CONSTRAINTS ON THE ORIGIN OF PALEOLAKE EXPANSIONS IN THE CENTRAL ANDES
Troy A. Blodgett Department of Geological Sciences, Cornell University, Ithaca, NY E-mail: [email protected] John D. Lenters Atmospheric Science Program, Cornell University, Ithaca, NY E-mail: [email protected] Bryan L. Isacks Department of Geological Sciences, Cornell University, Ithaca, NY E-mail: [email protected] Corresponding Author: Troy Blodgett Abstract During the late Pleistocene, at least one episode of lake expansions occurred in the internally draining high plateau region of Bolivia. Some researchers have advocated that a wetter climate associated with a change in atmospheric circulation caused the development of the large paleolakes, while others have hypothesized that deglaciation contributed to the water source for the expanding lakes. From estimates of the potential meltwater stored in the glaciers during their maximum extents, we conclude that insufficient meltwater was available to fill the large paleolakes. However, the meltwater hypothesis remains viable south of the main plateau region where, in five small drainage basins, the volume of available glacial meltwater was 3 to 16 times greater than the volume of water in the paleolakes. Pollen, dunes, and other eolian features indicate that the region surrounding the Altiplano was much drier during at least one interval of the Late Pleistocene. Although the timing of the dry period with respect to the paleolakes is still unknown, a pluvial explanation for the existence of paleolakes seems unlikely. Decreased evaporative loss, however, remains a possible explanation. To understand what factors could have been associated with a decrease in evaporation rates over the drainage basin, an evaporation model is developed based on the energy balance and bulk-transfer methods. The model indicates that a 10 °C drop in air temperature or a doubling in cloudcover could have caused the paleolakes to reach their highest levels. Alternatively, a 50% increase in precipitation rate could have also maintained the paleolakes. 1. Introduction 1.1. Evidence of paleolakes Evidence of former lakes in the Central Andes was first documented in the mid 1800’s when explorers observed lacustrine deposits and high shorelines around Lake Titicaca and identified algal-carbonate beach ridges four meters above the basin floor near Lake Poopó (Figure 1) (Clapperton, 1993). Distinct erosional shore platforms and depositional gravel beach surround most of the salars and lakes between 15˚ and 23˚ south latitude and throughout much of the internally drained portion of the Central Andes. The modern climate encompassing the region of paleolake development is diverse and include both arid regions receiving less than 200 mm of precipitation per year and more humid areas receiving over 800 mm per year. Within the numerous internally draining basins on the high plateau (Altiplano), deposits and shorelines corresponding to several episodes of paleolake expansions have been identified. The oldest and highest of the paleolakes identified is Lake Mataro in the basin containing Lake Titicaca. Lake Mataro is tentatively dated as Late Pliocene because it lies just above a 2.8 ± 0.4 Ma. ash bed (Lavanu, 1986). Evidence of two other paleolakes named Lake Cabana and Lake Ballivian have also been found within the Titicaca basin and are roughly mid to late Pleistocene in age (Clapperton 1993). During high paleolake phases in the southern Altiplano, the present Poopó, Coipasa, and Uyuni drainage basins were integrated into one basin (Figure 1, 2, 3). Within the Poopó-Coipasa-Uyuni drainage basin, Ahlfeld and Branisa (1960) identified conspicuous erosional benches cut into the flanks of the volcanic stock Cerro Lipillipi at the elevations of 3680 m, 3685 m, 3710 m, and 3735 m. Based on radiocarbon ages, the four terraces and a higher depositional shoreline at 3760 m were originally associated with Lake Minchin, while a shoreline consisting of clayey and calcareous diatomites 2-5 m thick at 3720 m was considered evidence of another 3950 Lake Mataro
3900 Lake Cabana
Lake Ballivian 3850 Titicaca highstands 3800 Titicaca elevation (m) 3750 Lake Tauca (3760 m) Lake Tauca (3720 m) 3700 Poopó Coipasa Uyuni 3650 Fig. 3 Schematic profile of modern drainage. Thick black lines represent relative modern lake/salar areas. Thin black lines show relative slope length and slope gradients (slopes are vertically exagerated). paleolake named Lake Tauca (Clapperton, 1993; Servant and Fontes, 1978). However, the original Lake Minchin dates are now considered suspect because of the likelihood of contamination by older carbon. Since dates acquired by Bills et al. (1994) show that the maximum level of Lake Minchin (3760 m) was attained by the Tauca lake phase at 13,790 yr BP, a distinction no longer exists between the maximum elevation of Lake Minchin and Lake Tauca. Additional AMS and Th/U series dates corresponding to several lower lake levels indicate that either subsequent lesser high lake phases developed or the Tauca lake phase persisted until about 9,500 yr BP (Clayton and Clapperton, in press; Servant et al., 1995). To avoid confusion associated with new shoreline dates collected within the Uyuni-Coipasa- Poopó basin, we simply consider the highest shoreline level (3760 m) to be the highest elevation attained by the Tauca lake phase. Lower shoreline elevations are considered lower levels of the Tauca lake phase. Although the age of the highest shoreline at 3825 m near the shores of Lake Titicaca has not been established, for our calculations we consider the 3825 m level part of Tauca (3760 m), because a highstand in the Lake Titicaca basin is likely to have been contemporaneous with a highstand in the Titicaca-Uyuni-Poopó-Coipasa basin (Wirrmann and Mourguiart, 1995). Servant and Fontes (1978) also dated shorelines within a small basin to the south that now contains Laguna Khota (Figure 4) at 11,000 to 12,500 yr BP. This small paleolake’s age suggests that the cause of the Tauca phase may also have been felt outside of the Uyuni-Coipasa-Poopó drainage basin. Glacial meltwater was one of the first hypotheses proposed to account for the paleolake expansions (Servant and Fontes, 1978). Kessler (1984) proposed as an alternative hypothesis that pluvial conditions accounted for the expanded lakes. He argued that the glaciers melted during a regressive lake phase and thus contributed no meltwater to the expansive lake phase. He further proposed that a southerly shift in the intertropical convergence zone had increased the amount of Amazonian -21.5
Laguna Yapi
Laguna Cachi
Laguna Capina
Laguna Khara
-22
10 km
salars/lakes Laguna Colorada paleolakes
former glaciers drainage divides
-67.5
Fig. 4 Map showing extent of former glaciers, paleolakes, and present salars/lakes in the Southern Altiplano. moisture reaching the plateau, and estimated that at least a 30% increase in precipitation would have been required to account for the Lake Tauca (3720 m) shorelines. A similar model was constructed by Hastenrath and Kutzbach (1985), who also rejected the idea that glacial meltwater could account for the paleolakes. They estimated that the expansion of two of the Lake Tauca phases at 3720 m and 3760 m resulted from precipitation increases of greater than 50% and 75%, respectively. A definitive test of the hypothesis that melting glaciers filled the basins would be to reconstruct the former glacier and lake water volumes to see which is greater. This is undertaken in the present study for the Titicaca-Uyuni-Coipasa-Poopó basin as well as for five small basins in the southern Altiplano (Figure 4). Furthermore, since reduced evaporation remains a potential explanation for the lake highstands, an evaporation model is constructed to investigate the sensitivity of evaporation rates to altered atmospheric conditions.
