What was the point?!

Having read previous posts, you might be forgiven for thinking to yourself: 'What's the relevance of all this stuff?' or 'Why should I care about what happened over 1,200 years ago?'. These are very valid questions and while they have in-part been answered before, hopefully I can do it once-and-for-all here.

The crux of the matter is that understanding past climate changes helps us to predict future ones and potentially mitigate any great damages or losses that they might bring. Discovering that a serious drought occurred across Mesoamerica at approximately 900 AD helps us to become aware of the potential dangers that inhabitants face today. What's more important, though, is understanding why the drought occurred when it did and the main process through which it happened. This allows us to gain greater understanding of the environment in which we live and predict when future changes might occur. This knowledge may even be able to be applied to different areas of the globe.

Understanding the role that humans play is also important. As mentioned in this blog, for example, it has been modelled that there is great potential for anthropogenic deforestation to increase the severity of drought conditions in Central America. This finding was based on data from the past and can help direct environmental management policies that should be implemented today. This example is especially relevant as the area in question does see high levels of deforestation that could prove extremely costly in the future.

Hopefully the point of the research covered in this blog has now become a bit clearer.

Environmental Deterioration: Conclusions

Looking at the evidence, some research has suggested that degradation, through deforestation and soil erosion, may well have played an important role in Classic Maya collapse. Abrams and Rue (1994) correlated deforestation and resultant soil erosion to the time of Mayan population decline in Copan, Honduras. They stated that such environmental degradation served as a primary factor in, what they saw was a gradual, Mayan demographic decline. Shimkin (1973) established the possible impact that such degradation might have as eventual shortages in fuel wood, necessary for heating and cooking, may have led to increased endemic respiratory and gastrointestinal diseases, contributing to a higher mortality rate and local population decline.

On the other hand, according to other research, it is apparent that, at their peak population, the Maya were actually very adept at conservation and degradation was not an influencing factor in Classic Maya collapse. Beach et al. (2006) discovered that, in many areas, terracing was employed and soil erosion was lower than at earlier times in the history of the Maya. McNeil et al. (2010) found that prior to the time of the Mayan population collapse, forest cover was actually increasing. They also suggested that erroneous chronology, found using poor dating methods, and an insufficient sediment core helped to disprove Abrams and Rue’s (1994) findings.

These conflicts revolve around whether deforestation was actually occurring around the 900 AD and not whether deforestation had the potential to cause demographic decline. The link between deforestation and its potential impacts on the climate and Mayan society were highlighted by Oglesby et al. (2010). They found that widespread deforestation does have the potential to induce drought conditions that may well have impacted the Maya and bolstered a natural drought cycle. However, without substantial evidence for widespread deforestation across the region, this paper does not prove that the potential of this activity was realised and actually served as a factor in the population decline of the Classic Maya.

It therefore seems that forest clearance had the potential to, at least, exacerbate drought conditions. However, in order to establish this capability was actually realised, more robust evidence that deforestation actually occurred is needed. 


References


Shimkin, D. B. (1973). 'Models for the downfall: Some ecological and culture-Historical considerations.' In Culbert, T. P. (ed), The Classic Maya Collapse. University of New Mexico Press, Albuquerque, pp. 269-299.

Collapse due to soil erosion?

Ignoring the deforestation debate, Beach et al. (2006) concentrated solely on the levels of soil erosion across the temporal and spatial extents of the Mayan empire.

Their paper provided new data from two sites (Blue Creek and Cancue´n) and synthesised more than a decade of the authors’ research in Guatemala, Belize, and Mexico. These research projects analysed more than 100 excavations in upland and depression sites, cored lakes and wetland sediments, and studied sediments in the field and laboratory using radiocarbon dating, a battery of soil chemistry tests, stratigraphic analysis, magnetic susceptibility, elemental analyses, and artefact identification. Their objective was to date when sedimentation and soil erosion occurred, identify stable surfaces, and correlate them with the state of knowledge about past land use.

Beach et al.’s (2006) findings indicated three general epochs of accelerated soil erosion. These occurred in the Pre-classic period (c. 1000 BC to AD 250), the Late Classic (AD 550 to 900), and in the last several decades.

