Info

FIGURE 7 Smoothed, 5-point running mean, oxygen isotope (518O) records from the Lake Peten-Itza core, Peten, Guatemala, are plotted against 14C years B.P., and based on two gastropods (Cochliopina sp. and Pyrgophorus sp.) and two ostracods (Cytheridella ilos-vayi, (solid line) and Candona sp., (open circles). (From Curtis, J. H., et al. (1998), with permission.)

et al., 1994), as did the Zacapu basin, ca. 1000 14C B.P. (A.D. 1020) (Metcalfe, 1995).

6.1.3. The Andean Altiplano

We focus on the region of the Bolivian Altiplano (Fig. 3) that surrounds the southern basin of Lake Titicaca (Lago Wiñaymarka) (16°20' S, 68°50' W) (Figs. 2a and 2b) and lies between 3800 and 4200 m asl. Climate conditions in the Lake Titicaca watershed present special challenges for Andean agriculturists. At very high elevations (—5100 m) on peaks at the edge of the Titicaca basin, agriculture is precluded by mean annual temperatures <0°C (Roche et al., 1992). The large area (8560 km2) and volume (—900 X 109 m3) of Lake Titicaca (Wirrmann, 1992) and its relatively warm water temperatures (10°-14°C) have profound thermal effects on the drainage basin, helping maintain mean annual temperatures around the lake between 7° and 10°C (Roche et al., 1992). Nevertheless, the mean minimum monthly temperature near the lake, which occurs in July, is about 1.8°C. Crops are at risk of nighttime freezing during the austral winter.

Long-term, annual rainfall over the entire Titicaca basin averages about 758 mm/year, but is spatially quite variable and influenced by lake effects and orography (Roche et al., 1992). Values range from —500 mm/ year at sites far from the lake to >1000 mm/ year over the water body. Precipitation in the Altiplano is derived largely from north-easterly winds that transport moist air from the Amazon basin over the Eastern Cordillera. Rainfall delivery is highly seasonal, with a wet period centered on January and extending from December to March. June marks the middle of the dry season, which runs from May to August. Seasonal rainfall in the Andean Altiplano is out of phase with the timing of precipitation in the Northern Hemisphere Maya lowlands. Intraannual precipitation variability in the Altiplano is best expressed by the percentage of annual rainfall delivered to the Titicaca basin during the 4-month wet season (70%), the 4-month dry season (5%), and the two intermediate 2-month periods (25%) (Roche et al., 1992). Mean monthly evapotranspiration exceeds average rainfall from March to December, leading to soil moisture deficit and salinization (Binford et al., 1997).

Interannual climate change in the Altiplano is expressed as yearly rainfall, which is controlled by the strength of summer monsoon circulation, the position of the ITCZ, and El Niño/Southern Oscillation (ENSO) events. Strong El Niño years are associated with dry conditions in the Altiplano (Roche et al., 1992). Lake Titicaca stage levels have been measured at Puno, Peru, since 1914. Although the relation between lake stage and rainfall is confounded somewhat by inputs of glacial meltwater, the lake level serves as a good proxy for rainfall and illustrates interannual variability in precipitation. The lake outflow sill to the Río Desaguadero is at 3804 m, and the mean lake stage for the period of measurement, 1914-1989, was —3809 m. The lowest stage over the period of measurement was recorded in 1943 and was 6.37 m below the highest stage, measured in 1986. There was a lake level decline of nearly 5 m between the mid-1930s and mid-1940s (Roche et al., 1992). Such rapid shifts in rainfall and lake level create problems for farmers in the drainage basin. Crops receive insufficient water during very dry periods, but rot or are drowned during wet episodes. Furthermore, extensive, nearshore cultivable flatlands are inundated or exposed by lake level excursions of only a few meters (Fig. 8 [see color insert]). Ion-rich lake waters (conductivity = 1400 ^S/cm) retreat during dry periods, leaving soils encrusted with salts.

