Isotopically distinct modern carbonates in abandoned livestock corrals in northern Kenya

Abstract

We report δ18O and δ13C data from modern carbonate in soils and dung samples from 3 recently abandoned livestock corrals in northern Kenya. Calcium carbonate content is higher within ∼5 cm depth that contains a mixture of dung and surface soils of corrals than in soils below 5 cm depth. We radiocarbon dated carbonates from 0.5 to 40 cm depths in two corrals and one control site. Surface carbonates (0.5 cm) from the two corrals were formed from modern carbon (>1955) when the corrals were active, while all other carbon is >16,000 years (BP) old. Shallow carbonate is also enriched in 18O (δ18O up to 3.0‰) and depleted in 13C (δ13C up to −12.0‰) with respect to carbonate at deeper levels and at two control sites. The δ18O and δ13C of soil carbonates (δ18OSC and δ13CSC respectively) in corrals are inversely correlated for depths up to about 15 cm where organic carbon is greater than 0.5%. Below that depth, there is a positive correlation between δ18OSC and δ13CSC values, similar to that observed in a control site.In concordance with the increase in δ18OSC and the decrease in δ13CSC values in corral surface soils, the δ15N of soil organic matter (SOM) (δ15NSOM) decreases with depth in corral soils, but in a control site shows a slight increase within the first 5 cm and then becomes relatively constant with depth. Dung-laden organic matter at corral surfaces is enriched in 15N by ∼5‰ relative to surface SOM of control sites. The δ15NSOM values imply that dung enriches the surface soils of livestock corrals in15N.The observed δ15NSOM and δ18OSC trends suggest microbially-mediated carbonate precipitation in the dung, a conclusion that is supported by δ13CSC and δ18OSC trends and the radiocarbon data. The calcium carbonate from the dung is released in the soil as dung mixes with the mineral phases of the soil.Changes in land use have resulted in more sedentary lifestyles among many pastoral communities, so corrals are likely to become increasingly important in conferring long-lasting transformations of the organic and inorganic components of soils that may lead to shifts in soil properties. The δ13CSC and δ18OSC therefore add to the toolbox for identifying former animal encampments in archaeological sites occupied by pastoral communities.