At some sites (the Petexbatun, the Three Rivers, and the Belize River) a higher than expected soil erosion in the Pre-classic period, due to the region’s first pioneer farmers, was found. These sites also showed less than expected soil erosion in the Late Classic when population peaked and land use was the most intensive. It is stated that this resulted from the wide diffusion of many types of terracing that may have conserved soils when ancient Maya populations were greatest, but may also be partly the result of sediment exhaustion, in which erosion may have already removed the readily erodible component of upland soils.

In other regions like Cancue´n, Guatemala, however, most soil erosion occurred during the Maya Late Classic (AD 550–830). Erosion here was intense but short-lived, whereby depressions record 1-3 m of aggradation in two centuries.

Beach et al. (2006) conclude that though there is some isolated evidence for a rise in soil erosion during the Classic Mayan period when population pressure was highest, in fact erosion had started before this time. It is apparent that there is also ample evidence for lower soil erosion at several sites during the periods of highest human populations and intensive land uses. Though the Mayans did cause widespread geomorphic change, both in terms of soil erosion and soil conservation, Beach et al. (2006) state that their impacts started near the beginning of their civilization, which persisted more than a thousand years beyond the start of the population decline.

It therefore seems unlikely that soil erosion could account for the devastating population decline that was experienced by the Maya at around 900 AD.

Deforestation: Evidence challenged

McNeil et al. (2010) directly challenged the results obtained and conclusions reached by Abrams and Rue (1994).

Their first criticism stems from the longevity of Abram ad Rue’s core, where the oldest sediments date to A.D. 1010 , at least 100 years after the well-documented collapse of Copan’s population and political system. McNeil et al. (2010) also found the “gradual collapse” thesis to be problematic because scholars have demonstrated that the method of dating used incorrectly models the complex weathering processes undergone by the dated artefacts. They state that this erroneous modelling produced erroneous dates that created false indications of extended collapse. In addition, McNeil et al. (2010) state that a slow demographic decline is not supported by other lines of archaeological evidence at the site. Thus, it is concluded that Abrams and Rue’s (1994) sediment core is from the Post-classic period and does not provide information concerning the environmental impact of human populations during the Classic period.

In order to disprove Abrams and Rue’s (1994) findings completely McNeil et al. (2010) analysed a longer sediment core taken from the same pond. It demonstrated that forest cover actually increased from A.D. 400 to A.D. 900, with arboreal pollen accounting for 59.8% - 71.0% of the pollen assemblage by approximately A.D. 780 - 980. The highest levels of deforestation were actually found about 900 B.C. when, at its peak, herb pollen made up 89.8% of the assemblage. It was suggested that this event likely coincided with the widespread adoption of agriculture, a pattern found elsewhere in Mesoamerica.

McNeil et al. (2010) conclude that deforestation was not widespread in Copan around the time of Classic Maya collapse and, thus, it could not have caused population decline.

Evidence of deforestation

Abrams and Rue (1994) analysed the link between deforestation and Maya collapse in Copan, Honduras (Figure 1). They state that the population of Copan grew by 5,000 people from 550 AD to 700 AD and then by a further 20,000 by 850 AD, when it was at its peak. However, during the period from 850 AD until 1000 AD the population declined dramatically by 50% and then by a further 50% during the following 150 years. Copan became completely uninhabited by 1200 AD and was not occupied again until the 19^th Century. 

Figure 1: Map of Central America showing location of Copan, Honduras (Source: Adapted from Image)

Evidence for deforestation was gained from palynological data obtained from a core extracted from the Agua-da de Petapilla, a small bog located just north of Copan. Figure 2 shows the expected decrease in arboreal species during the Late and Terminal Classic periods and a gradual reforestation period associated with an increasingly reduced population. It can be seen that the significant reduction of arboreal species, particularly that of the dominant pine, Pinus oocarpa Schiede, correspond temporally to the Late Classic period.