Rocky, upland soils in the Altiplano are nitrogen deficient (Binford et al., 1996) and prone to erosion on steep slopes. Raised-field canal mucks and intermittently inundated soils have higher total N content (Bin-ford et al., 1996) as a consequence of nitrogen fixation (Biesboer et al., 1999). The construction of raised agricultural fields by the Tiwanaku was a strategy that simultaneously addressed problems of water control, freezes, soil fertility, and salinization. Prehistoric raised fields in the combined Catari basin and Tiwanaku valley covered 130 km2 (Kolata and Ortloff, 1996) and bore testimony to the efficacy of this intensive farming practice. Both the widespread use and long persistence (ca.

A.D. 600-1100) of this agricultural technique are evidence of its importance.

In the late Pleistocene, 13,000-12,000 14C B.P. (13,500-12,050 B.C.), Lake Titicaca was deeper and fresher than it is now (P. Baker, personal communication). The extensive paleolake, or Tauca stage, was attributed to a 30-50% increase in precipitation over the Altiplano (Hastenrath and Kutzbach, 1985). At ca. 12,000 14C B.P., a drying trend began, and water levels in Lake Titicaca consequently fell as much as 85 m (Seltzer et al., 1998). Lago Wiñaymarka essentially desiccated, leaving only scattered, water-filled depressions. The low stage lasted until ca. 3600 14C B.P. (1930

B.C.) (Mourgiart et al., 1995; Wirrmann et al., 1987, 1992). An increasing E/P trend from the late Pleistocene to the middle Holocene is also inferred from studies of glacial retreat and higher snow lines (Seltzer, 1992), as well as lake sediment records from the Eastern Cordillera (Abbott et al., 1997b). As is predicted by insolation forcing, this long-term history of moisture availability from the Andean Altiplano is out of phase with conditions in the Northern Hemisphere Maya lowlands.

Late Holocene changes in Lake Titicaca's water level were inferred from paleolimnological study of lake sediment cores and serve as proxy estimates of moisture availability in the Altiplano over the last —350014C years. Cores were taken at six sites throughout Lago Winaymarka (Fig. 2). Retrieved lacustrine sediment came from elevations between —0.5 and 14.5 m below the lake outlet level at 3804 m. Lake stage reconstruction was based on lithostratigraphy of the cores and 60 AMS14C dates on gastropod shells and sedge (Schoeno-plectus tatora) achenes found near erosion surfaces (Abbott et al., 1997a). Hard water lake dating error was shown to be about 250 years in Lake Titicaca (Abbott et al., 1997a), and this value was subtracted from 14C ages before dates were calibrated with CALIB 3.0 (Stuiver and Reimer, 1993).

All cores collected from Lago Winaymarka displayed laminated lake sediments overlying gray clay soils (Binford et al., 1997). Underlying clay deposits were formed by gleization, implying that the soils developed under anoxic, waterlogged conditions. This conclusion is supported by the presence of littoral S. tatora achenes at the clay/organic sediment boundary (Abbott et al., 1997a). At all six coring sites, inception of lacustrine deposition following the protracted low-stage episode dates to between 3560 and 3160 14C B.P. (2030-1420 B.C.) (Binford et al., 1997). The timing of this lake level increase is consistent with the earlier findings of Wirrmann et al. (1987). Between the initial late Holocene lake level rise and the current high stand of Lake Titicaca, there have been four dry episodes (Fig. 9) when the lake level declined: 2900-2800, 2400-2200, 2000-1700, and 900-500 cal. B.P. (Abbott et al., 1997a).

In the early and middle Holocene, riparian agriculture in the Altiplano was precluded because of prevailing dry conditions. As the climate ameliorated ca. 1500 B.C. (3210 14C B.P.), Chiripa culture emerged in the basin. By ca. 400 B.C., Tiwanaku civilization began to develop, and raised-field cultivation was established by A.D. 600. The agricultural technology was widespread in the region by A.D. 800 and persisted until the Tiwanaku collapse, ca. A.D. 1150. Archaeological excavations in raised-field contexts indicate the technology fell into disuse after A.D. 1150, coincident with drought conditions on the Altiplano (Kolata and Ortloff, 1996) that are documented in sediment records from Lago Winaymarka (Abbott et al., 1997a; Binford et al., 1997) and in ice core records from the Quelccaya ice cap in Peru (Thompson et al., 1985).