1.2. Modern and late Pleistocene drainage patterns Because many of the drainage basins on the Altiplano overflowed and coalesced during the paleolake expansions, the modern series of drainage basins have been grouped into just two basins connected by the Desaguadero river (Figure 1, 3). Evidence of basin coalescence has also been documented in modern time. During the rainy season in the wet year of 1986, water not only reached Lake Poopó as is typical, but the inundation also flooded the Coipasa and Uyuni salars (Roche et al., 1991). One complication to this simple approach is that presently a small drainage area south of Lake Titicaca drains northward when Lake Titicaca is low, but drains southward into the Desaguadero river watershed when Lake Titicaca is high. For modeling present conditions, we include this alternating drainage as part of the Titicaca basin (Figure 1). However, since the highstand around Lake Titicaca was probably coincident with the Tauca lake phase, and the alternating drainage region area was likely to have drained directly into the Uyuni-Poopó-Coipasa basin during inundation episodes, the alternating drainage area is considered part of the Uyuni- Coipasa-Poopó basin during the Tauca lake phase. The combined area of the Titicaca-Uyuni-Coipasa-Poopó drainage basin is 198,000 km2.
2. Comparison of former ice volume and paleolake volume 2.1. Methodology The maximum extent of moraines and their associated glacier headwalls were digitally mapped on Landsat Thematic Mapper (TM) images within the Uyuni- Coipasa-Poopó and Titicaca drainage basins (Figure 1, 2). Both TM images and topographic maps were used to digitize the basin divides. Fox (1993) reconstructed the volume of 69 representative Pleistocene glaciers on the Southern Bolivian Andes using a digital version of the method described by Porter (1975) (Figure 5). Porter's method reconstructs former glacier surfaces using knowledge about modern glacier surfaces. Ice-surface contours of topographic maps of typical existing alpine glaciers show that the accumulation zone's slope is concave and the slope of the ablation zone is convex. The contours cut straight across the glacier near the equilibrium line altitude (ELA). A toe-to-headwall altitude ratio (THAR) method was used to estimate the ELA by Charlesworth (1957) and Meirding (1982). The THAR method was chosen in lieu of the accumulation area ratio (AAR) method, because AAR values for Andean glaciers range from .38 to .95 (Jordan, 1991). A THAR of 0.5 reliably determines the ELA of existing glaciers in the Bolivian cordillera (Oyarzun, 1987). Figure 6 shows the empirical relationship between former glacier area and ice volume, as derived by Fox (1993). Meltwater volume was determined by multiplying each glacier volume estimate by an assumed ice-to-water density ratio of 0.9. A. Reconstructed glacier profile
5.0 estimated ELA
reconstructed glacier surface 4.5 elevation (km) elevation present valley floor
1.0 2.0 3.0 4.0 distance up valley (km)
B. Reconstructed glacier surface 4900
4700
ELA C. Dimensions of reconstructed glacier 4500 4600
length 3.60 km area 3.32 km2 4400 maximum depth 147 m mean depth 62 m volume 0.206 km3 4300
4200 1 km 4100 contour interval: 100 m 4000
Fig. 5 Longitudinal profile of reconstructed glacier on Cerro Tunupa (A), reconstructed glacier surface (B), and physical dimensions of the glacier calculated using a geographic information system (C). (Fox, 1993, fig. 3.12) 9
8 y = 0.067x + 0.003x2 R2 = .997 7
6 ) 3
5
Volume (km 4
3
2
1
10 20 30 40
Area (km2)
Fig. 6 Relationship between the area and volume of the 69 reconstructed late Pleistocene glaciers. (Fox 1993, Fig 3.13) The relationship between lake area and lake elevation was determined based on digitized 20 m contour intervals and the extent and elevations of lakes/salars from 1:50,000 maps (Figure 7). Islands of elevations higher than the lake surface were subtracted out. Because the lake expansions within the Titicaca basin are assumed to have been coincident with the Tauca lake phase of the Uyuni-Coipasa-Poopó basin, the paleolake surfaces for each of the basins were combined to determine the total area inundated by the Tauca phases. Digitized 1:50,000 maps were used to determine the hypsometry of the Uyuni-Coipasa-Poopó basin, but in the Titicaca basin, data gaps in the Peruvian region required some contour data to be inferred from maps at a scale of 1:500,000 . For the five isolated basins in the south, shorelines identified and mapped on TM images were used to determine paleolake extent, since no field measurements of the height of the paleolake highstands are available (Figures 1, 4). The shoreline elevation was determined by superimposing the mapped shorelines on 1:50,000 topographic maps. The paleolake reconstruction represents the maximum extent of lake levels. Topographic contours (20 m interval) coinciding with or bracketing the highest shoreline were used as guides to map the perimeter of the paleolakes. To determine the relationship between lake volume and elevation, the average of paleolake areas bracketing each elevation increment was multiplied by the increment. The volumetric layers were then summed to produce a basinwide elevation vs. volume relationship (Figure 8). The paleolake volume is given as the amount in excess of the modern lake volume. To account for the total volume attributed to the two levels of the Tauca lake phase, the volume of inundation in the Uyuni-Coipasa-Poopó basin is combined with the increase in volume associated with two highstands identified in the Titicaca basin (Figure 3). Uyuni-Coipasa-Poopó
50000 elevations Salar de Uyuni 3653 m 40000 Salar de Coipasa 3657 m Lake Poopo 3686 m
30000 Titicaca lake area (km2)
20000 Tauca highstand terrace 4 terrace 3 terrace 2 terrace 1
10000
3700 3750 3800
elevation (m)
Fig. 7 Relationship between paleolake area and paleolake level (I.G.M., 1991). 5000
4000 ) 3
3000 volume (km
2000 glacier meltwater volume Tauca (3760 m) Tauca (3720 m) basin overflow
1000
3700 3800
elevation (m) Fig. 8 Hypsometric curve of Uyuni-Coipasa-Poopó basin showing relationship between paleolake volume (in excess of the modern volume) and paleolake level. 2.2. Results: Titicaca-Coipasa-Uyuni-Poopó drainage basin The volume of glacier ice in the Titicaca-Uyuni-Coipasa-Poopó drainage basin after the meltwater conversion yields a water volume of 1637 km3 (Table 1).
Table 1 Volume of former glaciers and Tauca lake phase volumes (in excess of the modern lake volumes). .
Basin glacier paleolake volume water volume ratio: equivalent (km3) glacier/ (km3) paleolake
Titicaca 1033 77 (3720 m) 13 (3720 m) 135 (3760 m) 7.7 (3760 m)
Uyuni- 604 1795 (3720 m) 0.34 (3720 m) Coipasa- 3675 (3760 m) 0.16 (3760 m) Poopó
Titicaca- 1637 1872 (3720 m) 0.87 (3720 m) Uyuni- 3810 (3760 m) 0.43 (3760 m) Coipasa- Poopó This amount of water would have been available for filling the paleolakes assuming no loss of glacial meltwater to evaporation. The calculated volume of Lake Tauca at the 3760 m highstand is 3810 km3, whereas the Tauca (3720 m) volume was 1872 km3 (Figure 8). Glacial meltwater was insufficient to fill Lake Tauca to the 3760 m level, but may have contributed substantially to filling Lake Tauca to the 3720 m level (Table 1). Hastenrath and Kutzbach (1985) calculated that a deglaciation period of a century or less would have been required for glacial meltwater to fill Lake Tauca to its 3760 m level, implying that the rate of deglaciation would have to be implausibly fast. According to our more precise measurements of the volume of ice occupied by glaciers and the volume occupied by Tauca (3760 m) and Tauca (3720 m), even if the ice had melted instantaneously, glaciers could have only contributed a maximum of 43% of the water for paleolake Tauca (3760 m), but meltwater may have contributed 87% of the water for Tauca (3720 m). Moreover, our lake volume estimates probably underestimate the true volume of the paleolakes because isostatic rebound is not considered here (Bills et al., 1994). Nevertheless, meltwater was perhaps responsible for a 20 meter lake level rise from 3740 m to the Tauca highstand of 3760 m because there was sufficient meltwater, and the event was interpreted by Clapperton (1993) to be short-lived. The lack of erosional benches and stromatolite development of the degree seen at lower levels indicate a pulse of lake water out of equilibrium with climate (Clapperton, 1993). To account for paleolake Tauca (3760 m), either more precipitation fell within the drainage basin, or less water evaporated from the watershed. In section 3, we quantitatively explore these mechanisms through water budget and evaporation modeling.
2.3. Results: Five Small Drainage Basins on the Southern Altiplano The extent of former glaciation mapped from TM images shows that voluminous glaciers not only drained into the Altiplano drainage basin, but that many smaller lake basins just south of the Altiplano also supported glaciers which could have contributed significantly to paleolake formation (Figure 4). Therefore, meltwater and lake volume comparisons were also made for this region. Stromatolite samples collected from an intermediate shoreline level of one of the small lake basins, Laguna Chiar Khota (Laguna Khara), have ages of 10,740 to 11,200 yr BP which implies that the stromatolite development was coincident with the development of Lake Tauca (Servant and Fontes, 1978). In all five basins, the volume of water in the former glaciers greatly exceeded the additional paleolake volume (Table 2). Meltwater could have filled the lakes if the
Table 2 Volume of former glaciers and paleolakes (in excess of modern lake volume) in five drainage basins .