Figure 2: Pollen profile (Source: Abrams and Rue 1994)

  

It was estimated that at least 23 km² of the upland pine forest may have been completely cleared by the end of the Late Classic period. An assessment of human needs for arboreal resources suggests that deforestation was the result of extensive clearing from the foothill zone for agricultural and habitational purposes and from the upland forest zone for domestic purposes of cooking and heating. Abrams and Rue (1994) believed the data suggested that three was not a sudden collapse in population but a more gradual decline

Having correlated deforestation with the time of Maya collapse, Abrams and Rue (1994) then studied its effect on the soils. It was found that surface runoff, sediment and nutrient loss greatly increased following the burning of pine from upland slopes. In fact, vegetation cover was discovered to be the primary factor in affecting these rather than slope angle. It was concluded that these factors led to the reduced productivity of the agricultural infrastructure and thus the decrease in population at Copan.

Aside

Interestingly, Abrams and Rue (1994) alluded to some lessons that may be learnt from the Mayan system that may prove very beneficial today. They state that the process of deforestation is best considered as a consequence of the broader process of urban growth, evidenced by increasing population size and density at Copan and other Late Classic centres. In this way, the relatively dispersed catchment areas of pre-urban settlements formed a larger single urban catchment area, leading to increased exploitation and denudation concentrated around centres. Many urban areas in less developed countries are evolving in this way and, it is warned, that if unchecked such processes could result in the same way as with the Maya.

Environmental deterioration as a cause for Mayan collapse

Aside from the climate-based theories for the collapse of the Classic Maya are those that deem environmental deterioration, as a result of rising population densities, as a major cause. It is said that such ecological mismanagement would have led to reduced productivity of the agricultural system, which in turn would have been largely responsible for the depopulation of urban centres. Among these, deforestation has occasionally been cited as a playing a major role.

Deforestation is the long-term reduction in the aerial extent of arboreal forest through the removal of tree species, either for the trees themselves or four some other resource(s) associated with the forest (Oldfield, 1981, p. 280). It is necessary for the rate of removal to exceed the rate of regrowth in order to be considered as deforestation.

The link between deforestation and the collapse of the Mayan empire was first alluded to by Cooke (1931), who stated that the rate of soil erosion was enormously accelerated when forest was cut and the cultivated soil was exposed to the torrential rains. This model gained support over the years, but the first empirical evidence was presented by Sanders (1973). Studying an area of Guatemala, he found that over 40% of soils had high to very high susceptibility to erosion and of these 37% had high fertility and thus attractive to an expanding agricultural system. It was added that these soils are found in areas which, prior to cultivation, were covered with forest. Despite this evidence for the link between deforestation and agricultural decline, however, the rate, absolute chronology and specific causes of ecological deterioration remained unclear. Furthermore, it remained necessary to qualify the relationship between agricultural decline and demographic collapse.

Shimkin (1973) suggested that the eventual shortages of fuel wood, necessary for heating and cooking, may have led to increased endemic respiratory and gastrointestinal diseases, contributing to a higher mortality rate and local population decline. 

References

Cooke, C. W. (1931). 'Why the Maya Cities of the Peten District, Guatemala, were abandoned.' Journal of the Washington Academy of Sciences 21(13): 283-287.

Oldfield, M. L. (1981). 'Tropical deforestation and genetic resources conservation.' In Sutlive, V. H., Altshuler, N., and Zamora, M. D. (eds.), Blowing in the Wind: Deforestation and Long-Range Implications. Studies in Third World Societies Pub. 14, College of William and Mary, Virginia, pp. 277-346.

Sanders, W. (1962). Cultural ecology of the Maya lowlands (Part I). Estudios de Culture Maya 2: 79-121.

Shimkin, D. B. (1973). 'Models for the downfall: Some ecological and culture-Historical considerations.' In Culbert, T. P. (ed), The Classic Maya Collapse. University of New Mexico Press, Albuquerque, pp. 269-299.

Conclusions: What caused the drought?

Past research (analysed in previous blogs here, here and here) does not provide a definitive reason for the drought that may well have caused the Classic Mayan collapse. Hodell et al. (2001) discovered a very convincing correlation between periods of drought and high levels of solar activity in a 206-year cycle. However, no explanation of the processes behind this link were proven and so this theory still remains in doubt.

