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Airborne gamma-ray spectrometry to quantify chemical erosion processes
Journal of Geochemical Exploration 88 (2006) 266 – 270 www.elsevier.com/locate/jgeoexp
Airborne gamma-ray spectrometry to quantify chemical erosion processes F. Carrier a,b,c,*, B. Bourdon b, E´. Pili c, C. Truffert a, R. Wyns a a
BRGM-CDG / Mode´lisation et Applications-45060 Orle´ans La Source, France IPGP-Laboratoire de Ge´ochimie et Cosmochimie-75252 Paris cedex 05, France CEA-De´partement Analyse, Surveillance, Environnement-91680 Bruye`res-le-Chaˆtel, France b
c
Received 11 April 2005; accepted 19 August 2005 Available online 23 November 2005
Abstract Airborne gamma-ray spectrometry data (uranium, potassium and thorium contents) reveal geochemical heterogeneities within the monolithological Hyroˆme watershed (ca. 150 km2) in the Armorican massif (western France). Our observations and computations provide important constraints on the spatial distribution and the associated magnitudes of chemical erosion processes at the scale of a small watershed. Two distinct, partially preserved, weathering profiles exhibit a strong correlation between regolith evolution and airborne-derived K/Th ratios, suggesting that the variability is linked to supergene processes. Using both airborne data and laboratory measurements on rock samples, the total net export of potassium has been estimated at 422 F 50 kg/m2 and the chemical weathering rate of potassium at 17 F 2 kg/km2/a. D 2005 Elsevier B.V. All rights reserved. Keywords: Airborne gamma-ray spectrometry; Weathering profile; Chemical erosion rates; Watershed
1. Introduction Chemical weathering studies use punctual samplings that make it often difficult to provide spatially distributed information (e.g., Heimsath et al., 1997; Anderson et al., 2002; Riebe et al., 2004). For the first time, chemical erosion has been explored in a small watershed (150 km2) using airborne radiometric data of the Armorican massif (western France, Bonijoly et al., 1999). Uranium, potassium and thorium content in the basement rocks were mapped, but elemental composition heterogeneities have also been observed within * Corresponding author. BRGM-CDG / Mode´lisation et Applications-45060 Orle´ans La Source, France. Tel.: +33 2 38 64 39 85; fax: +33 2 38 64 33 34. E-mail address: [email protected] (F. Carrier). 0375-6742/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2005.08.053
highly weathered monolithological units (Carrier, 2005). Variations in the content of a mobile element, potassium, and of an immobile element, thorium, along ancient weathering profiles, provide mass balance estimations at any point of the Hyroˆme watershed. Here, we present 48,406 airborne radiometric data points, rock and soil laboratory gamma spectrometry data, field observations, and computed digital elevation data used to quantify erosion fluxes. 2. Materials and methods The parent rocks of the study area (the Hyroˆme watershed) mainly consist of a series of Precambrian micaschists. Chemical weathering has deeply affected the micaschists over several tens of meters to build the regolith, which has then been intensely incised. Two
F. Carrier et al. / Journal of Geochemical Exploration 88 (2006) 266–270
weathering periods can be distinguished within the Hyroˆme watershed: 1) the first, during early Cretaceous (ca. 145.5 F 2.0 Ma to 99.6 F 0.5 Ma, Wyns, 1991). The early Cretaceous weathering profile now occupies ca. 40% of the total surface area and is associated with a saprolite layer up to 10 m thick; 2) the second, during early Eocene to middle Eocene (ca. 65.5 F 0.5 Ma to 40.4 F 0.1 Ma, Wyns, 1991). The Paleogene weathering profile corresponds to ca. 49% of the total surface area, with a 30-m thick saprolite layer.