salar/lake glacier water paleolake volume name equivalent volume ratio: (km3) (km3) glacier/paleolake Yapi 5.0 1.6 3.1 Cachi 2.9 0.2 14.5 Khara 2.1 0.8 2.6 Capina 9.0 1.8 5.0 Colorada 62.0 4.0 15.5 glaciers had melted instantaneously, but did melting occur fast enough to offset the effects of evaporation over the expanding lake areas? Given that the glaciers in each basin are situated not far from the valley floors where each of the paleolakes developed, we assume that little meltwater was lost enroute from the glaciers to the lakes. To address this question, we applied a simple model to the Laguna Khara basin (which has the smallest glacier/paleolake volume ratio). The glacial melting rate that would be needed to balance the evaporation from the paleolake was estimated using the maximum area of the lake as the surface area and the modern evaporation rate from Lake Titicaca. Changes in precipitation rates are not considered. Given the area occupied by the former glaciers, 7.48 x10-3 m of melt per day would have been required to balance evaporation from the paleolake. This melting rate is more than eight times less than a typical summer glacier-surface melting rate observed in the Alps of .0622 m per melt-day (Oerlemans, 1986). We therefore conclude that sufficient meltwater was available to fill the lakes in the small basins even when evaporation from the lake during deglaciation is considered. In internally draining basins to the west of the Titicaca-Uyuni-Coipasa- Poopó drainage basin, changes in the covariance between d13C and d180 during the lake highstands imply that more meltwater was contributed to the paleolake during the highstands thus implicating the glaciers as a primary water source for those paleolakes (Grosjean, 1994).
3. Water budget/evaporation model 3.1 Model description Having determined glacial meltwater to be of insufficient volume to fill Lake Tauca to its 3760 m or 3720 m level, we now examine the possibility that the enlarged lakes were the result of reduced evaporation or enhanced precipitation. The steady-state hydrologic balance for the closed Altiplano basin can be written as (Hastenrath and Kutzbach, 1985):
Pbas = Ewaw + El(1- aw ) = Ebas , (1)
Pbas - El or aw = , (2) Ew - El
where aw (referred to hereafter as the "lake area ratio") is the fraction of the basin's surface area which is covered by water, Pbas is the basin-averaged climatological precipitation rate, and E denotes the area-averaged evaporation rate over water (w), land (l), or the entire basin (bas). To investigate the effects of evaporation on aw we assume Pbas to be constant and equal to the modern value. The problem, then, is to model Ew and El in terms of atmospheric variables and determine whether reasonable changes in atmospheric conditions yield a lake area ratio, aw, which matches that of the paleolake levels Tauca (3720 m) or Tauca (3760 m). Similarly, to examine the sensitivity of lake area to changes in basin-averaged precipitation rate, we fix Ew and El at their modern values and vary Pbas . The sensitivity of Altiplano evaporation rates to changes in atmospheric conditions has been previously investigated by Kessler (1984) and Hastenrath and Kutzbach (1985), but in a simplified framework. Kessler (1984) uses a bulk transfer formula to estimate the sensitivity of Altiplano evaporation rates to changes in temperature and relative humidity. He concluded that a temperature depression of 3 °C and a 5% increase in relative humidity would reduce evaporation rates by 27% — not enough to fill Lake Tauca to the 3720 m level. However, Kessler's model makes the simplifying assumption that the surface temperature of the lake and the local air temperature are the same, which is not true for the modern case and may be even more inappropriate for past conditions. Hastenrath and Kutzbach (1985) estimated evaporation rates using the energy budget method and concluded that evaporation on the Altiplano is relatively insensitive to changes in temperature, relative humidity, or cloudiness. Again, the surface temperature and air temperature were taken to be identical. Furthermore, the Bowen ratio (the ratio of sensible heat to latent heat) of the water surface was assumed constant (i.e., insensitive to changes in atmospheric conditions). For this study we have constructed an evaporation model based on both the bulk transfer and energy budget methods. Doing so allows one to directly calculate the surface temperature and Bowen ratio, thereby eliminating the above simplifying assumptions. The evaporation rate is (Brutsaert, 1982):
0.622CDUf E = es(Ts ) - rh ×es(Ta) , (3) RTa [ ]
where es is the saturation vapor pressure evaluated at the air temperature, Ta , or the temperature of the water or land surface, Ts . The moisture availability function, f , varies from 0 for a completely dry surface to 1 for a saturated surface. The remaining variables in (3) are defined in Table 3. The surface temperature is determined by solving the energy budget equation,
Rsw - Rlu + eRld - H - LE = 0 , (4)
pCDUcp where H = Ts - T a , (5a) RTa ( )
Rsw = Rswc (1 -a )[1- (a+ bc)c], (5b)
4 Rlu = es Ts , (5c)
1 rh×es(Ta ) 7 4 b ¢ and Rld = 1.24 s Ta 1 + a¢ c . (5d) [ 100×Ta ] ( ) (Equations are adopted from Budyko, 1974 and Brutsaert, 1982 and are in MKS units; See Table 3 for variable definitions.) A long-term average is assumed in (4), so no heat storage term is required. Equations 3–5 represent the model's governing equations and, given climatological inputs (U , Ta , rh, p, and c ), provide estimates of annual mean surface temperature and heat fluxes (including evaporation rate).
3.2 Model calibration
The solution of (3)–(5) requires knowledge of the surface drag coefficient, CD , and moisture availability function, f . These parameters are determined for the
Altiplano basin by following a simple calibration procedure. First, the value of CD over water ( f =1) is determined from (3)–(5) using Ts =13.0 °C, the observed, annual mean surface temperature of Lake Titicaca (Carmouze, 1991). Climatological Table 3. Definitions and values of variables used in the water budget/evaporation model for water (w) and land (l) surfaces. Parameters whose values are allowed to vary are indicated by an asterisk, in which case the value given is for modern annual mean conditions for the Titicaca-Uyuni-Coipasa-Poopó basin.
