Environmental Change and Prehistoric Agriculture in the Mirador Basin

Click on image to enlarge.

Methods

A total of 7.28 m of sediment was recovered at Lago Puerto Arturo (17º 32' N, 90º 11' W; Figure 1), a crescent shaped lake (~1.5 km²) located 22 km northwest of the town of Carmelita in the northern Petén. The lake occupies an extensive depression along the edge of an east-west trending scarp. The lake has held water since it started to fill ca. 9500 cal yr ago. The center is quite shallow and is dominated by emergent sedges. The northern part is ~8 meters deep, with at least one depression reaching 12 meters near the eastern shore. A small island on the lake contains the ruins of structures that appear to date to the Late Classic, though no archaeological investigations have been carried out on it. Cores were raised from an anchored raft using a 5-cm diameter Livingstone piston corer modified to accept butyrate liners (Figure 2, shown above). A replicate core, vertically offset by 50 cm, was taken to ensure complete recovery. The sediment/water interface was captured in a 3-inch diameter PVC tube using a micro-Kullenburg gravity corer. The cores were stored in a 5ºC cold room at U.C. Berkeley.

Prior to sampling the cores, a complete series of x-radiographs was taken and whole core magnetic susceptibility determined with a Bartington Magnetic Sensor MS2C coil. The cores were then split and imaged using a Nikon digital camera. The digital images were spliced together to create a high-resolution composite. The x-radiographs, digital images and magnetic susceptibility were then used to correlate of overlapping cores. Core density was determined by analyzing scanned images of the x-radiographs with NIH Image 1.63, a public domain image analysis program.

Sediment composition was determined on by loss on ignition (LOI) (Heiri, et al. 2001). Sediment samples of 1.25 cc were oven dried at 100ºC for 24 hours to determine H₂O content (% wet weight) and combusted at 550ºC for two hours to determine organic content (% dry weight). Further combustion at 1000ºC determined carbonate content (% dry weight).

Samples were processed for pollen analysis using standard procedures (Faegri and Iverson 1989). Known quantities of exotic Lycopodium spores were added prior to digestion to allow calculation of pollen concentration and accumulation rates (Stockmarr 1971). Pollen was counted at 625x magnification with 1250x used to determine fine detail. Pollen grains and fern spores were identified to the lowest possible taxonomic level using the U.C. Berkeley Museum of Paleontology's collection of over 10,000 modern pollen samples, reference material collected in the field, and published pollen keys (Colinvaux, et al. 1999; B.C.S. Hansen 1990; Roubik and Moreno P. 1991). A minimum of 350 grains was counted in each sample. Zea mays was differentiated from other Poaceae pollen by size (>60 µm), long axis/pore ratio (5-9) and phase contrast light microscopy (irregular spacing of intertectile columella) (Irwin and Barghoorn 1965; Whitehead and Langham 1965). Zea grains ranged from 60-100 µm with a mean of 68 µm.  To determine the first appearance of Zea in the record, the entire area of the cover slip was scanned at 125X. Three slides were scanned for Zea at each of the levels below 2.46 m. Pollen counts were compiled and plotted using CALPALYN (Bauer, et al. 1991).

Oxygen isotope ratios were measured on gastropod shells (Pyrgophorus sp.) from 136 levels and ostracod carapaces from 64 levels. Samples were run on a GV IsoPrime mass spectrometer in the Earth and Planetary Science Department, U.C. Berkeley. Results are presented in standard notation (δ¹⁸O) relative to Pee Dee belemnite (PDB). Overall analytical precision is ±0.07‰ (internal precision ±0.007‰) for ¹⁸O. Multiple adult individuals were selected to create an aggregate sample for each level.

Twelve samples were taken for ¹⁴C AMS radiocarbon age determinations (Table 1). Each sample was obtained by sieving through a 100-µm screen and sorting the larger fraction under a binocular microscope. Charcoal, macroscopic plant fragments, wood, and macroscopic insect fragments were selected. Only terrestrial or emergent aquatic plant material was selected for dating, thus avoiding "old carbon" contamination (Deevey, et al. 1954).

Previous Page  |  Table of Contents  |  Next Page

Return to top of page