Cultural Implications of Late Holocene Climate Change in the Cuenca Oriental, Mexico

Cultural implications of late Holocene climate change in the Cuenca Oriental, Mexico Tripti Bhattacharyaa,1, Roger Byrnea, Harald Böhnelb, Kurt Wogaub, Ulrike Kienelc, B. Lynn Ingrama,d, and Susan Zimmermane Departments of aGeography and dEarth and Planetary Science, University of California, Berkeley, CA 94720; bCentro de Geociencias, Universidad Nacional Autónoma de México, Querétaro 76230, Mexico; cClimate Dynamics and Landscape Evolution, Helmholtz Centre Potsdam, German Research Centre for Geosciences, 14473 Potsdam, Germany; and eCenter for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA 94550 Edited by Karl W. Butzer, The University of Texas at Austin, Austin, Texas, and approved January 1, 2015 (received for review March 26, 2014) There is currently no consensus on the importance of climate agriculture (9). Moreover, this area was controlled by powerful change in Mesoamerican prehistory. Some invoke drought as a pre-Columbian city-states in the Classic and Post-Classic periods, causal factor in major cultural transitions, including the abandon- and served as an important site of contact between highland ment of many sites at 900 CE, while others conclude that cultural Mexico and the cultures of the Atlantic Gulf Coast (10). factors were more important. This lack of agreement reflects The site of Cantona, in the northern Cuenca Oriental, is the fact that the history of climate change in many regions of particularly intriguing because of its size and mysterious aban- Mesoamerica is poorly understood. We present paleolimnological donment. The city drew its economic importance from the ex- evidence suggesting that climate change was important in the ploitation of obsidian at the site of Oyameles-Zaragoza, and was abandonment of Cantona between 900 CE and 1050 CE. At its a key supplier of obsidian to settlements along the Gulf of peak, Cantona was one of the largest cities in pre-Columbian Mexico (11, 12). The site covered over 12 km2 and housed an Mesoamerica, with a population of 90,000 inhabitants. The site is estimated population of 90,000 at its peak, but was abandoned located in the Cuenca Oriental, a semiarid basin east of Mexico City. between 900 CE and 1050 CE (Fig. 1). Several authors have We developed a subcentennial reconstruction of regional climate speculated that drought was responsible for this abandonment from a nearby maar lake, Aljojuca. The modern climatology of the (11), but before the present study, no detailed paleoclimate region suggests that sediments record changes in summer mon- records have been available for the Cuenca Oriental. The present EARTH, ATMOSPHERIC, soonal precipitation. Elemental geochemistry (X-ray fluorescence) climate is strongly seasonal, with rainfall primarily occurring from AND PLANETARY SCIENCES and δ18O from authigenic calcite indicate a centennial-scale arid May through October (Fig. S1). This summertime increase in interval between 500 CE and 1150 CE, overlaid on a long-term convective activity is driven by the seasonal heating of the land drying trend. Comparison of this record to Cantona’schronology surface, and is linked to the broader circulation regime known as suggests that both the city’s peak population and its abandonment the North American, or Mexican, Monsoon (13). High-resolution occurred during this arid period. The human response to climate paleoclimatic records can help us understand the long-term vari- change most likely resulted from the interplay of environmental ability of this monsoonal system. and political factors. During earlier periods of Cantona’s history, Here we report on a subcentennially resolved paleoclimate ANTHROPOLOGY increasing aridity and political unrest may have actually increased record from a maar lake, Aljojuca, in the eastern Cuenca Ori- the city’s importance. However, by 1050 CE, this extended arid ental, 30 km south of Cantona (Fig. 1). We discuss the impli- period, possibly combined with regional political change, contrib- cations of the record for our understanding of late Holocene uted to the city’s abandonment. Significance Mesoamerica | paleoclimate | late Holocene | Cantona | paleolimnology Researchers have long invoked drought to explain the demise key uncertainty in the history of pre-Columbian Meso- of many pre-Columbian Mesoamerican sites. However, the cli- Aamerica is the precise relationship between climatic change matic history of many regions of Mesoamerica remains poorly and major cultural transitions. While some scholars argue for understood. This includes the region around Cantona, a large a direct causal link between aridity and the abandonment of fortified city in highland Mexico that was abandoned between many sites (1–4), others argue that anthropogenic landscape 900 CE and 1050 CE. We used stable isotopes and elemental transformations were more important in facilitating cultural concentrations from lake sediments to reconstruct past climate, change (5, 6). A focal point of this debate is the dramatic cultural and found evidence of regional aridity between 500 CE and change that occurred