By combining Eqs. (1) and (2), one finds CTh;r dCK;w CK;r dqr DK;Th ¼ CTh;w
267
ð3Þ
Eq. (3) was computed for every 48,406 data point within the Hyroˆme watershed, where subscripts w and r and denote airborne radiometric and rock sample data, respectively. Bedrock sample concentrations (metapelites and metagreywackes) have been measured by laboratory gamma-ray spectrometry. Uncertainties for uranium, potassium and thorium concentrations are about 15%. 3. Results and discussion
To localize vertically the airborne radiometric data within the weathering profile layers, the limit between the saprolite and the fissured micaschists (hereafter referred to as the saprolite footwall) has been carefully mapped on the field. A virtual surface corresponding to the saprolite footwall elevation has been interpolated from the outcrop locations, within the Hyroˆme watershed with a 50-m resolution. Then, the difference in altitude, for each pixel, between the present-day topography and the interpolated saprolite footwall surface was calculated to obtain the Saprolite Footwall Level (SFL in meters). An estimated position on the saprolite footwall is set to zero; when positive, SFL value corresponds to the present-day preserved saprolite thickness; when negative, SFL value corresponds to a position below the base of the saprolite. We used the contrasted behaviors of potassium and thorium with respect to chemical erosion to quantify weathering rates. Airborne radiometric measurements were the basis to estimate weathering-induced changes in volume, density and concentrations, assuming mass conservation of Th (McLennan et al., 1980) in the fresh rock and in the weathered material as well; hence, CTh;r dVr dqr ¼ CTh;w dVw dqw
ð1Þ
where C Th, V, q, are the thorium concentration (in ppm), the volume (in cm3), the dry bulk density (in kg/m3), respectively, and subscripts r and w denote the parent rock and weathered material, respectively. Here, we have considered Th as an insoluble element relative to K (Stallard and Edmond, 1983) and used massbalance equations (Brimhall et al., 1985) to derive the net export of potassium D K,Th in kg/m3 from the mass loss of potassium (m K) relative to the initial rock volume (Vr) as follows Vw dqw dCK;w Vr dqr dCK;r mK ¼ ð2Þ DK;Th ¼ Vr Vr
48,406 Airborne K/Th ratios were plotted in Fig. 1 with Saprolite Footwall Level values, together with rock and soil data. An overall negative correlation between K/Th ratios and SFL values is observed in Fig. 1, corresponding to a zone of progressive loss of potassium with rising location in the weathering profile, called the saprolitization layer. However, this trend is lost below and above the saprolitization layer, where K/Th ratios are roughly constant. For SFL values under 20 m, K/Th ratios correspond to those found in fresh rocks (~2000). For SFL values above + 10 m, K/Th ratios correspond to a zone where the potassium becomes less and less leachable (~1200), as more resistant minerals contain significant potassium amounts (e.g., sericite). The variability in space of the saprolite footwall, added to potential solifluxion processes of this loose material, may contribute to the broad dispersion of the data (K/Th and SFL uncertainties: F 20% and F 10 m, respectively). To minimize this latter effect, the downslope deposits have been mapped and excluded from the data set. The strong fractionation (Fig. 1) of a mobile element (K) relative to an immobile element (Th) illustrates the contrasted geochemical properties of K and Th that we used to estimate chemical paleoerosion processes at the scale of the Hyroˆme watershed. During the chemical erosion process, potassium has been lost from the erosion surface down to negative SFL values (here, down to SFL= 20 m, Fig 1), indicating that chemical erosion processes begin to alter the fresh rock under the saprolite footwall observable in the field. We then applied Eq. (3) to all the weathered micaschists with SFL values z 20 m. However, our open-system model considers export of matter from the weathering profile, but has not been corrected from potential inputs of Th or K. D K,Th estimations have been restricted to 48,406 pixels of weathered micaschists. The rock sample
268
F. Carrier et al. / Journal of Geochemical Exploration 88 (2006) 266–270
Fig. 1. Saprolite Footwall Level (SFL, see text for definition) as a function of K/Th ratios calculated from airborne gamma-ray spectrometry. The 48,406 airborne values are represented with small gray dots. The maximum density of the airborne data has been represented with a superimposed black zone (43% of the data points). SFL values equal to zero correspond to the limit between the saprolite and the fissured layer observed in the field. Soil samples are represented with white squares, rock samples with triangles.
