The devil is in the detail...

With drought being correlated to the Classic Mayan collapse in so many studies, Escobar et al. (2010) searched for increased detail in the nature of these events. Using oxygen isotope records from Lakes Punta Laguna (Curtis et al. 1996) and Chichancanab (Hodell et al. 1995) they first proved, with statistical certainty, that the two increases in δ¹⁸O found during the Maya Terminal Classic Period reflected significant climate shifts. They next investigated the “within-horizon” stable isotope variability (of δ¹⁸O and δ¹³C) measured on ostracod valves and gastropod shells within the lakes. Their results indicated that not only was the Maya Terminal Classic period (around 910-990 AD) the driest mean conditions in the last 3,000 years, but was also a consistently dry climate.

According to Escobar et al. (2010), variation information can be obtained by looking at δ¹⁸O variability among single shells/valves from a stratigraphic horizon. The data from the Punta Laguna core seems to show that high mean δ¹⁸O values are associated with low variability, whereas low mean δ¹⁸O values are associated with high variability (Figure 1).

Figure 1: A: Oxygen isotopic composition of the ostracod Cytheridella ilosvayi in the sediment core from Lake Punta Laguna, Mexico (Curtis et al. 1996). Horizontal lines represent the range of δ¹⁸Ο measurements on individual C. ilosvayi valve at several stratigraphic horizons. B: Detail of the Classic Maya collapse time period (Source: adapted from Escobar et al. 2010)

Escobar et al. (2010) state that this indicates that sub-decadal, relatively dry periods were constantly dry, whereas relatively wet periods consisted of wet and dry times. Furthermore, it is said that one might expect relatively larger fluctuations in δ¹⁸O during dry times when lake level is low as, during these times small variations in rainfall can generate rather substantial fluctuations in lake volume and the δ¹⁸O of water. However, as this was not found, the low variability during the dry episodes indicates persistent dry conditions, uninterrupted by periods of higher precipitation.

Thus, Escobar et al. (2010) concluded that the Terminal Classic period from approximately 910-990 AD was not only the driest period in the last 3,000 years, but also a persistently dry period. This assessment adds greater severity to the dry spell that occurred. Persistent drought increases its potential to have a significant impact on the Maya as water would be not be available for prolonged periods of time. This therefore heightens the probability of drought being a principle factor behind the population decline.