One of Hodell et al.'s (2001) proposed mechanisms was through the movement of the Hadley cell. The potential for this to cause drought was picked up by Gill et al. (2007). It is stated that the southwest–northeast travel of the North Atlantic High, a product of the Hadley cell, has the potential to bring drought to Mesoamerica. However one major contradiction prevents the two theories being complementary. Gill et al. (2007) state that movement of the Hadley cell would result from periods of reduced solar activity, but Hodell et al. (2001) found correlation between high levels of solar activity and drought. This disagreement means that no clear conclusion can be drawn as to whether the Hadley cell plays an important role in inducing drought in Central America.  

Moving on to potential anthropogenic causes of drought that might compliment the natural ones, Oglesby et al. (2010) investigated the possible role of deforestation. Using a climate model, it was found that increased deforestation in Central America would lead to stabilisation of the atmosphere and a reduction in precipitation. As no dated evidence was assessed, the actual reasons behind the drought acannot be proved using this discovery. However, it does highlight the potential of anthropogenic environmental degradation, and deforestation in particular, to strengthen natural drought. 

While it is yet to be known exactly what caused the Mayan drought, it seems likely that solar forcing played some role in the process due to its striking correlation with dry periods.  Due to the complexity of the climate system, the mechanisms by which fluctuating solar activity is tranformed into times of reduced rainfall are still contentious. More research is needed if this link is to be totally understood. However, it does appear that increased levels of deforestation, if they existed, may well have complemented natural drought conditions and, even if only slightly, exascerbated the impact of the dry period.

Deforestation: An anthropogenic cause of drought?

Oglesby et al. (2010), though admitting that natural drought was a known recurring feature of the Mayan area, highlighted the potential of anthropogenic deforestation in inducing warmer, drier drought-like conditions.

Using a climate model (MM5), various aspects of climate, including precipitation and temperature, were predicted from varying degrees of vegetation cover over the entire Mayan area. It was found that deforestation had two major effects (Figure 1). The first was that surface albedo increases, leading to a cooling and stabilisation of the atmosphere. The second was a large reduction in evapotranspiration from the surface, leading to warming and further stabilisation of the atmosphere. This is as a result of the energy once used for evapotranspiration being used to heat the surface, which then heats the air above it. As the atmosphere is stabilised, precipitation is reduced.   

Figure 1: Surface temperature differences (in ⁰C) for the MM5 simulation with all grassland minus all forested MM5 runs for (a) dry season and (b) wet season. Precipitation differences in (cm) for (c) dry season and (d) wet season (Source: Oglesby et al., 2010).

The biggest impacts were found to be in the wet season, where temperatures increased by 3⁰C-5⁰C and precipitation reduced by 15%-30%. Approximately 78% of the overall Maya region showed precipitation decrease, with a mean decrease of 17%. It is said that both the reduction in rainfall and increase in temperatures would have been detrimental to Mayan life. The reduction in rainfall means it would have been more difficult for the Maya to store enough water to survive the dry season, while the warmer conditions put more stress on evaporation, vegetation, livestock, and people.

The model’s results were validated and were found to be credible. However, it was also established that simulations of precipitation magnitude were less accurate, despite good predictions of spatial patterns.

Oglesby et al. (2010) hypothesised that the drought conditions that devastated the Maya resulted from a combination of natural variability and human activities. They state that neither the natural drought nor the human‐induced effects alone were sufficient to cause the collapse, but the combination created a situation the Maya could not recover from.

Oglesby et al. (2010) also point out that these results may have sobering implications for the present and future state of climate and water resources in Mesoamerica as ongoing massive deforestation is again occurring

This paper makes no headway in understanding what might have caused the Mayan collapse as no dated evidence assessed. However, it does highlight the potential of anthropogenic environmental degradation, and deforestation in particular, to strengthen natural drought.

Movements of the Hadley Cell: A cause of drought?

As mentioned by Hodell et al. (2001), a possible mechanism through which fluctuations in solar activity might have caused the Classic Maya drought is through the position and strength of the Hadley cell. The significance of this potential forcing factor was highlighted by Gill et al. (2007).

According to Gill et al. (2007), the position and strength of the Hadley circulation and the annual migration of the Inter-Tropical Convergence Zone (ITCZ) are very important factors in determining the annual rainfall at locations such as the Maya Lowlands, which lie at the northern limit of tropical rainfall.