During the late Tiwanaku IV and Tiwanaku V peri-

FIGURE 9 Water level record for Lago Winaymarka over the last —3600 calendar years plotted relative to the outlet elevation at the Rio Desaguadero (3804 m). The cultural sequence is plotted on a B.C./ A.D. scale to show the correlation with the lake-level curve. Ti-wanaku raised fields were widely utilized from about A.D. 600-1150. Fields fell into disuse and populations declined coincident with the onset of the last low-water-level event in the record. (Modified from Abbott et al., 1997.)

FIGURE 9 Water level record for Lago Winaymarka over the last —3600 calendar years plotted relative to the outlet elevation at the Rio Desaguadero (3804 m). The cultural sequence is plotted on a B.C./ A.D. scale to show the correlation with the lake-level curve. Ti-wanaku raised fields were widely utilized from about A.D. 600-1150. Fields fell into disuse and populations declined coincident with the onset of the last low-water-level event in the record. (Modified from Abbott et al., 1997.)

ods (A.D. 600-1100), dense population centers in the Catari basin and Tiwanaku valley were established near raised-field complexes. Following the state collapse, small populations were dispersed on the landscape and had no apparent relation with raised fields (Kolata, 1993). Populations declined as a consequence of the agricultural collapse (Ortloff and Kolata, 1993), and thereafter, Tiwanaku inhabitants, and later Inca populations, relied on terracing and flatland cultivation.

The protracted dry conditions in the Altiplano lasted more than a century and had negative impacts on raised-field agriculture. Low direct rainfall on crop planting surfaces reduced soil moisture beyond the permanent wilting point. Lack of groundwater recharge reduced or eliminated water flow to input springs and streams that fed canals between raised fields, exacerbating the dry conditions faced by crops. In addition, canal drying removed the benefits of freeze protection and nitrogen fixation. Without continuous flushing, fields were also prone to soil salinization.

6.1.4. Climate Forcing in the Circum-Caribbean

Modern studies of interannual rainfall variability in the tropical Atlantic demonstrate that wet years are associated with an enhancement of the annual cycle, driven by greater than normal summertime northward

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FIGURE 10 (a) Summer (August) and winter (February) insolation curves for the Maya lowlands (20°N) over the last 20,000 calendar years. (b) Summer (February) and winter (August) insolation curves for the Ti-wanaku area (15°S) over the last 20,000 years. Values are from Berger and Loutre (1991). (c) Seasonal insolation difference (summer minus winter) at 20°N and 15°S, illustrating that long-term trends in seasonal insolation between the Northern Hemisphere and Southern Hemisphere tropics were out of phase. Since the end of the Pleistocene, the greatest seasonality at 20°N occurred in the early Holocene, whereas the greatest seasonality at 15°S occurred in recent millennia.

FIGURE 10 (a) Summer (August) and winter (February) insolation curves for the Maya lowlands (20°N) over the last 20,000 calendar years. (b) Summer (February) and winter (August) insolation curves for the Ti-wanaku area (15°S) over the last 20,000 years. Values are from Berger and Loutre (1991). (c) Seasonal insolation difference (summer minus winter) at 20°N and 15°S, illustrating that long-term trends in seasonal insolation between the Northern Hemisphere and Southern Hemisphere tropics were out of phase. Since the end of the Pleistocene, the greatest seasonality at 20°N occurred in the early Holocene, whereas the greatest seasonality at 15°S occurred in recent millennia.

