Drought in the south?

Following early work in the northern parts of the Mayan region, studies have been conducted in the southern areas in order to establish that drought was the cause of collapse for the entire Mayan people. One such study was conducted by Curtis et al. (1998). They produced a climate record for Lake Peten-Itza, Peten, Guatemala, in the southern lowlands of the Mayan empire. They used oxygen isotopic records from a 5.5 m core as a proxy for climate change. However, unlike the studies in the northern regions of the Mayan territory, the core displayed no evidence for a Terminal Classic drought in the southern lowlands (Figure 1). 

Figure 1: Oxygen isotopic composition in snail shells (Cochliopina sp. and Pyrgophorus sp.) and ostracod valves (Cytheridella ilosvayi and Candona sp.) versus radiocarbon age in the Peten-Itza core (Source: Curtis et al. 2008)

 

Curtis et al. (1998) conclude that several factors, or a combination of factors, may account for the apparent climatic disparities between the northern and southern areas. First, it might be possible that the drought was local in extent and affected only the more northerly portion of the Yucatan Peninsula, but did not extend into the Peten lowlands. However, it is stated that this scenario is unlikely given the emerging evidence for dry conditions around 900 AD in southerly areas, such as Costa Rica. Second, Curtis et al. (1998) suggest that the sampling resolution of the Peten-Itza core may have been insufficient to record the drought event. However, this is said to be unlikely because the mean sample spacing in the Peten-Itza core for the period in question is about 15 years which exceeds the resolution at of Hodell et al.’s (1995) core from Lake Chichancanab where the drought was clearly evident and was recorded in many samples. It is said that the most plausible explanation for the lack of a drought signal in the Peten-Itza core is that the lake is simply too large (99 km²) and deep (~60 m) to record climatic changes that persist for less than several centuries. In comparison, Lakes Punta Laguna (0.9 km², ~12 m) and Chichancanab (10 km², ~12.5 m) have sufficiently small volumes that their lakewater. Curtis et al. (1998) state that ¹⁸O responds quickly to changes in E/P and Peten Itza’s large volume, and consequent long residence time, make it relatively insensitive to all but the most dramatic, long-term shifts in E/P.

It seems that in order to establish whether drought affected the southern lowlands, where Classic Mayan collapse was most pronounced, it will be necessary to conduct studies in smaller southern-Mayan lakes with high sedimentation rates and continuous records of preserved carbonate microfossils as proxies.

One such study was performed by Rosenmeier et al. (2002) and suggested that drought conditions were apparent at the time of Mayan collapse. They acquired a 4000-yr, 15 m sediment core record from Lake Salpet´en, Guatemala. It is said that Lake Salpet´en is a small, closed-basin lake that lies 104 m above sea level and has a maximum depth of 32 m. It is located in the southern portion of the Mayan empire.

High levels of δ¹⁸O, drought conditions, were found between 850 and 900 cal yr A.D. (Figure 2) and are concordant with the Classic Maya population decline between 800 and 900 A.D.

Figure 2: Oxygen isotope records from Lake Salpet´en (Source: Rosenmeier et al. 2002)

 

Rosenmeier et al. (2002) state that this period of high δ¹⁸O may well have been caused by greater aridity, documented in other northern Yucatan lakes. However they also state that the proxy used may just be recording the decreased hydrologic input to the lake as a consequence of forest recovery.

The evidence for drought conditions presented by Rosenmeier et al. (2002) goes some way in confirming that such conditions were experienced throughout the Mayan empire. It seems that the contradictory results produced by Curtis et al. (1998), discussed previously, were indeed erroneous. The reasoning Curtis et al. (1998) provided for their unexpected findings also seems to be proven correct by Rosenmeier et al. (2002).  It was said, by Curtis et al. (1998), that the lake they studied was too large to detect decadal climate shifts as it experience long residence times, making it relatively insensitive. In a similar area, Rosenmeier et al. (2002) studied a much smaller lake and such smaller scale climate shifts seem to have been picked up.