- Email: [email protected]
Characteristics of a paleosol and its implication for the Critical Zone development, Rocky Mountain Front Range of Colorado, USA
Applied Geochemistry 26 (2011) S72–S75
Contents lists available at ScienceDirect
Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem
Characteristics of a paleosol and its implication for the Critical Zone development, Rocky Mountain Front Range of Colorado, USA Matthias Leopold a,⇑, Jörg Völkel a, David Dethierc, Juliane Huber a, Markus Steffens b a
Geomorphology and Soil Science, Technische Universität München, D-85354 Freising-Weihenstephan, Germany Soil Science, Technische Universität München, D-85354 Freising-Weihenstephan, Germany c Williams College, Dept. Geoscience, Williamstown, MA 01267, USA b
a r t i c l e
i n f o
Article history: Available online 22 March 2011
a b s t r a c t Activity and stability phases as well as geomorphic processes within the Critical Zone are well known. Erosion and deposition of sediments represent activity; soils represent geomorphic stability phases. Data are presented from a 4 m deep sediment section that was dated by luminescence techniques. Upslope erosion and resulting sedimentation started in the late Pleistocene around 18 ka until 12 ka. Conditions at the study site then changed, which led to the formation of a well-developed soil. Radiocarbon dating of the organic matter yielded ages between 8552 and 8995 cal. BP. From roughly 6.2 to 5.4 ka another activity phase accompanied by according sediment deposition buried the soil and a new soil, a Cambisol, was formed at the surface. The buried soil is a strongly developed Luvisol. The black colors in the upper part of the buried soil are not the result of pedogenic accumulation of normal organic matter within an A-horizon. Nuclear magnetic resonance spectroscopy clearly documents the high amount of aromatic components (charcoal), which is responsible for the dark color. This indicates severe burning events at the site and the smaller charcoal dust (black carbon) was transported to deeper parts of the profile during the process of clay translocation. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Erosion and deposition of weathered material constantly shape and alter hillslopes. Intensities and chronology of these processes vary from site to site and depend on several factors including slope, geologic and geomorphic conditions, climate and vegetation cover. Erosion of weathered material and its deposition in adjacent depressions is a well known process in many areas throughout the world. It indicates a time of enhanced geomorphic activity along a slope. The stabilization of such a system can have many causes such as reduction in relief, changes within the source area or shifts in climatic conditions, which often are associated with alterations of the vegetation. The formation of a dense vegetation cover slows erosion and helps the formation of a soil cover. Especially in unstable semi-arid areas, soils effectively stabilize the surface and they have other positive effects on the infiltration rates, the water storage or the exchange capacity for nutrients or pollutants. Soils are a major part of the Critical Zone (CZ) and their existence, absence or alteration influences the CZ as a whole (Anderson et al., 2007). The Boulder Creek Critical Zone Observatory (BC-CZO), Colorado, USA offers the possibility to study such pedo-geomor-
phic variations especially well at one of its study sites within the Betasso watershed (Fig. 1). Here, a formerly v-shaped 4 m deep gully was filled with gravel-rich material which was eroded from weathered granodiorite rocks upslope. A well preserved paleosol at a depth of about 1 m, indicates a stability phase of these geomorphic processes within the CZ. However, additional deposition of material and re-establishment of modern soil and vegetation buried the old soil and preserved its characteristics. Optical stimulated luminescence (OSL) and radiocarbon dating techniques are used to establish a chronology of activity and stabilization phases within the CZ. Furthermore, chemical and physical laboratory analyses such as texture and CNS-analysis, X-ray fluorescence and diffraction measurements together with thin-section descriptions help to characterize the buried soil and sediments. Additionally, nuclear magnetic resonance (NMR) spectroscopy has been conducted to better understand the composition of the organic matter within the buried soil.
2. Results and discussion 2.1. Geomorphic situation and sediment chronology
⇑ Corresponding author. Tel.: +49 8161712506. E-mail address: [email protected] (M. Leopold). 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.03.034
The study site is situated at 1920 m a.s.l. some hundred meters SW of the parking lot at Bummers Rock (N40°000 46.300 /
S73
M. Leopold et al. / Applied Geochemistry 26 (2011) S72–S75
Fig. 1. (a) The study site within the Betasso watershed, one of the study areas of the Boulder Creek Critical Zone Observatory (BC-CZO). (b) View from SE into the erosion gully which was formed due to a failure in a water pipeline. (c) Panoramic overview of the sedimentologic conditions. The upper dark horizon of the buried soil is indicated by the two dotted white lines. OSL = samples for optically stimulated luminescence techniques. RC = samples for radiocarbon dating. (d) Enlargement of the buried soil. Note the black color of the upper horizon and the polyhedral micro structures of the lower Bt-horizon.