parameters (q r, C Th,r, C K,r) have been set to 2600 F 110 kg/m3, 6.3 F 0.8 ppm and 14000 F 2000 ppm (weighted means), respectively, and computed with airborne radiometric parameters (C Th,w and C K,w) with Eq. (3). Still, D K,Th values had to be reported according to the preserved profile’s thickness, for D K,Th values correspond to the total amount of potassium lost during the entire weathering periods. For that purpose, we used the linear relationships between K/Th and D K,Th in Eq. (3), also observed between K/Th and SFL in Fig. 1, to integrate the net export of potassium over the thickness of the remaining paleoprofile. Placed on the Hyroˆme watershed map (Fig. 2), these 48,406 integrated D K,Th values show the distribution of the total export of potassium during the weathering periods. The calculated net potassium export D K,Th can reach to 22 F 3 kg/m3 and the integrated export of potassium, for a weathered column, is quite similar for the two weathering profiles, reaching 411 F 50 kg/m2 for the early Cretaceous surface and 422 F 50 kg/m2 for the Paleogene surface. In Fig. 2, the considered pixels belong to either one or the other of the two weathering profiles whose areas are roughly similar. The early Cretaceous weathering profile developed during ca. 45 F 5 Ma and is located in the NE part of the watershed while the Paleogene
weathering profile has a maximum duration of 25 F 5 Ma and is located in the SW part of the watershed (Carrier, 2005). Although active weathering phases occurred at different times, there is no sharp contrast between the weathering products in the two weathering profiles (Figs. 1 and 2). Chemical erosion has led to comparable potassium exports, up to 422 F 50 kg/m2. As a consequence, the saprolitization of the micaschists may have occurred within at least 25 Ma, assuming a chemical weathering rate equal to 17 kg/km2/ a. However, the Paleogene climate is said to have induced more severe hydrolyzing conditions (e.g., Kennett and Stott, 1991; Yap, 2004) than the early Cretaceous one. Such a distinction cannot be observed with our data, characterized by comparable ranges. Corollary, similar weathering processes are observed, but, despite discontinuous development of the weathering profiles submitted to probably different climates, the weathering products are identical (Fig. 2). The specific time-scale for the weathering profile deepening is thus larger than 10 Ma (Meybeck, 1979), many orders of magnitude greater than climate change. The weathering profile developments are thus controlled by external forcing with the same order of magnitude. A long period with low vertical tectonic deformations, continuous continental exposure added to sufficient
F. Carrier et al. / Journal of Geochemical Exploration 88 (2006) 266–270
269
Fig. 2. Map showing the net export of potassium (in kg/m3) from the micaschists above the fissured layer (50-m resolution). The net loss of potassium reaches 22 F 3 kg/m3 in the upper parts of the paleoweathering profile on the plateaus areas, represented in dark. The white zones correspond to the fissured layer in the micaschists and to other lithologies, locations with no export of potassium ascribable to weathering processes.
amounts of water favor high degree of chemical weathering, associated with low physical erosion. 4. Conclusions Airborne gamma-ray spectrometry brings a new perspective to weathering profile studies. Chemical erosion intensity was quantified at the scale all of a 150-km2 watershed using natural radioelement contrasted geochemical behaviors. The progressive loss of potassium has been observed throughout the saprolitization layer, from the unweathered micaschists to the mature saprolite, thanks to remote sensing measurements. Then, a simple mass balance, based on potassium and thorium geochemistry, was used to estimate paleoweathering rates of potassium. There is no distinct evolution for the Paleogene and early Cretaceous weathering profiles, suggesting that weathering intensity should have been comparable over these successive periods. The chemical weathering rates of potassium are estimated to ca. 17 F 2 kg/km2/a with a 50-m preserved weathering
profile for the Paleogene period. The development of such thick profiles requires extended periods of high weathering conditions (low tectonic activity, in a continental environment), coupled with inefficient physical erosion. Direct airborne gamma-ray measurements capture the long-term bedrock weathering history. References Anderson, S.P., Dietrich, W.E., Brimhall, G.H., 2002. Weathering profiles, mass balance analysis, and rates of solute loss: linkages between weathering and erosion in a small, steep catchment. Geol. Soc. Amer. Bull. 114, 1143 – 1158. Bonijoly, D., Perrin, J., Truffert, C., Asfirane, F., 1999. Couverture ge´ophysique ae´roporte´e du Massif Armoricain. BRGM report, vol. R40471. 75 pp. Brimhall, G.H., Alpers, C., Cunningham, A.B., 1985. Analysis of supergene oreforming processes using mass balance principles. Econ. Geol. 80, 1227 – 1254. Carrier, F. 2005. Utilisation de la spectrome´trie gamma ae´roporte´e (uranium, thorium, potassium) pour quantifier les processus dTe´rosion et dTalte´ration. The`se de doctorat de lTinstitut de Physique du Globe de Paris, 263 pp.
