40 Part III -Abstracts PALEOSOLS AS INDICATORS OF PALEOCLIMATIC CONDITIONS IN THE QUATERNARY DEPOSITS OF NEW MEXICO H. Curtis Monger* and David R. Colet * Agronomy Department, New Mexico State University, Las Cruces, NM 88003, U.S.A. + Chemist~ Division, Oak Ridge National Laborato~, Oak Ridge, TN 37831, U.S.A. Many regions of the southwestern United States have experienced alternating periods of aridity and more effective moisture (pluvials) during the Quater- nary. In southern New Mexico, the pluvial periods were sufficient to form lakes and cause vegetational changes. The topic explored in this paper is whether the alternating arid and pluvial periods are recorded by paleosols in the form of their stratigraphic sequences in fan-piedmont areas, and other predictable character- istics such as polygenetic profiles, mineralogy, and stable isotopes. Sequences of buried paleosols are common in fan- piedmont areas surrounding mountains in southern New Mexico. The aggrading nature of the fan-pied- mont areas records the landscape's response to alter- nating episodes of landscape stability and instability. Landscape stability, which is characterized by soil formation, is thought to correspond to humid glacial intervals when increased vegetative cover prevented erosion and sedimentation. In contrast, landscape in- stability is characterized by erosion and sedimentation and appears to be associated with arid conditions of interglacial periods when vegetative cover decreased. Rising levels of atmospheric CO2 also associated with interglacial periods may have contributed to land- scape instability as well by causing a reduction of C4 plants. Unburied paleosols in southern New Mexico gener- ally have polygenetic soil profiles composed of two or more carbonate horizons. The depths of these horizons reflect different moisture regimes. During pluvial periods, carbonates were deposited deeper in the pro- file. During more arid periods, the depth of leaching would have shifted upward and deposited carbonates at shallower depths. Detrital sand and silt mineralogy was examined as a potential paleoclimatic indicator with the hypothesis that pedochemical weathering would have been greater during pluvial periods. However, there was little change in the sand and silt mineralogy of Quaternary paleosols. Apparently, the climatic shifts from arid to pluvial conditions were not extreme enough or were too short (~ 10 4 to 105 yr) to have produced significant mineralogical differences. Clay mineralogy is conceivably a more sensitive indi- cator of climate change than sand and silt mineralogy. The clay mineralogic record in southern New Mexico extends from the late Holocene to the middle Pleisto- cene and shows relatively clear trends that correspond to age and parent material. Deriving paleoclimatic evid- ence from clay mineralogy, however, is complicated and questionable because arid paleosols generally contain both eolian-derived and authigenic clay minerals. Soil carbonates can also be both eolian-derived and authigenic. However, if it can be demonstrated that carbonates precipitated in situ, have stable isotopic signatures which are sensitive indicators of environ- mental conditions that existed during the time of their formation. The 613C values in pedogenic carbonates preserve the vegetational response to climate change and atmospheric CO2 levels, including a relative measure of C4 and C3 plants. Provided that no evapor- ative enrichment has occurred in the soil, 61so values in pedogenic carbonates contain information about the amount of l So in ancient rain water which, in turn, is related to paleotemperatures. A MODIFIED MACK-JAMES-MONGER CLASSIFICATION OF PALEOSOLS W.D. Nettleton, B.R. Brasher and R.J. Ahrens u.s. Natural Resource Conservation Service, Lincoln, NB 68508, U.S.A. A new taxonomy is needed for pateosols. Except for relict and exhumed soils, they are buried and the burial has altered the usual relationships between many of their properties. Soil taxonomy may be applied to relict and exhumed paleosols, but needs extensive modifica- tion to fit buried paleosols. In addition, properties of buried soils are not considered at higher levels of soil taxonomy. The need for a taxonomy for paleosols has been recognized by others and a system has been pro- posed by Mack et al. (1993). (See James et al., this volume, Paleosol Classification). Our approach, like theirs, follows soil taxonomy orders. It uses the prefix paleo with some soil taxonomy orders and coins names for new orders. We use as criteria those properties of