The history of maize agriculture in the Dominican Republic between about AD 500-1500 is examined through analysis of lake bed sediments in a new paper by researchers Chad S. Lane, Claudia I. Mora, Sally P. Horn and Kenneth H. Orvis, appearing in an issue of the Journal of Archaeological Science to be published in August 2008.
The Laguna Castilla region in the Dominican Republic has not been surveyed archaeologically, and the only evidence for human use of the region detected to date is maize pollen. Nevertheless, researchers have been able to trace prehistoric maize agriculture in the area between about AD 500-1500, the period when the agricultural people called by archaeologists the Ostionoid culture are known to have been intensifying their dependence on agriculture.
Mapping Maize Agriculture
What researchers did was take a 7.8 meter deep sediment core from the small lake of Laguna Castilla. A sediment core is a metal tube inserted into wetland sediments, removing a small sample of the intact soils. Lane and colleagues analyzed each strata of the soil core for the presence of maize pollen, the composition of the carbon stable isotopes, and the rate of mineral influx. Mineral influx is the amount of soil deposited in a lake within a specific period: a sharp increase in the amount of soil deposited within a given period is evidence of increased localized erosion, which may have resulted from intensive agricultural use. Stable isotopes identify when the dominant species in the region is maize (a C4 plant) rather than tropical flora (typically C3 plants); pollen identifies the dominant species definitively as maize. See the discussion on C3/C4/Cam plants for more details. AMS radiocarbon dates taken on tiny charcoal inclusions within the samples allow the researchers to date the events represented in the core.
The research suggests that maize agriculture began in the valley between about 900 and 815 cal BP (or rather, ~AD 1050-1135)--that's the level at which the first maize pollen appears. The highest period of intensification of corn use in the Laguna Castilla region was between 730 and 765 cal BP (~AD 1185-1220). In the sediment level that came from lake deposits 414-382 centimeters below the surface of the water, a high maize pollen concentration occurs in combination with an increase of stable isotope values to close to C4 levels and an influx of soil at the rate of 370 mg/cm3/year (from a low of 20 mg/cm3/year). The core sampling suggests that prehistoric agricultural use of the valley steadily decreased after that until 680-550 cal BP (~AD 1270-1400), when it abruptly ceased.
Significance of the Study
To be perfectly frank, none of this is sensational news: you're not going to see the results blazoned on the front page of the New York Times this week. Ostionoid culture is documented in the broader region of the Dominican Republic, and the dates of agricultural increase have been mapped in the region by traditional archaeological study. But, what the report by Lane et al does is illustrates the utility of the method: they were able to increase information about the prehistoric occupation and use of a region without excavating a single potsherd or piece of debitage.
This is a clear example of why sediment core analysis has become an important tool of archaeologists these days.
Source and More Information
Lane, Chad S., Claudia I. Mora, Sally P. Horn, and Kenneth H. Orvis (2008). Sensitivity of bulk sedimentary stable carbon isotopes to prehistoric forest clearance and maize agriculture. Journal of Archaeological Science, 35(8), 2119-2132. DOI: 10.1016/j.jas.2008.01.019
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