W105°200 28.000 ). Several outcrops of the local coarse grained granite rock frame the rim of the watershed. The slopes consist of coarse gravel and sands and they reach slope inclinations of more than 20°. Due to a failure of a nearby water pipeline a deep gully eroded in a sediment body that filled the upper part of a small vshaped valley. This gully cuts through the sediments, including the buried soil, down to the bedrock (Fig. 1). Several phases of erosion and deposition at the study site have been identified (Völkel et al., 2010). OSL dating techniques were used to establish a sediment chronology at the site (Table 1). The sedimentation started around 18–16 ka at 3–4 m depth below the recent surface. Fine layered bands within the sediment complex indicate low energy flow. This general erosion and deposition
phase lasted until around 12.2 ± 0.7 ka. Sometime afterwards the geomorphic conditions at the site changed, which is indicated by the formation of a well-developed Luvisol. Radiocarbon dating of the bulk organic matter in the upper soil horizon yielded ages of 8649–8995 cal. BP and 8404–8552 cal. BP. A new phase of upslope erosion and deposition at the study site is represented by the OSL ages yielded from a sample slightly below 1 m of the surface. It started around 6.2 ± 0.3 ka and ended around 5.4 ± 0.3 ka at a depth of 55 cm. Above this depth there is no further time control, but recently the site has been covered by vegetation and a weak humic A-horizon above an oxidized B-horizon has developed. No indications of erosion or deposition of material under recent conditions have been found.
Table 1 Results from optical stimulated luminescence (OSL) and radiocarbon techniques used to establish a chrono-stratigraphy at the site. Calib. Rev 6.01 was used to calibrate Radiocarbon ages (Reimer et al., 2009). Riso No.
Sample
Depth (cm)
Age (ka)
Dose (Gy)
n
Dose rate (Gy/ka)
w.c.%
08 08 08 08 08
OSL OSL OSL OSL OSL
65 200 110 270 410
5.4 ± 0.3 12.2 ± 0.7 6.2 ± 0.3 18.0 ± 1.1 16.1 ± 1.1
25.3 ± 0.9 63 ± 2 29.3 ± 0.6 87 ± 3 77 ± 3
26 22 27 17 21
4.70 ± 0.21 5.16 ± 0.24 4.70 ± 0.21 4.83 ± 0.22 4.78 ± 0.22
4 4 4 4 4
54 54 54 54 54
40 41 42 43 44
1 2 3 4 5
Lab. No.
Sample
Material
Radiocarbon Age
cal. BP (2 sigma)
BETA248802 BETA248801
RC 1 RC 2
Organic matter Organic matter
7690 ± 40 7970 ± 40
8404–8552 8649–8995
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M. Leopold et al. / Applied Geochemistry 26 (2011) S72–S75
Relative signal intensity [%]
80 60 40 20 0
0.5 10.27 10.42 10.47 10.58 10.63 10.61 9.94 9.56 10.41 10.49 10.42 10.53 10.46 10.44 10.41 10.49 0.2 0.1
OM (%)
250
200
150
100
50
0
-50
-100
Chemical shift [ppm] Fig. 2. Chemical composition of the organic C in the upper blackish soil horizon as measured by solid state 13C nuclear magnetic resonance spectroscopy. Results show that a large part of the C is bound in aromatic structures. Carboxyl-C 220–160 ppm; Aromatic-C 160–110 ppm; O-Alcyl-C 110–45 ppm; Alcyl-C 45–10 ppm.