270
F. Carrier et al. / Journal of Geochemical Exploration 88 (2006) 266–270
Heimsath, A.M., Dietrich, W.E., Nishiizumi, K., Finkel, R.C., 1997. The soil production function and landscape equilibrium. Nature 388, 358 – 361. Kennett, J.P., Stott, L.D., 1991. Abrupt deep-sea warming, palaeoceanographic changes and benthic extinctions at the end of the Palaeocene. Nature 353, 225 – 229. McLennan, S.M., Nance, W.B., Taylor, S.R., 1980. Rare earth and element-thorium correlations in sedimentary rocks, and the composition of the continental crust. Geochim. Cosmochim. Acta 44, 1833 – 1839. Meybeck, M., 1979. Concentrations des eaux fluviales en e´le´ments majeurs et apports en solution aux oce´ans. Rev. Ge´ogr. Phys. Ge´ol., 31215 – 31246.
Riebe, C.S., Kirchner, J.W., Finkel, R.C., 2004. Erosional and climatic effects on long-term chemical weathering rates in granitic landscapes spanning diverse climate. EPSL 224, 547 – 562. Stallard, R.F., Edmond, J.M., 1983. Geochemistry of the Amazon: 2. The influence of the geology and weathering environment on the loa. J. Geophys. Res. 88, 9688 – 9761. Wyns, R., 1991. Structural evolution of the Armorican basement during the Cenozoic deduced from analysis of continental paleosurfaces and associated deposits. Ge´ol. Fr. 3, 11 – 42. Yap, C.J., 2004. Fe(CO3)OH in goethite from mid-latitude North American oxisol: estimate of atmospheric CO2 concentration in the early Eocene bclimatic optimumQ. Geochim. Cosmochim. Acta 68, 935 – 947.
Airborne gamma-ray spectrometry to quantify chemical erosion processes F. Carrier a,b,c,*, B. Bourdon b, E´. Pili c, C. Truffert a, R. Wyns a a
BRGM-CDG / Mode´lisation et Applications-45060 Orle´ans La Source, France IPGP-Laboratoire de Ge´ochimie et Cosmochimie-75252 Paris cedex 05, France CEA-De´partement Analyse, Surveillance, Environnement-91680 Bruye`res-le-Chaˆtel, France b
c
Received 11 April 2005; accepted 19 August 2005 Available online 23 November 2005
Abstract Airborne gamma-ray spectrometry data (uranium, potassium and thorium contents) reveal geochemical heterogeneities within the monolithological Hyroˆme watershed (ca. 150 km2) in the Armorican massif (western France). Our observations and computations provide important constraints on the spatial distribution and the associated magnitudes of chemical erosion processes at the scale of a small watershed. Two distinct, partially preserved, weathering profiles exhibit a strong correlation between regolith evolution and airborne-derived K/Th ratios, suggesting that the variability is linked to supergene processes. Using both airborne data and laboratory measurements on rock samples, the total net export of potassium has been estimated at 422 F 50 kg/m2 and the chemical weathering rate of potassium at 17 F 2 kg/km2/a. D 2005 Elsevier B.V. All rights reserved. Keywords: Airborne gamma-ray spectrometry; Weathering profile; Chemical erosion rates; Watershed
1. Introduction Chemical weathering studies use punctual samplings that make it often difficult to provide spatially distributed information (e.g., Heimsath et al., 1997; Anderson et al., 2002; Riebe et al., 2004). For the first time, chemical erosion has been explored in a small watershed (150 km2) using airborne radiometric data of the Armorican massif (western France, Bonijoly et al., 1999). Uranium, potassium and thorium content in the basement rocks were mapped, but elemental composition heterogeneities have also been observed within * Corresponding author. BRGM-CDG / Mode´lisation et Applications-45060 Orle´ans La Source, France. Tel.: +33 2 38 64 39 85; fax: +33 2 38 64 33 34. E-mail address: [email protected] (F. Carrier). 0375-6742/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2005.08.053
highly weathered monolithological units (Carrier, 2005). Variations in the content of a mobile element, potassium, and of an immobile element, thorium, along ancient weathering profiles, provide mass balance estimations at any point of the Hyroˆme watershed. Here, we present 48,406 airborne radiometric data points, rock and soil laboratory gamma spectrometry data, field observations, and computed digital elevation data used to quantify erosion fluxes. 2. Materials and methods The parent rocks of the study area (the Hyroˆme watershed) mainly consist of a series of Precambrian micaschists. Chemical weathering has deeply affected the micaschists over several tens of meters to build the regolith, which has then been intensely incised. Two
