Multiproxy palaeoenvironmental and palaeolimnological analyses of two Holocene-age sediment cores from the margin of Lago Cardiel, a 76 m deep, closed-basin lake in southern Patagonia (latitude 49°S), provide information on lake-level changes that can be related to regional palaeoclimate scenarios. Sedimentol ogic (magnetic susceptibility, organic and inorganic carbon content) and environmental indicators (pollen, dia toms, ostracodes and stable isotopes on ostracodes) show lake levels markedly higher than today during the early Holocene, following a rapid lake-level rise after a desiccation phase prior to 11000 BP. After about 6000 BP, lake levels were generally lower, but underwent repeated fluctuations. These inferred changes support the previously proposed view that the southern westerly stormtracks were focused (zonal) north of latitude 50°S during the early Holocene, allowing for Antarctic cold fronts to bring easterly moisture to southern Patagonia, whereas during the late Holocene the stormtracks shifted seasonally, with an overall more meridional behaviour, resulting in less and more variable moisture at these latitudes.

Chemical, paleontological, and mineralogical analyses of a 7.5-meter core from the middle of Lake Valencia, Venezuela, have provided information on the paleoclimatic history of this low-elevation, low-latitude site for the last 13,000 years. The data show that dry climates existed in this region from 13,000 years before present (B.P.) until about 10,000 years B.P. The Lake Valencia Basin was occupied by intermittent saline marshes at that time. About 10,000 years B.P., a permanent lake of fluctuating salinity formed and arboreal plant communities replaced the earlier dominant xeric herbaceous vegetation and marsh plants. By 8500 years B.P., Lake Valencia reached moderate to low salinities and discharged water; the modern vegetation became established at that time. After 8500 years B.P., the lake twice ceased discharging as a result of reduced watershed moisture. The second of these drying episodes is still in progress and has been aggravated by human activities in the watershed.

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A b s t r a c tFossil diatom assemblages in short cores of lake sediment from nine lacustrine environments (seven lakes) in Minnesota and South Dakota show the ecological reaction of freshwater diatoms to limnologic changes associated with enrichment following European-American settlement. The time of settlement is stratigraphically determined by an increase in the proportion of Ambrosia (ragweed) pollen, which signals late 19th century land clearance and cultivation in this region. In some cases other stratigraphic profiles, such as phosphorus or mining wastes, are used to date settlement activities near the lakes. Marked changes in the diatom stratigraphy frequently correlate with the time of settlement around the lakes, and in most cases it has been possible to interpret these changes in the context of limnologic modifications caused directly or indirectly by the settlement activities of man.Initially, diatom diversity decreases as the lakes become enriched by increased erosion and (or) by disposal of municipal wastes. Littoral (epiphytic and benthic) diatoms become underrepresented in the sedimentary record compared to predistur bance times, perhaps because of excessive shading by blue-green algae, or simply because they are numerically swamped out by massive blooms of planktonic diatoms. As enrichment increases, the more or less even distribution of spring, summer, and fall planktonic diatoms changes to a planktonic diatom flora dominated by species that bloom in the early spring-sometimes even under the ice. Foremost among these are Stephanodiscus minutus and S. hantzschii, the latter characterizing the most eutrophic lakes studied. Apparently the summer and fall diatom plankton cannot compete with the massive blooms of floating, blue-green algae that occur in the warmer seasons. Only in the shallow turbulent lakes do the heavy summer and fall diatom plankters maintain sizable populations that effectively compete with the buoyant blue-green algae. The stratigraphic record of the blue-green algae is inferred by the stratigraphy of Chydorus sphaericus, a normally littoral cladoceran that utilizes blue-green algal filaments to suspend itself in the limnetic regions of a lake, thus vastly increasing its habitat and population size.There are several variations on this theme, depending on the initial trophic state of the lake and other limnologic characteristics such as basin morphometry, but overall, the diatom stratigraphy of lake sediment is an effective way to assess man's impact on the lake ecosystem.

Increased aridity throughout the Great Plains and Rocky Mountain region during the middle Holo cene has been documented from pollen records, aeolian proxy variables in lake cores, and active sand dune migration. Varve calibration provided by a continuously varved record of the Holocene from a core from Elk Lake, northwestern Minnesota, shows that the influx of aeolian clastic material increased beginning about 8 ka and ended about 3.8 ka, with peak aeolian activity at about 6 ka. If aeolian influx to Elk Lake corresponds in time to aeolian influx in other lakes and to maximum dune activity in Minnesota dune fields, then the varve calibration in Elk Lake provides precise time calibration of periods of peak aeolian activity in Minnesota. Palaeowind studies from the Minnesota dune fields show that the dominant wind direction when the dunes were active was from the northwest, the same as the dominant wind direction in dune fields throughout the Great Plains and Rocky Mountains. If the mid-Holocene aeolian activity in Minnesota was driven by an increase in westerly zonal winds, then the varve calibration can be extended to more precisely determine the timing of activity of dunes over a much broader area. We suggest that an increase in the westerly zonal wind field might have a solar-geomagnetic cause.

Sediment traps placed in the profundal region of Elk Lake, north central Minnesota during the 1979 spring and 1983-84 fall and spring seasons monitored seasonal diatom production for two climatically distinctive periods. The spring of 1979 was one of the coldest and wettest on record. Ice out at Elk Lake was 10 days later than average, and spring circulation was short. Fragilaria crotonensis dominated the late spring and early summer diatom production in association with Synedra and Cyclotella species, perhaps because rates of phosphorus supply were low compared to silica. The winter of 1983-84 was drier than usual, and the early but cold spring of 1984 caused ice out at Elk Lake to be about 1 week earlier than normal. Spring storms promoted a long and full circulation that allowed Stephanodiscus minulutus to bloom, presumably in response to increased phosphorus loading related to deep and vigorous circulation. The two dominant diatoms in Elk Lake, F. crotonensis and S. minutulus may reflect climatic patterns that control lake circulation. The climatically regulated occurrence of these diatoms is generally, but not specifically, comparable to their distribution in lake surface sediments throughout Minnesota. A combination of regional and lake-specific studies on the controls of diatom succession and distribution provides climatic insights for interpreting paleolimnogical records of diatoms.

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