0.31 0.90 0.73 0.85 0.82 0.41 0.26 0.18 0.17 0.14 0.20 0.19 0.19 0.24 0.14 0.14 0.15 0.11 0.04
TC (%) pH (CaCI2)
300
SI3 Ls4 Ls4 Lts Lts Lts Lts Lts Lts Lts Lts Ls4 Ls4 Ls4 St3 St3 Ls4 Sl4 gS 11.4 22.3 22.4 25.6 29.0 30.7 34.3 29.1 26.0 25.2 25.3 23.7 23.8 22.0 18.6 18.4 18.6 13.3 1.5 15.1 24.1 27.4 27.9 23.0 24.3 24.0 24.7 25.0 21.8 21.2 20.0 20.9 17.6 13.9 14.3 16.9 15.5 6.5 3.1 7.0 8.0 7.3 5.6 4.9 5.7 5.0 5.6 4.5 4.3 3.8 4.6 4.0 3.1 3.2 3.7 3.8 0.7 5.3 9.2 11.0 10.9 8.8 8.8 8.5 9.5 6.0 7.0 7.0 6.3 7.3 5.8 4.9 4.5 5.9 4.5 1.7 6.7 7.8 8.4 9.7 8.6 10.6 9.8 10.3 13. 4 10.3 9.9 9.8 9.1 7.8 5.9 6.6 7.3 7.2 4.1 73.5 53.7 50.2 46.5 48.0 45.0 41.7 46.1 49.0 53.0 53.5 56.4 55.3 60.4 67.6 67.3 64.4 71.2 92.0 9.8 7.5 5.8 7.9 11.5 11.5 12.0 14.1 13.2 13.7 14.8 14.7 14.2 13.4 17.2 14.7 15.1 17.9 8.8 21.1 13.5 9.5 11.6 13.5 15.8 12.4 15.4 17.0 18.6 19.8 21.3 21.5 23.6 23.2 24.5 21.5 24.0 28.2 42.6 32.7 34.9 27.0 23.0 17.7 17.3 16.6 18.8 20.7 18.9 20.4 19.6 23.4 27.2 28.1 27.8 29.4 55.0 a
According to ad-hoc-AG Boden (2005).
23.6 9.7 9.5 6.6 7.3 6.9 6.1 9.8 10.2 9.2 11.5 95.0 10.30 12.50 10.50 18.20 22.10 17.6 48.8 M Bht Bht Bht Bht Bht–Bt Bt Bt Bt Bt Bt Bt Bt Bt Bt III Bt III Bt IVICj IVICj III III III III III III III III III III III III III III III III III III III
SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE
1/1 11/1 11/2 11/3 11/4 11/5 11/6 11/7 11/8 11/9 11/10 11/11 11/12 11/13 11/14 11/15 11/16 2/1 4/1 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964
Sample depth (cm) Soil horizona
110–130 150–155 155–160 160–165 165–169 169–175 175–180 180–185 185–190 190–195 195–200 200–205 205–210 210–215 215–220 220–225 225–230 350–370 370–400
cS
mS
fS
Sand
cSi
mSi
fSi
Silt
Clay
Soil texturea
6.25 5.97 6.06 6.09 6.15 6.18 6.17 5.78 5.56 6.05 6.10 6.06 6.12 6.08 6.07 6.05 6.10 6.38 6.54
2.2. Physical and chemical properties of the buried soil
>2 mm wt%
Grain sizea wt.%
100
-20 350
Sample
Table 2 Analytical data of the buried soil and overlaying colluvium. M = colluvium, Bht = Bt-horizon enriched with organic matter, Bt = clay enriched B-horizon, lCj = loose parent material.
79.0 77.0 86.5 86.5 86.0 65.5 84.0 104.5 102.0 113.0 139.0 138.0 188.5 619.0 313.5 202.0 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 11 9 8 7 8 5 3 2 3 2 3 3 3 3 2 3 2 6 6
TN
0.03 0.10 0.10 0.12 0.11 0.09 0.08 0.08 0.06 0.06 0.08 0.07 0.06 0.08 0.07 0.06 0.07 0.02 0.01
TS (%) C/N
Magn. susc. SI (105)
120
X-ray fluorescence analysis on the fine matrix (<2 mm) showed no major change in element distribution throughout the entire profile, which indicates a common sediment source. X-ray diffraction analysis of the phyllosilicates within the clay fraction also shows expandable smectite and illite as well as kaolinite throughout the profile. At the sampling area the buried soil starts at a depth of 150 cm below the surface (Table 2). This is indicated by a sharp sedimentologic change towards the overlying colluvium. The colors change from light brown to black and additionally the percentage of included gravels and stones drop from 23% in the colluvium to less than 9% in the buried soil. Also, the coarse sand fraction drops from 46% to as low as 14% in the buried soil which ends at around 220 cm. In general the buried soil consists of two horizons, a black to very dark brown (10YR 2/1 – 2/2) upper horizon and a dark yellowish brown lower horizon (10YR 3/6 – 4/4). Whereas the lower horizon is clearly identified within any soil classification system (here the German Ad-hoc AG Boden 2005 is used) as a Bt-horizon, the upper one is more complex. Based on diagnostic features it must be classified as a buried A-horizon which contains organic matter up to 1.6%. This is lower than the contents of modern A-horizons (OM 3.0%) in the vicinity. The C/N-ratio has values from 7 to 9, which is narrow and indicate well decomposed organic matter. The distribution of clay particles rises from 12% in the colluvium immediately above the buried soil to 22.3%. The clay contents constantly rise to nearly 30% in the upper soil horizon and