Late Quaternary fluvial landscape evolution in desert grasslands of northern Chihuahua, Mexico

摘要

Presented here is the first comprehensive late Quarternary alluvial stratigraphic record from desert grasslands in northern Mexico. The stratigraphic history was temporally constrained by radiocarbon dating, whereas the ecology of C4 plants was inferred by stable carbon isotopes from buried solids. Environmental relationships within the Cases Grandes and San Pedro River basins of the Juanaquena study area provide a picture of evolving landscapes where fluvial behavior is driven by climate change and ecosystem instability. Seven periods of flood-plain construction and six episodes of paleosol formation and channel erosion occurred during the last 14,000 ~(14)C yr B. P. In the latest Pleistocene, channel erosion ensured in response to a general warming trend and landscape instability, although C3 plants still dominated in the alluvial valleys. In the Holocene when C4 plants began to dominate, channel erosion was typically initiated by a shift from relatively warmer and drier to cooler and wetter climates. The timing of erosion events exhibits a remarkable correlation to arroyo formation and paleosol formation of ephemeral streams in the North American southwest. In contrast to arroyos, the Casas Grandes and San Pedro Rivers created flood plains having mixed to bedload mean-derbelts that persisted throughout the late Quaternary. However, bedrock valley constrictions accelerated flood-plain building, leading to periodic channel avulsion. Although intervals of landscape instability occurred numerous times in the late Quaternary, fluvial style was largely governed by the dominance of C4 grasslands, which appears to have maintained sufficient upland vegetative cover to mitigate flash flooding typical of arid environments.