LATE PLEISTOCENE-HOLOCENE PALEOENVIRONMENTS AND EARLY PEOPLING IN SONORA DESERT, NE MEXICO: STUDY CASE IN LA PLAYA ARCHAEOLOGICAL SITE

Early peopling has been well documented in NW Mexico in sites as Fin del Mundo and El Bajío and in la Playa in Late Glacial-Holocene. We have conducted a paleopedological research precisely in the archeological site “la Playa”, densely settled around 1500 BD, in order to provide information about the environmental conditions during the time of settlement. In this area two paleosol pedocomplexes were distinguished: the older, San Rafael (SRP), with a buried sequence of reddish A/Bw/BCk/2BCgk/2Ck horizons and the youngest one, Boquillas (BOP) consisting of several Ah horizons separated by alluvial sediments. SRP shows moderate clay accumulation and the presence of illuvial carbonates with a minimum content in Bw and the maxima in BCk and 2BCgk horizons. In contrast, BOP is a pedosediment, which has weak humus accumulation and the development of pedogenic structure over alluvial sediments. The age of SRP, has been established by AMS dating of carbonates concretions found in BCk (Cal BP 14910 to 14230), as well as charcoal found in A horizon (Cal BP 4440 to 4250), evidencing the cycles of soil formation. BOP is 3660 yr. BP., obtained from organic matter of one of the. A horizons This age coincides with the archaeological evidences. We consider that differences in pedogenetic trends are related to different environmental conditions. SRP properties (rubification; moderate clay accumulation; higher values of magnetic susceptibility, revealing a higher weathering degree; the presence of illuvial carbonates; and the reductomorphic features in the base) evidence a more humid environment. This condition is in good agreement with the higher lacustrine levels found in the area during Late Glacial. In middle Holocene, environment changes drastically and presence of fires were detected, coinciding with mid-to-late Holocene peak in fire activity reported by Brunelle et al. (2010). Actually, Holocene soil is less developed and strongly affected by fires thus revealing a high landscape instability caused by both natural and human influence.

Brunelle, A., Minckley, T.A., Blissett, S., Cobabe, S.K., Guzman, B.L., 2010. A ~8000 year fire history from an Arizona/Sonora borderland ciénega. Journal of Arid Environments 74:475-481.