further to 34% within the Bt-horizon at 180 cm depth before it starts to decrease to 18.9% at the base of the Bt-horizon at 220 cm depth (Table 2). A constant rise of clay particles within an A-horizon up to 30% would be unusually high and contradicts the formation factors known for such soil horizons. However, the analysis of the thinsection also documents well-developed clay cutans in the upper soil horizon, which indicates the clay to be of pedogenic and not of sedimentologic origin. Therefore, NMR spectroscopy was used in order to classify the organic matter. The results of one of the samples are presented in Fig. 2. Cross polarization 13C nuclear magnetic resonance spectroscopy (NMR) was applied to elucidate the chemical composition of the organic matter in the buried blackish upper soil horizon (Schmid et al., 2001; Fig. 2). Results show a very heterogeneous signal with the highest intensity originating from aromatic structures between 160 and 110 ppm. This implies that large parts of the organic matter are composed of these aromatic structures, which mainly consist of black carbon.
M. Leopold et al. / Applied Geochemistry 26 (2011) S72–S75
S75
Fig. 3. Chronostratigraphic scheme of the erosion and accumulation phases together with the soil formation. Major soil horizons are displayed in the white column at the right.
3. Conclusions
Acknowledgements
The data document a continuous sedimentation at the study site since the late Pleistocene (Fig. 3a and b). After 12.2 ± 0.7 ka a shift in the geomorphic system resulted in re-vegetation and the formation of a strong Luvisol (Fig. 3c). A forest vegetation is suggested as the translocation of clay presumes a stable surface which normally is best guaranteed under tree cover. Between 8649– 8995 cal. BP and 8404–8552 cal. BP a fire event occurred and produced large amounts of macro and micro charcoal (Fig. 3d). It is assumed that the finest particles of this black carbon were transported together with the clay during the lessivation process into deeper parts of the profile (nowadays the Bht-horizon in 1.5 m depth, Fig. 3e). The blackish upper horizon of the buried soil is not the fire event horizon itself which is also corroborated by the uniform distribution of the magnetic susceptibility values, which would have caused higher values due to the thermal formation of maghemite. However, the former A- and E-horizons of the Luvisol have been completely eroded (Fig. 3f) before additional sediments were deposited at the site between 6.2 ± 0.3 ka and 5.4 ± 0.3 ka and the actual Cambisol could form on the surface (Fig. 3g). This section documents the heterogeneous sedimentologic and pedologic history of the Critical Zone, which is important to know for any landscape evolution model.
The studies were carried out under the funding framework of ‘‘Boulder Creek Critical Zone Observatory (CZO)’’ sponsored by the US National Science Foundation (NSF) for which we are thankful. References Anderson, S.P., von Blanckenburg, F., White, A.F., 2007. Physical and chemical controls on the critical zone. Elements 3, 315–319. Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Bronk Ramsey, C., Buck, C.E., Burr, G.S., Edwards, R.L., Friedrich, M., Grootes, P.M., Guilderson, T.P., Hajdas, I., Heaton, T.J., Hogg, A.G., Hughen, K.A., Kaiser, K.F., Kromer, B., McCormac, F.G., Manning, S.W., Reimer, R.W., Richards, D.A., Southon, J.R., Talamo, S., Turney, C.S.M., van der Plicht, J., Weyhenmeyer, C.E., 2009. Intcal09 and Marine09 radiocarbon age calibration curves, 0– 50,000 years cal BP. Radiocarbon 51, 1111–1150. Schmid, E.M., Knicker, H., Baumler, R., Kogel-Knabner, I., 2001. Chemical composition of the organic matter in neolithic soil material as revealed by CPMAS C-13 NMR spectroscopy, polysaccharide analysis, and CuO oxidation. Soil Science 166, 569–584. Völkel, J., Huber, J., Leopold, M., Dethier, D.P., 2010. Young quaternary slope sediments and paleosoils in the Colorado Front Range, process and age. Geological Society of America Abstracts 42, No. 5, 177553.
