Lake Mascardi

Lake Mascardi (30 km2 and 200 m maximum depth) is a typical proglacial lake located ca. 800 m above sea level (asl) and 15 km from the glacier front of a tongue of the Tronador ice cap (41°10' S, 71°53' W; 3554 m asl). Precipitation falls mostly as snow in the winter season, reaching maximum values of ca. 4000 mm/year (Drago, 1973). The western branch of the lake is fed directly by glacial meltwater through the Upper Manso River (Fig. 1B). Previous work showed that the extent of the Tronador ice cap is sensitive to both winter precipitation derived from the southern Pacific westerlies and to mean summer temperatures (Villalba et al., 1990). Thus, the Lake Mascardi sediments record fluctuations in glacial meltwater activity that provide a record of Southern Hemisphere postglacial climate variability (Ariztegui et al., 1997 and references therein).

Approximately 60 km of 3.5-kHz profiles allow a reconstruction of the lake bathymetry and sediment structure, as well as the separation of the effects of climate and neotectonics on lake sedimentation. This is important because the lake is located in an area of significant Holocene volcanic activity. The profiles record up to 50 m below the lake floor, representing approximately the last 15,000 years of infill history. Sedimentation is characterized by a paucity of chaotic debris and a relatively simple infill stratigraphy. The bedrock surface and the thick proglacial sediments reflect glacial erosion and the impact of proglacial meltwater influxes. Although the predominant pattern of sedimentation comprises simple and continuous basin infilling, variable sedimentation rates, as well as hiatuses, were identified in certain areas of the lake.

Comparison of the seismic results with multiproxy analyses from a suite of sediment cores established a well-dated lithostratigraphy (Ariztegui et al., 1997). Figure 2 shows uninterpreted and interpreted seismic sections with seismic units in position A-B at a water depth of 30 m (see Fig. 1B for locations of cross sections). Assuming a p-wave velocity of 1460 m/sec in the sediments, 10 m/ sec of two-way travel time in the seismic profiles corresponds to 7.3 m in the sediment cores. Different shades of gray in the interpreted seismic section outline seismic sequences, which were mapped by tracing reflection terminations abutting unconformities. Radiocarbon ages could be assigned to sediment layers equivalent to seismic reflectors. Distinctive seismic facies were used to distinguish Holocene (coherent high-amplitude reflections) and late glacial deposits (low amplitude to transparent). These differences in the seismic facies are caused by differences in their physical properties. The mapping of reflector patterns that define seismic unconformities indicate two major environmental events that probably represent lake-level changes (Fig. 2). The first of these unconformities corresponds to the late glacial/ Holocene transition and is marked by a distinct on-lap

Northern Hemisphere Lakes
FIGURE 1 (A) Map of North and South America with the locations of the lacustrine basins with seismic profiles (Figs. 2-9) discussed in the text. Maps of (B) Lake Mascardi (Argentina), (C) Laguna Cari-Laufquen Grande (Argentina), and (D) Lake Titicaca (Bolivia-Peru).

surface. The sedimentation at the relatively shallow site of Fig. 2 is very sensitive to changes in water depth. Consequently, this change in lake level was recorded in the sediments not only as a change in lithology, but also as a change in sediment geometry, documented by the on-lapping reflections. Both the acoustic and lithologic features indicate an abrupt rather than a smooth change in the depositional environment. The second unconformity seems to be associated with an event in the mid-Holocene. It shows a less prominent impedance contrast than the Holocene boundary and, thus, may reflect a more gradual change in environmental conditions.

Figure 3 shows the uninterpreted (left) and interpreted (right) seismic section from position C-D at a water depth of 100 m. Compared to the A-B section, this section lies closer to the primary inflow to the lake and, thus, is closer to the source of sediments. The seismic profile in section C-D combined with core data clearly shows that this area of the lake is characterized by much higher sedimentation rates, therefore providing a higher resolution record of the Holocene. Because this section lies in deeper water than section A-B, the sedimentary processes are less influenced by environmental change than those from the shallower areas of the lake. Consequently, the sedimentary geometry is rather conformable. The late glacial/ Holocene transition in Lake Mascardi is marked by a change in core lithology from fine-grained uniform muds to muds with more clastic intercalations. This

Northern Hemisphere Lakes
FIGURE 2 Lake Mascardi: uninterpreted (left) and interpreted (right) seismic profile in position A-B (refer to Fig. 1 for location). The late glacial/Holocene transition is seismically defined by a distinct on-lap surface, documenting a significant change in environment.

change is registered by changes in the seismic facies from low-amplitude internal reflections in the late glacial period to a facies characterized by layered internal reflectors of gradually increasing amplitudes for the Holocene.

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