migration of the ITCZ and Azores-Bermuda High (Hastenrath, 1984). As a result, wet years occur when sea level pressure (SLP) is low on the equatorward side of the Azores-Bermuda High, the trade winds are displaced poleward (northward), SSTs between 10° and 20°N are warmer, and convergence and cloudiness are enhanced (Hastenrath, 1984). Dry years occur when there is a reduction in the annual cycle, associated with conditions opposite to those described previously (Hastenrath, 1984). Hodell et al. (1991) suggested that long-term E/P changes recorded in the stable isotope record from Lake Miragoane, Haiti, were driven by or-bitally forced (Milankovitch) variations in solar insolation, which in turn controlled the intensity of the annual cycle. Long-term changes in seasonal insolation (Fig. 10) may explain the general pattern of a dry late Pleistocene followed by a moister early Holocene in the circum-Caribbean. The dry late Pleistocene-moister early Holocene transition has been documented by paleolimnological studies at sites around the Gulf of México and Caribbean, including Haiti (Hodell et al., 1991); Yucatán, México (Hodell et al., 1995; Whit-more et al., 1996); Guatemala (Leyden et al., 1994); northern Venezuela (Bradbury et al., 1981; Leyden, 1985); and Florida (Watts and Hansen, 1994). Orbital forcing, however, does not fully account for the mag-

nitude of aridity inferred for the late Pleistocene (Hodell et al., 1991), nor the abrupt onset of moister conditions in the early Holocene (e.g., Leyden et al., 1994).

Milankovitch forcing is also thought to be responsible for reduced intensity of the annual cycle and the consequent drying trend in the late Holocene (Fig. 10). Empirical evidence for late Holocene drying around the Caribbean is reported from Lakes Miragoane, Haiti (Hodell et al., 1991); Chichancanab, Yucatán Peninsula, México (Hodell et al., 1995); and Valencia, northern Venezuela (Bradbury et al., 1981). The gradual reduction in the intensity of the annual cycle, however, cannot explain the rapid onset of drier conditions at ca. 3400-3000 14C B.P., documented by the 518O records from Lakes Miragoane (Hodell et al., 1991) and Chichancanab (Hodell et al., 1995). Also, orbital forcing cannot explain the dramatic decadal to centennial E/P fluctuations that are so apparent in the late Holocene portion of the paleoclimate records from Lakes Mi-ragoane, Chichancanab, and Punta Laguna (Curtis et al., 1996). Other, as yet unexplained, forcing factors are responsible for these shorter-term excursions in moisture availability that had such devastating consequences for Maya agriculturists in the ninth century A.D.

6.1.5. Climate Forcing in the Andean Altiplano

Since the late Pleistocene, long-term changes in Lake Titicaca's water level have been driven by insolation forcing that influenced both annual rainfall and glacial advance and retreat. Wet conditions on the northern Altiplano are associated with the Bolivian High, which develops as a consequence of convective precipitation during the Southern Hemisphere summer (Aceituno and Montecinos, 1993; Lenters and Cook, 1997). Perihelion occurred during the Southern Hemisphere winter (July) at 8450 14C B.P. (7500 B.C.), and seasonality was reduced relative to the present (Berger, 1988; Kutzbach and Guetter, 1986). Abbott et al. (1997b) argued that reduced summer insolation and increased winter insolation at 8450 14C B.P. would have caused cooler summers and warmer winters, decreased moisture transport over the continent, and a net increase in E/P, with the latter caused largely by a decrease in precipitation. Low summertime insolation over the Altiplano lasted from ca. 10,000-7600 14C B.P. (9160-6420 B.C.) (Fig. 10). Likewise, the highest JuneAugust insolation at the same latitude occurred from ca. 11,000-6600 14C B.P. (11,000-5500 B.C.), probably causing increased ablation of glaciers in the winter dry season. The combined effects of reduced summertime precipitation and increased disappearance of glacial ice in winter contributed to deglaciation and to the mid-Holocene dry episode on the Altiplano (Abbott et al., 1997b).

For the past —8000 calendar years, austral winter insolation on the Altiplano has decreased and summer insolation has increased (Fig. 10). Higher summertime insolation was accompanied by greater precipitation, which could account for the filling of Lago Wiñaymar-ka ca. 3600 14C B.P. Orbital forcing, however, does not explain the abrupt shifts in water balance that the lake has experienced over the last three millennia. The lake level has been shown to be correlated with rainfall, which tends to be lower during El Niño events (Roche et al., 1992; Binford and Kolata, 1996). Protracted periods of strong El Niño activity, which established its modern periodicity ca. 5000 cal. B.P. (Rodbell et al., 1999), might have been responsible for the documented low stands.