F. Carrier et al. / Journal of Geochemical Exploration 88 (2006) 266–270
weathering periods can be distinguished within the Hyroˆme watershed: 1) the first, during early Cretaceous (ca. 145.5 F 2.0 Ma to 99.6 F 0.5 Ma, Wyns, 1991). The early Cretaceous weathering profile now occupies ca. 40% of the total surface area and is associated with a saprolite layer up to 10 m thick; 2) the second, during early Eocene to middle Eocene (ca. 65.5 F 0.5 Ma to 40.4 F 0.1 Ma, Wyns, 1991). The Paleogene weathering profile corresponds to ca. 49% of the total surface area, with a 30-m thick saprolite layer.
By combining Eqs. (1) and (2), one finds CTh;r dCK;w CK;r dqr DK;Th ¼ CTh;w
267
ð3Þ
Eq. (3) was computed for every 48,406 data point within the Hyroˆme watershed, where subscripts w and r and denote airborne radiometric and rock sample data, respectively. Bedrock sample concentrations (metapelites and metagreywackes) have been measured by laboratory gamma-ray spectrometry. Uncertainties for uranium, potassium and thorium concentrations are about 15%. 3. Results and discussion
To localize vertically the airborne radiometric data within the weathering profile layers, the limit between the saprolite and the fissured micaschists (hereafter referred to as the saprolite footwall) has been carefully mapped on the field. A virtual surface corresponding to the saprolite footwall elevation has been interpolated from the outcrop locations, within the Hyroˆme watershed with a 50-m resolution. Then, the difference in altitude, for each pixel, between the present-day topography and the interpolated saprolite footwall surface was calculated to obtain the Saprolite Footwall Level (SFL in meters). An estimated position on the saprolite footwall is set to zero; when positive, SFL value corresponds to the present-day preserved saprolite thickness; when negative, SFL value corresponds to a position below the base of the saprolite. We used the contrasted behaviors of potassium and thorium with respect to chemical erosion to quantify weathering rates. Airborne radiometric measurements were the basis to estimate weathering-induced changes in volume, density and concentrations, assuming mass conservation of Th (McLennan et al., 1980) in the fresh rock and in the weathered material as well; hence, CTh;r dVr dqr ¼ CTh;w dVw dqw
ð1Þ
where C Th, V, q, are the thorium concentration (in ppm), the volume (in cm3), the dry bulk density (in kg/m3), respectively, and subscripts r and w denote the parent rock and weathered material, respectively. Here, we have considered Th as an insoluble element relative to K (Stallard and Edmond, 1983) and used massbalance equations (Brimhall et al., 1985) to derive the net export of potassium D K,Th in kg/m3 from the mass loss of potassium (m K) relative to the initial rock volume (Vr) as follows Vw dqw dCK;w Vr dqr dCK;r mK ¼ ð2Þ DK;Th ¼ Vr Vr
48,406 Airborne K/Th ratios were plotted in Fig. 1 with Saprolite Footwall Level values, together with rock and soil data. An overall negative correlation between K/Th ratios and SFL values is observed in Fig. 1, corresponding to a zone of progressive loss of potassium with rising location in the weathering profile, called the saprolitization layer. However, this trend is lost below and above the saprolitization layer, where K/Th ratios are roughly constant. For SFL values under 20 m, K/Th ratios correspond to those found in fresh rocks (~2000). For SFL values above + 10 m, K/Th ratios correspond to a zone where the potassium becomes less and less leachable (~1200), as more resistant minerals contain significant potassium amounts (e.g., sericite). The variability in space of the saprolite footwall, added to potential solifluxion processes of this loose material, may contribute to the broad dispersion of the data (K/Th and SFL uncertainties: F 20% and F 10 m, respectively). To minimize this latter effect, the downslope deposits have been mapped and excluded from the data set. The strong fractionation (Fig. 1) of a mobile element (K) relative to an immobile element (Th) illustrates the contrasted geochemical properties of K and Th that we used to estimate chemical paleoerosion processes at the scale of the Hyroˆme watershed. During the chemical erosion process, potassium has been lost from the erosion surface down to negative SFL values (here, down to SFL= 20 m, Fig 1), indicating that chemical erosion processes begin to alter the fresh rock under the saprolite footwall observable in the field. We then applied Eq. (3) to all the weathered micaschists with SFL values z 20 m. However, our open-system model considers export of matter from the weathering profile, but has not been corrected from potential inputs of Th or K. D K,Th estimations have been restricted to 48,406 pixels of weathered micaschists. The rock sample
