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Climate and the Dolores river anasazi
QUATERNARYRESEARCH
33,259(1990)
BOOK REVIEW Climate and the Dolores River Anasazi. By Kenneth Lee Petersen. University of Utah Anthropological Papers Number 113, 152 pp. Most Quatemary palynology in the southwestern United States is performed in connection with archaeological excavations. One recent compilation of this literature contains over t300 titles, over two-thirds of them archaeological. The research is supported by consulting agencies who evaluate the environmental impacts of various public works, and unfortunately most of the data he buried in unpublished site reports. Ken Petersen’s book is a welcome departure from this practice of data disposal. It makes available to the scientific community the results of the Dolores Archaeological Program, an extensive project that combined palynological, dendroclimatological, and meteorological data to provide a paleoenvironmental context for the occupation of the region by Pueblo Indians from 700 to 1300 A.D. For the non-archaeologist, the volume is a useful compilation of much of the Colorado Plateau paleoenvironmental data, and it provides a plausible lateHolocene climatic sequence for the Southwest. Petersen’s environmental reconstructions are extremely detailed-perhaps to a fault. Pollen percentages and intlux values from Twin Lakes and Beef Pasture in the La Plata Mountains are compared with tree-ring indices on a century time scale. As Petersen acknowledges, this chronologic precision is dubious, because the pollen sequence has just three radiocarbon dates for the period of comparison (Pueblo I-IV). Nonetheless, the climatic chronology is generally supported by other climatic sequences from the Colorado Plateau. Cold periods at 5600 and 4500 yr B.P. coincided with reduced summer precipitation, but during a 3600 yr B.P. cold spell summer precipitation remained high. A major climatic change at 2800 yr B.P. was followed by warm, wet summers. During the Pueblo period, phases of wet winters (600-8kl and lOO&llOO A.D.) alternated with wet summers, particularly from &IO to 1000 A.D. Summer temperatures were generally warmer than today from 700 to 1200 A.D., with particularly long growing seasons 750-#70 and 1000-1100 A.D. Petersen uses pollen ratios as indicators of summer temperature and winter rainfall, and the influx of pinyon pine pollen as an index of summer precipitation. For example, one of the ratios for summer temperature (Pinus + Abies + Picea / nonarboreal pollen) produces a curve similar to a bristlecone pine chronology from the nearby Almagre Mountains. Changes both in the pollen ratio and in ring width are inter-
preted as reflecting growing-season temperature at up per timberline. Although the correlations appear convincing, the critical evaluation of the climatic indices does not go beyond the correspondence. There is no application of the ratios to modem surface sample data, nor are the various ratios evaluated in other than a heuristic sense, i.e., they match other climate indicators. An interesting application of the climatic data is to use the warm periods as analogs for the anticipated 21st century greenhouse warming. On a longer time scale, increased insolation in the Northern Hemisphere during the last deglaciation has been inferred to control enhanced monsoonal precipitation in the American Southwest. The climax of the Pueblo period on the Colorado Plateau coincides with the Viking Period or Medieval Warm Period (110&1300 A.D.) of the European climatic chronology, which has been proposed as a possible analog for greenhouse warming. Petersen’s pollen ratios indicate that this interval was warmer than average, and that episodes of elevated summer temperature therein were generally correlated with increased monsoonal precipitation, thereby confirming the relationship between warming and monsoon activity suggested by the early-Holocene analog. The volume concludes with a detailed climatic explanation for human settlement in the Dolores area during the Medieval Warm Period. The Pueblo people were dependent on dry-land cultivation of corn. The maximum population in the Dolores area was during the mid-800s A.D.-a period others have identified as an intense (winter precipitation) drought. Petersen explains that the Dolores region, at relatively high elevation where summer precipitation was greater, became particularly attractive during this drought. He also notes that the growing season would have been longer at high elevation during this warm period, and he correlates the abrupt decline in population with cooler summers ca. 900 A.D. Climate and the Dolores River Anasazi compares quite favorably with other recent studies of climate and people of the Colorado, but it clearly excels in its explicit portrayal of paleoenvironmental data. Perhaps Petersen is guilty of overinterpreting these data, but the work will remain the primary referenced a topic of discussion-for those interested in the climate and prehistoric people of the Southwest. OWEN K. DAVIS Department of Geosciences University of Arizona, Tucson, Arizona 85721
259 0033-5894/90 $3.00 Copyright 0 1990 by the University of Wa&&ton. All rights of reproduction in my form reserved.