6.1.6. Interhemispheric Correlations

Since the late Pleistocene, long-term patterns of moisture availability in the circum-Caribbean and Andean Altiplano have been out of phase, i.e., negatively correlated. Low-elevation, circum-Caribbean sites were characterized by a dry late glacial period, a moist early and middle Holocene, and a general drying trend over the last —3000 years. In contrast, the Andean Altiplano was relatively wet during the late Pleistocene period, Tauca stage. Lago Winaymarka desiccated when the Altiplano became drier in the early and mid-Holocene. Moist conditions may have returned as early as 4500 14C B.P. (Baucom and Rigsby, 1999). Insolation is arguably the driving force behind the general trends, assuming that periods of greater summer insolation and increased seasonality were accompanied by higher rainfall in each of the respective hemispheres. Nonetheless, short-term, secular climatic variations in the two localities north and south of the equator may have been influenced by geographic characteristics or processes such as forest cover, orography, and glacial melting.

Sedentary agriculture developed in the two cultural regions of interest ca. 3000 years ago, when the climate became drier in the circum-Caribbean and wetter in the Altiplano. Although agricultural sedentism may not have been a direct response to climate change, environmental conditions together with agricultural innovations enabled population growth and cultural development in both areas. In the centuries that followed, raised-field technology and terracing in the Altiplano increased crop yields and permitted sustainable farming of planting surfaces. In the Maya lowlands, intensive practices such as raised-field construction, terracing, and perhaps arboriculture (Puleston, 1978) supplemented yields from slash-and-burn activities. In both regions, agriculture was probably under state-level control because intensive techniques required infrastructure and organized labor. Innovative technologies greatly enhanced the natural agricultural potential of the regions. These technologies, in combination with storage and long-distance trade, probably protected human populations against minor disruptions in agricultural output.

6.1.7. Abrupt Climate Change and Cultural Response

Abrupt, but persistent droughts are implicated in the collapse of the Maya (Hodell et al., 1995; Gill, 2000) and Tiwanaku cultures (Binford et al., 1997). General temporal correlations between late Holocene dry episodes in the two cultural areas have been commented upon previously. Curtis et al. (1996) noted that several dry periods between A.D. —600 and 1400, identified isotopically in the Punta Laguna sediment core (Fig. 6), correlate with periods of high, large (>0.63 ^m) microparticle (dust) concentrations in the Quelccaya, Peru, ice core. High concentrations of microparticles in the ice core are attributed to aeolian transport of soils that were exposed during initial raised-field construc tion (Thompson et al., 1988) as well as long-distance transport of soil and desiccated lake sediment during dry events.

Chepstow-Lusty et al. (1996) noted that palynologi-cal and sedimentological shifts in a core from Lake Marcacocha in the central Peruvian Andes (13°3' S, 72°12' W) are correlated with Peruvian ice core records from Quelccaya and Huascarán (Thompson et al., 1988, 1995). This suggests that these climatic shifts were regionally significant. Influx of silt and charcoal to Lake Marcacocha between A.D. 600 and 700 may correlate with a major dust event recorded at A.D. 620 in the 1500-year Quelccaya ice core record. The Quelccaya ice core displays a second dust event at A.D. 920, which marks the inception of drier, warmer conditions that prevailed from A.D. 1000-1400, during the Medieval Warm Period (MWP) (Lamb, 1982). This dry episode is correlated with the decline of Tiwanaku agriculture (Ortloff and Kolata, 1993; Binford et al., 1997). At Mar-cacocha, this climatic episode is recorded by the establishment of Alnus after A.D. 1040. A dust event in the 15,000-year Huascarán record occurred 2000 years ago and is correlated with a silty, charcoal-rich layer in the Marcacocha profile that is attributed to the effects of flooding and riparian agriculture. At both Quelccaya and Huascarán, relatively depleted S18O values reflect the Little Ice Age (LIA) (A.D. 1490-1900). This cool period is marked at Marcacocha by the Alnus decline. The LIA is also reflected by relatively depleted S18O values between ca. A.D. 1400 and 1800 measured on ostracod (Limnocythere) shells in core D (Fig. 2a) from Lago Wiñaymarka (Binford et al., 1996).