Variable Definition Value a shortwave cloud parameter 0.39 a ¢ longwave cloud parameter 0.22 a surface albedo 0.07 (w), 0.2 (l) aw lake area ratio* 0.085 b shortwave cloud parameter 0.38 b¢ longwave cloud parameter 2.0 B Bowen ratio* 0.15 (w), 1.4 (l) c cloud fraction* 0.42 -3 CD surface drag coefficient 1.25 x 10 -1 -1 cp specific heat of dry air 1004 J K kg E evaporation rate* 1470 mm yr-1 (w), 407 mm yr-1 (l) e surface emissivity 0.97 (w), 0.92 (l) es saturation vapor pressure* 1640 Pa (w), 2350 Pa (l) f moisture availability function 1.0 (w), 0.165 (l) H sensible heating rate* 17.6 W m-2 (w), 44.0 W m-2 (l) L latent heat of vaporization 2.5 x 106 J kg-1 p air pressure 6.519 x 104 Pa -1 Pbas basin-averaged precipitation rate* 497 mm yr R gas constant for dry air 287 J K-1 kg-1 rh relative humidity* 0.45 -2 Rld downward longwave radiation* 271 W m -2 -2 Rlu upward longwave radiation* 376 W m (w), 386 W m (l) -2 -2 Rsw net shortwave radiation* 247 W m (w), 213 W m (l) -2 Rswc cloudfree shortwave radiation 345.7 W m s Stefan-Boltzmann constant 5.67 x 10-8 W m-2 K-4 Ta air temperature* 10.6 °C Ts surface temperature* 14.4 °C (w), 20.1 °C (l) U windspeed 4.6 m s-1 measurements of Ta , rh, and p are obtained from 15 years of station data (Carmouze, 1991) at Puno, Peru on the shore of Lake Titicaca, while average cloud fraction is taken from the 8-year dataset of the International Satellite Cloud Climatology Project (ISCCP; Rossow, 1991). The annual mean windspeed over Lake Titicaca, is U =4.6 m s-1, the average value obtained by Kirkish and Taylor (1984; station Rocas Misteriosas). Although this estimate represents only an 11-month average (January – November, 1982), the measurements of Kirkish and Taylor (1984) were actually obtained over Lake Titicaca. Land-based observations in the Titicaca region, while spanning a longer time period, generally show wind speeds which are 2–3 times weaker than the over-lake measurements. Using these climatological -3 conditions as input, (3)–(5) are solved for a drag coefficient of CD =1.25 x 10 , which is within the typical range of 1–2 x 10-3 (e.g., Imberger and Patterson, 1990).
The second step in the calibration procedure is to determine values of CD and f for land areas of the Altiplano. Because of the lack of good ground temperature data for this region, we are forced to assume a value for CD . But since drag coefficients over land are often taken to be 2–3 times larger than over water (at least in large- scale atmospheric models; e.g., Cook and Gnanadesikan, 1991), and CD and U always appear as a product in the model equations, we simply assume CDU =5.75 x 10-3 m s-1, which is the same as that over water. Given that land-based wind speed measurements in the region of Lake Titicaca are roughly 2–3 times smaller than over water (so the product of CD and U should be roughly the same), this assumption is not unreasonable. Land values of f are primarily a function of soil moisture, which varies significantly from the dry, southern Altiplano to the more humid climate of the northern Altiplano. To account for this geographic variation we first calculate f separately for the Titicaca and Uyuni-Coipasa-Poopó basins (Figure 1) and then form an area-averaged f , which is utilized in the model. For the Titicaca basin, using -3 -1 CDU =5.75 x 10 m s and a land evaporation rate of E=585 mm/yr (from water
1 budget estimates ), (4) can be solved for Ts =15.9 °C, yielding f =0.346 (from Eq. 3). H This corresponds to a Bowen ratio of B = =0.76, which is reasonable for the semi- LE humid climate of this region. For the drier southern Altiplano we simply assume a Bowen ratio of 2.45 H (Hastenrath and Kutzbach, 1985) and solve (4) (with E = ) for T =22.6 °C, yielding LB s f =0.087. Climatological temperature, pressure, and humidity data representative of the southern Altiplano is taken from two years of measurements from a meteorological station in Oruro, Bolivia (see Figure 1; data obtained from the National Climatic Data Center), while cloud fraction is taken from the ISCCP dataset. To complete the calibration procedure, we calculate an area-averaged moisture availability function of f =0.165, which corresponds to a Bowen ratio of B=1.4 for basin-averaged climatic conditions. The calibration procedure described above is rather elementary and is meant only to capture the gross climatological characteristics of the Altiplano region. Improved calibration would require measurements of ground surface temperature and a much longer-term meteorological database. Given the uncertainty in our estimates of CD and f (especially over land), it is worthwhile to examine the sensitivity of the model results to the values of these and other empirically-derived parameters. We provide such an analysis at the end of this section.
-1 1 Land evaporation rates for the Titicaca basin are calculated from El = [Pbas(1-l )- Ew aw ](1- aw ) (see Eq. 1), where l =0.09 is the fraction of precipitation lost to groundwater and river outflow (value taken from Serruya, 1983). A 20-year annual mean precipitation map for the Titicaca basin (Roche et al.,
1991) has been digitized and integrated to give a basin-averaged precipitation rate of Pbas =737 mm/yr.
