Records of Ancient Human-Environmental Interaction from Mayan Southern Petén, Guatemala
Part 1:  A 1300-year record of human activity and climate change from Cancuén

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Methods

Utilising a GIS of the Cancuén region, the 'port' area's dimensions were established, as were those of the surrounding watershed that drains into the gully. Excavations located towards the centre of the gully provided access to over 2 m of sedimentary record, which following examination appeared to span the entire history of Cancuén's occupation. The sedimentary exposure's stratigraphy was recorded in detail on site. The most striking feature of the excavation into the gully's sediments was the high-density of human skeletal remains uncovered (Figure 4, shown above) (see Results, next section). Accordingly, some preliminary soil samples were collected from immediately around skeletal remains before any significant subaerial exposure and possible contamination could take place. These samples were collected for dating or soil forensic tests in the future (second part of this report) that may provide information on the circumstances of these people's deaths.

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Sediment samples of approximately 10 g mass were collected down the sedimentary sequence at 100 mm intervals from an exposed wall of a 4 x 4 m pit (Figure 5, shown above), while additional spot samples were collected when unique features were identified. All samples were collected with nylon tools where possible to avoid metalliferous contamination. Duplicate samples were collected every ten samples. An additional excavation pit located at the eastern edge of the gully, dug into the levee bank of the Pasión, was also sampled in this manner. Wherever possible, organic or charred remains in the sediments were collected to provide material for radiocarbon dating. In particular, an extensive and continuous layer of burnt sediments (including ceramic fragments and plant matter) that underlay those sediments containing the greatest number of skeletal remains was sampled with extreme care.

Excavations undertaken in the grand palace at Cancuén (Figure 2) revealed what was likely to represent a termination event in the structure, characterised by an extensive layer of charcoal several metres in length, and upwards of 10 mm depth in parts. Architecturally, this layer was considered to date from the latter period of Cancuén's history. A large volume of this material was sampled for radiocarbon dating by radiometric methods. As all charred materials sampled for dating had been burnt in situ, the problem of older material being transported into stratigraphically younger sediments (Blong and Gillespie, 1978), and thus generating erroneously old ages, was avoided. All samples collected from Cancuén were sealed in plastic and returned to Australia for laboratory analysis.

For each sample, Munsell colour, particle size information, and detailed descriptions were recorded. Those samples (n=6) which potentially were suitable for radiocarbon dating were examined microscopically to ensure no post-sampling growth of bacteria or fungus had occurred. On this basis, two samples were rejected from the study. The remaining four samples displayed no evidence of post-sampling contamination, which may severely distort radiocarbon measurements. Both conventional (radiometric, n=1) and AMS (n=3) radiocarbon dates were undertaken by Beta Analytic Inc. (U.S.A.). One sample, an animal femur bone from the base of the eastern pit in the gully failed to produce sufficient carbon after pre-treatment, and was therefore unable to provide an age estimate (Ron Hatfield, Deputy Director of Beta Analytic Inc., personal communication, 2005).

Each sediment sample was sub-sampled using plastic tools, and sub-samples prepared for chemical analysis following a modification of the procedure of Cook et al. (2005), using 10 ml of concentrated 9:1 HNO₃:HCl acids and test tube digestion at 100ºC for four hours. The concentration of 16 elements was determined simultaneously for each sample using Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES).

Estimates of analytical precision and accuracy for individual samples, and for the procedure were made based on the methods outlined in Cook et al., (2005); rapid, triplicate measurements of every 10^th sample, repeat digestion and analysis of every 10^th sample collected, and the analysis of four standards of known concentrations in sequence with the Cancuén samples. In addition, procedural blanks were prepared and analysed to determine the concentration of each element as a product of the reagents used and general procedural contamination.

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