16-13 Elemental and Isotopic Signatures of Natural and Cultural Soil Formation on San Salvador Island, Bahamas

Session: From Thin Section to Outcrop: Exploration of Undergraduate Research (Posters)

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Abstract:

The heterogeneity of soils and archaeological deposits on San Salvador Island, Bahamas, raises important questions about local pedogenesis and the influence of climate, and human activity on soil development. Distinct differences between modern and Early Lucayan (c.AD 700-c.1100) above ground residential occupational sites provide an opportunity to examine changes in elemental, carbon and nitrogen isotopic composition through time and assess the interaction between natural and cultural soil-forming processes.

Bahamian soils develop primarily on calcium carbonate bedrock composed of lithified oolites and fossil coral reefs, overlain by varying contributions of African dust rich in aluminum, iron, and silicon. This dust is transported from the Sahara during periods of increased aridity. As a result, soils on San Salvador are typically thin, overlie highly permeable substrates, and retain little moisture. They are strongly alkaline (pH ~7.8–8.8) and contain low concentrations of plant-available nutrients, making them generally unsuitable for intensive agriculture.

A total of 65 soil and sediment samples were collected between 2016-2018 from a range of environmental and archaeological sites, including excavations at the Pigeon Creek residential occupation archaeological site, caves and dissolution pits, dunes, wetlands, and coppice land. Samples were analyzed for elemental composition, bulk organic carbon, and stable carbon and nitrogen isotopes. Elemental concentrations were measured using portable X-ray fluorescence, while bulk organic stable isotopes were determined using gas chromatography with flame-ionization detection. Data were assessed using non-metric multidimensional scaling in PAST v.5.0.

The sites were distinguishable based on elemental composition, particularly with respect to the Early Lucayan and modern periods. Aluminum exhibited the greatest variability among elemental data, indicating substantial pedogenic influence from Saharan dust deposition across many of the sites. Red clay–rich samples contained elevated concentrations of Fe, Al, Si, and Ti relative to other environments.

Nitrogen isotopic enrichment is most pronounced within archaeological sites, particularly within cultural layers, which may reflect soil amendment practices such as the addition of bat guano in horticultural areas, decomposition of organic remains, or the reflection of the ‘original’ soil before cultivation. Overall, these results demonstrate 1) a strong Saharan dust signature in Bahamian soil development 2) heterogeneity between the sites, and 3) that Lucayan archaeological deposits possess distinctive elemental and isotopic characteristics that can be resolved spatially and temporally.

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