The modern Lake Titicaca evaporation rate of Ew =1270 mm/yr is calculated from the evaporation model, and the modern lake area ratio for the Titicaca basin is aw =0.1245. 3.3 Model results Using basin-averaged climatological conditions (from Puno and Oruro), the calibrated evaporation model yields a modern basin-averaged evaporation rate of
Ebas=497 mm/yr for a lake area ratio of aw=0.085 (estimated from digitized maps and satellite imagery). To investigate the effects of changes in atmospheric conditions and basin-averaged precipitation rate, we varied the air temperature, cloud fraction, relative humidity, and precipitation rate and calculated the corresponding lake area ratio from (2). The results are shown in Figure 9, with the paleolake levels Tauca (3720 m) ( aw=0.25) and Tauca (3760 m) ( aw=0.31) lake area ratios shown by the blue lines. Although ice cover is not considered in the evaporation model, in none of the climatic regimes shown in Figure 9 does the annual mean water or land surface temperature drop below freezing. In fact, the lowest annual mean surface temperature attained in the scenarios shown in Figure 9 is a water temperature of 4.1 °C for Lake Tauca (3760 m) (c »0.7, rh»0.05). Presently, the annual mean surface temperature of Lake Titicaca is roughly 2 °C higher than the lowest monthly mean value (Carmouze, 1991), so an annual mean surface temperature of 4.1 °C is not likely to result in significant ice formation at any time of the year. Lake area increases with decreasing air temperature (Figure 9a) as a result of lower surface temperatures and associated evaporation rates. According to the model, temperature depressions of 7.4 °C and 9.7 °C would be sufficient to fill Lake Tauca to the level of 3720 m and 3760 m, respectively. Figure 9a also indicates that, under modern evaporation rates, 35% and 48% increases in basin-averaged precipitation rate could also account for paleolake levels Tauca (3720 m) and Tauca (3760 m), respectively. These values are lower than previous estimates of 45–50% (Tauca 3720 m) and 73–78% (Tauca 3760 m) as determined by Hastenrath and Kutzbach (1985). This primarily reflects the fact that our evaporation model 50
0.6 45 0.4 40
35 0.5
30
25
20 Tauca (3760 m)
0.3 0.2 change in precipitation (%) 15 Tauca (3720 m) 10
5
0 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 change in air temperature (°C)
Fig. 9a Lake area ratio as a function of precipitation and air temperature. simulates a lower water-to-land evaporation ratio, although Hastenrath and Kutzbach (1985) also use lake area ratios for the paleolake level Tauca (3720 m) and modern lake phases which are lower than those used in this study. Our estimate of the required precipitation increase for the level Tauca (3720 m) is also less than that found by Kessler (1984), who calculated an 85% increase (with no change in evaporation), a 40% increase (with a 3 °C temperature depression), and a 30% increase (with a 3 °C temperature depression and 5% increase in relative humidity). The results of the present model indicate that the latter two scenarios would only require 20% and 19% increases in precipitation, respectively. Cloud cover has a relatively strong impact on the lake area ratio through its influence on surface temperature and, consequently, evaporation rate. For example, in the absence of other changes in atmospheric conditions, an increase in cloud fraction from 0.42 (modern) to 0.7 (Tauca (3720 m)) or 0.76 (Tauca (3760 m)) would reduce evaporation rates enough to fill the paleolakes (Figure 9b). Although these represent large (and perhaps unreasonable) changes in cloud cover, the point is that the effects of cloudiness on evaporation rate are significant and cannot be overlooked. Relative humidity, on the other hand, has comparatively little influence on the lake area ratio (Figure 9b) as a result of the competing effects of relative humidity on evaporation rate through downward longwave radiation (Eqs. 4 and 5d) and the vapor pressure gradient (Eq. 3). As a result, paleolake levels Tauca (3720 m) and Tauca (3760 m) cannot be accounted for by a change in relative humidity alone. Nevertheless, an increase in humidity would reduce evaporation rates slightly and could be incorporated into the above scenarios to help explain the origin of the paleolakes. For example, 10% increases in cloud fraction and relative humidity (e.g., 0.45 becomes 0.495) together with decreases in temperature of 6.2 °C and 8.5 °C 0.4 0.5 0.4 0.35
Tauca (3760 m) 0.3 0.3 Tauca (3720 m) 0.25 0.2 0.2
0.15 change in cloud fraction
0.1
0.05 0.1
0 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 change in relative humidity Fig. 9b Lake area ratio as a function of cloud fraction and relative humidity. would reduce evaporation rates enough to maintain paleolake levels Tauca (3720 m) and Tauca (3760 m), respectively. It is interesting to note that for a 10 °C drop in air temperature, the modeled lake surface temperature drops by only 7.2 °C as a result of the reduction in evaporative cooling. Thus the modern lake-to-air temperature differential of roughly 4 °C actually increases as the air temperature is lowered. This indicates that the questionable approximation adopted by Kessler (1984) and Hastenrath and Kutzbach (1985), which assumes identical surface and air temperatures, is even less appropriate for colder conditions. Associated with this increased temperature differential is an increase in the sensible heat loss from the lake surface. Together with the reduction in evaporative cooling, this results in a significantly increased Bowen ratio. For example, the Bowen ratio of the water surface increases from 0.15 to 0.38 for a 10 °C drop in air temperature. Thus, the assumption of a constant Bowen ratio in an energy budget model (as in Hastenrath and Kutzbach, 1985) tends to underestimate the sensitivity of lake evaporation to changes in air temperature, at least for the Altiplano region.