268
F. Carrier et al. / Journal of Geochemical Exploration 88 (2006) 266–270
Fig. 1. Saprolite Footwall Level (SFL, see text for definition) as a function of K/Th ratios calculated from airborne gamma-ray spectrometry. The 48,406 airborne values are represented with small gray dots. The maximum density of the airborne data has been represented with a superimposed black zone (43% of the data points). SFL values equal to zero correspond to the limit between the saprolite and the fissured layer observed in the field. Soil samples are represented with white squares, rock samples with triangles.
parameters (q r, C Th,r, C K,r) have been set to 2600 F 110 kg/m3, 6.3 F 0.8 ppm and 14000 F 2000 ppm (weighted means), respectively, and computed with airborne radiometric parameters (C Th,w and C K,w) with Eq. (3). Still, D K,Th values had to be reported according to the preserved profile’s thickness, for D K,Th values correspond to the total amount of potassium lost during the entire weathering periods. For that purpose, we used the linear relationships between K/Th and D K,Th in Eq. (3), also observed between K/Th and SFL in Fig. 1, to integrate the net export of potassium over the thickness of the remaining paleoprofile. Placed on the Hyroˆme watershed map (Fig. 2), these 48,406 integrated D K,Th values show the distribution of the total export of potassium during the weathering periods. The calculated net potassium export D K,Th can reach to 22 F 3 kg/m3 and the integrated export of potassium, for a weathered column, is quite similar for the two weathering profiles, reaching 411 F 50 kg/m2 for the early Cretaceous surface and 422 F 50 kg/m2 for the Paleogene surface. In Fig. 2, the considered pixels belong to either one or the other of the two weathering profiles whose areas are roughly similar. The early Cretaceous weathering profile developed during ca. 45 F 5 Ma and is located in the NE part of the watershed while the Paleogene
weathering profile has a maximum duration of 25 F 5 Ma and is located in the SW part of the watershed (Carrier, 2005). Although active weathering phases occurred at different times, there is no sharp contrast between the weathering products in the two weathering profiles (Figs. 1 and 2). Chemical erosion has led to comparable potassium exports, up to 422 F 50 kg/m2. As a consequence, the saprolitization of the micaschists may have occurred within at least 25 Ma, assuming a chemical weathering rate equal to 17 kg/km2/ a. However, the Paleogene climate is said to have induced more severe hydrolyzing conditions (e.g., Kennett and Stott, 1991; Yap, 2004) than the early Cretaceous one. Such a distinction cannot be observed with our data, characterized by comparable ranges. Corollary, similar weathering processes are observed, but, despite discontinuous development of the weathering profiles submitted to probably different climates, the weathering products are identical (Fig. 2). The specific time-scale for the weathering profile deepening is thus larger than 10 Ma (Meybeck, 1979), many orders of magnitude greater than climate change. The weathering profile developments are thus controlled by external forcing with the same order of magnitude. A long period with low vertical tectonic deformations, continuous continental exposure added to sufficient
F. Carrier et al. / Journal of Geochemical Exploration 88 (2006) 266–270
269
Fig. 2. Map showing the net export of potassium (in kg/m3) from the micaschists above the fissured layer (50-m resolution). The net loss of potassium reaches 22 F 3 kg/m3 in the upper parts of the paleoweathering profile on the plateaus areas, represented in dark. The white zones correspond to the fissured layer in the micaschists and to other lithologies, locations with no export of potassium ascribable to weathering processes.