Contents lists available at ScienceDirect
Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem
Characteristics of a paleosol and its implication for the Critical Zone development, Rocky Mountain Front Range of Colorado, USA Matthias Leopold a,⇑, Jörg Völkel a, David Dethierc, Juliane Huber a, Markus Steffens b a
Geomorphology and Soil Science, Technische Universität München, D-85354 Freising-Weihenstephan, Germany Soil Science, Technische Universität München, D-85354 Freising-Weihenstephan, Germany c Williams College, Dept. Geoscience, Williamstown, MA 01267, USA b
a r t i c l e
i n f o
Article history: Available online 22 March 2011
a b s t r a c t Activity and stability phases as well as geomorphic processes within the Critical Zone are well known. Erosion and deposition of sediments represent activity; soils represent geomorphic stability phases. Data are presented from a 4 m deep sediment section that was dated by luminescence techniques. Upslope erosion and resulting sedimentation started in the late Pleistocene around 18 ka until 12 ka. Conditions at the study site then changed, which led to the formation of a well-developed soil. Radiocarbon dating of the organic matter yielded ages between 8552 and 8995 cal. BP. From roughly 6.2 to 5.4 ka another activity phase accompanied by according sediment deposition buried the soil and a new soil, a Cambisol, was formed at the surface. The buried soil is a strongly developed Luvisol. The black colors in the upper part of the buried soil are not the result of pedogenic accumulation of normal organic matter within an A-horizon. Nuclear magnetic resonance spectroscopy clearly documents the high amount of aromatic components (charcoal), which is responsible for the dark color. This indicates severe burning events at the site and the smaller charcoal dust (black carbon) was transported to deeper parts of the profile during the process of clay translocation. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Erosion and deposition of weathered material constantly shape and alter hillslopes. Intensities and chronology of these processes vary from site to site and depend on several factors including slope, geologic and geomorphic conditions, climate and vegetation cover. Erosion of weathered material and its deposition in adjacent depressions is a well known process in many areas throughout the world. It indicates a time of enhanced geomorphic activity along a slope. The stabilization of such a system can have many causes such as reduction in relief, changes within the source area or shifts in climatic conditions, which often are associated with alterations of the vegetation. The formation of a dense vegetation cover slows erosion and helps the formation of a soil cover. Especially in unstable semi-arid areas, soils effectively stabilize the surface and they have other positive effects on the infiltration rates, the water storage or the exchange capacity for nutrients or pollutants. Soils are a major part of the Critical Zone (CZ) and their existence, absence or alteration influences the CZ as a whole (Anderson et al., 2007). The Boulder Creek Critical Zone Observatory (BC-CZO), Colorado, USA offers the possibility to study such pedo-geomor-
phic variations especially well at one of its study sites within the Betasso watershed (Fig. 1). Here, a formerly v-shaped 4 m deep gully was filled with gravel-rich material which was eroded from weathered granodiorite rocks upslope. A well preserved paleosol at a depth of about 1 m, indicates a stability phase of these geomorphic processes within the CZ. However, additional deposition of material and re-establishment of modern soil and vegetation buried the old soil and preserved its characteristics. Optical stimulated luminescence (OSL) and radiocarbon dating techniques are used to establish a chronology of activity and stabilization phases within the CZ. Furthermore, chemical and physical laboratory analyses such as texture and CNS-analysis, X-ray fluorescence and diffraction measurements together with thin-section descriptions help to characterize the buried soil and sediments. Additionally, nuclear magnetic resonance (NMR) spectroscopy has been conducted to better understand the composition of the organic matter within the buried soil.
2. Results and discussion 2.1. Geomorphic situation and sediment chronology
⇑ Corresponding author. Tel.: +49 8161712506. E-mail address: [email protected] (M. Leopold). 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.03.034
The study site is situated at 1920 m a.s.l. some hundred meters SW of the parking lot at Bummers Rock (N40°000 46.300 /
S73
M. Leopold et al. / Applied Geochemistry 26 (2011) S72–S75
Fig. 1. (a) The study site within the Betasso watershed, one of the study areas of the Boulder Creek Critical Zone Observatory (BC-CZO). (b) View from SE into the erosion gully which was formed due to a failure in a water pipeline. (c) Panoramic overview of the sedimentologic conditions. The upper dark horizon of the buried soil is indicated by the two dotted white lines. OSL = samples for optically stimulated luminescence techniques. RC = samples for radiocarbon dating. (d) Enlargement of the buried soil. Note the black color of the upper horizon and the polyhedral micro structures of the lower Bt-horizon.
