Extreme climates and current analogues : from the last millennia to the Holocene optimum (1)

Abstract

The Maya civilization developed over a wide area of Mesoamerica, from the wooded regions of Peten and Chiapas to the lowlands of the Yucatan peninsula. Throughout the Maya country, rainfall follows a marked seasonal cycle, wet in summer and dry in winter, caused by the latitudinal migration of the Intertropical Convergence Zone (ITCZ). Nevertheless, access to fresh water is often difficult both in the high tropical jungle zones and on the Yucatan karst plateau. This region is characterized by the absence of surface rivers, compensated for by a network of interconnected natural wells (cenotes).

Maya civilization lasted over four millennia, reaching its apogee in the ^1st millennium AD, characterized by the emergence of numerous city-states in competition with one another. The recent use of airborne lidar has revolutionized the study of Mayan sites, making it possible to map inaccessible areas beneath the rainforest. This recent archaeological research has revealed a landscape modified on a massive scale by intensive agriculture designed to feed a large population, whose estimates have been revised upwards (around 10 million).

The Terminal Classic period came to a relatively abrupt end between 800 and 900 AD. After their respective apogee phases, the main cities were abandoned following complex social and political events (e.g. Copan, Caracol, Tikal, Calakmul, and Coba from south to north). Dates of monument construction and warfare are known from Mayan calendars and hieroglyphs. Epigraphic data have been compiled in the form of statistical indices of intercity wars, making it possible to synthesize the ups and downs of Mayan civilization in the Classical period.

Unfortunately, Mayan texts do not describe the climatic variations of these periods, but it is possible to reconstruct them from natural archives with high temporal resolution, such as laminated lake sediments, stalagmites in caves, tree rings or the growth rings of massive corals. In particular, it has been possible to measure the ^18O/16Oratio of stalagmites (e.g. Yok Balum cave in Belize), calcareous microfossils (ostracods, gastropods), as well as ^18O/16Oand/or D/H ratios of gypsum crystals and molecular biomarkers contained in lake sediments (Chichancanab lake in Yucatan, Salpeten lake in Peten).

These well-dated series, notably the U-Th method for stalagmites, make it possible to trace rainfall variations over the last few millennia : wet periods correspond to the expansion phases of Maya civilization, while droughts interrupt these developments, particularly at the end of the Terminal Classic. However, comparison of the various paleoclimatic records reveals considerable variability, probably linked to different local responses or imperfections in the indicators. Nevertheless, the most significant climatic episodes are found in several records extending to areas beyond the Maya country (dendroclimatic series from Mexico, massive corals from Bonaire, marine sediments off Venezuela), which would be compatible with regional variations affecting the ITCZ.

The importance of climatic forcing therefore seems undeniable in explaining, at least in part, the societal crises that led to the collapse of Mayan cities at the end of the ^1st millennium, as suggested by the correspondences between paleoclimatic indicators and statistical indices of warlike conflicts mentioned in Mayan texts.