Gill et al. (2007) state that the Hadley circulation is best developed over the oceans where there are no local topographical features such as mountain chains to interfere with its flow. Every ocean in the world, therefore, has a high-pressure cell at around 308 latitude which results from the descending branch of the Hadley Cell circulation. In the North Atlantic Ocean, the high-pressure cell is known as the North Atlantic High, although some meteorologists refer to it as the Bermuda High or the Azores High. Gill et al. (2007) state that various meteorologists have related rainfall in Mexico, and the circum-Caribbean region, to the position of this North Atlantic High. It is said that research indicates that the centre of the high-pressure cell travels from year to year in a southwest–northeast direction. When the High is displaced towards the northeast, it lies closer to Europe and warm temperatures prevail over the continent. When it moves to the southwest towards the Caribbean and the Maya Lowlands, cold weather moves in over the European continent and drought comes to Mesoamerica.

This southwest–northeast travel of the North Atlantic High would, according to Gill et al. (2007) indicate that its movement is a response to the expansion and contraction of the Hadley Cell circulation. Although it is yet to be definitively established, it is said that as the Hadley Cell is thermodynamically driven, it would be logical to believe that its areal extent responds to changes in the energy driving the system. In warm periods, then, when ample energy is available, the cell expands, and the High moves toward Europe. In cold periods, when there is less energy, it contracts, bringing the High closer to Mesoamerica, diminishing rainfall in the region.

However, this theory disagrees with that of Hodell et al. (2001), who found that droughts in the Maya region were, in fact, correlated with increased solar activity.

This contradiction makes it hard to establish whether movements of Hadley Cell were responsible for the drought that played a part in the Classic Maya collapse. Without greater historical evidence, Gill et al.’s (2007) theory cannot be proved. It is therefore clear that the climatic processes behind the droughts that occur in Mesoamerica are yet to be fully understood and theories are yet to be universally accepted. This is the case because the responses of the climate system to various forcing factors are extremely complex, vary across the globe and are difficult to attribute to a particular cause. Thus, more research is necessary if the climatic reasons for Mayan drought are to be fully comprehended.

Solar forcing

Having correlated drought with Classic Mayan collapse and highlighted it as a potential cause, it is then important to understand what prompted the drought itself. This understanding is significant as it potentially allows prediction of such events in the future and mitigation of their impacts.

One theory surrounding the cause of Classic Mayan drought was presented by Hodell et al. (2001). By undertaking a time series analysis of oxygen isotopes and gypsum precipitation, taken from an earlier core in Punta Laguna (Curtis et al. 1996), they revealed a recurrent pattern of drought with a dominant periodicity of 208 years. This cycle was said to be similar to the documented 206-year period in records of cosmogenic nuclide production (carbon-14 and beryllium-10) that is thought to reflect variations in solar activity. They concluded that a significant component of century-scale variability in Mayan droughts can be explained by solar forcing.

According to Hodell et al. (2001), the 206-year cycle of cosmogenic nuclide production is believed to reflect solar variability or a combination of solar forcing and oceanic response. It is said that periods of higher solar activity correspond to times of lower cosmonuclide production. It was found that the Punta Laguna δ¹⁸O and the ¹⁴C production show a significant relationship for the past 2000 years (Figure 1). It is apparent that higher E/P (δ¹⁸O) coincides with lower ¹⁴C production, implying that drought occurred during times of increased solar activity.

Figure 1: (A) Comparison of δ ¹⁴C (red line) and δ¹⁸O record from Lake Punta Laguna (black line). M, S, and W denote the Maunder, Sporer, and Wolf sunspot minima, respectively. (B) Bandpass filter centred at 208-years of the δ ¹⁴C record (red line) and δ¹⁸O signal from Lake Punta Laguna (black line). The blue line refers to Gamma-ray attenuation bulk density signal from Lake Chichancanab (Source: Hodell et al. 2001)

Hodell et al. (2001) state that the processes by which changes in solar activity cause climate shifts in Mesoamerica is not certain. However, it is known that in order to obtain a significant climate response from rather small variations in solar output an amplifying mechanism is required. Hodell et al. (2001) mention some suggestions for such mechanisms. One of these includes changes in the ultraviolet part of the solar spectrum, which affects ozone production and stratospheric temperature structure, and the effect of cosmic ray intensity on cloud formation and precipitation. Another possible mechanism has been found from sensitivity experiments conducted with atmospheric general circulation models. These suggest that changes in solar output may affect global mean temperature, humidity, convection, and intensity of Hadley circulation in the tropics.