W105°200 28.000 ). Several outcrops of the local coarse grained granite rock frame the rim of the watershed. The slopes consist of coarse gravel and sands and they reach slope inclinations of more than 20°. Due to a failure of a nearby water pipeline a deep gully eroded in a sediment body that filled the upper part of a small vshaped valley. This gully cuts through the sediments, including the buried soil, down to the bedrock (Fig. 1). Several phases of erosion and deposition at the study site have been identified (Völkel et al., 2010). OSL dating techniques were used to establish a sediment chronology at the site (Table 1). The sedimentation started around 18–16 ka at 3–4 m depth below the recent surface. Fine layered bands within the sediment complex indicate low energy flow. This general erosion and deposition
phase lasted until around 12.2 ± 0.7 ka. Sometime afterwards the geomorphic conditions at the site changed, which is indicated by the formation of a well-developed Luvisol. Radiocarbon dating of the bulk organic matter in the upper soil horizon yielded ages of 8649–8995 cal. BP and 8404–8552 cal. BP. A new phase of upslope erosion and deposition at the study site is represented by the OSL ages yielded from a sample slightly below 1 m of the surface. It started around 6.2 ± 0.3 ka and ended around 5.4 ± 0.3 ka at a depth of 55 cm. Above this depth there is no further time control, but recently the site has been covered by vegetation and a weak humic A-horizon above an oxidized B-horizon has developed. No indications of erosion or deposition of material under recent conditions have been found.
Table 1 Results from optical stimulated luminescence (OSL) and radiocarbon techniques used to establish a chrono-stratigraphy at the site. Calib. Rev 6.01 was used to calibrate Radiocarbon ages (Reimer et al., 2009). Riso No.
Sample
Depth (cm)
Age (ka)
Dose (Gy)
n
Dose rate (Gy/ka)
w.c.%
08 08 08 08 08
OSL OSL OSL OSL OSL
65 200 110 270 410
5.4 ± 0.3 12.2 ± 0.7 6.2 ± 0.3 18.0 ± 1.1 16.1 ± 1.1
25.3 ± 0.9 63 ± 2 29.3 ± 0.6 87 ± 3 77 ± 3
26 22 27 17 21
4.70 ± 0.21 5.16 ± 0.24 4.70 ± 0.21 4.83 ± 0.22 4.78 ± 0.22
4 4 4 4 4
54 54 54 54 54
40 41 42 43 44
1 2 3 4 5
Lab. No.
Sample
Material
Radiocarbon Age
cal. BP (2 sigma)
BETA248802 BETA248801
RC 1 RC 2
Organic matter Organic matter
7690 ± 40 7970 ± 40
8404–8552 8649–8995
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M. Leopold et al. / Applied Geochemistry 26 (2011) S72–S75
Relative signal intensity [%]
80 60 40 20 0
0.5 10.27 10.42 10.47 10.58 10.63 10.61 9.94 9.56 10.41 10.49 10.42 10.53 10.46 10.44 10.41 10.49 0.2 0.1
OM (%)
250
200
150
100
50
0
-50
-100
Chemical shift [ppm] Fig. 2. Chemical composition of the organic C in the upper blackish soil horizon as measured by solid state 13C nuclear magnetic resonance spectroscopy. Results show that a large part of the C is bound in aromatic structures. Carboxyl-C 220–160 ppm; Aromatic-C 160–110 ppm; O-Alcyl-C 110–45 ppm; Alcyl-C 45–10 ppm.