3.4 Model sensitivities The evaporation model contains six empirically-derived parameters that introduce some degree of uncertainty to the model results. To test the sensitivity of the model to these parameters we calculated (one at a time) the air temperature depression ( dT ), cloud fraction increase ( dc ), relative humidity increase ( dr ), and percent precipitation increase ( dP) that would be required to maintain a lake area
2 ratio of aw =0.31 (Tauca 3760 m) and then reran the calculations after applying a 10% increase to one of the parameters. This was done for each of the six parameters,
2A lake area ratio of 0.13 was used for the relative humidity calculations, since changes in relative humidity alone cannot maintain lake areas much greater than this value. and the percent changes in dT , dc , dr , and dP were calculated and tabulated (Table 4). Temperature depression ( dT ) and precipitation increase ( dP) show minimal sensitivity to a 10% increase in the shortwave and longwave cloud parameters, but greater sensitivity to the drag coefficient and moisture availability function. The cloud fraction increase ( dc ) is only moderately sensitive to three out of the four cloud parameters, while the relative humidity increase ( dr ) is most strongly affected by changes in the land surface drag coefficient and moisture availability function (Table 4). The results of this sensitivity analysis indicate that the model calculations are not hypersensitive to any of the empirical parameters. In only one instance in Table 4 is there a greater than 1:1 sensitivity to one of the parameters, and that sensitivity is only 1.2:1. Most of the model results show a less than 0.5:1 sensitivity. The sensitivities listed in Table 4 cannot be used to assign quantitative uncertainties to the model results, as the uncertainties in the parameters themselves are not known. However, one can determine an approximate range of suitable values for CD and f from more easily estimated uncertainties in the modern climatological conditions, since these conditions were used in the calibration procedure which determined CD and f . For example, a 10% increase in
CD (either water or land) corresponds to a 0.6 °C drop in surface temperature. Since the annual mean surface temperature of Lake Titicaca is fairly well constrained
(within about 1 °C), the value of CD over water might, therefore, have a range of ±17%. Land temperatures, on the other hand, are more poorly constrained (within about 3°C), so the value of CD over land might only be known to within 50%. With most of the sensitivities in Table 4 being less than 0.5:1, that would imply that the model results are certain to within about 25%. Similar arguments could be used to Table 4. Model sensitivity to 10% increase in parameter value. Sensitivities are given in percent, with asterisks indicating less than 1% change.
D(dT ) D(dP) D(dc) D(dr) Parameter dT dP dc dr a * * – 6.4 – 4.2 a ¢ * * 3.5 1.9 b * * – 6.4 – 1.7 b¢ * * * – 1.7 CD (water) 3.5 4.2 4.1 – 2.1 CD (land) – 4.8 – 5.3 – 2.6 – 12 f (land) – 6.7 – 8.2 – 5.2 – 6.1 estimate the uncertainty in f , given that a 10% increase in its value corresponds to a 10% drop in Bowen ratio.
4. Discussion 4.1 Model limitations The results from the evaporation model clearly show that other climatic factors besides increased precipitation could have caused the lake expansions. However, it is important to note some of the model deficiencies. First of all, the lack of a spatially and temporally comprehensive climatological dataset with which to calibrate the model results in only a first-order calibration. Improvements could be made by including land surface temperatures in the model calibration as well as explicitly accounting for geographic variability by extending the model to two- dimensions. The inclusion of temporal variability (i.e., the seasonal cycle), which would require accounting for heat storage in the lake, would also be a worthwhile addition. Furthermore, more sophisticated bulk transfer formulations which include effects such as atmospheric stability could add significant realism to the model. Finally, it would be worthwhile to validate the model's radiation parameterizations with observed radiation over the Altiplano to see if such parameterizations are indeed appropriate for this particular region. Despite the model's shortcomings, the results are meaningful and can be used to constrain the climatic explanations for the late Pleistocene paleolakes.
4.2. Overflow hypothesis The well developed paleolake terraces at four levels were all accompanied by stromatolite reefs. The reefs are clear of sediment implying that the levels represent a falling succession of lake stages (Clapperton, 1993). Clapperton hypothesized that either static climatic periods or the altitude of an eroding overflow spillway caused the apparent steady state conditions (Clapperton, 1993). Our extensive topographic digitization and analysis of Altiplano relief indicates that a topographically controlled overflow did not regulate the height of the terrace levels in the Uyuni- Coipasa-Poopó basin. According to the 1:50,000 topographic maps of the region, the lowest passage leading out of the basin is at about 3830 m (Figure 2), 70 m above the highest recognized shoreline. Still, the possibility remains that either the topographic maps are incorrect or that some tectonic or volcanic activity has sealed off an exit since Lake Tauca’s development.
4.3. Timing of glaciers and lakes Based on the following geomorphic relationships and dating, we conclude that a major glacial stage coincided with the Tauca lake phase. Hanging deltas, identified on Landsat Thematic Mapper (TM) images at several sites, can be traced back to end moraines demonstrating that the paleolake level was high during a glaciation or deglaciation phase (Figure 10a). An ice-marginal fan-delta has been identified and mapped in the field by Clayton and Clapperton (in press). Shorelines identified on TM truncate glacial outwash showing that the major glaciations and glacier recession preceded the end of the Tauca lake phase (Figure 10b). Radiocarbon dates from the Cordillera Real and Chile indicate that a late glacial stage was coincident with the Tauca lake phase (Seltzer, 1992; Lowell et al., 1995). After the glacial stage which culminated between 14,000 and 12,000 yr BP, many of the valleys in the Eastern Cordillera of Bolivia and Peru were deglaciated by 8000 yr BP (Seltzer 1992). Similarly, the Tauca lake level reached 3760 m at 13,790 yr BP and by 9000 yr BP the Tauca lake phase was over (Servant, et al., 1995; Bills et al., 1994).
