The environmental history of the Maya attracts attention since climate changes appear to be linked with the management of resources and, in particular, with the collapse of their civilisation at the end of the Mesoamerican Classic period, (1140-1040 B. P), when, according to palaeorecords from the region, such as lake sediments and speleothems, a series of droughts occurred. However, attention has been focused mostly on the central lowlands and northern Mayab (but barely in the Highlands and other areas) where ca. seventy records have played an important role in our understanding of climate change and the role of drought in societal change, including the Maya Collapse. This includes some stable isotope records, in particular, δ18O from lake sediments and speleothems as proxies of water balance and precipitation amount, respectively. The most emblematic record comes from Lake Chichancanab, whose sediments contain gypsum deposits at specific points indicating the existence of droughts during crucial moments in Mayan history (e. g. The Maya Abandonment at 1200 years B. P. and the Maya Collapse from 1190 to 1040 B. P). In this thesis, research involves lake core sediments obtained both in the lowlands and the highlands.
First, an isotopic record on bulk carbonate for a site in the highlands (over 1000 m.a.s.l.) was obtained from a sediment core from Lake San Lorenzo in the Lagunas de Montebello Lake Complex. In addition, a density record was developed as well as a record based on the organics, carbonate, and residual content of the loss on ignition. Today, Lake San Lorenzo is hydrologically open. The isotope record δ18O in Lake San Lorenzo is a proxy of summer rainfall amount, indicating that the lake has always been hydrologically open. Episodes of major organic production appeared after periods when the surroundings were very densely populated, according to Franco Gaviria et al. (2018). Changes in the sedimentation rate through the record, including the abrupt change after 610 – 553 years B. P., as well as changes in the vegetation, are in part linked with changes in the summer rainfall amount but might also be driven by changes in the land use during the Colonial period.
Second, core sediments collected from Lake Esmeralda, a sister lake of Chichancanab, are studied in the lowlands. Isotopic analysis of waters and modern gastropods (family Hydrobiidae) from both lakes (Chichancanab and Esmeralda) were studied, showing that L. Esmeralda is today a more open basin in comparison to L. Chichancanab but still shows an important evaporative effect.
Samples for isotope analyses based on shells of gastropods (Pyrgophorus coronatus, family Hydrobiidae) from Holocene aged sediments from Lake Esmeralda were compared with the isotopic composition of samples of carbonate bulk sediment for assessing the quality of the environmental signal recovered from them. Overall both records should be very similar patterns. Therefore, a complete isotope record based on sieved sediment samples was used as a proxy of effective rainfall (water balance) in Lake Esmeralda (due to its lower cost).
Results based on a multiproxy approach (CaCO3 content, organic content, loss on ignition residuals content, the isotopic composition of bulk sediments, elemental abundance, elemental ratios, density, grey and colour scale) on a sediment core dated by radiocarbon suggest that Lake Esmeralda's sediments are predominantly made up of carbonates from 6500 to 3400 B. P. Lake Esmeralda became a more closed system, after ca. 4200 B. P. After 2500 B. P., there appears to be a further tendency to be a close system. Lake Esmeralda has become more organic-rich in the last 2500 years B. P. This coincides with periods when the human population across the Mayab turned to permanent settlement and developed urban centres. However, settlements (Shaw, 2000) and human impact on vegetation (Bermingham, 2020) near Lake Esmeralda existed practically only during the years of the Terminal Classic Horizon (950 years B. P.), suggesting an increase of the organics in the lake without major direct human disturbance. There is clear evidence for a dramatically increased organic amount in sediments from Lake Esmeralda, mostly produced outside the lake due to the loss of soil cover, during the critical moments of the Maya History, e. g. Maya Abandonment (1800 to 1750 years B. P.), the Maya Hiatus (1360 years B. P) and the Maya Collapse (1190 to 1040 years B. P.) at moments when the δ18O and the K/Sr show dry periods (at 1140 to 1040 years B. P.). No evidence of gypsum deposit was found before the drought at 167 years B. P., indicating a different catchment and chemistry of Lake Esmeralda compared to Chichancanab.
Finally, records of Lake Esmeralda and Lake San Lorenzo were compared, showing that in both lakes, the terrigenous intake is associated with the rainfall. But in Lake San Lorenzo, this process might also be linked with the erosion enhanced by the abandonment of agriculture. Besides, the CaCO3 precipitation is autogenic in both lakes, but such precipitation increases during dry periods in Lake San Lorenzo, whilst it decreases during dry periods in Lake Esmeralda. An opposite tendency in the hydrobalance at the millennial-scale to the lowlands happens in Lake San Lorenzo, in comparison to Lake Esmeralda. However, at decadal scale, the presence of recurrent droughts as it is registered in Esmeralda is still observed in San Lorenzo, indicating that the droughts during the critical moments of the Maya History were meteorological droughts.