0.31 0.90 0.73 0.85 0.82 0.41 0.26 0.18 0.17 0.14 0.20 0.19 0.19 0.24 0.14 0.14 0.15 0.11 0.04
TC (%) pH (CaCI2)
300
SI3 Ls4 Ls4 Lts Lts Lts Lts Lts Lts Lts Lts Ls4 Ls4 Ls4 St3 St3 Ls4 Sl4 gS 11.4 22.3 22.4 25.6 29.0 30.7 34.3 29.1 26.0 25.2 25.3 23.7 23.8 22.0 18.6 18.4 18.6 13.3 1.5 15.1 24.1 27.4 27.9 23.0 24.3 24.0 24.7 25.0 21.8 21.2 20.0 20.9 17.6 13.9 14.3 16.9 15.5 6.5 3.1 7.0 8.0 7.3 5.6 4.9 5.7 5.0 5.6 4.5 4.3 3.8 4.6 4.0 3.1 3.2 3.7 3.8 0.7 5.3 9.2 11.0 10.9 8.8 8.8 8.5 9.5 6.0 7.0 7.0 6.3 7.3 5.8 4.9 4.5 5.9 4.5 1.7 6.7 7.8 8.4 9.7 8.6 10.6 9.8 10.3 13. 4 10.3 9.9 9.8 9.1 7.8 5.9 6.6 7.3 7.2 4.1 73.5 53.7 50.2 46.5 48.0 45.0 41.7 46.1 49.0 53.0 53.5 56.4 55.3 60.4 67.6 67.3 64.4 71.2 92.0 9.8 7.5 5.8 7.9 11.5 11.5 12.0 14.1 13.2 13.7 14.8 14.7 14.2 13.4 17.2 14.7 15.1 17.9 8.8 21.1 13.5 9.5 11.6 13.5 15.8 12.4 15.4 17.0 18.6 19.8 21.3 21.5 23.6 23.2 24.5 21.5 24.0 28.2 42.6 32.7 34.9 27.0 23.0 17.7 17.3 16.6 18.8 20.7 18.9 20.4 19.6 23.4 27.2 28.1 27.8 29.4 55.0 a
According to ad-hoc-AG Boden (2005).
23.6 9.7 9.5 6.6 7.3 6.9 6.1 9.8 10.2 9.2 11.5 95.0 10.30 12.50 10.50 18.20 22.10 17.6 48.8 M Bht Bht Bht Bht Bht–Bt Bt Bt Bt Bt Bt Bt Bt Bt Bt III Bt III Bt IVICj IVICj III III III III III III III III III III III III III III III III III III III
SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE
1/1 11/1 11/2 11/3 11/4 11/5 11/6 11/7 11/8 11/9 11/10 11/11 11/12 11/13 11/14 11/15 11/16 2/1 4/1 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964 4964
Sample depth (cm) Soil horizona
110–130 150–155 155–160 160–165 165–169 169–175 175–180 180–185 185–190 190–195 195–200 200–205 205–210 210–215 215–220 220–225 225–230 350–370 370–400
cS
mS
fS
Sand
cSi
mSi
fSi
Silt
Clay
Soil texturea
6.25 5.97 6.06 6.09 6.15 6.18 6.17 5.78 5.56 6.05 6.10 6.06 6.12 6.08 6.07 6.05 6.10 6.38 6.54
2.2. Physical and chemical properties of the buried soil
>2 mm wt%
Grain sizea wt.%
100
-20 350
Sample
Table 2 Analytical data of the buried soil and overlaying colluvium. M = colluvium, Bht = Bt-horizon enriched with organic matter, Bt = clay enriched B-horizon, lCj = loose parent material.
79.0 77.0 86.5 86.5 86.0 65.5 84.0 104.5 102.0 113.0 139.0 138.0 188.5 619.0 313.5 202.0 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 11 9 8 7 8 5 3 2 3 2 3 3 3 3 2 3 2 6 6
TN
0.03 0.10 0.10 0.12 0.11 0.09 0.08 0.08 0.06 0.06 0.08 0.07 0.06 0.08 0.07 0.06 0.07 0.02 0.01
TS (%) C/N
Magn. susc. SI (105)
120
X-ray fluorescence analysis on the fine matrix (<2 mm) showed no major change in element distribution throughout the entire profile, which indicates a common sediment source. X-ray diffraction analysis of the phyllosilicates within the clay fraction also shows expandable smectite and illite as well as kaolinite throughout the profile. At the sampling area the buried soil starts at a depth of 150 cm below the surface (Table 2). This is indicated by a sharp sedimentologic change towards the overlying colluvium. The colors change from light brown to black and additionally the percentage of included gravels and stones drop from 23% in the colluvium to less than 9% in the buried soil. Also, the coarse sand fraction drops from 46% to as low as 14% in the buried soil which ends at around 220 cm. In general the buried soil consists of two horizons, a black to very dark brown (10YR 2/1 – 2/2) upper horizon and a dark yellowish brown lower horizon (10YR 3/6 – 4/4). Whereas the lower horizon is clearly identified within any soil classification system (here the German Ad-hoc AG Boden 2005 is used) as a Bt-horizon, the upper one is more complex. Based on diagnostic features it must be classified as a buried A-horizon which contains organic matter up to 1.6%. This is lower than the contents of modern A-horizons (OM 3.0%) in the vicinity. The C/N-ratio has values from 7 to 9, which is narrow and indicate well decomposed organic matter. The distribution of clay particles rises from 12% in the colluvium immediately above the buried soil to 22.3%. The clay contents constantly rise to nearly 30% in the upper soil horizon