North American Tree Rings, Climatic Extremes, and Social Disasters - Dendroclimatology - page 320

subfossil logs of both species, which allowed Grissino-Mayer to develop an exactly
dated chronology extending continuously from 136 BC to AD 1992, for a total
length of 2129 years.
El Malpais is an extreme moisture-limited site, and the derived chronology has
been used to estimate annual rainfall totals over west-central New Mexico for the
past two millennia (Grissino-Mayer 1996 ) . The El Malpais reconstruction sug-
gests that the multidecadal droughts in the eighth and sixteenth centuries may
have been the most severe and sustained droughts to impact the Southwest in
the past 1500 years. The eighth-century megadrought extended from AD 735 to
765 at El Malpais, coincidental with the approximate timing of the abandonment of
Teotihuacan, 600 km to the southeast on the Mesa Central of Mexico. We do not
know that the eighth-century drought extended into central Mexico (Fig. 10.16 ) , but
the 1950s drought in the instrumental record (Fig. 10.2 ) and a few other tree-ring-
reconstructed droughts (e.g. Figs. 10.9 , 10.10 , and 10.12 ) indicate that annual and
decadal droughts can simultaneously impact the entire region from NewMexico and
Texas down into central Mexico (Acuna-Soto et al. 2005 ) .
Fig. 10.16 Tree-ring-reconstructed summer PDSI is mapped across the available grid points for
the severe sustained drought during the mid-eighth century (AD 734-760; see Fig. 10.2 for map-
ping details). The predictor tree-ring chronologies are restricted to the western United States, North
Carolina, and West Virginia during this time period, and the eastern and southern margins of this
drought are not well defined by the available data. The sharp decline in reconstructed summer
PDSI in northern Mexico is entirely an artifact of the mapping software and the absence of tree-
ring chronologies. Other proxies indicate drought conditions over Mesoamerica from AD 750 to
950 and have implicated climate in the decline of Classic Period cultures (e.g., Hodell et al. 1995 ,
2005 ; Gill 2000 ; Haugetal. 2003 )