amounts of water favor high degree of chemical weathering, associated with low physical erosion. 4. Conclusions Airborne gamma-ray spectrometry brings a new perspective to weathering profile studies. Chemical erosion intensity was quantified at the scale all of a 150-km2 watershed using natural radioelement contrasted geochemical behaviors. The progressive loss of potassium has been observed throughout the saprolitization layer, from the unweathered micaschists to the mature saprolite, thanks to remote sensing measurements. Then, a simple mass balance, based on potassium and thorium geochemistry, was used to estimate paleoweathering rates of potassium. There is no distinct evolution for the Paleogene and early Cretaceous weathering profiles, suggesting that weathering intensity should have been comparable over these successive periods. The chemical weathering rates of potassium are estimated to ca. 17 F 2 kg/km2/a with a 50-m preserved weathering
profile for the Paleogene period. The development of such thick profiles requires extended periods of high weathering conditions (low tectonic activity, in a continental environment), coupled with inefficient physical erosion. Direct airborne gamma-ray measurements capture the long-term bedrock weathering history. References Anderson, S.P., Dietrich, W.E., Brimhall, G.H., 2002. Weathering profiles, mass balance analysis, and rates of solute loss: linkages between weathering and erosion in a small, steep catchment. Geol. Soc. Amer. Bull. 114, 1143 – 1158. Bonijoly, D., Perrin, J., Truffert, C., Asfirane, F., 1999. Couverture ge´ophysique ae´roporte´e du Massif Armoricain. BRGM report, vol. R40471. 75 pp. Brimhall, G.H., Alpers, C., Cunningham, A.B., 1985. Analysis of supergene oreforming processes using mass balance principles. Econ. Geol. 80, 1227 – 1254. Carrier, F. 2005. Utilisation de la spectrome´trie gamma ae´roporte´e (uranium, thorium, potassium) pour quantifier les processus dTe´rosion et dTalte´ration. The`se de doctorat de lTinstitut de Physique du Globe de Paris, 263 pp.
270
F. Carrier et al. / Journal of Geochemical Exploration 88 (2006) 266–270
Heimsath, A.M., Dietrich, W.E., Nishiizumi, K., Finkel, R.C., 1997. The soil production function and landscape equilibrium. Nature 388, 358 – 361. Kennett, J.P., Stott, L.D., 1991. Abrupt deep-sea warming, palaeoceanographic changes and benthic extinctions at the end of the Palaeocene. Nature 353, 225 – 229. McLennan, S.M., Nance, W.B., Taylor, S.R., 1980. Rare earth and element-thorium correlations in sedimentary rocks, and the composition of the continental crust. Geochim. Cosmochim. Acta 44, 1833 – 1839. Meybeck, M., 1979. Concentrations des eaux fluviales en e´le´ments majeurs et apports en solution aux oce´ans. Rev. Ge´ogr. Phys. Ge´ol., 31215 – 31246.
Riebe, C.S., Kirchner, J.W., Finkel, R.C., 2004. Erosional and climatic effects on long-term chemical weathering rates in granitic landscapes spanning diverse climate. EPSL 224, 547 – 562. Stallard, R.F., Edmond, J.M., 1983. Geochemistry of the Amazon: 2. The influence of the geology and weathering environment on the loa. J. Geophys. Res. 88, 9688 – 9761. Wyns, R., 1991. Structural evolution of the Armorican basement during the Cenozoic deduced from analysis of continental paleosurfaces and associated deposits. Ge´ol. Fr. 3, 11 – 42. Yap, C.J., 2004. Fe(CO3)OH in goethite from mid-latitude North American oxisol: estimate of atmospheric CO2 concentration in the early Eocene bclimatic optimumQ. Geochim. Cosmochim. Acta 68, 935 – 947.