Soil memory of bioclimatic changes in the northern Chihuahuan Desert, USA

Abstract

The soil memory concept of palimpsest-wise memory, supplemented with an analysis of punctuated sedimentation (book-wise memory), provides a systematic method for thinking about soil-geomorphic evolution in the context of bioclimatic change. The purpose of this paper is to examine the soil memory concept at three landforms (piedmont slope, Rio Grande valley border, and basin floor) in a region in the northern Chihuahuan Desert that has been studied for six decades as part of a USDA Desert Soil Geomorphology Project and Jornada Experimental Range.

On each of the three landforms, the soil memory concept supports previous chronosequence and progressive-regressive pedogenic models that describe the transformation of a lithomatrix into a pedomatrix. Palimpsest-wise and book-wise memory, combined with landform memory (stepped fan-terraces), provide evidence of alternating landscape stability (soil formation) and instability (erosion) ranging in magnitude from overgrazing within the last 180 years to the major glacial/interglacial cycles within the last 800,000 years. Carbon isotopic analysis in combination with the soil memory model indicates an increase in C3 desert shrubs during the recent overgrazing high-erosion period and during the mid-Holocene aridity period, but otherwise indicates a system dominated by C4 grasses. That the isotopes did not detect an increase in C3 shrubs during the earlier interglacial periods might be the result of overprinting by a C4 signature during the longer and more stable glacial periods.

In addition to memory acquisition, memory loss is an important concept. In the Chihuahuan Desert study area, memory loss occurs mainly by the microbial oxidation of organic matter during increased aridity, by erosion that truncates soil horizons during dry periods, and by the dissolution of carbonate during wetter periods.