during the Terminal Classic, from ∼850 1150 CE. In the initial phase of drought, Cantona’s population CE through 1000 CE. Much of this debate has focused on the grew, possibly as a result of regional political instability. How- Maya lowlands (3, 7), although researchers have also explored ever, by 1050 CE, long-term environmental stress likely con- linkages between climate and cultural change in Terminal Classic tributed to the city’s abandonment. Our work highlights the highland Mexico (1, 2, 4). However, these comparisons have been interplay of environmental and political factors in past human hindered by a lack of high-resolution paleoclimate records: While responses to climate change. paleoclimate records from across Mesoamerica suggest arid conditions at some point during this interval (1–3, 6–8), patterns of Author contributions: T.B., R.B., H.B., and U.K. designed research; T.B., R.B., H.B., K.W., U.K., B.L.I., and S.Z. performed research; B.L.I. contributed new reagents/analytic tools; climatic change may have been regionally heterogeneous. T.B., R.B., H.B., K.W., and S.Z. analyzed data; and T.B. and R.B. wrote the paper. The Cuenca Oriental, or Oriental Basin, offers an important set- The authors declare no conflict of interest. ting in which to assess hypotheses about past human−environment This article is a PNAS Direct Submission. relationships. Located east of the Basin of Mexico, in the eastern Data deposition: Radiocarbon dates and raw proxy data have been deposited in the Trans-Mexican Volcanic Belt (TMVB), the region is marginal National Oceanic and Atmospheric Administration/National Climatic Data Center Paleo- for maize agriculture today. Paleoclimate records from this area climatology database (www.ncdc.noaa.gov/paleo/study/17735). therefore have broad implications for our understanding of the 1To whom correspondence should be addressed. Email: [email protected]. “ importance of past climate change in altering the northern This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. frontier of Mesoamerica,” defined as the northern limit of maize 1073/pnas.1405653112/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1405653112 PNAS Early Edition | 1of6 Downloaded by guest on September 27, 2021 North Most core sections featured laminated sediments (Fig. S2). Our N Central Bayesian radiocarbon-based age model indicates that the cores 24˚N cover the past 6,200 y and feature a high sedimentation rate, with South a mean sedimentation rate of 3.5 mm/y, although the sedimentation rate varies over time (Fig. S3 and SI Materials and Meth- 0 5 km ods). In this paper, we focus on the last 3,800 y of the record, 22˚N when multiple proxies provide robust evidence of a dry phase between ∼500 CE and 1150 CE (Fig. 2). While the paleoclimate patterns recorded at Aljojuca may have implications for several pre-Columbian sites, we focus on Cantona, since it is the closest Tula 20˚N major pre-Columbian site to the lake. Teotihuacan Cantona In closed basin lakes, the oxygen isotopic ratio of carbonates Mexico City Aljojuca reflects changes in the ratio of evaporation to precipitation (E/P), Cholula Maar as a high ratio of E/P preferentially concentrates 18Oinlakewater 18˚N (14). In samples of authigenic carbonates from Aljojuca, we found that oxygen and carbon isotopes covary strongly, indicating closed basin conditions (Fig. S4) (14). We therefore interpret higher values of δ18O[‰ relative to Vienna Pee Dee Belemnite (VPDB)] 16˚N as indicative of an increase in the E/P ratio and a reduction in 0 200 km summer monsoon strength. We also interpret changes in sediment geochemistry as an 106˚W 104˚W 102˚W 100˚W 98˚W 96˚W indicator of changes in the strength of the summer monsoon. Increases in the concentration of aluminum, expressed as mass % Fig. 1. Locations of Cantona (red square) and other major sites in central Al2O3, reflect terrigenous material (i.e., feldspars and secondary highland Mexico (black circles), along with location of Aljojuca maar. To- clay minerals) washed into the lake during periods of high pre- pography from 1,500 m above sea level (masl) to 4,500 masl, contoured by 500 m. Gray line shows outline of the Cuenca Oriental. (Inset) Map shows cipitation (Fig. S5). The full suite of elemental geochemistry data Google Earth satellite imagery of region around Cantona, with outline of supports this interpretation, as fluctuations in concentrations of the city shown in gray. Labels designate the southern, central, and northern aluminum covary positively with fluctuations in immobile, ter- architectural clusters within the city. Modified from ref. 11. rigenous elements like titanium (Fig. S5). However, this terrigenous flux may also increase during periods of anthropogenically induced erosion. Our moving average-filtered proxy data show climate variability in Mesoamerica. We also explore the cultural that starting at 0 BCE/CE, values of δ18O become progressively implications of these paleoclimate changes for Cantona and more enriched, with the most enriched values occurring between other important pre-Columbian sites in highland Mexico (Fig.

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