Chepstow-Lusty et al. (1996) also point out that changes in the Marcacocha core correlate with events recorded in the profile from Lake Chichancanab, México. During the Maya Late Preclassic period (300 B.C.-A.D. 250), a dry episode is reflected in the Chichancanab core at about A.D. 1 by high gypsum concentrations, relatively enriched S18O values, and a change in ostracod taxa (Fig. 5). This dry episode in Yucatán is contemporaneous with the deposition of a silty, charcoal-rich layer in Marcacocha, above which cold-adapted Plantago spp. become established. The ninth-century dry episode on the Yucatán Peninsula may be linked to warming in Peru that is reflected by the somewhat delayed expansion of Alnus, ca. A.D. 1040. If these climatic events at distant sites on both sides of the equator prove to be synchronous, it would suggest global-scale disruption of atmospheric and oceanic fields.

Temporal correlation between climatic drying and cultural collapses at tropical sites north and south of the equator raises fascinating questions for paleoclimatol-ogists and anthropologists alike. The data indicate that climatic thresholds for agricultural production can be exceeded, with disastrous consequences for human populations. Recent paleoenvironmental studies indicate that climate surprises during the Holocene, i.e., significant decadal to centennial climate variability, were more common than was previously believed (Over-peck, 1996). Paleoclimatologists must date late Holo-cene climate changes accurately, evaluate their causes, assess the geographic extent of areas affected, determine the quantitative reduction in moisture availability that occurred during drought events, and investigate the role of human activity (e.g., deforestation) in compounding the effects of climate change. Social scientists must study factors that make cultures resistant or vulnerable to environmental perturbations such as climate changes. Sheets (2000) notes that simple societies tend to recover from environmental stresses such as explosive volcanism more readily than complex societies. Complex societies, with their state-run trade, agriculture, economies, and other facilities, are unable to cope with sudden, unanticipated environmental perturbations. The Maya and Tiwanaku civilizations instituted intensive agricultural methods that overcame natural limits to food production. These technologies increased yields, promoted sustainability, permitted human populations to reach high densities, and created a vast state-level infrastructure. Thus, intensive agricultural methods may have ultimately contributed to the cultural collapses that occurred as a consequence of sudden, unpredictable climate changes.

Paleoclimatic and archaeological information from the Maya lowlands and Bolivian/Peruvian Altiplano sheds new light on the concept of environmental determinism. Moisture availability in the climatically marginal Maya and Tiwanaku regions fluctuated appreciably over time, illustrating that past regional agricultural potential cannot be assessed by using modern climate variables, but rather should be based on historic climate conditions inferred from paleoclimate proxies. The decadal to centennial E/P variability revealed by the paleoclimate record must have presented serious challenges for pre-Columbian agriculturists. Cultural continuity in light of documented climatic variability provides evidence for both the resilience of the Maya and Tiwanaku food production systems and the ingenuity that went into their development. The multidisciplinary approach to investigating human-environment interactions permitted us to demonstrate a strong temporal correlation between protracted drought and cultural collapse in the high-altitude Tiwanaku region and the Maya lowlands. Drought was a major stressor in both regions, which contributed to agricultural and subsequent cultural declines. The findings suggest that cultural development and survival are profoundly influenced by environmental conditions.

Acknowledgments

This work was supported by National Oceanic and Atmospheric Administration (NOAA) grants NA36GP0304, NA56GP0370, and GC-95-174; National Science Foundation (NSF) grants ATM-9709314, DEB-9207878, DEB-9212641, and BNS-8805490; National Endowment for the Humanities (NEH) grant R0-21806-88; and a grant from the National Geographic Society. We thank G. Seltzer for discussions, and Sarah Metcalfe and Paul Baker for their constructive and helpful reviews of the manuscript. Finally, we are most grateful to Vera Markgraf for organizing the excellent PEP 1 meeting in Mérida, Venezuela, and for inviting us to contribute this chapter.

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