Alluvial archives of the Nochixtlan valley, Oaxaca, Mexico: Age and significance for reconstructions of environmental change

A reconnaissance of the Nochixtlan valley confirms that thick alluvial deposits of the late Quaternary are both well preserved and well exposed along the heavily incised modern streams. Their geometry and age were ascertained by logging representative cutbank exposures and radiocarbon dating buried palaeosols. The last four cut-and-fill cycles recorded cover the period since 14,000 BP, but older alluvial insets are also present. Contrary to previous reconstructions, the streams have had an arroyo-type morphology throughout the late Quaternary. The climatic oscillations at the transition from the Pleistocene to the Holocene seem to be expressed in the alternating formation of clayey organic-rich cumulic A horizons, and the precipitation of secondary carbonates. Rapid though intermittent aggradation set in after 10,300 BP in response to the establishment of a rather dry, warm and strongly seasonal climate and of open canopy forest or scrub on slopes. The unstable arroyo floodplains offered a favorable niche for the establishment of secondary vegetation and foragers exploiting annual and heliophilous plants. After 4000 BP sedentary farmers cleared the valley of its natural vegetation increasing sediment transfers and the frequency of local cut-and-fill cycles. They modified streams intentionally by building long flights of cross-channel agricultural terraces known as lama-bordos, the remains of which are preserved in many cutbanks. The oldest specimen dated goes back to ca. 3000 BP. A remarkably synchronous incision swept through the fluvial system close to 800 BP, possibly as a result of a marked climatic shift or valley-wide changes in land use. The widespread gullying of the latest pre-Conquest settlements and fields suggests the introduction of grazing or the collapse of hillside terraces as cause, but coeval alluvial fills have not yet been positively identified.

► Alluvium of the last 15 kyr is preserved and well exposed in the Nochixtlan valley. ► Incised channels and narrow floodplains have persisted throughout this timespan. ► Rapid climate change during deglaciation is well expressed in the stratigraphy. ► Cut-and-fill cycles accelerated markedly after the arrival of sedentary farmers. ► Construction of cross-channel terraces modified the streams since at least 3000 BP.