4.4. Paleoclimate explanation In general, the paleolake highstands coincided with colder intervals and the paleolake lowstands coincided with warmer intervals. The level of Lake Tauca reached 13,790 yr BP during a colder interval according to interpretations from both pollen cores in Europe and ocean cores in the North Atlantic (Woillard and Mook, 1981; Bard et al., 1987). Pollen cores extracted from Peru and Colombia concur (Markgraf, 1989). A lake-level lowstand, judged from the deposition of gypsum evaporites and a 54 m drop in lake Titicaca’s level, is associated with the Holocene hypsithermal interval of greater warmth dated at 7700-7000 yr BP (Clapperton, 1993). In addition, Lake Poopó was a salar during the hypsithermal (Wirrmann and Mourguiart, 1995). The correlation of lake highstands with cold periods emphasizes the contribution of decreased evaporation to high lake levels. However, not all cold periods appear to be associated with highstands. Lake Titicaca depressed to a level equivalent to the hypsithermal lowstand between 20,000 to 21,000 yr BP, a period which coincides with a hiatus in paleolake expansions in the South based on the lack of ages for high shorelines in that age range (Wirrmann et al., 1991). The lack of an evaporite layer associated with the older lowstand suggests that the cause was due to a decrease in precipitation rather than an increase in evaporation. Several climatologists have argued that fundamental changes in atmospheric circulation patterns were responsible for the observed "pluvial episodes" (Kessler 1984; Hastenrath and Kutzbach, 1985). Advocates of this hypothesis cite the existence of paleolakes and the expansion of the glaciers as evidence for a pluvial climate. However, the extent of both glaciers and the lakes are sensitive to changes not only in precipitation, but to changes in air temperature as well. The climate proxy data collected to date demonstrate that 1) at least one dry interval existed in the late Pleistocene, 2) the regional circulation pattern in the past was similar to the present, and 3) the period coinciding with late Tauca phase was not significantly wetter. Markgraf interprets from Altiplano pollen cores that the effective moisture was greater prior to 10,000 yr BP because of an increase in abundance of Gramineae and herbaceous taxa relative to Compositae (Markgraf, 1989). However, Markgraf's interpretation does not support an extreme increase in basinwide precipitation on the order of 50%, as inferred by Hastenrath and Kutzbach (1985). Based on pollen studies from the Junin region of the central Peruvian Andes, the glacial interval between 24,000 and 12,000 yr BP was interpreted to be cold and dry (Hansen et al., 1984). The Pantanal region directly east of the central Andes (57°, 17°) has relict dunes and deflation basins, oriented NNE and NNW. Although the age of the eolian landforms is unknown, the composition of vegetation which occupied the alluvial fans indicate that the predominant air masses of Amazon origin were dry (Klammer, 1982; Clapperton, 1993). Similarly, the Beni Basin to the northeast of the plateau has eolian features indicative of a climate drier than present (Clapperton, 1993). The Chaco-Pampas plains south of the Altiplano also has relic sand dunes and loess deposits in which Pleistocene megafauna have been found (Iriondo, 1989; Bloom, 1990). Peruvian ice cores from Huascarán contain more atmospheric dust during the late glacial stage, which is suggestive that land surface conditions during that period were drier (Thompson, et al., 1995). Alnus and Podocarpus pollen derived from the eastern slopes of the Andes found in sediments dated about 14,000 yr BP in Laguna Lejia (67°42’’, 23°30’’) is indicative of a northeasterly air mass flux which is similar in direction to the present moisture flux (Grosjean, 1994). In Chile, high lake levels and increased amounts of Gramineae, Compositae, and Chenopodiaceae pollen were dated at 12,000 yr BP (Grosjean, 1994). The lake expansions were restricted to elevations of approximately 3500-4000 m on the western side of the Altiplano, whereas no indicators for humid conditions such as high shorelines have been found below 3500 m (Grosjean, 1994). The elevation threshold implies that no additional moisture approached the central Andes from the west. The maximum extent of moraines are confined to higher elevations on the western side than on the eastern side of the Altiplano, demonstrating that an east-to-west decrease in precipitation existed then as it is observed today (Fox and Bloom, 1994). 4.5. Lake-atmosphere feedback Temperature depression for the last glacial stage based on the dO18 record of the Huascarán cores was 8-12 °C (Thompson et al., 1995). Our model shows that a depression of 10°C would have decreased evaporation rates enough to raise Lake Tauca to its highest level. In addition, we have identified a possible source of increased localized precipitation caused by the differential depression of lake and land surface temperatures in the past relative to the present. Today the highest annual precipitation rates occur over the center of Lake Titicaca (Kessler and Monheim, 1968). One factor which contributes to this phenomenon and which could have been accentuated during the paleolake intervals is the convergence of nightly land breezes over the lake. The evaporation model predicts less temperature depression over the lake surface relative to land, indicating that land breezes are likely to have increased in strength. Consequently, more convection may have occurred over the paleolakes. These effects would have served to induce greater precipitation over the paleolakes with perhaps little effect on the rest of the basin. However, an offsetting mechanism is that an increased lake to air temperature differential in the past may have increased evaporation rates by reducing atmospheric stability over the lake. Local lake-atmosphere interactions have been cited as a contributor in the development of paleolakes Bonneville and Lahontan (Hostetler, 1994).
4.6. Summary In summary, our model demonstrates that if air temperatures were depressed by 10 °C, the lake surface area would have swelled to encompass paleolake Tauca (3760 m). Alternatively a doubling in cloudcover or a 50% increase in precipitation could have maintained Lake Tauca at its highest level. Changes in relative humidity have little influence on evaporation rates over the lake. Differential changes in lake/land surface temperatures may have also contributed to a positive mass balance of the lake by altering the local climate. Paleoclimate proxies indicate that the Altiplano was cooler at the time of the Tauca highstand (3760 m) and warmer during the latest Titicaca lowstand. During at least one period in the late Pleistocene, the Amazon which feeds moisture to the Altiplano was drier, and drier conditions have been interpreted for the central Peruvian Andes during the Tauca lake phase. Nevertheless, proxies for precipitation rates on the Altiplano during the Tauca lake phase remain inconclusive. According to our volume calculations, melting glaciers probably played little or no role in the development of paleolake Tauca, but can account for the paleolakes inferred to have existed in five smaller lake basins south and west of the Titicaca-Uyuni-Coipasa-Poopó lake basin. 5. References Ahlfeld, F. and Branisa, L., Geología de Bolivia, Inst. Boliviano de Petróleo, Don Bosco Edit. La Paz, 1960.
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