33,259(1990)
BOOK REVIEW Climate and the Dolores River Anasazi. By Kenneth Lee Petersen. University of Utah Anthropological Papers Number 113, 152 pp. Most Quatemary palynology in the southwestern United States is performed in connection with archaeological excavations. One recent compilation of this literature contains over t300 titles, over two-thirds of them archaeological. The research is supported by consulting agencies who evaluate the environmental impacts of various public works, and unfortunately most of the data he buried in unpublished site reports. Ken Petersen’s book is a welcome departure from this practice of data disposal. It makes available to the scientific community the results of the Dolores Archaeological Program, an extensive project that combined palynological, dendroclimatological, and meteorological data to provide a paleoenvironmental context for the occupation of the region by Pueblo Indians from 700 to 1300 A.D. For the non-archaeologist, the volume is a useful compilation of much of the Colorado Plateau paleoenvironmental data, and it provides a plausible lateHolocene climatic sequence for the Southwest. Petersen’s environmental reconstructions are extremely detailed-perhaps to a fault. Pollen percentages and intlux values from Twin Lakes and Beef Pasture in the La Plata Mountains are compared with tree-ring indices on a century time scale. As Petersen acknowledges, this chronologic precision is dubious, because the pollen sequence has just three radiocarbon dates for the period of comparison (Pueblo I-IV). Nonetheless, the climatic chronology is generally supported by other climatic sequences from the Colorado Plateau. Cold periods at 5600 and 4500 yr B.P. coincided with reduced summer precipitation, but during a 3600 yr B.P. cold spell summer precipitation remained high. A major climatic change at 2800 yr B.P. was followed by warm, wet summers. During the Pueblo period, phases of wet winters (600-8kl and lOO&llOO A.D.) alternated with wet summers, particularly from &IO to 1000 A.D. Summer temperatures were generally warmer than today from 700 to 1200 A.D., with particularly long growing seasons 750-#70 and 1000-1100 A.D. Petersen uses pollen ratios as indicators of summer temperature and winter rainfall, and the influx of pinyon pine pollen as an index of summer precipitation. For example, one of the ratios for summer temperature (Pinus + Abies + Picea / nonarboreal pollen) produces a curve similar to a bristlecone pine chronology from the nearby Almagre Mountains. Changes both in the pollen ratio and in ring width are inter-
preted as reflecting growing-season temperature at up per timberline. Although the correlations appear convincing, the critical evaluation of the climatic indices does not go beyond the correspondence. There is no application of the ratios to modem surface sample data, nor are the various ratios evaluated in other than a heuristic sense, i.e., they match other climate indicators. An interesting application of the climatic data is to use the warm periods as analogs for the anticipated 21st century greenhouse warming. On a longer time scale, increased insolation in the Northern Hemisphere during the last deglaciation has been inferred to control enhanced monsoonal precipitation in the American Southwest. The climax of the Pueblo period on the Colorado Plateau coincides with the Viking Period or Medieval Warm Period (110&1300 A.D.) of the European climatic chronology, which has been proposed as a possible analog for greenhouse warming. Petersen’s pollen ratios indicate that this interval was warmer than average, and that episodes of elevated summer temperature therein were generally correlated with increased monsoonal precipitation, thereby confirming the relationship between warming and monsoon activity suggested by the early-Holocene analog. The volume concludes with a detailed climatic explanation for human settlement in the Dolores area during the Medieval Warm Period. The Pueblo people were dependent on dry-land cultivation of corn. The maximum population in the Dolores area was during the mid-800s A.D.-a period others have identified as an intense (winter precipitation) drought. Petersen explains that the Dolores region, at relatively high elevation where summer precipitation was greater, became particularly attractive during this drought. He also notes that the growing season would have been longer at high elevation during this warm period, and he correlates the abrupt decline in population with cooler summers ca. 900 A.D. Climate and the Dolores River Anasazi compares quite favorably with other recent studies of climate and people of the Colorado, but it clearly excels in its explicit portrayal of paleoenvironmental data. Perhaps Petersen is guilty of overinterpreting these data, but the work will remain the primary referenced a topic of discussion-for those interested in the climate and prehistoric people of the Southwest. OWEN K. DAVIS Department of Geosciences University of Arizona, Tucson, Arizona 85721
259 0033-5894/90 $3.00 Copyright 0 1990 by the University of Wa&&ton. All rights of reproduction in my form reserved.