua, a summer diatom species, dominates with increasing numbers of other shallow-water taxa. These data indicate a general shallowing and warming of the lake after 10,000 14C B.P. and a shift of nutrient input to the summer months by lake circulation at that time and (or) by nutrient influx during the summer precipitation season as happens today in central México.

16.2.8. Laguna Quexil: Guatemala (16.930N, 89.820W; 110 m)

Sediment mineralogy and pollen investigations on a core from Laguna Quexil in the Petén district of Guatemala allow important late Wisconsin limnologi-cal interpretations despite inadequate chronological control (Leyden et al., 1993) (Fig. 9). Gypsiferous clays bearing abundant grass and juniper pollen, but devoid of Botryococcus indicate extremely arid conditions during intervals presumed to represent the full and early postglacial periods. Analog vegetation communities live at higher elevations in environments 6.5°-8°C cooler than now at Laguna Quexil. Carbonate sedimentation and Chenopodiineae pollen increased afterwards, peaking by an estimated 12,500 14C B.P. These changes indicate that Laguna Quexil was alkaline and slightly saline, with fluctuating levels that responded to modest changes in effective moisture. By the early Holo-cene, organic sedimentation with abundant lowland rain forest pollen assemblages (Moraceae) characterize the record, indicating substantial increases in moisture and temperature to modern levels.

16.2.9. La Yeguada: Panama (8.450N, 80.850W, 650 m)

The 14,000 14C B.P. lake core record at La Yeguada (Bush et al., 1992) contains diatom, pollen, geochem-ical, and mineralogical evidence of a cool and dry (but seasonally wet) climate between 14,300 and 10,800 14C B.P. Shallow-water (attached) and saline diatoms characterize the calcium-rich levels between 14,300 and 12,800 14C B.P. (Fig. 10). This record corre-

Diatoms Explain
FIGURE 8 Selected diatom profiles from Lago de Pátzcuaro, Michoacán, México. (Data from J. P. Bradbury, in press.)

sponds to similar environments at Lake Valencia and indicates a modest precipitation deficit in the early late glacial period. However, strong fluctuations between Aulacoseira granulata early in this interval suggest submillennial-scale climatic fluctuations that require further chronological and paleontological resolution. After 12,800 14C B.P., A. granulata and Cy-clotella stelligera dominated and replaced indicators of shallow, saline conditions in a productive, seasonally fluctuating lake. The laminated sediments and C. stel-ligera imply seasonal stratification under warm climates. Smectite deposition predominated throughout the late glacial period, indicating moderate chemical weathering and therefore somewhat dry, cool environments, as did the presence of Quercus and other montane plants.

Warm and wet conditions indicated by a dominance of kaolinite followed (10,500-8200 14C B.P.), but an absence of diatoms during this interval remains unexplained. Silica limitation—perhaps related to leached, silica-poor tropical soils and persistent lake stratification and reduced nutrient influx—may have been relevant in this case. Therefore, the early Holocene pa-leolimnological record contains little information of value in evaluating paleoclimate. La Yeguada appears to have freshened earlier than Lago Valencia, perhaps reflecting a west-to-east gradient in moisture availability in the early late glacial period. The mid- to late Holocene dominance of Aulacoseira ambigua may suggest lower rates of nutrient supply (Cumming et al., 1995) to La Yeguada than before, perhaps as a result of progressive soil weathering.

16.2.10. Laguna Los Lirios: Merida,

Geochemical and clay analyses from the 17,500 14C B.P. Los Lirios core (Weingarten, 1988) broadly divide the record into periods before and after 9300 14C B.P. Be-

FIGURE 9 Selected geochemical, algal, and pollen profiles from Laguna Quexil in the Peten district of Guatemala. (Data from Leyden et al., 1993.)
FIGURE 10 Selected diatom, geochemical, and mineralogical profiles from La Yeguada in Panama. (Data from Bush et al., 1992.)

Weight %

FIGURE 11 Selected geochemical profiles from Laguna Los Lirios in Venezuela. (Data from Weingarten, 1998.)

Weight %

FIGURE 11 Selected geochemical profiles from Laguna Los Lirios in Venezuela. (Data from Weingarten, 1998.)

fore 9300 14C B.P., crystalline clays (principally chlorite and illite) and relatively lower amounts of organic matter and of extractable Fe and Mn characterize the late glacial period. After 9300 14C B.P., clays are largely amorphous, and organic matter and extractable Fe and Mn increase (Fig. 11). Crystalline clays reflect cold and relatively dry climatic conditions in which physical weathering predominated. The increased abundances of organic matter and of extractable Fe and Mn after 9300 14C B.P. indicate increased lake productivity coupled with greater development of surrounding vegetation; the chemical reduction and subsequent deposition of Fe and Mn were thereby increased. Amorphous clays that characterized the Holocene document increased temperatures and moisture and, consequently, increased chemical weathering.

Fluctuations of these parameters in the record, especially during the late glacial period (Fig. 11), almost certainly reflect climatic variations (Weingarten, 1988). The record suggests that cold and dry conditions at 17,000 14C B.P., which become cold and wet at 15,00014,000 14C B.P., possibly coincide with maximum glaciation. Variable, but generally dry climates between 13,400 and 11,300 14C B.P. became quite dry between 11,300 and 9800 14C B.P. Increasingly warmer and wetter climates followed, to become very warm and wet by 8000 14C B.P. Mid-Holocene climates are cooler and wet, reflecting neoglacial climates.

Cold and wet climates in the Venezuelan Andes at 15,000-14,000 14C B.P. contrast with presumed dry climates at Lake Valencia at the same time, possibly reflecting the effect of elevation and (or) the more westerly location of the Los Lirios record. Records for both Lake Valencia and Laguna Los Lirios document increased moisture levels by and following 10,000 14C B.P., which probably indicated the development of easterly (trade wind) source areas of moisture (e.g., the Gulf of México).

16.2.11. Lago de Valencia: Venezuela (10.18oN, 67.7oW; 404 m)

A 7.3-m core from the deepest part of Lake Valencia has a basal 14C age of 12,900 B.P. (Fig. 12). Stiff clays,

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