Eric V. McDonald

Factors and processes governing the 14C content of carbonate in desert soils

EPSL ELSEVIER Earth and Planetary Science Letters 125 (1994) 385-405 Factors and processes governing the 14C content of carbonate in desert soils Ronald Amundson a Yang Wang a, Oliver Chadwick b Susan Trumbore c, Leslie McFadden d, Eric McDonald d, Steven Wells e, Michael DeNiro f a Division of Ecosystem Sciences, 108 Hilgard, University of California, Berkeley, CA 94720, USA b Earth Sciences Division, Jet Propulsion Laboratory, Pasadena, CA 91109, USA c Department of Geosciences, University of California, Irvine, CA 92717, USA d Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA e Department of Earth Sciences, University of California, Riverside, CA 92521, USA f Department of Geological Sciences, University of California, Santa Barbara, CA 93106, USA Received 1 September, 1993; revision accepted 14 April 1994 Abstract A model is presented describing the factors and processes which determine the measured 14C ages of soil calcium carbonate. Pedogenic carbonate forms in isotopic equilium with soil CO 2. Carbon dioxide in soils is a mixture of CO 2 derived from two biological sources: respiration by living plant roots and respiration of microorganisms decomposing soil humus. The relative proportion of these two CO z sources can greatly affect the initial 14C content of pedogenic carbonate: the greater the contribution of humus-derived CO 2, the greater the initial 14C age of the carbonate mineral. For any gwen mixture of CO 2 sources, the steady-state CO 2 distribution vs. soil depth can be described by a production/diffusion model. As a soil ages, the 14C age of soil humus increases, as does the steady-state 14C age of soil CO 2 and the initial 14C age of any pedogenic carbonate which forms. The mean 14C age of a complete pedogenic carbonate coating or nodule will underestimate the true age of the soil carbonate. This discrepancy increases the older a soil becomes. Partial removal of outer (and younger) carbonate coatings greatly improves the relationship between measured 14C age and true age. Although the production/diffusion model qualitatively explains the 14C age of pedogenic carbonate vs. soil depth in many soils, other factors, such as climate change, may contribute to the observed trends, particularily in soils older than the Holocene. 1. Introduction R a d i o c a r b o n d a t i n g o f s e d i m e n t a r y d e p o s i t s in d e s e r t s is h i n d e r e d by a lack o f o r g a n i c carbon. Paradoxically, most o f the d e p o s i t s in t h e s e re- [MK] gions a r e b l a n k e t e d with i n o r g a n i c c a r b o n in the f o r m o f p e d o g e n i c c a r b o n a t e . Since t h e early 1960s, 14C ages have b e e n c a l c u l a t e d for t h e s e c a r b o n a t e s [1]. Fig. 1 illustrates t h a t an a p p r o x i - m a t e 1 : 1 r e l a t i o n s h i p exists b e t w e e n 14C ages of soil c a r b o n a t e a n d coexisting o r g a n i c m a t e r i a l s a n d ages d e t e r m i n e d by o t h e r m e a n s (see also [2,3]). T h e s e r e l a t i o n s h i p s a r e n o t perfect: s o m e c a r b o n a t e is y o u n g e r t h a n i n d e p e n d e n t ages sug- 0012-821X/94/

An isotopic study of soils in chronological sequences of alluvial deposits, Providence Mountains, California

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Interaction between aggrading geomorphic surfaces and the formation of a late pleistocene paleosol in the palouse loess of eastern Washington state

The carbon ({sup 14}C and {sup 13}C) and oxygen isotopic composition of pedogenic carbonate was determined for two soil chronosequences on limestone and granitic alluvium in the Providence Mountains area in the Mojave Desert, California. The measured {sup 14}C ages of pedogenic carbonate coating on clasts were interpreted in the light of a diffusion-reaction model developed in our recent studies. Model ages of soil formation calculated from the measured {sup 14}C ages of pedogenic carbonate are in correct relative order as determined by geomorphic evidence, and are also consistent with model ages from the measured {sup 14}C ages of soil organic matter. {sup 14}C model ages suggest that the order geomorphic surfaces we studied are of late Pleistocene age (ca. 47-17 ka) and the younger surfaces formed during the Holocene (ca. 11-4 ka). These age estimates of the geomorphic surfaces are older than the previously assigned ages based on a combination of soil development, geomorphic relationships, and several infrared-stimulated luminescence dates, but they are within a few thousand years of these other age estimates. Stable carbon isotopic composition of the soil carbonate indicates either a slight increase in C{sub 4} or CAM (crassulacean acid metabolism) plants or a decrease inmorexa0» plant density in this area during the Holocene. Both the carbon and oxygen isotopic composition of soil carbonates suggests that the climate in the eastern Mojave Desert has, in general, become warmer and drier during the Holocene. 57 refs., 9 figs., 5 tabs.«xa0less

Geochemistry of Background Sediment Samples at Technical Area 39, Los Alamos National Laboratory

Abstract Variable rates of loess deposition contributed to dramatic regional variation in a soil-stratigraphic unit, the Washtucna Soil, in the Palouse loess deposits in the Channeled Scabland of eastern Washington state. Throughout most of the Channeled Scabland, the morphology of the Washtucna Soil is that of a single buried soil, but it bifurcates into two well-developed and pedologically distinct buried soils in areas immediately downwind of the major source of loessial sediment. Regional loess stratigraphy confirms that the two well-developed soils formed during the same interval of time during which only one soil formed in areas that are distal to loess source areas. The variable and perhaps rapid rates of soil formation suggested by the stratigraphy resulted from an interaction between variable rates of loess deposition and the formation of superimposed calcic soils. Petrocalcic horizons with weak Stage IV morphology formed as the zone of carbonate accumulation moved up into former A and cambic horizons that had been profusely burrowed by cicadas. The development of cicada burrows in one phase of soil development that were subsequently engulfed by pedogenic carbonate under a rising land surface seems to have greatly accelerated the development of the petrocalcic horizons. Accelerated rates of formation of the petrocalcic horizons occurred when extrinsic (pulses of loess deposition) and intrinsic (engulfment of burrowed horizons) thresholds were exceeded. Stratigraphic evidence suggests that the soil formation that accompanied the rise in the land surface due to additional loess deposition may have occurred during the late Wisconsin glaciation when giant glacial outburst floods in the channeled Scabland triggered a new cycle of loess deposition.

General soil-landscape relationships and soil-forming processes in the Pajarito Plateau

This report presents results of chemical analyses of 24 analytes in 16 background sediment samples collected from Ancho Canyon and Indio Canyon at Technical Area (TA) 39, Los Alamos National Laboratory. Preliminary upper tolerance limits (UTLS) for sediments are calculated from this data set but, because of the small sample size, these UTLs exceed the maximum values in the data set by up to 50ZO and will require revision as more background sediment data are obtained.