New Mexico Geological Society Annual Spring Meeting — Abstracts

Landscape evolution in Palo Duro Canyon, a Rio Grande tributary, has implications for carbon storage and cycling times in semi-arid drainage basins. The drainage basin of Palo Duro is large enough to collect the discharge. needed to down cut through the Llano de Manzano pediment/piedmont surface that bounds the southeastern side of the Albuquerque-Belen Basin. As a result of this process, the lower valley of Palo Duro contains a nested suite of terraces estimated to range from late Pleistocene to latest Holocene in age.

Following terrace formation, the first stage of landscape development is manifested by truncation of soils at the front of terrace, while soils at the back and middle are buried by colluvium. Erosion of previously formed terraces contributes colluvial material to the younger terrace surface. The colluvial clasts have rinds of pedogenic carbonate which enhance carbonate accumulation in the younger terrace soils. This represents carbon "recycling" within the terrestrial reservoir.

As base level drops gullies incise the terrace. The gullies establish a dendritic drainage pattern that exploits the Junction between the terrace and the adjoining scarp. At this second stage, the terrace is isolated from the scarp and no longer receives colluvium. The gullies intercept colluvium and divert it from the terrace surface into the main channel. This event is significant because accumulation of pedogenic carbonate in terrace soils is no longer augmented by colluvial contributions. In a third stage of landscape development; colluvium is stripped from the back of the terrace by the intercepting gully.

Early Holocene terrace treads are spatially isolated but the latest Holocene terrace is not; this indicates that 10 years are required for stage two to be completed. Late Pleistocene terrace treads are isolated and colluvium has been stripped from the terrace back so the third stage of development takes approximately 105 years to attain. These time scales of landscape development are critical because in stage one there is an overall carbon gains and carbon as pedogenic carbonate on colluvial pebbles is recycled back into the soils. At stage two, however, these clasts are diverted from the surface by the intercepting gullies. This process contributes to carbon loss from the landscape. In stage three of landscape development, the rate of carbon loss increases because colluvium containing carbonate is lost from the landscape. These stages of landscape evolution indicate initial gains of carbon followed by gradually accelerating losses of carbon from the drainage basin for late Pliestocene to present.