and further to 34% within the Bt-horizon at 180 cm depth before it starts to decrease to 18.9% at the base of the Bt-horizon at 220 cm depth (Table 2). A constant rise of clay particles within an A-horizon up to 30% would be unusually high and contradicts the formation factors known for such soil horizons. However, the analysis of the thinsection also documents well-developed clay cutans in the upper soil horizon, which indicates the clay to be of pedogenic and not of sedimentologic origin. Therefore, NMR spectroscopy was used in order to classify the organic matter. The results of one of the samples are presented in Fig. 2. Cross polarization 13C nuclear magnetic resonance spectroscopy (NMR) was applied to elucidate the chemical composition of the organic matter in the buried blackish upper soil horizon (Schmid et al., 2001; Fig. 2). Results show a very heterogeneous signal with the highest intensity originating from aromatic structures between 160 and 110 ppm. This implies that large parts of the organic matter are composed of these aromatic structures, which mainly consist of black carbon.
M. Leopold et al. / Applied Geochemistry 26 (2011) S72–S75
S75
Fig. 3. Chronostratigraphic scheme of the erosion and accumulation phases together with the soil formation. Major soil horizons are displayed in the white column at the right.
3. Conclusions
Acknowledgements
The data document a continuous sedimentation at the study site since the late Pleistocene (Fig. 3a and b). After 12.2 ± 0.7 ka a shift in the geomorphic system resulted in re-vegetation and the formation of a strong Luvisol (Fig. 3c). A forest vegetation is suggested as the translocation of clay presumes a stable surface which normally is best guaranteed under tree cover. Between 8649– 8995 cal. BP and 8404–8552 cal. BP a fire event occurred and produced large amounts of macro and micro charcoal (Fig. 3d). It is assumed that the finest particles of this black carbon were transported together with the clay during the lessivation process into deeper parts of the profile (nowadays the Bht-horizon in 1.5 m depth, Fig. 3e). The blackish upper horizon of the buried soil is not the fire event horizon itself which is also corroborated by the uniform distribution of the magnetic susceptibility values, which would have caused higher values due to the thermal formation of maghemite. However, the former A- and E-horizons of the Luvisol have been completely eroded (Fig. 3f) before additional sediments were deposited at the site between 6.2 ± 0.3 ka and 5.4 ± 0.3 ka and the actual Cambisol could form on the surface (Fig. 3g). This section documents the heterogeneous sedimentologic and pedologic history of the Critical Zone, which is important to know for any landscape evolution model.
The studies were carried out under the funding framework of ‘‘Boulder Creek Critical Zone Observatory (CZO)’’ sponsored by the US National Science Foundation (NSF) for which we are thankful. References Anderson, S.P., von Blanckenburg, F., White, A.F., 2007. Physical and chemical controls on the critical zone. Elements 3, 315–319. Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Bronk Ramsey, C., Buck, C.E., Burr, G.S., Edwards, R.L., Friedrich, M., Grootes, P.M., Guilderson, T.P., Hajdas, I., Heaton, T.J., Hogg, A.G., Hughen, K.A., Kaiser, K.F., Kromer, B., McCormac, F.G., Manning, S.W., Reimer, R.W., Richards, D.A., Southon, J.R., Talamo, S., Turney, C.S.M., van der Plicht, J., Weyhenmeyer, C.E., 2009. Intcal09 and Marine09 radiocarbon age calibration curves, 0– 50,000 years cal BP. Radiocarbon 51, 1111–1150. Schmid, E.M., Knicker, H., Baumler, R., Kogel-Knabner, I., 2001. Chemical composition of the organic matter in neolithic soil material as revealed by CPMAS C-13 NMR spectroscopy, polysaccharide analysis, and CuO oxidation. Soil Science 166, 569–584. Völkel, J., Huber, J., Leopold, M., Dethier, D.P., 2010. Young quaternary slope sediments and paleosoils in the Colorado Front Range, process and age. Geological Society of America Abstracts 42, No. 5, 177553.