Furthermore, Hodell et al. (2001) assert that mean annual rainfall on the Yucatan Peninsula alone varies by a factor of 5 over a distance of only 500 km between the semiarid northwest coast (500 mm/year) and the southern lowlands of northern Guatemala and Belize (2500 mm/year). Consequently, it is said that any solar-forced change in the strength or position of Hadley circulation or tropical convective activity would be expected to affect rainfall in the region.

Though the exact processes that allow increased solar activity to result in drought conditions are not yet fully understood, Hodell et al. (2001) revealed a strong correlation between the two. The strength of this correlation allows the conclusion that, through whatever mechanism, solar activity does play an important role in determining climate in the Mayan region. Consequently, these multi-decadal to multi-century-scale oscillations may well have initiated the detrimental impact on Maya food production and culture.

"War! Huh!..."

This short video suggests that the Maya collapse was anything but uniform and rejects the drought hypothesis. 

With recently uncovered written evidence, it states that large-scale war between Mayan cities was the cause of population decline. However, while such battles may well have occurred and resulted in loss of life, this theory cannot be extrapolated for the entire Mayan empire and all urban centres based on evidence from one site. Furthermore, the overwhelming evidence that there was a significant period of drought at the time of collapse cannot be simply ignored.

While Mayan cities may well have fought battles, as this evidence suggests, it is most likely that they were only localised and did not cause the collapse across the entire empire. Furthermore, such fighting may well have occurred as a direct result of the drought. As food and water supplies declined, the Maya may well have engaged in battles for resources. It is also known that as drought conditions worsened many Mayan cities intensified their human sacrificing and so such battles may well have been waged in order to sustain supplies of these victims.

Civil war may well have played a part in the collapse of the Mayan civilisation but such strong evidence for a widespread and intense dry spell cannot be ignored. It seems likely that it was the drought itself that may have sparked the violence.

Widespread drought, uniform collapse?

Looking at all the evidence presented so far in this blog, it appears that drought conditions may well have been experienced throughout the Mayan region. However, as mentioned previously, some anthropologists do suggest that Maya from different areas reacted differently, resulting in uneven collapse.

According to Inomata (2010), the robustness of the northern Maya meant they could adapt to drought better than the southern Maya and so they experienced a more moderate decline.  

Curtis et al. (1996) also state that the collapse of the Maya was most profound in the southern lowlands, including areas of Guatemala, Belize and northern Honduras, and was accompanied by an apparent shift in population northwards into the Yucatan. Although some cities in the northern lowlands appeared to have survived the drought, their populations were also greatly reduced. Curtis et al. (1996) cites Gill (1995), from an unpublished thesis, as suggesting that cities in the northern lowlands might have survived the worst of the collapse because they had easy access to groundwater in the shallow water table. It is said that even if the drought lowered the water table by several metres, many northern Yucatan lakes and cenotes would still have held water. In the southern lowlands, however, Gill (1995) notes that the water table is located far below the ground surface and cities relied on rainwater cisterns, artificial lakes and reservoirs for water. Many of these would have dried up during the drought.

This information provides a general understanding of the spatial impact of drought throughout the Mayan region and a preliminary explanation for this pattern. It seems Maya that inhabited southern areas were harder hit by the conditions due to their reduced accessibilty to water. However, in order to appreciate this better, at each individual site, there is a need for much more detailed site-specific research. In this way, the chronology of collapse, an identification of the magnitude of drought, and the potential impacts of drier conditions could be linked and the population decline further understood.

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.

There's no doubt about drought

In addition to those already mentioned, Hodell et al.’s (1995) research prompted several paleoclimatic investigations into the Mayan region that suggest drought conditions were a feature of that period. Each focuses on different locations throughout the Mesoamerica and several proxies are employed. Instead of reviewing each in detail, a selection has been chosen and a summary of their findings is presented here.   

