Humble Beginnings: The Early Evidence

There is no better way to begin the accademic side of this blog than by reviewing an early paper that was instrumental in rethinking the Maya collapse, as featured in the previous post's video. In 1995, Nature published a journal article by Hodell et al. that put climate change firmly onto the table of possible causes behind the drastic population decline of the Maya.

Though climate change had been previously considered as a possible trigger of Mayan ruin, prior to this paper’s publication much of the evidence cited had been particularly ambiguous. For example, the suggested use of vegetation records as a proxy for drought were compromised by the vast anthropogenic influence in the area. The extent of human-induced deforestation in the region has altered vegetation such that it mimics past climate shifts, thus making it difficult to discriminate between the effects of natural climate variation and anthropogenic factors.

Here, Hodell et al. (1995) present evidence from sediment cores taken from Lake Chichancanab in Mexico in order to reconstruct a continuous Holocene climate record for the central Yucatan Peninsula. This is done through the study of temporal changes in Oxygen isotope and sediment composition found in a 4.9 metre core. Importantly, the lake is closed-basin and located in the Yucatan, a populous area in Mayan times (Figure 1).

Figure 1: Map of Central America showing location of Lake Chichancanab in the Yucatan Peninsula (Source: Hodell et al. 1995)

 

Hodell et al. (1995) state that the character of past climate can be ascertained by studying the ratio of evaporation to precipitation (E/P), whereby climates have a higher E/P when drier and a lower E/P when wetter. In order to infer the relationship of evaporation to precipitation here, the hydrologic budget of the lake is studied. This is done through two proxies.

The first proxy for climate change applied is that of Oxygen isotope records. It is proposed that when it is dry and E/P is high, there is a greater evaporative loss of H₂¹⁶O from the lake and therefore greater ¹⁸O enrichment.  Therefore, by studying the ratio of ¹⁸O/¹⁶O past climate can be inferred. In this paper, such changes in the ratio of ¹⁸O/¹⁶O are recorded in the δ¹⁸O of carbonate in ostracod and gastropod shells preserved in lake sediment.  

The second proxy used is that of changes in the ratio of gypsum/calcite that can be recorded in sediment cores. It is said, as gypsum does not precipitate into the lake today, that changes in the relative proportions of these minerals in the sediment indicate past variations in saturation of Lake Chichancanab’s water. This therefore reflects on the state of the lake’s hydrologic budget, the ratio of E/P and the overall climate of the time. Drier climates, then, are reflected by a more saturated lake with a greater proportion of gypsum to calcite in the sediments.

To summarise, it is proposed that a drier climate, with a higher E/P, can be reflected by an increase in δ¹⁸O values and an increased proportion of gypsum to calcite found in lake sediments. The opposite is true for wetter climates.

In order to date the sediment in the core, radiocarbon analysis was performed on both aquatic shells and terrestrial material, such as seeds and charcoal. The radiocarbon ages were then converted to calendar years using the decadal tree ring dataset and a specific conversion program called CALIB. Approximate errors of between 30 and 65 years (±) were certified, which appear acceptable.

Figure 2 portrays the result of the sediment core taken from Lake Chichancanab. It can be seen that there is a general consistency between the Oxygen isotope and the gypsum/calcite analyses. It was calculated that the driest conditions, portrayed by the peaks in both δ¹⁸O values and Sulphur (S) content, the measure of gypsum/calcite, occur between AD 800 and 1000 with a peak at approximately AD 922. The timing of this dry spell corresponds well with the collapse of the Classic Maya civilization between AD 750 and 900. The full continuous Holocene climate record produced by Hodell et al. (1995) also sets this period into context as the driest episode in an 8,000 year period. 

Figure 2: Proxy signals of paleoclimate plotted over calendar years to 1000 BC and against subdivisions of Mayan cultural evolution. S content (%) represents the gypsum/calcite ratio, while δ¹⁸O values are represented separately for ostracods and gastropods. (Source: Hodell et al. 1995)

These results presented the first unambiguous evidence for climatic drying at the time of the Mayan collapse. The findings do indicate a causal link between climate change and the dramatic population decline that occurred. However, as the Mayan empire was so geologically, ecologically and climatically diverse, the climate response may have varied throughout the region, suggesting a less than uniform decline than was experienced. Even if the drying occurred throughout the area, its effects may well have varied spatially due to differing sensitivities of cultural and ecological responses.

It is suggested by Hodell et al. (1995) that further study is needed in order to determine the full magnitude and geographical extent of the dry period that occurred between AD 800 and 1000, and in order to explain the intraregional pattern of collapse of the Classic Mayan civilization.

And so the investigation continues...