Weathering Profiles

Pliler R, Adams JAS (1962) Xhe distribntion of thorium and uranium in a Pennsylvanian weathering profile. Geochim Cosmochim Acta 26 1137-1146... 

The thermodynamic properties of U-Th series nuclides in solution are important parameters to take into account when explaining the U-Th-Ra mobility in surface environments. They are, however, not the only ones controlling radionuclide fractionations in surface waters and weathering profiles. These fractionations and the resulting radioactive disequilibria are also influenced by the adsorption of radionuclides onto mineral surfaces and their reactions with organic matter, micro-organisms and colloids. 

RADIOACTIVE DISEQUILIBRIA IN WEATHERING PROFILES DATING AND TRACING OF CHEMICAL MOBILITY... 

Characterization and time scale of chemical mobility in weathering profiles... 

A different approach for recovering weathering time information from U-Th series nuclides is to interpret depth variations of radioactive disequilibria in weathering profiles by simple but realistic modelling of U-Th series nuclides during formation and evolution... 

Distribution of U-Th-Ra in weathering profiles. The first U-Th studies (Pliler and Adams 1962 Rosholt et al. 1966 Hansen and Stout 1968) generally showed a U loss relative to Th at the base of the profiles, and an enrichment in the uppermost horizons and/or in some accumulation layers. The development of weathering studies, however, point out that this situation is not to be generalized and that reverse trends can be observed even at the scale of a single toposequence (Fig. 11). 

Compared to Th, U is generally much more mobile. In weathering profiles, however, its behavior is different from that of the purely soluble/weatherable elements such as Na, K (e g., Dequincey et al. 2002). Processes involved in the partial immobilization of U in... 

Figure 11. Distribution with depth of U/Th normahzed to the bedroek in two lateritic profiles of the Kaya toposeqnenee, about 300m apart (Burkina Faso) (Deqnineey et al. 2002 submitted). One profile is located downhill (Kaya 5) and the other one at the top of a residual hill (Kaya 1). The laterite consists of an uppermost fermginous hardtop, an intermediate pink clay nnit and a lowest pistachio unit. For Kaya 5 profile, U/Th distribntion shows a relative enrichment of U in the nppermost horizon and depletion in the lower part of the profile. This kind of distribution is quite conunon in weathering profiles bnt is not systematic as illnstrated by the Kaya 1 profile. In the latter, a relative depletion of U is observed in the npper part and a U-enriched level in the intermediate horizon. This lateral difference in U distribution is explained by vertical redistribntion of U from the ferruginons top to the nnderlying horizons, whose intensity is controlled by the evolntion of the iron oxides from the nppermost horizons (Dequincey et al. snbmitted).

Figure 12. Profiles of Th to Nb ratios for four Hawaiian soils developed on lava flows with ages ranging from 20 ka to 4100 ka (Kurtz et al. 2000). In these soils, Nb is assumed to be an irmnobile element. Variations of Th/Nb ratios, with lower values than those of basalt (greyed area) in the upper part of the profile and higher values in the lower part indieate an internal downward migration of Th in these fom weathering profiles. Cireles = 20 ka Laupahoehoe site, triangles = 150 ka Kohala site, squares = 1400 ka Molokai site, diamonds = 4100 ka Kauai site.

Assuming that ( " U/ U) fractionation variations, Boulad et al. (1977) proposed (230Th/238u) variations with depth in a laterite from Cameroon. These authors assumed the occurrence of two main U-Th fractionation fronts one at the base of the weathering profile with U loss, and one in the upper part of the profile where U is released and redeposited deeper in the profile (Fig. 15). By fitting theoretical curves to the data, they estimated weathering rates ranging from 50 to 70 mm/ka. An estimate of 50 mm/ka was also obtained with a similar approach by Mathieu et al. (1995) for a Brazilian laterite. 

Figure 17. and activity ratios in different weathering profiles (a) Rosholt et al. (1966)... 

Thiel et al. (1983) and Scott et al. (1992) have proposed more quantitative approaches, based on mathematical modelling of instantaneous and/or continuous U gain and loss. These models are applicable to weathering and account for the position of the data in the forbidden zones of the ( " U/ U) and ( °Th/ U) diagram. Several studies used this approach to interpret data in soils and weathering profiles (e.g.,... 

The study of separate mineral phases or of granulometric fractions is another approach which can be used to recover temporal information from radioactive disequilibria in weathering profiles. Such approaches rely on the assumption that the fractions only contain or concentrate minerals phases specific of a single or of few stages of formation and evolution of weathering profiles, and hence can help to characterise the time constants of the corresponding stages. 

This review highlights the important potential of U-series disequilibria in understanding the continental alteration and related mass transfers. The recent analytical developments, including TIMS and MC ICP MS techniques, for measuring small amounts of U, Th and Ra in geologic samples, offer today new possibilities and new perspectives for analysing U-series disequilibria in weathering profiles and river waters, and could lead to new and, as yet, unanticipated advances in the field of continental alteration. 

Bischoff JL, Fitzpatrick JA (1991) U-series dating of impure carbonates An isochron technique using total-sample dissolution. Geochim Cosmochim Acta 55 543-554 Bonotto DM (1998) Implication of groundwater weathered profile interactions to the mobilization of radionuclides. J South Am Earth Sci 11 389-405... 

Dequincey O, Chabaux F, Clauer N, Liewig N, Muller J-P (1999) Dating of weathering profiles by radioactive disequilibria contribution of the study of authigenic mineral fractions. CR Acad Sci Paris... 

Plater AJ, Ivanovich M, Dugdale RE (1995) Ra contents and Ra/ Ra activity ratios of the Fenland rivers and the Wash, eastern England spatial and seasonal trends. Chem Geol 119 275-292 Pliler R, Adams JAS (1962) The distribution of thorium and uranium in a Pennsylvanian weathering profile. Geochim Cosmochim Acta 26 1137-1146... 

Rates of Feldspar Weathering in Natural Weathering Profiles from Geochemical Mass Balance of Small Watersheds... 

Geochemical mass balance studies (also known as input-output budgets) invoke a simple conservation-of-mass principle. If the flux of any element leaving a watershed (e.g., via streams), and the flux of that element into the watershed (e.g., via atmospheric precipitation) are known, the difference between the two can be calculated, and this difference must be due to the sum of all reactions and transformations involving that element which took place within the watershed. Pioneering mass balance studies on weathering profiles and/or small watersheds include those of Garrels and Mackenzie (,51, 52) and Cleaves and Bricker and their... 

Four weathered profiles in different landscape settings and of different ages were examined. All profiles were developed on siliclastic metasedimentary rocks (siltstone-sandstone). 

The fourth weathering profile is in an area of post Miocene erosion, which has exposed areas of saprock. This profile... 

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