Curtis et al. (1996) reconstructed 3,500 years of climate for the Yucatan Peninsula using a 6.3 m lake sediment core taken from Lake Punta Laguna, Mexico (Figure 1). The proxies measured were δ¹⁸O in mono-specific ostracods and gastropods. 

Figure 1: Map showing location of Lake Punta Laguna in Quintana Roo, Mexico (Source: Curtis et al. 1996)

 The oxygen isotopic record from Punta Laguna indicated that the terminal Classic and earliest Post-classic period (800 to 1050 AD) was one of the driest intervals of the last 3,500 years. The high resolution record produced from Punta Laguna allows identification of peak drought conditions at 862 AD during the terminal Classic period (Figure 2). 

Figure 2: Comparison of the oxygen isotopic record from the Punta Laguna sediment core with Maya cultural periods (Source: Curtis et al. 1996)

 

Islebe and Sanchez (2002) also found there to be a drier period coincident with the period of Maya demographic decline. They analysed the pollen record of a sediment core taken form the Mexican Caribbean coast, showing the development and changes in the mangrove system (Figure 3). 

Figure 3: Location of the coring area along the Mexican Caribbean
coast (Source: Islebe and Sanchez 2002)

 

They found that the mangrove species Conocarpus erecta dominated during the period approximately 1500-1200 ¹⁴C yr BP while R. mangle almost disappeared and other taxa emerged, suggesting drier climatic conditions and generally more open vegetation.

A study by Carrillo-Bastos et al. (2010) once again found there to be drought conditions coincident with the collapse of the Classic Maya. They studied oxygen isotope measurements on a 2.5 m sediment core recovered from Lake Tzib central Quintana Roo, southeast Mexico (Figure 4). 

Figure 4: Map showing the location of Lake Tzib (Source: Carrillo-Bastos et al. 2010)

 

Their data indicated dry conditions between ~1300 and 1200 cal yr BP. They correlated the dry peak at 1200 cal yr BP with a dry period found using a marine core taken in the Cariaco Basin, north of Venezuela (Figure 5).

Figure 5: Correlation of  isotopes from Lake Tzib with Ti% data from the marine Cariaco Basin core (Source: Carrillo-Bastos et al. 2010)

  

It can be seen that the peaks in drought conditions found do correlate with the collapse of the Classic Maya.

 

With so many studies correlating drought conditions to the time of the Classic Mayan collapse, from such diverse parts of the empire, using such a wide variety of proxies, it seems very likely that drought conditions were experienced by all the Maya and caused population decline. Where there may be slight discrepancy in the chronology and exact dating of these is probably due to the differing dating techniques employed. It remains, however, to understand why this drought occurred and how much it affected different groups of Maya from around Central America.

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.

Summary poster

In order to reinvigorate your interest, and futher appeal to your senses, I have produced a poster that summarises what has been learnt so far during this accademic escapade. It may also prove useful as a convenient starting point for those who have not read previous posts. (Click to enlarge)

Conclusions so far...

 It is often stated that this debate has two sides, those with the social arguments for downfall and those who see climate as its major cause. However, it seems that the correlation between Mayan collapse and climate drought is not simply causal. It is this assumption that often leads some sociologists to pigeon-hole climate scientists as having over-emphasis on environmental determinism. It is evident that there are much more complex relationships occurring, with droughts potentially varying greatly over space and in intensity, affecting groups of Maya with vastly different tolerances to them. It seems then that in order to fully understand why the Mayan civilisation collapsed, there is a need for a complete climate record for the entire Mayan region as well as a knowledge of how the environmental changes might have impacted social processes of the different groups of Maya. Collaboration between paleo-environmentalists and anthropologists/archaeologists seems vital.

News From the Front – A Japanese Rambo!?

Refreshingly, the debate surrounding the collapse of the Classic Maya has arisen elsewhere in cyberspace this week. There is a section of the New York Times website called ‘Scientist at Work – Notes From the Field’, a blog described as the modern version of a field journal – a place for reports on the daily progress of scientific expeditions. On 27^th February Takeshi Inomata, a professor of anthropology specialising in the ancient Maya, draws the reader’s attention to the on-going field science aiming to put the debate surrounding the Mayan population breakdown to rest once and for all.

