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Stable carbon isotopes (delta C-13) and C/N ratios in bulk sediment organic matter were used as indicators of C-3 and C-4 vegetation functional types in a ca. 40-kyr, predominantly alluvial sequence from Arroyo La Estacada (33.5 degrees S, 69 degrees W), in the arid Andean piedmont of central Argentina. Although this sediment sequence has the potential to contribute to the knowledge of past vegetation dynamics through its fossil pollen content, previous palynological studies provided scant information, constrained to a few time windows, because of poor pollen preservation. Stable isotope results for the total organic carbon fraction from three lithostratigraphic sections that span the past 40 kyr show shifts in delta C-13 values associated with changing contributions from C-3 and C-4 plants, suggesting this proxy can detect changes in these photosynthetic pathways, representative of Patagonian (and/or stream margin macrophytes) and Monte phytogeographic units, respectively. Climate and local factors, such as water availability in the basin, are inferred to be the main drivers of vegetation dynamics. The C/N ratios in a few samples show that organic matter sources are algae and/or phytoplankton, not solely C-3 or C-4 vascular plants. The delta C-13 record from Arroyo La Estacada is an example of how isotope geochemistry has overcome difficulties with pollen analysis and provided a reliable tool to investigate past vegetation changes in regions where C-3 and C-4 are differentially distributed. Isotope values during part of the Marine Isotope Stage 3 (MIS 3) and the middle Holocene are attributed to C-3 local floodplain plants, with a little or no contribution from regional terrestrial (Patagonian) vegetation. Late Pleistocene-Holocene palaeosols have a clear C-4 signal of terrestrial dominance, representative of Monte plants, thus, carbon isotope values constrain the early times of MIS 1, particularly where the Pleistocene Holocene transition occurs. A mixture of C-3 and C-4 values during the last 3-4 kyr is indicative of environmental conditions that prevail in the region today.