Stalagmighty Evidence

Here is yet another paper that adds to the growing body of evidence for major droughts in the Maya Lowlands... Webster et al. (2007) used a stalagmite from Macal Chasm, a cave in western Belize to provide a 3300-year record of climate history of the region for the period from 1225 BC to present (so includes the Preclassic, Classic and Postclassic periods).

Location of Macal Chasm, along with other important Maya sites. Source: modified from Webster et al. (2007)

You must be thinking how on earth can a stalagmite record such changes? Well, a stalagmite can actually capture a cave’s response to changes in climate very well, by encoding the signals in various ways. The interior of a cave can in turn reflect outside temperatures and conditions and so it is an indirect way of inferring past climate. As this particular stalagmite was located near the entrance of the cave, it is thought to better reflect environmental conditions outside the cave.

By looking at the colour, luminescence and carbon (and oxygen stable isotope records encoded in the stalagmite, they showed how a series of significant droughts impacted the Maya, with the most sustained dry period (lasting from 700 to 1135 AD) – again coinciding with the Classic Maya collapse.

Luminescence: This can be used as a proxy for moisture availability, because it is produced by organic acids and therefore relates to productivity of soil and the amount of vegetation cover above the cave. The record generally showed a period of greater luminescence punctuated by intervals of lesser luminescence.  

Colour: This correlates with luminescence well, where greater values of the colour index correlate with lesser luminescence and vice versa. Higher values (darker colours) are indicative of dust or fine detritus that probably accumulated as a result of inadequate water flow that would have otherwise kept the tip of the stalagmite dirt-free (suggesting a dry interval). Basically, the more continuous flow of water meant the more translucent the stalagmite (see figure 1 below).

Figure 1.  Variations in grey colour (A) and in luminescence (D) with digital grey-scale images of visible colour (B) and UV-stimulated luminescence (C) in the upper-44 cm of the stalagmite. You can see how the two anti-correlate in the images quite well! 

Oxygen isotopes: Any increase in evaporative conditions would have led to greater oxygen isotopes (δ¹⁸O) values in the calcite. In contrast, greater rainfall characterised by lower δ¹⁸O values would have led to less evaporation and thus lower δ¹⁸O values in the calcium carbonate.  

Carbon isotopes: These are determined by the carbon isotopes (δ¹³C) of the CO₂ in the soil above the cave, affected by plant cover. Stable carbon isotopes will be lower when the plant cover is largely the C₃ vegetation typical of wetter climates (e.g. tropical forests and shrubs), than if the vegetation is predominantly C₄ found in hotter, drier climates (e.g. savannah grasslands). So, during dry periods with reduced plant growth, less soil CO₂ can be expected in percolating waters and therefore less exchange of soil C with C derived from the limestone and we end up with higher ratios of δ¹³C.

What did they show?

During a drought, we would expect a browner colour, lesser luminescence, and higher δ¹³C and δ¹⁸O ratios. These four records show exactly this, and considering they were all measured at different resolutions, they correlate fairly well (the R² coefficient was 0.82).

The highest isotope ratios combined with minima in luminescence are shown in the record,  highlighting four major droughts centred about 780, 910, 1074 and 1139 AD, the first two of which correspond with the Classic collapse (shown in Figure 2). The dates also match well with the 3 phases of abandonment hypothesised by Richard Gill.

Figure 2. Stable isotope, luminescence and colour records from the stalagmite are compared with  Maya cultural periods. 

Peak drought conditions are shown for 754-798, 871, and 893-922, with successive droughts increasing in severity. These correspond with the increasing decline of monument building.

The dates match well with the other two records reviewed last week, indicating that the droughts that affected the Maya were widespread rather than just local. An important thing to note is that these papers only review the physical evidence and do not explore the potential social factors arising from drought that can be applicable to contemporary situations (something that I plan to discuss later on in this blog).