Whilst primarily studying the anthropological aspects of the ancient Maya, the writer describes the arrival of a team of Japanese geologists and plant scientists who aim to uncover more about the area’s past climate (Figure 1). The group, including a lake-corer known as ‘the Japanese Rambo’, aim to collect sediment from 3 lakes in Guatemala, comprising of Lakes Petexbatun, Las Pozas and Quexil in the central Guatemalan lowlands. Results will stem from the analysis of geochemistry, isotopes, pollens, diatoms and other remains and appear to be successful already, with a series of four-metre-long cores extracted. However, the rich amounts of data on environmental change contained within each core will not be analysed until they are transported back to Japan.  

Figure 1: The group of Japanese scientists with a core sample taken from Lake Petexbatun (source)

 

Building on what we learnt from the paper by Hodell et al. (1995), it is claimed by the writer that though apparent that there were major droughts in the northern parts of the empire at the time of collapse, archaeological evidence suggests that centres prospered in this region during these supposed dry spells. Furthermore, it is stated that the southern regions witnessed a far greater abandonment, but evidence of droughts in this area still remains unclear.  

It is clear to see the need for a greater depth of study into the spatial extent of the defined droughts and the ability of different Mayan groups to resist such conditions. It is as a result of this need that groups of scientists, such as the Japanese, are working to discover new evidence of drought in the southern regions of the Mayan civilization and why anthropologists and archaeologists, like Takeshi Inomata, are working to find out how different sets of people were affected by climate changes.

"Stalagtites hold tight, stalagmites..."

Webster et al. (2007) also outlined a link between drought and Classic Maya collapse. This was done by presenting a climate record for the Maya region using evidence obtained from a stalagmite once located in the Macal Chasm in the Vacal Plateau, Belize (Figure 1).

Figure 1: Location map showing Macal Chasm and other important Mayan sites (Source: Webster et al. (2007))

Past climate was reconstructed from analysis of the reflectance, colour, luminescence, and carbon and oxygen stable isotope records for the period from 1225 B.C. to the present. The record thus encompassed the Maya Pre-Classic, Classic, and Post-Classic periods. 

The stalagmite record indicates that a series of droughts, which collectively form the most prolonged dry interval in the 3300-year record, lasted from A.D. 700 to 1135 and thus coincided with the collapse of the Maya civilization (Figure 2).  

Figure 2: Droughts in Belize indicated by a reduction in stalagmite luminescence compared with Maya monument production (Source: Webster et al., 2007)

For comparison and validation Webster et al. (2007) referenced the work of Keigwin (1996). Keigwin (1996) produced a record of the Holocene variability of sea surface temperatures (SSTs), salinity and flux of terrigenous material in the Sargasso Sea, within the North Atlantic Ocean. A sea-sediment core was taken in order to accomplish this, due to the high rates of sediment accumulation in the region. 

Keigwin (1996) observed that variation in SSTs in the record showed separate distinct rises in temperatures that correlate with the evidence for Mayan collapse. Though the Sargasso Sea is located a significant distance from the location of the Mayan empire, it is claimed that the record, most probably, corresponds to climate change at a “basin or hemispheric scale”. However, this conclusion is yet to be wholly substantiated and is open to debate.

Webster et al. (2007) compiled their own findings with those of Keigwin (1996) and Hodell et al. (1995) (discussed earlier in the blog), as shown in Figure 3.

Figure 3: Comparison of the Belize stalagmite record with Sargasso Sea and Lake Chichancanab data (Source: Webster et al., 2007)

Webster et al. (2007) state that these correlations demonstrate that the Macal Chasm stalagmite provides a regional record of climate, rather than a local record of land-use change or a record only of changes within the cave itself. More significantly, they demonstrate that the dry periods in the Macal Chasm record were widespread events that would have affected the entire Mayan civilization

It is apparent that all three studies show periods of drought occurring at the same time – the Classic Maya collapse. Webster et al. (2007) conclude that severe dryness affected a broad region of Mesoamerica most likely contributed to the collapse of the Maya civilization during the Late Classic period.

While this evidence is still not definitive for the entire Mayan region and does not explore the social implications of the drought, it can be said that the greater the number and spatial extent of the proof that drought did occur at the time of Mayan collapse, the more readily this factor can be accepted as a playing a key role in the demographic decline.